NdSrPb isotopic constraints on metal and fluid s

Authors: Bo Peng Robert Frei

Publish Date: 2004/03/12

Volume: 39, Issue: 3, Pages: 313-327

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Abstract

This paper presents NdSrPb isotope data on scheelite inclusion fluids and residues of gangue quartz and sulfides from the WSbAu ore deposits at Woxi and Liaojiaping LJP in the Xuefeng Uplift Belt XUB Western Hunan China Sm and Nd concentrations in scheelite from Woxi are much lower than in scheelite from LJP and can be distinguished by their high 147Sm/144Nd ratios of ~125 from the much lower ratios around 026 in scheelites from the LJP εNd values compared to values at 200 Ma which is the average timing of granitoid emplacement during the IndosinianYanshanian periods in the XUB are around −10 for the LJP and compare well with the range of −5 to −11 defined by the granitoids whereas they are around −27 for scheelite from Woxi This might indicate that REEs in the mineralizing fluids at LJP originated from granitoids that are concentrated along the southern border of the XUB whereas in the case of Woxi the original fluids might have been masked by REEs released during intense hightemperature wall rock alteration of unexposed Precambrian basement rocks at depth Sr isotopes of scheelite from these two deposits show similar relations to host / nearby rocks in that 87Sr/86Sr T=200 Ma ratios of ~0721 for LJP scheelite agree with values ranging between 0718 – 0726 for granitoids whereas these ratios are much higher ie 0745 for scheelite from Woxi and correspond to the 87Sr/86Sr T=200 Ma ratio range of 0743 – 0749 for Precambrian host slates Crushing experiments to release inclusion fluids from gangue quartz and sulfides deposited during later stages of ore deposition in both deposits failed to provide accurate and geologically meaningful twopoint fluidresidue tie lines in RbSr isochron diagrams However Sr released from fluid inclusions generally reveals lower initial 87Sr/86Sr ratios than the respective residues and shows affinities to 87Sr/86Sr T=200 Ma values of IndosinianYanshanian granitoids both at Woxi and LJP Pb stepwise leaching of scheelite and sulfides did not result in sufficient spreads in Pb isotope diagrams and therefore no information regarding exact mineralization ages in the two deposits could be deduced Overall ore Pb isotopes reveal upper crustal signatures and are compatible with Pb isotope signatures of the predominant Precambrian slates in the Woxi area Steep trajectories through late stage quartzsulfide mineralization in Pb isotope diagrams may hint at mixing scenarios involving Pb from the host rocks and a component with lower 207Pb/204Pb and 208Pb/204Pb ratios relative to 206Pb/204Pb ratios which cannot be linked to any known reservoir in the XUB mining district Sr isotopes of four out of seven residual sulfide samples from Woxi plot along a paleomixing line at an age of 199 ± 8 Ma supporting a mixing scenario for the fluids indicated by the Pb isotopes and pointing to a possible genetic relationship with the emplacement of Indosinian—Yanshanian granitoids The budgets of REEs RbSr and Pb in the original fluids were severely affected by contamination of these elements apparently during intense wall rock alteration but after “sealing” of the major pathways the mineralizing fluids tend to have better preserved their original signatures These attest a genetic relationship between the metallogeny in the XUB WSbAu province and the emplacement of IndosinianYanshanian granitoids during Mesozoic intracontinental tectonic uplift and thrustingThis study was made possible by funding from the Natural Science Foundation Committee of China grant nr 40002021 The first author acknowledges the Danish government for granting a Cirius fellowship to study at the Geological Institute University of Copenhagen We are grateful to Birthe Møller and Maria Jankowski for assistance and help during the lab work to John Bailey for an unofficial review and help with XRF analytics and to Peter Venslev for help in mineral separation procedures Toby Leeper is thanked for help with mass spectrometrical analyses Britta Munch significantly improved the graphic design of some of the figures in this manuscript Mr Chuanchu Pan Yongjun Xiao Dingliang Guo and Zhaoliang Song are thanked for joining the fieldwork We are grateful to Mr Mingbo Lei Binfu Tan Pingsheng Chen and Shengyou Liu at the Woxi Gold Mine for access permission and guidance during the field campaign We are grateful for official and very constructive reviews by Dongsheng Ma and Jens Schneider and we also thank the editor Bernd Lehman for insightful comments and suggestions

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