Abstract

The orebodies at the Getchell and Twin Creeks mines were studied through mineral paragenesis, geologic relationships, and 40 Ar/ 39 Ar dating. Mineral paragenetic relationships are based on observations made during the logging of 18,000 m of drill cuttings and core and crosscutting relationships recognized in the field and Main pit at the Getchell mine. Ages for igneous and mineralizing events were determined through 40 Ar/ 39 Ar incremental heating analyses of 15 samples of biotite, K feldspar, sericite, and vein adularia. A thermal history for the area was also developed using K feldspar multiple diffusion domain results and age determinations on cogenetic minerals, which have different argon closure temperatures.Mineral paragenetic relationships and 40 Ar/ 39 Ar dating at the Getchell and Twin Creeks deposits document that gold mineralization occurred in five stages between 95 and 42 Ma.Stage 1 pyrrhotite-quartz-arsenopyrite-chalcopyrite-biotite gold mineralization in skarn is associated with 95 Ma dacite dikes and granodiorite plugs. Stage 2 chalcopyrite-sphalerite-galena-pyrite-arsenopyrite + or - gold and silver mineralization in skarn is related to the 92 Ma Osgood Mountains stock. Secondary K feldspar and sericite in stage 3 quartz-pyrite-kaolinite-gold mineralization formed at 83 Ma and are not related to 95 or 92 Ma igneous activity. Stage 4 low-grade quartz-pyrite-gold mineralization formed at about 75 Ma, in the matrix of a breccia pipe that cuts 83 Ma silicic and argillic ores. Adularia intergrown with stibnite-orpiment-pyrite-quartz-gold + or - realgar mineralization yields precise 40 Ar/ 39 Ar plateau ages of 42.11 + or - 0.43 and 41.90 + or - 0.25 Ma for stage 5.This study documents the temporal complexity of mineralization and igneous activity at the Getchell and Twin Creeks deposits. Minor gold in skarn-type mineralization (stages 1 and 2,) associated with 95 and 92 Ma igneous intrusions preceded and was not genetically related to Carlin-type gold mineralization (stages 3 and 5). The Carlin-type deposits at Getchell and Twin Creeks therefore represent a composite orebody formed by the overprinting of 83 Ma stage 3 quartz-pyrite-gold by 42, Ma stage 5 orpiment-stibnite-pyrite-gold mineralization.

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