METAMORPHISM AND SUPERGENE ALTERATION OF
Four principal types of sulfide ore are recognized at the Thiegry'mine, located in the Uchi volcanic belt (Superior Province), northwestern Ontario. These are: disseminated Cu-Ni-Fe sulfides, breccia ore, mylonite ore and bornite ore. The sulfides are interpreted to be of magmatic origin, but modified by strong shearing to produce mylonite and breccia ore. The bornite ore represents enrichment in Cu of blocks of amphibolite schist in the mylonite. Supergene alteration removed iron from the iron-nickel sulfides to produce iron oxides and sulfides, especially violarite enriched in cobalt and nickel. Violarite (Ni-Fe-rich) appears to coexist with pentlandite where it formed from pentlandite. Violarite (Ni-Co-rich) generated as the result of metamorphism does not coedst with pentlandite. This strongly suggests that cobalt plays a major role in defining the lowtemperature stability fields and explains the two conflicting interpretations of assemblages involving violarite in the system Fe-Ni-S. The modification, during deformation and metamorphism, of primary magmatic sulfides of subeconomic grade to an ore deposit at Thierry and other areas implies that an ulfiamafic-gabbroic body in an intensely metamorphosed and deformed terrane might offer a better target for er<ploration than ore in lower-grade rocks. Furthermore, the enrichment of platinum-group elements @GE) in mylonite and bornite ores at Thierry indicates that mobilization into shear zones could produce PGE deposits.
The Dumont serpentinite; a model for control of nickeliferous opaque mineral assemblages by alteration reactions in ultramafic rocks
Various distinctive nickel-rich minerals are commonly encountered in barren and weakly mineralized ultramafic rocks. The minerals include awaruite, heazlewoodite, pentlandite, millerite, and violarite but few, if any, iron sulfides. Using the Dumont ultramafic body in northern Quebec as an example, it is demonstrated that certain assemblages of these opaque minerals tend to be spatially associated with particular types of alteration, namely, serpentinization and talc-carbonate alteration. An analysis of nickeliferous opaque mineral assemblages on the one hand and alteration reactions on the other leads to a general model in which the former are controlled by the latter through Fe-related redox mechanisms. The model predicts that incipient serpentinization which generates H 2 should be accompanied by reduced assemblages characterized by low-sulfur minerals such as awaruite and heazlewoodite, while carbonate alteration which generates O 2 should be accompanied by oxidized assemblages characterized by high-sulfur minerals such as millerite. The general applicability of the model was tested by compiling bulk mineral assemblages from many locales, as reported by numerous investigators; the results showed a gratifying confirmation of the model.Extension of the model to consider higher sulfur contents was explored in order to explain the common assemblages found in conventional nickel deposits. These sulfide-rich occurrences, in contrast to the barren or weakly mineralized ultramafic rocks, consist mainly of iron sulfides (pyrrhotite + or - pyrite) in addition to pentlandite, and lack awaruite and heazlewoodite. The difference in assemblages can be explained by the model. Because of greater sulfide content and hence higher sulfur, (1) the sulfide assemblages are self-buffered with respect to oxygen and sulfur, therefore remaining largely unaffected by alteration reactions; and (2) the assemblages are required by compatabilities at high S:Ni ratios to include iron sulfides.An important consequence of the model is that serpentinization, by converting some silicate nickel into a sulfide form, may be economically significant in the case of low-grade deposits.
Local modification of rock chemistry by deformation
Metabasalts subjected to progressive deformation in large-scale shear zones at Yellowknife display corresponding changes in major element abundances. Deformation, under conditions of greenschist facies metamorphism, has involved grain size reduction from 1200 μm to <20 μm, depletion of SiO2 (≃5%) and Na2O, together with hydration, and a decrease in specific gravity from 2.97 to 2.80. Chemical redistribution by deformation has been accomplished through a decrease in grain diameter of quartz and albite by intercrystalline diffusive mass transport (pressure solution), with concomitant transfer of material into extension veins. The degree of chemical modification is related to the finite strain. Deformation has involved a redistribution of ∼7.1015g of SiO2 over a volume of about 50km3.The microstructure of an adamellite deformed in a shear zone at higher temperature, under conditions of amphibolite facies metamorphism is indicative of dominant dislocation creep. A low degree of tectonic grain refinement is present. Constant values of major element abundances and specific gravity determined across the shear zone at increasing states of strain imply isochemical and isovolumetric deformation. These results are taken to support the precept that crustal deformation is characterised by a low temperature deformation regime dominated by pressure solution, with local changes of rock chemistry and volume; and a high temperature regime in which strain is accommodated principally by dislocation creep, an isochemical and isovolumetric deformation mechanism.