Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
- 150 The Bassett Lake Mafic Intrusion: Preliminary Report by I. Graham1, B.R. Watters1 and L. Hulbert2 Graham, I., Watters B. R. , and Hulbert L. (1986): The Bassett Lake mafic intrusion: preliminary report; in Sunmary of Investigations 1986 , Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report"lJ6- 4. The Bassett Lake mafic intrusion is situated on and immediately south of Bassett Lake in the southernmost part of the Central Metavolcanic Belt of the La Range Domain. The Bassett Lake area mainly comprises a suite of plutonic mafic-ultramafic rocks bounded by a predominately intermediate to mafic volcanic country rock assemblage to the south, a compositionally variable granodioritic body to the northwest and a diorite intrusion to the east. Present Project Detailed mapping and sampling of the mafic intrusion has been carried out and will form the basis for mineralogical and geochemical studies. Samples of drill core from the Triangle Lake area in the central part of the body will constitute an important component of these studies. Major aims of the study will be to determine the nature of the intrusion, its origin and relationships to other rock units in the La Range Domain, and the potential for mineralization involving elements of the platinum group, vanadium, titanium and chromium. A variety of analytical techniques, including electron probe microanalysis, will be employed to investigate the mineralogy, mineral zoning, chemical characteristics and phase relations of oxide and sulphide minerals. Previous Work The earliest reference to work done in the area is that of Mclarty (l 936) who mapped the area as part of a regional project. An airborne magnetic survey over the "basic and ultrabasic" rocks in the area for the Saskatchewan Department of Mineral Resources in 1953 indicated magnetic highs in the Triangle Lake area (Fig. 1). surveys were flown by the Great Plains Development Co. of Canada Ltd., and the Don Fisher Syndicate undertook a ground electromagnetic survey. Regional mapping by Padgham ( 1960) included a report on the "Bassett Lake Gabbro", in which he referred to chloritized and epidotized hornblende gabbro and the development of some anorthosite. He also noted chalcopyrite, pentlandite, pyrrhotite and pyrite mineralization. Richards (1964) descr ibed pyrrhotite and pyrite mineralization in a variety of gabbroic rocks from five diamond - drill hole cores from the Bassett Lake body. Reynolds (l 967) subsequently mapped the area and reported a significant development of pyroxenites in a predominantly dioritic body, the surrounding rocks including granulites, granitic gneisses and volcanics. Recent work by Harper ( 1982) included an investigation of both drill core and surface geology in the Triangle Lake area. The study was primarily an economic evaluation, and particular attention was given to the magnetite-rich rocks and their potential as an iron ore. Layered gabbro, olivine gabbro, minor peridotite and anorthosite are described by Harper ( 1982). Intrusive Mafic Rocks The most common rock types in the Bassett Lake Intrusion are variably uralitized and hornblendic medium-grained to very coarse-grained gabbros. These are commonly foliated though locally massive or display only a weak fabric. Trenching of the sulphide showings in 1957 and 1961 covered the claim area held by A. Studer. Beck ( 1959) reported pyrite, pyrrhotite and chalcopyrite from a pit blasted at the main showing. In 1965, further airborne electromagnetic and magnetic On weathered surface the rock appears very dark, except where 'leucogabbro' is developed. The leucogabbro has well-developed plagioclase laths typically altered to aggregates of 'saussurite' as a result of deuteric processes and/or incipient metamorphism. Alteration of the plagioclase also occurs in the more mafic gabbros, although these rocks commonly display clearer and in places visibly zoned plagioclase crystals. Such zoning is due to inclusions of pyroxene, spinel and iron oxide. l Department of Geology, University of Regina 2 Economic Geology and Mineralogy Division, Geological Survey of Canada Project funded under the Canada component of the Canada-Saskatchewan Subsidiary Agreement on Mineral Development 1984-89, with some logistic support from the Saskatchewan Geological Survey. The coarse-grained gabbroic rocks contain hornblende, occurring as large poikilitic plates. Secondary amphibole ("uralite") observed in some thin sections is predominately actinolite, with very minor nonpleochroic tremolite. Carlsbad twinning of the plagioclase is recognized in some hand specimens. A thin section from a core sample of gabbro from the Triangle Lake area shows - 151 - Legend fa~§~~~ Glacial cover Intermediate intrusive Metagabbro and metanorite Magnetite gabbro/norite Magnetite gabbro (magnetic intensity greater than 10,000 gauss) -.., """" Shear zone ,·-·-·- - Conductor Diamond-drill hole sites Main trench sites Foliation Granodiorite and granitoid rock Topographic contours (10 m interval) Mafic and Intermediate metavolcanic rock 10,000 gauss magnetic contour Felsic metavolcanic rock Inferred lithological contacts Figure 1 - Preliminary geological map of the Bassett Lake mafic intrusion. - 152 development of augite (with possible pigeonite exsolution lamellae), zoned calcic labradorite and possibly late-magmatic hornblende. Norite has been identified in the field and in thin section from the Triangle Lake area and may represent the "mottled type metagabbros" referred to by Harper (1982). These rocks are commonly weakly and more rarely strongly magnetic. In hand specimen they show oikocrystic development of hypersthene. The orthopyroxene crystals are medium grained, idiomorphic and bronze toned, and are responsible for the highly reflective, mottled outcrop surfaces common in the Triangle Lake area. Orthopyroxene observed in noritic horizons along the north shores of Bassett Lake is somewhat finer grained and 'sugary' textured with poorly formed crystals. Both ortho- and clinopyroxenes show evidence of high temperature alteration by hydration to amphibole. These rocks also contain plagioclase, commonly zoned and in the range calcic andesine to sodic bytownite, small flakes of hematized dark brown biotite, various amounts of opaque minerals and accessory apatite. Opaque minerals have not yet been fully identified but include magnetite, ilmenite, possibly chromite and trace sulphides such as pyrrhotite, pyrite and chalcopyrite. Areas underlain by norite and gabbro commonly have smooth, rolling and densely wooded hills. Outcrop is not abundant except along the shores of Bassett Lake and on steep hillsides. Magnetite-rich gabbro and norite are concentrated in the Triangle Lake area and along the western flank of the body. These melanocratic rocks contain up to 30 percent opaque minerals, the major component of which is probably magnetite. Although most of the noritic rocks contain opaque minerals, 'normal' norite is regarded here as that containing less than about l 5 percent opaques in the mode. The textures are granular and somewhat anhedral, except where individual crystals are enclosed by magnetite and where subhedral crystal development occurs. Very highly magnetic ultramafic rocks (Fig. l), typically exhibit a surface magnetic intensity in excess of 10,000 gauss (Great Plains Development Co. of Canada Ltd., surface EM survey). One thin section of such an ultramafic rock (from diamonddrill core in the Triangle Lake area) contains approximately 60 percent opaque minerals, 20 percent olivine (including alteration products such as iddingsite and minor antigorite), augite and minor oligoclase or andesine. Reynolds (1967) mapped 'pyroxenites' in the Bassett Lake intrusion, but no rocks of this type were identified during the recent field study. Anorthosite occurs in most parts of the Bassett Lake body where layering is well developed. This rock type has a well-developed granular cumulate texture and is more leucrocratic than the more common leuco- or anorthositic gabbro. It contains as little as 5 percent mafic minerals (usually hornblende), as well as saussuritized plagioclase feldspar. The anorthite content of the feldspars cannot be determined because of the alteration. "Magnetite anorthosite" containing approximately l O to 15 percent magnetite, occurs at a locality near Triangle Lake Creek. Well-developed layering is present mainly in the northern and western parts of the body, and occurs on a scale varying from 1 and 2 cm to over 2 m. The layering is commonly compositional, but textural layering is also developed. To the west of Bassett Lake, in the area of very high magnetic intensities (Fig. l ), rhythmic layering on a scale of several centimetres is very well developed. The layers alternate in composition between an intensely magnetic "magnetite gabbro" and completely nonmagnetic leucogabbro. A series of large-scale blocky "breccias" occur along the eastern contact zone of intrusion; angular blocks composed of fine- to medium-grained gabbro are set in a matrix of quartz dioritic to "leucogabbroic" material. The angular gabbroic blocks may be the result of disruption of the wall rocks of the Bassett Lake body during intrusion of the dioritic magma. Veins of graphic-textured leucocratic granodiorite intrude the Bassett Lake body mainly in the north and west, but also in the northern-central parts of the intrusion. Variably developed in these regions, the vein margins range from sharp to di ff use. Quartz is concentrated towards the centres of the veins, whilst large hornblende crystals up to 7 cm in length are developed adjacent to the contacts. Pockets of predominantly uralitized mafic minerals occur in gabbroic host rocks adjacent to the veining, suggesting a genetic relation with the veins. Geology of the Country Rocks The intermediate and mafic metavolcanics to the southwest of the Bassett Lake Intrusion are composed predominantly of crystal tuffs with lesser mafic flows and minor volcanogenic sedimentary units. Rocks along the shores of Thorton Lake have been intensely sheared, and minor shearing occurs elsewhere in the volcanics, en echelon to the main shear zone. Contact relationships between the volcanic rocks and the gabbroic rocks of the Bassett Lake body could not be established due to the absence of suitable outcrops. Diorite and quartz diorite observed to the east of the mafic intrusion are medium to coarse grained, generally hornblendic and foliated. The predominant diorite contains more hornblende than the quartz diorite which in places is extremely leucocratic. Quartz diorite also occurs in - 153 association with the breccias at the contact zone between the Bassett Lake gabbro and the diorite. Granodioritic rocks outcropping to the east of Bassett Lake are compositionally variable, in places appearing granitic. A number of rafted "blocks" are present in the granodiorite, the largest xenolith recognized being a rhyolitic metavolcanic (indicated as "acid volcanics" in Fig. l). The north-trending shear zone in the west, well developed in the granodiorite, is a major zone of mylonitization over 50 m wide in places. This shear zone extends northwards through Beaudry and Fault Line Lakes (Thomas, pers. comm.) and southward through Thorton Lake (Padgham, 1960). support from the Saskatchewan Geological Survey for some aspects of the field work is gratefully acknowledged. K. Haynes, D. Thomas and C. Harper are thanked for their assistance during the field season. References Beck, L.S. (1959): Mineral occurrences in the Precambrian of northern Saskatchewan (excluding radioactive minerals); Sask. Dep. Miner. Resour., Rep. 36, l34p. Harper, C. T. (1976): Core descriptions of DOH core holes 1PCH BL US DOH 1/2; Sask. Energy Mines. Summary The Bassett Lake body is intrusive into predominantly metavolcanic rocks and is bounded to the east and northwest by later diorite and granodiorite respectively. The intrusion mainly comprises gabbro, norite and minor anorthosite. Magmatic magnetite is variously developed, resulting in a region of anomalous positive magnetic intensity. Much of the intrusion has been deuterically altered and a graphic granodioritic veining is commonly developed. The central parts of the body characteristically show primary magmatic layering. Ack now ledge men ts The project is supported by a research contract with the Geological Survey of Canada. Logistic _ _ _ _ (1983): Iron ores of northern Saskatchewan; Sask. Energy Mines Rep. 220, 102p. Mclarty, D.M.E. (1936): Lac la Range sheet (east half), Saskatchewan; Geol. Surv. Can., Map 358A. Padgham, W.A. (1960): The Geology of the Otter Lake area (west half) Saskatchewan; Sask. Dep. Miner. Resour., Rep. 41, 34p. Richards, B.R. ( 1964): Description of DOH core; Sask. Dep. Miner. Resour. Reynolds, N.W. (1967): Report on geology of Claim Block CBS 622, Bassett Lake area, Saskatchewan; Sask. Dep. Miner. Resour ., 4p.