Mineralogical Setting of Precious Metals at the Assarel Porphyry Copper-Gold Deposit, Bulgaria, as Supporting Information for the Development of New Drill Core 3D XCT-XRF Scanning Technology

Abstract

A petrographic investigation of ore samples from the Assarel porphyry copper deposit in the Srednogorie metallogenic zone (Bulgaria) constrains the setting and character of precious metals (Au, Ag, PGE) and related minerals within the deposit. This work supports renewed interest in understanding the deportment of precious metals and provides mineralogical knowledge during the testing and validation of novel drill core 3D X-ray computed tomography–X-ray fluorescence (XCT-XRF) scanning technology being developed as part of the X-MINE project. Scanning electron microscopy–energy dispersive spectrometry (SEM-EDS) results indicate precious metals occur in their native state (Au, Ag), as sulfides (Ag), sulfosalts (Au), tellurides (Ag, Pd), and selenides (Ag), and typically form micron-sized inclusions in pyrite and chalcopyrite or are disseminated in the groundmass of the rock. Preservation of early Fe oxide–chalcopyrite ± bornite assemblage as relics in the more dominant pyrite-chalcopyrite mineralization assemblage supports mineral disequilibrium relationships and multi-stage mineralization events. Several rare minerals (e.g., merenskyite, acanthite, sorosite, tetra-auricupride, auricupride, greenokite, bismuthinite, nagyagite, native Ni) are reported for the first time at Assarel and highlight the mineralogical diversity of the ore. The occurrence of precious metals and related minerals at Assarel attest to a complex hydrothermal system that underwent progressive physicochemical changes during the evolution of the mineralizing system (e.g., redox conditions, fluid chemistry).

Precious and Critical Metal-Bearing Assemblages in the Ann Mason Copper Porphyry Deposit, Yerington, Nevada

“…Previous research has described the association of Cu-sulfide bearing assemblages with potassic alteration . Our petrographic results support the association of the chalcopyrite-bornite assemblage with potassic alteration, and find that the majority of precious metal microparticles are also hosted by bornite within this assemblage, as also seen at Elatsite and Assarel deposits in Bulgaria (Cioacă et al, 2020).…”

“…More rarely, gold is hosted in pyrite (e.g. Assarel deposit; Cioacă et al, 2020) or by silicate minerals as 'free gold'. In some deposits, including the Red Chris, Granisle and Bell deposits, Au and Ag are predominantly present as electrum (>10% Ag), mostly hosted on the edge of bornite crystals (Cuddy and Kesler, 1982;Rees et al, 2015).…”

“…Other minor forms of gold, silver and bismuth, such as tellurides and selenides, are reported at deposits such as Assarel, Rio Blanco, Santo Tomas II and Bingham (Ballantyne et al, 1998;Cioacă et al, 2020;Crespo et al, 2018;Tarkian and Koopmann, 1995). Deposits such as Santo Tomas II show relatively high concentrations of PGE, primarily held as PGM including merenskyite, kotulskite and moncheite (Tarkian and Koopmann, 1995).…”

Assessing the potential of Assarel porphyry copper deposit for critical raw materials: mineral-geochemical data for combination with agile exploration methods and better geo-modeling

“…Recent studies by Cioacă et al (2020) indicate the occurrence of Au-, Ag-, Bi-, Te-, and Se-bearing minerals as minute inclusions in pyrite and chalcopyrite from the main chalcopyrite-pyrite ore assemblages and in the upper epithermal and supergene alteration zones. Platinum-group elements are scarce and reported as rare Pd-bearing tellurides occurring in pyrite.…”

Combined X-Ray Computed Tomography and X-Ray Fluorescence Drill Core Scanning for 3-D Rock and Ore Characterization: Implications for the Lovisa Stratiform Zn-Pb Deposit and Its Structural Setting, Bergslagen, Sweden

“…In addition to the above challenges, XCT-XRF scanning adds another step to the core characterization procedure of an exploration or mining operation (see also Cioaca ˘ et al, 2020). However, scanning (1,500 m at 3 m/h) and subsequent digital logging (~25 m/h) of a moderate amount of core at Lovisa has provided a significantly enhanced level of structural and mineralogical-geochemical detail not previously obtainable via conventional methods, thereby allowing for refinement of 3-D geologic models at multiple scales.…”