CHARACTERISTICS OF SEMI-CONFORMABLE ALTERATION ZONES ASSOCIATED WITH VOLCANOGENIC MASSIVE SULFIDE DISTRICTS

Semi-comformable alteration zones are present within the footwall stra tigraphy of base metal horizons hosted within various volcanic environ ments. In large-scale, subaqueous, continental rifts (Bergslagen and I berian Pyrite Belt mining base-metal districts), semi-conformable alte ration zones may be tens of kilometres in strike length and hundreds o f metres thick. In base-metal camps hosted within smaller tectonic dep ressions (oceanic rifts and cauldrons) the semi-conformable alteration zones are of smaller scale, but still affect much of the volcanic str atigraphy within the boundaries of the down-faulted terrane. The semi- conformable alteration systems consist of vertically stacked zones tha t superficially resemble regional metamorphic facies. Their formation includes chloritization, spilitization, silicification and epidotizati on. Chemical gradients present across these zones indicate that their formation is a result of a series of simultaneous and consecutive meta somatic reactions between seawater and the volcanic pile that take pla ce at progressively higher temperatures with depth in the stratigraphy . The isotherms controlling the reactions are parallel to the sub-hori zontal upper contact of the underlying synvolcanic intrusion. The boun daries of these reaction zones are therefore semi-conformable with the volcanic stratigraphy. The semi-conformable alteration zones form by seawater-rock reactions that take place along the downwelling segment of a hydrothermal fluid convection system. Low temperature reactions ( 50-degrees to 140-degrees-C) take place in the shallow subseafloor to form a Mg-K enriched zeolite metasomatic facies. Diffuse hydrothermal discharge associated with this early stage of alteration results in th e accumulation of regional-scale Fe-rich chemical sediments. Further d ownwards circulation of a chemically evolving seawater results in Na-M g enrichment of the rocks at moderate temperatures (140-degrees to 300 -degrees-C), followed by an Na-enriched greenschist metasomatic facies (300-degrees to 400-degrees-C). Reactions at the greenschist-amphibol ite metasomatic boundary include intense leaching of base-metals and t he silicification of the rocks. The permeability reduction due to sili cification isolates the underlying amphibolite metasomatic facies rock s to form a Ca-Fe enriched, base-metal rich reservoir zone. Periodic b reaching of the reservoir allows metal-rich fluids to rise to the seaf loor to form massive sulphide deposits. The recognition of the chemica l gradients across these semi-conformable alteration zones will allow explorationists to more easily target and locate massive sulphide hori zons.