The Ni–Cr–Cu content of biotite as pathfinder elements for magmatic sulfide exploration associated with mafic units of the Sudbury Igneous Complex, Ontario, Canada

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Highlights

  • We characterized the trace element chemistry of biotite from a Ni-Cu-PGE deposit, Sudbury, Ontario.

  • We used in-situ methods (LA-ICP-MS, SEM–EDS) to quantify and map the metal content of the biotites.

  • Ni, Cr, and Cu in biotite vary as a function of host lithology and proximity to mineralization.

  • Biotite in the ore-hosting lithology hosts ~ 1400 ppm < [Ni]Bt < ~ 2700 ppm and has a Ni/Cr ratio > 2.

  • Lithologies cannot be readily differentiated based on bulk rock composition or macroscopic textures.

Abstract

Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) was used to evaluate the dissolved Ni–Cr–Cu content of biotite as an exploration pathfinder for magmatic Ni–Cu–platinum-group element (PGE) sulfide deposits associated with mafic igneous units of the Worthington quartz diorite offset dyke and their host rocks at the Totten Mine (Vale Canada Ltd), Sudbury Igneous Complex (SIC), Ontario, Canada. Enrichment in Cu in biotite (up to two orders of magnitude higher than background) occurs within barren to weakly mineralized, sublayer quartz diorite and adjacent Huronian metasediments within ~ 200 m of massive sulfide. With respect to Ni and Cr, three distinct populations of biotite were distinguished based on textural and chemical criteria: (i) type I — isolated, euhedral laths within only inclusion-rich sublayer quartz diorite (IQD; the main mineralized host lithology), and high Ni (~ 1400 ppm < [Ni]Bt < ~ 2700 ppm), low to moderate Cr (~ 50 ppm < [Cr]Bt < ~ 2450 ppm) and variable Ni/Cr ratios (always > 2). (ii) Type II — coarse-grained poikiloblasts (enclosing amphibole, chlorite, and type III biotite) within country rocks, and having moderate to high Ni (~ 500 ppm < [Ni]Bt < ~ 1400 ppm), moderate to very high Cr (~ 1750 ppm < [Cr]Bt < ~ 6000 ppm) but consistently low Ni/Cr ratios (always < 0.5); and (iii) type III — subhedral to euhedral laths, intergrown with amphibole in dense, foliated aggregates and having low Ni ([Ni]Bt < ~ 300 ppm), very low to moderate Cr (~ 4 ppm < [Cr]Bt < ~ 1200 ppm), and variable Ni/Cr ratios (< 20), found within Huronian country rocks, sublayer quartz diorite (QD), and cross-cutting diabase dykes (Sudbury dyke swarm). Type I biotite within IQD is compositionally distinct from those observed in all other lithologies associated with the Sudbury Igneous Complex and its footwall rocks, and can be most readily discriminated in a Ni/Cr vs. Ni binary diagram or in a Ni–Cr–Cu ternary diagram by anomalously high Ni content and Ni/Cr ratio > 2.

Application of biotite chemistry to routine exploration requires establishing local “background” metal-in-biotite concentrations for each potential host lithology, and scrutiny of anomalous or heterogeneous metal contents in biotite resulting from discrete sulfide microinclusions that contaminate the analytical volume, and chloritization or coeval sulfide minerals in direct contact with biotite causing localized modifications to primary biotite metal abundance.

In the Sudbury environment, the Ni–Cr–Cu chemistry of biotite can be used (i) in drill core to identify proximity to mineralization, and to differentiate QD (hosting minimal sulfides) from IQD (primary host to sulfide ore bodies), rock types for which bulk textural and compositional discrimination is problematic; and (ii) in soils and tills through the analysis of biotite and its weathering products to locate buried or surface-exposed IQD. The results may be extended to other mafic–ultramafic systems where sulfide-saturated or metal-enriched intrusive phases grew metal-enriched biotite during primary crystallization, or through secondary processes of metasomatic enrichment involving remobilization of base metals by magmatic-hydrothermal fluids.

Introduction

The incorporation of specific divalent (e.g., Co2 +, Cu2 +, Ni2 +) and monovalent (e.g., Ag1 +, Cu1 +) metal cations into the structure of micas and amphiboles is well documented in experimental studies (e.g., Della Ventura et al., 1993, Hazen and Wones, 1972, Klein and Ito, 1968, Klingsberg and Roy, 1957). In micas, the substitution of divalent cations occurs within the octahedral M(1–3) sites (that contain Fe2 +, Mg2 +, Co2 +, Cu2 +, Ni2 +, Zn2 +, and Pb2 +), whereas monovalent cations replace K in the interlayer sites (Hazen and Burnham, 1973, Hazen and Wones, 1972, Klingsberg and Roy, 1957, Takeda and Morosin, 1975). The capacity for trioctahedral micas to accommodate structural substitutions is strongly influenced by both the ionic radius of the substitute, and the structural parameters of the occupancy site within the mica unit cell (Hazen and Wones, 1972, Volfinger and Robert, 1980, Volfinger et al., 1985). Early experimental work has shown that these unit cell parameters are not significantly influenced by the temperature or pressure at which the biotite crystallizes, and that variations in unit cell dimensions are due to the nature of the cations occupying the interlayer and octahedral sites (Crowley and Roy, 1960, Hazen and Wones, 1972, Klingsberg and Roy, 1957). However, more recently it has been shown that biotite that experiences reequilibration at T and ƒO2 different than those conditions of formation shows changes in unit cell dimensions due to exchange reactions involving common cations within the biotite structure, promoting structure substitutions (e.g., Chon et al., 2003, Russell and Guggenheim, 1999, Takeda and Morosin, 1975). Further complexity in substitution behavior occurs due to exclusionary and acceptance phenomena (e.g. “Fe–F”, “Mg–Cl”, and “Ni–Cl” avoidance; Hanley and Mungall, 2003, Mason, 1992, Morrison, 1991, Munoz, 1984, Ramberg, 1952, Rosenberg and Foit, 1977, Volfinger et al., 1985). Analytical techniques, such as laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) and EMP, allow quantification of these geochemical variations in-situ in samples where multiple generations of metal bearing micas might be present.

In felsic settings, the metal content of sheet silicates has been summarized in, for example, intrusion-related hydrothermal Pb–Zn–Ag deposits (Wilson, 1961), quartz–carbonate–associated and disseminated/replacement gold deposits in Archean greenstone belts (Fleet and Pan, 1998, Pan and Fleet, 1992a, Pan and Fleet, 1992b) and porphyry deposits (e.g., Ahn et al., 1997, Hendry et al., 1981, Ilton and Veblen, 1988, Ilton and Veblen, 1993). There has also been considerable effort to characterize the retention/enrichment of Ni and Cu in sheet silicates (mainly serpentine, chlorite and clay minerals) in the lateritic weathering profile associated with barren and sulfide-mineralized mafic–ultramafic rocks (e.g., Brazil: Colin et al., 1990, Barros de Oliviera et al., 1992; Ivory Coast: Nahon et al., 1982a, Nahon et al., 1982b, Noack and Colin, 1986; New Caledonia: Trescases, 1975, Pelletier, 1996, Wells et al., 2009; Australia: Elias et al., 1981, Gilkes and Suddhiprakarn, 1979; Spain: Suárez et al., 2011). However, there have been few descriptions of the metal content of hydrous alteration silicates in unweathered, fresh or hydrothermally altered (at non-surficial conditions) mafic–ultramafic settings. The majority of these studies have focused on magmatic Ni-Cu-PGE ore styles and associated alteration assemblages at Sudbury, Ontario, Canada, where elevated concentrations of Ni have been reported in biotite, amphibole and chlorite in proximity to sulfide deposits (Ames and Kjarsgaard, 2013, Farrow, 1994, Hanley and Mungall, 2003, Li and Naldrett, 1993, Magyarosi et al., 2002, Stewart, 2011, Tuba, 2012; and others therein). Of these minerals, biotite is the most appropriate mineral for study as a pathfinder/indicator mineral, because it is ubiquitous throughout both the SIC and all surrounding country rocks, having grown in association with specific and easily discernible magmatic, metamorphic, and hydrothermal events that are well documented in the literature (e.g. Ames and Kjarsgaard, 2013, Coats and Snajdr, 1984, Corfu and Andrews, 1986, Farrow, 1994, Farrow and Watkinson, 1992, Hanley and Mungall, 2003, Li and Naldrett, 1993, Magyarosi, 1998, Magyarosi et al., 2002, Noble and Lightfoot, 1992, Stewart, 2002, Thomson et al., 1985, Tuba, 2012). However, there has been no robust evaluation of the factors controlling the Ni enrichment in biotite, and its reliability as a routine exploration tool for Ni-Cu-PGE sulfide ore deposits at Sudbury, or any other mafic–ultramafic-associated sulfide deposit.

In this study we investigate the chemistry of biotite in the Totten deposit (southeast corner of Sudbury Igneous Complex) and its surrounding country rocks. The deposit is a magmatic Ni-Cu-PGE sulfide system hosted within a radial offset dyke of multiphase quartz diorite. Despite evidence of complex interaction of the dyke with its metasedimentary and metavolcanic country rocks, and overprinting by syn- to post-emplacement igneous, hydrothermal and metamorphic events, it remains a petrographically distinct unit comprised of several phases of quartz dioritic rocks, with an inclusion-rich quartz dioritic phase quartz hosting discrete magmatic Ni-Cu-PGE sulfide ore bodies (Cochrane, 1984, Grant and Bite, 1984, Lightfoot and Farrow, 2002, Lightfoot et al., 1997, Murphy and Spray, 2002, Stewart, 2011, Tuchscherer and Spray, 2002, Zurbrigg et al., 1957). Within and surrounding the Totten deposit, we characterize the trace metal content of biotite and reconcile variations in biotite metal chemistry to host lithology, proximity to sulfides, biotite origin, and a variety of analytical variables that may introduce large degrees of scatter in data sets and false anomalies unrelated to ore proximity. Applications of biotite chemistry to routine exploration are discussed.

Section snippets

Regional geology of the Sudbury Igneous Complex

The elliptical (60 × 30 km; Ames et al., 2008) Sudbury structure in Ontario, Canada, occurs at the contact of the Archean Superior Province, and the Early Proterozoic Southern Province (Ames et al., 2008, Card et al., 1984; Fig. 1A). Contained within the Sudbury structure is the Sudbury basin comprised of the Whitewater group and the Sudbury Igneous Complex (SIC). The Sudbury structure was produced through a bolide impact at 1.85 Ga (Krogh et al., 1982), supported by observations of impact-related

Geology of the Worthington offset

Detailed descriptions of the geology of, and genetic models for, the Worthington offset were presented by Lloyd (2001), Stewart (2002), and Lightfoot and Farrow (2002). The Worthington offset is located near the southwest margin of the SIC between the Denison and Drury townships (Fig. 1B). The Worthington offset is a radial-type dyke which extends more than 15 km away from the SIC. The dyke pinches and swells along its length, ranging in thickness from 30 to 100 m and dips approximately 80° to

Sampling

In order to study the variations in the trace metal content of biotite as a function of distance from ore, host rock lithology, and to define the normal “background” levels of ore metals contained within biotite from the Worthington area, samples were collected from a diamond drill core intersecting the Worthington offset dyke and associated sulfide mineralization within country rocks, QD, and IQD, with the intersection being sub-perpendicular to the dyke. A selection of 19 representative

Biotite composition and petrography

The results of the SEM–EDS analyses of biotite composition from the Totten mine area are summarized in Table 2. Using the principle component classification scheme for biotite (Fig. 2; Deer et al., 1966), there is no compositional distinction in biotites from QD, IQD, or Huronian country rocks. All of these compositions fall along or close to the annite–phlogopite join with comparable ranges in Mg#, and low ivAl p.f.u. Biotites from the Sudbury diabase dykes have slightly lower Mg# but also lie

Comparison of [Ni]Bt with deposits and rock types of the SIC environment

The ranges in [Ni]Bt of type I biotite from IQD, type II and III biotite from the Huronian country rocks, and type III biotite from QD (i.e., all biotite types, all rock types) were compared to a variety of other barren and ore-forming environments at Sudbury for which [Ni]Bt concentration data are available in the literature but for which little or no information is provided by those studies further characterizing the samples (Fig. 5; compiled from Ames and Kjarsgaard, 2013, Hanley and

Conclusion

The study of biotite chemistry in an offset dyke deposit setting at Sudbury provides insight into its potential use as an exploration indicator mineral for magmatic Ni-Cu-PGE deposits in mafic–ultramafic settings. Combining textural classification with major/trace element analysis it was possible to discriminate biotite associated with a mineralized lithology (highly elevated Ni and Ni/Cr ratio) from other forms of biotite related to post-ore regional metamorphism, and to identify anomalous Cu

Acknowledgments

The authors would like to thank Vale Canada Ltd. (Ontario Operations), the Targeted Geoscience Initiative 4 program (GSC), and NSERC (Discovery Grant 160751039 to JH) for financial support. Thoughtful reviews by Joe Petrus and Martin Tuchscherer significantly improved the manuscript.

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