Fj. Nie, LEAD AND SULFUR ISOTOPE STUDIES OF THE WULASHAN QUARTZ-K FELDSPAR ANDQUARTZ VEIN GOLD DEPOSIT, SOUTHWESTERN INNER-MONGOLIA, PEOPLES-REPUBLIC-OF-CHINA, Economic geology and the bulletin of the Society of Economic Geologists, 89(6), 1994, pp. 1289-1305
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.