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Gray, J.E., 1996

Exploration, genesis, and environmental geochemistry of mined and unmined mercury-antimony vein lodes in southwestern Alaska

Bibliographic Reference

Gray, J.E., 1996, Exploration, genesis, and environmental geochemistry of mined and unmined mercury-antimony vein lodes in southwestern Alaska: University of Colorado, Boulder, Ph.D. dissertation, 297 p., illust., maps.


Numerous epithermal Hg-Sb lodes are located throughout a large region covering several tens of thousands of square kilometers in southwestern Alaska. The Hg-Sb lodes generally consist of small, discontinuous veins in sedimentary or igneous rocks, or at their contacts. The mineralogy of the lodes is dominated by cinnabar and stibnite with subordinate realgar, orpiment, native mercury, pyrite, gold, and hematite, as well as solid and liquid hydrocarbons; quartz, calcite, limonite, dickite, and sericite are alteration gangue minerals. About 41,000 flasks of mercury have been produced from mines in the region. Mercury concentrations in stream-sediment samples and cinnabar in heavy-mineral-concentrate samples collected downstream from the lodes are diagnostic for exploration of this deposit type. Formation of the Hg-Sb lodes is closely correlated with igneous activity of a Late Cretaceous and early Tertiary magmatic arc in southwestern Alaska. Isotopic data from sulfide and alteration minerals of the Hg-Sb lodes indicate that ore fluids were derived from mixed sources, with most fluids indicating a nonmeteoric source, primarily from local sedimentary rocks. Radiometric 40Ar/39Ar ages of 70 +/- 3 Ma obtained from hydrothermal sericites in the Hg-Sb lodes indicate a temporal association of igneous activity and mineralization. These data suggest that Hg-Sb ore fluids were generated in local sedimentary rocks as they were intruded by magmas. The igneous activity provided the heat to initiate mineral dehydration reactions and expel formation waters in the sedimentary rocks, causing thermal convection and hydrothermal fluid flow along fractures and faults. The mercury mines in southwestern Alaska represent a potential environmental hazard in the region because of the highly toxic nature of mercury. Stream-sediment, stream-water, and fish samples collected downstream from mines generally contain mercury concentrations higher than those from background sites. However, all mercury concentrations in fish are below the 1 ppm wet-weight concentration for edible fish when mercury advisories are listed and sale of fish is restricted. In addition, all concentrations of mercury in stream-water samples collected in the study are below the 2 ppb drinking-water maximum-contaminant level recommended by the State of Alaska.

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