Genesis of the Lass vein system, Beaverdell silver camp, south-central British Columbia

Abstract

The Lass vein is in the Beaverdell silver, lead, zinc (gold) vein camp in south-central British Columbia. Veins in this camp are generally hosted within propylitized Westkettle granodiorite of Jurassic age, but mineralization is related to the Beaverdell quartz monzonite stock of Late Paleocene age (based on a K–Ar biotite date of 58.8 ± 2.0 Ma). Galena lead isotopes, interpreted using the recent "shale," "Bluebell," and "mixing-line isochron" models for the Canadian Cordillera, confirm a Tertiary age for all major vein mineralization in the Beaverdell camp.Examination of metal zoning, mineralogy, fluid inclusions, and sulfur isotopes indicates that the Lass vein system can be divided into two distinctly different parts, an upper western portion and a lower eastern portion. Differences between the two parts are related to the dominance of one of two mineralizing events.Event 1, the earlier, was most dominant in the lower portion of the Lass vein system. By comparison with event 2, event 1 is characterized by (i) relatively gold-, zinc-, and lead-rich but silver-poor ore (Au, 3163 ppb; Zn, 5.34%; Pb, 2.34%; Ag, 208 ppm); (ii) thicker veins (20 cm); (iii) sulfides with abundant pyrite, arsenopyrite, and dark sphalerite with exsolved chalcopyrite; and (iv) fluid inclusions with higher salinities (15 wt. equiv. wt.% NaCl), local CO2 phases, and higher temperatures of homogenization (287 °C) with matching equilibrium temperatures indicated by sulfur-isotope geothermometry from galena and sphalerite (294 °C).The younger event 2 is responsible for most of the mineralization in the upper portion of the Lass vein. Many characteristics of this event are statistically different from those associated with event 1. Namely, event 2 (i) is relatively silver rich and gold, zinc, and lead poor (Ag, 291 ppm; Au, 764 ppb; Zn, 3.07%; Pb, 1.27%); (ii) has narrower veins (10 cm); (iii) has sulfides characterized by silver–sulfosalt-bearing galena and pale sphalerite; and (iv) is represented by fluid inclusions that are variable but on average lower in salinity (7 equiv. wt.% NaCl), have lower temperatures of fluid homogenization (225 °C), and do not contain CO2 phases.Estimates from fluid inclusions indicate that event 1 could have occurred at depths equivalent to those of event 2 if the former was under lithostatic pressure at temperatures near the boiling point and if the latter formed at hydrostatic pressures near boiling temperatures. Sulfur-isotope data indicate that event 1 was nearly boiling; no similar definition is available for event 2. Thus, only minimum depth estimates are available for event 2.The most likely scenario for formation of the Lass vein system starts with event 1 and is followed by event 2. During event 1, minerals were deposited in a confined system under lithostatic pressures from high-salinity fluids where chloride complexing could have been important in the transportation of gold. Fracturing of the vein system to the surface changed the pressure regime to hydrostatic, and CO2 was released. Consequently, temperatures and salinities of the fluids dropped, presumably mainly in response to mixing with cooler and less saline groundwater. Thus, gold solubilites concomitantly decreased, and silver deposition became more significant.Fluid-inclusion and lead-isotope analyses appear to be useful exploration procedures in the Beaverdell area in the identification of gold-rich systems. A further practical observation is that gold should continue to depth in the Lass system, if the vein can be followed.