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Brenda |
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British Columbia, Canada |
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Mo Cu
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Super Porphyry Cu and Au
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The Brenda porphyry molybdenum-copper deposit is located 225 km ENE of Vancouver and 22 km west of the Okanagan Valley, in British Columbia, Canada. (#Location: 49° 52' N, 120° 0'W)
Published reserve and production figures include:
164 Mt @ 0.16% Cu, 0.026% Mo, (Prod.+Res. 1984, incl. Prod. 131 mt, 1970-84, Dawson, etal. 1991).
159 Mt @ 0.183% Cu, 0.049% Mo, which includes a 'high grade' core of,
25 Mt @ 0.212% Cu, 0.063% Mo, (Res. pre-production, 1969, @ 0.3% Cu eq. cut-off,
Soregaroli & Whitford, 1976).
Geology
The Brenda Cu-Mo deposit is within the Brenda Stock, a composite zoned body of Jurassic age within the Brenda Mine area that forms part of the larger Pennask Batholith which in turn intrudes upper Triassic sedimentary and volcanic rocks of the Nicola Group. No clear boundary between the 'stock' and the 'batholith had been established in 1976. The Brenda Stock is sub-divided into two units, a quartz-diorite with abundant mafic minerals, and a porphyritic granodiorite. Pre- and post-ore dykes with widely divergent compositions cut the stock (Soregaroli & Whitford, 1976).
The quartz-diorite shows considerable variation in texture and composition. Near the contact with the Nicola Group it is some what foliated, and generally becomes increasingly, but not uniformly, more mesocratic away from the contact. Generally it comprises 10 to 25% quartz, 10 to 20% K-feldspar, 50 to 60% plagioclase, 10 to 30% hornblende, 1 to 15% biotite and 1 to 2% magnetite, sphene and apatite. Quartz occurs as angular interstitial grains, while poikilitic K-feldspar encloses quartz and plagioclase. The gradation from the quartz-diorite to the porphyritic granodiorite is generally diffuse, with the contact defined by the appearance of subhedral quartz and biotite. The porphyritic granodiorite has a chilled finer grained phase, both of which are characterised by subhedral quartz grains that range from 2 to 6 mm in diameter, and by well defined 3 to 10 mm biotite phenocrysts. This unit is lighter coloured than the quartz-diorite. Its composition is generally 20 to 30% quartz, 10 to 20% poikilitic K-feldspar, 45 to 60% plagioclase, 5 to 15% biotite, 2 to 10% hornblende and minor magnetite, apatite and sphene. The mineralisation is predominantly within the quartz-diorite (Soregaroli & Whitford, 1976).
Dykes cutting the Brenda Stock include aplite-pegmatite, andesite, quartz-diorite, dacite-porphyry and felsite, all of which are pre-ore, while subsequent trachyte-porphyry dykes are only weakly mineralised, cutting most stages of veining, suggesting they are late inter-mineral. The last dykes are post mineral and of basaltic composition (Soregaroli & Whitford, 1976).
The Nicola Group comprises volcanic breccias and flows and greywackes, which adjacent to the stock have been altered to schistose hornfels over a width of up to 450 m. The hornfels is characterised by the development of bands and aligned lenses of felted brown to black biotite. Siliceous bands and matrix in the hornfels consist of quartz with minor plagioclase, K-feldspar and occasional euhedral brown garnet (Soregaroli & Whitford, 1976).
Mineralisation & Alteration
'Economic mineralisation', as defined by the 0.3% Cu equivalent grade line has a plan outline of some 720 x 360 m, and persists to at least 300 m below the surface, with all drill holes bottoming in ore grade at that depth. The lateral boundaries are very close to vertical. Hypogene mineralisation is generally confined almost entirely to veins, with disseminated sulphides being rare, except in some mineralised dykes and in areas of intense hydrothermal alteration. The grade of the orebody is a function of the fracture/vein density and of the thickness and mineralogy of the filling material. The average sulphide content of the ore zone is generally 1% or less (Soregaroli & Whitford, 1976).
At least five stages of veining, each with unique attitudes and mineralogy, were developed in fractures created by east-west regional compression. The grade of mineralisation is a function of the fracture density and mineralogy of the veins. Chalcopyrite and molybdenite are the most abundant sulphides, generally accompanied by minor but variable quantities of pyrite and magnetite in a gangue of quartz, potash feldspar, biotite and/or calcite. Bornite, specular hematite, sphalerite and galena are rare constituents of the ore. Pyrite is most abundant in altered andesite dykes and in quartz-molybdenum veins. The ratio of pyrite:chalcopyrite in the orebody is around 1:10, with the chalcopyrite content diminishing beyond the ore boundaries (Soregaroli & Whitford, 1976).
The density of veining is not homogeneous within the orebody, ranging from <9 per metre near the periphery of the ore to 63 and occasionally 90 per metre near the centre. Some veins have very sharp contacts with the wall rock, but most are irregular with replacive margins. Individual veins may show fracture characteristics in one area and replacive margins in another. Five stages of veining are recognised within the orebody, as follows (after Soregaroli & Whitford, 1976), from oldest to youngest, with stage 5 representing a later, probably un-related event:
Biotite - chalcopyrite veins, most of which do not exceed 1.5 mm in thickness. Biotite filled fractures contain disseminated patches and crystals of chalcopyrite and more rarely pyrite, molybdenite and K-feldspar. Quartz is absent. The biotite shows evidence of slip along the fractures. Four directions of veining are recognised, representing three phases of veining, from oldest to youngest. The strikes and dips are as follows, phase 1A - 62°/78°S; phase 1B - 284°/86°S; and phase 1C - 348°/58°E and 312°/18°N. Biotite-chalcopyrite veins account for about 20% of the total number of veins within the orebody. However because of their narrow thickness and patchy mineralisation , they probably contain less than 5% of the total sulphides.
Quartz - K feldspar - sulphide veins account for the bulk of the mineralisation in the Brenda deposit. Two ages of veins with different characteristics have been recognised. Phase 2A, oriented at 68°/74°S and 70°/vertical, is the earlier, and is characterised by quartz and K-feldspar, with highly variable quantities of chalcopyrite, molybdenite and pyrite. Quartz is the most abundant vein mineral, while K-feldspar varies from 1 to 25%. Sulphide distribution within the vein is erratic, generally comprising between 10 and 25%, but may locally exceed 80% of the vein space. Pyrite is a minor but constant accessory, while magnetite course as rare octahedra in chalcopyrite and more rarely as bands at vein walls. Bornite and sphalerite are occasional accessories. The NE striking set are the most abundant, comprising 60% of all veins in the pit, with all other phase 2A veins making up about 5% of the total veining. The younger phase 2B veins, oriented at 338°/vertical and 35°/20°NW, are vuggy with sulphides occurring as discrete crystals and crystal groups in interstices between quartz and K-feldspar crystals. Biotite occurs as a minor constituent of all veins, as does epidote. Chalcopyrite is the principal sulphide, and is accompanied by minor quantities of molybdenite and pyrite.
Quartz - Molybdenite - Pyrite veins range from 2.5 to 35 cm in thickness. They contain quartz and molybdenite with disseminated cubes of pyrite and chalcopyrite on fractures. Banding in some veins is caused by seams of molybdenite within the quartz. Calcite is a minor constituent. These veins strike and dip predominantly at 72°/86°S, with a lesser set at 284°/vertical.
Epidote - sulphide - magnetite veins, which strike and dip at 310°/vertical, comprise epidote and magnetite with minor molybdenite, chalcopyrite or pyrite. These veins occur throughout the area, but are no where abundant. The adjacent wall rock is irregularly replaced by epidote and chlorite.
Biotite, calcite and/or quartz veins of several ages are later than the mineralising episode.
Hydrothermal alteration is particularly weak, with mineralisation being confined almost entirely to veins in relatively fresh homogeneous host rocks. The different styles of alteration include (after Soregaroli & Whitford, 1976):
Potassic alteration (K-feldspar and biotite) forms envelopes adjacent to stages 2 and 3 mineralised veins, and is directly related to sulphide mineralisation. Generally K-feldspar and biotite are separated, but locally occur together. Pink to white K-feldspar replaced plagioclase next to most stage 2, and to a lesser extent stage 3 veins, forming irregular envelopes ranging from 1 cm to 1 m, but averaging 2 cm in thickness. Hydrothermal biotite replaced magmatic mafic minerals in wall rocks adjacent to stage 2, and particularly stage 3 veins, forming envelopes ranging in width from <1 mm, to several cm's. In local highly irregular areas it may form up to 50% of the rock. Replacement biotite only occurs within the Brenda orebody, while stage 1 veins of hydrothermal biotite extends over a broad area extending beyond the deposit.
Propylitic alteration formed both before veining, and accompanied some later stage veins, but is only of local significance. It varies from weak to intense, and is characterised by the development of chlorite and epidote, as well as microscopic sericite and carbonate. Large areas within the orebody have not been propylitised, while K-feldspar selvaged veins cut across earlier pre-ore propylitised quartz-diorite. The second stage propylitisation accompanied the development of stage 4 veining and is reflected as envelopes of epidote and chlorite, but because these veins are very minor in occurrence the propylitisation is also sparse.
Argillic alteration is only locally developed and is restricted to post mineral fault zones where the host rock has been highly shattered. Kaolinite, sericite and epidote almost replace the comminuted host rocks, with only the original quartz remaining.
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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Logan, J.M. and Mihalynuk, M.G., 2014 - Tectonic Controls on Early Mesozoic Paired Alkaline Porphyry Deposit Belts (Cu-Au ± Ag-Pt-Pd-Mo) Within the Canadian Cordillera : in Econ. Geol. v.109, pp. 827-858.
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Soregaroli A E and Whitford D F, 1976 - Brenda: in Sutherland Brown A (Ed.) 1976 Porphyry Deposits of the Canadian Cordillera Canadian Institute of Mining and Metallurgy, Special Volume 15, pp 186-194
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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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