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.
Supergene alteration of the Mt Windarra nickel sulphide ore deposit, Western Australia
Three main zones of progressive oxidation, termed the transition, violaritepyrite and oxide zones, can be delineated in the supergene profile of the Mt Windarra massive/matrix ore deposit. In the broad transition zone from pure primary ore, pentlandite is progressively oxidised to an iron rich violarite of composition Co0.02Fe1.38Ni1.60 S4, releasing Fe2+ and Ni2+ ions into solution. Up to 43% of this Ni2+ moves to nearby pyrrhotite margins which are replaced firstly by nickeliferous smythite and then by a second lamellar-textured violarite with an even higher iron content but lacking in cobalt (approximately Fe1.6Ni1.4S4). On completion of violaritisation of the pentlandite, violaritisation of the pyrrhotite also ceases and the remainder of the pyrrhotite is rapidly replaced by secondary pyrite/marcasite, siderite and void space, this reaction defining the top of transition zone. Both sulphur and nickel are extracted from solution and further Fe2+ ions are released into solution. The violarite-pyrite zone is characterised by the absence of pentlandite and pyrrhotite and continued stability of violarite and secondary iron disulphides. Most, if not all, of the iron generated by these oxidation reactions precipitates as magnesian siderite at the expense of magnesite, giving rise to solutions containing mainly Mg2+ and Ni2+ ions. At and just above the water table atmospheric oxygen is reduced while the sulphides are oxidised to sulphate and hydroxides. Much of the iron remains in situ as characteristic goethite relicts while nickel and copper are leached, producing the enrichment below the water table. The overall genetic model proposed is electrochemical and is analogous to the corrosion of a piece of metallic iron partially immersed in differentially aerated water.
Dolomite-magnesian calcite relations at elevated temperatures and CO2 pressures
Abstract The equilibrium thermal decomposition curve of dolomite has been determined up to a CO2 pressure of 20,000 lb/in.2, at which pressure dolomite decomposes at 857°C. Equilibrium was approached from both directions, by the breakdown and by the solid-state synthesis of dolomite. At elevated temperatures and pressures, calcites in equilibrium with periclase as well as those in equilibrium with dolomite contain Mg in solid solution. In the former, the Mg content increases with increasing CO2 pressure, and decreases with increasing temperature. In the latter, it is a function of temperature only. The exsolution curve of dolomite and magnesian calcite has been determined between 500° and 800°C; at 500° dolomite is in equilibrium with a magnesian calcite containing ~6 mol per cent MgCO2; at 800°, ~22 mol per cent. There appears to be a small but real deviation from the ideal 1 : 1 Ca : Mg ratio of dolomite, in the direction of excess Ca, for material in equilibrium with magnesian calcite at high temperature. The experimental findings indicate that very little Mg is stable in the calcites of sedimentary environments, but that an appreciable amount is stable under higher-temperature metamorphic conditions, if sufficient CO2 pressure is maintained.