Precious metals associated with Late Cretaceous-early Tertiary igneous rocks of southwestern Alaska

Epithermal mercury-antimony and gold-bearing vein lodes of southwestern Alaska

Epithermal mineral deposits and occurrences of southwestern Alaska consist of Hg-Sb and gold- and sulfide-bearing 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 mafic to felsic intrusive rocks. Mineralized Hg-Sb vein and vein breccia lodes are found in the sedimentary or igneous rocks or at their contacts. The minerology of the Hg-Sb lodes is dominated by cinnabar and stibnite, with subordinate realgar, orpiment, and native mercury, pyrite, gold, and hematite, as well as solid and liquid hydrocarbons; quartz, carbonate, limonite, dickite, and sercite are alteration gangue minerals. The largest mercury mine in Alaska, Red Devil, produced about 36,000 flasks of mercury, but the Hg-Sb lodes of southwestern Alaska generally consist of small, discontinuous veins that rarely 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 than 1 percent Sb and As but are generally poor in base emtals 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 arc in southwestern Alaska. Geologic and geochemical characteristics of the Hg-Sb lodes suggest that ore fluids were generated in local sedimentary rocks as they were intruded by magmas. These intrusions provided the heat to initiate dehydration reactions and expel fluids from hydrous 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 originated from 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 , N2, 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 sercites in the Hg-Sb lodes indicate a temporal association of igneous activity and mineralization, 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-arc 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, northeast-striking, steeply dipping faults and fractures. The Alaska-Apollo mine produced about 500,000 metric tons (t) of ore 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, barsite, clay, rhodonite, and adularia. Ores generally have Au-Ag-Te-Pb-Zn-Mn-Cu geochemical signatures, with wide As-Hg aureoles around some veins. Geologic and mineralogical characteristics of these lodes are similar to adularia-sericite volcanic-hosted epithermal deposits. The gold-bearing vein lodes may be related to arc porphyry systems, but more data are required to verify this association.