The mineralogy and genesis of the lead-zinc-vanadium deposit of Abenab West in the Otavi mountains, S. W. A

Abstract

Abstract by author:

The ore body is considered to be the oxidized and extensively contaminated hood zone of a sulphide vein that had a low copper content but must have carried appreciable pyrite. The sulphides had probably crystallised under mesothermal conditions and were subjected to subsequent deformation. The oxidation of the ore body took place mainly during an arid climatic period that is held responsible for the characteristic suite of secondary minerals. The vanadium mineralisation was one of the last events in the geologic history of the ore body and is separated from the sulphide mineralisation by a long period of time. Two generations of vanadinite and mimetite are distinguished in the Abenab West and surrounding deposits, the earlier vanadinite having undergone solution and extensive replacement by descloizite. Pseudomorphs and casts of several other minerals indicate that the oxidation of the ore body was characterised by a series of transformations and replacements due to changing supergene conditions. Slumping and deposition of bedded clay and sand played an important part in the formation of the ore body, both before and after the deposition of the vanadates. The alterations effected during the present-day humid oxidation period are insignificant compared with those effected beforehand

There is little doubt that the vanadates were deposited by supergene weathering solutions. New evidence of the conditions of deposition of the vanadates of the Otavi Mountain Land, including spectrographic data and the results of decrepitation tests, appears to support this conclusion. The most likely source of the vanadium is thought to be the shale horizons in the Otavi System, the leaching of vanadium having taken place during the arid oxidation period most probably from shale that was totally decomposed and afterwards removed by erosion

From a systematic mineralogic study of mine samples, three concentric vanadinite-descloizite zones could be distinguished in the ore body, with vanadinite predominating towards the centre and descloizite towards the periphery. These are superimposed on a twofold core of galena-cerussite, characterised by an increased lead content and a concomitant decrease of vanadium. Willemite has a well-defined marginal relationship