ORIGIN OF HYPOGENE NON-SULFIDE ZINC DEPOSITS: EVIDENCE FROM THE VAZANTE (BRAZIL), FLINDERS RANGES (AUSTRALIA), AND STAR (ZAMBIA) DEPOSITS (Invited Presentation)

Hypogene non-sulfide zinc (HNSZ) deposits are an unusual type of zinc-rich sediment-hosted base metal mineralization characterized by the predominance of willemite over sphalerite. Like sphalerite-dominant Mississippi Valley-type, Irish-type, and sedimentary exhalative deposits, HNSZ deposits occur in sedimentary basin and platform geological settings and apparently precipitated from saline hydrothermal fluids. These similarities raise the question of what geochemical factors led to the distinctive mineralogy of HNSZ deposits. The present study represents an ongoing effort to characterize the composition of HNSZ deposits and their parent fluids in order to find the factors that governed their origin and that distinguished HNSZ from sulfide-dominant deposits.

Fluid inclusion and isotopic thermometry suggest a wide range of formation temperatures for HNSZ mineralization from <40° to >300° C, though the solubility of willemite is much less sensitive to temperature than the solubility of sphalerite. Fluid inclusion salinities also range widely, mainly from about 6 to 20 equivalent weight % NaCl, though willemite and sphalerite solubility are about equally sensitive to salinity. Like sphalerite-dominant sediment-hosted base metal deposits, HNSZ deposits appear to have formed from fluids with slightly potassium-enriched but otherwise typical sedimentary brine compositions. Lead isotope studies of the Vazante and Flinders Ranges HNSZ deposits indicate that they likely derived their Pb from U-Th-rich source rocks and are genetically related to nearby sulfide deposits. Willemite is very soluble in quartz-saturated fluids, meaning that Zn concentrations in HNSZ ore fluids should be high. However, the concentrations of Zn and other ore metals in HNSZ ore fluids have not yet been successfully quantified to test this hypothesis. If high ore fluid Zn concentrations are needed to precipitate willemite and because Zn is not very soluble in the presence of sulfide, which is relatively abundant in sedimentary environments, then this may explain the relative rarity of HNSZ deposits worldwide. Reaction path models suggest that HNSZ deposits most likely precipitated as a result of mixing between hot, acidic, reducing, Zn-rich brine with cooler, pH-neutral, oxidizing, dilute meteoric water.