Pre‐Klondikean oxidation prepared the ground for Broken Hill‐type mineralization in South Africa

Abstract

New Cu isotope data obtained on chalcopyrite from the Black Mountain and the Broken Hill deposits in the medium‐ to high‐grade metamorphic Aggeneys‐Gamsberg ore district (South Africa) require a revision of our understanding of the genesis of metamorphic Broken Hill‐type massive sulphide deposits. Chalcopyrite from both deposits revealed unusually wide ranges in δ65Cu (−2.41 to 2.84‰ NIST 976 standard) in combination with distinctly positive mean values (0.27 and 0.94‰, respectively). This is interpreted to reflect derivation from various silicate and oxide precursor minerals in which Cu occurred in higher oxidation states. Together with the observation of a typical supergene base metal distribution within the deposits and their spatial association with an unconformity only meters above the ore horizon, our new data are best explained by supergene oxidation of originally possibly SEDEX deposits prior to metamorphic sulphide formation, between the Okiepian (1,210–1,180 Ma) and Klondikean (1,040–1,020 Ma) orogenic events.

Syn-metamorphic sulfidation of the Gamsberg zinc deposit, South Africa

“…This opens up the possibility that the metamorphic sulfidation suggested here affected not only previously existing sulfides with a lower sulfidation state but also supergene alteration products such as base metal silicates, -oxides and -hydroxides. Such reduction/sulfidation of previously oxidized base metal minerals would also explain recently obtained Cu isotope ratios of the different deposits presented by Höhn et al (2020). • Our findings suggest that sulfur-rich metamorphic fluids were able to (re-)-sulfidize base metal-rich rocks on a deposit scale and thereby to camouflage previous oxidation as expected beneath a chemically weathered land surface.…”

“…The suggested metamorphic sulfidation fills a hitherto unexplained gap in the complex metallogenetic history of the Aggeneys-Gamsberg ore district. The position of the orebodies only a few meters below a major unconformity and Cu isotopic characteristics of the orebodies from Black Mountain and Broken Hill (Höhn et al 2020) indicate a pre-Klondikean oxidation event. A phase of strong oxidative chemical weathering on a pre-Klondikean paleosurface is supported by the widespread occurrence of peraluminious rocks around Aggeneys (Willner et al 1990), the high quantities of base metals in silicates and oxides (Walters 1998), and the unusually strong enrichment in Mn in this area (McClung and Viljoen 2011).…”

“…Over this time, the Bushmanland Group was affected by weathering, erosion and peneplanation. This opens up the possibility that the ore bodies situated below the unconformity, although sulfidic in nature, underwent a phase of oxidation as suggested by a recent Cu isotope study (Höhn et al 2020). The protracted thermal history of the Bushmanland Group had two peaks that correspond to the Okiepian (1210( -1180 and Klondikean (1040-1020 Ma) orogenic events (Clifford et al 2004;Cornell et al 2009).…”

“…Based on our new results, the observed trend in δ 34 S could be the product of variations in the amount of sulfur-rich fluid that reacted with the already metamorphosed rocks and/or a consequence of differences in the oxidation state of the precursor. Considering its stratigraphic position right below the pre-Klondikean unconformity and its Zn-dominated base metal content, the Gamsberg deposit probably experienced the most intense pre-Klondikean weathering-induced oxidation at the time of the hiatus (Höhn et al 2020) and consequently the most intense resulfidation thereafter. In that case, the δ 34 S ratios of its sulfides should not describe the primary but rather the metamorphic sulfur source.…”

“…Furthermore, various geochemical anomalies in the immediate vicinity of the ore district (e.g., Willner et al 1990) and several studies, which point to inconsistencies in the redox conditions of the deposits (e.g., Stalder and Rozendaal 2005b;Cawood and Rozendaal 2020), all suggest a more complicated metallogeny. An oxidation event affecting the whole ore district between the Okiepian (1210( -1180 and Klondikean (1040-1020 Ma) orogenic events (Clifford et al 2004) has been suggested based on Cu isotope data (Höhn et al 2020). The current study concerns detailed mineral chemical analyses of hitherto unexplained textures of the sulfidic ore, which will be used to demonstrate the possibility of metamorphic sulfidation.…”