The O’okiep Copper District, Namaqualand, South Africa: a review of the geology with emphasis on the petrogenesis of the cupriferous Koperberg Suite

Abstract

The O’okiep Copper District is the oldest formal mining area in South Africa. Between 1852 and 2002, the 2,500 km2 area yielded two million tons of copper from 32 mines ranging in ore tonnages from 140,000 to 37 million tons. This paper summarizes the calendar of events from the formation of the first primitive crust 1,700–2,000 Ma ago to early Cambrian times ~500 Ma ago, with particular emphasis on the Namaquan (Grenville) Orogeny, notably: the O’okiepian Episode (1,180–1,210 Ma ago) of alpine-type folding, regional granite plutonism, and granulite facies metamorphism and the Klondikean Episode (1,020–1,040 Ma ago) of open and tight folding and the intrusion of the Rietberg Granite and the Koperberg Suite. Almost all of the copper in the O’okiep District occurs in the Koperberg Suite, of which there are 1,700 small bodies that constitute 0.7% of the outcrop area. The suite comprises jotunite, anorthosite, biotite diorite, and hypersthenic rocks ranging from leuconorite to hypersthenite, and it is one of only two world examples of economic copper mineralization in rocks of the anorthosite–charnockite kindred; the second example is Caraiba, Brazil. High I Sr and low e Nd (for a 1,030 Ma intrusion age), and high μ 2 of 10.1, for Koperberg rock-types indicate a crustal progenitor for the suite, and the presence of jotunite suggests a (subducted) crustal source at ca. 40–50 km depth. The magmatic sulphide paragenesis in the Koperberg Suite is chalcopyrite + pyrrhotite (Narrap-type ore) that, in a number of ore-bodies, has been inverted under upper amphibolite facies conditions to bornite + Ti-free magnetite (Carolusberg-type ore). Meteoric fluids resulted in supergene Cu enrichment in Koperberg bodies to ~500 m below the pre-Nama peneplane, and lower greenschist facies metamorphism 500–570 Ma ago is reflected by inter alia Hoits-type ore bearing second-generation bornite + chalcopyrite(± covellite ± chalcocite).