Genesis of epizonal Ag vein mineralization at San Bartolomé in central Ecuador; textural evidence, fluid inclusions, and stable isotope geochemistry
Epizonal Ag-Pb-Zn vein mineralization at San Bartolome, in the eastern Andes of Ecuador, is one of a series of mineral occurrences which makes up the Canar-Azuay Ag belt.The mineral assemblage in the veins crystallized in five stages: comb quartz selvages on vein margins; coarse, crystalline aggrega...
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| Published in: | Economic geology and the bulletin of the Society of Economic Geologists Vol. 92; no. 2; pp. 210 - 227 |
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| Main Authors: | , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
Economic Geology Publishing Company
01-04-1997
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| Subjects: | |
| Online Access: | Get full text |
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| Summary: | Epizonal Ag-Pb-Zn vein mineralization at San Bartolome, in the eastern Andes of Ecuador, is one of a series of mineral occurrences which makes up the Canar-Azuay Ag belt.The mineral assemblage in the veins crystallized in five stages: comb quartz selvages on vein margins; coarse, crystalline aggregates of sphalerite associated with subsidiary amounts of freibergite (avg 23.9% Ag) and galena, with minor stannite; spectacular interpenetrating networks of bladed pyrrhotite with interstitial galena, freibergite, arsenopyrite, sphalerite, and minor stannite; coarse galena and Ag-bearing sulfosalts dominated by plumose masses of owyheeite (Pb5Ag2Sb6S9), boulangerite, and jamesonite; and finally, a gangue assemblage of quartz, rhodochrosite, and dolomitic carbonates. Pyrrhotite is now almost completely replaced by hypogene marcasite and pyrite pseudomorphs. Sulfur isotope geothermometry indicates ore-forming temperatures of 219° to 354°C and δ34SCDT (Canyon Diablo troilite) values in the range -3 to +2 implies a magmatic origin for a dominantly reducing, sulfidic fluid. Relative δ34S values in the principal vein sulfides suggest that crystallization proceeded under conditions close to equilibrium. Gradual variation in δ34S values along individual bladed structures further suggests that steady crystal growth occurred within a system which was closed to sulfur replenishment. The δ13C and δ18O values in rhodochrosite are consistent with carbonate-depositing fluids also having a primarily magmatic source with a minor meteoric component. Microthermometry of fluid inclusions in quartz, carbonate, and sphalerite shows that three fluids have contributed to mineralization: a hypersaline fluid (30 wt % NaCl equiv), and an intermediate-salinity fluid (6-15 wt % NaCl equiv), both associated with inclusion homogenization temperatures (Th) in the range of 200° to 400°C; and a low-salinity fluid (<6 wt % NaCl equiv) associated with lower Th in the range of 100° to 300°C. On the basis of these observations, it is most likely that the San Bartolome veins formed from magmatohydrothermal fluids possibly linked to the intrusion of an S-type pluton, close to the palaeocontinental margin. Mineralizing fluids invaded a dilatant fracture system as a series of discrete pulses, where they were subsequently diluted by the later introduction of cooler, meteoric ground waters. |
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| ISSN: | 0361-0128 1554-0774 |
| DOI: | 10.2113/gsecongeo.92.2.210 |