Abstract:
The evolution of the ore-forming fluids in the Rooiberg tin-field has been reconstructed from their source, through the migration history and to the final site of ore deposition. The granites in the west and in the east of the Rooiberg Fragment have been mapped, sampled and chemically analysed. Four granite types are present: Nebo, Klipkloof, granophyric and Bobbejaankop Granite. The granites are considered to be high heat producing. The Bobbejaankop Granite is geochemically the most evolved and supplied the ore-forming fluids. Pb-Pb isotope analyses of ankerite yielded dates of 2181 - 2712 Ma, which are older than 2054 Ma of the Bobbejaankop Granite. These anomalous dates are ascribed to assimilation with extraneous Pb in the arkosite during fluid migration. A prolonged period of pervasive migration of the fluids took place through the arkosite, and was probably in the order of tens of million years. Feeder channels for the ore-forming fluids only attained importance at the site of ore-deposition when the fluids migrated from the arkosite into them. Alteration of the arkosite on a large scale represents the geochemical footprint of the ore-forming fluids. Polyphase mineralisation is manifested in at least four ways: 1. Ore deposition first took place in the pockets and subsequently in the lodes, but both types of ore-bodies were formed from the same ore-forming fluids, liberated as a single batch from their source, and residing in the arkosite for a long period of time; 2. Two pulses of ore-forming fluids led to two periods of tourmaline formation during the early stages of pocket mineralisation; 3. Early-formed cassiterite in the pockets was redissolved and redeposited as a later phase of high-grade mineralisation; and 4. A separate pulse of fluids was responsible for late stage development of ankerite in fractures.
