Jeffrey R. Fitzmayer

Abstract

The Kori Kollo Au-Ag deposit, located within the La Joya mining district of the Central Bolivian Altiplano, is hosted within an intensely sericitized, Miocene dacite intrusion emplaced within shales of the Silurian Catavi Formation.

Wall-rock alteration consists of a phyllic assemblage of phengitic sericite (~38 vol. %) + pyrite (up to 14 vol. %) + quartz (44-51 vol. %) accompanied by minor (1-3 vol. %) hypogene alunite and APS minerals preferentially replacing apatite.   Kaolinite occurs locally as a secondary overprint of earlier phyllic alteration.   A distinctive, positive europium anomaly is characteristic of the altered wall-rocks.

High grade Au-Ag mineralization is contained within northeast-trending, 1- to 15-cm wide, steeply dipping polymetallic sulfide veins.   These veins, characterized by an abundance of polymetallic sulfide minerals and a lack of quartz and carbonate gangue, infill fractures in a 200m wide x 600m long zone within the dacite.   Vein mineralization consists of stage I (60-80 vol. %) pyrite, arsenopyrite, high-fineness electrum and minor Bi, bismuthinite and jamesonite,   stage II (10-20 vol. %) base-metal sulfides (marcasite, chalcopyrite, galena, Fe-rich sphalerite) and sulfosalts (jamesonite, tetrahedrite, bournonite, stannite) with minor bismuthinite and moderate-fineness electrum, and stage III (10-20 vol. %) alunite-family and APS minerals accompanied by late sphalerite (low-Fe), chalcopyrite, marcasite and minor covellite.   Electrum grains (1- 200 microns) occur as encapsulations within stage I pyrite and arsenopyrite and to a lesser extent within stage II base metal sulfides and sulfosalts.   Lower grade Au-Ag mineralization is disseminated within the wall-rock as sub-microscopic inclusions in pyrite.

Ore fluid temperatures (350-250^oC) and salinities (5-11 wt. % NaCl eq.), derived from arsenopyrite geothermometry and fluid inclusions in sphalerite and rare vein quartz, are higher than those typically observed in epithermal systems.   In addition, the deposit shares several characteristics with both low- and high-sulfidation precious metal systems but does not fit neatly in either category.   Geothermometric data indicate a minimum depth of formation of 0.8 km, though the deposit may have formed as deep as 2 km. The sheeted dacite intrusion hosting the ore body emanates from a much larger intrusive body at depth and porphyry style fracturing and hydrothermal fluid release from this deeper body is believed to have played an important role in the deposit's formation. It is anticipated that more deposits belonging to this type will be recognized in the region as detailed deposit descriptions become available.

Jeff logging core at Kori Kollo

Complex paragenesis

3-D rendition of Kori Kollo system

BSE and X-ray element images of REE-APS in alunite.

Kori Kollo geological staff.

Jeff explains the intracacies of logging core to undergraduate SEG Student Chapter members at Chuquicamata, Chile (SEG Student Chapter Field Trip, Chile '97)

Fitzmayer, J.R. and Petersen, E.U., 2006, Geology and Genesis of the Kori Kollo Gold-Silver Deposit, Bolivia, Mineralium Deposita, (in preparation)

Petersen, E.U. and Fitzmayer, J.R., 2006, REE-APS minerals in Sericitic Alteration at the Kori Kollo, Au-Ag Deposit, Bolivia, Canadian Mineralogist, (in preparation)

Fitzmayer,J.R. 1999, Sulfide Mineralogy, Wall-rock Alteration and Geologic Model of the Kori Kollo Gold-Silver Deposit, Bolivia, Unpbul. M.S. Thesis, University of Utah, 237 p.

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

Fitzmayer,J.R. and 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. and 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, Denver, 28, 7, A-334. Theme Session on Transition between porphyry copper and epithermal systems.