LEAD ISOTOPE ANALYSIS OF NON-SULFIDE ZINC AND SULFIDE ZINC-LEAD DEPOSITS IN THE FLINDERS RANGES, SOUTH AUSTRALIA

 

Hypogene non-sulfide zinc (NSZ) deposits represent a mode of zinc enrichment in the Earth's crust that until recently has largely been overlooked relative to supergene NSZ and sulfide Zn-Pb deposits.  NSZ and sulfide Zn-Pb mineralization commonly occur in close proximity to one another in sedimentary basins, in some cases occurring even within the same deposit, suggesting the possibility of a shared diagenetic origin. The Flinders Ranges of South Australia are an excellent setting in which to investigate the hypothesis of a genetic relationship between NSZ and sulfide Zn-Pb deposits because both deposit types are well represented there. In the present study, Pb isotope compositions of Zn and Pb ore minerals from NSZ and sulfide Zn-Pb deposits from throughout the Flinders Ranges were analyzed. All of the Pb isotope compositions measured were anomalous, ranging from 18.324-21.125 for ²⁰⁶Pb/²⁰⁴Pb, from 15.731-16.020 for ²⁰⁷Pb/²⁰⁴Pb, and from 38.285-40.985 for ²⁰⁸Pb/²⁰⁴Pb.  The data form a linear trend in both ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb space, most likely indicating mixing of a more radiogenic and a less radiogenic source.  Assuming a two stage model of evolution, the less radiogenic end member would have been derived from a source no younger than ~940 Ma.  The more radiogenic end member could have been removed from its source at either 977 Ma (instantaneous model), or was derived from a 1416 Ma source in which it evolved until sequestration in the Zn-Pb deposits around 435 Ma (continuous model).  The 940-977 Ma ages correspond with the time of a rifting event that initiated the formation of the Adelaide Geosyncline, the basin that hosts the deposits.  The 1416 Ma age corresponds with the age of much of the younger igneous rock in the Gawler Craton, which flanks the Adelaide Geosyncline on the west, and may comprise part of the basement of the Adelaide Geosyncline.  The Pb isotope data also show a strong regional pattern, with the most radiogenic compositions generally occurring in the largest deposits in the northwestern Flinders Ranges, implying a likely regional control on the mixing proportions of the two sources.