The Kori Kollo gold deposit is hosted by a c.15 Ma sericitized dacite stock in the La Joya district on the Bolivian Altiplano. The mineralization is of the Bolivian polymetallic vein (BPV) type, with a significant amount of the gold reserves contained in the oxide cap.
Field and microscopic studies, K-Ar dating, delta(34)S values of pyrite, alunite and jarosite, and delta(18)O values of alunite and jarosite demonstrate that some of this gold is supergene. Two types of supergene gold are present: minute gold grains, 1 to 10 microns across, of high fineness and associated with jarosite and iron oxy-hydroxides; and 'invisible' gold, which is probably sorbed onto minerals such as jarosite and iron oxy-hydroxides.
Pyrite delta(34)S values fall in the limited range -3.5 +/- 0.6 parts per thousand, consistent with a magmatic origin. Sulfate delta(34)S values are homogeneous and overlap closely with those of pyrite (-1.2 +/- 2.l parts per thousand), which supports petrographic evidence that sulfates replaced pyrite. By contrast, sulfate delta(18)O values are inhomogeneous with a large range (alunite +2 to 26 parts per thousand, jarosite -5 to +18 parts per thousand). These data are interpreted as reflecting kinetically controlled reactions involving various proportions of atmospheric oxygen and local meteoric water. This is typical of supergene alteration and is due to the seasonally dry climate of the Altiplano.
The oxide-zone mineralogy is dominated by sulfates and iron oxy-hydroxide minerals with some phosphates and halides, which indicates that the groundwaters were acidic, oxidizing and relatively saline. Under these conditions the most likely complexes responsible for gold mobilization are halides.
Geological data and K-Ar ages indicate that oxidation of the BPV deposits began at least around 11 Ma and possibly up to 15 Ma ago, and that supergene alteration lasted until around 4 Ma. During this period, erosion was minimal. Although supergene processes have been active on the Bolivian Altiplano since the Middle Miocene, in contrast to Northern Chile, enrichment of gold has not occurred. (C) 1999 Canadian Institute of Mining, Metallurgy and Petroleum. Published by Elsevier Science Ltd. All rights reserved.
|Number of pages||13|
|Journal||Exploration And Mining Geology|
|Publication status||Published - 1997|
- LATERITIC WEATHERING PROFILES
- STABLE ISOTOPE GEOCHEMISTRY
- ATACAMA DESERT
- NORTHERN CHILE