Sideritization and silification of unconformity-related hydrothermal baryte veins near Grünau, south Namibia

The development of economic mineralization within unconformity related hydrothermal vein type deposits is a topic of basic (but also economic) significance. In particular late-stage processes like pseudo- or perimorphic replacements can significantly influence the mineralogy and hence processability of ore deposits. This study aims to shed light on such late stage processes leading to the mineralogical modification of primary hydrothermal veins by pseudomorphous and perimorphous replacements of quartz after hydrothermal gangue minerals like carbonates, baryte and fluorite; whereas the genesis of pseudomorphic replacements of baryte by siderite and a contemporaneous perimorphic overgrowth of quartz has not been studied in detail so far. To study this process, hydrothermal veins of the south Namibian hydrothermal vein type district, which are related to the breakup of Pangea are chosen as natural laboratories. Fluid inclusion data together with a detailed petrography of the paragenetic sequence and fluid inclusion assemblages reveal a temperature drop from early quartz I at ∼170 °C down to ∼80 °C in quartz III at almost constant salinities of 23.1 to 24.5 wt% (NaCl+CaCl 2 ). The chemistry of the observed fluid inclusion assemblages is in accordance with previous microthermometry studies carried out in the same hydrothermal vein type district indicating an identical provenance of the fluids recognized in the other deposits (e.g., Aukam and Garub). Hence it is likely that a high salinity basement brine of cryogenic origin has been mixed with a Nama Group limestone derived fluid to form the primary mineralization. Mixing of two chemically-contrasted fluids is also depicted by the trace elements studied in the youngest quartz generation whereas the application of the TitaniQ thermometer provide evidence for a temperature of about 320 °C in the deep-seated reservoir which became afterwards mixed with Nama Group limestone derived fluids. Thermodynamic modelling based on the gathered fluid data and data from analogue studies, provide evidence that siderite pseudomorphs after baryte form under reducing conditions, under which sulfate is reduced and the dissolution of baryte promotes siderite and pyrite precipitation. Hence, the present study contributes to the still weakly developed understanding how post-precipitation processes influences the mineralogy of hydrothermal veins.