Abstract

A lithofacies change from terrestrial arenites to marine Oamites Formation sediments and possible volcanics is a major feature of the southern margin of the late Precambrian Damara eugeosyncline in the Oamites area. The MO sequence, which is the predominantly rudaceous middle section of the Oamites Formation, has a maximum thickness of 55 meters and hosts the 600-m-long Oamites Cu-Ag orebody near the major lithofacies change. Upper greenschist facies metamorphism and structural deformation have not obscured the main primary characteristics of this stratiform sulfide deposit.The weakly mineralized lower MO sequence, consisting mainly of conglomerate, was deposited during rapid marine transgression, possibly following a volcanic episode. The basal MO pink pebble rudites contain iron oxides and ilmenite, but in the overlying reduced white pebble conglomerates and white quartzites virtually complete sulfidization of iron in these detrital components has been effected. The upper MO sequence quartzites, conglomerates, and dolomites were deposited during the initial stages of regression and are often well mineralized. The reduced lagoonal facies grades into a pink oxidized facies at the eastern limit of the orebody, indicating proximity to the paleostrandline. Sulfide mineral zonation cuts across bed boundaries but tends to parallel oxidized lithofacies/reduced lithofacies boundaries. The Cu content of sulfides increases upward stratigraphically and toward the paleostrandline laterally. As is the case in many other stratiform deposits, Pb and Zn are concentrated above the Cu orebody in a stratigraphic sense and in a deeper water depositional environment laterally. Mineralogic investigation revealed a well-defined paragenetic sequence for the MO sulfides: Pyrite is first replaced by chalcopyrite which is, in turn, progressively replaced by bornite, neodigenite, and chalcocite.A basically syndepositional genetic model for the Oamites sulfide deposit is proposed: Base metals and Ag were carried by rivers into a lagoon where H 2 S in the anoxic zone caused their selective precipitation. Algal reefs developed near the strandline played an important role in the development of restrictive environments favoring concentration of Cu and Ag. Albitization of K-feldspar, which is characteristic of the orebody, suggests that the lagoonal waters were highly saline. It is concluded that the Oamites deposit is in many ways similar to the Zambian Copperbelt deposits, although the copper content at Oamites is relatively low.

First Page Preview

First page PDF preview
You do not currently have access to this article.