|
Bell / Newman |
|
|
British Columbia, Canada |
| Main commodities:
Cu Mo
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All available as eBOOKS
Remaining HARD COPIES on
sale. No hard copy book more than AUD $44.00 (incl. GST) |
|
|
Bell/Newman had published reserve and production figures of:
59 Mt @ 0.37% Cu, 0.35 g/t Au (Prod.+Res. 1984, incl. Prod. 42 mt, 1972-82, Dawson, etal. 1991).
116 Mt @ 0.48% Cu, 0.35 g/t Au (Original geol. res., Carson, etal., 1976).
71.75 Mt @ 0.46% Cu, 0.23 g/t Au, 0.48 g/t Ag (Pit Reserve Noranda. Ann. Rep. 1990)
Geology - The Bell/Newman Cu-Au deposit is crescent shaped in plan and dips steeply, with dimensions of 1000 x 150 to 300 m, and has been shown to extend to at least 300 m below the surface. The orebody follows and overlaps the western and northern edges of a typical 'Babine-type' Eocene plug of biotite-hornblende-plagioclase porphyry, which has a surface outline of some 700 x 400 m. This plug, which was emplaced along a major block fault, the Newman Fault, intruded lower Jurassic and mid Cretaceous volcanic and sedimentary rocks and Eocene rhyodacite (Carson, etal., 1976).
The lower Jurassic rocks belong to the lower Hazelton Group and comprise light green andesitic flows, aquagene tuffs, lapilli tuffs and breccias, overlain by green tuffaceous argillite and siltstone. Outside of the mine area middle Jurassic upper Hazelton Group greywacke, siltstone and tuffs are also present, as are succeeding subaerial red, maroon and purple andesitic to dacitic lapilli tuffs. West of the Hazelton Group and in fault contact with it are the middle Cretaceous rocks of the Skeena Group, comprising fine grained gritty shales in the orebody area, but also including argillite and shale outside of the mine area, where they are also overlain by andesite tuffs and breccias. The Babine Suite volcanic rocks, which are cut by and are plausibly comagmatic with the mineralised plug, comprise two types of coarse pyroclastics of lahar or mudflow origin. One is dominated by angular, light coloured, flow banded, dense, massive, quartz-albite porphyry clasts 0.6 to 1.2 cm in diameter, while the other contains a chaotic mixture of un-altered mauve, purple and light green, fine grained biotite-hornblende-plagioclase porphyry clasts up to 0.6 m in diameter. Massive rhyodacite and biotite-feldspar porphyry dykes and small sills have intruded the volcanics. Mineralisation is developed in the main Babine biotite-hornblende-plagioclase porphyry plug described below, and in each of the Jurassic, Cretaceous and Eocene lithologies in the mine area (Carson, etal., 1976).
Two sets of intrusives are known from the Babine Igneous Suite (from Carson, etal., 1976), namely,
1) Rhyodacite intrusions, which where exposed near Bell Copper comprise large bodies of white, light brown, pale greyish-green or buff coloured rocks. Some are coarsely to finely porphyritic with phenocrysts of quartz and albite, while others are dense, massive, flow banded, and many are in part breccia bodies. Most rhyodacites at Bell are probably coeval with the biotite-hornblende-plagioclase porphyries. Both are closely associated in the area, while in many localities, including Bell and Morrison, the porphyry is central to, and intrudes the rhyodacites.
2) Biotite-hornblende-plagioclase porphyry, which includes the main mineralised dumbbell shaped plug at Bell which is divided by the NNW trending Newman Fault into a western half with a 200 m diameter and an eastern section which is 600 m across. Several contacts in the western lobe are observed to be steep, with apophyses intruded along joints between jostled blocks of country rock. The western lobe of the plug has intense quartz-sericite alteration, unlike most of the known Babine plugs. This lobe, and the neighbouring rocks also contains the known orebody. Unaltered porphyry south of the Bell alteration is medium grey, with abundant 0.25 to 5 mm phenocrysts of zoned oligoclase-andesine, biotite and hornblende in a fine grained aphanitic matrix of the same minerals plus quartz and K-feldspar.
3) Breccia pipes, occur as a small cluster in the western part of the mineralised biotite-hornblende-plagioclase porphyry plug at Bell. Nearly all are adjacent to the concave perimeter of the crescent shaped orebody in rock which is very low in Cu. Most are aligned along the projected trace of the Newman Fault, but one is near the western edge of the plug, and another is well within the orebody. These pipes are 2 to 10 m in diameter and have very sharp vertical contacts. Most contain angular to sub-rounded fragments that are <1 to 10 cm in diameter, reflecting the character of the adjacent wall rock, namely bleached, silicified and sericitised porphyry, although near the edges of the porphyry they contain a variety of rock types. The matrix is either compact grey to brownish clay or a porous mixture of quartz and well crystallised pyrite, although in some the fragments are welded together with little cement and numerous voids. These pipes are post ore, with in some cases Cu depletion in adjacent rocks.
Three normal NNW trending normal faults dissect rocks in the Newman Peninsular into four blocks, with the western side consistently displaced downwards. The greatest displacement is across the Newman Fault with a total throw of 700 to 1300 m, although this is believed to largely be prior to intrusion of the main mineralised plug (Carson, etal., 1976).
Mineralisation & Alteration - The crescent shaped Bell orebody is largely associated with the western lobe of the porphyry intrusion. The southern arm terminates abruptly just west of the Newman Fault, while the northern extends over that fault for 500 m to the east. Grades on the concave side of the orebody decline sharply to <0.2% Cu, while on the convex side they decrease much more gradually, as shown on Figure 37, while an north-western trending arm of low grade mineralisation follows a large dyke like offshoot of porphyry from the north-western section of the ore zone. The orebody contains two internal high grade zones that are 80 to 100 m in diameter and average 0.9% Cu. Within the orebody Mo averages around 50 ppm, with maximum values of up to 150 ppm found peripheral to the inner edge of the higher Cu grades. In contrast Au and Ag shows a very close correspondence with Cu, with averages of 0.45 g/t Au and 1.5 g/t Ag within the 0.3% Cu contour in the upper sections of the orebody. Zn and Pb both average <100 ppm within the orebody, although they form an anomalous halo at the outer edge of the pyrite halo (Carson, etal., 1976).
The major part of the ore zone in the open pit consists of very intense quartz-sericite-pyrite-chalcopyrite mineralisation in biotite-hornblende-plagioclase porphyry and rhyodacite. Chalcopyrite is the main Cu mineral, with more than half being finely disseminated, while the remainder is occurs as fracture coatings and in 2 to 8 mm quartz stringers with pyrite. Pyrite is largely within fractures and in quartz stringers. Minor to moderate amounts of bornite occur throughout the orebody, but a well formed zone of bornite is not evident, other than that the bornite:chalcopyrite ratio is highest within the 0.3% Cu contour. Bornite generally occurs in biotitic and quartz-sericite alteration assemblages and does not vary systematically with depth. Supergene chalcocite as coatings on pyrite and as replacements of chalcopyrite, occurred to depths of 50 to 70 m and enriched grades from 12 to 15% above the hypogene grade. Primary chalcocite, digenite and covellite occur in trace quantities in the bottom of the known ore zone (Carson, etal., 1976).
Sulphide mineralisation and alteration style changes with depth in the high grade central part of the south-western segment of the ore zone. Light grey porphyry with quartz-sericite-pyrite-chalcopyrite extends to depths of about 250 m. From approximately 250 to 400 m this assemblage diminishes as veins and irregular patches in darker porphyry containing biotite-chalcopyrite±pyrite develop. The latter persists to near 600 m, below which it changes to biotite-chalcopyrite-anhydrite with traces of pyrite. The ratio of disseminated:veinlet chalcopyrite is higher in the biotite bearing and pyrite poor sections than in the quartz-sericite zone. In some cases it can be demonstrated that pyrite and pyrite-sericite stringers have cut through the biotite-chalcopyrite-anhydrite assemblage. The Cu content of the quartz-sericite-pyrite-chalcopyrite ore is some 15% higher than that in the deeper biotite-chalcopyrite±pyrite interval, with a decrease evident across the overlap zone. The deeper biotite-chalcopyrite-anhydrite assemblage has the lowest grade of all. In the north-eastern limb of the orebody across the Newman Fault the quartz-sericite-pyrite-chalcopyrite assemblage is not abundant, and the lower grade biotite-chalcopyrite±pyrite zone predominates (Carson, etal., 1976).
The Bell orebody is surrounded by a prominent annular pyrite halo about 2200 m in diameter. All rock types within this halo contain abundant disseminated pyrite and stringers that are 5 to 20 mm wide, generally accompanied by subordinate sericite, chlorite, carbonate and quartz. The halo averages 10% pyrite by volume, ranging from 5 to 25%. The outer boundary is well within the limits of hydrothermal alteration and is marked by a decrease in pyrite from >5% to <2%. The inner boundary of the annulus has a diameter of around 800 m, enclosing the orebody, and is relatively low in pyrite, except for the overlap of the quartz-sericite-pyrite-chalcopyrite zone (Carson, etal., 1976).
The halo of hydrothermal alteration surrounding the Bell Copper orebody has a dimensions of around 3500 x 2500 m at the surface. This alteration envelope is characterised by an inner zone of biotite, surrounded by a concentric annulus of chlorite-carbonate. In contrast to other Babine deposits there is however a superimposed irregularly shaped zone of moderate to intense quartz-sericite overlapping the biotite zone, surrounded by a shell of sericite-carbonate alteration. In plan the quartz-sericite alteration envelopes much of the orebody and the sericite-carbonate alteration extends outwards into the pyrite halo. Each of the alteration styles can be summarised as follows (from Carson, etal., 1976):
o Biotite zone, replaces the mafic minerals of the porphyry and the adjacent sediments and volcanics, as deep brown coarse biotite, which becomes lighter and finer with more associated chlorite outwards. The biotite zone is centred on the Bell Copper orebody. Magnetite which is an original constituent of the biotite-hornblende-plagioclase porphyry is stable in the biotite-chalcopyrite association, although in other assemblages it is altered to hematite.
o Chlorite-carbonate zone, at Bell is characterised by the replacement of the original mafic minerals by chlorite and carbonate. Epidote is found throughout, but is more abundant on the outer fringes of the zone. The porphyry is typically greenish-grey in this zone in contrast to the dark grey in the biotite zone. The carbonates comprise dolomite, calcite and some siderite.
o Quartz-sericite zone, occurring as intense quartz-sericite alteration within the upper western section of the ore zone, has obliterated the biotite alteration assemblage. Parts of the porphyry plug and the adjacent rhyodacite have been cut by closely spaced quartz-sulphide stringers and flooded with quartz and sericite, with most pre-existing textures destroyed. This zone extends to depths of around 250 m, where a transition zone that continues to 400 m marks the lower sections of the quartz-sericite zone, passing into the deeper overprinted chalcopyrite bearing biotite zone. A similar but sharper transition occurs at the surface to the east. While the biotite, chlorite-carbonate and sericite-carbonate zones are all centred on the Bell Copper orebody, the quartz-sericite zone overlaps the orebody and shows a strong structural control. It is within a highly fractured zone within the western nose of the porphyry plug, while the eastern and north-eastern limits coincide closely with the Newman Fault and subsidiary north-east trending faults.
o Sericite-carbonate zone, generally envelopes the quartz-sericite zone with relatively sharp boundaries to the west, south and north, and overlaps the pyrite zone to the south-east. This alteration is best developed in the intrusives, particularly the rhyodacite, and is weakest in the sediments. The carbonates comprise dolomite, calcite and some siderite.
o Clay alteration, other than illite, is not abundant. Minor to moderate kaolinite is found in most of the alteration zone, while montmorillonite is restricted to the orebody. No peripheral argillic zone is evident. Primary clay minerals within the quartz-sericite zone range from illite in the upper sections to kaolinite and montmorillonite at depth. Kaolinite dominates in the deep biotite-chalcopyrite±pyrite zone, whereas montmorillonite occurs in both this and the transition zone to sericite-quartz-pyrite-chalcopyrite zone. Illite is found throughout the interval, but is strongest in the upper 400 m of the orebody
Moderate supergene chalcocite enrichment, as described below in the 'supergene enrichment' segment occurs in the upper 50 to 70 m of the quartz-sericite-pyrite-chalcopyrite segment of the ore during pre-Pleistocene time (Carson, etal., 1976).
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:
|
Wilson J W J, Kesler S E, Cloke P L, Kelly W C 1980 - Fluid inclusion geochemistry of the Granisle and Bell porphyry copper deposits, British Columbia: in Econ. Geol. v75 pp 45-61
|
Zaluski G, Nesbitt B, Muehlenbachs K 1994 - Hydrothermal alteration and stable isotope systematics of the Babine Porphyry Cu deposits, British Columbia: Implications for fluid evolution of Porphyry systems: in Econ. Geol. v89 pp 1518-1541
|
|
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.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
