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Berg |
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British Columbia, Canada |
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Cu Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Berg deposit in west-central British Columbia, Canada, is related to a middle Eocene (49±2.4 Ma) stock which intrudes middle Jurassic Hazelton Group volcanics, some 14 km to the east of the Coast Plutonic Complex.
Regionally however, numerous stocks ranging in age from lower Jurassic to upper Eocene are found to the east of the Coast Plutonic Complex. The multi-phase Berg Stock commenced with porphyritic quartz-monzonite (adamellite), followed by quartz-plagioclase porphyry which formed a funnel shaped neck and a pipe through the former phase. These were in turn cut by a third phase comprising a plagioclase-biotite-quartz porphyry which formed a cylindrical sheath around the northern half of the original porphyritic monzonite. The fourth and final phase was of quartz-feldspar porphyry which cuts all of the foregoing types, largely as dykes. The composite stock is some 1500 x 300 to 700 m. A satellite breccia some 300 m to the south with dimensions of 600 x 150 m cuts both the quartz-diorite and the Hazelton hornfels. A few hundred metres to the east of the Berg Stock, a larger middle to upper Eocene quartz-diorite body is mapped, which is elongated north-south and has dimensions of 9 x 1 to 3 km. Within the district the Hazelton Group comprise mainly coarse to medium grained andesitic tuffs with subordinate flows and breccias, and a sedimentary component of coarse re-worked volcanic arenites (greywackes), with minor shale and siltstone (Panteleyev, et al., 1976).
Major mineralisation was associated with the plagioclase-biotite-quartz porphyry. A quartz-sulphide stockwork was developed in which molybdenite, chalcopyrite and pyrite were zonally deposited around the margins of the porphyritic quartz-monzonite (adamellite) and plagioclase-biotite-quartz porphyry bodies. Sulphide minerals are distributed in annular zones, which are vertical cylinders, coaxial with and partly overlapping with the two phases detailed above. The 0.2% Cu cutoff define an annulus 200 to 350 m wide, with an outer diameter of 1300 m and inner low grade to barren core of around 600 m. The >0.05% MoS2 molybdenum annulus is only 100 to 150 m wide and lies within the copper zone. A pyrite halo extends outwards from the stock, being traced over a distance of at least 700 m to the southwest where it is most extensive. The final stage of the stock, the quartz-feldspar porphyry, which is pyritic, post-dates the main mineralisation. Similarly the breccia pipe appears to post date the ore, and contains fragments of the mineralised lithologies (Panteleyev, et al., 1976).
Alteration takes two forms, an earlier hornfelsing, and later hydrothermal alteration. The hornfels ranges from biotite, to albite-epidote outwards from the stock, and is apparently more extensive and stronger than expected for a stock of this size. The hydrothermal alteration reflects the distribution of the mineralisation. In a generalised sense the alteration stages include: i). quartz-sericite-pyrite, forming a cylindrical zone overlapping the intrusive contact and border of the porphyritic quartz-monzonite (adamellite) and plagioclase-biotite-quartz porphyry phase, and extending irregularly outwards into the hornfels for up to 200 m; ii). kaolinite and secondary biotite, which are distributed throughout the entire stock to where it overlaps and melds with the quartz-sericite-pyrite zones and possibly into the hornfels for irregular distances of up to 450 m from the stock contact; much of the kaolinite associated with secondary biotite above the gypsum zone is of supergene origin; iii). propylitic zone, comprising an assemblage in chloritised hornfels rocks, extending outwards from the zone in which biotite is abundant (Panteleyev, et al., 1976).
Published reserve plus production figures include:
238 Mt @ 0.39% Cu, 0.03% Mo, 0.05 g/t Au (Res. 1984, Dawson, et al. 1991).
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1995.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Heberlein D R, Godwin C I 1984 - Hypogene alteration at the Berg Porphyry Copper-Molybdenum property, north-central British Columbia: in Econ. Geol. v79 pp 902-918
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