D2 vein-halo-style actinolite-rich metasomatism occurred within or proximal to rocks of the Mg-Ca association (Fig. 3G). Mineralization in the Au-rich lenses is ...
Hydrothermal history and gold mineralization at the Lalor volcanogenic massive sulphide deposit, Snow Lake camp, Manitoba, Canada Antoine Caté Institut national de la recherche scientifique – Centre Eau, Terre et Environnement, 490 rue de la Couronne, Québec, QC G1K 9A9 Patrick Mercier-Langevin Geological Survey of Canada, GSC-Québec, 490 rue de la Couronne, Québec, QC G1K 9A9 Pierre-Simon Ross Institut national de la recherche scientifique – Centre Eau, Terre et Environnement, 490 rue de la Couronne, Québec, QC G1K 9A9 Shamus Duff Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5 Mark Hannington Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5 Benoît Dubé Geological Survey of Canada, GSC-Québec, 490 rue de la Couronne, Québec, QC G1K 9A9 Simon Gagné Manitoba Geological Survey, 360-1395 Ellice Ave., Winnipeg, MB R3G 3P2 Abstract. The Lalor deposit is a newly developed Volcanogenic Massive Sulphide (VMS) deposit in the Snow Lake camp, Manitoba. The deposit is hosted in a complexly deformed and metamorphosed rock package referred to as the Lalor volcanic succession and located in the VMS-prospective Chisel sequence of the Snow Lake arc assemblage. The Lalor deposit consists of stratigraphically and structurally stacked Zn-rich, Au-rich and Cu-Au-rich ore lenses gently dipping toward the NNE. The Lalor volcanic succession has undergone extensive syn-VMS hydrothermal alteration and subsequent polyphase deformation and amphibolite grade metamorphism. Metamorphosed hydrothermal alteration styles have been classified into chemical associations (K, K-Mg-Fe, Mg-Fe and Mg-Ca). Mapping of the host volcanic rocks, chemical associations and ore lenses at Lalor indicates that: 1) the Zn-rich massive sulphide lenses are preferentially associated with K and Mg-Ca alteration zones, 2) the Zn-rich massive sulphide lenses formed at two distinct stratigraphic positions as a result of protracted seafloor/sub-seafloor hydrothermal activity, 3) the Cu-Aurich zones at depth are associated with transposed discordant Mg-Fe altered rocks and presumably represent footwall feeders, and 4) Au was introduced as part of the VMS-forming event, probably during a late, lowtemperature pulse and was locally remobilized into sulphide-poor ore zones during the main deformation events. Keywords. VMS; gold; alteration; metamorphism; Snow Lake; Lalor
1 Introduction The Lalor Volcanogenic Massive Sulphide (VMS), which is located in the Snow Lake mining camp in northern Manitoba (Fig. 1), is exploited by HudBay Minerals Inc.
Production started in 2012, and the deposit has combined reserves and resources of 25.3 Mt with grades of 2.9 g/t Au, 25 g/t Ag, 5 wt% Zn and 0.79 wt% Cu (HudBay Minerals Inc., 2014), including 8.8 Mt at 4.6 g/t Au, making it the largest and richest VMS deposit of the Snow Lake camp (Mercier-Langevin et al., 2014).. Lalor belongs to a restricted subgroup of large, Auenriched VMS deposits (c.f. Mercier-Langevin et al. 2011). Such deposits are challenging exploration targets because the Au-enrichment processes and their diagnostic characteristics are complex and variable, and can be further complicated by superimposed deformational and metamorphic effects that have modified or obliterated primary relationships and patterns. The study of metamorphosed hydrothermal alteration and mineralization at Lalor indicates that Au was introduced as part of the VMS-forming hydrothermal activity, and was partly remobilized during the subsequent deformation and metamorphic events.
Figure 1. Location of the Lalor deposit in North America (map from Galley et al. 2007).
2 Geological setting The Lalor deposit is hosted in the Lalor volcanic succession (Caté et al. 2014), which is part of the Chisel
mature arc sequence of the Paleoproterozoic Snow Lake arc assemblage (Bailes and Galley 1999). The ca. 1.89 Ga Snow Lake arc assemblage occurs in the easternmost part of the Paleoproterozoic Flin Flon greenstone belt (Galley et al. 2007). The Chisel arc sequence consists of intercalated, geochemically fractionated mafic to felsic flows, volcaniclastic and volcanosedimentary units and related subvolcanic intrusive phases. A total of six producer or past-producer bimodal felsic-type VMS deposits are located in the Chisel sequence, and most of them are at the contact between the lower and upper parts of the sequence (Bailes et al. 2013; Engelbert et al. 2014). However, the Lalor deposit is situated at a slightly lower stratigraphic position (Caté et al. 2014).
3 Geology of the Lalor deposit The Lalor deposit consists of stratigraphically and structurally stacked gently-dipping Zn-, Au- and Cu-Aurich ore lenses that are transposed into the main foliation (Fig. 2). The host volcanic succession is composed of mafic to felsic, mainly transitional to calc-alkaline in affinity, extrusive to intrusive volcanic rocks, and the ore is hosted in both mafic and felsic rocks (Caté et al. 2014). Extensive syn-volcanic hydrothermal alteration has affected most of the volcanic rocks of the Lalor volcanic succession. The deposit and its host rocks have undergone several deformation events corresponding to the regional D1 to D3 events described by Kraus and Williams (1999), with D2 being the main deformation event. Regional polyphase metamorphism reached amphibolite grade peak conditions syn- to late-D2 at Lalor (Froese and Gasparrini 1975; Menard and Gordon 1997).
Figure 2. Schematic representation of the ore lenses of the Lalor deposit looking NW (interpreted from Hudbay data).
4 VMS-related hydrothermal alteration The mineralization at Lalor is spatially associated with metamorphosed VMS-related hydrothermally altered rocks that are grouped in four distinct chemical associations (K, K-Mg-Fe, Mg-Fe and Mg-Ca) based on their mineralogy and geochemistry. The K chemical association (Fig. 3A) corresponds to metamorphosed low temperature (~200-250°C: MercierLangevin et al. 2014) sericitic alteration. The K-Mg-Fe
chemical association (Fig. 3B) corresponds to metamorphosed moderate temperature (~250°C) chloritesericite alteration. The Mg-Fe association (Fig. 3C) corresponds to metamorphosed high temperature (~200350°C) chloritic alteration, and the Mg-Ca chemical association (Fig. 3D) is a metamorphosed moderate to high temperature (~200-300°C) chlorite-carbonate alteration. Zn-rich massive to semi-massive sulphide lenses (Fig. 3E) are mainly hosted in rocks of the K association for the uppermost ore lenses and in rocks of the Mg-Ca association for the lowermost Zn-rich ore lenses. Au-rich lenses mainly occur in rocks of the Mg-Ca association but also in rocks of the Mg-Fe association, and are usually proximal (tens of meters) to the lowermost Zn-rich ore lenses. Semi-massive- to disseminated-sulphide Cu-Aurich ore lenses (Fig. 3F) are hosted in rocks of the Mg-Fe association deep in the VMS system. Rocks of the K-MgFe association generally define a distal alteration halo to the mineralization. The geometry of the ore lenses and the nature of their host rocks indicate that the Zn-rich ore lenses are concordant massive to semi-massive sulphide mineralization formed on the seafloor or in the subseafloor environment at two distinct stratigraphic horizons and that Cu-Au-rich ore lenses are transposed discordant footwall stringer zones.
5 Syn-metamorphic metasomatism and Au remobilization The mineralogy of hydrothermally altered rocks has undergone important changes due to metamorphism, but the bulk geochemistry of most of the Lalor-deposit host rocks has been preserved. Resulting metamorphic mineral assemblages contain variable amounts of quartz, amphibole, chlorite, talc, cordierite, biotite, muscovite, staurolite, gahnite, garnet, kyanite, sillimanite, diopside and/or carbonate. Dehydration and decarbonation of hydrothermally altered rocks caused the circulation of fluids during metamorphism (Tinkham 2013). A protracted, pre-D1, to post-D2 syn-metamorphic Ca-rich metasomatism locally overprinted unaltered rocks west of the deposit and part of the hydrothermally altered host rocks. The metasomatic assemblage is characterized by epidote, hornblende, grossular, diopside, calcite and/or anhydrite. Syn- to late D2 vein-halo-style actinolite-rich metasomatism occurred within or proximal to rocks of the Mg-Ca association (Fig. 3G). Mineralization in the Au-rich lenses is preferentially hosted in these actinolite-rich assemblages (Fig. 3G and H), even though it is also present in rocks of the Mg-Ca and Mg-Fe associations. The Au-Ag-Pb-Cu±Zn sulphide-poor mineralization is characterized by the presence of sulphosalts. The mineralization can be hosted in syn-D2 quartz-sulphide veins, disseminated in the host rock (Fig. 3H), or in millimetric syn- to post-D2 fractures in the most competent rocks (Fig. 3G).
Figure 3. A. Mineral assemblage of the K chemical association with biotite, muscovite and sillimanite porphyroblasts in a quartzmuscovite-biotite matrix. B. Mineral assemblage of the K-Mg-Fe chemical association with biotite, kyanite and staurolite porphyroblasts in a quartz-biotite matrix. C. Mineral assemblage of the Mg-Fe chemical association with garnet, staurolite and Mg-Fe amphibole porphyroblasts in a quartz-Mg-Fe amphibole-cordierite matrix. D. Mineral assemblage of the Mg-Ca chemical association with deformed carbonate spheroids and actinolite porphyroblasts in a Mg-chlorite matrix. E. Massive sulphide Zn ore with pyrite porphyroblasts in a sphalerite matrix. F. Cu-Au ore with remobilized garnet porphyroblasts. G. S2-perpendicular sulphide-filled fractures in F2-folded actinolite layers in a chlorite matrix. Sulphides are pyrrhotite, chalcopyrite and trace galena. This mineralization is part of the Au-rich ore. H. Disseminated sulphide Au ore with chalcopyrite, pyrrhotite and galena in a quartz-actinolite matrix. Abbreviations: Act = actinolite, Ath = Mg-Fe amphibole, Bi = biotite, Cb = carbonates (calcite and/or dolomite), Chl = chlorite, Cpy = chalcopyrite, Crd = cordierite, Gn = galena, Gt = garnet, Ky = kyanite, Mu = muscovite, Po = pyrrhotite, Py = pyrite, Qz = quartz, Sil = sillimanite, Sph = sphalerite, St = staurolite, Sulph = sulphides.
Au-rich mineralization is locally present in unaltered mafic dykes that crosscut massive-sulphide ore. The same dykes are barren away from the massive-sulphide ore lenses. The textures and current setting of the Au-rich mineralization indicate that it results from a syn- to lateD2 local remobilization of Au, which was already present in the VMS system, and that this Au remobilization is genetically associated with the Ca-metasomatism that formed the vein-halo-style actinolite-rich alteration. No syn-metamorphic external input is necessary to explain the Au endowment at Lalor. The Au-Ag-Pb-Cu±Zn association may result from a syn-VMS, late low temperature pulse (Duff et al., 2015), whereas the presence of sulphosalts in the Au-rich mineralization is compatible with sulphide anatexis during metamorphism (Frost et al. 2002; Tomkins et al. 2006).
6 Conclusions The formation of the Lalor VMS deposit and its Au zones are the result of the combination of synvolcanic hydrothermal alteration and mineralization, and subsequent deformation and syn-metamorphic processes. Despite important deformation and transposition, primary zonation within the host succession has been largely preserved. Remobilization of Au occurred during metamorphism and caused the formation of Au-rich sulphide-poor ore lenses in Ca-rich assemblages. However, this remobilization is only local (centimetres to tens of meters) and can be the result of mechanical remobilization, syn-metamorphic metasomatism, sulphide anatexis or a combination of all these processes.
Acknowledgements The authors thank HudBay Minerals Inc. and its staff for the opportunity to study the Lalor deposit, permission to publish, support and constructive discussions. Our research at Lalor is funded by the Geological Survey of Canada (GSC) through the Targeted Geoscience Initiative 4 program (TGI-4) and by HudBay Minerals Inc. A. Bailes, D. Tinkham, H. Gibson, B. Lafrance, M. Engelbert, J. Lam, V. Friesen and A. Galley are thanked for their collaboration and for sharing their knowledge of the Snow Lake camp.
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