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Granisle |
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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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Granisle had published reserve and production figures of:
52 mt @ 0.41% Cu, 0.12 g/t Au (Prod.+Res. 1984, incl. Prod. 52 mt, 1966-82, Dawson, etal. 1991).
Geology - The Granisle Cu-Au deposit is associated with a series of Eocene age 'Babine-type' porphyry intrusions which cut lower Jurassic volcanic and sedimentary rocks of the Hazelton Group (see Figure 38). In the mine area the Hazelton Group comprises two distinct members. The first of these is composed of green to purple water-lain andesitic tuffs and breccias with intercalated chert pebble conglomerates, overlain to the west by massive amygdaloidal andesite and intercalated thinly bedded shales (Fahrni, Kim, etal., 1976).
Copper mineralisation is associated with a series of Eocene porphyry intrusions which occur in the central part of the small island on which the orebody occurs. The oldest of these is a vertical cylindrical, NE-SW elongated elliptical plug, of grey, finely porphyritic quartz-diorite with plan dimensions of the order of 300 x 500 m, which is locally referred to as the 'earlier porphyry phase'. It is commonly a micro-porphyry, with 1 mm phenocrysts of zoned andesine set in a fine grained quartz-plagioclase-biotite matrix. The largest and most prominent phase is an adjacent, similarly aligned, 120 to 200 m wide dyke which is several kilometres long. It is composed of biotite-feldspar porphyry, and is cut by a number of 'inter-mineral' and 'nearly post mineral' dykes of very similar composition. These two intrusives are shown on Figure 38. The earlier quartz-diorite (which is also a biotite-feldspar porphyry) plug includes some breccia like zones of volcanic and sedimentary rocks within the quartz-diorite (Fahrni, Kim, etal., 1976).
The most important intrusives are a sequence of biotite-feldspar porphyries of several distinct, but very similar phases, which includes the prominent dyke mentioned above. This dyke cuts the earlier quartz-diorite with a near vertical contact. It is a light to dark grey rock and ranges in composition from quartz-diorite to granodiorite, depending on the amount of K-feldspar present, most of which is secondary. Characteristically this lithology is a crowded porphyry with 35 to 50% by volume of 2 mm, euhedral, fresh, zoned plagioclase phenocrysts, and 1 mm flakes and books of fresh brown biotite. These phenocrysts are set in a fine grained matrix, consisting essentially of quartz, plagioclase, patches of fine grained biotite, some of which is pseudomorphed after amphibole, as well as fresh K-feldspar and apatite. Outside of the pit it is a uniform grey colour and contains hornblende phenocrysts as well as biotite and plagioclase. The main dyke on the island is bounded to the north-east and south-west by two major NW-SE trending regional faults (Fahrni, Kim, etal., 1976).
Within the pit area the main fractures are vertical to steeply dipping and include sets with the following strikes - 20° to 40°, 70° to 85° and 300° to 330°. Horizontal to slightly inclined fractures are also common. In general the resulting fracture spacing may vary from 0.1 to 1 m. Movement has occurred along many of the fractures, although the most common faulting direction is 20, and slightly east of north (Fahrni, Kim, etal., 1976).
Mineralisation & Alteration - The ore deposit is centred on the nearly vertical contact between the earlier quartz-diorite plug and the biotite-feldspar plug dyke. The principal sulphide minerals are chalcopyrite, bornite and some pyrite. Medium to coarse chalcopyrite is the most widespread, occurring mainly in quartz filled fractures which vary in width between 1 and 5 mm. The mineralised fractures are steeply dipping and have preferred orientations of 35° to 60° and 300° to 330° (Note: - the first of these directions is not consistent with the fracture directions quoted above from the same paper). The horizontal fracture set is only weakly mineralised. Chalcopyrite is also disseminated within the earlier quartz-diorite phase and the inclusions of meta-sediments and volcanics. Bornite is most widespread in the southern half of the ore zone, particularly the upper 75 m, where it occurs with chalcopyrite and quartz in fractures. Gold and silver are recovered from the copper concentrate. Molybdenite occurs locally within the ore, most commonly as drusy quartz veinlets which appear to be later than the main mineralising stage. Magnetite and specularite are common in the northern half of the ore zone, where they occur in fractures with chalcopyrite and pyrite. The greatest concentration of pyrite is peripheral to the Cu orebody, where it occurs as blebs, fracture coatings, stringers and disseminations (Fahrni, Kim, etal., 1976).
Alteration patterns are well developed and include a central, crudely oval shaped zone of
K-silicate alteration (secondary biotite) which is coincident with, but of greater areal extent than the Cu orebody. Within this zone, intrusive rocks appear relatively fresh and plagioclase phenocrysts are essentially un-altered. The main alteration product is a dark brown secondary biotite which occurs as very fine grained aggregates entirely replacing original hornblende in all but the latest intrusive phase. Fine grained biotite is also spread through the matrix of the intrusives. Near the outer limits of the K-silicate zone chlorite begins to replace biotite. Minor hydrothermal K-feldspar is also present in the Cu orebody as fine grains in the matrix and thin envelopes to veins (Fahrni, Kim, etal., 1976).
The K-silicate alteration zone is gradationally surrounded by a phyllic, or quartz-sericite-carbonate-pyrite zone in which sericite, carbonate and minor quartz form an inner, partial ring around the deposit, with a pyrite halo extending 150 to 250 m outwards from the ore zone. Pyrite occurs both as disseminations and as fracture fillings. It appears to merge with a similar alteration along the regional fault south-west of the pit. Outside of the pyrite halo, most of the rocks on the island display varying degrees of propylitic alteration, principally comprising an assemblage of chlorite-carbonate-epidote. This alteration halo has an overall dimension of around 1200 m in diameter within both the Eocene intrusions and the Jurassic volcanics and sediments, while the potassic biotitic core is approximately 500 x 300 m. The ore zone, defined by a 0.25% Cu cut-off is of the order of 600 x 200 m in plan at the surface and is within both the earlier quartz-diorite plug and the later biotite-feldspar porphyry dyke phases. At depth minor amounts of the Hazelton Group also has ore (Fahrni, Kim, etal., 1976).
The most recent source geological information used to prepare this decription was dated: 1996.
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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Selected References:
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Wilson J W J, Kesler S E, Cloke P L, Kelly W C 1980 - Fluid inclusion geochemistry of the Granisle and Bell porphyry copper deposits, British Columbia: in Econ. Geol. v75 pp 45-61
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Zaluski G, Nesbitt B, Muehlenbachs K 1994 - Hydrothermal alteration and stable isotope systematics of the Babine Porphyry Cu deposits, British Columbia: Implications for fluid evolution of Porphyry systems: in Econ. Geol. v89 pp 1518-1541
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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