LEAD AND SULFUR ISOTOPE STUDIES OF THE WULASHAN QUARTZ-K FELDSPAR ANDQUARTZ VEIN GOLD DEPOSIT, SOUTHWESTERN INNER-MONGOLIA, PEOPLES-REPUBLIC-OF-CHINA

The newly discovered Wulashan gold deposit in southwestern Inner Mongo lia, People's Republic of China, is hosted by Archean metamorphosed vo lcano-sedimentary sequences of the Wulashan Group and surrounded by a number of late Paleozoic granitoid dikes and a batholith. The gold min eralization occurs in quartz-K feldspar and quartz veins related to th ese intrusions. Sulfur isotope analyses of 21 sulfide mineral (galena and pyrite) samples from the gold-bearing quartz-K feldspar and quartz veins, hornblende-plagioclase gneiss, sillimanite-biotite gneiss, and granitoid intrusions in the Wulashan district suggest that the sulfur of the ore-bearing fluids was mainly derived from a mixing of sources from late Paleozoic granitoid intrusions and Archean metamorphosed vo lcano-sedimentary rocks. Lead isotope data for amphibolite, one of the major host rocks of these gold-bearing veins, define a correlation li ne with a slope of 0.1641 +/- 0.0003 (2sigma) which corresponds to an age of 2498 +/- 76 Ma (2sigma). This lead isotope line also overlaps w ith the late Paleozoic granite line of the Dahuabei granitoid batholit h defined by five K feldspar samples. Lead isotope data of 13 galena a nd pyrite samples from the gold-bearing quartz-K feldspar and quartz v eins plot in between data for the amphibolite and Dahuabei granitoid b atholith samples, and constitute a mixing line. Plots of all these lea d isotope data (Pb-206/Pb-204 vs. Pb-207/Pb-204 and Pb-206/Pb-204 vs. Pb-208/Pb-204) display a linear trend, with K feldspar of the Dahuabei granitoid batholith at the most radiogenic end and amphibolite of the Wulashan Group at the least radiogenic end. This trend is probably a result of the mixing of mantle-derived lead with lead from a radiogeni c granitoid source. The results of lead isotope studies indicate that protoliths of the amphibolite and the Dahuabei granitoid batholith hav e identical or similar initial lead isotope compositions but that they may possess quite different mu values. The amphibolite was derived fr om mantle-related mafic igneous rocks erupted or intruded at 2498 Ma, whereas the Dahuabei granitoid batholith was probably derived by selec tive remelting of previously formed volcano-sedimentary rocks during a late Paleozoic (Variscan) tectonic event. The lead and sulfur of the gold-bearing quartz-K feldspar and quartz veins were derived from mult iple sources, including Archean amphibolite and the late Paleozoic gra nitoid batholith.