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Invisible Gold in Pyrite (NanoSIMS)

Revealing invisible gold in pyrite with NanoSIMS sulfur isotope analyses
In Carlin-type gold deposits, refractory gold often occurs in Fe-sulfide host minerals. The host grains are typically very small and the concentration of Au too low to be detected by EPMA, giving rise to the term “invisible gold”. The NanoSIMS uniquely permits to complement mapping of trace elements invisible to EPMA (~ten time better spatial resolution than EPMA and with better sensitivity) with isotopic ratio mapping of sulfur revealing unambiguously two successive gold-rich fluid inclusion identified by their distinct 34S/32S ratios. Understanding the chemistry of the Au-bearing fluid is fundamental to targeting other potentially viable deposits in the local vicinity!

The NanoSIMS was used to determine the distribution of Au, Cu, As, Sb, and S, as well as stable isotopes of S (34S/32S) in gold-bearing pyrite from two refractory Carlin-type gold deposits: West Banshee, Northern Carlin Trend, and Turquoise Ridge, Getchell Trend, located in Northern Nevada. (The results shown above were obtained from ore-stage pyrite sample 664 211 from Turquoise Ridge, Northern Nevada, USA). The NanoSIMS maps reveal that gold occurred in two discrete episodes in each deposit.

Elevated gold concentrations correlate with elevated concentrations of As, Sb, Cu ± Te, and lower 34S/32S ratios, compared to periods when gold was not deposited. Precipitation of elevated gold and trace elements at West Banshee was followed by precipitation of pyrite with lower gold concentrations, whereas at Turquoise Ridge precipitation of trace element-rich pyrite was followed by precipitation of late ore-stage minerals.
These NanoSIMS results are consistent with formation of the deposits by the episodic incursion of gold-rich fluids into hydrothermal systems otherwise dominated by gold-poor fluids. Such gold-rich fluid pulses may be related to incursions of magmatic fluid, which have been shown to have high concentrations of gold, arsenic, and copper in porphyry and high-sulfidation epithermal systems.

From : Uncloaking invisible gold: use of NanoSIMS to evaluate gold, trace elements, and sulfur isotopes in pyrite from Carlin-type gold deposits. Shaun L.L. Barker, Kenneth A. Hickey, Jean S. Cline, Gregory M. Dipple, Matt R. Kilburn, Jeremy R. Vaughan, and Anthony A. Longo. Economic Geology, 2009, vol. 104, pp. 897–904