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Cargill |
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Ontario, Canada |
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P
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Super Porphyry Cu and Au
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The Cargill carbonatite-phosphate deposit is located 32 km southwest of Kapuskasing, north of the Timmins Mining District, northern Ontario, Canada, and lies within the Superior Province of the Canadian Shield, #Location: 49° 17'N, 82° 49'W.
The Cargill carbonatite complex is overlain by a very high grade secondary residual phosphate deposit, where Cretaceous weathering and dissolution of the soluble minerals in the host carbonatite rock has left behind a residue of the insoluble minerals, largely apatite crystals, accompanied by a well-developed karst topography, now buried under glacial lake deposits of lacustrine clays, and boulder tills of Pleistocene age.
The Cargill carbonatite complex lies within the Ontario Carbonatite Province of northern Ontario and western Quebec, the only known concentration of carbonatite complexes in North America (Erdosh, 1979). This petrographic province is made up of some 50 known carbonatite complexes over an area of 1.3 million sq. km, almost all of which occur along recognisable major tectonic features. They all contain 5 to 25% apatite in the carbonatite phase, with some having undergone significant enrichment of apatite through leaching of carbonates, as at Cargill. Based on their ages, the carbonatites belong to four groups; the two younger, dated at 120 and 570 Ma, are restricted to the Ottawa graben, whereas the two older groups, dated at 1100 and 1700 Ma, are distributed along the Kapuskasing and the Albany Forks gravity highs. All of these gravity highs/structures are major tectonic features that are probably related to the Mid-Continent Gravity High, the geophysical expression of the Mid-continental Rift mafic to ultramafic intrusive and extrusive masses.
The Cargill carbonatite complex occurs within the Kapuskasing Sub-province of the Archaean Superior Province. This sub-province is geophysically characterised by a northeast- to NNE-trending zone of gravity highs and pronounced linear aeromagnetic trends. It is one of several carbonatite complexes that occur within the sub-province. The intrusion has been dated at 1740 Ma (K-Ar) and 1907±4 Ma (U-Pb). The location of the carbonatite complex was controlled by regional NE-trending faults characteristic of the subprovince. The complex has in turn been dislocated by the later Cargill Fault that has split the original near circular, ~1.25 to 1.5 km diameter intrusion into two parts, and moved the northwestern third ~2.5 km to the NE to form a NE-SW elongated dumbbell shape in plan-view. In detail the two portions of the disrupted complex are connected by slivers of carbonatite between strands of the broad, up to 250 m thick, Cargill Fault Zone. In aeromagnetic data, the dumbbell-shaped body is ~7.2 km long consisting of two pronounced aeromagnetic anomalies separated by a narrow band of lower magnetic intensity. A third aeromagnetic anomaly is evident 4 km west of the main intrusive mass.
The carbonatite was intruded into an Archaean to Palaeoproterozoic gneissic terrane composed of quartz diorite gneiss and amphibolite. These rocks are mottled white and black on weathered surfaces and have a banded pink and black appearance on fresh surfaces. The northeast trending and steeply northwest dipping banding comprises up to 2 cm thick segregations reflecting the variation in the relative amounts of the mafic (dominantly biotite) and felsic components. Fenitisation is distinctly developed marginal to the west sub-complex, but is absent in quartz diorite rocks in contact with the complex to the south.
The carbonatite complex consists of arcuate to curvilinear bands of siderite, calcite and dolomite carbonatite enclosed in a mass of pyroxenite, hornblendite and related rocks. The peripheral ultramafic to mafic rocks have an intrusive contact with the enclosing gneissic complex, and include olivine clinopyroxenite, olivine-rim magnetite clinopyroxenite, magnetite-oligoclase clinopyroxenite, hornblendite, hornblende-oligoclase pyroxenite, soda-pyroxene hornblendite, late stage veins and segregations, phlogopite clinopyroxenite, carbonatite and ultramafic hybrid rocks. The rock is massive, dark-grey to black, medium- to coarse-grained, and strongly magnetic.
A narrow contact zone that varies from a few to several tens of metres in width separates the pyroxenite-amphibolite intrusive from the carbonatite. It is composed of a series of alternating bands of carbonatite and pyroxenite. The presence of pyroxenite xenoliths within the carbonatite, suggest the latter intrudes the pyroxenite.
The carbonatite is multi-phased and zoned. The outer zone is calcite carbonatite rock (sovite) and the core is dolomite carbonatite (rauhaugite). Twyman (1983) subdivided the sovites into olivine-, clinohumite- and arfvedsonite-carbonatites, based on their silicate content. With increasing accessory mineral content, sovite grades into zones of silicocarbonatite. Arcuate or lens-shaped sideritic bodies, which are probably the latest, intrude the earlier carbonatite phases. Contacts between calcite and dolomite carbonatites are gradational, although the contacts between siderite carbonatite and the other carbonatites are sharp.
Twyman (1983) proposed that the source magma was a carbonated mafic alkalic silicate magma that underwent immiscibility at ~27 kbar in the upper mantle, giving rise to an olivine sovite magma containing approximately 8% alkalies at 1200 to 1100°C which differentiated to a natrocarbonatite magma by crystal fractionation. The carbonate phases form as cumulates through heterogeneous nucleation on magma chamber walls, from the walls inward, with the sovites and rauhaugite being progressively developed cumulates from the original alkaline carbonatite magma. Late stage crystallisation of a carbonatite magma evolves to a hydrothermal fluid that forms ankerite breccia zones, veins and replacement of calcite by ankerite. Olivine sovites are the most primitive and arfvedsonite-aegirine carbonatites the most evolved.
The sovite carbonatite (with more than 50% carbonate), where fresh, is a white or light-grey, coarse- to medium-grained, moderately banded, inequigranular-seriate, hypidiomorphic to allotriomorphic rock, with straight to curved grain boundaries. Calcite is the dominant carbonate species, accompanied by trace to 15% phlogopite, 1 to 15% magnetite, 0 to 10% clinohumite, 50 to 100% carbonate, 2 to 15% apatite, 0 to 15% olivine, 0 to 5% pyrrhotite and 0 to 10% amphibole.
In thin section the silicocarbonatite is fine to medium grained, equigranular, hypidiomorphic to allotriomorphic, with straight to curved grain boundaries. An estimate of the mode is 5 to 55% phlogopite, 1 to 30% magnetite, 0 to 10% clinohumite, 15 to 50% carbonate, 0 to 5% apatite, 0 to 30% olivine, and 2 to 70% amphibole.
Rauhaugite is largely confined to the core of the complex, where dolomite becomes the most abundant carbonate phase. Where fresh, it is a massive, fine grained, dense, beige to tan coloured rock in which dolomite is the dominant carbonate species. The rock contains a visually estimated 95% dolomite, 5% apatite, and minor biotite, while magnetite was locally observed.
The carbonatite complex is overlain by a residual accumulation that resulted from weathering of the carbonatite, most likely during the Cretaceous. It is a light- to dark-grey, sometimes brownish, unconsolidated material, predominantly of sand-size material, composed of white or colourless apatite crystas, crystal fragments or rounded grains, with minor goethite, siderite, magnetite, crandallite and pyrite. Apatite locally constitutes up to 100% of the residuum and is diluted in places by clay, vermiculite, quartz, iron oxides and chlorite. Near the pyroxenite contact the residuum consists entirely of biotite, clay and chlorite derived from weathering of the pyroxenite, while in other places there is little apatite and the main constituent is goethite. Even though apatite is present in all phases of the complex, the protore for the apatite residuum is principally sideritic and dolomitic carbonatite (Pressacco, 2002). A crandallite-rich blanket near the top of the apatite residuum is high in rare earth values which may ultimately prove of economic interest (Sandvik and Erdosh 1977; Erdosh 1979).
The residuum is separated from the primary unweathered carbonatite by a layer of saprolite, whose thickness varies, depending upon the composition of the underlying carbonatite. Cemented residuum is sometimes found immediately above bedrock, overlain by unconsolidated residuum. Where observed, it is a yellow- to brownish-yellow with the appearance of highly weathered, strongly leached, medium-grained consolidated rock and is composed of loosely cemented apatite (20 to 80%), magnetite and other iron oxides. Mostly it is only a few metres thick, but may be as much as 100 m in bedrock troughs.
The residuum is very variable in thickness, being as much as 170 m in glacial troughs, and thinning to only a few metres over intervening ridges.
The initial mining extracted two principal types of residuum, A and B. The A type ore is characterised by its low ironand relatively high phosphate contents (0 to 8% total Fe, and >30 % P2O5). It can vary in colour from white (virtually pure apatite sands) through tan-sandy-brown to grey and dark grey to black. Pyrite and pyrrhotite are believed to be the dominant iron minerals in this ore. The B ores have higher iron and relatively lower phosphate contents (8 to 30% total Fe, and 15 to 30% P2O5). Its colour varies depending upon the dominant iron minerals (magnetite, hematite, limonite/goethite, and ilmenite) and can be earthy brown, chocolate brown, wine-red, tan and rusty (Pressacco, 1998).
The residuum is unconformably overlain by Late Cretaceous to Early Tertiary pre-glacial well-sorted silica/kaolinite sand, organic clay and peat deposits; Pleistocene homogenous grey-blue clay and varved brown glacial lake clays, and Recent swamp and alluvium deposits. This overburden has low-strength and high-moisture contents requiring pit slopes slopes as shallow as 6:1.
Using a 15% P2O5 cut off grade, the total mineral inventory in 1998 was 49.078 Mt @ 24.4 % P2O5 (Pressacco, Agrium Inc., 1998).
This summary, except where otherwise acknowledged, is largely based on Sage, 1988, Ontario Geological Survey Study 36, and Erdosh, 1989.
The most recent source geological information used to prepare this decription was dated: 2012.
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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Erdosh G, 1979 - The Ontario Carbonatite Province and its phosphate potential: in Econ. Geol. v.74 pp. 331-338
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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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