Structural history and metamorphic modification of Archean volcanic-type nickel deposits, Yilgarn Block, Western Australia

Abstract

The Archean volcanic-type Fe-Ni sulfide ores of Western Australia occur at the base of relatively small, lenslike peridotitic flows or subvolcanic sills that occur beneath thick sequences of ultramafic flows. Massive ores, breccia ores, matrix ores, and disseminated ores occur in varying proportions in the major deposits.The economic volcanic-type ores known to date are confined to amphibolite facies metamorphic domains, commonly of dynamic style, although the important Kambalda deposits occur in a static-style environment. The orebodies have an essentially tabular form elongate subparallel to the penetrative linear fabrics in host rocks (where present) and/or to the trend and plunge of regional and parasitic folds. Locally, orebodies may be confined to footwall embayments or to prominent shear zones in the ore environment. The ores themselves have been metamorphosed and complexly deformed; most massive ores exhibit metamorphic layering and tectonite fabrics, breccia ores are common, and most contacts between massive ores and more disseminated ores appear to be tectonic boundaries.There are gross similarities in the structural-metamorphic histories of a number of deposits (Juan and Lunnon shoots, Kambalda; Windarra, Redross, and Nepean mines). All ores have undergone a complex interplay of largely ductile deformation and heating events during which massive ores were generated from more disseminated ores in some instances and were mobilized relative to less massive ores in all cases. Repeated deformation of ores is indicated by their crosscutting relations with late-metamorphic, largely posttectonic dikes. All ores reverted, at least in large part, to metamorphic monosulfide solid solution (mss) during the metamorphic climax; relict phases within mss included pyrite and spinels. Highly variable Fe/Ni ratios characterize ores that suffered largely premetamorphic deformation as pyrrhotite-pentlandite + or - pyrite aggregates, whereas ores deformed as essentially mss during the metamorphic climax are typified by constant Fe/Ni ratios. Stress-induced diffusion of Cu accompanied the deformation of massive ores under all metamorphic conditions, whereas sulfur diffusion via the vapor phase was common during waning metamorphism in some ores. Mechanical segregation of pyrite and spinels appears to have been important during deformation of mss-rich ores. Considerable modification, and possibly generation, of sulfide ores occurred in metasomatic reaction zones developed at the contact of the host ultramafic unit with footwall rocks in some ores, particularly those from dynamic-style environments.The gross structure of the ores records the major part of the ductile deformation of the sulfides, whereas their small-scale structures and textures record only deformation or annealing events at low metamorphic temperatures following unmixing of the Fe-Ni-Cu sulfides from metamorphic mss.This study demonstrates the significance of metamorphism in modifying preexisting sulfides and emphasises the importance of considering such effects before erection of models for the magmatic stage. Allowing for this metamorphic imprint, there is still considerable evidence favoring the prior existence of magmatic sulfides, although at least in some deposits they may have been less concentrated than the present ores. A remaining, as yet unexplained, anomaly is the variable mean Fe/Ni ratio between individual ore shoots, which contrasts with the relatively constant (magmatic?) mean Ni/Cu ratio of the same shoots.