Abstract

Detailed geologic, metal zonation, paragenetic, fluid inclusion, and stable isotope studies have been undertaken of the Cu-rich Gecko K44 and the Au-rich White Devil Deeps Zone orebodies in the Proterozoic Tennant Creek district of the Northern Territory, Australia. Despite differences in the grade and mineral abundances in the ores, three paragenetic stages are present in both deposits: (1) an early, synkinematic oxide stage at which time the ironstones formed, (2) a syn- to postkinematic sulfide stage at which time Cu, Au, and Bi were introduced, and (3) a late, minor carbonate stage with minor Pb and Zn. The oxide stage involved precipitation of magnetite, hematite, quartz, and chlorite; this stage was subsequently foliated. The sulfide stage involved the precipitation of gold, chalcopyrite, Bi minerals, chlorite, and quartz and the hematization of magnetite. The carbonate stage involved precipitation of carbonate, talc, galena, and sphalerite and the alteration of chalcopyrite to bornite and covellite.Fluid inclusion studies indicate that oxide-stage minerals formed from low-temperature (250 degrees C), high-salinity (15-20 wt % NaCl equiv) fluids, whereas the sulfide stage formed from high-temperature (350 degrees C), high-salinity (20 wt % NaCl equiv) fluids that show evidence of phase separation. Sulfur isotope studies indicate that lower delta 34 S values occur in Au-rich pods, whereas higher delta 34 S values occur in Cu-rich halos.Metal zonation studies indicate that in the White Devil Deeps Zone orebody a basal Au-rich zone is surrounded by a Bi-rich shell; this zonation is broadly similar to zonation at the Juno deposit (Au --> Bi --> Cu --> pyrite; Large, 1975). Owing to its very low grades, the distribution of Cu in the White Devil Deeps orebody is erratic. Conversely, the Gecko K44 orebody is only weakly zoned with Au enrichment at the top of the orebody in the "sulfide tongue."Mineral deposition occurred in well-zoned, Au-rich (e.g., Juno and White Devil) and weakly zoned chalcopyrite-hematite-rich (e.g., Gecko) deposits, owing to the reduction of highly oxidized sulfide-stage fluids through hematization of magnetite in ironstones. The fluids evolved through the hematite field (in f (sub O 2 ) -pH space) to the hematite-pyrite join and then followed the hematite-pyrite join toward the hematite-pyrite-magnetite triple point. This fluid evolution caused early gold precipitation in the hematite field, but chalcopyrite precipitated only along the hematite-pyrite join, which accounts for metal zonation in well-zoned, Au-rich deposits. In chalcopyrite-hematite-rich deposits, fluid evolution was restricted to the hematite-pyrite join, and minor gold precipitated with chalcopyrite. The K44 sulfide tongue is inferred to be a small-zoned, Au-rich system adjacent to the main K44 orebody.Differences in metal abundance and zonation can be interpreted in terms of varying proportions of highly oxidized magmatic hydrothermal fluids relative to more reduced connate basinal fluids introduced into the ore-forming systems. A large component resulted in well-zoned, Au-rich deposits, an intermediate component resulted in chalcopyrite-hematite-rich, Au-poor deposits, and a small component resulted in unzoned chalcopyrite-rich, pyrrhotite-bearing deposits with intermediate Au grades (e.g., Peko).

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