The role of evaporites in the genesis of base metal sulphide mineralisation in the Northern Platform of the Pan- African Damara Belt, Namibia: geochemical and fluid inclusion evidence from carbonate wall rock alteration

摘要

The Otavi Mountain Land is a base metal sulphideore province in northern Namibia where deposits are hosted byplatform carbonates of the Otavi Group in a foreland fold-and-thrust belt on the northern edge of the Pan-African Damara Belt.Deposits have been classified as the Berg Aukas- or Tsumeb-types, based on differences in ore association, stratigraphicposition and geochemistry of ores and gangue carbonates.Mineralisation at these deposits is accompanied by carbonatealteration in the form of dolomite and calcite veins, carbonaterecrystallisation, calcitisation and carbonate silicification. Basedon cathodoluminescence imaging, trace and rare earth element(REE), O and C isotope, and fluid inclusion data, a series ofcarbonate generations, constituting wall rock alteration aroundthe Tsumeb and Kombat (Tsumeb-type) and Berg Aukas (BergAukas-type) deposits, was established. Similar data obtained onthe recently discovered Khusib Springs deposit indicate a strongaffinity to Tsumeb-type deposits. Tsumeb-type deposits aredistinguished from Berg Aukas-type deposits by having traceelement and REE concentrations that are significantly higher inthe alteration products compared to the carbonate host rocks.Only around Tsumeb-type deposits a relative enrichment in lightREE is noted for the hydrothermal carbonate generations thatare cogenetic with the main stage of mineralisation.Microthermometric results from fluid inclusions in carbonatealteration phases and associated quartz indicate relatively highsalinity (17-23 wt percent NaCl equivalent) for the mainmineralising and subsequent sulphide remobilisation stages atthe deposits investigated. Estimated mineralisation temperaturesare significantly higher for Tsumeb-type deposits (370-405deg C)with early sulphide remobilisation in Tsumeb at 275deg C,whereas they are lower at Berg Aukas (up to 255deg C). Fluidinclusion leachate analysis suggests that most of the observedsalinity can be ascribed to dissolved, predominantly Ca- andMg-carbonates and chlorides with subordinate NaCl. Na-Cl-Brleachate systematics indicate a derivation of the fluid salinityfrom the interaction with evaporitic rocks en route. Tsumeb-typemineralisation is interpreted to be derived from fluids expelledduring Pan-African orogeny in the more intensely deformedinternal zones of the Damara Belt further south. When the highsalinity fluids reached the carbonate platform after havingscavenged high concentrations of base metals, base metalsulphide precipitation occurred in zones of high porosity,provided by karst features in the carbonate sequence. Resultsobtained for the Berg Aukas-type deposits emphasise theirderivation from basinal brines, similar to Mississippi Valley-type deposits, and confirm that mineralisation of the BergAukas- and Tsumeb-types are both spatially and temporally distinct.