The geological setting of porphyry-type copper mineralization in the Haib River area, South West Africa

Abstract

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In terms of geochemical classifications, the Orange River Group has been divided by Reid (1977) into four categories represented by four rock-types, viz. basaltic andesite, andesite, dacite, and rhyolite. In the study area the Tsams Formation consists predominantly of andesites, dacites and rhyolites with minor amounts of basaltic andesite. The evolution of the basaltic andesites is considered to be related to fractional crystallization of minerals such as olivine and amphibole from a primitive magma, whereas andesites probably originated by fractional crystallization of amphibole and plagioclase from a basaltic andesite parent. The acid end-members of this volcanic suite probably evolved by plagioclase, K-feldspar, and biotite fractionation. In general, the Orange River Group volcanics are comparable in terms of their geochemistry with other calc-alkaline volcanic suites in the world, but they do display a notable enrichment of potassium and depletion of sodium. Other unique features of the Orange River Group include its extreme age, limited areal extent, and tectonic setting

The petrography of the Vioolsdrif Suite has revealed a mineralogical evolution of the various phases. Notable mineralogical changes within this suite include the disappearance of hornblende, but the appearance of biotite in the acid, leucocratic end-members. In addition the change in the nature and morphology of the K-feldspar, from an interstitial mineral in the diorite-tonalite-granodiorite phases, to an anhedral, cumulus phase in the adamellite and leucogranite end-members, has a direct influence on the trace-element composition of the rocks. Distribution of Ba, Rb, and Sr trace-elements among the different phases of the Vioolsdrif Suite have been examined and provide a means of tracing the crystallization history of the suite. The appearance of K-feldspar on the liquidus of the cooling magma had a pronounced effect on the trace-element composition of the solids which were subsequently formed. The older mafic phases, viz. diorite, tonalite, and granodiorite, evolved by fractional crystallization of quartz, plagioclase, and amphibole, whereas the younger phases evolved by fractionation of quartz, plagioclase, K-feldspar, biotite, and minor amounts of amphibole

The volcanic rocks of the Orange River Group and the plutonic rocks of the Vioolsdrif Suite are considered to be genetically related by processes of fractional crystallization. Petrology and the major- and trace-element composition of the volcanic and plutonic rocks indicate that the Vioolsdrif Suite was derived by accumulation of phases fractionated from a magma which extruded residual liquids that cooled to form the Orange River Group

Detailed investigations of the Haib copper prospect have shown that the porphyry stock developed in a tensional tectonic environment and that it intrudes the floor of a graben structure. The quartz-feldspar porphyry comprising the stock is a minor phase of the Vioolsdrif Suite, which, probably originated by high degrees of partial melting of adamellite. In terms of a classification, the Haib copper prospect is the oldest occurrence of its kind in the world and is similar to the class of plutonic porphyry deposits which characterize the Guichon Creek Batholith at the southern end of the Canadian Cordillera in British Columbia. Plutonic porphyry deposits are developed internally in medium-sized plutons what exhibit petrologic zoning. The plutons intrude coeval volcanic rocks and the intrusions related to mineralization are emplaced near the middle stages of orogenesis. Breccias are notable by their rarity, but high fracture density is ubiquitous. Hydrothermal alteration is generally very weak and is often distributed along veins and fractures. In general the older potassic alteration is weak, with younger phyllic assemblages superimposed on it. Chaleopyrite-molybdenite-pyrite mineralization is superimposed on the phyllic alteration and the mineralization is predominantly controlled by fracture systems

The distribution of silicate alteration assemblages has been examined on a regional scale involving the whole of the Haib porphyry stock and on a local scale involving the main mineralized area. In terms of the regional alteration patterns the Haib porphyry stock can be equated with the potassic core of the classic model, proposed by Lowell and Guilbert (1970), which is surrounded by successive alteration haloes of phyllic and propylitic assemblages. Within the main mineralized area a late zone of moderate-to-intense phyllic alteration overprints and is superimposed on an early phase of potassic alteration which affected the whole of the Haib porphyry stock. The main zone of mineralization is restricted to the north-central flank of the stock and is superimposed on the phyllic alteration assemblages. Two main phases of alteration and mineralization are proposed. The early phase is characterized by potassic alteration assemblages and chalcopyrite-molybdenite-pyrite mineralization, while the late phase is represented by phyllic alteration and chalcopyrite-pyrite mineralization. Very late-stage percolation of convective hydrothermal solutions through the main mineralized area gave rise to clay-mineral assemblages and low-sulphur sulphide assemblages, such as bornite, covellite, and digenite. Metal transport was accomplished by hydrothermal solutions which constituted a regionally-developed convective system, with the Haib porphyry stock providing the heat-source for the system. It is concluded that multiple events of tectonism, intrusion, alteration, and mineralization were more-or-less superimposed on one another to produce the Haib copper prospect which represents the intergrated effect of these processes