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The Salmo lead-zinc deposits : a study of their deformation and metamorphic features MacDonald, Alan Stratton


The problematic history of the strata-bound Salmo lead-zinc deposits, which occur in highly deformed, low-grade metasediments of the southern Kootenay Arc of British Columbia, has been investigated via structural analyses, fabric studies, and minor element determinations in three of the deposits: Reeves MacDonald, Jersey and H. B. mines. Three phases of folding have been distinguished: an early phase of near-isoclinal folding, overturned toward the west, produced the major structures; a second phase of upright, more open folding modified the first phase folds by near coaxial refolding, by further closure and local flattening, and ultimately by westward-directed thrusting; a third phase of conjugate mono-clinal folds and kink bands was produced by north-south compression related to northward-directed thrusting. The essentially tabular sphalerite-pyrite-galena ore bodies are involved, on all scales, in the folding and the ores exhibit internal structures ascribed to the differential movement of sulphides and host dolomite and calcite marbles. Regional metamorphism, to lower greenschist facies, was synchronous with Phase 1 folding. Contact metamorphism by granite stocks (K-Ar age: circa 100 m.y. B.P.) postdates all phases of folding, and affects Jersey and H. B. deposits, and possibly also Reeves MacDonald; estimated temperatures are in the range 425-600°C (at 1.5 kb). Seventy-one analyses of minor elements in the sulphides show that pyrite has Co:Ni<l, and that sphalerite has increasingly high Fe, Mn and Cd contents with increasing grade of contact metamorphism, except for an anomalous enrichment in Fe, Cd, Cu and Ag with depth in Reeves MacDonald mine. Pyrite retains brittle deformation textures until hornblende hornfels facies is attained when recrystallization becomes increasingly important, and ultimate breakdown to pyrrhotite occurs. Sphalerite and galena typically have granoblastic-polygonal recrystallization textures but exhibit widespread slight lattice bending and subgrain development, believed to postdate annealing recrystallization; local deformation twinning and kinking, recognized only in sphalerite, may pre-date annealing recrystallization. X-ray fabric analyses of sphalerite, mainly from Reeves MacDonald mine, show well developed preferred orientation of (111) parallel with composition layering, attributed to syntec-tonic recrystallization. Sphalerite from Jersey mine has more varied (111) subfabrics showing development toward small-circle patterns of [ill] which probably represent annealing fabrics. Mylonitic sphalerite, from thin zones in Jersey mine, has a distinctive (111) subfabric with orthorhombic symmetry and a pattern approaching (110) [001] . Galena, which has variable mylonitic and recrystallized textures, has either a random fabric, or weak (200) subfabrics which may reflect plastic deformation on the system (001) [110]. Quartz c-axis subfabrics and orientations of deformation lamellae (determined optically) suggest that, in quartzitic rocks, syntectonic recrystallization occurred during Phase 1 folding, whereas plastic deformation, and at least local recrystallization (in fine-grained material), was produced essentially by flattening during Phase 2, and possibly during Phase 3 folding. c-axis subfabrics in host dolomite marbles may also be indicative of recrystallization during flattening. It is concluded that the sulphides exhibit structures, on all scales, equivalent to those recognized in the host rocks and that these indicate involvement in all phases of deformation, in regional metamorphism, and in contact metamorphism. The deposits are interpreted as being originally of Mississippi Valley type.

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