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Highland Valley - Lornex
British Columbia, Canada
Main commodities: Cu Mo

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The Lornex, zoned, structurally controlled porphyry Cu-Mo deposit is contained entirely within the Skeena Quartz Diorite, which is a member of the Bethlehem Phase of the Guichon Batholith, near Kamloops in British Columbia, Canada.

For details of the geological setting see the Highland Valley overview record.

The host Skeena Quartz Diorite is a slightly porphyritic, medium to coarse grained rock. It consists of 20% quartz, 50% plagioclase, 10% orthoclase, 5 to 10% biotite and 5 to 10% hornblende, with accessory sphene, apatite, zircon and magnetite. It is cut by a younger north-westerly trending pre-mineral quartz porphyry dyke within the ore zone (see the section above for a geological description of the phases and setting of the Guichon Creek Batholith). The contacts of the dyke are indistinct because of over-printing silicification and sericitisation. The dyke is presumed to have intruded one of a series of structural zones parallel to the Highland Valley Fault. Quartz phenocrysts normally compose 20 to 25% of the dyke and plagioclase phenocrysts occur locally. The grey aphanitic matrix is composed of 60 to 70% plagioclase and 10% quartz. The Lornex Fault, a north striking and west dipping regional structure forms the north-western boundary of the orebody, and separates the host from younger, virtually barren Bethsaida Granodiorite west of the orebody (Waldner, et al., 1976).

The orebody is approximately 1900 x 500 m in plan, and geological interpretations suggest it plunges at 30 to 40° to the north-west to a depth in excess of 750 m. Mineralisation is controlled by the density and distribution of fractures, and commonly occurs as fracture coatings or veins. Mineralised and post-mineral fractures were controlled by at least three periods of deformation, one mineralised and two barren. These are from oldest to youngest, 1). mineralised system striking 22° dipping at 55°SE, 64° dipping 57°SE and 90° dipping 58°S; 2). post mineral faults and fractures which strike at 272 to 274°, and 21 to 32°; and 3). faults and fractures which strike 292 to 297°, 316° and 352° (Waldner, et al., 1976).

The major sulphides in order of decreasing abundance are chalcopyrite, bornite, molybdenite and pyrite. Minor amounts of sphalerite, galena, tetrahedrite and pyrrhotite also occur. The total sulphide content ranges from 1 to 1.5% by weight in the ore zone, gradually decreasing from the centre of the ore zone to its peripheries. The common gangue minerals include quartz, calcite, epidote, hematite, magnetite and gypsum. Sulphide mineralisation occurs primarily as fracture fillings with quartz and as fracture coatings. Only an estimated 5% of the total bornite, chalcopyrite and pyrite occurs as disseminations or as partial replacement of the mafic constituents of the host rock. Veins average 5 to 15 mm in width, but vary from hairline to more than a metre. The larger veins, some of which have been mapped along strike lengths of over 200 m, are commonly composed of quartz, molybdenite and chalcopyrite. Molybdenite may occur as rosettes in vuggy quartz veins, but is normally found as thin laminae in banded quartz veins. An erratic band of post ore gypsum occurs at an elevation of below 1100 m, being deeper in the centre of the orebody and higher on the fringes. It occurs as veins from 5 to 10 mm thick. Anomalously high values of Zn, Ag and Bi occur in the Lornex Fault, with up to 1200 ppm Zn. Sphalerite and discontinuous pods of massive pyrite are also found within this fault zone, although bornite, chalcopyrite and molybdenite are absent (Waldner, et al., 1976).

Four stages of alteration are recorded, as follows, 1). K-silicate alteration which is erratically distributed with no well defined zone. It occurs as veins that average approximately 5 mm in width; 2). Phyllic alteration which consists of quartz-sericite envelopes to the mineralised veins. A grey mixture of quartz and sericite commonly forms borders on quartz-copper sulphide and quartz-molybdenite veins within the argillic alteration zone. These envelopes, which commonly form sharp boundaries with pervasive moderate to intense argillic alteration, average approximately 3 mm in width; 3). Argillic alteration which is pervasive throughout the ore zone, is characterised by the presence of quartz, sericite, kaolinite, montmorillonite and chlorite. Sericite and kaolinite, with minor montmorillonite and chlorite form pseudomorphs after plagioclase. The cores of these crystals are more strongly altered than the rims, although with more intense alteration the whole crystal is converted to sericite and clays. Kaolinite, sericite and minor montmorillonite also replace orthoclase, but in contrast from the rim inwards. Biotite and hornblende alter to chlorite and sericite. The argillic alteration of the Skeena Quartz-diorite has produced a cream (kaolinite dominant) to apple green (sericite dominant) rock; 4). Propylitic alteration is pervasive and peripheral to the argillic zone. The typical assemblage consists of epidote (zoisite), chlorite and carbonates (calcite), with minor sericite and hematite. Epidote and calcite are most common in veins. Quartz and orthoclase are fresh, but plagioclase alters to calcite and epidote with minor amounts of sericite and chlorite. Mafic minerals alter to chlorite, calcite and sericite, with minor hematite and epidote.

The sulphide minerals and hydrothermal alteration zones are distributed in a roughly concentric pattern. The bornite core is surrounded by a zone of chalcopyrite and molybdenite mineralisation, while pyrite is peripheral. Phyllic and pervasive argillic alterations are occupy the ore zone and propylitic alteration occurs in the peripheral zone of pyrite and lower copper grades. Intensity of hydrothermal alteration increases with fracture density and sulphide content. All phases of the Guichon Creek Batholith have been dated at 198±8 Ma, although evidence suggests relative ages decrease inwards. Hydrothermal alteration products at Lornex have been dated at 190±4 Ma, suggesting the ore is slightly younger than the youngest intrusive phase (Waldner, et al., 1976).

Published reserve and production figures include:

577 Mt @ 0.39% Cu, 0.013% Mo (Prod.+Res. 1984, incl. Prod. 228 Mt, Dawson, et al., 1991).

For detail consult the reference(s) listed below.

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.

    Selected References
DAngelo, M., Alfaro, M., Hollings, P., Byrne, K., Piercey, S. and Creaser, R.A.,  2017 - Petrogenesis and Magmatic Evolution of the Guichon Creek Batholith: Highland Valley Porphyry Cu ±(Mo) District, South-Central British Columbia: in    Econ. Geol.   v.112, pp. 1857-1888.
McMillan W J,  2005 - Porphyry Cu-Mo Deposits of the Highland Valley District, Guichon Creek Batholith, British Columbia, Canada: in Porter, T.M. (Ed), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide,   v.1 pp. 259-274

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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