1_README.TXT

Geological and Geophysical Setting of the Gold-Silver Vein 
Systems of Unga Island, Southwestern Alaska

Edited by James R. Riehle(1)

With chapters by

James R. Riehle(1), Introduction
James R. Riehle(1), Frederic H. Wilson(1), Nora Shew(1), and 
     Willis H. White(2), Geology of Unga Island and the 
     northwestern part of Popof Island
James R. Riehle(1), Chemical Compositions of the Volcanic 
     Rocks on Unga Island and Some Inferences About Their 
     Origins
James R. Riehle(1), Geologic Structures of Unga Island, their 
     Relations to Mineralization, and some Speculations on 
     their Origins
John W. Cady(3) and Bruce D. Smith(4), Geologic 
     Interpretation of DIGHEMIV Airborne Aeromagnetic and 
     Electromagnetic Data over Unga Island, Alaska
Frederic H. Wilson(1), Stanley E. Church(5), and Elizabeth A. 
     Bailey(1), Rock, Stream Sediment, and Heavy-Mineral 
     Concentrate Geochemical Data from Unga and Western Popof 
     Islands, Alaska Peninsula, Alaska
Donald A. Singer(6), Classifying the Shumagin and Alaska 
     Apollo Deposits


Open-File Report 99-136
Version 1.3

1999


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

U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

(1) U.S. Geological Survey, 4200 University Drive, Anchorage, 
     AK 99508-4667
(2) 12779 Flat Meadow Lane, Herndon, VA 20171
(3) GeoPeregrino, 3955 Douglas Mountain Drive, Golden, 
     CO 80403
(4) U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, 
     CO 80225
(5) U.S. Geological Survey, P.O. Box 25046, MS 973, Denver, 
     CO 80225
(6) U.S. Geological Survey, 345 Middlefield Rd, MS 901, Menlo 
     Park, CA 94025

ISBN 0-607-92654-6

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DISCLAIMERS

This Compact Disc Read-Only-Memory (CD-ROM) publication was 
prepared by an agency of the United States Government.  
Neither the United States Government nor any agency thereof, 
nor any of their employees, makes any warranty, expressed or 
implied, or assumes any legal liability or responsibility for 
the accuracy, completeness or usefulness of any information, 
apparatus, product, or process disclosed in this report, or 
represents that its use would not infringe on privately owned 
rights.  Reference therein to any specific commercial 
product, process, or service, by trade name, trademark, 
manufacturer, or otherwise does not necessarily constitute or 
imply its endorsement, recommendation, or favoring by the 
United States Government or any agency thereof.

Although all data and software published on this CD-ROM have 
been used by the USGS, no warranty, expressed or implied, is 
made by the USGS as to the accuracy of the data and related 
materials and (or) the functioning of the software.  The act 
of distribution shall not constitute any such warranty, and 
no responsibility is assumed by the USGS in the use of this 
data, software, or related materials.

OVERVIEW OF THE CD-ROM CONTENTS

   The topic of this CD-ROM is the geologic framework of 
gold-silver vein deposits on Unga Island, in the Shumagin 
Islands, southwestern Alaska.  The core of the publication is 
a new geologic map at a scale of 1:63,360 and aeromagnetic 
and electromagnetic survey data acquired by industry over the 
area of mineralization.  Both the geologic map as well as a 
preliminary interpretation of the geophysical data--which are 
included by permission of the owner--are aimed towards 
deciphering the relations among volcanism, tectonism, and 
mineralization.

   Data and discussions are organized in seven chapters, 
titles of which are outlined in the table of contents.  The 
chapters consist of viewable text and figure images; 
postscript versions of the frontispiece figures and all 
chapter figures are included on the CD-ROM as well.  The 
geologic map is a large viewable figure (Plate 1) that 
accompanies chapter 2; printable .tif and .jpg versions are 
provided in the Geology directory.  The map was constructed 
in ARC and its component coverages are provided in the folder 
"Geology" for users who may wish to modify the geologic data 
or add their own data.  

Export and related files that are available on the CD-ROM 
are: 
   altera.e00; a polygon coverage of alteration areas
   faults.e00; a line coverage of faults and topographic 
     lineaments
   geology.e00; a polygon coverage of geologic units
   strkdip.e00; a point coverage of strike and dip 
     observations
   symbol.e00; a point coverage of symbols such as mines 
     and drill holes
   topogr.e00; a line coverage of topographic contours; 
     100 ft interval except partial 50-ft contours are 
     shown at the 
     coast of northern Unga Island
   topo_dem; a USGS-formatted DEM, 30-m cellsize, integer 
     option ungadip; a text file of strike and dip 
     observations: latitude, longitude, azimuth oriented 
     so that dip is to the right in the direction viewed, 
     and dip in degrees

   Geologic and alteration polygons in the geologic map are 
shaded according to the item "class" using the shadeset 
geology.shd.  Lines are coded to the item "arc-code" and are 
plotted using the lineset geology.lin; topographic contours 
are plotted using an edited version of geology.lin for 
suitable line weight.  The textset used to print the viewable 
map is font.txt and the markerset is an edited version of 
geology.mrk keyed to the item "type".  The Digital Elevation 
Model that is the basis for the shaded-relief images shown as 
frontispieces and as background for some of the geophysical 
plots was constructed from scanned images of 1:63,360 USGS 
topographic maps.  The images were imported as TIF files, 
registered to Albers coordinates, and rectified using 
IMAGEGRID.  They were then converted to a coverage, which was 
edited by removing unwanted lines such as section boundaries 
and linking contour segments.  Contours were then coded to 
elevation, the coverage converted to a lattice, and the 
lattice to a USGS-formatted DEM.

   A geologic interpretation of the geophysical survey data 
is presented in chapter 5.  The  gridded survey data are 
available in the folder "Geophysics", together with 
WordPerfect files of the chapter text and embedded tables.  
Files in the Geophysics folder are in the subfolders Unga and 
Popov, which denote the separate areas of the two surveys, 
and the subfolder ERMAPPER.  In the different subfolders are 
the following:

   anomalies; this is an ascii table of anomaly 
characteristics described in the contractors report (see 
report folder). 

   archive; this is the flight line data for the survey.  
Format is in the ascii file. 

   grids; these are xyz grids in geosoft format for the EM 
apparent resistivities (geosoft can be contacted at 
http://www.geosoft.com).

   The subfolder called ERMAPPER contains ERMAPPER(6) files; 
all the geophysical grids (Unga.MDS) in an ERMAPPER grid.  
More information on ERMAPPER is given in the 1_README.TXT 
file in that subdirectory.  File types in the subdirectory 
include:

   format (*.ers files). This gridded data set is created so 
that each geophysical channel (magnetics, vlf, 
electromagnets) is a separate band.

   Algorithms (*.alg) files are given which process the grids 
for present in the figures given in the geophysics chapter. 
The various *.erv files are used by the algorithms for line 
work in the figures.  (A free viewer is available on the 
ERMAPPER web site, http://www.ermapper.com).

   Rock, stream-sediment, and pan-concentrate geochemical 
data are discussed in chapter 6, which includes a map as a 
large viewable figure (Plate 2).  The geochemical data and 
component files of the map and related figures are available 
in the folder "Geochemistry".

   Lastly, a metadata file created to meet Federal 
requirements for geospatial data is included in two versions: 
a text version and an html version.  The file is written 
chiefly for the geologic map, which is the heart of this 
publication; reference is made to related data such as the 
Digital Elevation Model and the geophysical survey grids in 
appropriate sections of the geologic map metadata file.  The 
geochemical chapter has its own metadata file.


EXECUTIVE SUMMARY OF GEOLOGIC RESULTS

1. Geologic mapping at 1:63,360 scale confirms that Unga and 
Popof Islands were sites of recurring and intensive magmatism 
during mid-Tertiary time and provides new details about 
relations among magmatism, tectonism, and mineralization.  
Nonindurated marine sediments on the continental shelf were 
deformed and buried by andesitic lava flows or were 
interbedded with andesitic and dacitic ash-flow tuffs.  
Initially effusive volcanism (31-38 Ma) was joined by 
hypabyssal activity (31-34 Ma) as domes of basaltic andesite 
to rhyolitic composition were extruded from numerous vents.  
Ash-flow tuffs occur, but they are of small volume and 
probably originated at domes.  There is no explicit evidence 
for a caldera structure.

2. Gold and silver mineralization is concentrated in two 
major, northeast-trending zones of faulting, brecciation, and 
quartz veining that extend across southeastern Unga Island.  
Total offset is minor, indicating only incipient faulting 
although multiple vein sets imply repeated movements.  A K-Ar 
age of vein adularia (34 m.y.) and another of sericitic vein 
alteration (32 m.y.) show that veining was at least partly 
contemporaneous with magmatism.  

3. Other physiographic lineaments having northeast trends 
also occur on southeastern Unga Island, which are also sites 
of silicification and alteration.  The lineaments have been 
the focus of exploratory activity by industry.  The multitude 
of northeast-trending lineaments is suggestive of northwest-
directed extension.  Northwest-trending lineaments cross-cut 
the northeast-trending lineaments; the younger lineaments are 
not as well developed and those that have demonstrable 
offsets appear to have been subjected mainly to strike-slip 
movements.  

4. The reduced-to-the-pole magnetic field over southeastern 
Unga Island reflects mainly different rock types, such as 
lava flows, volcaniclastic rocks, or domes.  Broad areas of 
anomalously conductive bedrock appear to reflect alteration 
of the volcanic rocks, which at least locally was probably 
selective of permeable volcaniclastic rocks.  Discrete 
bedrock conductors define linear trends, most of which are 
northeasterly and coincide or are parallel with physiographic 
lineaments and mapped faults.  Such linear conductive 
anomalies may be the result of fault-controlled sulfide 
mineralization.  One such linear anomaly ends at the mapped 
contact with a hypabyssal dome, supporting the inference that 
faulting and veining were contemporaneous with magmatism.

5. Gold and silver abundances in stream-sediment and heavy-
mineral-concentrate samples reflect one but not both of the 
major mineralized trends.  Anomalous concentrations of Au, 
Ag, As, Mo, and Pb, however, occur in volcanic rock samples 
from throughout Unga and northwestern Popof Islands.

6. Vein mineralogy, geologic setting, and grade-tonnage data 
from the main mineralized trends were used to infer the 
likely deposit type.  Types that were considered are Sado, 
Comstock, and Creede epithermal vein, polymetallic vein, and 
no existing model.  Neither the Comstock nor the Sado types 
can be rejected, so the "no existing model" option is 
unnecessary.  The Sado model seems the most appropriate.

7. The precise nature of faulting that controlled 
mineralization is uncertain: Slickenlines indicate both 
strike-slip and normal faulting on the northeast-trending 
lineaments.  Marine-seismic data, however, document 
northeast-trending growth faults on the continental shelf 
near Unga Island that are interpreted to have formed during 
early to mid-Tertiary time in response to northwest extension 
(Bruns and others, 1987).  Possibly the Unga Island 
lineaments initially formed in response to extension and were 
later reactivated by strike-slip movements.  In any case, 
minor but repeated movements on the lineaments would have 
provided recurring pathways for circulating geothermal waters 
above and adjacent to active hypabyssal domes. 

CD STRUCTURE

This CD-ROM contains directories for each of the seven 
chapters.  In addition to text, we have included digital data 
files for some of the chapters.  The chapter directories and 
their contents are as follows:

C1 Introduction:  Chapter 1, Introduction

C2 Geology:  Chapter 2, Geology of Unga Island and the 
northwestern part of Popof Island.  The geologic setting and 
history; includes map as Plate 1; the detailed descriptions 
of map units and the table of K-Ar ages are included in one 
file

C3 Chemistry:  Chapter 3, Chemical Compositions of the 
Volcanic Rocks on Unga Island and Some Inferences About Their 
Origins.  Includes a lengthy table 

C4 Structure:  Chapter 4, Geologic Structures of Unga Island, 
their Relations to Mineralization, and some Speculations on 
their Origins

C5 Geophysics:  Chapter 5, Geologic Interpretation of 
DIGHEMIV Airborne Aeromagnetic and Electromagnetic Data over 
Unga Island, Alaska.  Includes nine some of which are nearly 
50 MB

C6 Geochemistry:  Chapter 6, Rock, Stream Sediment, and 
Heavy-Mineral Concentrate Geochemical Data from Unga and 
Western Popof Islands, Alaska Peninsula, Alaska.

C7 Deposit types:  Chapter 7, Classifying the Shumagin and 
Alaska Apollo Deposits

Additional directories consist of the Acrobat Reader 
installers and the index search-engine files.

SYSTEM REQUIREMENTS

This CD-ROM (OF99_136) was produced in accordance with the 
ISO 9660 Level 2 standard and Apple Computer's(1) 
hierarchical file system (HFS) standard.  The data and text 
on this CD-ROM require either a UNIX work station, 
Macintosh(1) or compatible computer, or an IBM(2) or 
compatible personal computer, all equipped with a CD-ROM 
drive.

Macintosh
- 68030 or higher processor (PowerPC recommended)
- 8 megabytes RAM (64 MB recommended)
- Apple System Software version 7.1.2 or later (8.1 or later 
     recommended)
- 13-inch color monitor that can display 256 colors (16.7 
     million colors recommended)

Windows
- 80386 or higher processor (Pentium(3) recommended)
- Microsoft Windows(4) 3.1 or higher (Windows 95, 98, or NT 
     recommended)
- 8 megabytes RAM (64 MB recommended)
- VGA color monitor that can display 256 colors (16.7 million 
     colors recommended).

All platforms
- Adobe Acrobat(5) Reader 3.0 or higher (3.01 and 4.0 
     included on this disc for Macintosh and Windows) or 
     other software that can translate PDF files

Registered Trademarks
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(5) Adobe Systems, Inc.
(6) Earth Resources Mapping

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open Notepad or a word-processing application, change to the 
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