METALLOGENESIS OF THE WORLD’S LARGEST SILVER DEPOSIT:
CERRO RICO, BOLIVIA
Bolivian polymetallic vein (BPV) deposits are large, intrusive-related, sulfide-dominated polymetallic vein systems. Typically they span the mesothermal to epithermal environment. They are significant producers of silver, tin, zinc and gold.
Cerro Rico is a classic example, and is the world's largest silver deposit.
Since its (re)discovery in 1544 by the conquistadores it has produced at least 1 billion ounces of silver.
The town of Potosi which sits beneath the mountain had a population of approaching 200,000 in the 17th century during it’s heyday - bigger than London or Paris - when it was the financial engine of the Spanish Empire.
A cost of this enormous wealth was the terrible toll of slaves: an estimated one million (possibly more) lost their lives. Today it is mined by local miners’ cooperatives - Approved project IP/427/0994 with Dr. C. Rice Aberdeen University has investigated the metallogenesis of this truly giant ore deposit.
There are many BPV deposits, but one question was at the centre of this study -
WHAT MADE CERRO RICO SO BIG?
Cerro Rico dominates the town of Potosi, and its Ag production has never been matched.
When one compares Cerro Rico to other BPV deposits, three features stand out as being exceptional:
* The phenomenal silver resource. Cerro Rico has produced almost five times more silver than any other BPV deposit.
* Development of a thick acid-sulfate lithocap. The acid-sulfate lithocap at Cerro Rico is at least twice as thick as any other described from a BPV deposit.
* Development and preservation of a particularly deep zone of oxidation. This is almost four times as thick as any other developed at a BPV deposit.
The extraordinary richness of the Cerro Rico silver deposit is not due to weathering processes - as is the case with many of the Tertiary porphyry Cu deposits of northern Chile. Instead the enrichment of Ag must be related to volcanic-related processes. Recent studies of melt inclusions have shown that the magmas at Cerro Rico were highly fractionated and enriched in incompatible elements such as Ag and Sn.
However, fractionation alone is unlikely to account for the unusual abundance of silver at Cerro Rico.
Critically, the combination of accurate dating and the use of stable isotopes – both based upon excellent field and petrographic observations – allowed us to observe that the mineralizing episode was protracted, by comparison to other deposits related to processes in the upper parts of volcanoes.
Our studies suggest that it was this exceptional duration of magma-related hydrothermal activity that was perhaps the critical factor.
It is probable that this was sustained by multiple injections of fractionated Ag-enriched magma from a deep reservoir into a high level magma chamber underlying Cerro Rico - (Au & Ag treasure chests - well protected)
This work has now been accepted for publication in Economic Geology - and has also been the subject of an article in - the last issue of NERC’s public bulletin - Planet Earth.
DEVELOPMENTS IN UNDERSTANDING LASER-INDUCED S ISOTOPE FRACTIONATION -
Laser heating of sulfides in the presence of O2 - a technique pioneered by ICSF and SUERC - with part funding from NSS capital budget - is the most commonly used method of in situ sulfur isotope analysis. Previous work indicated that a small, but reproducible fractionation of 34S/32S exists between the product SO2 gas, and the mineral. The magnitude of this fractionation varies with bond strength, as reflected in Gibbs free energy of formation. The correction factors are known for common sulfides and anhydrite, but not hitherto for stibnite and the sulfosalt minerals, which are important constituents of many classes of ore deposits. In our paper with Dr. Thomas Wagner (University of Tübingen: Wagner et al, 2002 - see Publication List) the correlation of ΔGo298 with the laser correction factor for the sulfosalts fits well with the trend previously established for simple sulfides. There is an excellent correlation between the fractionation factor and mineral composition, a parameter that does result in bond strength variations (e.g. mole fraction of PbS in sulfosalts), allowing estimation of the correction factors for simple intermediate compositions. Finally, bond strength also varies with variation in inter-atomic distances, and we have therefore investigated the behaviour of stibnite, a strongly anisotropic mineral. Our results indicate that there is a significant variation in fractionation factor depending on crystal orientation. The fractionation factor along the prismatic b-axis, which displays the strongest chemical bonds (as monitored by the shortest bond lengths), is more negative (-1.7‰) than along the other crystallographic directions (-0.7‰ to -1.0‰), in full agreement with theoretical predictions. Reviewing the manuscript, Prof. Mike McKibben (UC Riverside) said of the paper “…..it develops a very useful analysis and discussion of the significant effects of bond strength, solid-solution composition, and crystallographic orientation on the mineral-gas isotopic fractionation that is observed during laser ablation and isotopic analysis.”
Output
I have a strong commitment to the public dissemination of data, particularly through publication in peer-reviewed journals. I thank the many colleagues and their students who have worked with me through ICSF over the years.
Scottish Universities - Research Centre - Rankine Avenue, Scottish Enterprise Technology Park East Kilbride G75 0QF, Scotland, UK
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