LEAD ISOTOPE AND FLUID INCLUSION MICROTHERMOMETRY CONSTRAINTS ON THE ORIGIN OF NON-SULFIDE ZINC AND SULFIDE ZINC-LEAD DEPOSITS IN THE FLINDERS RANGES OF SOUTH AUSTRALIA

Hypogene non-sulfide zinc (NSZ) deposits represent a distinct mode of zinc enrichment in the crust.  In contrast to other better known types of sediment-hosted zinc deposits, hypogene NSZ mineralization is dominated by willemite.  However, NSZ deposits also share important features with some of these other types of deposits, such as occurrence in carbonate host rocks and precipitation from saline hydrothermal fluids.  NSZ and sulfide Zn-Pb mineralization commonly occur in close proximity to one another, in some cases even within the same deposit.  The present study is seeking to examine the genetic relationship between NSZ and sulfide Zn-Pb deposits in the Flinders Ranges of South Australia, and to characterize more fully the nature of the mineralizing fluids.

Lead isotopes measurements have been made from numerous NSZ and sulfide Zn-Pb deposits in the Flinders Ranges.  All of the compositions measured were anomalous and form linear trends in both ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb space, with data ranging from 18.332-21.125 for ²⁰⁶Pb/²⁰⁴Pb, from 15.729-16.022 for ²⁰⁷Pb/²⁰⁴Pb, and from 38.264-40.967 for ²⁰⁸Pb/²⁰⁴Pb.  The linearity of the data suggests the deposits are genetically related and most likely contain varying mixtures of the same end member components.  The origin of the less radiogenic Pb component could not be determined from the data, but the more radiogenic Pb component could have been derived from rocks either about 940-978 Ma or 1428-1502 Ma.  The younger set of ages corresponds to a period of tectonic quiescence but the older set of ages corresponds to a prominent phase of granitic magmatism in the region.

Fluid inclusion microthermometry has been conducted on minor subeconomic deposits of both NSZ and sulfide Zn-Pb mineralization.  Earlier studies had focused on the major economic deposits within the region.  Newly measured homogenization temperatures in the NSZ deposits vary widely from 64º to >325º C, consistent with published findings from the major deposits.  Sulfide Zn-Pb deposits have a narrower range of homogenization temperatures of 154-200º C.  Last ice melting temperatures in minor NSZ and sulfide Zn-Pb deposits range from -2º to -16º C and are on average higher than the major deposits, suggesting that formation of the minor deposits involved a higher proportion of a dilute fluid.