Determining the Source of Metals and Timing of Mineralization through Osmium Isotopes

The University of Arizona has a long tradition of studying base metal porphyry deposits through the work of John Guilbert, Spence Titley, and their students. We continue that tradition by applying new methodologies to the problem of why large accumulations of metals occur in the earth's crust. We are using a new isotopic system to date and to determine metal sources of base metal porphyry deposits. Base metal porphyry deposits are interesting because they represent major accumulations of chalcophile elements within the earth's crust, and are set within large hydrothermal systems where prolonged interaction between fluids derived from magmas and country rocks have concentrated these metals well above average crustal values. Mineralization is spatially and temporally associated with subduction zone magmatism, and the magmas are believed to be a product of mixing of upper mantle and crustal material. Determining the proportion of mantle and crustal components for the mineralization will constrain some of the processes that are ultimately responsible for the ore formation, which is of academic and economic importance.

Arizona and chile contain world class base metal porphyry deposits which have been studied extensively for decades. Previous studies, involving whole rock, mineral, fluid inclusion, and stable isotope analysis, have not been able to uniquely determine the source of metals. Some argue that the metals must originate from mantle-derived magmas, whereas others believe crustal geochemical provinces control metallogenesis. The most significant correlation between mineralization and metal source was recognized by Titlye, who demonstrated that the Ag/Au ratios of ore deposits in southern Arizona and New Mexico correspond to two Proterozoic crustal domains that differ in age and lithology. This correlation suggests that the crust influences metal ratios in ore deposits regardless of deposit age or style of mineralization. The Pb isotope ratios of some of these deposits also suggest that the crust is influencing metallogeny, but neither study can determine the influence of the mantle on metallogenesis, which must be present as the deposits are linked to subduction processes beneath continental crust and mantle.

Rhenium-Osmium Isotopes Applied to Base Metal Porphyries

The Re-Os isotope system is uniquely suitable for studying mineralization because Re and Os are strongly chalcophile, and are higher in concentrations in sulfides relative to silicate rocks. If one assumens that Os is a good proxy for the sulfide ore-forming metals, Os becomes a significant tracer for the source of the metal of interest, i.e., Cu, Zn, etc. Os is also compatible relative to Re in partial melting of the mantle, leading to high Re/Os ratios in the products of mantle melting that in turn evolve to greatly elevated 187Os/188Os ratios over time due to the decay of 187Re to 187Os. Thus, Re-Os isotopes are a potentially powerful tool for documenting interactions between the crust and mantle in sulfide mineralization.

Problems that have hindered the full application of Re-Os isotopes, such as difficulty in concentration and separating the elemnts from samples with low (ppb-ppt) concentrations, and a lack of instrumentation with detection levels in this range, have been resolved in our Re-Os lab with the development of new techniques. We have obtained Re-Os ages for molybdenite from Laramide base metal porphyry deposits in Arizona that form two groups, distinguished by the age of the crustal domain in which they occur (see figure), suggesting that the crust has influenced mineralization.

In determining metal source, the mantle contains more Os than continental crust, but continental crust can contribute significant radiogenic Os by virtue of being older and having higher Re/Os ratios, which is expressed in base metal sulfides with high initial 187Os/188Os ratios. Our preliminary data from Magma, Arizona and El Teniente, Andacollo and El soldado, chile, indicates a large crustal component for the mineralization (see figure). Interestingly, the 187Os/188Os ratios of Chilean porphyry base metal deposits, regardless of age, are less radiogenic than the 187Os/188Os ratio of Magma, which is situated in older, probably more radiogenic lower crust. At El Soldado, Chile, a ~100 Ma Cu-Au manto deposit, sulfides are much more radiogenic than in the Chilean base metal porphyry deposits. this indicates a different source for the Os, with a plausible source being metalliferous black shales. These preliminary data suggest a significant role for the Re-Os isotopic system in the further study and exploration of copper deposits worldwide.

by Joaquin Ruiz, Fall 1996.

Go Back to the Ruiz Group Homepage

Return to U.of A. Geosciences Home Page