Geochemical fingerprinting of carbonate wall rock alteration at major base metal sulphide deposits in the Otavi mountain land, Namibia

Abstract

Abstract provided by author:

Carbonate generations within each deposit can be grouped into barren and mineralised carbonates. The mineralised carbonates show slight differences which can be explained by changes in physicochemical conditions such as Eh, pH, temperature, stratigraphic controls and fluid rock interaction. These indicate that the ore-bearing fluids precipitated sulphides under reducing, acidic, high temperature conditions. These were followed by oxidising, alkaline, lower temperature conditions during which ore remobilisation took place. Stark differences, especially in trace element concentrations, compared to host rocks for Tsumeb-type deposits indicate that fluids which precipitated the alteration phases and associated mineralisation, were derived from a source outside the Otavi Mountain Land, in contrast to fluids responsible for Berg Aukas-type mineralisation. Fluid salinities, on the other hand, appear to be very similar for both Berg Aukas- and Tsumeb-type mineralisation, in contrast to previous findings. The salinities for both fluid types were likely derived from evaporitic formation waters. Former evaporite deposits probably also provided the sulphur source for the mineralisation. Similarities in trace element content, formation temperatures, ore associations, and structural relations point to a strong Tsumeb-type affinity for the Khusib Springs deposit. Furthermore, stable isotope ratios for carbonate alteration phases at Khusib Springs are distinct from those for carbonate alteration at Berg Aukas. Enrichments in Mn, Sr, Y and REE in carbonate alteration phases relative to the host rock compositions, and higher temperatures of formation, are diagnostic of economic Tsumeb-type mineralisation. Integration of all geochemical aspects, in conjunction with structural and stratigraphic controls, may help in identifying and further exploiting economic mineralisation in the Otavi Mountain Land