Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits?

Neumayr, Peter; Walshe, John L.; Hagemann, Steffen; Petersen, Klaus; Roache, Anthony; Frikken, Peter; Horn, Leo; Halley, Scott

doi:10.1007/s00126-007-0170-2

Cited by 93 publications

(34 citation statements)

References 6 publications

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“…The presence of two different sources for CO 2 -bearing fluids in Archean lode gold deposits has previously been suggested by Walshe et al (2003) and more recently Neumayr et al (2008) and Kendrick et al (2008) and is our favored model for the Sunrise Dam deposit (see below). CO 2 -dominated, nearly water free, fluids were interpreted as the ore-forming fluids in other lode gold deposits (Schmidt-Mumm et al, 1997;Chi et al, 2006).…”

Section: Implications For the Interplay Between Deformation And Fluidsupporting

confidence: 62%

P-T-X Conditions of Fluids in the Sunrise Dam Gold Deposit, Western Australia, and Implications for the Interplay between Deformation and Fluids

Baker¹,

Bertelli²,

Blenkinsop³

et al. 2010

Economic Geology

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The Late Archean Sunrise Dam gold deposit (~10 Moz) is hosted within greenschist-facies rocks and is characterized by extreme structural complexity, resulting from a protracted deformation history with evidence of structural reactivation and multiple phases of gold mineralization. Early Group I orebodies are hosted within shallow-to moderately dipping northwest-trending shear zones and occur in foliation parallel veins within a strong penetrative fabric. Group II orebodies occur within steeply dipping shear zones and are characterized by veins and breccias, and Group III and IV orebodies comprise dominantly stockwork veins. The orebodies formed in response to multiple episodes of deformation. Many structures were created during D1 and D2 shortening whereas the bulk of the gold was deposited during D3 and D4 that marks the transition from a dominantly lithostatic pore fluid regime to hydrostatic conditions. Two main fluid inclusion types were recognized at Sunrise Dam: low-salinity CO2-H2O inclusions and CO2 inclusions. CO2-H2O inclusions are more common in the early Group I orebodies. These fluids were trapped under high P-T conditions during D3 (minimum estimates suggest conditions were likely >~300°C and >1 to 3 kbars predominantly within the one-phase field). CO2-H2O inclusions that occur in later Group II and III orebodies were trapped predominantly within the two-phase field at temperatures < 300°C and pressures of ~1 kbar. CO2 inclusions crosscut the early CO2-H2O inclusions and are the dominant inclusion type in Group II, III, and IV structures. They likely represent a major late influx of separately sourced fluid (magmatic or mantle?) during D4 and were trapped at lower pressure conditions than the earlier CO2-H2O fluids. The combined structural and fluid history for the deposit suggests that the early CO2-H2O fluid ponded beneath moderately dipping shear zones during late D2, early D3, and that fluid pressures increased to near-or supralithostatic conditions (>1−3 kbars) at a temperature >300°C. Shear failure along these structures resulted in widespread precipitation of moderate-grade gold mineralization from the CO2-H2O fluid. Continued deformation and exhumation modified P-T-X conditions, and cooling of the host rocks below 300°C resulted in the CO2-H2O fluid entering the two-phase field. A combination of temperature decrease, a transition from lithostatic to suprahydrostatic and/or hydrostatic pressure conditions, fluid immiscibility, and the influx of a second CO2-rich fluid, resulted in fracture during D4 and precipitation of high-grade gold mineralization in steeply dipping structures forming veins, breccias, and stockworks.

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Section: Implications For the Interplay Between Deformation And Fluidsupporting

confidence: 62%

P-T-X Conditions of Fluids in the Sunrise Dam Gold Deposit, Western Australia, and Implications for the Interplay between Deformation and Fluids

Baker¹,

Bertelli²,

Blenkinsop³

et al. 2010

Economic Geology

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“…Chi et al (2009) A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 10 Uemoto et al (2002) presented a model of an ore fluid that had been slightly modified by reaction with rock types of differing oxidation state along subsidiary structures before mixing at the depositional site. More recently, mixing of fluids with different sources has been proposed (Walshe et al, 2003;Neumayr et al, 2007), but this appears physically unreasonable because the ore fluid is clearly overpressured and would drive resident fluids out of the system, not mix with them. Also, much of the mixing hypothesis is based upon the presence of iron oxide minerals in association with more reduced gold-related assemblages, which Evans (2010) has shown may be the product of a single fluid undergoing changing water-rock ratio during infiltration into the orehosting country rocks.…”

Section: Accepted Manuscriptmentioning

confidence: 97%

Orogenic gold: Common or evolving fluid and metal sources through time

Goldfarb

Groves

2015

Lithos

739

256

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“…In many Phanerozoic and Paleoproterozoic gold provinces, there is an association of ores with carbonaceous and organic material. Fluid mixing has been invoked by some workers (e.g., Neumayr et al, 2005Neumayr et al, , 2008 as important for orogenic gold formation based on observations of oxidized and reduced mineral phases in deposit halos. Shifts in pH and temperature decreases are not important factors in the formation of orogenic gold (e.g., Phillips and Powell, 2010).…”

Section: Ptx Constraints On Ore Depositionmentioning

confidence: 99%

“…Pyrite is the dominant sulfide phase in the ores, but there are minor amounts of arsenopyrite, tetrahedrite, and base metal sulfides. Many workers have stressed a mixing of fluids or two distinct fluids to help explain assemblages with coexisting hematite, magnetite, siderite, ankerite, pyrite, and anhydrite (e.g., Neumayr et al, 2008). Gold fineness throughout the deposit is >900 C. Silver and Sb both increase relative to gold with a depth (Bateman et al, 2001), which is atypical of most hydrothermal systems.…”

Section: Late Archean Greenstone-hosted Golden Milementioning

confidence: 99%