Mafic basement of the Mesoarchaean Witwatersrand basin fill: Possible source of the world's largest known accumulation of gold?
Final Report Abstract
A comprehensive lithogeochemical, mineral chemical and isotopic study of selected pre-Witwatersrand basement rocks in the immediate vicinity to the 2.90-2.78 Ga Central Rand Basin, the world’s largest known depository of gold, and of various ore components, notably pyrite and uraninite, in the Mesoarchaean conglomeratic gold placer deposits that had been deposited in the Central Rand Basin revealed new insights into the origin and formation of the Witwatersrand gold - the supergiant amongst gold provinces. For the first time, trace element concentration data and maps could be obtained for uraninite and for Witwatersrand pyrite. These data strongly support the hypothesis of a placer origin of these minerals (and by inference of the gold) in the Witwatersrand ore bodies and argue against a post-depositional hydrothermal introduction of the ore into the host conglomerates as previously suggested by several workers. Furthermore, a magmatic provenance could be established for the detrital uraninite grains, and based on anomalously high Au concentrations in the uraninite, it may be speculated that the same magmatogenic hydrothermal source also applies to the detrital gold particles. With the help of new U-Pb and Lu-Hf isotope data obtained on detrital zircon grains that occur together with the gold (and uraninite), together with previously published Re-Os ages obtained on the gold, the most likely source rocks could be constrained as c. 3060 Ma granitoids. In three drill cores, likely equivalents of these source rocks in the pre-Witwatersrand basement could be found and characterized geochemically as well as isotopically. They constitute part of a 3.06 Ma magmatic arc along the northern margin of what later became the Central Rand foreland basin. The investigated basement rocks comprise calc-alkaline granite, granodiorte, diorite as well as hornblende metagabbro. The geochemical and mineralogical characteristics of the premetamorphic protoliths point to emplacement of hydrous melts with >4 wt% H2O - a feature that is well known to facilitate the transport of many metals, including Au, into the crust. The studied basement rocks display evidence of a low-temperature alteration, which can be found across the Witwatersrand Basin and adjacent rocks and thus is most likely of post-Witwatersrand age, but also of a high-temperature alteration that can be ascribed to late-magmatic metasomatism along the interface of granite and hornblende gabbro. The latter is significant in so far as it bears charateristics of magmatogenic-hydrothermal alteration (enrichment in K, P, F, U, rare earth elements) typically found in some of the most fertile ore deposit systems known elsewhere in the world (e.g. porphyry-copper and ironoxide-copper-gold deposits). Remnants of higher crustal, volcanic equivalents with variable degree of alteration could be found as clasts within the auriferous conglomerates. A compilation and analysis of reported gold reserves and resources showed that volcanic arc settings have been particularly conducive for the formation of gold deposits throughout Earth history but also that by far most of the globally known gold entered the continental crust at c. 3.1- 3.0 Ga at a time when the Earth’s mantle reached its thermal maximum and highest partial melting rates occurred. A considerable proportion (c. 40 %) of that Mesoarchaean gold became preserved in the Witwatersrand deposits, most of the rest became (repeatedly) re-processed by erosion, sedimentation, subduction and orogenic recycling that shaped much of our continents over the past 3 billion years, thus leading to the great variety of gold deposit types known today.
Publications
- 2007, Gold Endowment of the Earth’s Crust Over Time. Andrew, C. et al. (eds.), Digging Deeper, Proc. 9th Biennial SGA Meeting, 20-23 August 2007, Irish Assoc. Econ. Geol., Dublin, v. 1, p. 11-14
Frimmel, H.E.
- 2008, Earth's continental crustal gold endowment: Earth Planet. Sci. Letters, 267, 45-55
Frimmel, H. E.
- 2009, Geochemical and geochronological constraints on the nature of the immediate basement beneath the Mesoarchaean auriferous Witwatersrand Basin, South Africa. Journal of Petrology, 50, 2187-2220
Frimmel, H.E., Zeh, A., Lehrmann, B., Hallbauer, D.K., Frank, W.
- 2009, The source of Witwatersrand Gold: Evidence from uraninite chemistry. In Williams, P. et al. (eds.) Smart Science for Exploration and Mining. Proc. 10th Biennial SGA Meeting, 17-20 August 2009, Townsville, v.1, p. 353- 355
Frimmel, H., Emsbo, P., Koenig, A.E.
- 2009, Trace element characteristics of different pyrite types from Mesoarchean to Palaeoproterozoic placer deposits. In Williams, P. et al. (eds.) Smart Science for Exploration and Mining. Proc. 10th Biennial SGA Meeting, 17-20 August 2009, Townsville, v. 1, p. 363-365
Koglin, N., Frimmel, H., Minter, W.E.L., Brätz, H.
- 2010, New constraints on the auriferous Witwatersrand sediment provenance from combined detrital zircon U-Pb and Lu-Hf isotope data for the Eldorado Reef (Central Rand Group, South Africa). Precambrian Research, 183, 817-824
Koglin, N., Zeh, A., Frimmel, H.E., Gerdes, A.
- 2010, Trace-element characteristics of different pyrite types in Mesoarchaean to Palaeoproterozoic placer deposits. Mineralium Deposita 45, 259-280
Koglin, N., Frimmel, H.E., Minter, W.E.L., Brätz, H.
- 2011, LA-ICP-MS trace element analysis of pyrite from the Xiaoqinling gold district, China: Implications for ore genesis. Ore Geology Reviews, 43: 142-153
Zhao, H.-X., Frimmel, H.E., Jiang, S.-Y., Dai, B.-Z.
