New geochemical constraints on the genesis of the Gamsberg zinc deposit, Namaqualand Metamorphic Province, South Africa

摘要

The base metal massive sulfide deposits of the Aggeneys-Gamsberg (A-G) District are hosted within the Mesoproterozoic Bushmanland Group of the Namaqua-Natal Metamorphic Complex in the Northern Cape Province of South Africa. The district displays an apparent eastward trend in the economic concentration of base metals (+ barite) from relatively Cu-Pb-rich, Ba-poor mineralisation at Black Mountain to Zn- and Ba-rich ores at Gamsberg. Base metal sulfides at Gamsberg are restricted to the so called Gams (Iron) Formation which comprises a sulfidic mineralized unit (“B”) enveloped within a sequence of meta-sedimentary units (“A” and “C”). The aim of the study was to shed further light on the genesis and chemical evolution of the sulfide mineralisation at Gamsberg in the context of the entire A-G District, by interrogating further the apparent district-wide trend in base metal distribution. The Gams Iron Formation was sampled and studied from one key drill core intersection (“G1”) which intersects the largest part of it as described elsewhere; a small number of additional samples from a second drill core (“G2”) complemented the main sample suite. Minerals that make up the silicate assemblages across the studied section include quartz, garnet, pyroxene, pyroxenoid, phyllosilicates, carbonates, amphiboles, oxides (chiefly magnetite) and graphite. In a stratigraphic context, the mineralogical variations conform directly to those documented in the relevant literature from the Gamsberg locality. These are coupled, where possible, with mineral-chemical profiles of selected silicate species which replicate those of bulk-rock compositions, particularly with respect to Mn, Fe and Ca in the upper C Unit of the studied section. These signals collectively track the characteristic transition from a terrigenous, siliciclastic sediment-dominated footwall to an exhalative sediment-dominated hanging wall to the sulfide mineralisation as also seen in similar deposits elsewhere, particularly with respect to the characteristic Mn-rich signature increasingly observed in the hanging wall C Unit. The foregoing suggests that the examined section faithfully records the interpreted primary stratigraphy of the deposits, despite the complex structural and metamorphic overprint that characterises the region. This facilitates a stratigraphic analytical approach on the sulfidic Unit B, through a combination of mineral-chemical and stable isotope analyses. Dominant sulfides in Unit B are sphalerite and pyrite, with lesser pyrrhotite and minor galena. Sphalerite shows high and generally invariant contents of Fe (mean 12.18wt%, as FeS) whereas Zn anti-correlates with Mn (mean 5.58wt%, as MnS). Isotopic analyses for S, Fe and Zn in hand-picked sphalerite and pyrite separates were used with a view to providing new evidence for chemical and isotopic variation within the sulfide ore-body in a vertical (i.e. stratigraphic) sense, discuss the implications thereof, and ultimately interpret the new data in light of similar existing data from the A-G District and elsewhere. The δ³⁴S data for pyrite (plus a single pyrrhotite grain) and sphalerite from both cores G1 and G2 show comparable compositional ranges between 22.9 and 30.4‰ and between 27 and 30.1‰ respectively. The δ⁵⁶Fe data for pyrite show a range between -1.85 and 0.19‰, whereas seven sphalerite separates have a very narrow range of δ⁶⁶Zn from 0.06 to 0.20‰. The atypically high sulfur isotope data reported in this study are interpreted to reflect sedimentary deposition of primary sulfide ore at Gamsberg from an isotopically highly evolved seawater sulfate source through large-scale Rayleigh fractionation processes. Thermogenic sulfate reduction is proposed to have been the main reductive mechanism from seawater sulfate to sulfide, given the absence of very low δ³⁴S data for sulfides anywhere in the A-G District. By contrast, the δ⁶⁶Zn values for sphalerite are for all intents and purposes invariant and very close to 0‰, and therefore suggest little Zn isotope fractionation from an original exhalative fluid source. On this evidence alone, Zn isotopes therefore appear to hold little promise as a proxy of the chemical and isotopic evolution of SEDEX deposits in space and time, although this can only be verified through further application in the broader A-G District and similar deposits elsewhere. The apparent decoupling of Zn and S isotopes in the Gamsberg sulfide deposit, however, points towards diverse sources of these two components, i.e. ascending metalliferous brines versus seawater respectively. Finally, pyrite δ⁵⁶Fe data do show a stratigraphic trend of generally declining values up-section, which are interpreted to reflect the influence of broadly coeval precipitation of isotopically heavy Fe-oxides on a broader-scale – now preserved as abundant magnetite through metamorphism. Further work on the iron isotope composition of silicate-and oxide-hosted Fe on a local-to-district scale will assist in testing this interpretation.