Petrology and mineral chemistry of sulphide ores and associated metalliferous rocks of the Gamsberg Zn-Pb deposit, South Africa : implications for ore genesis and mineral exploration

[摘要] ENGLISH ABSTRACT: The Gamsberg Zn-Pb deposit is a metamorphosed and multiply deformed sediment-hosted base metaldeposit in the central Namaqua Province of South Africa. The deposit is hosted by the BushmanlandGroup, a late Palaeoproterozoic (2000-1600 Ma) supracrustal succession of quartzite, metapelitic schistand interbedded metavolcanic rocks. Mineralisation occurs within the central part of the Gams Formation,a heterogeneous sequence of metamorphosed metalliferous sediments and fine-grained organic-richshales. The ore horizon is subdivided into a lower unit of metapelite-hosted ore, an intermediate layerof phosphorite-hosted ore, and an upper unit of banded garnet-apatite ore. The ore body is enveloped byunmineralised silicate-, carbonate- and oxide-facies metalliferous rocks, which originally representedmixtures of Fe-Mn-rich hydrothermal precipitates, authigenic carbonate, and variable concentrations ofdetrital material. Based on mineralogical and geochemical characteristics, the metalliferous host rocks aresubdivided into iron formations, coticules, Fe-Mn silicates, impure marbles and barite/Ba-rich quartzite.Minerals of the Gams Formation mostly represent solid solution between the Fe and Mn end-membersof garnet, pyroxene, pyroxenoid, amphibole, olivine, spinel and ilmenite. Calcium-rich rock types are atypical feature and characterized by the occurrence of manganoan calcite, clinopyroxene, andradite-richgarnet and titanite. A successive increase in the (Mn+Ca):Fe value of rocks and minerals is evident withincreasing distance from the ore horizon. Amphibole is restricted to Fe-rich ore-bearing assemblages,whereas orthopyroxene, clinopyroxene, Fe-rich pyroxenoid and olivine are present in intermediateassemblages, and Mn-rich rhodonite and pyroxmangite in the most manganiferous assemblages. Thesevariations are mimicked by an increase in the Mn:Fe value of coexisting garnet and ilmenite groupminerals with increasing distance from ore. LA-ICP-MS analyses have been used to constrain the REEpatterns of garnet and apatite. In the ore-body, these minerals display a positive Eu anomaly, which isinterpreted to reflect a distinct hydrothermal signature. In contrast, garnet and apatite in unmineralisedmetalliferous rocks display nil or a negative Eu anomaly.Primary features of the Gams Formation, such as REE patterns, the banded nature of garnet-apatite ore,the presence of diagenetic apatite nodules, and the distribution of the redox-sensitive elements Ba andMn have been used to constrain palaeo-environmental conditions. The results indicate that metapelitehostedore has been deposited in a stratified ocean that was characterised by anoxic bottom waters andprecipitation of Fe and Zn sulphides into organic matter-rich shales. These rocks were superceded byphosphorite-hosted ore, garnet-apatite ore and metalliferous host rocks that developed in a suboxic tooxic environment. The large size of the deposit, the internal lamination of the ores and the predominanceof sphalerite and barite are consistent with a vent-distal setting and precipitation of the ore-formingconstituents from dense and reduced hydrothermal fluids, which originated due to reactivation of dormantgrowth faults. Collectively, the geological evidence indicates that Gamsberg is bridging the gap betweenthe SEDEX and BHT classifications. The relationships demonstrate that differences between these twoclasses of sediment-hosted Zn-Pb deposits are predominantly related to environmental conditions withinlocalised third order basins and not to fundamental differences in ore-forming processes.

[发布机构] Stellenbosch University