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Gibraltar

British Columbia, Canada

Main commodities: Cu Mo
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The Gibraltar porphyry copper deposit is located on the western slopes of Granite Mountain, 20 km NE of Williams Lake and 362 km NE of Vancouver, British Columbia, Canada.

Published reserves and production are as follows:

315 Mt @ 0.35% Cu, 0.004% Mo (Prod.+Res. 1984; incl. Prod. 152 mt, 1972-84, Dawson, et al. 1991)
327 Mt @ 0.37% Cu, 0.016% Mo (Res., pre-production, 1972).
The operation comprises a series of separate bodies, with pre-production reserves in 1972,
    using a 0.25% Cu cut-off, including,
    Gibraltar East - 136 Mt @ 0.37% Cu, (with 0.0072% Mo in 1993 reserve).
    Gibraltar West - 9 Mt @ 0.4% Cu,
    Granite Lake - 109 Mt @ 0.37% Cu, (with 0.0096% Mo in 1993 reserve).
    Pollyanna - 73 Mt @ 0.36% Cu. (with 0.0102% Mo in 1993 reserve).
    Cuisson - 11 Mt @ 0.37% Cu, 0.008% Mo (Res., 1980, USBM).
    Gibraltar North - 40 Mt @ 0.5% Cu, 0.3 g/t Au (Resource, 1992).
    McLeese Lake - 75 Mt @ 0.3% Cu (Res., 1993, AME, 1994).

Geology

All four of the main mineralised zones at Gibraltar fall within the Granite Mountain Pluton, one of several Triassic to Cretaceous age intrusives in the Cache Creek Terrane on the east side of the Fraser River Fault. The Cache Greek Terrane is characterised by heavily disrupted mélange developments of lower Carboniferous to middle Jurassic radiolarian cherts and argillite, shallow water carbonate, basalt, gabbro and Alpine type ultra-mafic rocks separating the Quesnellia and Stikine Terranes.

At Gibraltar, multiple intrusion of the Granite Mountain pluton, deformation, greenschist metamorphism and mineralisation are interpreted as having taken place, even overlapped, over a short period of time at about 204 Ma. The late stage veining in the deposit has undergone only minor deformation. This pluton is one of several which are found on the east side of the Fraser River Fault (Drummond, et al., 1976).

On the margin of the pluton a sheared and chloritised diorite occurs peripherally to a saussuritised (albite-epidote altered) and cataclastically foliated quartz diorite, which forms the main phase of the pluton and hosts the four main orebodies. Pre-mineral phases of the main body include i). leuco-cratic porphyritic quartz-diorite, ii). quartz-feldspar porphyry and iii). aplite which makes up the core of the pluton. The four main ore zones occur peripherally to a later fourth stage of intrusive leucocratic and porphyry phases and indicate an elliptical shaped sulphide bearing stockwork which is 4000 x 1600 m in plan. The later leucocratic phase intrusives have both sharp and gradational contacts with the saussuritised earlier intrusives, and comprise quartz (30%) and saussuritised plagioclase with albitic rims (45%) in grains which range from 1 to 5 mm. The remaining 25% of the rock is a fine grained mosaic of quartz and albitic plagioclase with 1 to 2% chlorite. A fifth more definite porphyry cross-cuts the saussuritised quartz-diorite and apparently also the leucocratic phase. It has 3 to 5 mm phenocrysts of quartz (30%) and albitic plagioclase (10%) in a white aphanitic matrix of quartz, albitic plagioclase and some carbonate, muscovite and zoisite. Both the porphyry and leucocratic phase are still pre-mineralisation (Drummond, et al., 1976).

Mineralisation & Alteration

The orebodies are irregular tabular stockwork masses up to 1200 m long that dip approximately 35°SSW and plunge 10° to the WNW. This orientation is controlled by the regional foliation and the orebodies positions appear to be controlled by north-south-normal faults.

The stockwork which comprises four stages of vein development, has been imposed upon, and modified by, continuing deformation of the saussuritised main quartz-diorite phase. Earlier vein stages have been folded while the last (or fourth) vein stage filled tensional fractures. The main mineralisation coincided with a period of intense north-westward shearing, that was probably generated during the amalgamation of the Cache Creek Terrane with Quesnellia and Stikinia to form the Eastern Super-Terrane (Drummond, et al., 1976; Dawson, et al., 1991).

While Drummond, et al., (1976) only describe Cu and associated Mo mineralisation within veins and their selvages, as detailed below, other sources indicate that the mineralisation is primarily chalcopyrite (with minor molybdenite) within veins along foliation planes and crosscutting the foliation, as well as disseminations throughout the rock.

There are at least four stages of veining recognised in the Gibraltar deposits. These stages are as follows:

i). Sub-divided into a). quartz-pyrite±chalcopyrite with a sericite envelope of sericite-quartz-pyrite±chalcopyrite with the original 'saussuritised' feldspars being converted to sericite and clay; b). quartz-chlorite-pyrite-chalcopyrite-magnetite±carbonate with a chlorite envelope of quartz-chlorite-pyrite±chalcopyrite with an absence of epidote.
ii).  Comprising a). quartz-chlorite-pyrite±magnetite; b). quartz-chlorite-pyrite-chalcopyrite-epidote±magnetite; c). quartz-chlorite-pyrite-epidote±magnetite; d). quartz-chlorite-pyrite-chalcopyrite-magnetite; e). quartz-chlorite-bornite±pyrite which is restricted to the porphyry between Pollyanna and Granite Lake. All have ±carbonate.
iii).  Quartz-molybdenite-bornite±magnetite±carbonate.
iv). Massive white quartz with blebs of fine grained chlorite and chalcopyrite blebs.

Veins with sericite and chlorite envelopes are cross-cut by stage two veins which do not have envelopes, while chloritic veins have not been detected being cut by one with a sericitic margin. Similarly sericitic and chloritic alteration parallels as well as cross-cuts the foliation in the host quartz-diorite. It has been noted in the Pollyanna and Gibraltar East zones that the amount of sericite relative to the quantity of epidote in the saussuritised plagioclase could be correlated reasonably well with the Cu grade (Drummond, et al., 1976).

Several stages of deformation are observed, with the first two stages of veining generally following S2, and less commonly cross-cutting it. The fourth stage of veining is formed in the S3 structures in the axial planes of deformed S2. S1 was a primary igneous foliation (Drummond, et al., 1976).

The 'alteration' of the host quartz-diorite is apparently uniform, pervasive and basically propylitic, with the main assemblage being sericite, chlorite, clinozoisite, iron carbonates and garnet. The chlorite could be a retrograde alteration of biotite while garnet occurs as euhedral crystals in chlorite. There is no recorded K-silicate or argillic alteration, nor have any breccia pipes been encountered. This alteration most likely represents the saussuritisation that affects the whole Granite Mountain quartz-diorite. Within the pluton there are mineralogical and textural variations associated with the intensity of the cataclastic deformation. Where foliation is weakly developed the mineral association is characterised by quartz-albite-muscovite-chlorite-epidote-zoisite-magnetite. As the intensity of deformation increases the quartz-diorite becomes strongly foliated and eventually schist-like, with a mineral assemblage of quartz-oligoclase-chlorite-muscovite-magnetite. The increase in deformation and schistosity coincides with chlorite becoming progressively more dominant, while quartz shows increasing evidence of crushing and rotation. Epidote becomes less abundant as the foliation increases. These changes and assemblages are taken to represent regional metamorphism rather than alteration (Drummond, et al., 1976).

Geophysics indicates that the sulphide mineralisation is broadly elliptical developed peripherally to the area in which the leucocratic quartz-diorite and quartz-feldspar porphyries are most commonly found. The overall dip of mineralised veins, which both cut and parallel the foliation, is around 30°S, which also marks the dip of the associated sulphide mineral zonation within the deposit. The overall zoning comprises a low sulphide central core of leucocratic porphyry with chalcopyrite and bornite and negligible amounts of pyrite. This is rimmed by the ore zone with a chalcopyrite-pyrite mineralogy, which passes outwards into a pyritic halo with only minor chalcopyrite (Drummond, et al., 1976). In addition there is a lateral zonation from east to west of Cu-Mo dominant mineralisation in the Pollyanna Pit to predominant Cu±Mo in the Gibraltar East Pit to Cu-Ag and Zn in the Gibraltar West Pit.

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


  References & Additional Information
   Selected References:
Drummond A D, Sutherland Brown A, Young R J and Tennant S J,  1976 - Gibraltar - Regional metamorphism, mineralization, hydrothermal alteration and structural development: in Sutherland Brown A (Ed.), 1976 Porphyry Deposits of the Canadian Cordillera, Canadian Institute of Mining and Metallurgy,   Special Volume 15, pp 195-205
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.


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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