Age and fluid chemistry of sediment-hosted disseminated gold deposits in the Great Basin, Nevada.

The Post/Betze sediment-hosted disseminated gold (SHDG) deposit is the largest known deposit of its type in the world. Alteration and mineralization assemblages and patterns are typical of SHDG deposits described in the literature. Because of extensive lateral and vertical exposures, this deposit is one of the best available examples of this type of mineralization for the study of unweathered ores. Pre- and post-mineralization igneous events at Post/Betze are 157 and 39 Ma as determined by $\sp{40}$Ar/$\sp{39}$Ar and fission-track methods on mineral separates from fresh rock samples. Gold-related hydrothermal activity is recorded in hydrothermal sericite and resetting of fission tracks in zircons, and probably took place near 117 Ma. Available evidence suggests that other SHDG deposits most likely are of similar age. The common occurrence of alunite in SHDG deposits has led to the suggestion that it is primary and must reflect hydrothermal fluid compositions. However, on the basis of textures, K/Ar dates, and stable isotope measurements alunite in SHDG deposits is determined to be of weathering-related origin. Therefore, alunite cannot be used to constrain any of the hydrothermal fluid chemistry. Gold is present in arsenic-rich pyrite overgrowths on pre-existing pyrite crystals in SHDG deposits. By comparison with other studies of gold in arsenopyrite and arsenian pyrite, it is inferred that gold is likely present as a charged species (Au$\sp{+3})$ in arsenian pyrite. Ion microprobe measurements made on the same samples as gold and arsenic measurements suggest that the sulfur in the ore fluid originated at least in part as a result of thermochemical reduction of seawater-derived sulfate. A separate study of fluid inclusions in native copper also was undertaken. Analyses of fluids from supergene native copper from Arizona establish the viability of the method. Results from sediment-hosted copper deposits at White Pine, MI suggest deposition from meteoric water; in addition, these results suggest that $\delta\sp{18}$O of ocean water was 0 per mil. However, fluids extracted from basalt-hosted amygdaloidal copper from the Keweenawan peninsula, MI are more enigmatic and suggest deposition from evolved meteoric fluids rather than seawater.PhDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105838/1/9226833.pdfDescription of 9226833.pdf : Restricted to UM users only