Mechanisms for Pd-Au enrichment in porphyry-epithermal ores of the Elatsite deposit, Bulgaria
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Platinum-group elementsPorphyry copperTelluride meltsHydrothermal pyriteElatsite
J.M. González-Jiménez, et al. Mechanisms for Pd-Au enrichment in porphyry-epithermal ores of the Elatsite deposit, Bulgaria. Journal of Geochemical Exploration 220 (2021) 106664 [https://doi.org/10.1016/j.gexplo.2020.106664]
SponsorshipSpanish projects - "Ministerio de Ciencia, Innovacion y Universidades" RTI2018-099157-A-I00 CGL2015-65824-P CGL2016-81085-R; Spanish projects - "Ministerio de Economia y Competitividad" (MINECO) RTI2018-099157-A-I00 CGL2015-65824-P CGL2016-81085-R; Spanish Government RYC-2015-17596
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 (Cusingle bondFe)-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 (Cusingle bondFe)-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 (Cusingle bondFe)-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 (Pdsingle bondTe and Au). These observations are further confirmed by careful investigations of the PGE-rich (Cusingle bondFe)-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.