ZINC AND LEAD MOBILITY DURING LP/HT METAMORPHISM: A POTENTIAL METAL SOURCE FOR PB-ZN DEPOSITS?

Through extensive study of sediment hosted Pb-Zn deposits, we have gained a general idea about metal sources and formation mechanisms for typical MVT and SEDEX deposit types. However, many Pb-Zn deposits do not follow the principles established for the above deposit type genesis. Compared to fluid properties and metal mobility involved in SEDEX and MVT deposit formation, there are few studies on Zn and Pb behavior during regional metamorphism, and of these, the results are rather contentious. To help to resolve this lack of knowledge, we systematically studied Zn and Pb behavior on a whole rock- and mineral-scale during prograde metamorphism using a set of well-characterized psammo-pelite samples from the Eastern Mount Lofty Ranges, South Australia. Metamorphic conditions range from ~350 ˚C to the onset of partial melting in the presence of excess aqueous fluid at ~ 650–700 ˚C (3 to 5 kbar). Stable isotope studies indicate extensive up-temperature fluid flow during metamorphism, which favored significant element mobility.

Our results show that in staurolite-absent rocks, biotite contains >85 % of the bulk rock Zn inventory, and therefore controls the Zn budget during metamorphism, additionally, biotite is a major Pb host in low-grade rocks. We observe that biotite in low-grade (<450 ˚C) samples have a wide range of Zn (e.g., 200–900 ppm) and Pb (e.g., 5–50 ppm) concentrations, which greatly contracts at temperatures ≥450 ˚C. Moreover, the Zn and Pb content of biotite decreases upon prograde metamorphism and reaches their lowest value in migmatitic rocks. This leads to an overall loss of ~75 % of the bulk rock Zn, and ~50 % loss of bulk rock Pb content during prograde metamorphism.

Our study suggests that for the formation of Pb-Zn deposits, pre-metamorphic enrichment of Pb and Zn is not a prerequisite. Moreover, the geochemical trends observed in this study might be very similar to those that lead to the production of metal-rich fluid end-members prior to fluid mixing and subsequent formation of Pb-Zn deposits in metamorphic systems that do not fit SEDEX/MVT deposit styles.