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).
Highlights
Porphyry-type deposits are the main source for global Cu and Mo production, while several strategic and high-tech elements such as Re, W, Bi, In, Te and Se may reach economic concentrationsMinerals 2020, 10, 946; doi:10.3390/min10110946 www.mdpi.com/journal/mineralsMinerals 2020, 10, 946 and be extracted as co-products
This study presents a petrographic investigation of precious metals and related minerals at the Assarel porphyry copper deposit, one of four actively mined test sites being used to validate drill core 3D X-ray computed tomography (XCT)-X-ray fluorescence (XRF) scanning technology within the X-MINE project [14]
Mineral assemblage types described in this paper represent petrographically characteristic groupings of two or more hydrothermal minerals formed under either equilibrium or disequilibrium conditions, with key constituent minerals listed in decreasing relative abundance (e.g. Gifkins et al [56])
Summary
Porphyry-type deposits are the main source for global Cu and Mo production, while several strategic and high-tech elements such as Re, W, Bi, In, Te and Se may reach economic concentrationsMinerals 2020, 10, 946; doi:10.3390/min10110946 www.mdpi.com/journal/mineralsMinerals 2020, 10, 946 and be extracted as co-products. The mineralogical assessment of porphyry Cu ± Au mineralization commonly relies on “conventional” methods of petrographic investigation including optical microscopy, scanning electron microscopy–energy dispersive spectrometry (SEM-EDS), X-ray diffraction analysis, electron microprobe analysis, and fluid inclusion studies [6,7,8]. These methods are typically time consuming, sample destructive, and yield results that may not immediately influence exploration/mining decision making. Recent advances in drill core scanning and imaging methods based on hyperspectral infrared spectroscopy [9], laser-induced fluorescence spectroscopy [10], laser-induced breakdown spectroscopy [11], laser Raman spectroscopy [12], and X-ray computed tomography [13] offer the potential for relatively rapid and non-destructive mineralogical characterization of ore and rock samples
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