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Island Copper

British Columbia, Canada

Main commodities: Cu Au Mo
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The Island Copper deposit is located on Vancouver Island, south-west British Columbia, Canada.

Published reserve and production figures include:

Production + Reserves 1984 - 257 Mt @ 0.45% Cu, 0.01% Mo, 0.11 g/t Au
      - includes Production of 114 Mt, 1971-84, (Dawson, etal. 1991).
Reserve 1993 - 59 Mt @ 0.36% Cu (AME, 1994).

Geology

The Island Copper deposit is hosted by the Lower Jurassic Bonanza Volcanics, with the ore zones being within andesitic pyroclastics in both the hangingwall and footwall of a quartz-feldspar porphyry dyke. The Bonanza Volcanics in the mine area are andesitic pyroclastics which include lithic, crystal and lapilli tuffs and intra-formational breccias. Bedding and graded bedding are common in tuffs and lapilli tuffs. Primary textures of the volcanics become increasingly vague as the dyke is approached, and disappear within 120 m of the contact. Alteration of the volcanic rocks near the orebody is so intense that it is difficult to determine the original mineralogy. Plagioclase phenocrysts are albitised, while pyroxene and amphiboles phenocrysts are pseudomorphed by chlorite. The matrix is a very fine grained mass of chlorite and sodic feldspars (Cargill, etal., 1976).

The quartz-feldspar porphyry dyke is approximately 100 m thick, strikes at 290°, and dips 60° to the north-west. It is intensely altered and has an extremely fine grained matrix, although near the centre where it is least altered it has a granodioritic composition. In general the phenocrysts of the porphyry consists of 5 to 15% quartz, 20 to 30% plagioclase and 5 to 10% mafic minerals, pseudomorphed by chlorite. The fine grained matrix is composed of 15 to 20% quartz, 10 to 25% plagioclase and 15 to 25% K-feldspar. Plagioclase phenocrysts are glomeroporphyritic and around 2 to 3 mm in size. Mafic minerals are largely converted to chlorite, but have also been altered to epidote, carbonate, magnetite and leucoxene or to sericite, clay minerals and leucoxene. Most of the fine grained matrix has been strongly altered, but where less modified it consists of a micro-granitic assemblage of equant quartz, subhedral sodic plagioclase and anhedral orthoclase. It has been suggested that the dyke is a feeder for the upper Bonanza Volcanics (Cargill, etal., 1976).

Breccias with both volcanic and intrusive fragments occur along the margins of the dyke and cap it on the north-west end. Three styles of breccia are recognised, namely, pyrophyllite, marginal and ferroan dolomite rich varieties, as follows (from Cargill, etal., 1976):

1). Pyrophyllite breccia is approximately 110 m thick and can be traced for more than 1100 m along strike, thickening to the north-east. It contains 1.5 to 45 cm matrix supported fragments of both highly altered quartz-feldspar porphyry and of massive fine grained altered rock, presumably of volcanic origin. The texture of the altered porphyry fragments is largely preserved, with quartz phenocrysts being preserved and plagioclase and mafic minerals being pseudomorphed by sericite and quartz. The presumed volcanic clasts are composed of fine grained quartz completely surrounded by sericite. The matrix is similar to the volcanic fragments, but more altered and finer grained, comprising quartz eyes surrounded by white mica;
2). Marginal breccias are tabular bodies which roughly parallel the contact of the porphyry dyke. All breccias occurring between un-brecciated dykes are included in this grouping. The width of these breccias is extremely variable, commonly with 15 to 30 m in the hangingwall of the dyke, but locally the whole dyke is brecciated. They continue to depths of more than 550 m. The marginal breccias are less open textured than the pyrophyllite breccia, because the fragments are separated by vein quartz. Fragment compositions range from 100% volcanic on the 'volcanic' margin to all porphyry within the dyke, with mixtures of varying proportions in between. Breccias surrounded by un-brecciated porphyry contain only porphyry clasts, while those with mixed clasts exhibit movement and rotation of fragments. Near the contacts of the breccia zone is a crackle breccia. In general 'crackle breccia' predominates on the outer, and rotated fragments in the interior of the breccia zone; and
3). Ferroan dolomite breccias, which ranges from 15 to 60 m in thickness and widen with depth, are exposed over a length of 250 m. This breccia consists of highly altered volcanic fragments separated by several stages of quartz and carbonate veins, which at surface are generally rusty brown ferroan dolomite. Because the fragments are not apparently rotated this breccia resembles a 'crackle breccia'.

Mineralisation & Alteration

Although chalcopyrite and molybdenite occur in all rocks, ore grade concentrations are predominantly within volcanic rocks and the marginal breccias. In volcanic rocks the Cu is present as fracture fillings in narrow, closely spaced fractures. In the marginal and ferroan-dolomite breccias, it occurs in relatively large quartz veins. Although minor molybdenite is present in the Cu veins, the majority is present as a later stage in quartz veins with sericite envelopes, and on fracture surfaces which cut these envelopes (Cargill, etal., 1976).

The orebody is divided into two by the dyke. The hangingwall ore zone is a roughly tabular body 60 to 180 m wide and approximately 1700 m long, continuing essentially unchanged to 300 m depth. It plunges downwards on either end. The footwall orebody does not outcrop and is less well known. Chalcopyrite and molybdenite are the only recovered ore minerals, although pyrite is the most abundant sulphide. Pyrite is the most abundant sulphide, and is two to three times more abundant in the ore zone than chalcopyrite. Sphalerite and galena are found sporadically in the carbonate veins and bornite is negligible. Magnetite is the most abundant oxide and is found in both the volcanics and porphyry dyke, occurring as 0.1 mm disseminations with chlorite pseudomorphs of mafic minerals (Cargill, etal., 1976). Locally the magnetite content may be greater than 10% (Arancibia & Clark, 1990). Chalcopyrite occurs as dry veinlets, on slip surfaces and locally as disseminations. Molybdenite is principally on slip surfaces and appears to post-date the Cu mineralisation. A complex hydro-carbon, gilsonite is found in the northern part of the orebody, mainly in carbonate veins and may be related to one of the intruded sedimentary beds which is saturated with hydro-carbons (Cargill, etal., 1976).

Alteration is complex, with seven types recognised, divided into two groups, the earlier contact alteration caused by intrusion of the dyke, and subsequent wallrock alteration related to the mineralisation. The contact alteration is pervasive through large volumes, unrelated to brecciation or fracturing and are symmetrical about the dyke, comprising inner biotite zone approximately 100 m wide; fringing transition chloritic zone around 180 m in thickness and outer epidote rich zone which is up to 350 m wide. These zones are gradational and apparently developed symmetrically on either side of the dyke. The wallrock alteration linked to mineralisation characterises small volumes of rock and is closely related to fracturing and brecciation. There are four types of alteration in this grouping, namely sericite, chlorite-sericite, pyrophyllite and quartz-ferroan dolomite. This alteration is developed as zoned envelopes on fractures and veins. In breccias and zones of closely spaced fractures, the envelopes coalesce and wall rock alteration pervades small volumes of rock (Cargill, etal., 1976).

In volcanic rocks fractures have envelopes characterised by an inner pyrophyllite zone, a central sericite and an outer chlorite-sericite zone. Widths of envelopes range from 1 to 50 mm's. In the quartz-feldspar porphyry, envelopes consist of an inner sericite and an outer chlorite-sericite zone. In the upper part of the dyke and near the contacts the inner sericite envelopes have coalesced, while at greater depths the outer chlorite-sericite sections of the envelope have met, but the inner sericite bands remain distinct. Within the pyrophyllite breccias, plagioclase has been altered to pyrophyllite, muscovite and kaolinite, mafic minerals to a similar assemblage and the matrix to quartz, pyrophyllite, kaolin, muscovite and dumortierite. In the ferroan-dolomite breccia the plagioclase is altered to muscovite and kaolinite, mafic minerals are converted to chlorite and muscovite and the matrix alters to quartz, sericite, kaolinite, carbonate and plagioclase. The last stage of the mineralising system produced carbonate-zeolite veins cutting all other features (Cargill, etal., 1976).

The alteration described above overprints an earlier phase of Fe-Na metasomatism that produced a magnetite rich, sulphide poor, zone with a width of up to 650 m, which persists along strike beyond the limits of the pit. This phase of alteration has produced three main assemblages, namely: 1). quartz-magnetite-albiteħamphiboleħapatite within the main porphyry dyke; 2). quartz-magnetite-amphibole-albite or oligoclase to andesine±apatite±trace scapolite within the Bonanza Volcanics adjacent to the dyke; and on the outer margins, 3). amphibole-magnetite ±plagioclase±quartz±apatite. The amphibole ranges from magnesio-hornblende, through actinolitic-hornblende to actinolite. The subsequent biotite-rich K-silicate alteration which accompanied the Cu-Mo mineralisation is less extensive than, and occurs within, the essentially barren magnetite halo, and is best developed peripherally to the quartz-magnetite core (Arancibia & Clark, 1990).

There is a 3000 nT magnetic anomaly over the orebody, extending 460 m SE of and 760 m NW from the centre of the ore, within a larger 6000 x 1500 m anomaly (Cargill, etal., 1976).

For detail consult the reference(s) listed below

The most recent source geological information used to prepare this summary 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.


  References & Additional Information
   Selected References:
Arancibia O N, Clark A H  1996 - Early Magnetite-Amphibole-Plagioclase alteration-mineralization in the Island Copper Porphyry Copper-Gold-Molybdenum deposit, British Columbia: in    Econ. Geol.   v91 pp 402-438


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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