Mineral-resource assessments in Alaska; background information to accompany maps and reports about the geology and undis

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Books Books and Maps

By J.R. Riehle, S.E. Church, R.L. Detterman, and J.W. Miller

For sale by U.S. Geological Survey, Map Distribution Box 25286, MS 306, Federal Center Denver, CO 80225

Abstract........................................................................................... Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Mineral-Resource Assessments.........................................................

Geographic and Physiograhpic Setting of the Katmai Study Area . . . . . . . . . . . . . . . Geologic Investigations in the Katmai Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Previous Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Present Study.............................................................................. Definitions and Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recent Mineral Assessments in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Map I-2032: Quaternary geologic map of the Mount Katmai quadrangle

Map I-2204: Geologic map of the Mount Katmai quadrangle and

Open-File Report 86-586: Analytical results and sample locality map

Open-File Report 87-615: Mineralogy and sample-locality map of the

Open-File Report 88-422: Analytical data and sample locality map for

Open-File Report 89-570: Sample locality maps, analytical data, and

MF-2021-A: Geochemical map showing the distribution of selected

MF-2021-B: Geochemical maps showing the distribution of selected

MF-2021-C: Geochemical map showing the distribution of selected

MF-2021-D: Mineralogic map showing the distribution of selected and adjacent parts of the Naknek and Afognak quadrangles, Alaska....................................................................

adjacent parts of the Naknek and Afognak quadrangles, Alaska....................................................................

of stream-sediment and heavy-mineral-concentrate samples from the Mount Katmai quadrangle, and portions of the Naknek, Afognak, and Iliamna quadrangles, Alaska.............

nonmagnetic heavy-mineral-concentrate samples from the Mount Katmai quadrangle and portions of the Naknek, Afognak, and Iliamna quadrangles, Alaska........................

aqua-regia leachates of stream sediments analyzed by ICP from the Mount Katmai quadrangle, and portions of the Naknek, Afognak, and Iliamna quadrangles, Alaska.............

statistical summary of analyses of rock samples from the Mount Katmai quadrangle and adjacent portions of the Naknek and Afognak quadrangles, Alaska...................................

elements determined in stream sediments from the Mount Katmai and portions of the Afognak and Naknek quadrangles, Alaska....................................................................

leachable metals determined by ICP in stream sediments from the Mount Katmai and portions of the Afognak and Naknek quadrangles, Alaska....................................................

elements determined in nonmagnetic heavy-mineral concentrates from the Mount Katmai and portions of the Afognak and Naknek quadrangles, Alaska. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

minerals identified in nonmagnetic heavy-mineral concentrates from the Mount Katmai and portions of the Afognak and Naknek quadrangles, Alaska. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Map showing location of Mount Katmai study area on the eastern Alaska Peninsula.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Map showing physiographic features and tracts having favorable potential for mineral deposits, Mount Katmai study area... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Geologic, geochemical, and mineral-resource publications of the Mount Katmai study.................................

Bulletin 2020: Interpretation of exploration geochemical data from the

MF-2021-E: Map and table showing K-Ar ages from the Mount Katmai

MF-2021-F: Mineral and energy resource assessment maps of the

MF-2021-G: Macrofossil locality map, checklists, and pre-Quaternary

METRIC CONVERSION FACTORS

Mount Katmai quadrangle and adjoining parts of the Afognak and Naknek quadrangles, Alaska . . . . . . . . . .. . . . . . . . . .. . . . . . . . . . . . . . . .

and adjacent parts of the Naknek and Afognak quadrangles, Alaska....................................................................

Mount Katmai, Naknek, and western Afognak quadrangles, Alaska....................................................................

stratigraphic sections of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska....................................................................

TABLE

v

GLOSSARY OF TERMS

Quartz diorite. An intrusive igneous rock having little quartz or alkali feldspar, and a high content of plagioclase feldspar and dark accessory minerals.

Quartz monzodiorite. An intrusive igneous rock having contents of quartz and alkali feldspar that are between those of granite and quartz diorite.

Quartzite. A metamorphic rock consisting essentially of quartz grains; the original rock was typically a quartz sandstone.

Reverse fault. A steeply dipping fault along which the rocks above the fault have moved up relative to those below.

Rhyolite. A compositional term for a volcanic rock having a silica content (Si02) of between 68 percent and 77 percent by weight.

Silicification. The process of adding silica to a rock by chemical precipitation from fluids.

Sill. An intrusive igneous rock having a tabular form, of which the two long dimensions are parallel to bedding in the adjacent wall rocks. Stratovolcano. A volcano, typically cone shaped and more than 300 m above its base, consisting of alternating layers (strata) of lava, cinders, and ash.

Subduction. A process of plate tectonics in which one crustal plate, commonly but not necessarily an oceanic plate, slides beneath another plate; the site of contact between the plates is usually a physiographic trench, and the region behind the trench is one of elevated seismicity and active volcanism caused by the downgoing plate. Tonalite. An intrusive igneous rock having the same content of quartz as does granite, but less alkali feldspar than does granodiorite. Vein. A fracture in a rock filled by secondary minerals, typically formed by chemical precipitation from hydrothermal fluids; such deposits commonly appear veinlike when viewed on rock faces.

Volcanic arc. A belt of aligned volcanoes that follows an arcuate trace in map view; volcanic arcs are typically formed above subducting plates.

Wall rock. Preexisting rock that hosts a younger rock such as a magmatic intrusion or a vein.

Geologic and geochemical field studies were carried out from 1983 to 1987 in the Mount Katmai 1oX 2° quadrangle and adjoining region, at the northeast end of the Alaska Peninsula. The region is nearly entirely within Katmai National Park and Preserve and has had almost no mineral production, so prior to this study there were few data by which to assess the mineral potential of the region. This report describes the folio of publications that have resulted from the study: geologic maps, geochemical results, fossil identifications, radiometric rock ages, and an assessment of the undiscovered-mineral-resource potential of the region.

The Katmai region is inferred to potentially have three types of undiscovered mineral deposits: porphyry copper (molybdenum), precious-metal vein, and hot-springs gold. These deposit types occur elsewhere on the Alaska Peninsula in similar geologic units. Evidence suggesting their occurrence in the Katmai region is the presence of trace amounts of metals typically associated with these kinds of deposits in bedrock of certain tracts and in sediments of streams draining those tracts. Magma to provide heat, fractures to provide pathways for mineralizing fluids, and altered rock are required by genetic models of these deposit types. Such features do occur in the Katmai tracts. Confirmation of any mineral deposit in the Katmai region requires detailed followup sampling and acquisition of subsurface information, which is beyond the scope of this study. However, producing porphyry deposits are unknown elsewhere on the Alaska Peninsula in similar rocks, so if any such deposits occur in the Katmai region, they are likely to be few in number. Conversely, vein deposits are typically small in size so there may be several of such deposits.

Undiscovered-Mineral-Resource Potential of the Mount Katmai Quadrangle and Adjacent Parts of the Naknek and Afognak Quadrangles, Alaska Peninsula Mineral-Resource Assessments in Alaska-Background Information to Accompany Maps and Reports about the Geology and

ABSTRACT

The properties and thermal history of the sedimentary rocks that could serve as reservoirs for oil or gas are unfavorable in adjacent regions. Thus the potential of the Katmai region for producible quantities of fossil fuels is low. In theory the region has shallow concentrations of geothermal fluids, but specific evidence for their presence is obscured by heavy precipitation and cold young rocks or deposits. Small volumes of coal occur at tidewater sites on the Pacific coast.

INTRODUCTION

This report describes a set of maps and reports about the geology, geochemistry, and mineral resources of the Mount Katmai quadrangle and adjacent portions of the Naknek and Afognak quadrangles on the Alaska Peninsula. The reports, published under the auspices of the former Alaska Mineral Resources Assessment Program, provide information for decisions about landownership, for landuse planning, and for policymaking concerning the national mineral endowment. Such reports should be useful to State and Federal agencies, to private landowners, and to the mineral industry in Alaska.

The study area occupies 15,000 km that extend east-west across the Alaska Peninsula at its northeast end (fig. 1). Most of the study area is within Katmai National Park and Preserve (fig. 2); a small area at the south margin is within Becharof National Wildlife Refuge. Mineral production in a national park, part of which is also wilderness, may be unlikely. Regardless, the study has three main benefits. First, geologic trends and rock units in adjacent areas were either traced through the study area or were

Figure 1. Location of Mount Katmai study area on the eastern Alaska Peninsula.

based on mineral occurrences at the ground surface about the potential that mineral deposits occur at depth.

Three kinds of data were newly gathered for the Katmai assessment. (1) Geologic mapping permits identification of different geologic terranes and, by analogy with better studied areas elsewhere, suggests the types of deposits that are likely to occur in those terranes. (2) Stream sediments ideally provide a composite sampling of an entire; drainage basin. (3) Samples of mineralized bedrock provide direct evidence for the occurrence, and something about the nature, of mineralization. Mineralized bedrock was specifically sought throughout the course of fieldwork. Based on these kinds of information, small tracts having both permissive and specific evidence of mineralization at the ground surface were identified.

The Katmai study lacks subsurface information and so does not rigorously assess the size or number of potential deposits. Geophysical data can help to constrain the size of favorable host rocks for certain deposit types. In the Katmai· region, however, aeromagnetic data (Andreasen and others, 1963) are the only regionally extensive, geophysical data that are available, and these are available only for the Naknek quadrangle where bedrock is deeply buried beneath glacial drift. Moreover, the aeromagnetic data are too widely spaced to delineate any but the largest areas of magnetically anomalous rock.

By 1971, two National laws regarding landownership in Alaska had been enacted. The Alaska Statehood Act (1958) allowed the State to select about one-quarter ofthe landmass for State ownership. The Alaska Native Claims Settlement Act (ANCSA; 1971) allowed Alaskan Natives to select one-eighth of the State for private ownership and further authorized the Secretary of the Interior to withdraw another one-quarter for potential inclusion in parks or other preservation units. ANCSA required identification of the withdrawals by December 1978. Consequently, in 1974 Congress requested that the U.S. Geological Survey (USGS) provide information about the mineral potential of the probable withdrawals in time for Congress to use in the final decisions. Initially multidisciplinary teams undertook GEOGRAPHIC AND PHYSIOGRAPIDC SETTING

GEOLOGIC INVESTIGATIONS IN THE

OF THE KATMAI STUDY AREA

KATMAI REGION

PREVIOUS STUDIES on brief prospecting activity at Kukak Bay for copper and gold, and prospecting occurred sporadically after 1920 particularly in the area immediately south of Kulik Lake and along Shelikof Strait (see claims summary in Cobb, 1980). But owing to the remoteness, lack of producible surface mineralization, and withdrawal from mineral entry, the Katmai region missed intensive exploratory activity that was typical of mining districts elsewhere in Alaska.

The earliest systematic field studies in the region were carried out by industry while exploring for hydrocarbons. The publicly available data through 1975 were compiled by Magoon and others (1976) as a map, which includes that part of the study area north of Cape Chiniak and east of Kulik Lake (fig. 2). The first geologic map of the Mount Katmai quadrangle (Keller and Reiser, 1959) was based on mapping carried out from five fieldcamps occupied over the course of one summer, supplemented by air-photo interpretation. Use of helicopters in our study provided vastly better access, especially to the more remote parts of the study area.

Numerous studies were carried out in the Valley of Ten Thousand Smokes after 1960, focusing on the volcanic deposits and history of the 1912 Novarupta eruption. These studies continue today (see Eichelberger and others, 1991); most have dealt with volcanic processes, although Hildreth (1983) mapped the boundary of the 1912 ash-flow deposit.

The Katmai region is part of the Peninsular terrane of Jones and Silberling (1979), which is adjacent to the Chugach terrane on the southeast. Wilson and others (1985) proposed renaming the Peninsular terrane the Alaska Peninsula terrane and subdividing it into the Chignik and Iliamna subterranes.

Fieldwork was carried out during the summers of 1983 through 1987. Due to the undeveloped nature and remoteness of the study area, self-supporting fieldcamps were needed throughout the period of fieldwork. The research vessel Don J. Miller served as both helicopter platform and living quarters on the Pacific coast (parts of two seasons). Fieldcamps were established at Becharof Lake (two seasons), Naknek Lake (one season), and Battle Lake (two seasons). Commercial facilities in King Salmon were also used briefly.

Quaternary deposits are those less than 1.65 million years old; that is, 1. 65 Ma. Quaternary deposits in the Katmai study area are mainly either glacial deposits, or volcanic rocks and deposits of the active Aleutian volcanic arc. Although the entire Katmai area was glaciated several times, thick glacial deposits occur only west of the Aleutian Range in the Bristol Bay lowlands. The mountain lakes Naknek, Brooks, Grosvenor, Kulik, and Nanwhyenuk are dammed by moraines left at their west ends after the last major glaciation. The lake basins were ice free by at least 10,300 yr ago. The glaciers that scoured these basins originated in the adjacent Aleutian Range to the east. Prominent moraines between the west end of Becharof Lake and Bristol Bay were formed by glaciers that had moved south in Shelikof Strait and poured northwest through a low in the Aleutian Range.

Volcanic deposits of the Aleutian volcanic arc are found chiefly at or near the Aleutian Range crest, which in part consists of active stratovolcanoes. Rhyolitic or dacitic ash-flow tuffs occur at the south margin of the Katmai study area, in the Valley of Ten Thousand Smokes, and at Kaguyak Crater. Airfall deposits of the 1912 eruption are more than a meter thick in the area from the head of the Valley of Ten Thousand Smokes east to Shelikof Strait (Judy Fierstein, USGS, written commun., 1991). Several warm springs near the crest of the Aleutian Range are additional evidence of continuing volcanic activity.

The Katmai study area is a continental margin that has been the site of several cycles of subduction-generated volcanism and associated intrusive activity. The oldest rocks in the study area of certain age are submarine basalt flows and diabase sills of the Cottonwood Bay Greenstone, now metamorphosed to greenschist facies. In the Katmai region, this unit occurs near the south margin of the study area as remnants of older rocks (roof pendants) that were intruded during Jurassic time (205-138 Ma). The basalt flows crop out near interbedded marine limestone and basalt flows; the limestone and flows are assigned to the Late Triassic (210-205 Ma) Kamishak Formation that overlies the Cottonwood Bay Greenstone in the Iliamna quadrangle north of the Katmai area (Detterman and Reed, 1980).

Some metamorphic rocks of Paleozoic(?) (570-240 Ma) or early Mesozoic (240-190 Ma) age are assigned to the Kakhonak Complex and may be older than the Cottonwood Bay Greenstone. The metamorphic rocks crop out in a northeast-southwest trending belt as roof pendants within Jurassic intrusive rocks. These rocks were probably metamorphosed by heat from the intrusions. Nonmetamorphosed parent rocks of the Kakhonak Complex in the Katmai area as well as in the Iliamna quadrangle include exposed, pre-Middle Jurassic rocks. However, quartzite and potassium-feldspar-bearing gneiss exposed on an island in Maps MF-2021-A through 2021-D show the spatial distribution of anomalies identified by the different analytical techniques reported in Open-File Reports 86-586, 87-615, and 88-422 for the set of stream-sediment samples. The analytical results of all techniques are interpreted together in Bulletin 2020.

Results are statistically analyzed to determine the concentration of each element that is unusually high (anomalous) in stream sediments according to the dominant type of bedrock that underlies the drainage basin of the sample. Spatial clustering of samples having anomalous concentrations and the occurrence of multiple elements in anomalous concentrations in those samples indicate likely sulfide (iron, copper, lead, zinc, or arsenic) and (or) precious-metal (gold, silver) mineralization in tracts near Cape Douglas and Kulik Lake and in the central part of the study area (fig. 2).

Results confirm most of the three tracts outlined in MF-2021-A. A fourth tract defined exclusively by leachates is the area underlain by Tertiary volcanic rocks north of Naknek Lake. Such an alteration signature may reflect widespread, low-grade cooling alteration of the lava flows; it may also reflect older deposits of glacial till, or buried altered rocks.

The high-density concentrates should contain most ore-forming minerals. Samples anomalous in copper, lead, gold, or silver occur near Cape Douglas; anomalous concentrations of copper, lead, zinc, molybdenum, and boron occur from Kukak Bay southwest to Katmai Bay, near Kejulik Pass at the south margin of the study area, and in the central part of the study area. Anomalous concentrations of copper, molybdenum, lead, and zinc occur immediately south of Kulik Lake. A few samples having anomalous concentrations of gold, molybdenum, and silver occur in the northwest part of the map area, in the tract underlain by Tertiary volcanic rocks.

Pyrite-, copper-, lead-, and zinc-sulfide minerals, and uncommon gold occur in samples from southwest of Cape Douglas. A few samples from a tract underlain by altered volcanic rocks at the south margin of the study area have copper- and zinc-sulfide minerals. A few samples from the center of the study area and from a tract south of Kulik Lake have copper-, lead-, or zinc-sulfide minerals and a tungsten mineral. The tract north of Naknek Lake, underlain by Tertiary volcanic rocks, yielded a few samples having tungsten, tin, or zinc minerals.

Interpretation of analytical results for the bedrock and stream-sediment samples is aided by comparison with results from mineralized tracts in adjacent areas. In the Chignik-Sutwik Island quadrangles south of the Katmai area, porphyry mineralization occurs near Tertiary or Quaternary intrusions in marine and nonmarine sedimentary rocks (Wilson and Cox, 1983). The metals of interest in a porphyry mineral deposit are chiefly copper and molybdenum but may include gold, silver, or zinc. The ore minerals occur in thin veins at the margins of plutons or in adjacent wall rocks. Areas of possible porphyry mineralization occur in the Ugashik and Karluk quadrangles as well, immediately south of the Katmai area (Church and others, 1989b). In these areas, stream sediments have anomalous concentrations of copper, molybdenum, and one or more of lead, zinc, arsenic, silver, and gold; rock samples have anomalous concentrations of these elements and tin or tungsten.

Precious-metal veins occur at the Apollo mine on Unga Island 470 km southwest of Katmai. The Apollo produced $3 million worth of gold and silver between 1893 and 1908. The occurrence is as near-vertical quartz veins in Tertiary lava flows and domes (Wilson and others, 1988). The Apollo and other prospects on Unga Island are sited on faults that have localized quartz veins, silicification, and alteration (White and Queen, 1989). Stream sediments in the area of the Apollo mine have anomalous concentrations of silver, copper, lead, molybdenum, and zinc; bedrock samples have anomalous concentrations of these elements as well as gold (J. Frisken, written commun., 1988).

Numerous prospects and mineral claims for gold, silver, copper, and iron have been filed in the Iliamna quadrangle north of the Katmai region. These occurrences are either replacement deposits (skarns) formed in limestone or volcanic rocks of the Talkeetna Formation at the contact with intrusive rocks or are magnetite in gabbro of the Jurassic batholith (Detterman and Reed, 1980). Gabbro and limestone are uncommon in the Katmai study area, and there are few appropriate geochemical anomalies. Thus, there is little evidence for similar deposits in the Katmai region. A gold-, silver-, and copper-bearing quartz vein occurs 13 km north of Kulik Lake in slightly altered, Tertiary volcanic rocks; the vein probably formed above a buried pluton.

Anomalous concentrations in both bedrock and stream-sediment samples of copper, molybdenum, lead, zinc, silver, arsenic, tin, and tungsten are suggestive of undiscovered porphyry copper or molybdenum deposits. Such anomalies occur in tracts at the center of the study area, at Cape Douglas in the northeast, and immediately south of Kulik Lake at the north margin of the study area. Less intensely developed patterns of anomalies occur as well at the south margin of the study area near Kejulik Pass and in the northwest. In all of these tracts there are porphyritic intrusive rocks or there is permissive evidence for the occurrence of such rocks at shallow depth.

Anomalous concentrations of copper, molybdenum, lead, zinc, silver, and arsenic may indicate the presence of polymetallic veins. Where gold, silver, mercury, or arsenic occur in anomalous concentrations, potential gold-silver (precious metal) veins are suggested. Samples having anomalous concentrations of such elements occur in the same tracts as those outlined for porphyry deposits: in the center of the study area, near Kulik Lake, and at Cape Douglas. Another tract having a large number of samples anomalous in the vein suite is the area from Cape Douglas south to Katmai Bay. This tract is underlain by altered Tertiary and Quaternary volcanic rocks that have abundant quartz veins, fractures, and dikes and other small intrusive masses that may have been sources of mineralizing fluids.

The occurrence of active fumaroles or warm springs near some of the Quaternary volcanoes is permissive of undiscovered hot-springs gold deposits. Some stream-sediment samples collected in basins draining the flanks of active volcanoes· have anomalous concentrations of elements characteristic of hot-springs deposits (such as mercury, lead, arsenic, and silver). However, specific sites having well-developed geochemical anomalies were not identified.

Analytical data are presented for 46 new potassiumargon ages determined on samples of intrusive and volcanic rocks collected during the course of this study. Data are included for 18 additional ages that were previously determined but some not previously published. Sample localities are plotted on a base geologic map. The ages define two main clusters: an early to middle Jurassic intrusive event (the Alaska-Aleutian Ranges batholith) and a middle Tertiary to Quaternary, intrusive and volcanic event that includes the modem Aleutian volcanic arc. The new sample ages have important implications for the mineral-resource assessment: for example, mineralization in areas underlain by batholithic rocks is preferentially concentrated near plutons of Tertiary age.

These maps summarize the evidence and interpretations in the other publications, and outline tracts having evidence permissive or suggestive of undiscovered mineral deposits. A major contribution of the Katmai study is the first systematic resource assessment of the region.

In the north-central part of the study area, near Kulik Lake, evidence is favorable for the occurrence of an undiscovered porphyry-copper deposit and (or) for polymetallic vein deposits. There is also potential for placer gold deposits in this tract. Bedrock in this tract includes Tertiary plutons that have intruded Jurassic plutons and their older wall rocks, similar to the types of bedrock that occur around claims and mineral occurrences in the Iliamna quadrangle. Specific evidence for mineralization includes quartz veins, disseminated pyrite, rock alteration, and anomalous concentrations of metals including copper, gold, and silver in both rock samples and stream sediments. Some claims and prospect pits occur in the area.

No direct evidence for replacement (skarn) deposits at the margins of Jurassic or Tertiary plutons was found in the Katmai region during the course of this study.

There is potential for a porphyry copper (molybdenum) deposit and (or) for polymetallic vein deposits in the central part of the study area. The geologic setting is Tertiary plutons that have intruded fractured and locally altered, Mesozoic sedimentary rocks. Anomalous quantities of copper, lead, zinc, silver, and gold occur in both bedrock and stream-sediment samples. Alteration and quartz veins are present in the plutonic rocks. A similar geochemical signature occurs in a tract of altered volcanic rocks near Kejulik Pass, at the south margin of the study area. There are, however, no exposed intrusive rocks at Kejulik Pass except for small sills and dikes, and the geochemical signature is less intensely developed than in the central tract.

The entire Aleutian Range, from Cape Douglas south to Katmai Bay, is judged to have potential for undiscovered deposits of porphyry copper, hot-springs gold, and (or) polymetallic veins. The underlying bedrock is late Tertiary and Quaternary volcanic rocks and small intrusive masses, and older Mesozoic and Tertiary sedimentary wall rocks. All rock types are altered or fractured, providing pathways for mineralizing fluids, and have quartz veins that are parallel to the prevalent northwest-trending fractures. Some geochemical anomalies are associated with areas of particularly intense alteration and veining. Anomalous concentrations of zinc, silver, copper, molybdenum, and gold are identified in both stream-sediment and bedrock samples.

The number and size of undiscovered mineral deposits that may occur in the Katmai region are unknown. No producing porphyry deposits occur in similar rocks elsewhere on the Alaska Peninsula, thus it is unlikely that many such undiscovered deposits would occur in the study area. On the other hand, vein deposits may extend for only tens to hundreds of meters along a narrow zone, so there could be numerous undiscovered vein deposits within the study area.

The only known mineral production from the study area has been from a gold-placer claim on the American River south of Nanwhyenuk Lake. A few tidewater claims for pumice (lightweight aggregate) are still valid on the Shelikof Strait; there has been no known production from these claims. Sand and gravel for construction are abundant in the inhabited western portion of the study area. The few known warm springs are too low in temperature for direct uses. Hydrocarbon potential within the study area is low, due to unfavorable rock characteristics and geologic history. Minor coal seams in Tertiary rocks along Shelikof Strait could constitute a small, locally marketable resource under favorable conditions of economics and land classification.

A total of 338 collections of macrofossils were made during the course of the study, many from measured stratigraphic sections. The report presents fossil identities, localities, ages, and measured sections. The combination of fossil and stratigraphic data is useful not only for determining the exact ages of the rocks but for describing precisely the sequence of environmental changes during deposition of the rock layers.

Most collections are from the Naknek Formation, the main rock unit of the Alaska Peninsula. Marine deposits of the upper part of the Naknek Formation are richly fossiliferous, the clamlike genus Buchia predominating. Different species of Buchia indicate different marine environments.

Ammonites- marine invertebrates related to modem squids-and the clam Inoceramus are important fossils in the Cretaceous rocks of the area. Ages of nonmarine Tertiary rocks are determined from plant fossils.

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