Abstract To stimulate interest and provide background information for future petroleum and mineral exploration in interior Alaska, the state of Alaska, Division of Oil and Gas, and the U.S. Geological Survey joined in a cooperative effort to compile and merge all of the publicly available magnetic data throughout interior Alaska. Interior Alaska extends from the Brooks Range in the north to the Alaska Range in the south and from the Peninsula on the west to the Alaska-Canadian border. Budget and time constraints restricted the initial compilation to the area of state-controlled lands within the Alaskan interior, extending from 61°N to 66°N and 144°W to 159°W (Figure 1). The primary sedimentary basins within this area consist of the Copper River Basin, Susitna Basin, Minchumina Basin, Holitna Basin, and Middle Tanana Basin (Kirschner, 1994). The data processing was performed by Paterson, Grant & Watson Ltd. of Toronto, Ontario, under contract to the state of Alaska. Aeromagnetic maps at a scale of 1:500,000 have been published jointly by the U.S. Geological Survey and the state of Alaska, Division of Oil and Gas (Meyer and Saltus, 1995). In this study, twenty-three magnetic surveys, comprising 280 000 line-kilometers, flown between 1954 and 1982, were merged to produce two digital data grids. The first grid, referred to as the composite grid (Figure 2), was produced to retain the closest resolution to the original data for each survey and was designed to be used for detailed modeling and local depth-to-basement determinations. The second grid, referred to as the merged grid

Abstract Epithermal mineral deposits and occurrences of southwestern Alaska consist of Hg-Sb and gold- and sulfidebearing vein lodes. Numerous Hg-Sb lodes are located throughout a region measuring several tens of thousands of square kilometers in and surrounding the Kuskokwim River basin in southwestern Alaska. The Hg-Sb lodes are hosted in sedimentary rocks of the Cretaceous Kuskokwim Group. The Triassic to Cretaceous Gemuk Group, and the Paleozoic Holitna Group, as well as in Late Cretaceous and early Tertiary maflc to felsie intrusive rocks. Mineralized Hg-Sb vein and vein breccia lodes are found in the sedimentary or igneous rocks or at their contacts. The mineralogy of the Hg-Sb lodes is dominated by cinnabar and stibnite, with subordinate realgar, orpiment, native mercury, pyrite, gold, and hematite, as well as solid and liquid hydrocarbons; quarlz, carbonate, limonite, dickite, and sericite are alteration gangue minerals. The largest mercury mine in Alaska, Red Devil, produced about 36,000 flasks of mercury, bul the Hg-Sh lodes of southwestern Alaska gencntally consist of small, discontinuous veins that rerely exceed a few meters in width and a few tens of meters in strike length. The Hg-Sb lodes generally contain about 1 to 5 percent Hg and less that 1 percent Sb and As but are generally poor in base metals and precious metals. Anomalous concentrations of gold in some lodes, however, suggest that gold deposits may be present in higher temperature environments below some of the Hg-Sb lodes. The formation of the Hg-Sb lodes is closely correlated with igneous activity of a Late Cretaceous and early Tertiary magmatic are in southwestern Alaska. Geologic and geochemical characteristics of the Hg-Sb lodes suggest that are fluids were generated in local sedimentary rocks as they were intnuled by magmas. These intrusions provided the heat to initiate dehydration reactions and expel fluids from hrdrous minerals and formational waters in the sedimentary rocks, causing thermal convection and hydrothermal fluid flow along fractures and faults. Isotopic data from sulfide and alteration minerals of the Hg-Sb lodes indicate multiple sources for the ore fluids; most fluids appear to have originattd in local sedimentary rocks. Hydrothermal fluids with isotopically heavy oxygen but isotopically light hydrogen and sulfur compositions indicate derivation of these species from sedimentary rocks. Isotopically shifted, evolved meteoric water was a primary component in ore fluids from a few Hg-Sb lodes. Geochemical, isotopic, and fluid inclusion data also indicate that Hg. CO 2 , CH 4 , N 2 , and local hydrocarbons were derived from breakdown of organic matter in Sedimentary rocks when they were heated by intrusions. Radiometric 40 Ar/ 39 Ar ages of 70 ± 3 Ma from hydrothermal sericites in the Hg-Sb lodes indicate a temporal association of igneous activity and minemlization, which is consistent with the geologic characteristics. Most epithermal gold-bearing vein lodes on the Alaska Peninsula and Aleutian Islands are located in Eocene to Pleistocene volcanic-are rocks, commonly andesite and dacite. These vein and vein breccia lodes, such as the Alaska-Apollo and Shumagin deposits on Unga Island, tend to be aligned along regional, northcast-striking, steeply dipping faults and fractures. The Alaska-Apollo mine produced about 500,000 metric tons (t) of are that yielded an estimated 3,500 kg (130,000 oz) of gold from veins that were as much as 12 m wide and extended for 1,500 m laterally and 420 m vertically, Ore minerals include gold, galena, sphalerite, chalcopyrite, pyrite, marcasite, arsenopyrite, and native copper; gangue minerals are quartz, sericite, calcite, and chlorite and locally, barite, clay, rhodonite, and adularia. Ores gcnemlly have Au-Ag-Te-Pb-Zn-Mn-Cu geochemical signatures, wilh wide As-Hg aureoles around some veins. Geologic and mineralogical characteristics of these lodes are similar to adularia-sericitc volcanic-hosted epithermal deposits. The gold-bearing vein lodes may be related to are porphyry systems, but more data are required to verify this association.

New collections of floral and faunal remains were recovered from late Paleozoic sediments of the Mount Dall conglomerate in the Alaska Range of south-central Alaska. This isolated unit's type section is ∼1500 m thick and comprises thick to very thick conglomerate beds with interbedded sandstones and siltstones in a series of fining-upward intervals each tens of meters thick. The unit is interpreted to be a coastal braidplain deposit of Early Permian age in the upper Farewell terrane (Mystic subterrane sequence). Genus-level taxonomic composition of paleobotanical collections from lenticular mudstones to siltstones is discussed with regard to taphonomy and the interpreted lowland paleoenvironment of deposition. Poorly to moderately preserved megafossil compressions and impressions of the foliage genera Pecopteris , Zamiopteris , Rufloria , Angaropteridium , Cyclopteris , and Cordaites are consistent through several hundred meters of section and suggest a locally dense floral community. Horizons with sideritic rhizoliths indicate the presence of immature soils. The co-occurrence of these foliar and reproductive organs in the Mount Dall conglomerate suggests a mixed phytogeographic affinity to both the temperate Angaran Floristic Province of northern Pangea and the Euramerican Province of lower paleolatitudes. The brachiopod genera ? Stenoscisma and ? Schuchertella also were recovered and indicate a coastal depositional setting. These new biogeographic data complement exclusively marine zoogeographic data from the Farewell terrane's older strata and may be used to test hypotheses regarding the paleogeography of this displaced continental fragment. The paleofloral data support the placement of this terrane within a midlatitude climate belt during the Early Permian.

Silurian rocks in Alaska have been identified in 12 accreted terranes and in the Tatonduk-Nation River area of east-central Alaska, which represents part of autochthonous North America. Most of the terranes are in situ or structurally imbricated portions of the North American (or Siberian) continental margin. An exception is the Alexander terrane of southeastern Alaska, which originated as an offshore island arc. Discontinuously exposed and (or) highly altered sequences have precluded detailed investigations of Silurian rocks in most parts of Alaska, but reconnaissance-level studies reveal that graptolitic shales of turbidite or hemipelagic origin record deep-water or “shale out” conditions west or north of the ancient continental margin of North America. Platform carbonates are also exposed in many areas and are particularly well represented in southwestern (Nixon Fork subterrane of Farewell terrane) and southeastern (Alexander terrane) Alaska, indicating that much of Alaska resided close to the paleoequator in the Silurian. Subtidal stromatolite reefs in southwestern and southeastern Alaska that are similar to those in Salair and the Ural Mountains, Russia, indicate a paleobiogeographic connection between these two parts of Alaska, Siberia, and eastern Baltica via the Uralian Seaway in the Late Silurian. Deposition of vast accumulations of red beds and other siliciclastic rocks beginning in the Late Silurian suggests that parts of Alaska may have been affected by late stages in Cale-donian orogenesis and (or) early stages in the Ellesmere orogeny during formation of the Laurussian landmass.