U.S. GEOLOGICAL SURVEY
Open-File Report 98Ð620
Oreshoot Zoning in the Carlin-type Betze 
Orebody, Goldstrike Mine, Eureka County, 
Nevada



By Stephen G. Peters1, Gregory C.  Ferdock2, Maria B.  
Woitsekhowskaya1, Robert Leonardson3, and Jerry Rahn3




This report is preliminary and has not been reviewed for conformity with U.S. 
Geological Survey editorial standards or with the North American 
Stratigraphic Code.  Any use of trade, product, or firm names is for descriptive 
purposes only and does not imply endorsement by the U.S. Government.



Abstract
	Field and laboratory investigations of the giant Betze gold orebody, the largest Carlin-
type deposit known, in the north-central Carlin trend, Nevada document that the orebody is 
composed of individual high-grade oreshoots that contain different geologic, mineralogic, and 
textural characteristics.  The orebody is typical of many structurally controlled Carlin-type 
deposits, and is hosted in thin-bedded, impure carbonate or limy siltstone, breccia bodies, and 
intrusive or calc-silicate rock.  Most ores in the Betze orebody are highly sheared or 
brecciated and show evidence of syndeformational hydrothermal deposition.  The interplay 
between rock types and pre- and syn-structural events accounts for most of the distribution 
and zoning of the oreshoots.  Hydrothermal alteration is scale dependent, either in broad, 
pervasive alteration patterns, or in areas related to various oreshoots.  Alteration includes 
decarbonatization (~decalcification) of carbonate units, argillization (illite-clay), and 
silicification.  Patterns of alteration zoning in and surrounding the Betze orebody define a 
large porous, dilated volume of rock where high fluid flow predominated.  Local restriction of 
alteration to narrow illite- and clay-rich selvages around unaltered marble or calc-silicate rock 
phacoids implies that fluid flow favored permeable structures and deformed zones.  Gold 
mainly is present as disseminated sub-micron-sized particles, commonly associated with 
AsÐrich pyrite, although one type of oreshoot contains micron-size free gold.

	Oreshoots form a three-dimensional zoning pattern in the orebody within a 
WNWÐstriking structural zone of shearing and shear folding, termed the Dillon deformation 
zone (DDZ).  Main types of oreshoots are:  (1) rutile-bearing siliceous oreshoots;  (2) illite-
clay-pyrite oreshoots; (3) realgar- and orpiment-bearing oreshoots;  (4) stibnite-bearing 
siliceous oreshoots; and (5) polymetallic oreshoots.  Zoning patterns result from 
paragenetically early development of illite-clay-pyrite oreshoots during movement along the 
DDZ, and subsequent silicification and brecciation, associated with formation of the realgar- 
and orpiment-bearing, and stibnite-bearing oreshoots.  Additional shear movement along the 
DDZ followed.  Polymetallic oreshoots, which contain minerals rich in Hg, Cu, Zn, Ag, and 
native Au, were the last ores to form and overprint most earlier oreshoots.  

	Ore textures, gouge, phyllonitic rock, alteration style, and previously documented 
isotopic and fluid-inclusion data, all indicate a weakly to moderately saline fluid that ascended 
and cooled during structural displacements.  Changing conditions, due to water-wall rock 
reactions and PÐT changes during deformation, are probably responsible for fluid variation 
that resulted in zoning of the different oreshoots during dynamic interaction of the Au-bearing 
fluid with the wall rock.  This investigation indicates that isolated AsÐ, SbÐ, and HgÐrich ores 
are separate parts of a larger single gold system.  This large gold system was 
contemporaneous with post-Jurassic brittle-ductile deformation, on the basis of deformed 
mineralized pods of the Jurassic Goldstrike pluton, and large-scale hydrothermal flow, and 
together they appear to be an integral part of the formation of some Carlin-type gold deposits 
in north-central Nevada.


INTRODUCTION

	The Betze gold orebody is one of more than 50 gold deposits along the Carlin trend, 
Nevada (Fig. 1), which together are conservatively estimated to contain more than 100 million 
oz (~3,200 tonnes) of gold (Christensen, 1996; Teal and Jackson, 1997).  Exploration activity 
in the Betze area began in 1962 and mining on the Goldstrike property, which took place 
between 1974 and 1986, exploited oxide gold orebodies in largely siliceous host rocks.  The 
deep hypogene Betze and Deep Post orebodies were discovered in 1987 and production 
began by Barrick Goldstrike Mines Inc. and Newmont Mining Co. Ltd. shortly thereafter.  
Over 10 million oz gold (~325 tonnes gold) have been produced to 1998.  The Betze orebody 
contains over 30 million oz (~1,000 tonnes) gold in hypogene ores averaging about 0.2 oz 
Au/t (6.4 g Au/t).  Mining rates at the Goldstrike Mine have been between 300,000 to 
500,000 tonnes per day, of which between 30,000 to 60,000 tonnes are ore.

	Carlin-type deposits are characterized by relatively uniform, low gold grades 
(Arehart, 1996); however, recent open pit exposures in deep hypogene parts of the Betze 
orebody and in other gold deposits along the Carlin trend suggest that some orebodies are 
composed of distinct high-grade oreshoots that are zoned complexly in three dimensions 
(Peters, 1996, 1997a).  Stratabound, structurally controlled, and complex orebodies were 
noted along the Carlin trend by Christensen (1993).  In structurally controlled type deposits, 
such as the Betze orebody, oreshoots contain micro- and mesoscopic-scale deformation 
fabrics and textures that can be linked directly with paragenetic development of the orebody.  
Geometric and structural characteristics of oreshoots in the Betze orebody indicate that some 
parts of the orebody were formed synchronously with deformation and have spatial and 
genetic relations to folds and faults.

	The results presented in this paper suggest that each oreshoot type in the Betze 
orebody is characterized by a combination of rock type, ore- and alteration-mineral 
assemblage, and structural fabric.  Understanding the nature of oreshoots and predicting their 
location and shape has always been a major concern of economic geologists (Penrose, 1910; 
Hulin, 1929; Blanchard, 1931; McKinstry, 1941, 1955; Peters, 1993a).  Oreshoots usually 
indicate zones of dilation and high fluid flow, as well as unique rock chemistry.  They are 
characterized by higher metal content than that in adjacent mineralized rocks.  The mass of 
most gold-bearing oreshoots in mesothermal and epithermal vein deposits ranges between 2 x 
104 and 1 x 106 tonnes with grades generally greater than 0.1 oz Au/t (3.1g Au/t) (Peters, 
1993b).  Oreshoots generally have a heterogeneous grade distribution, such that they are 
thicker and richer in the center or in a lobe along one side.  Oreshoots rarely terminate 
abruptly, commonly at geologic features, or their margins may taper in thickness or grade to 
assay cut-offs.  A characteristic feature of many oreshoots is their unique internal geologic 
complexity and mineralogy that reflect multiple episodes of formation.  Oreshoots in the 
Betze orebody have characteristics similar to most other oreshoots, but the large size of the 
entire orebody and the multiple types of oreshoots are unusual.  Documentation and analysis 
of this oreshoot variation in the Betze orebody is important to understand Carlin-type 
deposits because it is the largest gold orebody along the Carlin trend.  Differentiating oreshoot 
types also may prove to be important in ore sorting and mixing prior to processing in 
underground operations.


PREVIOUS WORK

	Geologic investigations of ore deposits along the Carlin trend and of the Betze 
orebody have been numerous (Teal and Jackson, 1997), but the origin of these deposits is still 
in dispute (see Hofstra and others, 1991; Ilchik and Barton, 1997; Arehart, 1996; Peters and 
others, 1996; Tosdal, 1998).  The geology of the Goldstrike Mine area has been described in 
some detail by Sampson (1993), Volk and Lauha (1993), Campbell (1994), Volk and

 others (1996), Peters (1996, 1997a), and Leonardson and Rahn (1996).  Additional 
documentation and summaries of the geology and ores of the area were given by Bettles 
(1989), Arehart and others (1993b,c), Lauha and Bettles (1993), Thoreson (1993), and Smith 
and Sharon (1994).  These workers have described a shallow WNWÐplunging, structurally 
complex orebody that is hosted in Paleozoic sedimentary rocks along a diorite contact zone.  
The gold ores are typified by disseminated gold-bearing arsenian pyrite.  Much of the Betze 
orebody is hosted in breccia (Peters and others, 1997), which is common in many Carlin-type 
deposits (Williams, 1992, 1993), particularly in orebodies of the north-central Carlin trend 
(Clode, 1993; Griffin and others, 1993; Leach, 1993).  

	Paragenetic study of ores in the Betze orebody and its associated orebodies have been 
conducted by Berry (1992), Bettles and Lauha (1991), Arehart and others (1993a), Lamb 
(1995), and Ferdock and others (1996, 1997), and it contains mineral sequences similar to 
those proposed for most other Carlin-type deposits (see Radtke, 1985; Kuehn, 1989; Bakken 
and Einaudi, 1986; Madrid and Bagby, 1986; Kuehn and Rose, 1992, 1996; Groff, 1995).  
These studies interpreted one or more gold events and recognized mineralogically distinct 
types of ore, but spatial relations between the various types of ore were not well established.  
Mineralogically distinct types of ore have been identified in most Carlin-type deposits, and 
temporal relations among these ore types have been proposed (Hausen and Kerr, 1966, 1968; 
Madrid and Bagby, 1988); however, the concept of distinct, high-grade, oreshoots in Carlin-
type deposits and their spatial and genetic relation to one another has not been well 
understood (see also Radtke and others, 1972a, b, 1974b, 1977; Radtke and Dickson, 1975).  


METHODS OF STUDY

	Geologic bench and broken ore pit bottom mapping at 1 in = 50 ft and at 1 in  = 5 ft 
(1:600 and 1:60) scales, supplemented by drill-core and drill-chip logging, provided the basis 
for observations made in this paper.  Blast hole assays that were collected for mine planning 
and metallurgical purposes on the 4,800 ft elevation level were compiled and were hand 
contoured.  Paragenetic relations in the Betze deposit were determined from mine-scale 
mapping, from slabbed hand specimens, and from thin section petrology as well as SEM 
studies.  Scanning-electron-microscope work was conducted at the U.S.  Geological Survey, 
Menlo Park, and at the Mackay School of Mines, Reno.  XRD analysis was conducted at 
Barrick Goldstrike and at Mackay School of Mines.  Illite crystallinity estimations followed 
the methods of Kirsch (1991) and used both the half peak width and area measurements of 
<2Ð_m clay material.  Geochemical modeling was conducted, as part of a larger study 
(Woitsekhowskaya and Peters, 1998), using the HCh program of Schvarov (1976).  
Geochemical analysis of ores was done by the U.S.  Geological Survey laboratories, Denver, 
and also was contracted through the Nevada Bureau of Mines and Geology, Mackay School 
of Mines, Reno.


REGIONAL GEOLOGIC SETTING

	In the Carlin trend area, northern Nevada, early and middle Paleozoic, deep-water, 
siliciclastic sedimentary and volcanic rocks were thrust eastward approximately 75 to 200 km 
during the Late Devonian to Early Mississippian Antler orogeny (Roberts and others, 1958).  
These rocks compose the approximately 5Ðkm-thick Roberts Mountains allochthon (Madrid 
and others, 1992), which lies upon coeval shallow-water, carbonate-rich rocks of the 
continental platform (Figs. 1 and 2, table 1).  The two packages of rocks, the upper and lower 
plates, are separated by the Roberts Mountains thrust fault.  The Golconda allochthon 
(Silberling and Roberts, 1962), consisting of uppermost Devonian to lower Upper Permian 
carbonate-rich turbiditic sands and basinal strata, was thrust on top of the continental margin 
and parts of the Roberts Mountains allochthon during the latest Permian and early Triassic.  
These tectonic events indicate crustal thickening accompanied by deformation and are 
generalized and combined with other, subsequent tectonic events in the region on table 1 as 
deformation events D0 through D4.  Gold mineralization in the Carlin trend is interpreted to 
be either Late Cretaceous or early Eocene (Arehart and others, 1993c; Teal and Jackson, 
1997), indicating that it would be synchronous with the Late Jurassic to Early Eocene Sevier-
Laramide D3 tectonic event (table 1).  A NEÐtrending lineament, the Crescent Valley-
Independence Lineament crosses the Carlin trend and is interpreted by Peters (1998) and by 
Theodore and others (1998) to have had geologic activity along it from the Late Paleozoic to 
the Middle Cenozoic, which overlaps the gold mineralizing time estimates of the Carlin trend 
(Fig. 2).

	Sedimentary rock-hosted disseminated gold deposits in northern Nevada are spatially 
associated with tectonic windows through the Roberts Mountains allochthon or with 
structural highs beneath the allochthon (Roberts, 1960; 1966; Thorman and Christensen, 
1991; Peters, 1997b,c).  The Carlin trend is a northwest-trending belt of gold deposits near 
these windows (Fig. 1).  The largest window in the Carlin trend area is the northwest-
plunging Lynn-Carlin window that hosts the Betze orebody and its associated deposits in its 
northern parts (Figs. 1, 2, 3A, B).  Lower-plate rocks exposed in the southern part of the 
window (see Evans 1974a, b, 1980) consist of Ordovician and Cambrian(?) thick-bedded 
dolomite and thin-bedded limestone, massive, thick-bedded quartzite, and sandy dolomite 
(Figs. 3C,D).  These rocks are overlain by Ordovician black to dark gray, fine-grained, thick-
bedded to massive dolomite with bioclastic debris and chert.  

	Gold deposits mainly are hosted in the three upper units of the lower plate:  (1) the 
Silurian and Devonian Roberts Mountains Formation, a silty laminated dolomitic limestone;  
(2) the Devonian unnamed limestone (Dl) of Evans (1980), locally called the Popovich 
limestone, composed of platy, laminated, silty dolomitic and micritic limestone; and (3) the 
locally recognized Devonian Rodeo Creek unit (see Christensen, 1993), which consists of 
laminated mudstone, siltstone, siliciclastic and cherty rocks, with local limestone.  Although 
gold is hosted in all these rock types, over 90 percent of the gold is present in a 350Ðm-thick 
stratigraphic interval encompassing the top and bottom of the Popovich limestone (Fig. 3C, 
D; see also Armstrong and others, 1997).  Upper-plate rocks locally are mineralized and 
include rocks of the siliceous (western) assemblage of Evans (1980), which are generally 
assigned to the Ordovician Vinini Formation in the area of the Carlin trend (Madrid, 1987; 
Madrid and others, 1992), but also include Silurian and Devonian rocks (Barrick, unpub. data, 
1996; Cluer and others, 1997).  

	Gold mineralized rocks along the Carlin trend are concentrated along a series of 
NNWÐ and NEÐtrending, medium- to high-angle district-scale shear zones and faults (Figs. 2, 
3B and 4).  In addition, several WNW-striking, medium to shallow, NEÐdipping shear folds 
have been recognized (Peters, 1997a, b, c).  In the area around the Carlin trend the strikes of 
common shallow-plunging F2 (table 1) fold axes in both the lower- and upper-plate rocks can 
be grouped into two domains.  Domain I contains a series of NEÐSWÐplunging fold axes, and 
Domain II is a local NNWÐSSEÐtrending belt of fold axes with common NNW strikes (Fig. 
4A).  Domain I is most common in upper plate rocks outside of the Carlin trend.  Domain II 
roughly coincides with lower-plate "windows," but also contains upper-plate rocks, which 
have NNWÐSSEÐtrending fold axes.  This geometry has been interpreted by Peters (1996, 
1997c) to result from shear folding, where WNWÐESEÐstriking Late Jurassic to Early Eocene 
D3 shear zones (table 1) form F3 shear folds.  These shear folds refolded and rotated original 
F2 folds from NEÐSW to NNWÐSSE orientations (Figs. 3A, 4B, C).  This refolding and 
rotation resulted in the preponderance of NNWÐSSEÐplunging fold axes in the mineralized 
areas in Domain II, because the F2 NEÐplunging fold axes were rotated by 
WNWÐESEÐplunging F3 shear folds.  The main F3 shear fold in the Goldstrike Mine is the 
Dillon deformation zone (designated as DDZ; Figs. 3A, 5A, 6A,B, and 7A,B,C), a zone of 
shallow, NE-dipping, WNWÐESEÐstriking brittle-ductile shear zones and WNW-
ESEÐplunging shear folds, which host the Betze orebody and is described by Peters (1996) 
and Leonardson and Rahn (1997).  The DDZ and other WNW-trending F3 shear folds and 
shear zones within Domain II in the Carlin trend area (see Figs. 3A and 4C) are important 
features for interpretation of syndeformational Carlin-type gold ores, because structural 
fabrics and ore textures in the orebody should be aligned with, and related to, these F3 
structures if mineralization were synchronous with D3 deformation.


GEOLOGY OF THE GOLDSTRIKE MINE AREA

	Host rocks in the Betze orebody are altered and deformed.  The Betze orebody in 
the Goldstrike Mine is surrounded by the upper Post and north Betze orebodies on the 
north and by the Deep Post orebody on the east side (Fig. 5).  These contiguous orebodies 
have NNW trends and shallow SE plunges, parallel to fold axes in the Post anticline.  The 
main Betze orebody, in contrast, has a WNW trend and a near horizontal plungeÑparallel 
to the DDZ and the Betze anticlineÑalthough some extensions of the orebody have NNW 
orientations and the lower parts of the orebody are more irregular.

	Rocks in the Goldstrike Mine area are upper-plate Ordovician through Devonian 
siliciclastic rocks of the Roberts Mountains allochthon, mostly assigned to the Vinnini 
Formation, and Devonian and Silurian lower-plate carbonate and siliciclastic rocks (Fig. 5).  
These rocks are in contact with the northern part of the Late Jurassic (158ÐMa) 
Goldstrike stock and sill complex (Fig. 5), which also locally is mineralized (Bettles, 1989; 
Arehart and others, 1993a).  Miocene Carlin Formation rocks and Quaternary alluvium 
also are present on the north and east sides of the mine (Fig. 5).

Sedimentary Rocks
	Sedimentary host rocks of the Betze orebody are lower-plate rocks of the lower 
Rodeo Creek unit, Popovich limestone, and upper Roberts Mountains Formation (Fig. 5).  
These units are composed of limestone, dolomite and siliciclastic material and also contain 
rare sedimentary breccias.  These three units served, not only as host rocks for the Betze 
orebody, but also as conduits for migrating fluids and as host rocks for other orebodies along 
most of the north-central Carlin-trend (Fig. 3B).  

	The Rodeo Creek unit is composed of locally carbonaceous, bedded, siliciclastic 
mudstone (locally termed argillite), chert, and calcareous rocks.  The hanging wall or 
northeastern part of the upper central Betze orebody is located along a deformed transitional 
contact between the Rodeo Creek unit and the underlying Popovich limestone.  The 
transitional zone is composed of bedded siltstone, sandstone, chert, argillite, slaty mudstone, 
phyllonitic rock, fossiliferous massive muddy limestone, limy mudstone, and red knobby 
breccia bodies, across an interval of 20 to 40 m.  The transitional contact zone also contains 
intricate flow-like folding of white, gray and black, clay-rich siltstone (decalcified limestone) 
that locally is structurally interleaved with white to creamy, tan, and black, pyritic cataclastic 
pods composed of deformed massive to laminated argillite, and decalcified limestone and 
mudstone.  Local dismembered phyllonitic bodies in the transitional sequence suggest that 
early deformationÑpossibly of Antler age (D1, table 1)Ñtook place along this transitional 
contact zone, prior to D2 folding and was followed by D3 shearing and mineralization (Peters, 
1997a).

	 The Popovich limestone is host rock for much of the Betze orebody and commonly 
is present as phacoidal-shaped, gray limestone and white to green-gray crystalline marble 
blocks that are surrounded by gray and black decalcified, clay-altered, deformed seams and 
breccia layers.  Unaltered upper strata of the Popovich limestone are medium- to thin-bedded 
to massive, carbonaceous, micritic, and dark gray to black with local carbonate veinlets and 
diagenetic pyrite.  The lower parts of the Popovich limestone and upper parts of the Roberts 
Mountains Formation contain horizons composed of both planar- and wispy-laminated, 
sandy, bioclastic dolomitic limestone.  Local horizons are accompanied by soft-sediment 
deformation.

	Sedimentary breccias in these host rocks are stratabound, lenticular facies composed 
of rip-up breccia, debris flow breccia, and fossil hash breccia (Griffin and others, 1993; Peters 
and others, 1997).  Rip-up breccia bodies, along the transitional contact between the Rodeo 
Creek unit and the Popovich limestone, contain 0.5Ð to 2Ðcm-diameter laminated, angular 
clasts of siliciclastic argillite and shell fragments cemented by a fine-grained, sandy to silty 
matrix of similar material.  Near the Betze orebody, these rip up breccia bodies are silicified.  
Debris flow breccia bodies and fossil hash breccia horizons are common in the middle and 
lower parts of the Popovich limestone and in the upper parts of the Roberts Mountains 
Formation over a 100Ð to 230Ðm-thick interval.  Fossil hash beds are 0.5 to 3 m wide and 
contain whole or fragmented shells and fossil debris.  These zones of sedimentary breccia 
constitute large parts of the Betze orebody and have been contact metamorphosed, 
decalcified, collapsed, and structurally deformed, which allowed them to provide ready 
passage to flow of hydrothermal fluids.


Igneous and Metamorphic Rocks

	The footwall or southwest part of the Betze orebody is hosted in the contact 
metamorphic zone of the Goldstrike stock (Figs. 5, 6A,B), which has intruded the Rodeo 
Creek unit, the Popovich limestone, the Roberts Mountains Formation, and rocks lower in 
the stratigraphic section.  The stock is composed of homogenous, medium-grained and locally 
siliceous, biotite- and hornblende-bearing, gray diorite to tan quartz diorite and minor 
granodiorite with sills, dikes, and apophyses that extend into the hornfels and calc-silicate 
contact zone forming thin, irregular shapes (Fig. 6).  Dikes and small intrusions of Late 
Jurassic lamprophyre and Eocene intermediate composition dikes also are present in the 
diorite and in the wall rocks (Leonardson and Rahn, 1996).

	Contact metamorphism near the Goldstrike stock consists of pyroxene-biotite and 
silica hornfels and marble.  These rocks locally are overprinted by diopside-grossular, 
diopside-grossular-vesuvianite, and tremolite-epidote-calcite KÐfeldspar-diopside skarn 
(Walck, 1989), and by sericite alteration in the intrusive rocks.  These contact metamorphic 
and metasomatic rocks are most common in a 230Ðm-wide zone adjacent to the main contact 
and near apophyses and epiphysis of the stock, along NNWÐSSEÐstriking faults, as 300Ðm-
wide inliers in the diorite, and in structurally isolated blocks (Fig. 6A).  The northeast contact 
of the Goldstrike stock parallels the WNWÐESE oriented trend of the Betze orebody.  
Textures and structural analysis of phacoids of diorite, calc-silicate rock, and marble indicate 
that the diorite-Popovich limestone contact has been deformed by F3 (table 1) shear folding 
along the DDZ.  Apophyses of diorite have been dismembered and isolated into phacoidal 
shapes along phyllonitic seams.  Bodies of 15Ð to 70Ðm-thick metamorphosed and 
metasomatized rock provided large areas of mottled, brittle rock and protobreccia that 
subsequently were fractured and brecciated, and commonly were surrounded and isolated by 
illite-clay phyllonitic seams.  Field relations strongly suggest that the main gold 
mineralization post-dates the Goldstrike stock (see also Arehart and others, 1993c; 
Leonardson and Rahn, 1996).  


Structure

	Most orebodies in the north-central part of the Carlin trend are associated with 
NNWÐstriking faults, such as the gouge-filled Post and JB faults, or with NNWÐtrending 
folds, such as the Post anticline and Peters syncline (Fig. 5) (see also, Resnell (1990).  The 
most important NNWÐSSEÐstriking fold in the Betze orebody is the pointed synclinal keel of 
the Peters syncline that contains rocks of the Rodeo Creek unit in contact with the more 
competent underlying rocks of the Popovich limestone (Fig. 5).  The deformed transitional 
sequence in this keel between the two units acutely crosses and is deformed by shear folding 
in the WNWÐESEÐstriking upper central Betze orebody and these transitional sequence rocks 
in the orebody are further brecciated and show evidence of dissolution, shearing, 
decalcification, fluid flow, and folding.  In lower parts of the orebody, the Roberts Mountains 
Formation is broadly folded below these units in the synclinal keel.  

	The WNWÐESE orientation of the Betze orebody is anomalous when compared to the 
NNWÐSSE orientation of the Carlin trend and its parallel faults and folds.  These two 
orientations are separated by 40 to 50 degrees of strike (Fig. 4C).  This consistent horizontal-
plunging WNWÐESE orientation of the Betze orebody has been attributed by several workers 
(Peters, 1996, 1997a; Leonardson and Rahn, 1996) to be related to the DDZ (Figs. 5, 6 and 
7).  Folds in the Goldstrike Mine all contain NNWÐstriking fold axes and axial planes outside 
the orebody, but contain WNWÐESEÐstriking orientations (F3 folds?) inside the orebody 
(Fig. 4B).  All WNW-ESEÐstriking fold axes in and proximal to the Betze orebody lie along a 
great circle that approximates a plane dipping 27¼ NE and striking 300ž (Fig. 4B), parallel to 
the DDZ.  The Betze anticline also lies in and is parallel to the DDZ (Fig. 5).  Kinematic 
indicators of symmetry in the DDZ show both right lateral and left lateral movement, and 
displacement across the zone is equivocal, which suggests that deformation is restricted to a 
volume that closely approximates the Betze orebody and is consistent with non-steady 
ductile flow in shear zones as described by Dazhi and White (1995).

	As interpreted by Peters (1996, 1997a) at the district scale in defining Domains I and 
II (Fig. 4A), the NNWÐSSEÐtrending Post anticline and Peters syncline, are specific examples 
of originally NEÐSWÐtrending regional-scale F2 folds, that have been refolded and rotated 
through shearing and refolding on WNWÐESEÐstriking F3 shear zones (in this case the DDZ) 
to their present NNWÐESEÐ and NWÐSEÐtrending orientations.  Some of this refolding can 
be seen in the change in strike of the axial planes of these folds as shown on Figure 5.  
Additional evidence of refolding can be seen as 100Ðm-wide internal fold closures on the 
western limb of the Post anticline.  An alternative interpretation is that these 
NNWÐSSEÐtrending folds could be due to folding related to strike-slip faulting along the Post 
and JB fault systems.  This last possibility is compatible with the conclusions of Evans and 
Theodore (1978) that these NNWÐSSEÐtrending folds along the Carlin trend were generated 
by Jurassic tectonism.

	Thick zones of tectonic breccia also serve as host rocks for much of the Betze 
orebody and are most common in the DDZ.  These breccia zones are present in or related 
to shear zones, brittle faults, and associated folds and contain rock types with textures 
such as breccia, cataclasite, gouge, and phyllonite that have characteristics of fault zone 
rocks described by Sibson (1997), Ramsey (1980a) and Tanaka (1992).  The main types 
include:  (1) breccia formed along the Popovich limestone-Rodeo Creek unit contact;  (2) 
cataclastic zones and fault gouge; and (3) breccia and phyllonite associated with strands of 
the DDZ.  Mineralized, silicified, 20Ðm-thick, white to black, locally pyritic, cataclastic, 
breccia bodies consist of broken and crushed argillite, decalcified limestone, and mudstone.  
Breccia and phyllonite in the central parts of the DDZ consist of undeformed phacoidal 
clasts, slabs, and blocks present in sheared pelitic zones, which anastamose around them.  
Brecciation in the DDZ postdates the igneous apophyses and dikes of the Goldstrike 
stock and fragments of igneous rocks are present in many of the breccia bodies.

	Strain in the DDZ was partitioned differently in each rock type.  For example, 
pervasively sheared, 1Ð to 10Ðm-thick, phyllonitic strands are common locally, but elsewhere 
undeformed phacoidal clasts, slabs, and blocks lie in sheared, pelitic layers that anastamose 
around them.  Breccia bodies are surrounded by seams of sheared, carbonaceous, illite-clay-
rich phyllonite.  This structural isolation of individual altered and mineralized cataclastic pods 
indicates multiple movements along individual fault planes.  Deformation styles throughout 
the DDZ also indicate that products of processes of dissolution, transport, fluid flow, 
plucking, decalcification, and high strain are spatially coincident with gold and its associated 
geochemical elements and further suggests that the hydrothermal and deformational events 
may have been linked in time and in space, such that mineralization and deformation were, at 
least in part, synchronous.  


Mineralization

	The Betze orebody is surrounded by as many as twenty smaller contiguous and non-
contiguous orebodies in an approximate 3,000 by 1,200 m area over a 700 m vertical extent.  
Mine-scale mapping and drilling has defined an ore outline as a 0.035 oz/t (~1.0 g/t) Au 
contour line (Fig. 5) that has an average thickness of 170 m, an average width of 200 m, and a 
strike length of over 1.5 km (Bettles and Lauha, 1991; Leonardson and Rahn, 1996).  Patterns 
of alteration zoning in and surrounding the Betze orebody define a large, porous, dilated zone 
where fluid flow was high.  Several different ore types and ore grades are distributed in 
discrete oreshoots.  A central 0.3 by 2 km core of high-grade gold-associated illite-clay and 
silica alteration in and adjacent to the high-grade oreshoots is surrounded by 500Ðm-thick 
lower-grade zone of decalcification and weak illite-clay.  Contours of gold and sulfide sulfur 
content in the orebody (Figs. 7A, and B) indicate that most oreshoots are concentrated along 
the WNWÐESEÐtrending diorite- Popovich limestone contact, the NNWÐSSEÐstriking 
Popovich limestone-Rodeo Creek unit contact in the keel of the Betze syncline, individual 
strands of the WNWÐESEÐstriking DDZ, and along ENEÐWSWÐstriking fault zones (not 
shown on Fig. 5) that cross cut the DDZ.  

	Oreshoots contain cataclastic mixtures of crushed, altered, and brecciated wall rock, 
gouge, and phyllonite that are spatially related to host rock types and to faults and folds.  
The oreshoots cluster in a zonal pattern in plan from the hanging wall to the footwall of the 
orebody and in section with pyrite- and arsenic-rich ores on the top or outside of the orebody 
and quartz (antimony)-rich ores in the bottom and center parts.  Analysis of mesoscopic 
distribution and crosscutting relations between these oreshoots indicates an orebody-scale 
paragenesis that can be used with other microscopic and geochemical tools and methods to 
interpret the timing and genesis of the orebody.  Because much of the Betze orebody is 
structurally controlled, the relation of ore deposition to the host breccia, phyllonite and folds 
determines processes of ore genesis in terms of the regional tectono- paleo-thermal history.


Hydrothermal Alteration 

	Alteration types associated with gold mineralized rocks in the Betze orebody include:  
carbonation (carbon introduction), decarbonatization (_decalcification), argillization (illite-
clay), and silicification, similar to alteration types in other Carlin-type ore deposits described 
by Radtke (1985), Bakken and Einaudi (1986), Kuehn and Rose (1992), and Arehart (1996).  
Pre-gold alteration assemblages include syngenetic or diagenetic minerals in Paleozoic 
sedimentary rocks, as well as contact metamorphic and metasomatic mineral associated 
spatially with the contact of the Goldstrike diorite.  Post-gold alteration effects are related to 
supergene processes and locally to Miocene hot-spring related events associated with Basin 
and Range development (Fig. 8).  The high-grade, upper 30 m of the Betze orebody contains 
an alteration assemblage of illite-clay minerals, whereas the lower parts of the orebody 
generally are silicified.  The outer alteration zone, outward from the 0.035 oz/t Au contour 
(Fig. 5), has an irregular shape and is composed of non-contiguous, black, carbonaceous 
bodies of weak illite-clay alteration and silicification.  The difference between alteration in the 
Betze orebody system and those in other Carlin-type deposits is (1) the large area of 
alteration and (2) the superposition of the alteration on zones of contact metamorphism and 
metasomatism, as well as intense structural deformation at Betze.

	Carbonation as pre-gold carbon (Fig. 8) is present in large masses of black, sooty 
sedimentary rocks that surround and lie in the orebody, and is similar to carbon associated 
with most Carlin-type deposits (see Radtke and Scheiner, 1970a, b; Kuehn, 1989).  Carbon 
content in the main orebody is generally less than 1 wt. percent, whereas areas surrounding 
the northern parts of the orebody contain carbon contents in excess of 10 wt. percent.  This 
distribution of carbon away from the main orebody has been attributed by Leonardson and 
Rahn (1996) to be caused by the mobilization of carbon outward from the Goldstrike stock 
during intrusion, and the tendency for gold to be precipitated proximal to the Goldstrike 
stock contact inside this mobilized carbon front.  The carbon usually takes the form of 2Ð to 
15Ð_m-size platy crystals and aggregates away from ore and is present as angular grains of 
less well-crystallized graphite or carbon in the ore.  Carbon may also have been remobilized 
during gold deposition (Ferdock and others, 1997), as suggested by bitumen-calcite veinlets.  
Carbon is present in illite-clay altered sedimentary horizons, in phyllonitic sheared seams in 
the hanging wall of the upper central orebody, along high-angle fault zones above ore, and in 
beds of the Rodeo Creek unit and Popovich limestoneÑparticularly the Post 
anticlineÑabove the orebody (Peters, 1997a).  Local areas of carbon inside the orebody may 
have been important in the deformation history of the Betze deposit, because they represent 
areas of low shear strength and were therefore commonly areas of high strain.

	Decarbonatization, and decalcification, have affected large parts of the Popovich 
limestone and to a lesser extent the Rodeo Creek unit and Roberts Mountains Formation, 
along a 100Ðm-wide zone in the central part of the orebody (Fig. 7C).  Decarbonatization has 
removed carbonate minerals and much of the Fe, Mg, and Ca.  Decalcification refers to the 
removal of calcite and is the general term used in field mapping.  Leonardson and Rahn (1996) 
indicate that decalcification expanded vertically along fractures into tabular, bedding-parallel 
layers, and therefore collapse breccia bodies are most common as stratabound zones in the 
central parts of the orebody.  During decarbonatization and decalcification the carbonate 
content in limestone decreased from 30 to 40 wt. percent to as much as less than 1 wt. 
percent.  Carbonate content in the orebody is spatially variable, but a relatively uniform 
central zone of generally less than 10 wt. percent carbonate lies along the central part of the 
DDZ (Fig. 7C).  Magnesium content in dolomitic rocks and lamprophyre dikes follows a 
similar pattern.  Several discrete areas in this WNWÐtrending central low carbonate zone (Fig. 
7C) have higher carbonate contents:  (1) northeastern areas coincide with dark gray, unaltered, 
relatively undeformed Popovich limestone (area a, Fig. 7C);  (2) southwestern and 
northwestern areas coincide with unmineralized and unaltered marble along the diorite-
Popovich limestone contact (areas marked b, Fig. 7C); and (3) the southeastern area 
represents seams of high-grade realgar-orpiment ore with calcite veining surrounding barren 
marble phacoids (area c, Fig. 7C).

	Decarbonatized collapse breccias contain 30 to 50 vol. percent, centimeter-size, 
anhedral clasts of multicolored rock fragments, contained in a 50 to 70 vol. percent fine-
grained matrix of small, millimeter-scale rock fragments and illite-clay minerals (Fig. 9).  
Contact metamorphism, illite-clay alteration, silicification, and deformation have overprinted 
early-formed collapse breccias, which may themselves have been superimposed over 
sedimentary breccia (Peters and others, 1997).  Decarbonatization of the limestone and 
marble resulted in increased permeability that provided conduits for gold-bearing fluids.  
Volume loss and collapse most likely accompanied phyllonite development and was 
associated closely in time with a hydrothermal event.  Many collapse- and tectonic-related 
megabreccias have oriented fabrics defined by illite-clay-rich brecciated fragments and 
phyllonite seams that surround phacoidal-shaped blocks of limestone or marble (areas A, B 
and C on Fig. 7C), indicating that fluid pathways were along these phyllonitic zones and that 
much of the dissolution and collapse was synchronous with strain (Fig. 9).

	Argillization as illite-clay alteration is pervasive forming soft, elongate, pervasively 
altered masses of rock in the upper 30 m of the Betze orebody.  Illite-clay alteration also is 
present as widespread mesoscopic stratabound layers and microscopic inter-layers with 
quartz, especially in the lower parts of the orebody.  Illite-clay alteration commonly is 
associated with gold in the north-central Carlin trend (see Drews-Armitage and others, 1996), 
where illite is the dominant clay mineral phase, although, dickite, smectite, and kaolinite 
locally are present separately in the rock or are interlayered in the illite crystals.  Diorite is 
transformed to soft, white- to green- to tan-colored rock with relict igneous textures.  Beds of 
dark gray Popovich limestone are altered to soft, creamy white, gray and black layers rich in 
clay, sulfide and carbon.  Altered parts of the Popovich limestone commonly are sheared or 
folded and these softer areas surround unaltered, hard, phacoidal or round 1Ð to 10Ðm-size 
blocks of undeformed massive limestone, marble, or calc-silicate rock.  Crystallinity of illite in 
the Betze orebody is similar in phyllonite (0.36_ to 0.54_ 2_), in clay fault gouge (0.38_ to 
0.41_ 2_), in decalcified rock (0.38_ to 0.57_ 2_) and in illite-clay altered silty limestone 
(0.39_ to 0.53_ 2_).  The range of 0.2 _2__ in these rocks is close the minimum value (0.1_ 
2_) for the degree of confidence of illite crystallinity measurements discussed by Robinson 
and others (1990). This suggests that clay minerals in these rocks at some time reached 
similar temperatures and is compatible with synchronous deformation and hydrothermal 
alteration.  Crystallinities of illite in altered diorite and calc-silicate rock (0.73_ to 1.05_ 2_), 
suggest that either lower temperatures were reached in these rocks or may indicate addition 
clay minerals, such as smectite, that are interlayered with the illite.

	Silicification is present in isolated breccia bodiesÑand to a lesser extent in 
unbrecciated rockÑin the hanging wall and footwall parts of the upper orebody and increases 
in the lower parts of the orebody (Fig. 11 B).  Silicification mainly consists of 
cryptocrystalline and microcrystalline quartz between and replacing breccia clasts, but also is 
present as 1Ð to 2Ðmm-thick veinlets that cut breccia clasts.  Multiple stages of silicification 
are common in local areas; for example, fine-grained quartz replaces breccia clasts and coarse-
grained quartz replaces or forms the matrix of the breccia.  Leonardson and Rahn (1996) 
record at least five stages of silicification, some of which are similar to the quartz veinlet 
stages recorded by Bakken and Einaudi (1986) and Kuehn and Rose (1995) in the Carlin 
deposit.  Quartz veinlets containing AsÐrich, gold-stage pyrite commonly parallel phyllonitic 
foliation and are broken and fractured, indicating shear zone movement during or after silica 
emplacement.  Silicified breccia bodies and cataclastic zones commonly are isolated 
structurally, surrounded by phyllonitic illite-clay seams or by fault gouge, also indicating 
post-silicification shear zone movement.  In some siliceous breccias, parts of the matrix are 
not filled or replaced by quartz but consist of angular voids, similar to crackle breccia.  
Silicified areas contain both micro- and mesoscopic-scale mixed zones of illite-clay that 
alternate with bands of silica along bedding planes or along deformation fabric layers.  
Silicification of diorite, hornfels and calc-silicate rock is rare; however, residual, pre-gold, 
microcrystalline silica in subsequently altered contact metamorphic and metasomatic rocks 
forms the protolith for most breccia fragments in the silicified oreshoots.


Paragenesis

	Mineral deposition stages in the Betze orebody are designated as pre-gold, syn-gold 
and post-gold stages on the basis of field mapping and crosscutting relations determined 
microscopically (Fig. 8).  The syn-gold stage also has been divided into three substages by 
Ferdock and others (1997).  Pre-ore stages are:  (1) Paleozoic detrital or diagenetic events;  (2) 
contact metamorphic, metasomatic or hydrothermal base metal-dominated mineral deposition 
associated with the Late Jurassic Goldstrike stock; and (3) early hydrocarbon mobilization.  
Post-gold ore mineral development consisted of minor hypogene and supergene events (Fig. 
8).

	Gold-stage ore deposition in the Betze orebody started with disseminated pyrite, 
followed by AsÐrich pyrite (Fig. 10A) with micron-scale gold deposited as a separate phase 
or contained in AsÐrich rims grown on early pyrites, similar to those described by Wells and 
Mullen (1973), Arehart and others  (1993a,b), Fleet and Hamid (1997), and Bakken and 
others (1989) in other Carlin-type deposits.  Temporal relations between mineral phases is 
indicated by most workers to start with the deposition of pyrite with gold-rich, AsÐrich rims, 
followed by ore rich in orpiment-realgar, barite, stibnite, and mercury.  This paragenetic 
sequence also is the same as that produced by geochemical modeling of typical Carlin-type 
systems due to fluid-rock interaction and to cooling of the hydrothermal system from 210_ - 
200_ C to 180  - 150C, at moderate salinity (<10 wt. percent NaCl equivalent) (Hofstra and 
others, 1991; Woitsekhowskaya and Peters, 1998).

	Geochemical modeling also indicates that the hydrothermal fluid contained gold 
complexed as Au(HS)2-, whereas the dominant arsenic complex was H3AsO3, and antimony 
was complexed as Sb(OH)3 (Woitsekhowskaya and Peters, 1998).  Gold, arsenic, antimony 
and other components in ore were transported by these chemical complexes to the 
depositional sites from outside the orebody at initial total pressures of approximately 1,200 
bars (Lamb and Cline, 1997).  The major features of the alteration zonation pattern observed 
in the orebody resulted from the reaction of the ore fluid with host rocks as the system 
cooled and pressure decreased.  During dissolution-precipitation reactions, including 
carbonate dissolution, both silica-leaching and silica-fixation, and sulfide mineral precipitation 
controlled mass transfer in the Betze orebody. Mass transfer calculations (Woitsekhowskaya 
and Peters, 1998) indicate that gold precipitation was associated with sulfidation of reactive 
iron in the system.  This sulfidation further created higher hydrogen and lower sulfide sulfur 
activities in the system, which then destabilized the H3AsO3 and Au(HS)2- complexes, and 
caused gold precipitation in association with AsÐrich pyrite.  This model is similar to 
precipitation mechanisms suggested for Carlin-type systems in general by Rytuba (1985), 
Hofstra and others (1991), and Stenger and others (1998) and confirms that sulfidation of 
host rock iron, during reaction of the ore fluid with host rock, was the primary gold 
precipitation mechanism.

	Combinations of different minerals, such as As-pyrite, orpiment and realgar, stibnite, 
quartz calcite and other minerals help distinguish different types of ore on a mesoscopic 
scale, although microscopic assemblages are not always definitive.  For example, at least five 
types of pyrite (Fig. 10B) in the Betze orebody, including oscillatory zoned (Fig. 10C) and 
rare reversibly zoned pyrite (Fig. 10D), suggest local complex paragenesis.  The AsÐrich 
pyrite was followed and over-printed by realgar, orpiment and calcite, and coevalÑbut 
spatially separateÑquartz, barite, stibnite, and sphalerite (Fig. 8).  These gold-stage 
assemblages have similar form, shape and style in the orebody, but are present in separate 
areas along the DDZ, where they are isolated from each other in separate masses by fault 
gouge and phyllonitic seams.  Structural and geometric similarity of each ore type suggests 
that they formed in a similar structural environment rather than from multiple mineralizing 
systems or events punctuated by time intervals.

	The evolving stages of hydrothermal mineral development associated with gold ore 
deposition overlapped due to cycling of the circulating hydrothermal fluid, such that different 
mineral assemblages formed both sequentially and simultaneously, depending on fluid-flow 
pathways through the orebody (Ferdock and others, 1997).  The gold mineralizing events 
evolved from initial preparation of an environment conducive to early gold deposition to a 
dominant gold-depositing event, which was followed by late deposition of calcite and quartz 
(Fig. 8).  Most hydrothermal gangue minerals were formed late and represent concentration 
and deposition of the elements remaining in solution, principally Sb, As, Ba, Fe, S and lesser 
amounts of Zn, W, In, Co, Cd, Se, Cs, Tl, and Hg.  Some sulfate minerals (barite and gypsum) 
and some carbonate minerals (calcite, dolomite, and siderite) precipitated late.  


Zoning and Control of Oreshoots

	Distribution of different ore mineral associations and alteration types in the Betze 
orebody is dependent on structural setting, host rock, and location in the orebody.  On the 
basis of morphology, mineralogy, and geologic setting, several spatially distinct styles of ore 
are recognized and cluster into elongate oreshoots, separated from one another by waste or 
low-grade rocks.  These oreshoots constitute three-dimensional geochemical, mineralogical, 
geological, and metallurgical zoning in the orebody.  Separate ore control mechanisms of 
individual oreshoots correlate with individual strands of the DDZ and with local shear folding 
in the Betze anticline.  Local control of the location and shape of the oreshoots also is due to 
features in the keel of the Peters syncline, petrology of the sedimentary rocks, and geometry 
and thickness of the diorite contact (Figs. 6A,B).  

	Classification and discrimination of different oreshoots is made on the basis of 
previous studies of the different ore types and host rock types in the orebody.  Six distinct 
ore types were recognized in the Betze orebody by Peters (1996, 1997a) and by Leonardson 
and Rahn (1996).  These ore types and their related rock types roughly correlate with 
oreshoot types and are portrayed on Figures 11 and 12.  Breccia is the most common host 
rock type in the oreshoots and breccia types were differentiated by Peters and others (1997) 
on the basis of geologic setting, mineralogy, geochemistry, and morphology.

	In addition to distinct petrologic features related to breccia type, oreshoots also are 
characterized by variable geometry, mineralogy, geochemistry, structural style, and location 
in the orebody.  None of these features are distinct for all oreshoots.  Oreshoots consist of 
five main types:  (1) rutile-bearing siliceous oreshoots;  (2) illite-clay-pyrite oreshoots;  (3) 
realgar- and orpiment-bearing oreshoots;  (4) stibnite-bearing siliceous oreshoots; and (5) 
polymetallic oreshoots (Figs. 11 and 12).  Characteristics of these oreshoot types are given 
on table 2.  Geochemical analyses of representative samples from each oreshoot type is given 
in table 3.  The following summary of the characteristics of the oreshoots is given from the 
hanging wall to the footwall of the orebody, or from the northeast to southwest parts (see 
Fig. 6B) and roughly coincides with the older to younger paragenetic development of the 
orebody.  About three dozen distinct oreshoots are present on the 4,800-foot elevation in the 
upper central Betze orebody and at least 60 percent of the orebody can be viewed as 
occurring in oreshoots.  


Rutile-Bearing Siliceous Oreshoots

	Rutile-bearing siliceous oreshoots (hanging wall siliceous breccia bodies of Peters, 
1996, 1997a) are present along a 300Ðm-long zone at the transitional contact between the 
Rodeo Creek unit and Popovich limestone.  They are characterized on a mesoscopic scale as 
6Ð to 40Ðm-long, 2Ð to 25Ðm-wide, knobby, red to dark gray, shallow-dipping, lenticular 
pods (Figs. 5, 6B, 11; table 2). These oreshoots are further characterized on a microscopic 
scale by mm-size or smaller mottled-textured, pisolitic, millimeter- to micron-scale grain 
aggregate intergrowths of different combinations of ilmenite, titanite, magnetite, pyrite, rutile, 
sphalerite, and pisolitic apatite (Fig. 14).  These oreshoots typically are anomalous in Zn, Ni, 
V, P, and Tl (table 3) and have gold grades between 0.5 and 2.0 oz Au/t (~15.5 to 64 g Au/t).  
Detrital minerals in the transitional sequence of rocks between the Rodeo Creek unit and 
Popovich limestone account for local minerals high in Ti, U, and REE.  Some zircon grains 
also are present intergrown with REE minerals, or with Ti minerals in cryptocrystalline 
groundmass.  These pre-gold minerals were common nucleation sites for AsÐrich pyrite and 
sphalerite during the main gold event.  

	Individual rutile-bearing siliceous oreshoots consist of tectonic breccia, fossil hash, or 
sedimentary breccia and contain collapse features present in an interval about 1 m above the 
oreshoots.  Silicification consists of a pervasive dark, cryptocrystalline groundmass and 
replacement of clasts, and late 0.5Ð to 10Ðmm-thick, clear or opaque quartz veinlets.  Pyrite 
is present as disseminations throughout the cryptocrystalline quartz and in veinlets as 
elongate vein-parallel aggregates of AsÐrich pyrite and hydrothermal rutile.  The rutile is 
presumably derived from the local detrital ilmenite in the wall rock.


Illite-Clay-Pyrite Oreshoots

	Illite-clay altered rocks with disseminated pyrite and carbonaceous material comprise 
the largest volume of ore in the upper central Betze orebody (Fig. 12).  They form as locally 
siliceous, brecciated, illite-clay-pyrite bodies mainly present in the DDZ.  The lower parts of 
the orebody have less abundant illite-clay alteration and are more typically silicified breccias. 
This type of ore predates all but the rutile-bearing siliceous oreshoots.  Because illite-clay-
pyrite alteration and mineralization are paragenetically early, this type of ore forms a 
mesoscopic and microscopic matrix oreshoots that were superimposed over the illite-clay-
pyrite ores.  Pods of illite-clay-pyrite ores that were not replaced by later ores have been 
locally structurally isolated along strands of the DDZ and form individual oreshoots.  Other 
illite-clay-pyrite bodies commonly wrap around the other oreshoots in gouge-filled or 
phyllonitic seams.  Most illite-clay-pyrite oreshoots contain 0.10 to 0.25 oz Au/t (~3.2 to 
7.8 g Au/t).  Geochemically this ore type has high values of Cr, Zn, Fe, Ba, Ti, and Cu with 
local Ba, Ni, and V (table 3).  The oreshoots commonly are isolated from one another, and 
from other oreshoot types low-grade zones (Figs. 6A,B and 11; table 2).  Three main textural 
types include:  (1) disseminated AsÐrich pyrite in relatively undeformed, unbrecciated 
decalcified limestone and silty limestone (Fig. 15A); (2) tectonized, phyllonitic rock, with 
disseminated gold-bearing AsÐrich pyrite contained in and peripheral to broken quartz 
veinlets (Fig. 15B); and (3) structurally complex isolated, commonly silicified, polygenetic 
breccia (Figs. 15C and D).  The ores typically are tectonized, dark, black, sulfide- and carbon-
bearing, and are locally but intensely phyllonitic.  Where bedding is recognizable, intense 
folding is common.  Black and gray, 10Ð to 50Ðcm-thick illite-clay-rich oreshoots contain soft 
dense gouge and breccia that commonly parallel the phyllonitic fabric or bedding on the flanks 
of isoclinal folds.  Sedimentary bedding slip zones are common and are filled with phyllonitic 
gouge.
	
	Pyrite is disseminated throughout the illite-clay-pyrite oreshoots in several textural 
forms and generations, including 0.01Ð to 2Ðmm-size euhedral crystals, anhedral grains and 
grain aggregates (Figs. 10 A, B, and 15A, B, C).  Pyrite is closely related to 1-mm to 1Ðcm-
thick cryptocrystalline quartz veinlets that typically parallel the phyllonitic matrix and are 
typically broken or dismembered.  Arsenic-rich pyrite is present as felt-textured grains in the 
quartz veinlets and as <0.01-mmÐdiameter disseminated grains and aggregates in phyllonitic 
wall rock.  Gold is micron sized and commonly is contained in 1Ð to 10еm-thick AsÐrich 
rims on pyrite (Arehart, 1993a).  Gold distribution in these oreshoots is not uniform.

	Illite-clay-rich, cataclastic, pyritic, gold-rich zones and breccia pods are common 
constituents of illite-clay-pyrite oreshoots in the hanging wall of the orebody in the DDZ; 
whereas in the footwall, unmineralized marble and limestone form mesoscopic phacoids that 
are separated by conjugate and anastamosing, gray, illite-clay seams.  These breccia bodies 
represent silicified end members of dissolution and collapse, cataclasis, fossil hash, 
sedimentary breccia and combinations of these primary breccia types that have been 
dismembered and isolated by shearing and folding in the DDZ.  These features are compatible 
with significant movement during formation of illite-clay-pyrite oreshoots, but prior to the 
formation of some other oreshoots.


Realgar- and Orpiment-Bearing Oreshoots

	Realgar- and orpiment-bearing oreshoots (arsenic seam ore of Peters, 1996, 1997a) 
contain abundant masses of realgar and orpiment, but also contain disseminated pyrite and 
marcasite and some apatite (table 2).  This ore type result from deposition of these minerals 
on earlier formed zones of illite-clay-pyrite ore.  The oreshoots were most common in the 
upper central Betze orebody in a 30Ð to 50Ðm-size zone away from the diorite contact, 
spatially associated with diorite apophyses or with contact metamorphic or metasomatic 
zones, or surrounding and lying above domal-shaped siliceous stibnite-bearing siliceous 
oreshoots (Figs. 11, 12, and 16A).  The realgar- and orpiment-bearing oreshoots are 
composed of dark gray to black, decalcified, tectonized seams of sheared limestone and calc-
silicate rock that surround unmineralized undeformed phacoids of crystalline, white limestone 
and marble (Fig. 17A, B).  In the southeastern upper central Betze orebody, realgar-and 
orpiment-bearing oreshoots average about 1 oz Au/t (~32 g Au/t).  The unmineralized nature 
of the limestone phacoids, which are 40 to 60 percent of the volume, suggest that the realgar-
orpiment seams average greater than 2 oz Au/t (~64 g Au/t) (Fig. 17B; table 2).  Realgar- and 
orpiment-bearing ores peripheral to the main orebody have low or trace amounts of gold, 
suggesting that much of the gold may be contained in illite-clay-pyrite ores that were 
overprinted by the later realgar and orpiment.

	Realgar usually is stratabound, but has a tendency at hand specimen-scale to form 
cross-cutting texturesÑespecially in siliceous zones or in breccia fragmentsÑwhich indicate 
small-scale injection or veining, whereas orpiment masses generally have more conformable 
replacement-style textures (Fig. 17C).  Realgar also is present as 0.5Ð to 5Ðcm-thick seams 
replacing bedding planes and phyllonitic layers and as 0.1Ð to 0.5Ðmm-diameter disseminated 
crystals.  Disseminated realgar also is present as 1Ð to 5Ðmm-diameter aggregates, as 
inclusions inside orpiment crystals, as replacements of fossil shells, and as rims on orpiment.  
Orpiment is more commonly associated with calcite than realgar and typically is present as 
rounded, more massive, 0.  5Ð to 1Ðcm-diameter radiating crystals; it also is present as 
disseminated 0.01Ð to 0.1Ðmm-diameter crystal aggregates, as interstitial crystals in calcite 
masses, in 1Ð to 10Ðmm-thick veinlets, as growths around lowÐAs pyrite, and as growths 
around breccia clasts.

	Pyrite in realgar and orpiment-bearing oreshoots is present in several textural habits 
that suggest most of it preceded realgar-orpiment-calcite precipitation.  Pyrite ranges from 2 
to 15 vol. percent in most ores as fine-grained disseminations in illite-clay- and silica-rich 
parts, as both euhedral and subhedral shapes, and as both lowÐAs and highÐAs types.  
Coarse euhedral pyrite also is present locally.  Pyrite and rare stibnite also are present as 
inclusions in orpiment and in vugs (Fig. 17D).

	Quartz is present as blue- to gray-colored, rounded, centimeter-scale breccia fragments 
of silica hornfels in a matrix of orpiment, realgar, and calcite, and as microcrystalline matrix 
and fine-grained crystals in millimeter-scale veinlets in local siliceous breccia.  Calcite is white, 
coarse-grained, in euhedral to subhedral crystals in veinlets and in matrix fill, and may occupy 
up to 10 vol. percent of the ore.  Calcite typically cross cuts realgar veinlets and masses.  
Realgar-and orpiment-bearing oreshoots also contain local pre-gold, syn-diorite(?) scheelite as 
inclusions in orpiment, and locally abundant fine-grained apatite near the diorite contact.  
Antimony and Tl contents locally are enriched in these oreshoots (table 3), but Sb and Tl 
minerals are rare, because they are possibly overprinted by realgar and orpiment, or they may 
also be due to the enrichment of these elements in orpiment and realgar mineral structures (see 
Radtke and others, 1973, 1974).  


Stibnite-Bearing Siliceous Oreshoots

	Stibnite-bearing siliceous oreshoots (siliceous breccia bodies of Peters, 1996, 1997a) 
are arcuate, homogeneous, hard, dark-gray bodies that occupy local areas in the orebody along 
1,000 m of strike length (Figs. 6A, 11, 12).  Gold contents of these ores is generally less than 
0.1 oz Au/t (~3.2 g Au/t) Au, and together with the restriction of most AsÐrich pyrite to pre-
silica clasts, suggests that these oreshoots may have overprinted the illite-clay-pyrite ores.  
The northwest end of the orebody contains a discontinuous area of stibnite-bearing siliceous 
breccia, approximately 70 by 200 m, surrounded on its sides and top by realgar- and 
orpiment-bearing ore (Fig. 6B).  This stibnite-bearing siliceous breccia is in contact with 
diorite on its western margin, decalcified clay-altered limestone on its northwest margin, and 
phacoidal blocks of limestone separated by carbonaceous shear strands of the DDZ on its 
eastern margin.  The domal shape of the tops of these oreshoots roughly conforms to bedding 
in the Betze anticline (Figs. 11, 12, 6A).  Two other stibnite-bearing siliceous oreshoots lie in 
or adjacent to the DDZ in the central and southeastern parts of the upper central Betze 
orebody (Fig. 6B).

	Textures in siliceous stibnite-bearing oreshoots resemble both crackle breccia and 
silicified collapse breccia and contain both hard, dense zones of silicification, and porous 
zones with irregular 0.1Ð to 1Ðcm-wide voids with 0.2Ð to 5Ðcm-diameter angular to partly 
rounded clasts.  Silica constitutes more than 70 wt. percent of the rock and is composed of 
several quartz types:  (1) massive cloudy, white, tan, or gray microcrystalline groundmass 
quartz and clast replacement quartz, possibly related to siliceous contact metamorphism or 
metasomatism;  (2) 5Ð to 15Ðmm-thick cryptocrystalline quartz veinlets and void fillings 
containing pyrite;  (3) veinlets of euhedral clear comb quartz containing stibnite, barite and 
some pyrite; and (4) late quartz veinlets with marcasite.  Brecciation commonly caused 
fracturing of contact metamorphic microcrystalline quartz fragments and early quartz-pyrite 
veinlets.

	Pyrite is present as disseminated, 0.01Ð to 0.3Ðmm-size, AsÐrich pyrite in quartz 
veinlets, cut by late gold-stage stibnite-quartz veinlets, and as euhedral to anhedral, lowÐAs 
pyrite that also is coeval with barite- and stibnite-quartz.  LowÐAs pyrite also is present as 
inclusions in stibnite.  Some pyrite is zoned with internal layers rich in Sb and As and 
contains AsÐrich inner cores.  Stibnite is most common in the upper 10 meters of the stibnite-
bearing siliceous oreshoots in subhorizontal 30Ð by 10Ðm-size zones.  Stibnite, calcite, and 
barite are coarse-grained, grow into vugs among breccia fragments, and also line fractures as 
striated, euhedral needles.  Stibnite occupies both clasts and matrices in dense silica.  Stibnite 
also is present in clear quartz veinlets in mmÐ to cmÐscale irregular masses and as micron-
scale inclusions in pyrite, as well as massive coatings or growths on euhedral pyrite crystals.  
Boulangerite (Pb5Sb4S11) and sphalerite are common 10еm-size inclusions in stibnite.  Planar 
marcasite-quartz veinlets crosscut the silica-stibnite stage.  Geochemically, the ore is rich in 
Sb and As and moderately rich in Hg, Tl, Ba, and Zn (table 3).  The occurrence of stibnite-
bearing siliceous oreshoots spatially separate from realgar- and orpiment-bearing oreshoots 
and the lack of cross cutting relations between them suggests that these two types of ore may 
be products of late zoning, which may be approximately coeval.


Polymetallic Oreshoots

	Polymetallic oreshoots (sulfidic breccia pods of Peters, 1996, 1997a) form high-grade 
oreshoots along the diorite contact.  Polymetallic oreshoots are black to dark gray, hard, 
irregularly shaped, 10Ð by 30Ðm-sized pods and siliceous breccia bodies localized by steeply-
dipping, narrow, phyllonitic faults, and contain illite-clay, decalcified limestone, and silica 
hornfels fragments (Fig. 16).  The pods are located between and over print both the upper 
realgar- and orpiment-bearing oreshoots and the lower siliceous stibnite-bearing oreshoots.  
They also cross cut the earlier illite-clay ores (Figs. 6B, 11, 12; table 2).  This ore type have 
relatively high gold contents [> 0.15 oz Au/t (5 g Au/t)], which is due to the presence of 
native gold in the oxidized micro vugs, and high-sulfide content; realgar, orpiment, and stibnite 
are generally absent, except as earlier formed minerals that have been overprinted.  These 
polymetallic ores are distinct from pre-gold base metal occurrences associated with diorite 
emplacement and clearly cross cut earlier stages of gold mineralized rocks.  

	Many polymetallic oreshoots coincide with high sulfide sulfur contents in blast hole 
assays  (Figs. 6B and 7B) and commonly account for high Zn and Cu contents in the mill 
circuit (Barrick Goldstrike, unpub. data, 1997).  Polymetallic oreshoots are typified by 10Ð 
to 100еm-size, late gold-stage sulfide mineral aggregates containing Hg, As, Cu, Sb, Zn, and 
Ag associated with pyrite, although the geochemical expression of these aggregates is weak 
(table 3).  Most common are aggregate sulfide grains composed of pyrite, Hg-rich sphalerite, 
chalcopyrite, and cinnabar. 

	The variety of minerals and elements present, their the proximity to the diorite 
contact, and their late paragenesis are important features of the polymetallic oreshoots.  
These oreshoots contain the main Hg minerals in the Betze orebody and may be similar to the 
mercury ores in the Carlin deposit noted by Radtke and others (1972a,b).  Cinnabar is 
present between crystals of realgar and as 1Рto 2еm-size rinds on pyrite-chalcopyrite grain 
aggregates  (Figs. 18A and B).  Chalcopyrite is common as disseminated euhedral to subhedral 
0.1Ð to 0.5Ðmm-diameter grains associated with illite-quartz microveinlets.  Pyrite is present 
as anhedral, ragged 10Рby 50еm-size grains interstitial to quartz and apatite fragments in 
microbreccia.  Arsenic-rich 1Рto 50еm-diameter, euhedral pyrite contains mm-scale 
inclusions of cinnabar, fluorite, galena and sphalerite.  Some pyrite contains PbÐ, HgÐ, and 
CuÐrich rims (Fig. 18C).  A common feature of this ore type includes local, oxidized, 
100еm-size microvugs that contain mm-size Ag sulfide (Fig. 18D), Ni and Fe oxide 
minerals, Cu, Hg, Se minerals, and native Au.  Disseminated apatite is abundant in parts of 
the breccia bodies as 5Рto 20еm-diameter, round crystals with cinnabar inclusions.  Other 
trace minerals are phosphates of Ce, La, Nd, and U, abundant Ti oxide minerals, and local 
micron-size Cd and Te minerals.  Geochemically, the ore is rich in Ba, Cu, Hg, Zn, Ag, and 
moderately rich in As (table 3).


DISCUSSION AND CONCLUSIONS

	The precise nature of oreshoot control and genesis in ore deposits, as Bateman (1942) 
discussed, is unsolved in most cases; however, interpretation of the detailed geology of 
oreshoots and comparison of characteristics of oreshoot types to one another, and to the 
main orebody, can shed light on how oreshoots, and the orebody as a whole, formed.  
Textural relations among gangue and ore minerals, wall rock alteration assemblages, and 
zoning characteristics of the mineralized rocks indicate reaction of ore fluid with wall rocks 
through time (see Ferdock and others, 1997; Woitsekhowskaya and Peters, 1998).  Therefore, 
spatial zoning of mineralogically distinct oreshoots in the orebody is likely to be related to 
geologic events that were present during the evolution of this fluid and together represent the 
ore-forming process.

	Strong field and laboratory evidence exists at Betze for the introduction of Au-bearing 
fluid during deformation, not unlike interaction of fluids with localized deformation events in 
the upper crust that have been documented elsewhere (McClay, 1977; Henley, 1973; Henley 
and Ethridge, 1994; Hickman and others, 1994; Logan and Decker, 1994).  These events 
commonly are regarded as extensions of deep crustal fluid-deformation interactions in the ore-
forming environment (see Fyfe and others, 1978; Ethridge and others, 1983; OÕHara, 1998; 
Hyndman, 1994).  The Betze orebody most likely formed during a tectonic event after the 
Late Jurassic (D3, table 1) that was accompanied by significant fluid flow.  Faulting, shearing, 
and gouge development along less competent illite-clay altered and decalcified 
zonesÑfollowed by continued hydrothermal alterationÑincreased porosity further and 
enhanced fluid flux along the DDZ,  and may also have developed zones of hydrofracture 
(Byerlee and Brace, 1972;  Phillips, 1972, 1986;  Engelder, 1974).  Fluctuations in pressure 
and temperature also caused variation in fluid chemistry across the DDZ.

	Ore textures in the orebody also indicate ore formation under elevated fluid pressures 
in a dynamic environment.  The illite-clay-pyrite oreshoots contain AsÐrich pyrite, illite and 
quartz gangue along bedding planes or phyllonitic seams.  Veins and veinlets are rare and 
open-space-filling texturesÑmore typical of shallow, low-pressure environments (Bodnar 
and others, 1985; Berger and Bethke, 1985)Ñare generally lacking.  Fibrous textures and 
pointed terminations of veinletsÑsuch as are locally present in the realgar- and orpiment-
bearing oreshootsÑare characteristic of crack-seal mechanisms (Secor, 1965; Beach, 1977; 
Ramsay, 1980b) that formed in deep settings under high fluid pressures in the ductile-brittle 
environment (Sibson and others, 1988; Sibson, 1990).  The oreshoots contain, or are 
separated by, complex cataclastic mixtures of crushed and brecciated wall rock, gouge, 
phyllonite, illite-clay seams, and altered wall rock, all of which indicate a complex dynamic 
ore forming system.  Textural variations among the oreshoots along the host DDZ conduit 
also suggest that local pressure and temperature fluctuations in the ore fluid may have 
occurred in conjunction with deformation and dilation.

	Patterns of alteration zoning in the orebody define a large zone where fluid flow was 
high defined by a central high-grade core of gold-associated illite-clay and silica alteration in 
and adjacent to the high-grade oreshoots that is surrounded by lower-grade zone s of 
decalcification and weak illite-clay alteration.  Restriction of alteration to narrow illite-clay-
rich selvages around unaltered marble or calc-silicate rock phacoids implies that fluid flow 
favored permeable deformed zones in the DDZ.  Alternating complex layers of illite-clay 
alteration and silicification may be due to:  (1) distinct, separate hydrothermal events;  (2) 
mixing of multiple fluids;  (3) evolution of a single fluid through fluid-rock interaction; or 
most likely (4) strong lithologic or deformation fabric control of fluid movement and 
hydrothermal alteration and mineralization.

	Gold deposition in the Betze orebody resulted from hydrothermal fluid reactions with 
the host lithologies, starting with decarbonatization, which resulted in increases in porosity 
and permeability.  Alteration proceeded through time from fresh, unaltered rock, through 
progressive decarbonatization, then to argillic alteration, and finally to silicification.  The main 
gold deposition stage involved iron sulfide and arsenic deposition, a transition from illite to 
kaolinite deposition with pulses of quartz deposition.  These events were accompanied by 
phyllonite development, fault gouge, dissolution-collapse, and tectonic brecciation.

	Pre-ore deformational fabrics or textures, such as broad folding, and formation of 
sedimentary and collapse breccias, can be thought of as structural ground preparation, 
whereas syn-ore forming deformational textures, such as phyllonite development and 
mesoscopic shear folding, were due to conduit-fluid interaction, similar to processes in kinetic 
oreshoots proposed by Poulsen and Robert (1989).  The stages of ground preparation 
present in textures in the oreshoots are:  pre-ore stage deformation (D1 to D2, table 1), 
decalcification, and chemical preparation of sheared rocks in the DDZ, and syn- and post-ore 
shearing, shear folding and faulting (D3, table 1) that resulted in development of complex ore 
deposition sites.  Interaction of the syndeformational shear folding with the evolving fluid and 
resulting paragenesis was responsible for the distribution and zoning of the various types of 
oreshoots as well as the surrounding ore.

	Zones of illite-clay alteration in the upper parts of the Betze orebody played an 
important role in development and distribution of the oreshoots.  Many illite-clay-pyrite ores 
contain phyllonitic seams and breccias accompanied by tight, mesoscopic folds that parallel 
the strike of the DDZ, suggesting that deformation accompanied or closely followed 
alteration and mineralization.  Similar illite crystallinity indices of illite in phyllonite, fault 
gouge zones, and undeformed altered sedimentary beds and phacoids suggest that these rocks 
may all have been altered under similar PÐT conditions (see also Pollastro, 1993), which also 
is compatible with interaction between wall rock and the hydrothermal fluid during 
deformation.  It is likely that these hydrated minerals in the shear zones and faults enhanced 
shearing.  Argillization also may have influenced the evolution of the fluid by releasing or 
absorbed saline fluids, and planar illite-clay phyllonitic zones may have formed local pressure 
seals that affected channeling of fluids (Wang and Mao, 1979; Wang and others, 1979; Moore 
and others, 1989).  

	Although local offset can be demonstrated inside the deformed, WNWÐtrending zone 
that contains the Betze orebody and the DDZ, evidence of major throw and shear sense 
across this zone are equivocal.  Lack of significant offset across this intensely deformed zone 
indicates that a direct correlation between the thickness of the zone and displacement does 
not exist (see Robertson, 1983; Hull, 1988; Walsh and Watterson, 1988, 1989).  This suggests 
that deformation, and accompanying hydrothermal alteration and mineralization, were 
confined to a finite volume adjacent to the contact of the Goldstrike stock and that folding 
and shearing were accommodated by dissolution, collapse, and brecciation within this volume 
of intensely disturbed rock.

	Lithologic contacts influenced development and distribution of oreshoots.  Many 
oreshoots are present on both sides of contacts between the Rodeo Creek unit and Popovich 
limestone and the Popovich limestone and Goldstrike diorite.  The plunge of the illite-clay-
pyrite, realgar- and orpiment-bearing and stibnite-bearing siliceous oreshoots coincides with 
the intersection of the DDZ and these lithologic contacts, similar to lithologic control 
discussed by Reid and others (1975) and Treagus (1988).  Intersection of the DDZ and the 
keel of the Peters syncline is spatially associated with rutile-bearing siliceous oreshoots that 
plunge parallel to the intersection of these structures.  Deformation and rotation of the 
previously deformed keel (D1 or D2?) of the Peters syncline by the DDZ (D3) caused an 
increase in porosity and fracture density, thereby providing conduits for ore fluids.  

	Permeability and dissolution in the DDZ are inferred to have been similar to that 
documented and discussed for most common shear zones by Brace (1980), Bell (1981), Logan 
and Decker (1994), and Scholz and Anders (1994).  Porosity networks in shear zones usually 
take the form of cracks or tubes along the volume of rock in the shear zone (G_raund and 
others, 1995), and these networks may be partially manifested by the geometry of the 
oreshoots in the Betze orebody.  Secondary faulting in the DDZ during the mineralizing event 
also may have generated tensional fields that localized oreshoots, because geometric 
complexity and multiple movement in splayed areas can result in local tensile stresses (Lajtai, 
1969; Gamond, 1987).  This may have caused fluid pressure and stress gradients to develop 
throughout the fault network, especially in the competent stibnite-bearing siliceous oreshoots 
hosted mainly by calc-silicate rocks, enhancing permeability and channeling fluids of variable 
composition through breccia and interconnected parts of the fault network.

	Fluid-wall rock interaction affected fluid flux and fluid chemistry along the DDZ that 
caused chemical gradients between calcareous, siliceous, carbonaceous and dioritic rocks that 
further produced chemical changes in the ascending fluid that crossed these lithologic contacts 
(see also Fyfe and Kerrich, 1984).  Chemical reactions, pH, and temperature changed around 
individual oreshoots as the fluid flowed into the adjacent rocks.  Channeling of fluid into 
separate, permeable structural zones and wall rock types isolated pockets of fluid and caused 
them to evolve independently due to separate wall rock interaction and local temperature and 
pressure conditions.  

	Gold deposition and decarbonatization resulted from fluid-rock reactions between the 
ore fluid and the host rock as small changes occurred in temperature and rock-fluid ratio, 
according to geochemical modeling by Woitsekhowskaya and Peters (1998).  This modeling 
shows that chemical reactions accompanied strain and consisted of (1) dissolution of 
diagenetic clay minerals, (2) silica-leaching and silica-fixation, and (4) carbonate dissolution.  
Fluid-rock interaction under strain led first to volume loss and formation of early 
kaolinite+pyrite+graphite and late illite-clay-pyrite mineral assemblages, which further 
weakened the rock mass, providing the favorable sites where major phyllonite seams could 
form and propagate.  The relatively low pH of the reactions and accompanying dissolution 
produced an aqueous phase with Si/Al ratios higher than those in the dissolving host rock, 
and therefore, aqueous silica concentration increased due to alteration of primary clay 
minerals in the host rock.  This finally led to clay mineral deposition, predominantly illite, 
dissolution and concurrent precipitation of quartz, which strengthened the rock mass, 
producing late brittle brecciation in the silicified parts and late gouge development in the 
unsilicified illite-clay rich parts of the orebody.

	The deposition of Au, Fe, As, and Sb minerals during the later stages of mineralization 
require that the ore fluid reacted at relatively high aqueous silica values with already formed 
alteration-mineral assemblages (see Woitsekhowskaya and Peters, 1998).  Silica-leaching 
reactions did not result in saturation of the system with respect to quartz, because the bulk of 
the host rocks are relatively silica-poor.  Gold that remained in the fluid and was not depleted 
during formation of AsÐrich pyrite also precipitated in the realgar- and orpiment-bearing and 
stibnite-bearing siliceous oreshoots and was accompanied by the replacement of calcite by 
quartz.  Gold and arsenic were consistently depleted from the inflowing fluid as it reacted 
with altered wall rock.  Realgar and orpiment precipitated at the lower temperatures, and 
eventually stibnite precipitated due to decrease in sulfur activity and temperature.  Leaching 
reactions were not important during development of the realgar- and orpiment-bearing and 
stibnite-bearing siliceous oreshoots and the mineralizing process was not chemically 
aggressive.  This enabled a preservation of textures in undeformed areas during the course of 
replacement.  The geologic expression of these relations is the interplay between changing 
mineral solubility and depletion of metals in the system.  Late deposition of native gold, 
mercury, base-metals and silver minerals precipitated in the polymetallic oreshoots.  Zoning 
of the oreshoots indicates that temporal and spatial chemical changes allowed precipitation of 
different minerals in different oreshoots as the fluid evolved and cooled.

	Ore textures, gouge, phyllonitic shear zone rocks, alteration style, and isotopic and 
fluid-inclusion data (see also Hofstra, 1997; Hofstra and Rye, 1998) all point to a weakly to 
moderately saline fluid that ascended and cooled in the DDZ and participated in local 
deformation in this zone.  Evidence for more than one fluid is generally lacking, although later 
influx of meteoric water in a waning, uplifted, collapsing system is possible.  Changing 
conditions in the DDZ, such as wall-rock reactions, and declining P-T are responsible for 
fluid variation, and account for most metal deposition.  Changes to the deposition sites of ore 
and alteration minerals in the Betze orebody occurred both prior to ore formation and during 
dynamic interaction of the gold-bearing fluid with the wall rock.  All of these processes 
promoted oreshoot growth and zoning.  


ACKNOWLEDGMENTS

	We would like to thank Barrick Goldstrike Mines Inc. for permission to publish the 
results of our studies.  Ferdock also would like to thank the U.S.  Geological Survey, Barrick 
Goldstrike Mines Inc., and the Ralph J.  Roberts Center for Research in Economic Geology, 
Geological Sciences Department, Mackay School of Mines, University of Nevada, Reno for 
funding of field and laboratory work.  Access to Barrick Goldstrike property and was 
facilitated by Keith H.  Bettles, Eric A.  Lauha, and Jeff A.  Volk who also provided 
discussion and support.  XRD of some clay minerals was performed by Ali Ucruum at 
Mackay School of Mines; other XRD analyses were conducted at the Barrick Goldstrike 
laboratory.  SEM photographs are by Robert L.  Oscarson, USGS Menlo Park.  Help with 
photography was provided by Mall J.  Hibbard and Zhiping Li.  Reviews of early drafts of 
the manuscript by Howard J.  McCarthy Jr.  , David A.  John, Dennis P. Cox and Ted G.  
Theodore are much appreciated.


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