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Title: Supergene mineralisation in gold-rich Bolivian polymetallic vein deposits
Author: Darke, K. E.
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1996
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The Bolivian Au-rich polymetallic vein deposits (BPV) are associated with high level dacitic intrusives and volcanics, usually show pervasive phyllic alteration, and consist of massive sulfide veins. The deposits which have been studied, include the Kori Kollo gold mine (the largest gold project in Bolivia with sulfide ore reserves estimated at 65Mt.), the nearby Kiska prospect, and the prospects of La Riviera in the Western Cordillera and Escala in the southern Altiplano. Through the first detailed study of the geochemistry and mineralogy of the oxide zones, evidence of supergene processes within these deposits has been established. Visible and invisible supergene gold have been recognised, with a strong spatial association with jarosite, iron oxides and mixed-metal oxide phases. Oxide zone mineralogy has been used to establish the prevalence of extremely acidic, oxidizing conditions, thus favouring gold mobilisation as the short-lived halide and thiosulfate complexes, and leading to precipitation of gold via an adsorptive, colloidal or solid solution mechanism. Geochemical data has also shown halogens to be present in anomalous quantities in oxidized samples, thus supporting the hypothesis that gold was mobilised as halide complexes. The oxidation zones are rich in alunite (KAl3(SO4)2(OH)6) and jarosite (KFe3(SO4)2(OH)6), and stable isotope studies have been used to elucidate the origin of these sulfates. Sulfur isotope studies show that sulfides at all the deposits probably had a magmatic sulfur source. Complete analyses (δ34S, δ18O, δD) of alunite and jarosite indicate a supergene origin for these minerals at Kori Kollo, Kiska and Escala, but suggest that some jarosite at Kiska, and alunite at La Riviera, are hydrothermal in origin. The latter probably formed in a magmatic steam environment, suggesting the deposits to be expressions of near-surface hydrothermal systems. Oxygen and hydrogen isotope studies of the sulfate minerals indicate that the Miocene and Quaternary meteoric waters of the Altiplano were depleted in 18O and deuterium compared to the present day. K-Ar dating of sulfates show that supergene processes have occurred from at least 12Ma to the Quaternary on the Bolivian Altiplano. An accumulation of evidence suggests that the Altiplano has experienced a seasonally-dry, and semi-arid climate, at least since the Middle Miocene. Calculations reveal that the rate of weathering has been very slow in this environment.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available