Geochemical and isotopic aspects of Rosh Pinah mineralisation and host rock, Namibia ― Relevance for metallogeny and as vectors to ore

摘要

Trace element and stable isotope geochemistry of selected parts of the Rosh Pinah orebody and host rocks in Namibia have been investigated to identify applicable vectors to ore and metallogenic conditions. Hydrotherma/detrital element ratios and multi-element alteration indices of host rocks appear to be viable proximity indicators to ore, even after lower amphibolite facies metamorphism. Host-rock minerals proximal to mineralisation tend to show enrichment in hydrothermal elements due to diadochic substitution. Chondrite-normalised rare-earth element patterns of sulphide-bearing argillite and arenite are typical of sedimentary rocks, whereas the mineralisation dominated by sphalerite displays flat, moderately enriched rare-earth patterns with a positive europium anomaly. δ~(34)S isotope ratios of base-metal sulphide minerals show that in remobilized ores, there is a strong sedimentary component of sulfur within a predominantly hydro-thermal origin. Sulphur isotope thermometry based on δ~(34)S ratios of cogenetic sulphide pairs reveal formation temperatures between 320 and 390℃, confirming microthermometric data of other investigators. High Ba contents of host-rock sediments and Ba and S contents of the mineralised rocks suggest an origin for early barite ore from hot, acid and reduced fluids, heat-driven by felsic volcanic centres. Cross-cutting structures of paragenetic and possibly late stage occurring carbonate and base metal sulphides support the view that subsequent brines were of lower temperature and oxidised, causing the formation of ankerite alteration halos. Apart from strong sedimentary facies changes, the trace element contents of mineralisation and host rocks suggest variable redox conditions in a turbulent depositional environment within an evolving rift graben.