Diapiric structures and regional compression in an Archean greenstone belt, Island Lake, Manitoba

The sequence of structures affecting rocks of the Island Lake greenstone belt suggests a progression from structures attributable to emplacement of granitic plutons to those that are of regional extent. Early homoclines (D0 deformation) and isoclinal folds (D1) are overturned or face away from marginal plutons which partly truncate the folds. Granitic offshoots display an S1 foliation. Syn-D1 emplacement of the granite is implied, and the pattern of homoclines and folds is consistent with deformation due to diapiric uplift.Later upright folds (D2) and a widespread subvertical schistosity S2 trend eastward throughout most of the region. S2 passes with little deflection into the granites. D1 folds are rotated towards S2 and clasts and volcanic vesicles are flattened, except within a zone of low strain extending 15 km east–west between two plutons. Evidently during D2 deformation the plutons acted as relatively rigid massifs between which a large 'pressure shadow' developed in response to regional north–south compression.

Mineralogy and textural relationships of ores from the Whalesback Mine, northeastern Newfoundland

The Whalesback Mine is one of many copper deposits associated with Ordovician volcanic rocks in the Notre Dame Bay area, Newfoundland. The deposit consists of veins, pods, and disseminated sulfides localized within a highly chloritized shear zone cutting basaltic pillow lavas. Porphyritic dikes cut the shear zone, sulfide deposit, and the surrounding pillow lavas; all of the rocks, including the sulfide-rich rocks, have been regionally metamorphosed. Ore minerals, in decreasing order of abundance, include pyrite, chalcopyrite, pyrrhotite, sphalerite, mackinawite, pentlandite, magnetite, cubanite, galena, and ilmenite. Marcasite, covellite, and goethite are supergene minerals. Chlorite and quartz are the predominant gangue minerals. Muscovite, carbonates, sphene, albite, and epidote are minor constituents. Banding and streaking of sulfides in massive ores, crushed pyrite, and the local occurrence of pressure-shadow phenomena in the ore are indicative of shearing stress post-dating original sulfide ore formation. Present sulfide assemblages are compatible with relatively low temperatures and are the result of re-equilibration and internal reaction among the sulfides with decreasing temperature.