Abstract
Modes of occurrence of Au- and Ag-bearing phases and their relation with associated hypogene ore minerals were examined with the objective to elucidate Au-Ag distribution at the Esperanza porphyry deposit in the Eocene Centinela copper belt, using ore-microscope modal analysis, semi-quantitative analyses by automated mineralogy, electron probe microanalysis, and secondary ion mass spectrometer. The Esperanza hypogene mineralization is characterized by early-stage chalcopyrite-rich veinlets in the potassic alteration zone and later polymetallic stage with tennantite and galena in the chlorite-sericitic alteration zone. Only the early-stage chalcopyrite contains fine-grained electrum (Au68Ag32 - Au81Ag19) and hessite (Ag2Te), and thus yields positive correlations in Cu vs. Au and Cu vs. Ag grades that are clearly recognized in the hypogene sulfide zone. The early-stage chalcopyrite grains frequently exhibit polysynthetic twinning suggestive of inversion from intermediate solid solution. These features suggest that the fine-grained electrum and hessite are products exsolved in the cooling process with the intermediate solid solution to chalcopyrite inversion. In contrast, tennantite and galena of the later-stage mineralization contain no detectable Ag, and it is thus proposed that the early-stage inverted chalcopyrite is the principal storage of economically important precious metals.
Original language | English |
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Pages (from-to) | 80-90 |
Number of pages | 11 |
Journal | Resource Geology |
Volume | 71 |
Issue number | 1 |
DOIs | |
Publication status | Published - 2021 Jan |
Keywords
- Cu-Au correlation
- Esperanza porphyry deposit
- chalcopyrite
- electrum
- microanalyses
ASJC Scopus subject areas
- Geology
- Geochemistry and Petrology