Sulfide-silicate textures in magmatic Ni-Cu-PGE sulfide ore deposits: Massive, semi-massive and sulfide-matrix breccia ores

摘要

Much of the value of magmatic Ni-Cu-PGE sulfide orebodies is contained within massive or semi-massive ores that show a wide variety of textural relationships to included or adjacent silicate rocks. We identify five mutually gradational textural types: (1) pure inclusion-free massive sulfide ores; (2) sulfide matrix ore breccias, of sharp-wall, soft-wall or mixed character; (3) emulsion textured ores formed by frozen mixtures of molten silicate and sulfide, most commonly developed as melt films at thermal erosion contacts; (4) vein-hosted sulfides formed at late magmatic or high temperature post-emplacement deformation stage close to the brittle-ductile transition in the country rocks or host igneous bodies; and (5) tectonic "durchbewegung" breccias, formed by mechanical inter-shearing of less-ductile silicate inclusions and more-ductile solid sulfides. Some deposits, the Moran Shoot at Kambalda being the type example, record the invasion of country rock footwall by downward- or sideways-percolating superheated molten sulfide liquid generating vertical sequences of pure massive sulfide, emulsion textured ores and finely-spaced invasive sulfide veins; these are referred to as sulfide melting-infiltration fronts and may provide a clue to the mechanism of formation of sulfide-rich magmatic ores as whole. Sulfide matrix ore breccias are particularly well developed in the Voisey's Bay and Aguablanca deposits, where they developed by flooding of percolating sulfide melt through the silicate matrix of magmatic intrusion breccias, displacing silicate melt. The lithology of the silicate or carbonate rock inclusions determines the nature of the inclusion-matrix relationships. Non-refractory inclusions typically disaggregate along original grain boundaries to leave coherent inclusions surrounded by clouds of inclusion-derived or matrix-derived crystals, with the low-melting silicate component preferentially displaced by sulfide liquid, whereas refractory inclusions retain sharp boundaries. Zonation of inclusions and overgrowths preserves reaction between inclusion and silicate matrix that pre-dates invasion of the intrusion breccia by sulfide liquid. The process of percolation of dense, low-viscosity sulfide liquid into pore space and fractures within partially molten (or melting) silicate rock is a unifying theme that links sulfide matrix ore breccias and emulsion textured ores with distinctive textures in less sulfide rich rocks such as net-texture (matrix ore texture), leopard texture (poikilitic net texture) and interspinifex ore. Vein-hosted massive sulfides may be emplaced under magmatic conditions where the excess pressure of the sulfide liquid column drives or enhances fracturing of the country rock and injection of sulfide into the cracks. Such veins are commonly referred to as "remobilised", a term which may obscure process understanding and should be reserved for cases where tectonic solid-state mobilisation of sulfide can be demonstrated on textural and structural grounds. The tendency of sulfide liquids to invade country rocks and potentially to drive the propagation of their own magmatic containers may be a critical feedback loop in the development of magmatic sulfide mineral systems.