Lead and sulfur isotope studies of the Wulashan quartz-K feldspar and quartz vein gold deposit, southwestern Inner Mongolia, People's Republic of China

Abstract

The newly discovered Wulashan gold deposit in southwestern Inner Mongolia, People9s Republic of China, is hosted by Archean metamorphosed volcano-sedimentary sequences of the Wulashan Group and surrounded by a number of late Paleozoic granitoid dikes and a batholith. The gold mineralization occurs in quartz-K feldspar and quartz veins related to these 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 volcano-sedimentary rocks.Lead isotope data for amphibolite, one of the major host rocks of these gold-bearing veins, define a correlation line with a slope of 0.1641 + or - 0.0003 (2Sigma ) which corresponds to an age of 9.498 + or - 76 Ma (2Sigma ). This lead isotope line also overlaps with the late Paleozoic granite line of the Dahuabei granitoid batholith defined by five K feldspar samples. Lead isotope data of 13 galena and pyrite samples from the gold-bearing quartz-K feldspar and quartz veins plot in between data for the amphibolite and Dahuabei granitoid batholith samples, and constitute a mixing line. Plots of all these lead isotope data ( 206 Pb/ 204 Pb vs. 207 Pb/ 204 Pb and 206 Pb/ 204 Pb vs. 208 Pb/ 204 Pb) 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 radiogenic granitoid source.The results of lead isotope studies indicate that protoliths of the amphibolite and the Dahuabei granitoid batholith have identical or similar initial lead isotope compositions but that they may possess quite different mu values. The amphibolite was derived from mantle-related mafic igneous rocks erupted or intruded at 2498 Ma, whereas the Dahuabei granitoid batholith was probably derived by selective 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 multiple sources, including Archean amphibolite and the late Paleozoic granitoid batholith.