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 Colu~nbia. 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 f 2.0 Ma). Galena lead isotopes, interpreted using the recent "shale," "Bluebell," and "mixing-line isochron" models for the Canadian Cordillera. confim~ 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 I , the earlier, was most dominant in the lower portion of the Lass vein system. By coinparison 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 CO, phases, and higher temperatures of homogenization (287°C) with matching equilibrium temperatures indicated by sulfur-isotope geothernlometry 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 em); (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. 76 NaCl), have lower temperatures of fluid homogenization (225"C), and do not contain CO, 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 fonned 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 fornlation 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 COz was released. Consequen...

Penetration of surface-evaporated brines into the Proterozoic basement and deposition of Co and Ag at Bou Azzer (Morocco): Evidence from fluid inclusions

“…As silver is transported more efficiently in high salinity NaCl fluids, as chloride complexes, the subsequent decrease in the fluid chlorinity results in the destabilisation of Agchloride complexes, because of the decrease in the ionic strength of the solution. Similar mechanisms have been described by Godwin et al (1986) for the Ag-(Co) Beaverdall deposit British Columbia, by Marshall et al (1993) at Cobalt and by Essarraj et al (1998) for the Zgounder Ag-deposit, or Imiter deposit (Baroudi et al, 1999) in the Anti-Atlas.…”

“…In both deposits: (i) veins cross-cut metamorphosed Precambrian rocks, (ii) the first Co-Ni sulphur-arsenides association is followed by silver ores, (iii) the average silver grade is similar (500-600 g/t Ag) and (iv) a K-feldspar-albite hydrothermal alteration affects the wall rocks (Leblanc and Lbouabi, 1988). At Cobalt-Gowganda, the Ag-deposit is interpreted to 0899 result from the cooling and/or dilution of brines (Godwin et al, 1986;Marshall et al, 1993). At Bou Azzer, Leblanc and Lbouabi (1988) have associated silver deposition with K-Na metasomatism at an inferred temperature between 200 and 350°C and a relatively high pressure of around 200-300 MPa.…”

Basinal Brines at the Origin of the Imiter Ag-Hg Deposit (Anti-Atlas, Morocco): Evidence from LA-ICP-MS Data on Fluid Inclusions, Halogen Signatures, and Stable Isotopes (H, C, O)

“…At Imiter, the Ag concentration in the brine is up to 30 mmol/kg (3,200 ppm, avg 900 ppm) and agrees with the experimental data from Pal'ynova (2008). The decrease in chlorinity due to dilution results in the destabilization of the Ag chloride complexes and subsequent Ag deposition (the Ag and Na contents are correlated in the brines), as suggested for numerous other Ag deposits (Beaverdall deposit in British Columbia, Godwin et al, 1986;Cobalt, Ontario, Marshall et al, 1993; and from the Anti-Atlas in Morocco-Zgounder Ag deposit, Essarraj et al, 1998, andthe Bou Azzer Co-Ni-Ag deposit, Essarraj et al, 2005, among others). Mixing processes were also proposed for the Pb-Zn-(Ag) deposits in the Irish Midlands (Wilkinson, 2010b), the Alaskan Brooke Range (Leach et al, 2004), the French Pb-F-Ba deposits from the Massif Central (Boiron et al, 2002), and Upper Silesia (Poland) (Heijlen et al, 2003), among others.…”

Genesis of porphyry and plutonic mineralisation systems in metaluminous granitoids of the Grampian Terrane, Scotland

“…Field & Gustafson 1976), and from plutonic-hosted Ag and/or Au-bearing veins in Western America (e.g. Godwin et al 1986), andW-bearing veins, mostly in Korea (e.g. So et al 1983).…”