Palladium–copper-rich platinum-group element mineralization in Legris Lake mafic–ultramafic complex,Western Superior Province,Canada

Abstract

The Legris Lake Complex is a northeast-trending, 7.3 km × 3.5 km, mafic-ultramafic intrusive complex located in the western Wabigoon Subprovince of the Archaean Superior Province. It is part of a circular series of mafic-ultramafic complexes, the most notable of which is the Lac des Iles Complex, ca 7 km northwest of the Legris Lake Complex, which hosts the Lac des Iles palladium mine. Four phases of exploration have been conducted on the Legris Lake Complex property since autumn, 1999, defining mineralized zones on surface and at depth, with drill-core intersections of up to 2.04 g/t Pd, 0.41 g/t Pt, 0.71 g/t Au, 0.42% Cu and 0.13% Ni over 9.95 m.

The Legris Lake Complex consists of mostly gabbroic rocks but also contains lithologies ranging from anorthosite to wehrlite and a variety of igneous breccias. The gabbroic rocks vary from melanogabbro to porphyritic leucogabbro. Medium-grained, massive, biotite-rich leucogabbro is the predominant exposed variety and probably caps the complex. The north-western margin of the complex (2 km × 600 m), which contains all the known platinum-group element (PGE) mineralization, is characterized by heterolithic breccia with abundant fragments of sedimentary rocks and numerous gabbroic dykes and sills.

The PGE mineralization in the Legris Lake Complex has different characteristics from many PGE deposits, where the ore occurs in sulphide-rich bodies at the base of mafic and ultramafic rocks. In the Legris Lake Complex PGE enrichment occurs in sulphide-poor (1-10 vol%, mostly <5 vol%), medium- to coarse-grained, porphyritic leucogabbro (termed ‘Main Showing-type') hosted within zones of intense magmatic brecciation. The mineralized leucogabbro is mineralogically highly evolved and exhibits a sill-like form near the stratigraphic top of the complex. The mineralization consists of disseminated to blebby sulphides rimmed by epidote and disseminated magnetite. The mineralized rocks typically contain Cu ranging from 0.2 to 0.4 wt% and Ni from 0.07 to 0.12 wt%, with low Pt/Pd ratios (ca 0.2) and high Cu/Ni ratios (ca 3). The PGE contents display a positive correlation with those of Cu and Ni. The origin of the mineralization is best explained by preferential partitioning of PGE into an immiscible sulphide melt in evolved silicate magma after fractional crystallization of olivine and clinopyroxene. The immiscible separation of sulphide melt may have been aided by incorporation of silica and sulphide from adjacent sedimentary rocks. Formation of magnetite and hydrous minerals in the mineralized zones suggests that the sulphide melt had high oxygen/(oxygen + sulphur) ratios and high contents of volatiles, most probably reflecting a high oxidation state and volatile-rich nature of the parental magmas. This primary magmatic PGE mineralization was followed by minor redistribution of PGE by deuteric hydrothermal fluids released from the parental magma.

Get full access to this article

View all access options for this article.

References

Barnes

S. J.

, Keays

R. R.

and Hoatson

D. M.

Distribution of sul-phides and PGE within the porphyritic websterite zone of the Munni Munni Complex, Western Australia, Aus. J. Earth Sci., 39, 1992, 289–302.

Boudreau

A. E.

Chlorine as an exploration guide for the plat-inum-group elements in layered intrusions.

J. geochem. Explor., 48, 1999, 21–37.

Brugmann

G. E.

, Naldrett

A. J.

and Macdonald

A. J.

Magma mixing and constitutional zone refining in the Lac des Iles Complex, Ontario: genesis of platinum-group element mineralization.

Econ. Geol., 84, 1989, 1557–73.

Davis

D. W.

and Edwards

G. R.

Crustal evolution of Archaean rocks in the Kakagi Lake area, Wabigoon Subprovince, Ontario as interpreted from high precision U—Pb geochronology.

Can. J. Earth Sci., 23, 1986, 182–92.

Fleet

M. E.

. Laboratory partitioning of platinum-group ele-ments (PGE) and gold with application to magmatic sulphide—PGE deposits. Lithos, 47, 1999, 127–42.

Francis

R. D.

Sulphide globules in mid-ocean ridge basalts (MORB) and the effect of oxygen abundance in Fe—S-0 liquids on the ability of those liquids to partition metals from MORB and komatiite magmas.

Chem. Geol., 85, 1990, 199–213.

Gaetani

G. A.

and Grove

T. L.

Wetting of mantle olivine by sul-phide melt; implications for Re/Os ratios in mantle peridotite and late-stage core formation.

Earth Planet. Sci. Lett., 169, 1999, 147–63.

Gaetani

G. A.

and Grove

T. L.

Partitioning of moderately siderophile elements among olivine, silicate melt and sulphide melt: constraints on core formation in the Earth and Mars.

Geochim. cos - mochim. Acta, 61, 1997, 1829–46.

Gupta

V. K.

and Sutcliffe

R. H.

Mafic—ultramafic intrusives and their gravity field; Lac des Iles area, northern Ontario.

Geol. Soc. Am. Bull., 102, 1990, 1471–83.

10.

Hrominchuk

J. L.

Geology, stratigraphy, geochemistry and copper—nickel—platinum group element mineralization in the River Valley intrusion. In Summary of field work and other activities 2000. Open File Rep. Ont. geol. Surv. 6032, 2000, 29-1-11.

11.

Lavigne

M. J.

. Field trip 2 and 4 Lac des Iles mine. Field trip guidebook, Superior PGE 2001, CIM Geological field confer-ence, 16–19 Sept. 2001. www.cim.org.

12.

Macdonald

A. J.

Platinum-group element mineralization and the relative importance of magmatic and deuteric processes; field evi-dence from the Lac des Iles deposits, Ontario, Canada. In Prichard

H. M.

. eds Geo-platinum 87, 1988, 215–36.

13.

MacTavish

A. D.

The mafic—ultramafic intrusions of the Atikokan—Quetico area, northwestern Ontario.

Open File Rep. Ont. geol. Surv. 5997, 1999.

14.

Mavrogenes

J. A.

and O'Neill

H. S. C.

The relative effects of pressure, temperature and oxygen fugacity on the solubility of sulphide in mafic magmas.

Geochim. cosmochim. Acta, 63, 1999, 1173–80.

15.

McDonough

W. F.

and Sun

S-S.

The composition of the Earth.

Chem. Geol., 120, 1995, 223–53.

16.

Michaud

M. J.

The geology, petrology, geochemistry and platinum-group element—gold—copper—nickel ore assemblage of the Roby Zone, Lac des Iles mafic—ultramafic complex, northwestern Ontario. M.Sc. thesis, Lakehead University, Ontario, 1998.

17.

Naldrett

A. J.

and Cabri

L. J.

Ultramafic and related mafic rocks, their classification and genesis with special reference to the concentration of nickel sulphides and platinum-group elements.

Econ. GeoL, 71, 1976, 1131–58.

18.

Paces

J. B.

and Miller

J. D.

Jr.

Precise U—Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights to physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga mid-con-tinent rift system.

J. Geophys. Res., 98, 1993, 13997–14013.

19.

Peck

D. C.

. Controls on the formation of contact-type platinum-group element mineralization in the East Bull Lake intru-sion. Econ. Geol., 96, 2001, 559–81.

20.

Peck

D. C.

and Keays

R. R.

Insights into the behavior of pre-cious metals in primitive, S-undersaturated magmas; evidence from the Heazlewood River complex, Tasmania.

Can. Miner., 28, 1990, 553–7.

21.

Pettigrew

N. T.

Report on 1999–2000 exploration Legris Lake palladium-platinum property, Avalon Ventures, Ltd./Starcore Resources, Ltd., Joint Venture company report, 2000.

22.

Prendergast

M. D.

The geology and economic potential of the PGE-rich Main Sulphide Zone of the Great Dyke, Zimbabwe. In Prichard

H. M.

. eds Geo-platinum 87 (London: Elsevier, 1988), 281–302.

23.

Puchtel

and Humayun

Platinum group elements in Kostomuksha komatiites and basalts. Implications for oceanic crust recycling and core-mantle interaction.

Geochim. cosmochim. Acta, 64, 2000, 4227–42.

24.

Puchtel

I. S.

and Humayun

Platinum group element frac-tionation in komatiitic basalt Lava Lake.

Geochim. cosmochim. Acta, 65, 2001, 2979–93.

25.

Pye

E. G.

and Fenwick

Atikokan-Lakehead sheet. Ontario Dept of Mines, Compilation Series, map

2065, 1964, scale 1 : 253400.

26.

Raedeke

L. D.

and McCallum

I. S.

Investigations of the Stillwater Complex, II. Petrology and petrogenesis of the Ultramafic series.

J. Petrology, 25, 1984, 395–420.

27.

Rehkamper

. Ir, Ru, Pt and Pd in basalts and komati-ites: new constraints for the geochemical behavior of the platinum-group elements in the mantle. Geochim. cosmochim. Acta, 63, 1999, 3915–34.

28.

Sassani

D. C.

and Shock

E. L.

Solubility and transport of plat-inum-group elements in supercritical fluids: summary and estimates of thermodynamic properties for ruthenium, rhodium, palladium and platinum solids, aqueous ions and complexes to 1000°C and 5 kbar.

Geochim. cosmochim. Acta, 62, 1998, 2643–71.

29.

Sutcliffe

R. H.

Regional geology of the Lac des Iles area, District of Thunder Bay.

Misc. Pap. Ont. geol. Sum, 132, 1986, 70–5.

30.

Sutcliffe

R. H.

Magma mixing in late Archaean tonalitic and mafic rocks of the Lac des Iles area, Western Superior Province.

Precamb. Res., 44, 1989, 91–101.

31.

Sutcliffe

R. H.

, Sweeny

J. M.

and Edgar

A. D.

The Lac des Iles Complex, Ontario: petrology and platinum-group elements mineral-ization in an Archaean mafic intrusion.

Can. J. Earth Sci., 26, 1989, 1408–27.

32.

Sweeny

J. W.

and Sutcliffe

R. H.

Geology and platinum group element mineralization of the Roby Zone, Lac des Iles Complex. In Summary of field work and other activities 1986.

Misc. Pap. Ont. geol. Sun'., 132, 1986, 82–4.

33.

Talkington

R. W.

and Watkinson

D. H.

Trends in the distribu-tion of the precious metals in the Lac des Iles Complex, northwestern Ontario. Can. Miner., 22, 1984, 125–36.

34.

Tatsumi

, Oguri

and Shimoda

The behaviour of plat-inum-group elements during magmatic differentiation in Hawaiian tholeiites.

Geochem. J., 33, 1999, 237–47.

35.

Todd

S. C.

. The J-M platinum-palladium reef of the Stillwater Complex, Montana: I. Stratigraphy and petrology. Econ. Geol., 77, 1982, 1454–80.

36.

Vogel

D. C.

and Keays

R. R.

The petrogenesis and platinum-group element geochemistry of the Newer Volcanic Province, Victoria, Australia.

Chem. Geol., 136, 1997, 181–204.

37.

Von Gruenewaldt

, Sharpe

M. R.

and Hatton

C. J.

The Bushveld Complex: Introduction and review.

Econ. Geol., 80, 1985, 803–12.

38.

Wendlandt

R. F.

Sulphide saturation of basalt and andesite melts at high pressures and temperatures.

Am. Miner., 67, 1982, 877–85.

39.

Williams

H. R.

Subprovince accretion tectonics in the south-central Superior Province.

Can. J. Earth Sci., 27, 1990, 570–81.

40.

Xiong

and Wood

S. A.

Experimental quantification of hydrothermal solubility of platinum-group elements with special ref-erence to porphyry copper environments.

Mineral. Petrol., 68, 2000, 1–28.