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.
Chemical Composition and Stability of Violarite
Violarites of the general composition (Fe, Ni) 3 S 4 from a variety of natural Fe-Ni sulfide assemblages have been studied by electron microprobe analysis. The violarite solid solution extends from the Ni-rich end member polydymite to at least 17 at. percent Ni, compositions more Fe-rich than FeNi 2 S 4 being characteristic of pyrrhotite-bearing assemblages and compositions more Ni-rich than FeNi 2 S 4 of Ni-rich assemblages that do not contain pyrrhotite. Most of the violarites have compositions in the range of 56 to 57 at. percent S and thus are slightly S-deficient relative to the ideal M 3 S 4 composition.The predominant mode of formation of violarite is by alteration of pentlandite. Violarite also forms from millerite, but alteration of pyrrhotite to violarite has not been confirmed in the present study. A single crystal X-ray diffraction study of a partially violaritized pentlandite grain has shown that violarite has the same crystallographic orientation as the host pentlandite and the alteration can be effected by removal of excess metal atoms from pentlandite and redistribution of the remainder. It is suggested that violarite forms readily from pentlandite because of the ease with which the pentlandite structure can be converted to that of violarite.The stability field of violarite in the Fe-Ni-S system must enlarge to more Fe-rich compositions, toward the Fe-S join, as the temperature decreases. Likely phase relations at temperatures sufficiently low to include smythite and the extension of the violarite field to 17 at. percent Ni have been schematically presented.
High‐precision U‐Pb zircon age calibration of the global Carboniferous time scale and Milankovitch band cyclicity in the Donets Basin, eastern Ukraine
High‐precision ID‐TIMS U‐Pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic, cyclostratigraphic, and biostratigraphic framework of the Carboniferous Donets Basin of eastern Europe. Chemical abrasion of zircons, use of the internationally calibrated EARTHTIME mixed U‐Pb isotope dilution tracer, and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of <0.05%, or ∼100 ka, for these Carboniferous volcanics. This precision allows the resolution of time in the Milankovitch band and confirms the long‐standing hypothesis that individual high‐frequency Pennsylvanian cyclothems and bundles of cyclothems into fourth‐order sequences are the eustatic response to orbital eccentricity (∼100 and 400 ka) forcing. Tuning of the fourth‐order sequences in the Donets Basin to the long‐period eccentricity cycle results in a continuous age model for the Middle to Late Pennsylvanian (Moscovian‐Kasimovian‐Ghzelian) strata of the basin and their record of biological and climatic changes through the latter portion of the late Paleozoic Ice Age. Detailed fusulinid and conodont zonations allow the export of this age model to sections throughout Euramerica. Additional ages for Mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations.