In situ Pb-Pb isotopic dating of sulfides from hydrothermal deposits: a case study of the Lala Fe-Cu deposit, SW China

Abstract

Precisely dating different generations of mineralization is important for understanding the origin and tectonic environment of ore formation. In this study, we have dated two generations of sulfides from the Lala Fe-Cu deposit, SW China, using in situ Pb-Pb isotopic data obtained by the LA-MC-ICPMS technique. The Lala deposit is composed of Fe-Cu ore bodies hosted in sedimentary-volcanic rocks of the Paleoproterozoic Hekou Group. It has a paragenetic sequence of pre-ore banded pyrite (Stage I) and Na alteration (Stage II), followed by early Fe (Stage III) and late Cu (Stage IV) mineralization. The sulfides in the deposit have dominantly formed in two stages, including pyrite from stratiform pyrite bands in the host rocks (i.e., Stage I) and epigenetic pyrite-chalcopyrite in Fe-Cu ore (i.e., Stage IV). The sulfides of both stages have highly radiogenic Pb isotopic ratios which form linear trends in the 207Pb/204Pb versus 206Pb/204Pb diagram. The regression trend for Stage I pyrite grains defines an errorchron age of 1669 ± 110 Ma (MSWD = 32), undistinguishable from zircon LA-ICPMS U-Pb ages of volcanic rocks in the Hekou Group (i.e., ~ 1700 Ma). The regressions for Stage IV pyrite and chalcopyrite yield younger errorchron ages of 1088 ± 88 Ma (MSWD = 93) and 1053 ± 220 Ma (MSWD = 15), respectively. Taking pyrite and chalcopyrite together, an errorchron age of 1053 ± 75 Ma (MSWD = 108) is obtained. These errorchron ages agree well with the molybdenite Re-Os age of the Fe-Cu ore (1086 ± 8 Ma). Despite the large error and MSWD values that are interpreted to be related to the small variation of initial ratios, or post-ore open system behavior, the good agreement with the precise zircon U-Pb and molybdenite Re-Os ages indicates that the Pb-Pb errorchron ages for Stage I and IV sulfides are geologically meaningful. Therefore, this study shows that the in situ Pb isotopic analyses can yield geologically meaningful Pb-Pb ages for radiogenic Pb-rich sulfide minerals. We also propose that in situ Pb-Pb isotopic dating may be more promising for Precambrian deposits containing sulfides with ingrown Pb, particularly for those involving multiple mineralization/hydrothermal events but without relevant minerals for precise Re-Os or U-Pb dating.