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Geological andGeophysical Set t ing ofthe Gold-Silver VeinSystems of Unga Island,Southwestern Alaska

Edited by James R. Riehle1

Open-File Report 99-136Version 1.3

1999

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

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

1 U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667

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CONTENTS

Frontispiece figures

List of Contributors

Executive Summary of Geologic Results

Chapter 1: Introduction to the present study, previous studies, and a descriptive

summary of the vein systems and their production history

James R. Riehle

Chapter 2: Geology of Unga Island and the northwestern part of Popof Island

James R. Riehle, Frederic H. Wilson, Nora Shew, and Willis H. White

Chapter 3: Chemical compositions of the volcanic rocks on Unga Island and someinferences about their origins

James R. Riehle

Chapter 4: Geologic structures of Unga Island, their relations to mineralization,

and some speculations on their origins

James R. Riehle

Chapter 5: Geologic interpretation of DIGHEMIV airborne aeromagnetic and

electromagnetic data over southeastern Unga Island, Alaska

John W. Cady and Bruce D. Smith

Chapter 6: Rock, stream-sediment, and heavy-mineral concentrate geochemical

data from Unga and western Popof Islands, Alaska Peninsula, Alaska

Frederic H. Wilson, Stanley E. Church, and Elizabeth A. Bailey

Chapter 7: Classifying the Shumagin and Alaska Apollo deposits

Donald A. Singer

Pl. 1: Geologic map of Unga Island and northwestern Popof Island, Alaska (scale

1:63,360

Pl. 2: Geochemical sample localities and drainage basins

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CONTRIBUTORS TO THIS REPORT

Elizabeth A. Bailey: U.S. Geological Survey, 4200 University Dr., Anchorage AK99508

John W. Cady, GeoPeregrino, 3955 Douglas Mountain Dr., Golden CO 80403;

johncady@geoperegrino.com

Stanley E. Church: U.S. Geological Survey, Box 25046 DFC, Denver CO 80225

James R. Riehle: U.S. Geological Survey, 4200 University Dr., Anchorage AK99508; jriehle@isdmnl.wr.usgs.gov

Nora Shew: U.S. Geological Survey, 4200 University Dr., Anchorage AK 99508

Donald A. Singer: U.S. Geological Survey MS 901, 345 Middlefield Rd., MenloPark CA 94025

Bruce D. Smith: U.S. Geological Survey, Box 25046 DFC, Denver CO 80225;

bsmith@usgs.gov

Willis H. White, 12779 Flat Meadow Lane, Herndon, VA 20171

Frederic H. Wilson, U.S. Geological Survey, 4200 University Dr., Anchorage AK99508

http://www.geoperegrino.com/http://www.geoperegrino.com/

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About the frontispiece

Shaded map: Hillshaded image of Unga Island and, at the upper right,northwestern Popof Island. The image is a computer rendering of the land surfaceas it would appear if illuminated by a sun at 45 degrees above the horizon, shiningfrom the northwest (upper left). Northwestern Unga Island is underlain by the

homoclinally NW-dipping, Miocene Unga Formation (smooth topography).Northeastern Unga Island and northwestern Popof Island are underlain by flat-lyingandesitic lava flows of the Oligocene Popof volcanic rocks. The prominent hill nearthe center of Unga Island is a Miocene dome encircled by carapace breccia and tuff;the encircling escarpment is artificially enhanced by the imaging process. Thesubcircular area of rugged topography, which extends from the center of UngaIsland to the east shore, is underlain by high-silica andesitic lava flows andintrusive sills and domes. Low terrain in southeastern Unga Island is underlain bylava flows and tuff of the Popof volcanic rocks, while most of the prominent peaksare shallow intrusive domes. Southeastern Unga Island is also cut by east- tonortheast-trending topographic lineaments. Two of these--the Apollo andShumagin trends--extend across the full width of the island and have localized goldand silver mineralization. These two important trends are marked by lines at their

ends; the Apollo trend bifurcates near the eastern coast of Unga Island. Othernortheast-trending lineaments can be seen between the Shumagin lineament and thehigh peaks at the center of the island. (Image created from a digital elevation modelthat was constructed for this study and subsampled in 30-m horizontal cells.)

Oblique map: Perspective view toward the northeast, of the hillshaded surface ofsouthern Unga Island. The Apollo lineament is labelled "1" and the Shumaginlineament "2". Other lineaments are marked but not named.

About the coverEarly view of the Apollo mine on Unga Island and, in the middle distance, probablythe Sitka mine. View is eastward. G.C. Martin, U.S. Geological Survey collection,

photo no. 147, September 3, 1904.

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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 (34m.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,

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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 growthfaults 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.