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Afton, Ajax |
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British Columbia, Canada |
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Cu Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Afton-Ajax group of alkalic porphyry ore deposits are concentrated within and area of 10 x 4 km, associated with the 30 x 5 km alkalic Iron Mask Batholith, a sub-volcanic multiple intrusion emplaced between 205 and 190 Ma. It is located to the south-west of Kamloops and 360 km NE of Vancouver in British Columbia, Canada.
Published reserve and production figures for the Afton-Ajax Group of deposits include:
71 Mt @ 0.75% Cu, 0.44 g/t Au, 4 g/t Ag (Prod.+Res. '84, incl. Prod. 15 Mt, 1977-84, Dawson, et al. 1991).
31 Mt @ 1% Cu, (Afton, Total dep. @ 0.25% Cu cutoff, Carr & Reed, 1976).
Other production / reserve / resource figures from various sources & mine visits include:
22.1 Mt @ 0.91% Cu, 0.69 g/t Au (Afton open pit - Prod., to 1992),
9.5 Mt @ 1.5% Cu, 1 g/t Au, 7 g/t Ag (Afton underground - Geol. Reserve),
18.3 Mt @ 0.47% Cu, 0.34 g/t Au (Ajax West - Prod.+Res.),
6.4 Mt @ 0.44% Cu, 0.34 g/t Au (Ajax East - Prod.+Res.),
3.3 Mt @ 0.71% Cu, 0.41 g/t Au (Big Onion - Open pit Res.),
2.6 Mt @ 0.38% Cu, 0.24 g/t Au (DM - Geol. Res.),
2.4 Mt @ 0.35% Cu, 0.72 g/t Au (Pothook - Prod. to 1992),
1.2 Mt @ 0.46% Cu, 0.21 g/t Au (Crescent - Prod. to 1992).
65.8 Mt @ 0.77% Cu, 0.55 g/t Au (Total Afton-Ajax - Res. + Prod).
Geology
The Afton-Ajax group of alkaline porphyry Cu-Au deposits comprise one larger (Afton) and a number of smaller nearby bodies (Ajax West, Ajax East, Big Onion, DM, Crescent and Pothook, Iron Mask, Erin, Python, Galaxy, Kimberley) with a size range from 1.23 to 31.6 Mt and grade ranging from 0.35 to 1.5% Cu and 0.24 to 1.0 g/t Au, as listed in the table above. The deposits lie within a 10 x 4 km area of the multi-phase Iron Mask batholith.
The elongate, north-west trending Iron Mask batholith is 30 km long and up to 5 km wide. It is a sub-volcanic multiple intrusion emplaced between 205 and 190 Ma, which is predominantly a diorite, but includes phases ranging from gabbro to syenite composition. The batholith lies length-wise in a major cross structure cutting the Quesnellia Terrane and intrudes comagmatic volcanics of the upper Nicola Group. The cross structure is controlled by a deep-seated, long active structure that has influenced the pattern of development of both intrusives and sediments (Carr & Reed, 1976; Dawson, et al., 1991).
The Iron Mask Batholith has been divided into a number of 'units', namely the Pothook (diorite and gabbro), Hybrid (hornblendite and magnetite rich gabbro), Sugarloaf (porphyritic diorite to andesite) and Cherry Creek (diorite to syenite and generally porphyritic) Phases. Concentrations of copper and magnetite, and associated alteration haloes, are known throughout the Iron Mask Batholith, although the main economic deposits are localised within the Cherry Creek and Sugarloaf Units. The Pothook diorite and gabbro is the oldest, followed by the finer grained more porphyritic units, culminating in the Cherry Creek Phase which is the youngest. The Cherry Creek phase has a wide variety of rock types, often occurring as intrusion breccias, cross-cutting dykes and sills and comprises mainly diorite, monzonite and syenite which are generally porphyritic and together form relatively large bodies (Carr & Reed, 1976). The Sugarloaf Phase is present as elongate bodies up to 1 km wide as well as narrow dykes. It is characterised by the presence of hornblende and/or augite phenocrysts and a fairly andesitic composition and is similar in composition to sections of the Nicola Volcanics with which it is interpreted to be co-magmatic. Both the Cherry Creek and Sugarloaf Units are indicated as being shallow sub-volcanic intrusives.
The intruded and co-magmatic Nicola Group around the Iron Mask Batholith consists of andesitic to basaltic tuffs, tuff breccias, interbedded flows and flow breccias. Elsewhere large amounts of greywacke, argillite and limestone occur within this sequence of alkaline volcanics.
Mineralisation & Alteration
Hypogene mineralisation is similar at all of the significant Cu-Au deposits. The dominant style of mineralisation is chalcopyrite with calcite infilling fractures and veinlets, often with chloritic selvages and as disseminations and blebs. Pure chalcopyrite veins up to 2 cm thick are known. Bornite and pyrite are minor constituents, although significant bornite does occur in the main Afton orebody. The Iron Mask Batholith has widespread saussuritic (propylitic) and K-feldspar alteration as well as high magnetite contents (up to 10% in the intrusive) and massive magnetite zones. The area within a 1 km radius of each deposit is marked by an epidote-albite-chlorite-magnetite-carbonate alteration assemblage, increasing towards ore, and by irregular weak pyritic haloes, generally comprising <1% pyrite, to a maximum of 5%.
The main alteration associated with chalcopyrite mineralisation is fracture controlled albitisation and K-feldspar development. Anhydrite is also a common alteration mineral in mineralised veins. Ore grade chalcopyrite-calcite-fluorite fracture and veinlet mineralisation can also occur in strong epidote-calcite-magnetite (propylitic) altered zones. Intense albitisation produces hard creamy-white albite and tends to seal rocks, decreasing fracturing and the Cu-Au grade. Fine grained secondary biotite tends to occur away from albite alteration zones. At Ajax albitisation has destroyed magnetite forming a magnetic low.
At the main Afton deposit the steeply dipping chalcopyrite-bornite-pyrite-magnetite orebody is developed entirely within breccias, fractured dioritic and latitic porphyry of the Cherry Creek 'unit' on the north-western extremity of the composite alkalic Iron Mask Batholith. The following rock types are encountered in the orebody area, namely i). diorite, dated at 190±6 Ma, which is known only in the footwall and comprises a coarse greenish-grey rock with phenocrysts up to 5 mm across of plagioclase, pyroxene and hornblende in an interstitial matrix of quartz and K-feldspar, with biotite patches; ii). diorite porphyry which predominates in the orebody area and is a grey-green to grey-pink, fine grained rock composed of plagioclase phenocrysts up to 1 mm in size and scattered smaller biotite contained in a finely granular to aphanitic groundmass of plagioclase and minor K-feldspar; iii). latite porphyry present as numerous dykes which increase in size upwards into the Cu deposit. It is more porphyritic than the diorite porphyry and exhibits euhedral plagioclase phenocrysts up to 3 mm in size in an aphanitic groundmass rich in K-feldspar with minor biotite; iv). intrusion breccia which occurs in narrow intersections throughout the deposit and consists of close spaced, sub-rounded fragments of all of the Cherry Creek Unit lithologies in a fine grained matrix of consisting of K-feldspar and lesser plagioclase. Clasts are generally <2 to 3 cm, although larger fragments are known (Carr & Reed, 1976).
Hypogene alteration in the main Afton deposit has no recognised pattern and includes the successively developed K-silicate, saussuritic and phyllic varieties. The K-silicate alteration is characterised by secondary K-feldspar and locally by hydrothermal biotite and is sporadically distributed, possibly related to the latite porphyries. The saussuritic or propylitic alteration, chiefly epidote-chlorite-magnetite and rarely quartz and calcite, and is apparently related to the pyritic zone to the south of the orebody. These are followed by a very minor localised phase of phyllic quartz-sericite alteration. All of these are overprinted by argillic alteration which includes some montmorillonite and intense pervasive introduction of hematite. This latter phase has been attributed to supergene processes (Carr & Reed, 1976).
A large scale zoning of magnetite, pyrite and Cu minerals is crudely evident in the vicinity of the orebody. Abundant hydrothermal magnetite (content not available in the literature) forms a 300 m wide zone trending north-westwards to the Afton orebody over a distance of 800 m. The magnetite zone contains the Afton orebody on its north-western end, and is flanked by barren pyrite zones. The south-western pyrite zone, which is around 1.5 km long and 300 m wide, forms the hangingwall at Afton, and also flanks the Pothook deposit around 1 km to the south-east. This pyrite zone contains up to 10% pyrite south of Afton, chiefly as fracture fillings in the Cherry Creek Phase and Nicola Volcanics (Carr & Reed, 1976).
The main Afton deposit as defined by the 0.25% Cu cut-off, is a tabular to vertically wedge shaped body striking at 290° and dipping at 55°S. In a simplified form it is 520 m long by an average of 90 m wide and persists to a depth of at least 600 m. The deposit comprises two distinct zones and upper oxidised (supergene) section to a depth of around 400 m below the surface, and a lower sulphide rich (hypogene) zone which is only preserved in the central and western sections of the deposit (Carr & Reed, 1976).
In the hypogene zone a few supergene minerals, mainly chalcocite and covellite, persist. Bornite predominates in the hangingwall, with lesser chalcopyrite, although this gradually changes towards the footwall where the reverse is true. Pyrite in the barren hangingwall disappears within 10 to 20 m of the orebody, but locally infringes on the orebody with both chalcopyrite and with bornite. There is no pyrite in the footwall. The orebody has a central higher grade core in its central and western sections. In the upper sections this splits into a series of shoots, while at depth to the west it occupies most of the ore zone from footwall to hangingwall (Carr & Reed, 1976).
The inferred supergene mineralisation has a sharp contact with the underlying hypogene zone. This boundary plunges steeply in places, with 'supergene' minerals persisting to 600 m below surface, while elsewhere steep narrow columns of hypogene mineralisation remain at high levels. Metallic native Cu defines the 'supergene' zone, comprising 65 to 85% or more of the Cu in the upper 100 m of the orebody, where it occurs as fine scales, dendrites, films and granules, but also in masses up to 5 mm thick. Chalcocite is present as 10 to 30% of the available Cu in the same interval and is both the sooty variety and as digenite, occurring both as disseminations and in veins up to 25 mm thick, where it replaces both bornite and chalcopyrite. Both native Cu and chalcocite are coated variously by cuprite, covellite and hematite, especially in the upper sections. Malachite, azurite, conichalcite and chrysocolla are very minor constituents, mostly within a few metres of the surface. The rocks in the 'supergene' zone are disintegrated and impregnated with hematite, which clogs fractures and stains the altered feldspars. In the western part of the orebody earthy hematite is spectacularly developed, probably from the original magnetite, veins of which remain partly preserved to the east. The average grade of the oxidised ore is a little lower than that of the hypogene mineralisation (Carr & Reed, 1976). Adjoining Eocene strata post date the supergene enrichment, while the orebody is terminated to the west by a fault. Geophysical and geochemical data fail to clearly distinguish the orebody from the widespread surrounding sub-economic mineralisation (Carr & Reed, 1976; Dawson, etal., 1991).
A feature of the Ajax West and East deposits, and the Crescent pit is the intense tectonism evident during and after the mineralisation. Fracturing is intense, with many fractures having slickensides and many veins being off-set. The Ajax West, Big Onion and Pothook deposits have central sulphide rich mineralised breccia pipes up to 50 m across. The dominant structural directions evident in the Ajax West pit are north-west trending faults and fractures associated with the structural grain of the batholith, and east-west trending fault zones along the boundaries between the Sugarloaf and Hybrid unit diorites.
The most recent source geological information used to prepare this decription was dated: 1998.
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.
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Selected References:
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Carr J M, Reed A J 1976 - Afton: A supergene copper deposit: in Sutherland Brown A, (Ed), Porphyry Deposits of the Canadian Cordillera CIM, Canada CIMM Spec Vol 15 pp 376-387
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Lang J R, Stanley C R 1995 - Contrasting styles of alkalic porphyry copper-gold deposits in the northern part of the Iron Mask batholith, Kamloops, British Columbia: in Schroeter T G (Ed), Porphyry Deposits of the Northwestern Cordillera of North America Can. Inst. of Min. Met & Pet. Spec Vol 46 pp 581-592
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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.
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Ross K V, Godwin C I, Bond L, Dawson K M 1995 - Geology, alteration and mineralisation of the Ajax East and Ajax West copper-gold alkalic porphyry deposits southern Iron Mask batholith, Kamloops, British Columbia: in Schroeter T G (Ed), Porphyry Deposits of the Northwestern Cordillera of North America Can. Inst. of Min. Met & Pet. Spec Vol 46 pp 565-580
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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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