Morphology and geochemistry of different pyrite types from the Upper Witwatersrand System of the Klerksdorp Goldfield, South Africa

Abstract

ZusammenfassungPyrite aus den proterozoischen, gold-und uranfiihrenden Konglomeraten des Oberen Witwatersrand Systems des Klerksdorp Goldfeldes (Siid-Afrika) wurden mit dem Rasterelektronermaikroskop (RUM) untersucht. Eine dreidimensionale Betrachtung der Erz-Pa~Jkel mit dem RUM wurde durch das Herausl6sen der Pyrite aus den Konglomerat-Proben mittels Flul3s~iure erm6glieht. Auf Grund morphologischer Kriterien wurden die Pyrite in drei Typen unterteilt. Spurenelementanalysen (Au, Co, Ni, Cu, Zn, Pb, Mn, As) dea" drei Typen mit der Atomabsorptionsmethode und deren statistischer Auswertung best~itigten die morphologische Klassifikation:Typ 1: Authigene, idiomorphe bis hypidiomorphe Pyrite oder Pyritaggregate, die sich sekund~ir w~ihrend der Diagenese oder Metamorphose bildeten. Auf Grund einer dreidimensionalen Betradatungsweise der Pyrit-Partikel mit dem RUM kann gezeigt werden, dal3 sich Pyrit-Typen, die aus dem Witwatersrand System von fr/iheren Verfassern besdarieben wurden, in die hier dargestellten Typen klassifizieren lassen. AbstractThe morphology of pyrites from the Proterozoic, auriferous and uranfferous conglomerates of the Upper Witwatersrand System of the Klerksdorp Goldfield (South Africa) was studied by means of scanning electron microscopy (SUM). The pyrite particles *) Autor's address: Dipl.-Geol. T. UTTER, 27 Walton Av., Auckland Park, Johannesburg, South Africa.Paper presented in German at the 67. annual conference of the Geologische Vereinigung 1977 in Tiibingen. 774Band 67, Heft 2, 1078, Seite 774--804 T. UT~a-Morphology and geochemistry of different pyrite types were recovered by hydrofluoric acid leaching, thus making a three-dimensional SEM examination possible. According to morphological criteria the pyrites were classified into three types. Trace-element analysis by atomic absorption spetrophotometry (Au, Co, Ni, Cu, Zn, Pb, Mn, As) and the statistical evaluation of the results confirmed the morphological classification:Type 1: Authigenic, idiomorphic to hypidiomorphic pyrites or pyrite accumulations, which were formed in the conglomerates during diagenesis or metamorphism.Type 2: Allogenic, rounded, compact pyrites. This type was eroded from primary deposits in the hinterland of the Witwatersrand basin and deposited with the Witwatersrand sediments. It shows the closest trace-element affinity to pyrites from the Barberton Mountain Land, the source area model for the Witwatersrand sediments. The recognition of this pyrite type from the Klerksdorp Goldfield is in agreement with observations on detrital compact pyrites described from other goldfields of the Witwatersrand.Type 8: Allogenic, rounded, porous pyrites. These were formed from pyritic muds and iron sulfide gels existing on the surface of the alluvial fan, and later were reworked as mud balls or fragments and deposited with the conglomerates. Indentations with radial fracture patterns point to transport partly in a plastic state. The occurrence of colloform pyrites among this type supports the postulation of pyritic muds or iron sul...

Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis

“…The investigated samples contain pyrite of three main types (Table 1): rounded massive pyrite, rounded porous pyrite, and authigenic pyrite, in accordance with previous observations from the same reef horizons (Ramdohr, 1958;Saager, 1970;Feather and Koen, 1975;Utter, 1978;Hallbauer, 1986;England et al, 2002). Rounded grains are spherical to discoidal, up to 1 cm in diameter and include massive and, less commonly, porous varieties, the latter with inclusions of various silicate minerals, such as quartz and phyllosilicates ( Fig.…”

Some aspects of the genesis of heavy mineral assemblages in Lower Proterozoic uranium-gold conglomerates

“…Investigations of the morphology and hydraulic equivalence of some of the ore particles and comparison with modern water-worn grains is also consistent with a placer origin (Hallbauer and Utter, 1977;Utter, 1980). However, some of the gold and pyrite textures are those of replacement (Davidson, 1957;Ramdohr, 1958;Saager, 1970) with uranium commonly occurring in brannerite; this is clearly not detrital and a 'modified placer' hypothesis (Liebenberg, 1955) which allows for limited mobility of elements during diagenesis and metamorphism has been developed to explain some of these features.…”

“…Pyrite, sulphidation events and precursor minerals. The pyrites of the Lower Proterozoic conglomerates and those of the Rand in particular have been discussed by several authors (Ramdohr, 1958;Saager and Mihalik, 1967;Saager, 1970;Simpson andBowles, 1977, 1981;Utter, 1977;Kimberley, 1978;Dimroth, 1979 (Dimroth 1979, Simpson andBowles 1981) or of volcanic/diagenetic origin (Kimberley 1978).…”

Understanding Mineral Deposits

“…Four types of pyrite have been recognized in the Witwatersrand conglomerates (Utter 1978, Hallbauer 1986, Hallbauer & Barton 1987: (a) detrital (allogenic) pyrite, derived from a source external to the Basin or from erosion of pre-existing reefs; (b) authigenic pyrite, formed in situ after deposition of the conglomerate by sulfidation of Fe-bearing minerals; (c) synsedimentary pyrite, which often contains significant concentrations of gold; and (d) framboidal pyrite, believed to have been formed by the action of reducing bacteria. Four types of pyrite have been recognized in the Witwatersrand conglomerates (Utter 1978, Hallbauer 1986, Hallbauer & Barton 1987: (a) detrital (allogenic) pyrite, derived from a source external to the Basin or from erosion of pre-existing reefs; (b) authigenic pyrite, formed in situ after deposition of the conglomerate by sulfidation of Fe-bearing minerals; (c) synsedimentary pyrite, which often contains significant concentrations of gold; and (d) framboidal pyrite, believed to have been formed by the action of reducing bacteria.…”