Depletion haloes in fresh rocks surrounding the Cobar orebodies, N.S.W., Australia: Implications for exploration and ore genesis

Abstract The orebodies at Cobar (central N.S.W., Australia) are steeply plunging lenses of considerable vertical extent, transgressive to bedding. They lie roughly parallel to the cleavage of turbidites in the Cobar Supergroup, within the confines of the Cobar Trough. Broadly there are Cu-Au and Pb-Zn mineralizations varying from aggregations of veins to submassive and massive types. A study was initiated to determine the nature and extent of any alteration haloes that might enlarge the drilling target in exploration for blind orebodies in the Cobar region. Results show that the mineralization is surrounded by unusually extensive depletions in Li and less extensive depletions in Na, K, Rb, Sr and Ba in the host metasediments. Apart from Li, these are co-extensive with zones marked by the virtual absence of feldspar and sericite and, proximal to ore, an increase in chlorite. The shapes and extent of the haloes suggest formation in vertical zones of high permeability by a process of fluid flow rather than diffusion. The mineralization, which is not accompanied by significant enhancement haloes of chalcophile elements, appears to have formed later, in discrete zones within the depletion haloes. A hypothesis for the formation of these haloes is proposed. Maturation of kerogens produced water-soluble carboxylic acids and/or phenols which in turn caused a secondary porosity in narrow, vertical zones overlying basement faults. Dewatering into a primitive cleavage took place during early diagenesis, leading to feldspar and mica dissolution by organic acids, transport of variable amounts of aluminium as an organic chelate and alkali and alkaline earth elements as soluble organic salts. Ore deposition probably took place later, just before the onset of greenschist metamorphism. It is speculated that deposition followed the release, upward movement and mixing of highly evolved and concentrated connate brines in the zone of earlier-formed dewatering cleavage, now focused into a tectonic cleavage.