Characteristics of semi-conformable alteration zones associated with volcanogenic massive sulphide districts

Abstract

Semi-comformable alteration zones are present within the footwall stratigraphy of base metal horizons hosted within various volcanic environments. In large-scale, subaqueous, continental rifts (Bergslagen and Iberian Pyrite Belt mining base-metal districts), semi-conformable alteration zones may be tens of kilometres in strike length and hundreds of metres thick. In base-metal camps hosted within smaller tectonic depressions (oceanic rifts and cauldrons) the semi-conformable alteration zones are of smaller scale, but still affect much of the volcanic stratigraphy within the boundaries of the down-faulted terrane. The semi-conformable alteration systems consists of vertically stacked zones that superficially resemble regional metamorphic facies. Their formation includes chloritization, spilitization, silicification and epidotization. Chemical gradients present across these zones indicate that their formation is a result of a series of simultaneous and consecutive metasomatic reactions between seawater and the volcanic pile that take place at progressively higher temperatures with depth in the stratigraphy. The isotherms controlling the reactions are parallel to the sub-horizontal upper contact of the underlying synvolcanic intrusion. The boundaries 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° to 140°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 the accumulation of regional-scale Fe-rich chemical sediments. Further downwards circulation of a chemically evolving seawater results in Na-Mg enrichment of the rocks at moderate temperatures (140° to 300°C), followed by an Na-enriched greenschist metasomatic facies (300° to 400°C). Reactions at the greenschist-amphibolite metasomatic boundary include intense leaching of base-metals and the silicification of the rocks. The permeability reduction due to silicification isolates the underlying amphibolite metasomatic facies rocks to form a Ca-Fe enriched, base-metal rich reservoir zone. Periodic breaching of the reservoir allows metal-rich fluids to rise to the seafloor to form massive sulphide deposits. The recognition of the chemical gradients across these semi-conformable alteration zones will allow explorationists to more easily target and locate massive sulphide horizons.