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 geology of the Thierry Cu-Ni mine, northwestern Ontario
The Thierry Cu-Ni mine is located in the Uchi Volcanic Belt, Superior Structural Province, 450 kilometres northwest of Thunder Bay, Ontario, near the town of Pickle Lake. The regional geology consists of an east-west-trending belt of Archean metavolcanic rocks, predominantly pilowed mafic flows with minor felsic volcanic and metasedimentary rocks. These have been intruded by granitoid stocks and mafic to ultramafic rocks that range in composition from gabbro to peridotite. Four main types of Cu-Ni sulfides are recogflized: breccia ore, mylonite ore, bornite ore and disseminated sulfides. The regional metamorphic grade is middle amphibolite facies; mineral and chemical data suggest that temperature and pressure reached approximately 600oC and 5.5 kbar. These rocks have undergone late dynarnic metamorphism, which produced a shear zone and a system of conjugate faults. Where mafic and ultramafic rocks were sheared, a mylonite was produced consisting of fragrnents of hornblende in a matrix of chlorite and biotite. Chemical alteration is associated with the dynamic metamorphism; whole-rock data indicate that Ca, Na and Si have been removed from the mylonite, and K was added. The sulfides are of magmatic origin, but their distribution within the mine was modified by dynamic metamorphism. Both mylonite and breccia ores were emplaced during the mylonitic event. The bornite ore was formed by the enrichment in copper of amphibolite-schist blocks in the mylonite.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.