The Red-Chris porphyry copper-gold deposit lies approximately 11 km east of Highway 37 and 82 kilometres south of Dease Lake, in the Stikine Terrane of the Intermontane tectonic belt in north-western British Columbia, Canada.
The country rock within the deposit area is composed of massive volcanic-wackes, siltstone and augite-porphyritic basalt of the Upper Triassic Stuhini Group which are overlain to the south by Middle Jurassic sedimentary rocks of the Bowser Lake Group.
The ore deposit is hosted by an ENE elongated, sub-volcanic, hornblende monzonite porphyry intrusion known as the Red stock (203.8 ±1.3 Ma). The stock has been faulted and pervasively altered to a phyllic assemblage of quartz-sericite-ankerite-pyrite. The southern margin of the stock is faulted against sediments of the Bowser Lake Group, while along its northern contact it intrudes augite phyric basalts (the 'Dynamite Hill volcanics'), of the Stuhini Group. The Dynamite Hill volcanics are predominantly composed of monolithic flow and pillow breccias with thick sections of medium-grained, massive felspathic volcanic wacke and occasional thinner intervals of bedded mudstone to fine volcanic sandstone.
In general, mineralisation occurs as a steeply dipping, high-grade core of quartz stockwork and zones of sheeted quartz veining associated with intense and pervasive carbonatisation, surrounded by barren to weakly mineralised, phyllic (quartz-sericite-ankerite-pyrite) altered host porphyry. The quartz stockwork and zones of sheeted quartz veining dip steeply to the north and trend parallel the long axis of the Red stock.
Two coalescing, ENE trending, mineralised zones, the Main and East zones constitute the Red-Chris deposit, with a total strike length of approximately 1700 m and width ranging from 250 to 700 m. The deposit becomes both wider and higher grade with depth, persisting to at least 750 m below surface in the East zone. The higher grades of Cu and Au are contained within quartz stockworks.
Two additional zones of mineralisation, the Gully and Far West zones, collectively referred to as theYellow-Chris zone are located within about 1 kilometre of the western limit of the Red-Chris deposit.
Within the stockwork zone, local, laterally discontinuous, intense quartz stockwork veining, with narrow zones of sheeted quartz veins, are flanked by a moderate to strongly developed quartz stockwork within the carbonate-sericite-pyrite altered plagioclase-hornblende porphyritic host rocks. The transition from intensely developed quartz stockwork mineralisation to sheeted material is generally gradational, but in some locations may be faulted.
The stockwork veins are composed of planar, grey quartz envelopes and vein-fill material with sharp contacts. Veins and veinlets vary from 2 to 20 mm in thickness forming a randomly orientated network pattern with at least two vein generations. Sheeted veins are composed of 2 to 4 mm thick alternating bands of light and dark grey microcrystalline quartz carrying chalcopyrite and pyrite, with minor bornite. Dark grey quartz bands contain skeletal hematite and remnants of host rock that are intensely altered to sericite, hematite and clay. Chalcopyrite with minor pyrite, hematite and bornite commonly occur as disseminations and thin veinlets in both the quartz veins and their selvages in the porphyry between the veins.
The dominant structural trend is ENE faulting which has controlled the orientation of the Red stock and the mineralised veining. Fault are normal with dominantly dip-slip movement, striking at 60 to 90° and dipping at approximately 75° S. The vein systems and ore deposits which are controlled by this fault system, are stepped down to the west and offset laterally along younger northerly faults. Thus, the western deposits represent progressively shallower parts of the hydrothermal system.
Faults active either before or during the mineralising event are usually healed and accompanied with intense silicification. Reactivation of these faults has produced gouge zones from several cms to 50 m in width contains rounded cm sized fragments of altered and pyrite-chalcopyrite mineralised Red stock in a matrix of clay, quartz and carbonate.
There appears to be a close correlation of high Cu with elevated Au and Ag, with the highest concentrations in quartz-rich samples, as either sheeted or stockwork veins. Other base metals are typically of a low tenor, with Zn being weakly anomalous and show no correlation with Cu, Au or Ag values.
The host porphyry within the stockwork zones has been modified by intense, pervasive carbonate alteration, chiefly ankerite and iron-rich magnesite, which occur as a fine-grained, anhedral granular intergrowth, with lesser fine-grained quartz, sericite and sulphides. Mafic minerals are converted to chlorite, sericite and ankerite, while plagioclase phenocrysts are locally kaolinised, but are more often strongly sericitised. In addition, in the East zone, several developments of sheeted quartz-sulphide veining are associated with zones of intense silica flooding and quartz stockwork. This alteration developed as a number of main overlapping styles, as follows:
i). Phyllic and carbonate - phyllic alteration is generally pervasive and is the most widespread alteration, extending over an area of 2 to 3 sq. km. Generally, the host rock is altered to a pale grey and retains some of the primary textures. a). In the interval of weak phyllic (to weak argillic) alteration, plagioclase has been altered to sericite and kaolinite and has a bleached appearance, while hornblende is typically intensely altered to completely destroyed and in places the groundmass appears to be silicified. Carbonate alteration takes the form of iron-magnesite and ankerite, with usually 10 to 20 per cent replacement of the host rock. Vein pyrite exceeds disseminated pyrite for a total content of 5 to 10%, while weak quartz-pyrite ±chalcopyrite stringers are cut by late, white calcite veins. b). Mottled phyllic alteration partially destroys the primary porphyritic texture of the host rock and is characterised by 3 to 7 mm spherical and irregular pale grey patches of intense quartz-sericite alteration that comprise from 10 to 15 per cent of the rock. The groundmass is beige, suggesting significant ankerite. Typically fine-grained to blebby pyrite occurs near the centre of these patches, while pyrite veins are common with well developed quarts-sericite selvages. The total pyrite content varies from 5 to 10%. c). Carbonate veining and alteration of the host porphyry groundmass to ankerite and iron-rich magnesite are widespread throughout the Red stock. The surrounding volcanics and sediments are also locally intensely carbonate altered, although these are barren of sulphides, and appear to be very late and probably unrelated to the main copper-gold mineralising event.
ii). Potassic - which is is sporadic and limited in both extent and intensity, representing only 5 to 10 per cent of the total alteration envelope. Potassic zones are generally only a few metres wide and are discontinuous, with gradational to sharp contacts with the phyllic-altered host and quartz stockwork. The porphyritic texture of the host is often obliterated and replaced by fine-grained K-feldspar, producing a light orange-brown to salmon coloured rock. The alteration assemblage includes 2 to 7% hematite (martite) after magnetite and magnetite as fine disseminations and rare veins. Pyrite generally comprises 2 to 4% of the rock as disseminations with a fine-grained to blebby texture, while narrow quartz stingers contain pyrite and chalcopyrite. Hornblende is locally altered to fine-grained, felted brown biotite. It has been pointed out that hematite and siderite impart a buff pink appearance to hand specimens that may be mistaken for K-feldspar flooding.
iii). Propylitic - which is prevalent in the mafic volcanics to the north of the Main and East zones and has been identified locally in late phase dykes. In general propylitic alteration is only poorly developed and is represented by up to 5% disseminated epidote and 2 to 5% finely disseminated pyrite that has only been identified in the augite porphyry of the 'Dynamite Hill Volcanics' immediately to the north of the main zones of stockwork mineralisation.
iv). Gypsum - observed to the west and south-west of the Main zone, where it occurs as a poorly defined zone of weak to strong gypsum veining which appears to be late and cut mineralisation
The recognised metal and sulphide zoning across the deposit is as follows, from east to west:
i). East zone - mainly bornite with intense silicic alteration with a Cu%:Au g/t ratio of approximately 1:0.8.
ii). Main zone - chalcopyrite greater than bornite, with 1 to 3% pyrite, occasional molybdenite specks, and a Cu:Au ratio of about 1:1
iii). Gully zone - bornite is uncommon, chalcopyrite:pyrite is about 1:1, with 3 to 4% pyrite, and a Cu:Au ratio of approximately 1:2.
iv). Far West zone - pyrite greater than chalcopyrite, 3 to 4% pyrite, a Cu:Au ratio of approximately 1:3, gypsum veining, and occasional base metal sulphides in quartz-carbonate veins.
Published resource and reserve estimates include:
225 Mt @ 0.42% Cu, 0.33 g/t Au, (Total resource, American Bullion Minerals Ltd., 1998),
185 Mt @ 0.414% Cu, 0.325 g/t Au, (Proven + probable reserves, bcMetals, 2004),
276 Mt @ 0.349% Cu, 0.266 g/t Au, (Proven + probable reserves, including low grade
stockpile ore, bcMetals, 2004)
312.6 Mt @ 0.54% Cu, 0.55 g/t Au, (Measured+Indicated Resource, 2010, @ 0.3% Cu eq. cutoff, Imperial Metals website), plus
237.7 Mt @ 0.46% Cu, 0.50 g/t Au, (Inferred Resource, 2010, @ 0.3% Cu eq. cutoff, Imperial Metals website)
86.9 Mt @ 0.88% Cu, 1.11 g/t Au, (Measured+Indicated Resource, 2010, @ 0.6% Cu eq. cutoff, Imperial Metals website), plus
39.2 Mt @ 0.81% Cu, 1.02 g/t Au, (Inferred Resource, 2010, @ 0.6% Cu eq. cutoff, Imperial Metals website)
The information in this summary is largely derived from the British Columbia Geological Survey online MINFILE record for this deposit.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2010)
