Abstract

Fluorite-hematite mineralization on the farms Kromdraai and Naaupoort near Rust de Winter is of volcanogenic origin and is related to a rhyolitic pyroclastic suite and associated ore deposits. These include the Vergenoeg mine orebody, which is a discordant breccia pipe vent set within the uppermost Rooiberg Group rhyolites, and several ore deposits which are interstratified concordantly within the pyroclastic suite. At surface the Vergenoeg orebody is made up of an oxidized gossan, which consists of massive to porous hematite-goethite cementing pelletal fluorite. Extensive secondary leaching and hydration-dehydration has led to classical colloidal textures. The gossan is a replacement after magnetite and siderite. Below the oxidation zone the ore consists of fresh magnetite-siderite-grunerite-fluorite and sulfides. Local vertical fracture zones exist with a prominent north-northwest orientation. High-grade "metspar" bodies of massive to coarsely crackle brecciated fluorite, veined by a ramifying network of hematite after siderite-magnetite, form elongated plugs within these fracture zones. Numerous near-vertical fissure dikes, also striking north-northwest and consisting of similar metspar originate from the apical part of these plugs. The contact of the massive ore and the metspar plugs and dikes is marked by intervening intrusive fluidized tuffisite. Trace and accessory minerals consist of rare earth phosphates and carbonates, cassiterite, and sulfides such as pyrite, pyrrhotite, and minor sphalerite, chalcopyrite, bornite, and galena.The Plattekop orebody is a fines-depleted, froth flow channel lag deposit, floored by the basal silicic, fines-depleted agglomeratic, lag-welded tuff and is covered by the fines-enriched, ash-fall tuff and agglomerate. The orebody is thus concordantly interstratified within the pyroclastic rock suite. It displays fine horizontal bedding and contains glass tuff lenses and bomblike vent ejecta. This unit probably represents a basal lag-surge ignimbrite. The more distal Naauwpoort orebody represents a conglomeratic placer fan deposit with overlying banded iron-formation. Petrographic evidence of shard and devitrified glass is abundant. All the evidence points to a close genetic relationship between the ore deposits and the pyroclastic rock suite.Vergenoeg is the end member of a series of genetically related fluorite-hematite deposits which are associated with the Bushveld granites. Rapid pressure release from centripetally fractionated Bushveld granite cells, due to structural failure of the roof, led to an iron-rich F-CO 2 -type exhalative or phreatic vent eruption. This interpretation is supported by the experimental petrological data of Ernst (1968).

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