High-grade copper ore

from the McCreedy West mine, Sudbury mining district, Ontario, Canada

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Above: A sawn piece of massive chalcopyrite ore, with a 10-gram offcut and polished thin section prepared from the specimen. The sample appears to be almost pure granular chalcopyrite, grain size 2-3 mm, with perhaps 1% of scattered grains of bronze-coloured pentlandite. The chalcopyrite has a characteristic tarnish. Main sawn piece is 197.36 g, magnetic susceptibility averaging 2.00x10-3 SI units, which is not appreciably magnetic in hand specimen.


"Rock of the Month # 115, posted for January 2011" ---

Chalcopyrite is one of the most widespread and economically important ore minerals of copper.

The sample was collected underground in June 2010 from a vein of visually massive sulphide ore, largely chalcopyrite, in the McCreedy West mine, known in its early days as Levack West.

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Above left: a single 40x40-micron inclusion of white PGM (sperrylite?) in fractured chalcopyrite host.
Above right: equant, euhedral magnetite inclusion, 125 microns across, in chalcopyrite.
Images at nominal 200X magnification, long-axis field of view circa 0.45 mm, in plane-polarized reflected light.

An optical examination of the massive chalcopyrite vein reveals traces of pentlandite and at least one white platinum group mineral (most probably sperrylite, platinum arsenide). Inclusions and late veinlets include carbonate and anhydrite, translucent yellow-brown sphalerite and magnetite. A remarkable ore, with an undiluted grade of circa 30 weight percent copper plus values of nickel, zinc, platinum and no doubt other metals. Although pentlandite and magnetite inclusions could have formed in a magmatic setting, the presence of carbonate, anhydrite and sphalerite as inclusions and along fractures speaks to a post-magmatic, lower-temperature deposition during vein filling in the footwall of the Sudbury igneous complex, as it cooled following an unimaginably large asteroidal impact event some 1,850 million years ago.

The McCreedy West mine lies on the west side of the North Range of the Sudbury structure, and is well-documented in terms of mining, geology, geophysics and mineralization (e.g., Hoffman et al., 1979; Coats and Snajdr, 1984; Naldrett, 1984; Whiteway, 1990; Naldrett et al., 1992; Pessaran, 1993; Farrow and Watkinson, 1997; Naldrett et al., 1999; Farrow and Lightfoot, 2002; Roussell et al., 2002; Morgan et al., 2002; Farrow et al., 2005).

The 8-km Hardy-Longvack section is the most intensely mineralized portion of the North Range, including the McCreedy West, Onaping, Strathcona and other deposits. Amongst several styles of mineralization are spectacular footwall veins which extend downwards and outwards from the Sudbury igneous complex into the host rocks, and contain often-spectacular grades of Cu and platinum group elements, justifying a) pursuit of these veins to depth, and b) paying a cost in dilution (the term for reduction of the grade of ore recovered, in taking out a minimum thickness of the barren or at least subeconomic wall rocks, in order to permit safe mechanized mining). The footwall vein deposits are enriched in copper, gold and silver, platinum, palladium and other metals. Total Pt+Pd+Au in massive sulphide veins is commonly 10-20 ppm (grams per tonne), a very desirable target, even neglecting the bonanza copper grades! The Sudbury igneous complex crystallized at 1850±1 Ma. Osmium isotope ratios show substantial variation, which may be explained if a large (>50%) proportion of the igneous complex is derived by impact-induced melting of local crust (Walker et al., 1991).

FNX, in a joint venture with Dynatec, acquired McCreedy West and four other Sudbury mines from Inco in 2002 (Scales, 2005). Commercial production was declared at McCreedy West in November 2003. The deposit was mined originally from 1970 to 1998, with total production of 15.8 MT grading 1.70% Cu, 1.44% Ni and 1.3 ppm Pt + Pd + Au across 7 known ore zones. Levack and McCreedy West, aka the Levack complex, contain foorwall veins with grades much higher than the above average of different ore types.

N.B. I have temporarily "misplaced" (i.e., buried) my field notes and the field guide for the trip during which this sample was collected, part of the 11th International Platinum Symposium at Laurentian University, June 2010. I'll add a short update when I unearth them! - GCW.

References

Coats,CJA and Snajdr,P (1984) Ore deposits of the North Range, Onaping-Levack area, Sudbury. In `The Geology and Ore Deposits of the Sudbury Structure' (Pye,EG, Naldrett,AJ and Giblin,PE, editors), OGS Spec.Vol. 1, 603pp., 327-346.

Farrow,CEG and Lightfoot,PC (2002) Sudbury PGE revisited: toward an integrated model. In `The Geology, Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements' (Cabri,LJ editor), CIM Spec.Vol. 54, 852pp., 273-297.

Farrow,CEG and Watkinson,DH (1997) Diversity of precious-metal mineralization in footwall Cu-Ni-PGE deposits, Sudbury, Ontario: implications for hydrothermal models of formation. Can.Mineral. 35, 817-839.

Farrow,CEG, Everest,JO, King,DM and Jolette,C (2005) Sudbury Cu(-Ni)-PGE systems: refining the classification using McCreedy West mine and Podolsky Project case studies. In `Exploration for Platinum-Group Element Deposits' (Mungall,JE editor), MAC Short Course 35, 512pp., 163-180.

Hoffman,EL, Naldrett,AJ, Alcock,RA and Hancock,RGV (1979) The noble-metal content of ore in the Levack West and Little Stobie mines, Ontario. Can.Mineral. 17, 437-451.

Morgan,JW, Walker,RJ, Horan,MF, Beary,ES and Naldrett,AJ (2002) 190Pt-186Os and 187Re-187Os systematics of the Sudbury igneous complex, Ontario. Geochim.Cosmochim.Acta 66, 273-290.

Naldrett,AJ (1984) Mineralogy and composition of the Sudbury ores. In `The Geology and Ore Deposits of the Sudbury Structure' (Pye,EG, Naldrett,AJ and Giblin,PE, editors), OGS Spec.Vol. 1, 603pp., 309-325.

Naldrett,AJ, Asif,M, Schandl,E, Searcy,T, Morrison,GG, Binney,WP and Moore,C (1999) Platinum-group elements in the Sudbury ores: significance with respect to the origin of different ore zones and to the exploration for footwall orebodies. Econ.Geol. 94, 185-210.

Naldrett,AJ, Pessaren,A and Li,C (1992) Variation in the Ni, Cu and PGE content of ore on the North Range at Sudbury, between the Hardy and Longvack mines. Can.Mineral. 30, 495-496.

Pessaran,A (1993) Compositional Variation in Sudbury Ores and Proximity of Footwall Copper-PGE Ore Bodies. MSc thesis, University of Toronto, 166pp.

Rousell,DH, Meyer,W and Prevec,SA (2002) Bedrock geology and mineral deposits. In `The Physical Environment of the City of Greater Sudbury' (Roussell,DH and Jansons,KJ editors), OGS Spec.Vol. 6, 228pp., 20-55.

Scales,M (2005) Mining breakthrough in Sudbury. Can.Min.J. 126 no.3, 14-17.

Walker,RJ, Morgan,JW, Naldrett,AJ, Li,C and Fassett,JD (1991) Re-Os isotope systematics of Ni-Cu sulfide ores, Sudbury igneous complex, Ontario: evidence for a major crustal component. Earth Planet.Sci.Letts. 105, 416-429.

Whiteway,P (1990) North rim nickel. Northern Miner Magazine 5 no.4, 32-37 (April 1990).

Graham Wilson, 07-08 July 2011

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