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