S, Pb, C AND O ISOTOPIC COMPOSITIONS AND ORE GENESIS OF THE STRATABOUND POLYMETALLIC SULFIDE DEPOSITS IN MIDDLE INNER MONGOL, CHINA
Li Zhaolong Xu Wendou Pang Wenzhong(Tianjin Geological Academy, Ministry of Metallurgical Industry)
The stratabound Cu-Pb-Zn polymetallic sulfide deposits occur in metamorphic rocks of greenschist facies of middle-upper Proterozoic Langshan Group in middle Inner Mon-gol. δ34S values of sulfides range from -3.1‰ to +37.3‰ and an apparent difference is noticed between vein sulfides and those in bedded rocks. For example, δ34S of bedded pyrite range from +10.6‰ to +20.0‰, whole those of vein pyrite vary from -3.1 ‰ to +14.1 ‰. δ34S of bedded pyrrhotite is in the range +7.9‰-+23.5‰ in comparison with +6.5‰-+17.1 ‰ for vein pyrrhotite. The wide scatter of δ34S and the enrichment of heavier sulfur indicate that sulfur may have been derived from H2S as a result of .bacterial reduction of sulfates in the sea water. Sulfur isotopic com-position also differs from deposit to deposit in this area because of the difference in environment in which they were formed. The mobilization of bedded sulfides in respon-se to regional metamorphism and magmatic intrusion led to the formation of vein sul-fides.δ180 and δ13C of ore bearing rocks and wall rocks are within the range typical of ordinary marine facies, with the exception of the lower values for ore-bearing marble at Huogeqi probably due to diopsidization and tremalitization of carbonate rocks.Composition of lead isotope is relatively stable and is characterized by lower radio-genetic lead. The age of basement rocks was calculated to be about 23.9 Ma and ore form-ing age is 7.8 Ma 207Pb/204Pb-206Pb/204Pb and 208Pb/204Pb-206Pb/204Pb plots indicate that Pb may probably be derived from the lower crust or upper mantle.It is believed that the deposits in this region are related to submarine volcanic ex-halation superimposed by latter regional metainorphism and miagmatic intrusion.