Pyrite as a record of hydrothermal fluid evolution in a porphyry copper system: A SIMS/EMPA trace element study
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Reich Morales, Martín
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Pyrite as a record of hydrothermal fluid evolution in a porphyry copper system: A SIMS/EMPA trace element study
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Abstract
Porphyry copper deposits are currently the world’s largest source of copper and molybdenum, and are also among the
largest reservoirs of gold in the upper crust. Despite the fact that pyrite is a ubiquitous mineral phase in these deposits
and secondary Cu enrichment processes are commonly controlled by the abundance of this sulfide, the major and trace element
chemistry of pyrite from porphyry systems remains unconstrained. In this study, we report the first comprehensive trace
element database of pyrite from the Dexing deposit, China’s largest porphyry Cu deposit. By combining high-spatial resolution
and X-ray mapping capabilities of electron microprobe analysis (EMPA) with low detection limits and depth-profiling
capabilities of secondary-ion mass spectrometry (SIMS) in a suite of samples from the Dexing deposit, we show that the concentrations
of precious metals (e.g., Au, Ag), metalloids (e.g., As, Sb, Se, Te) and heavy metals (e.g., Cu, Co, Ni, Zn, Hg) in
pyrite from porphyry systems are more significant than previously thought. Among the elements analyzed, Cu, As, Au and Ni
are the most abundant with concentrations that vary from sub-ppm levels to a few wt.% (i.e., 6 wt.% Cu, 3 wt.% As,
0.25 wt.% Au, and 0.2 wt.% Ni). Detailed wavelength-dispersive spectrometry (WDS) X-ray maps and SIMS depth vs.
isotope concentration profiles reveal that pyrite from the Dexing deposit is characterized by complex chemical zoning where
the studied elements occur in different mineralogical forms. While As occurs as a structurally bound element in pyrite, Cu and
Au can occur as both solid solution and micro- to nano-sized particles of chalcopyrite and native Au (or Au tellurides),
respectively, indicating that pyrite can control metal speciation and partitioning during porphyry Cu mineralization. The
well-developed oscillatory zoning detected in pyrite, where Cu-rich, As-depleted growth zones alternate with Cu-depleted,
As-rich layers, indicates that Cu is geochemically decoupled from As, suggesting that this selective partitioning of metals into
pyrite is most likely the result of changes in hydrothermal fluid composition.
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Geochimica et Cosmochimica Acta 104 (2013) 42–62
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