Sulfur Isotopes, Trace Elements, and Textural Analyses of Pyrite, Arsenopyrite and Base Metal Sulfides Associated with Gold Mineralization in the Pataz-Parcoy District, Peru: Paragenesis, Fluid Source, and Mechanisms

Mining & Minerals - Evaluations / Gold-Silver Exploration & Mining Projects

Voute, et al., report that in the Pataz-Parcoy district, current mining activity is focused on the mesothermal quartz-carbonate-sulfide veins hosted by the Pataz batholith. Total gold production yielded approximately 8 Million ozs. with grades in the mined ore shoots varying between 7 and 15 g/t Au, and locally reaching up to 120 g/t Au. High-grade ore shoots are extraordinarily enriched in sulfides, representing 10 to 20 modal vol% of the vein. Ore mineralogy is characterized by a complex paragenesis of pyrite, arsenopyrite, galena, sphalerite, chalcopyrite, and gold. Gold occurs mostly as electrum in equilibrium with base metals sulfides filling fractures of pyrite and arsenopyrite.

A novel combination of secondary ion mass spectrometry, laser ablation inductively coupled plasma mass spectrometry, and electron probe microanalysis is used to track the compositional evolution of ore fluid(s) and to investigate the mineralization processes occurring in the Pataz-Parcoy district. Contrasting with the classical two-stage ore sequence previously proposed for the Pataz-Parcoy district, we suggest a revised paragenetic sequence, i.e.,:

1) deposition of pyrite core (PyI) with homogeneously distributed base-metal sulfide inclusions,
2) progressive replacement of PyI by arsenian pyrite (PyII) and arsenopyrite associated with invisible gold deposition, and
3) deposition of sphalerite – galena ± chalcopyrite − electrum in fractured pyrite and arsenopyrite.

We propose two models for the formation of base metal sulfide inclusions in Py^I, i.e.:

1) coprecipitation of base metal sulfide with Py^I and later redistribution in cracks driven by partial As replacement of PyI to PyII and arsenopyrite and
2) preferential replacement of the Py^I along crystallographic planes by percolation of the fluid responsible for base metal sulfide deposition in fractured pyrite and arsenopyrite.

Batholith-hosted gold mineralization in the Pataz-Parcoy district exhibits spectacular sulfide-rich veins up to several meters wide and hundreds of meters long. Structural and geochemical features of the deposits are fairly uniform throughout the Pataz-Parcoy district with local mineralogical variations such as base metal sulfide (BMS; Schreiber et al. 1990; Haeberlin et al. 2004). However, the paragenetic sequence remains controversial. Previous workers have described a classical two-stage ore sequence composed of (1) early pyrite and arsenopyrite associated with coarse-grained euhedral quartz and (2) galena, sphalerite ± chalcopyrite, and native gold or electrum precipitated in pyrite fractures together with microgranular quartz (Schreiber et al. 1990; Haeberlin et al. 2004).

More recently, Witt et al. (2016) proposed that gold was introduced during deposition of an early, massive, fine-grained pyrite-arsenopyrite together with < 10 vol% clear, glassy quartz followed by the introduction of milky, white quartz-carbonate, and coarse-grained pyrite-arsenopyrite with up to 10 vol% galena and sphalerite. The discovery of a new, base metal-rich, gold-bearing vein system in the Santa Maria area, about 10 km south of the main mining center of Paraiso, illustrates the importance of understanding the relative timing of gold and base metal emplacement.

Interpretation of sulfide mineral textures and inclusion distribution is critical for assessing trace element and sulfur isotope data (Large et al. 2007; Cook et al. 2009; Large et al. 2009; Cook et al. 2013), especially to avoid over- or underestimation of elemental concentrations. Inferences based on equilibrium partitioning of trace elements in sulfides (e.g., element in the parent fluid, the redox, pH, temperature, and f_O2 of fluid) can only be applied to elements (e.g., As, Au, and Cu) that are known to substitute into the crystal lattice of a sulfide mineral (Chouinard et al. 2005; Tanner et al. 2016). Trace element concentrations in pyrite associated with gold mineralization have been commonly used to evaluate the fluid evolution (e.g., element concentration in the parent fluid, the redox, pH, temperature, and f_O2 of fluid) in parallel with the paragenetic sequence of mineral deposition (Deditius et al. 2009; Reich et al. 2013).

Sulfur isotope fractionation in pyrite is well established and has been used to discuss the fluid source(s), interaction with wall rocks as well as with previously deposited minerals, and fluid-mixing processes (Ohmoto and Rye 1979). Recent in situ coupled trace element and sulfur isotope analyses on sulfides in epigenetic, hydrothermal gold deposits (such as West Banshee and northern Carlin Trend Turquoise Ridge in northern Nevada) (Barker et al. 2009), Porgera in Papua New Guinea (Peterson and Mavrogenes 2014), and El Callao mining district in Venezuela (Velásquez et al. 2014) show clear covariation between δ³⁴S and trace elements values through the paragenetic sequence, possibly triggered by changes in oxidation state, pressure, temperature, and/or pH.

Arsenic-rich sulfides, such as arsenian pyrite and arsenopyrite, can carry gold in their crystal structure (Mumin et al.1994; Fleet and Mumin 1997; Palenik et al. 2004; Reich et al. 2005; Barker et al. 2009; Deditius et al. 2014; Fougerouse et al. 2016). Many different mineral systems are known to contain invisible gold in arsenopyrite and arsenian pyrite including Carlin-type Au, porphyry Cu, Cu–Au, epithermal Au, orogenic Au, volcanogenic massive sulfides (VMS), and iron oxide Cu–Au (IOCG) (Deditius et al. 2014). A maximum Au content of 0.64 wt% was obtained from synthetic arsenian pyrite, whereas gold values up to 3.0 wt% Au have been reported in synthetic arsenopyrite from the Carlin-type deposits in Nevada (Fleet and Mumin 1997). It has been established that gold can be present as structurally bound cations (Au¹⁺ or Au³⁺) or as nanoparticles of native metal (Au⁰) (Simon et al. 1999). In arsenian pyrite, invisible gold has been described to be spatially associated with local As enrichment, which highlights a possible substitution of anionic or cationic As (As¹⁻ or As³⁺) into the pyrite structure, which in turn enhances the incorporation of Au (Reich et al. 2005; Deditius et al. 2008).

In this study, trace element and sulfur isotope geochemistry of individual growth bands of pyrite, arsenopyrite, and BMS are used to track the evolution of the ore fluid(s) composition and the processes responsible for gold deposition. The authors also highlighted the presence and proportion of invisible gold which occurs together with pyrite and arsenopyrite.

Finally, we reexamined the paragenetic sequence in the Pataz-Parcoy district using in situ micro-analytical methods, including scanning electron microscope (SEM), electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS and NanoSIMS), and present a new, alternative model for sulfide and gold deposition, which contrasts with the two-stage ore sequence previously established in the Pataz-Parcoy district.