CHARACTERISTICS OF BARREN QUARTZ VEINS IN THE PROTEROZOIC LA RONGE DOMAIN, SASKATCHEWAN, CANADA - A COMPARISON WITH AURIFEROUS COUNTERPARTS

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: zon es are coincident with competency contrasts in the plutons, which may be attributed to the northeast-trendng boundary of igneous phase chang es, or the presence of northeast-trending dikes. Veins range from 30 c m to 2 m wide, up to hundreds of meters long, and exhibit a discontinu ous pinch and swell nature common to most shear zone-hosted mesotherma l quartz veins. Barren veins are paragenetically simple, quartz being the dominant hydrothermal mineral with sparse microcline of variable o rigin. Muscovite, a common gangue mineral in auriferous veins, is rare in barren veins; in barren veins its occurrence is correlated with sl ightly elevated gold values. Quartz microtextures indicate incremental vein emplacement and deformation in a dynamic environment. Dynamic re crystallization (subgrain formation > recovery rate) is the dominant d uctile 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 fl uid pressure cycling in the brittle-ductile transition. Regionally, ve ins have a bimodal distribution of delta(18)O quartz values with barre n and auriferous veins occupying both populations. Excepting the Jaspe r mine, on both a regional and hand sample scale, there is no shift in isotopic composition of vein quartz with increased ductile deformatio n. In fact the majority of the veins are remarkably homogeneous with r espect to delta(18)O quartz. Petrographically, fluid inclusions in bar ren veins are identical to those described in auriferous veins and exh ibit temperatures of total homogenization similar to those of inclusio ns in auriferous veins. Compositionaly, fluid inclusions in barren vei ns have similar wt percent NaCl and bulk CO2 as auriferous veins; howe ver, an important distinction is that auriferous veins may have up to 30 mole percent CH4, a phase absent in barren veins. Similarities betw een auriferous and barren veins such as field relationships, microstru ctures, stable isotopes, and fluid inclusions are interpreted to indic ate a broad temporal contemporanity of auriferous and barren fluids. D istinct mineral paragenesis and CH4 content may represent different re dox states of barren and auriferous fluids. Lack of reduced carbon spe cies in barren veins may reflect an oxidizing fluid with few reduced s ulfur ligands capable of transporting gold.