Carbonate-hosted base metal deposits; lead isotope data bearing on their genesis and exploration

Abstract

Lead isotope analyses from four Australian carbonate-hosted Pb-Zn deposits (Coxco, Sorby Hills, Pillara, and Wagon Pass) demonstrate that these deposits have different lead isotope properties. Whereas lead isotope compositions for Coxco and Wagon Pass exhibit linear trends, leads from Sorby Hills and Pillara are isotopically homogeneous although they have an excess of radiogenic lead when compared to sulfide ores. At Coxco the dispersion of the data is limited and most lead is identical to that of the nearby HYC deposit. The leads from Wagon Pass, however, are more radiogenic than the other deposits ( 206 Pb/ 204 Pb > 20) and there are measurable isotopic differences between galenas occurring only a few centimeters apart. Both Pillara and Wagon Pass occur on the Lennard shelf and this is one of the rare cases where homogeneous and heterogeneous leads occur within one carbonate province.Data from the ores and their weathered surface expressions demonstrate that the lead isotope compositions of the ores are maintained during oxidation. These results suggest that lead isotope analyses of a limited number of either surface or drill core samples can be of considerable assistance in assessing the exploration potential of suspected base metal prospects in carbonate environments.Comparison of the Australian results with published lead isotope data for a number of carbonate-hosted deposits in different geologic settings allows two main groups to be distinguished. One group is characterized by lead isotope compositions that are homogeneous within each deposit and the data are either (e.g., Pine Point, Silesia, eastern Alps) or radiogenic (Sorby Hills, Pillara). In the other group, the data are generally radiogenic and form linear arrays (Viburnum Trend, Wagon Pass).There is a lithological and isotopic transition between the carbonate-hosted deposits with homogeneous normal isotope ratios and shale-hosted massive sulfides which have a large component of carbonate in their host rocks (e.g., HYC, Mount Isa).Of the isotopically homogeneous deposits, the central European ores occurring in Mesozoic sediments on a Paleozoic basement exhibit lead compositions conforming to average global lead evolution. On the other hand, Sorby Hills and Pillara, which occur in Paleozoic host rocks on middle to lower Proterozoic basements, have a distinct excess of radiogenic lead. Of the deposits with heterogeneous lead, Coxco occurs over an only slightly older basement and exhibits the least radiogenic lead conforming to average lead evolution. In contrast, in the Viburnum Trend where ores occur in Paleozoic hosts immediately above a middle Proterozoic basement, even the least radiogenic leads are anomalous in as much as they do not fit standard lead evolution models. It thus seems that the radiogeneity of the leads in carbonate-hosted deposits varies as a function of the age differences between the crystalline basement and the time of ore deposition.The homogeneity of the leads from carbonate-hosted deposits seems to depend on the proximity of the ores to deep sedimentary basins. This is exemplified by the difference between data from Pillara and Wagon Pass, both of which occur on, but different sides of, the Lennard shelf. The isotopically heterogeneous Wagon Pass deposit lies only a few kilometers from the Proterozoic Kimberley basin, whereas the homogeneous Pillara prospect is on the edge of the Fitzroy trough which contains a 5-km-thick pile of Phanerozoic sediments. The homogeneity of the lead can be accounted for by the combined effects of sedimentation, diagenesis, and prolonged brine circulation. On geologic and isotopic grounds, we favor a lateral secretion origin for carbonate-hosted deposits.