Abstract

The Star and Island Lake plutons of the Central metavolcanic belt, La Ronge domain, host a variety of barren and auriferous quartz veins in northeast-trending shear zones (040 degrees 80 degrees NW). Shear zones are coincident with competency contrasts in the plutons, which may be attributed to the northeast-trending boundary of igneous phase changes, or the presence of northeast-trending dikes. Veins range from 30 cm to 2 m wide, sup to hundreds of meters long, and exhibit a discontinuous pinch and swell nature common to most shear zone-hosted mesothermal quartz veins. Barren veins are paragenetically simple, quartz being the dominant hydrothermal mineral with sparse microcline of variable origin. Muscovite, a common gangue mineral in auriferous veins, is rare in barren veins; in barren veins its occurrence is correlated with slightly elevated gold values.Quartz microtextures indicate incremental vein emplacement and deformation in a dynamic environment. Dynamic recrystallization (subgrain formation > recovery rate) is the dominant ductile deformation mechanism. Macro-and microfractures, preserved as veins and planar arrays of fluid inclusions, are a result of episodic brittle failure. The cyclic nature of these events suggests in turn fluid pressure cycling in the brittle-ductile transition.Regionally, veins have a bimodal distribution of delta 18 O quartz values with barren and auriferous veins occupying both populations. Excepting the Jasper mine, on both a regional and hand sample scale, there is no shift in isotopic composition of vein quartz with increased ductile deformation. In fact the majority of the veins are remarkably homogeneous with respect to delta 18 O quartz. Petrographically, fluid inclusions in barren veins are identical to those described in auriferous veins and exhibit temperatures of total homogenization similar to those of inclusions in auriferous veins. Compositionaly, fluid inclusions in barren veins have similar wt percent NaCl and bulk CO 2 as auriferous veins; however, an important distinction is that auriferous veins may have up to 30 mole percent CH 4 , a phase absent in barren veins. Similarities between auriferous and barren veins such as field relationships, microstructures, stable isotopes, and fluid inclusions are interpreted to indicate a broad temporal contemporanity of auriferous and barren fluids. Distinct mineral paragenesis and CH 4 content may represent different redox states of barren and auriferous fluids. Lack of reduced carbon species in barren veins may reflect an oxidizing fluid with few reduced sulfur ligands capable of transporting gold.

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