HYDROTHERMAL ALTERATION AND GOLD DEPOSITION IN THE HOMESTAKE IRON-FORMATION-HOSTED GOLD DEPOSIT, LEAD, SOUTH DAKOTA

The Homestake deposit located in Lead, South Dakota was one of the largest gold deposits, having produced over 40 million ounces of gold from 1876 to 2001. The deposit is hosted in the Proterozoic-aged Homestake Formation which is an iron formation consisting of either iron-carbonate or iron-silicate metasedimentary rocks. The unit is characterized by siderite in the greenschist facies and grunerite in the lower amphibolite facies, with minor chlorite, quartz, and graphite. The ore is located in and adjacent to quartz veins, and typically consists of arsenopyrite, pyrrhotite and native gold contained in a matrix of quartz±chlorite±biotite±garnet. Alteration is related to retrograde metamorphism and a post-metamorphic hydrothermal event, the latter coincident with gold deposition. Dominant alteration minerals mostly in halos adjacent to quartz veins are chlorite, carbonate (siderite and ankerite) and sericite. The chlorite grains in random to parallel orientation are believed to be directly related to the gold-bearing fluids. Sericite alteration occupies bleached zones consisting of sericite, siderite, carbonate, quartz and plagioclase. The objective of this study is to determine the extent of fluid-wall rock interaction in the Homestake Formation and to constrain the possible controls of ore deposition, specifically the influence of fluid-mineral reactions on gold deposition. Four different quartz veins from 3 drill holes were chosen and samples were taken at intervals away from the vein into unaltered country rock. Using petrography and geochemistry, the alteration assemblages will be distinguished from unaltered country rock assemblages. The different assemblages will elucidate changes in fluid chemistry that could have led to gold precipitation.