Preliminary Descriptive Deposit Model for Detachment-Fault-Related
By Keith R. Long
related to detachment faulting has only recently been recognized as a
distinct deposit type, even though such deposits have been mined since
the 1860's. These deposits have characteristic mineral assemblages, alteration
patterns, ore fluid types, and structural controls that differ considerably
from those of other deposit types found in the Basin and Range province
of the Western United States. However, detachment-fault-related mineralization
is not widely known, having been described but twice in widely circulated
journals (Spencer and
Welty, 1986; Roddy
and others, 1988); most of the detailed studies have appeared as publications
of the Arizona Geological Survey and the Arizona Geological Society.
of the unique character of these deposits has been hampered by confusion
with other types of epithermal mineralization that may or may not occur
near a low-angle or detachment fault, such as the Cyclopic deposit in
northwest Arizona (Myers
and Smith, 1986) or the Mesquite deposit in southeastern California
(Manske and others, 1988).
This discussion sets out the distinguishing characteristics of detachment-fault-related
mineralization vis-ŕ-vis other types of epithermal mineralization in the
region and provides a justification for the new deposit model presented
(K.R. Long, this volume). This deposit model is considered preliminary
because this deposit type has yet to be fully investigated and has, thus
far, only been recognized in a detachment-faulted terrane encompassing
parts of west-central Arizona, southeastern California, and southernmost
Nevada (fig. 35).
faults are low-angle (up to 30°) normal faults of regional extent that
have accommodated significant regional extension by upward movement of
the foot-wall (lower-plate) producing horizontal displacements on the
order of tens of kilometers. Common features of these faults are supracrustal
rocks in the upper-plate on top of
rocks that were once at middle and lower crustal depths, mylonitization
in lower-plate rocks that are cut by the brittle detachment fault, and
listric and planar normal faults bounding half-graben basins in the upper
plate (Davis and Lister,
detachment fault and structurally higher normal faults locally host massive
replacements, stockworks, and veins of iron and copper oxides with locally
abundant sulfides, veins of barite and (or) fluorite, and veins of manganese
oxides (Spencer and Welty,
1986; fig. 36). Bedded manganese oxides occur in sedimentary rocks
deposited in the half-graben basins and are generally associated with
fault veins of manganese oxides. These bedded manganese deposits should
be described separately as another model (lacustrine manganese). Intense
chloritic alteration of foot-wall mylonitic rocks and potassium feldspar
replacement of upper-plate rocks are common alteration types that are
not always accompanied by mineralization.
mineralization is termed detachment fault related not simply because it
is strongly controlled by detachment-fault structures, but also because
it is apparently related to the formation of detachment faults themselves
(Roddy and others, 1988).
Early chloritic alteration and associated sulfide mineralization appears
to result from retrograde metamorphism as hot lower-plate rocks are brought
up to shallower depths. Potassium feldspar alteration and oxide mineralization
appear to be related to the upward circulation of saline brines derived
from syntectonic basins along the detachment fault into more steeply dipping
upper-plate normal faults. This fluid movement may have been driven by
heat derived either from lower-plate rocks or from syntectonic microdiorite
to rhyolite intrusives (Reynolds
and Lister, 1987).
Figure 35. Major detachment faults and
detachment-fault-related mineral deposits in Arizona, southeastern California,
and southernmost Nevada.
of detachment-fault-related mineralization that distinguish it from other
deposit types are listed below. Further details are available in Spencer
and Welty (1986), Roddy
and others (1988), and Spencer
and Reynolds (1989).
- Deposits are controlled by structures formed during
detachment faulting. These include the low-angle, detachment-fault system,
high-angle faults in the lower-plate just below the detachment fault,
and low- to high-angle normal faults in the upper-plate.
DEPOSIT TYPES COMMONLY CONFUSED WITH DETACHMENT-FAULT-RELATED MINERALIZATION
- Deposits are often brecciated or deformed by movement
along or above the detachment fault.
- Chlorite-epidote-calcite alteration occurs along
and below the detachment fault. These altered zones sometimes contain
base-metal sulfides and barite.
- There is massive potassium feldspar replacement of
upper-plate rocks. This alteration appears to generally precede ore formation
and is not always spatially associated with mineralization.
- Weak sericite-silica alteration of wall rock is sometimes
present around barite-fluorite veins.
- Most mineralization consists of iron and copper oxides,
principally specular to earthy hematite and chrysocolla. Common gangue
minerals are chalcedonic to amethystine quartz, ferrous to manganiferous
calcite, barite, fluorite and manganese oxides. Distal barite-fluorite
veins consist of variable proportions of barite, fluorite, and manganese
oxides. Common gangue minerals are quartz and manganiferous calcite.
- Fluid inclusions have moderate homogenization temperatures
(150 to 350 °C) and salinities (10 to 23 equivalent weight percent NaCl),
compatible with precipitation from connate brines. Fluid inclusions from
barite-fluorite veins have lower homogenization temperatures (90 to 200
°C) and are somewhat less saline (6 to 20 equivalent weight percent NaCl),
compatible with precipitation from variably cooled and diluted connate
- Host rocks are enriched in Cu. Pb, Zn, Au, Ag, and
Ba and are depleted in Mn, Sr, Ni, and Rb. Elements characteristic of
epithermal environments, such as As, Sb, Hg, and Tl, occur in very low,
gold-silver deposits that occur along or near low-angle faults might be
mistaken for detachment-fault-related mineralization. Several possible
cases can be identified:
Figure 36. Schematic diagram (not to
scale) showing structural position of detachment-fault-related polymetallic
mineralization, Ba-F-Mn veins, and lacustrine manganese mineralization
in detachment-faulted terranes.
- Epithermal deposits found in metamorphic rocks (for
example, Mesquite, California; Manske
and others, 1988).
Epithermal deposits that are overprinted by younger detachment-fault-related
mineralization (for example, Cyclopic, Arizona; Myers
and Smith, 1986).
Epithermal deposits can be distinguished from detachment-fault-related
deposits by their characteristic ore mineralogy, alteration minerals and
patterns, geochemical signatures, and fluid-inclusion compositions, as
described in the deposit model for hot spring Au-Ag (Berger,
1986b). Principal distinguishing characteristics are the following:
- Epithermal deposits that overprint detachment-fault-related
mineralization or that were emplaced during detachment faulting (for example,
Bullfrog, Nevada; Jorgeson
and others, 1989).
- Epithermal deposits that are significantly younger
than detachment faulting but are controlled by detachment-fault structures
(no known examples in the published literature).
SIZES AND GRADES OF DEPOSITS
- Ore mineralogy consists of base- and precious-metal
sulfides with few or no primary oxide minerals. Gangue quartz is not usually
amethystine, and gangue calcite is poor in iron and manganese.
- Extensive propylitic and (or) argillic alteration
of upper-plate host rocks is observed with only local potassic alteration.
- Low-salinity (<6 equivalent weight percent NaCl),
moderate homogenization temperature (200 to 300 °C) fluid inclusions are
- Anomalous concentrations of the elements As, Sb,
Hg, and Tl, which are characteristic of epithermal deposits, are present.
data on sizes and grades of detachment-fault-related mineral deposits
consist mostly of production statistics originally collected by the U.S.
Bureau of Mines and reported by the Arizona Geological Survey (Keith
and others, 1983; Spencer
and Welty, 1989). The only reserve data available are for recently
explored deposits, such as Copperstone, Arizona (Spencer
and others, 1988). Attempts
model tonnages and grades for detachment-fault-related polymetallic deposits
using cumulative production data (table 10) were not successful. Few of
these deposits produced all of the metals that occur in this deposit type,
making it difficult to model deposit grades. In fact, indications are
that there may be two subtypes of detachment-fault-related mineralization—a
Cu-Au type and a Pb-Zn-Ag type—but further research is required to confirm
any case, grade and tonnage models based on the production data listed
in table 10 would not give an accurate indication of the range in sizes
and grades of these deposits that could be expected to be encountered
in a modern exploration program. Not only were not all metals recovered,
but also many of these ores were concentrated in part by hand. In hand
sorting, a large quantity of waste is typically rejected prior to sending
ore to the concentrator, and these rejects are not always included in
recorded production tonnages. Thus, the grades computed from production
statistics are not likely representative of the true grade of the ore
mined. Further, these were underground mines; thus, in comparison with
the tonnages and grades that might be estimated for a modern open-pit
operation, these older ore bodies were smaller in size and higher in grade.
better sense of the potential size and grade of these deposits is indicated
by recently reported reserves for deposits that have been excluded from
table 10 as a result of their lack of production history. These are Copperstone
(Spencer and others, 1988),
a recent producer with reserves of 4.2 million short tons of 0.077 troy
ounce per ton Au ore as of December 31, 1988, having produced 62,800 troy
ounces Au prior to that date (Cyprus
Gold Co., 1989); and Newsboy, a recent discovery in Arizona, with
reserves of 1.5 million short tons of 0.045 troy ounce per ton ore (H.
Dummett, oral commun., 1989).
number of deposits have been excluded from table 10 because their classification
as detachment-fault-related deposits is controversial. These include Picacho,
California (Van Nort and
Harris. 1984), and Silver, Arizona (Bradley,
Table 10. Grades and tonnages for detachment-fault-related
in short copper, lead, and zinc grades in percent; silver and gold grades
in troy ounces per short ton. Country and state abbreviations explained
in app. B]
Streak USAZ 189
River Mountains USAZ 15,000
1 (Keith and others, 1983),
2 (Spencer and Reynolds,
1989), and 3 (Spencer
and Welty, 1986).