The Anglo-Rouyn Mine, La Ronge Area: A Preliminary Report 1 R.G. Roberts 2 and K. Krey2 Roberts, A.G. and Krey, K. (1992): The Anglo-Rouyn Mine, La Ronge area: A preliminary report; in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92-4. The Anglo-Rouyn mine is a past-producer of copper and gold, located approximately 35 km northeast of La Ronge, on the Williams Peninsula of the northwest shore of Lac la Range (Figure 1). Between 1966 and 1972, Rio Algom Mines Ltd. extracted 1. 7 million tons of ore from three ore bodies, with an average grade of 1. 7 percent Cu, 1.2 g/ton Au, and 5.5 g/ton Ag. The mine development involved an open pit, and an underground operation developed on five levels, serviced by a 244 m shaft. The geology of the Anglo-Rouyn deposit has been described by Forsythe (1968, 1971, 1972), and recently by Coombe Geoconsultants Ltd.(1991). The property is also noted in publications by Mclarty (1936), Keith (1951), and Pearson (1957). In 1984-85, Noranda Exploration conducted an exploration program on the property which involved line cutting, geochemical and geophysical surveys, and 816.8 m of diamond drilling. Kristo Gold Inc. is presently investigating the gold poten tial of the mine tailings. The current project has as its objectives a description of the orebodies with emphasis on their relationships to the structural geology and stratigraphy of the area. The ultimate objective is the classification of the orebodies. In this past summer, geological mapping was carried out on the property, on a scale of 1 :2500. 104 ' -~r---r-----"; H?" +... G3 JEP SON LAti;E GRANITE IBj 8 10TIT E - FE LDSPAR SCHIST ( o I {b} ( c) 0 200 4 00 - - l O META MORPH OSED SEDIMENTAR Y ROCK S / / GEOLOG tcal CONTAC TS te] A MPHl8 0L E SCH I ST OPEN Pll / ORE BO DjES ~ SHAFT ~ MIN E tailin GS Figure 1 - Geological map of tht1 Anglo-Rouyn Mint1 all/a. (a) Stereographic plot of: (i) poles to S, foliations (n = 120), (ii) L1 lineation s (n =42); (b) Stereographic plot of poles to veins parallel to S,, associatijd with D1 (n = 14); (c) StiJreographic plot of poles to subvert/cal veins associated with D3 (n "' 12). (1 J Project is funded by the Geological Survey of Canada (2) Department of Earth Sciences, University of Waterloo, Waterloo, Ontario. N2l 3G1. 184 Summary of Investigations 1992
1. Geological Setting The property is in Aphebian rocks of what was once referred to as the Nut Bay Belt (Lewry and Slimmon, 1985), but which is now considered to be part of the Wapassini Sheet (Lewry et al., 1990). Lewry and Slimmon (1985) describe the Nut Bay belt as comprising foliated granodioritic gneisses intercalated with subordinate supracrustals, which include hornblende gneisses and amphibolite of volcanic origin, and lesser metasediments. Mafic and ultramafic plutons occur locally. The orebodies occur in a northeast-trending belt of metamorphosed sedimentary rocks (referred to below, as the metasedimentary rock unit) which, in the map area, has a strike length of 4 km, and is between 200 and 500 m wide (Figure 1). The Jepson Lake 'granite' lies to the northwest of the metasedimentary rocks, and between these units is a unit of foliated biotite-feldspar schist, which is probably a deformed variety of the granite. A belt of amphibole schist, at least 200 m wide, outcrops to the southeast of the metasedimentary rock unit. Irregular pegmatite veins and stocks occur throughout the area. All the rock units described above show evidence of deformation, the most prominent structural feature being a regional, penetrative foliation that strikes subparallel to the rock unit contacts, and dips from 40 to 85 to the northwest. The rocks have undergone amphibolite facies metamorphism. 2. Description of Rock Types a) Metasedimentary Rocks The metasedimentary rocks are characterized by dark and light layers, typically 1 to 2 cm thick and locally up to 30 cm thick. This is primary layering, or bedding, which is transposed into a regional penetrative foliation. The most abundant minerals are plagioclase feldspar (An35) and hornblende, which characterize the light and dark layers respectively. Other minerals in the metasedimentary rocks include: epidote, clinozoisite, sphene, scapolite (calcium-rich), quartz, calcite, apatite, and biotite. Epidote and sphene, in association with feldspar may each characterize layers that are recognized in the field by their respective green and red colourations. Quartz is normally subordinate to feldspar, but at the northwest contact with the biotite-feldspar schist, the metasedimentary unit is represented by a distinctive, pale gray siliceous unit with more than 70 percent quartz. Scapolite and calcite are generally minor components of feldspar-rich layers. Biotite is most abundant (up to 15 percent) in association with hornblende-rich layers in the metasedimentary unit within 100 m of the southeast contact with the amphibole schist unit. Elsewhere in the unit, biotite is comparatively rare, constituting less than 10 percent of the rock. The unit of metasedimentary rocks contains a range of lithologies within the constraints of the mineralogy described above. In general, it consists of a calc-silicate assemblage of amphibole, feldspar, epidote, and sphene. To the northwest this is in sharp contact with a siliceous unit, and to the southwest the calc-silicate unit grades to a more biotite-rich unit. However, the contact relationships are complicated by deformation, and at the present stage of the investigation, the unit has not been subdivided. b) Amphlbole Schist The amphibole schist is a dark green, well-foliated rock, which contains up to 60 percent amphibole, feldspar and biotite, and minor amounts of epidote, sphene, and calcite. Primary layering was not recognized in the unit. c) Jepson Lake 'Granite' The Jepson Lake granite is coarse grained, weakly to moderately foliated with some brecciated zones. It comprises albite, microcline, hornblende, biotite, and epidote. d) Biotite-feldspar Schist A unit of strongly foliated biotite-plagioclase feldspar schist lies at the contact between the Jepson Lake granite and the metamorphosed sedimentary rocks. The unit is of granodioritic composition and consists of coarse-grained quartz, potassic-feldspar, plagioclase, biotite, epidote and carbonate. 3. Structural Geology Three periods of deformation are recognized; the first giving rise to penetrative mesoscopic-scale structures throughout the area, and also to the major structures. The two later deformations are probably only locally significant, but they may have played a part in modifying the sulphide orebodies with the development of veins. a) First Deformation (D1) The S, foliation is a penetrative fabric that effects all rocks. It is defined by the preferred orientation of hornblende and the less abundant biotite, and, in the metasediments, by the transposition of the primary layering So into S1 (Figure 2). A lineation l 1 is defined by the preferred orientation of hornblende, and lies in the S1 foliation. The lineation plunges between 5 and 20. In the southwest part of the property it plunges to the southwest, and between Lake 'C' and the northeast boundary it plunges northeast (Figure 1 (a)). The S1 foliation is a plane of flattening characterized by the development of boudinage structures. The relationship between the boudinage structure and the L1 lineation is illustrated in Figure 3. Extension on the S1 plane, parallel to the mineral lineation has produced a boudinage structure involving necking, rupture, and veining. Extension perpendicular to the mineral lineation has resulted in pinch-and-swell structure. Saskatchewan Geologicaf Survey 185
Figure 3 - Diagram showing the relationship of the mineral lineatian L 1 to the boudinage and pinch-and-swell structures in S,. Figure 2 - The transposition of So layering (bedding) into tha S, foliation in the metasedimentary unit. Hornblende-rich layers are shown in the stippled pattern. (The sketch is from a photograph of an outcrop). Mesoscopic Ft folds in the metasedimentary unit are illustrated in Figure 2. The axial surfaces of the folds are parallel to St, and the axes plunge at shallow angles, parallel to the L1 mineral lineation. Megascopic F1 folds were not identified with certainty, but using the mesoscopic folds in Figure 2 as a model, an antiform may be postulated in the south and west part of the area. Here, the contact between the metasedimentary and the amphibole schist unit follows the northwest shore of a string of lakes, and the S1 foliation in the amphibole schist projects into the metasedimentary unit to the southwest. It is therefore probable that the western shore of the most southwesterly lake follows the northwest limb of a closure to an antiform, defined by the contact between the metasedimentary and the amphibole schist units. b) Second Deformation (D2) The S1 foliation is folded to form localized groups of intrafolial F2 folds, confined to a discrete thickness of S1 layers that rarely exceeds a metre. The folds are inclined to recumbent, verge consistently to the northeast, and axial surfaces curve to become approximately parallel with the overlying S1 foliation (Figure 4A and 48}. The folds are seen only in the metasedimentary unit. The surfaces of slip, such as thrust planes and the S1 planes that confine the interval of folded layers, are welded, suggesting that the deformation is early and probably coincident with metamorphism. The consistent asymmetry of the folds, their confinement to discrete intervals of S1 layers, and the curvature of the axial surfaces, suggest that they were formed by dextral shear movement along the S1 foliation planes. The precise orientation of movement in the plane of S1 has not been determined, largely because other kinematic indicators were not found. c) Third Deformation (D3) The f3 folds are present in the metasedimentary and amphibole schist units as isolated structures or groups of structures. They are approximately concentric (Figure SA}, their axial surfaces strike northwest and dip between 85 and 60 to the northeast and the folds axes plunge between 30 and 50 to the northwest. Variation in attitude of the axial surfaces within the limits described above, is characteristic. The folds die out rapidly along their axial surfaces. Folds in the metasedimentary rocks are associated with brittle-ductile reverse shears, approximately parallel to axial surfaces (Figure SA}. Prominent fractures, some with quartz-sulphide veins, that parallel the reverse shears, were probably also formed during the 03 deformation. Sinistral, brittle-ductile shears, with similar quartz. sulphide veins, occur sub-parallel to S1, but may be deflected into the reverse shears parallel to those associated with folding. The sense of movement on the 186 Summary of Investigations 1992
Figure 4 - A) Photograph of an F2 fold in the open pit. The folded surface is S,; the axial surface (trace shown by the dashed line) curves to become parallel to the overlying nonfo/ded S,. B) Cut block of an F2 fold. The asymmetry of the fold and the thrust structures indicate dextral shear. Pyrrhotite and chalcopyrite are disseminated along the S, planes and the thrust structures. shears is interpreted from the orientation of quartz fibres in vein-filled shears. The Fa folds, reverse shears, and the sinistral shears subparallel to S1, could have formed under conditions of shortening where the major principal stress was oblique to the folded layers, S1 (Figure 58). 4. Ore Zones and Mineralization The sulphide ore occurs in the metasedimentary rock unit within 50 m of the contact with the biotite-feldspar schist. The metasedimentary rocks are noticeably darker in the ore zones due to an increase in hornblende relative to feldspar, in compositional layers up to 30 cm thick. In the open pit, the contact of the calc-silicate unit with the siliceous unit of the metasediments is approximately 5 m above the ore zone. The dip of the contact is to the northwest, and is between 5 and 1 0 lower than the dip of the S1 foliation indicating that the contact is on the northwest limb of an antiform. The deposit consists of a number of sulphide-rich, fens-shaped ore zones, oriented parallel to the S1 foliation, and distributed over a strike length of approximately 3000 m (Figure 1}. For mining purposes, the ore zones are classified into three orebodies, 'A', 'B', and 'C'. The B and C orebodies each consist of a discrete ore zone, and A is made up of four ore zones. Coombe Geoconsultants Ltd. (1991) describe the ore zones as 'ruler shaped'. The largest ore zone is in the A orebody, where it is 15 m wide, has a down-dip dimension of 60 m, and a downplunge dimension of 1750 m. The whole deposit lies approximately on a plane (Figure 1). In detail, however, the sulphide lenses that constitute the A orebody lie on at least two planes within a width of 10 m. The ore zones plunge with the mineral lineation. The A and 8 orebodies plunge to the southwest at approximately 10, and the C orebody plunges to the northeast at approximately 13. The principal ore minerals are chalcopyrite, pyrite, pyrrhotite, and magnetite. Minor amounts of molybdenite are observed in hand specimens, and sphalerite, galena and pentlandite are reported from the ore by Coombe Geoconsultants Ltd.(1991). Much of the ore occurs as disseminations and irregular blebs (with dimensions up to several centimetres) which cannot be identified with a specific tectonic fabric. However, there are many ore structures that can be attributed to a specific period of deformation. These are described below. 01 deformation: Sulphides and magnetite occur as disseminations along $1 foliation planes, and in layers up to several centimetres thick parallel to the S1 foliation and following the curvature of boudinage and pinch-andswell structures. Veins consisting of sulphides and angular fragments of vein quartz occur parallel to S1, are up to at least 5 cm thick, and extend for tens of metres in the plane of S1 (Figure 1 (b)). It was noted that on the immediate wall-rock surface, sulphides define the Saskatchewan Geological SuNey 187
silicate minerals indicate that the protolith is an impure, bedded magnesian limestone. 3. The association of the sulphides with a specific lithology, and the structural relationships suggest a stratigraphic control for the ore; however, this will require further structural mapping if it is to be confirmed. 50m Figure 5A Sketch of an F3 fold in rnetasedirnents in the open pit. The southwest limb of the fold is replaced by a brittle-due tile shear. s, \ SI NI STRAL 8Al i TL - DIJCTIL( SHEAR/VEIN S Figure 58 Diagram illustrating the possible relationships of the brittle-ductile reverse shears and the sinistral shears to the F3 folds. mineral lineation in places. The enclosing wall rock contains irregular blebs and disseminations of sulhides.tension fractures in the boudinage structures may be filled with sulphides (mainly chalcopyrite) and vein quartz. D2 deformation: Sulphides and magnetite are disseminated along the folded S1 planes and thrust structures in F2 folds (Figure 48). 03 deformation: Quartz sulphide veins occur in reverse shear structures associated with f3 folds in the metasedimentary unit, and in the sinistral brittle-ductile shears subparaflel to S1 (Figure SB). 5. Summary and Conclusions At this stage of the investigation, the authors can make no statement concerning the genesis or classification of the deposit. However, the field work points to the following conclusions. 1. The sulphides are coeval with, or predate, the 01 deformation. 2. The identification of the primary layering in the metasedimentary unit, and the assemblage of calc- I 4. There is no evidence thus far to indicate a relationship between the granite pluton and the ore except that both were emplaced prior to, or during the 01 deformation. 6. Acknowledgments Jennifer Burgess assisted with the geological mapping. The authors' understanding of the structural geology was helped considerably following conversations with Ors. Francois Robert and Howard Paulsen of the Geological Survey of Canada, and with Dr. T.1.1. Sibbald of Saskatchewan Energy and Mines. 7. References Coombe Geoconsultants Ltd. (1991) : Base Metals in Saskatchewan; Sask. Energy Mines, Open File Rep. 91-1, p62 67. Forsythe, L.H. (1968): The geology of the Stanley area (west half), Saskatchewan; Sask. Oep. Miner. Resour., Rep. 115, pt1, 58p. (1971): The geology of the Nemeiben Lake area --(~e-as~t~h-alf) and the geology of the mineral deposits in the Nemeiben Lake-Stanley areas, Saskatchewan; Sask. Dep. Miner. Resour., Rep. 115, pt2, 178p. (1972): Anglo-Rouyn copper mine, Ore Bay, Lac -~,-a-r~o-ng-e, Saskatchewan; Geel. Soc. Amer. Bull., v83, p3405-3414. Keith, M.L. (1951): MacKay Lake, Saskatchewan; Geol. Surv. Can. Map 592A. Lewry, J.F. and SJimmon, W.L. (1985): Compilation bedrock Geology. Lac la Range, NTS area 73P/ 731; Sask. Energy Mines, Rep. 225, Sp. Lewry, J.F., Thomas, D.J., Macdonald, R., and Chiarenzelll, J. (1990): Structural relations in accreted terranes of the Trans-Hudson Orogen, Saskatchewan: Telescoping in a collisional regime?; in Lewry, J.F. and Stauffer, M.R. (eds.)., The early Proterozoic Trans-Hudson Orogen of North America, Geo!. Assoc. Can. Spec. Pap. 37, p75-94. Mclarty, D.M.E. (1936): Lac la Range sheet (east half); Geol. Surv. Can., Map 358A. Pearson, W.J. (1957): All investigation into the geological significance of some major anomalies in the Lac la Range area of northern Saskatchewan; Sask. Dep. Miner. Resour., Rep. 29, 3p. 188 Summary of Investigations 1992