La gene`se des nodules de mangane`se

Abstract

Lead isotope analyses have been undertaken as part of a program to evaluate the potential of geochemical methods for use in exploration in the Tennant Creek goldfield. Earlier exploration in this area was based on magnetic geophysical methods. Economic Au, Cu and Bi mineralization usually occurs in magnetic magnetite-chlorite lenses or pods (“ironstones”) which may be only 30 m across. Several hundred ironstones are found in the Tennant Creek field, of which only nine have been significant producers. Despite complications arising from the low Pb and relatively elevated U contents of the ore, determination of Pb isotope ratios in drill core material allows discrimination between economic magnetic ironstones and “barren” ironstones of similar mineralogy. A target signature for the Th-derived Pb isotope ratio, 208Pb/204Pb, is specific for lode mineralization, although it does not discriminate between Au-rich and Cu-rich lodes. The target signature is commonly found not only in the central Au-rich magnetite-chlorite zone, but also in the outer “barren” talc-magnetite and carbonate zones, offering up to a two-fold increase in the size of the target. The Pb isotope signature is retained in hematite-rich surface ironstones (termed “gossans” here) and it appears possible, at this stage of the project, to discriminate between gossans derived from mineralized magnetite lodes and “barren” magnetite lodes. A discrimination between weakly-mineralized (either Cu or Au) and economic deposits is equivocal at this stage. The difference between target and sample 208Pb/204Pb ratios (Δ208/204) ranges from −3 to +2% for the economic lodes, through weakly mineralized and “barren” magnetic ironstones (commonly in the range −20 to −40%) to the country rock magnetite shales (about −40 to −70%). These data are consistent with a significant Pb component in the “barren” ironstones being derived locally from the magnetite shales. Hematite shales commonly associated with economic lodes may have acted as limited channelways for the ore fluids as the target signature is discernible for up to 50 m along the hematite shale bands from the outer chlorite zones of the economic lodes. Further trace element and isotopic work is necessary to elucidate the genesis of the mineralization.