ORIGIN OF THE TERTIARY SANDSTONE-HOSTED JINDING ZN-PB-AG DEPOSIT, YUNNAN, CHINA

Sedimentary sequences deposited in nonmarine environments generally have not been considered favorable for hosting large sediment-hosted Zn-Pb deposits. However, the Jinding deposit occurs in a Paleocene alluvial fan sequence within the northern Lanping-Simao Basin in southwestern China that formed in the collision zone between the Indian plate and the Yangtze platform. The Jinding mineralized zone is reported to contain a resource of more than 200 million t at 7% Zn+Pb and 5.8 g/t Ag, with 60% of the mineralization in sandstones and the remainder in carbonate breccia and carbonate fragment-bearing sandstones. Jinding is unusual because it occurs within coarse siliciclastic strata deposited along the faulted margin of a lacustrine basin. Jinding is hosted by sandstones, but those sandstones are distinctly different in depositional environment and petrologic character from those that host “sandstone-type” Pb deposits. These SST Pb deposits are hosted in basal quartzarenites that formed following episodes of prolonged tectonic stability, whereas the Jinding host sediments are deposited in a tectonically active basin. Also, Jinding is Zn-rich in contrast to the typical SST Pb deposits. Jinding mineralization is associated with active faulting as commonly is the case for SEDEX deposits and may be related to orogenic uplift as commonly proposed to initiate MVT mineralization. Thus, Jinding may represent a new or hybrid type of sediment-hosted Zn-Pb deposit with significant economic potential. Existing geologic, fluid inclusion, organic geochemistry, and stable and radiogenic isotope evidence support a genetic model involving focused discharge of a metal- and petroleum-bearing brine sourced from a Mesozoic marine sequence at depth. Mixing of these heated basinal brines with shallow groundwater containing reduced sulfur from bacterial reduction of sulfate resulted in sulfide precipitation, dominantly by the replacement of calcite cement and limestone fragments in the alluvial fan.