Abstract
Porphyry Cu can contain significant concentrations of platinum-group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd). In this study, we provide a comprehensive in situ analysis of noble metals (PGE, Au, Ag) for (CuFe)-rich sulfides from the Elatsite, one of the world's PGE-richest porphyry Cu deposits. These data, acquired using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), indicate that Pd was concentrated in all the (CuFe)-rich sulfides at ppm-levels, with higher values in pyrite (~6 ppm) formed at the latest epithermal stage (i.e., quartz–galena–sphalerite assemblage) than in bornite and chalcopyrite (<5 ppm) from the hypogene quartz–magnetite–bornite–chalcopyrite ores. Likewise, Au is significantly more concentrated in pyrite (~5 ppm) than in the (CuFe)-rich sulfides (≤0.08 ppm). In contrast, Ag reaches hundreds of ppm in pyrite and bornite (~240 ppm) but is in much lesser amounts in chalcopyrite (<25 ppm). The inspection of the time-resolved spectra collected during LA-IPC-MS analyses indicates that noble metals are present in the sulfides in two forms: (1) structurally bound (i.e., solid solution) in the lattice of sulfides and, (2) as nano- to micron-sized inclusions (PdTe and Au). These observations are further confirmed by careful investigations of the PGE-rich (CuFe)-rich sulfides by combining high-spatial resolution of field emission scanning electron microscope (FESEM) and focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM). A typical Pd-bearing mineral includes the composition PdTe2 close to the ideal merenskyite but with a distinct crystallographic structure, whereas Au is mainly found as native element. Our detailed mineralogical study coupled with previous knowledge on noble-metal inclusions in the studied ores reveals that noble metal enrichment in the Elatsite porphyry ores was mainly precipitated from droplets of Au-Pd-Ag telluride melt (s) entrained in the high-temperature hydrothermal fluid. These telluride melts could separate at the time of fluid unmixing from the silicate magma or already be present in the latter either derived from deep-seated crustal or mantle sources. Significant enrichment in Pd and Au (the latter correlated with As) in low-temperature pyrite is interpreted as remobilization of these noble metals from pre-existing hypogene ores during the epithermal overprinting.
Highlights
Porphyry-epithermal mineral systems are defined as large volumes (10− > 100 km3) of hydrothermally altered rock centered on shallowintruded porphyry stocks with a typical stockwork and disseminated mineralization that may be accompanied by skarn, carbonate-replacement, sediment-hosted, and high- and intermediate-sulfidation epithermal base and noble metal mineralization (Sillitoe, 2010; Lee and Tang, 2020)
The results presented here indicate that Pd does not enter in solid solution with (CueFe)-rich sulfides, but it is preferentially concentrated into micro- to nano-sized particles of PdeTe, formed from immiscible telluride melts
Summarizing, we can conclude that our observations are consistent with previous mineralogical studies on Elatsite, confirming that noble metals such as Pd, Au, Ag were mostly incorporated during the earliest potassic alteration stage
Summary
Porphyry-epithermal mineral systems are defined as large volumes (10− > 100 km3) of hydrothermally altered rock centered on shallowintruded porphyry stocks (i.e., porphyry Cu deposit sensu stricto) with a typical stockwork and disseminated mineralization that may be accompanied by skarn, carbonate-replacement, sediment-hosted, and high- and intermediate-sulfidation epithermal base and noble metal mineralization (Sillitoe, 2010; Lee and Tang, 2020). These include PCDs of the subtypes CueAu (Mutschler et al, 1985; Tarkian and Stribrny, 1999; Economu-Eliopoulos and Eliopoulos, 2000; Thompson et al, 2001; Economou-Eliopoulos, 2005; Summerlin, 2014; Eliopoulos et al, 2014; Logan and Mihalynuk, 2014; McFall et al, 2018; David and Timms, 2018), Cu-Mo-Au (Tarkian and Stribrny, 1999; Core et al, 2006; Pašava et al, 2010; Wang et al, 2014) and CueMo (Cabri, 1981; Tarkian and Stribrny, 1999; Sotnikov et al, 2001; Berzina et al, 2007; Logan and Mihalynuk, 2014; Plotinskaya et al, 2018) These observations highlight that in addition to Cu, Au, Mo, Re or Ag, platinum-group elements (PGE) like Pt and Pd may be recoverable from some PCDs. The application of a range of techniques for mineral microanalysis —electron probe microanalysis (EPMA), scanning electron microscope (SEM), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), and high-resolution transmission electron microscopy (HRTEM) — have shown that noble metals can be found in PCDs forming specific minerals or micro/nano-particles. This improved understanding is used to provide insights on sources and mechanisms of enrichment of the valuable metals in porphyry-epithermal systems, highlighting the possible implications for other deposits of this style worldwide
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