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Genesis and zoning of silver-gold veins in the Beaverdell area, South-Central British Columbia

University of British Columbia

The Beaverdell silver, gold, lead, zinc, vein camp is located approximately 88 km south of Kelowna, in south-central British Columbia at 49.43° north latitude and 119.06° west longtitude. The camp has been a silver producer since the turn of the century and some gold was produced in the early part of the century. This thesis examines the deposits in the regional area and examines, in detail, zoning in the Lass vein system on Wallace Mountain, and represents the first comprehensive study of zoning and genesis of the veins. Galena-lead isotopes are examined within the regional setting of the deposits. Fluid inclusion, sulphur isotope, mineralographic and major and minor element zonation studies yield definitive information about the genesis of the deposits. Granodiorite of the Westkettle batholith, probably Jurassic, underlies much of the area and has been Intruded by stocks of Tertiary quartz monzonite, such as the Beaverdell stock. Remnants of pendants and/or screens of Wallace Formation metamorphosed volcanic and sedimentary rocks, believed to be Permian, are contained in the granodiorite. Silver mineralization occurs in the Beaverdell mines on Wallace Mountain mainly within the West-kettle batholith. Numerous showing and old workings of silver and/or gold mineralization are found throughout the surrounding region. The gold-bearing veins at Carmi contain a different mineral assemblage than the silver-bearing veins on Wallace Mountain. Galena-lead analyses of samples collected throughout the region fall into two distinct clusters on the ²º⁶Pb/ ²º⁴Pb versus ²⁰⁷Pb/ ²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁸Pb/²⁰⁴Pb diagrams. The first group is represented by the Carmi gold veins and the second by the Beaverdell silver veins. Models for the generation of lead in these deposits used Permian (0.27 Ga), Jurassic (0.15 Ga) or Tertiary (0.05 Ga) ages of mineralization on the basis of geological and K-Ar data. The model that is believed to be the best approximation of the system that formed these deposits assumes that the two groups of deposits formed at different times, under markedly different geological conditions. The parameters of this model indicate that: 1. the Carmi-type, gold-bearing vein mineralization is probably Jurassic and formed as a result of the intrusion of the Westkettle batholith, with the metamorphosed Wallace Formation as the probable lead source; 2. the Beaverdell-type, Silver-bearing vein mineralization is probably Tertiary and can be linked genetically to intrusions of that age, such as the Beaverdell stock; 3. ore fluid flow direction for the solutions that formed the Beaverdell-type mineralization was outward through the Westkettle batholith, away from the Beaverdell stock. Within the Lass vein system on Wallace Mountain, a distinctive, depth related, east-west zonation pattern in Au, Ag, Pb and Zn can be defined. Many of the other 11 elements analysed (Cu, Fe, Mn, Cd, Ca, Mg, Co, Ni, Hg, As, Sb) also show this pattern. Two zones are defined. The deeper portions of the orebody (at the east end of the vein system) contain high gold values, low silver values, and moderate to high zinc and lead values. High silver values, accompanied by moderate lead and zinc values, are found at a higher elevation in the system, in the western part of the vein system. Veins in the lower section have a greater average thickness than those in the western, upper section, and generally contain less gangue material. Fluid inclusions in sphalerite and quartz samples from the Lass vein system can be divided into three groups based on their homogenization temperatures. These are: Group 1: primary inclusion (with and without CO²), formed between 260°C and 310°C, from solutions with an average of 13 equivalent weight percent NaCl; Group 2: pseudosecondary inclusions formed between 230°C and 260°C, with salinities from 0.6 to 14 equivalent weight percent NaCl; Group 3: pseudosecondary and secondary inclusions formed between 180°C and 220°C, from solutions containing 0.4 to 14 equivalent weight percent NaCl. Arithmetic means of salinities for pseudosecondary, and secondary inclusions are, respectively, 8 and 6 equivalent weight percent NaCl. Sulphur isotope thermometers calculated for sphalerite-galena pairs (268°C to 320°C) are in close agreement with temperatures of homogenization of primary fluid inclusions. Seven stages of mineral paragensis can be recognized in the Lass vein system. Therfirst three stages (pyrite, arsenopyrite and dark sphalerite) are associated with the higher temperature, higher salinity, CO₂-bearing, primary inclusions. Pseudosecondary and secondary inclusions appear to be related to stages 4 to 6, which consist of galena, paler sphalerite, silver minerals and late quartz. Estimated depths of formation, based on a system under hydrostatic pressure, fall into two groupings. The minimum estimated depths of formation for primary, group 1 inclusions average 720 m, while depths calculated for groups 2 and 3 overlap in range, with-averages of 370 m and 175 m. The model-developed to explain the formation of this orebody accounts for the major and minor element zonation in the vein, the decreasing temperature, salinity and pressure (depth), and the loss of CO2 from the ore-forming fluid. The model explains two spatially distinct areas of mineralization represented by: 1. a zone of high temperature, high salinity, .arid" moderate pressure below a throttling point; and 2. a lower temperature, low salinity area caused by ground water mixing on the lower pressure side of the throttling point. CO₂ is present in the system below the throttling point, but is not found in any inclusions in Groups 2 and 3, on the lower temperature side of the throttling point. The association of CO₂ with gold deposition, suggests that gold would be expected in those areas where CO₂ is present in some of the inclusions. The definition of these two zones is critical for exploration. High silver values would not be expected to reappear further at depth to the east of the present workings, because this type of mineralization would only occur above the throttle point. Gold mineralization can be expected to continue for some time at depth if this model holds true. The abrupt change from the gold to the silver zone represents the throttle point in this model, and is highly visible in the major and minor element distribution patterns for the Lass vein system. Several different analytical procedures have been shown to differentiate between the two types of vein mineralization in the Beaverdell area. The use of these methods for exploration and development would allow the determination of key parameters concerning mineralization prior to extensive development of a showing or property. The level within the hydrothermal system, and therefore the type of ore expected can be determined by fluid inclusion studies for the younger, Tertiary veins. The age of vein mineralization, and therefore, the type of mineralization, also can be predicted by the use of galena-lead isotope ratios.

Text

2010-03-26

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http://hdl.handle.net/2429/22691

Geological Sciences

University of British Columbia

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