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Changqing Zhang, Chinese Academy Geological Sciences (China)
Jingwen Mao, Chinese Academy Geological Sciences (China)
Jinjie Yu, Chinese Academy Geological Sciences (China)
Houmin Li, Chinese Academy Geological Sciences (China)
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Geology
The stratiform Chipu Zn-Pb deposit is located on the southwestern margin of Yangtze craton, China. It is one of the most important deposits in this area. All of the orebodies are hosted in dolomite and comprise sphalerite, galena, pyrite, dolomite, quartz, and minor calcite, limestone. The host ore and gangue minerals and with their stable isotopic compositions provide a unique opportunity to investigate the nature of the ore-forming brines.
Results
The sulfer isotopes are measured on sphalerite and galena, and enriched in heavy sulfur. The ranges of δ34S for the samples of shpalerite and galena are from 8.6 to 13.2 per mil and 11.9 to 15.1 per mil with an average of 10.71 per mil and 13.23 per mil, respectively. The oxygen and carbon isotopic compositions of bitumen bearing dolomite are found within and vicinity of orebody. The δ18OV-SMOW and δ13C of bitumen bearing dolomite range from 18.3 to 26.1 per mil and -12.2 to -4.6 per mil, respectively. The 18OV-SMOW of quartz range from 18.2 to 19.5 per mil.
Discussion and conclusion
The δ13C values of bitumen bearing dolomite studied are in the range of unnormal dolomite marine carbonate. They have the lower value than dolomite marine carbonate (averge about 0 per mil) and magmatic rocks (averge about -5 to -8 per mil), which may be effected by the reduction from organic matter. However, theδ13C values are higher than those all from the organic matter (less than -20 per mil). It may be the result of mixing between metal-bearing fluid with the sulfur bearing fluid reducing by organic matter. The δ18O values in quratz and bitumen bearing dolomite have the similar ranges. This suggest that the oxygen isotope in quartz and dolomite had already reached isotopic equilibrium. The characteristic of enriched in heavy oxygen indicate that they should not be mantle-derived oxygen with theδ18OV-SMOW values of less than 10 per mil.
The characteristic of enriched in heavy sulfur is obviously different from mantle-derived sulfur with δ34S values of about 0 per mil. However it is similar to those of about 17‰ for the host rock of Dengying Fmantion, Sinian. Reduced sulfur can be derived from the thermochemical reduction of sulfate by organic matter, which yields sulfur isotope fractionations about 0 to 15 per mil of the sulfate source. In addition, there are some bitumen within or vicinity the orebody. So we propose that this deposit is interpreted to have involved reduced sulfur derived by thermochemical reduction of sulfate.
All the above indications, δ18O, δ13C and δ34S isotopic values, ore-forming fluids of the Chipu deposit is mainly the product of the thermochemical reduction of sulfates from the carbonate strata of different ages (marine sulfates) in the orefield. The sulfide precipitation was maybe related to the fluid reduced by organic matter. Organic matter maybe plays a key work on researching the genesis of this deposit. So research on it is a future issue.
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