Abstract

¹The Kori Kollo deposit is the largest gold deposit in Bolivia (150 metric tons Au; 3rd largest in South America). Hypogene mineralization is hosted in wallrock and a sheeted pyrite-polymetallic vein system developed in the apical portion of a porphyritic andesitic-dacitic intrusion emplaced within Silurian clastic sedimentary rocks.

Wallrock alteration consists of intense silicification (44-51 vol.% quartz), seritization (38-vol.% phengite), pyritization (up to 14 vol.% pyrite), (K,Na)-alunite+florencite [Fig. 1] (1-3 vol.%) and gold mineralization with local, minor kaolinite. A distinctive positive europium anomaly is characteristic of altered wallrock. The vein swarm strikes 20 to 30 degrees NE, dips 75-85 degrees NW and is exposed over an area of approximately 200m x 600m. Sulfide mineralization in the veins (1 mm to 15 cm thick) consists of Stage I (85-90 vol. %) arsenopyrite (05-10 vol %), pyrite (95-90 vol %) and electrum; Stage II (5 vol %) pyrite (0-10 vol %), jamesonite (5-10 vol %), chalcopyrite, galena, stannite, bournonite, bismuthinite-stibnite, sphalerite, low-Ag tetrahedrite (0-20 vol % each) and electrum; Stage III (5-10 vol %) sphalerite (10-20 vol %) with and minor quartz, marcasite and chalcopyrite and aluminum-phosphate-sulfate (APS) minerals (50-60 vol %) that replace earlier sulfides.

Electrum exhibits significant compositional differences between Stage I and II. In Stage I electrum contains 82-95 atomic % Au and occurs as 1 - 50 micron inclusions in pyrite. In Stage II electrum contains 55-79 atomic % Au and occurrs as inclusions in base-metal sulfides. Electrum within each paragenetic stage shows no significant compositional variation over a 200 m vertical extent. Arsenopyrite (28.5-33.5 atomic % As) geothermometry indicates that Stage I mineralization occurred between 300 and 400°C [Fig. 2]. Sphalerite geobarometry indicates pressures between 1 and 2 Kb. Geochemical modeling of electrum compositions indicates that gold precipitated in Stage II precipitated from higher Au/Ag solutions than Stage II gold.

Fluid inclusion studies show that magmatic fluids (700-300°C, 8-45 wt.% NaCl) are trapped in primary inclusions in quartz phenocrysts [Fig. 3]. Secondary inclusions record temperatures between 250 and 350°C (5-13 wt.% NaCl). Primary inclusions in rare, vein quartz yield T=250-350°C, 5-9 wt.% NaCl. Primary inclusions in sphalerite yield T=200-300°C, 7-15 wt.% NaCl with secondary inclusions yielding T=150-225°C, 7 wt.% NaCl. These data suggest minimum burial depths during mineralization between 0.4 and 0.8 km.

Field relations, mineralogy, initial fluid inclusion studies, and arsenopyrite geothermometry suggest that the Kori Kollo deposit may represent a transitional magmatic-epithermal precious metal system.

References

Fitzmayer, J.R. & Petersen, E.U., 1996, The Kori Kollo Au-Ag deposit: A transitional magmatic-epithermal precious metal system within the La Joya District, Bolivia. Geological Society of America Abstracts, 28, 7, A-334. Theme Session on Transition between porphyry copper and epithermal systems.

Fitzmayer, J.R. & Petersen, E.U., 1997, Electrum mineralization in Bolivian, polymetallic vein (BPV) deposits, Kori Kollo, Bolivia. Geological Society of America Abstracts, 29, 6, A-466.

Fitzmayer, J.R., 1998, Sulfide mineralization and wall rock alteration of the Kori Kollo Au-Ag Deposit, Bolivia. M.S. Thesis, The University of Utah X p.

Petersen, E.U. & Fitzmayer, J.R., 1998, alunite-sericite association: A new type of epithermal precious metal deposits? Geological Society of America Abstracts, 30, 6, A-4XX.

¹Petersen, E.U. & Fitzmayer, J.R., 2000, The Kori Kollo deposit, Bolivia: A porphyry-hosted gold deposit. Geoscience2000 Canada

Acknowledgements

This presentation on the geology and geochemistry of the Kori Kollo deposit is based on the MS work of Mr. Jeffrey R. Fitzmayer. I would like to thank Jeff for his generous support in making this presentation possible and Battle Mountain Gold for their generous support for our studies in the Kori Kollo District.