GEOPHYSICAL REPORT PART I, GRASSET LAKE AREA Zs Pota.4)
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GM 29947 GEOPHYSICAL REPORT PART I, GRASSET LAKE AREA zs pota.4) SELCO MINING CORPORATION LIMITED • GEOPHYSICAL REPORT GRASSET LAKE AREA PART I Ministère des Pichesses Naturelles, Québec W. J. Anderson SERVICE DE LA Toronto, Ontario DOCUMENTAT NTECHNIQUE • 13 - A0 U 1974 . May, 1974. Date: No GM: SELCO MINING CORPORATION LIMITED GEOPHYSICAL REPORT GRASSET LAKE AREA INTRODUCTION During the period January to April, 1974, magnetic and electromagn._`_ic surveys were completed on 37 grids in Grasset Lake area. This report describes the results on 30 of those grids LOCATION &ACCESS The claim groups are located in the townships of Bapst, Ste. Helene, Subercase, Grasset, La Gauchetiere, and Daniel. Access to the souther part of the area is by winter roads sponsored by the Quebec Department of Natural Resources, which extend west from New Hosco, and northwest from mile 80 on Highway 61. Northern claim groups are best reached by float or ski-equipped aircraft from Matagami. GEOLOGY The geology of the area west of longitude 78°00' has been mapped at one inch to one mile and described by Remick (1969), and the reader is referred to this report for details. At the extreme eastern edge of the area, in Daniel Township, felsic ana intermediate volcanics which 2 • host the massive sulphide deposits of the Matagami camp either terminate or form the western closure of an anticline about the Bell River Complex (Sharpe, 1968). With the exception of the Allard River area towards Matagami, most of the geology of the area is only known in a general fashion, due to very limited outcrop exposure. Briefly, the area is underlain by typical Archaean metavolcanics striking largely westerly, and intercalated with some greywacke-type sediments and mafic and intermediate intrusives. The area of interest is bounded tc the north in the vicinity of Grasset Lake by granite, and to the south below Bapst Township by sediments. The largest known accumulation of felsic volcanics occurs approximately midway up the boundary between Ste. Helene and La Gauchetiere Townships. PREVIOUS WORK The area has been subjected to moderately intensive exploration for base metals in the past, notably in the years 1959 through to 1965. All of the assessment work filed in this period is noted in Remick's (1969) report. More recently, in early 1973, Phelps Dodge Corporation of Canada intersected a copper-zinc massive sulphide deposit in southwest La Gauchetiere Township (Latulippe, 1974). Exploration is continuing on the zone and elsewhere, and the public announcement has resulted in renewed activity • by others in the general area. INSTRUMENTATION & PROCEDURES Grids of cut lines were laid out cri selected airborne targets. The lines were chained and picketed and the station and line intervals were 100 feet and 400 feet (with 200 feet on some smaller grids) respectively. Magnetic surveys were carried out using Sharpe MF-1 and McPhar M-700 fluxgate-type magnetometers. Both instruments measure changes in the vertical component of the earth's magnetic field to an accuracy of 10 gammas. Diurnal variations were controlled by establishing base stations along the baseline. Readings have been taken generally on a 100 foot separation with 50 foot stations on some anomalies. All electromagnetic surveys were conducted using horizontal loop array systems._ In this method readings are taken of both in-phase and quadrature components. Conductors are generally indicated by negative readings flanked by positive readings. Conductive horizontal sheets however, such as clay beds, may considerably alter responses over vertical sources, and also produce anomalous readings by themselves. Two'horizontal loop instruments were used; a McPhar VHEM using a frequency of 2400 Hz and a 300 foot cable, and a Geonics EM-17 operating at 1600 Hz and using a coil separation of 400 feet. The former instrument employs an audio-nulling system, while the latter encompasses a visual nulling feature. Readings were taken at 100 foot intervals with • closure to 50 feet over anomalies. DISCUSSION OF RESULTS Interpretation of the results is treated below by individual grids. Some comments at this time however,• on a common problem are appropriate. In much of the area, and particularly in the vicinity of Grasset Lake, the overburden includes conductive clays. In the absence of vertical conductors, horizontal sheets should normally merely shift the base level values of the components for horizontal loop array surveys. Linear discontinuities however, such as edges of sheets and troughs, can produce effects very similar to vertical conductors. Careful e noting of topography by the operator can often eliminate such anomalies. The more serious aspect of conductive overburden concerns its effect on the response due to a vertical or near-vertical conductor source,. A good discussion of such effects achieved through modelling is presented by Lowrie and West (1965). For overburden sheets of moderate conductivity they indicate that the apparent response for a ve.rt_.cal source indicates a "better" and deeper conductor than is the case. More spectacular effects result if the overburden is strongly conductive, in which case the in-phase component may become positive, and the quadrature shifted towards increasing • negativity, but inverting in profile across the conductor. 5 The problem in the Grasset area is complicated by the fact that some bedrock sources are probably buried close to the penetrative limit for the systems used even in a resistive overburden. Thus, on many grids it is difficult to determine whether weak poorly-defined anomalies are due to deep bedrock sources or spurious overburden effects. Grids showing such ambiguous results will requre further examination with another geophysical system, such as horizontal loop gear using_low frequency ztind a long (500 ft.) coil separation. GRID 80-1 Magnetic Survey Several broad positive anomalies occur which have amplitudes of 200 to 1000 gammas. The western and southern anomalies are possibly due to mafic flows or intrusives. A more discrete positive closure centred at 28E/7N coincides with two subparallel conductors and thus could in part be due to pyrrhotite. EM Survey Two subparallel conductors with good responses occur in the northeast half of the block. As noted above, there is an associated magnetic anomaly of up to 900 gammas. Both zones are short and are vertical or dip steeply south. A drill hole is recommended at 36E/0+50S (-45°N) . 6 IMO GRID 80-2 Magnetic Survey The only significant anomalies obtained are several broad west-striking positive highs occurring in the south half of the grid. They are consistent with a bedrock source of mafic flows. EM Survey A single weak fair response occurs in the extreme northwest section of the block on lines 0 to 8E. An additional line to the west should be surveyed, and the profiles across the source re-run with an En-17 with 400 foot cable to provide a more diagnostic response. Two other very weak ambiguous cones are indicated. They may be due to spurious overburden effects. Further checking with a different geophysical system is necessary. GRID 80-3 Magnetic Survey The most prominent feature on the grid is a continuous narrow positive high with amplitudes of 700 to 900 gammas, striking east-northeast. It is folded or faulted in the vicinity of 14E/5S. The continuity and linearity imply a sedimentary magnetite or pyrrhotite source. Broadening of the flanks toward the north and east may relate to a second less magnetic zone. EM Survey All indicated conductors on the plan are tentative, There are obvious overburden responses throughout the survey as shown by the positive in-phase and erratic quadrature values. At the same time, all the marked conductor zones display amplitudes of the same order of magnitude as the overburden noise (2 to 8%). The linearity of the zones was a factor in delineating them on the plan, however the eastern zone appears to transgress the magnetic anomaly described above. The broadish responses and poor phase relationships suggest the sources are more likely edges of clay sheets or troughs. No drilling is warranted until further geophysical checking is completed. • GRID 80-5 Magnetic Survey Maximum relief across the grid is limited to about 100 gammas. A local low positive closure in the center of the grid may relate to mafic volcanics. EM Survey Large variations in the quadrature component with similar in-phase shifts are due to overburden. The conductor zone indicated south of the baseline is a poc:,: response and shows high positive quadrature shoulders,, and a markedly asryrlmetrical in-phase profile. It is likely due to an edge effect in overburden..Further checking is required, however. • 8 GRID 80-6 Magnetic Survey There is no significant relief on the survey. EM Survey A single, very strong to good conductor occurs under Lac Lagauchetiére. Extreme amplitudes obtained on line 16E imply the source is probably at the bedrock surface at that point. Double-peaking of the anomaly profiles is a peculiar feature of the sources. The distance between the zero points is not sufficient to indicate a multi-conductor zone. The general character of the profile most closely approximates that of a model profile for a buried sphere. The source is possibly an irregular-shaped body with its vertical axis dipping steeply north. A drill hole is recommended at 16E/2+20S (-45°S). GRID 80-7 Magnetic Survey A single low broad positive anomaly occurs in the center of the grid. The source may be a more magnetite- rich flow or intrusive. EM Survey All anomalous areas on the grid appear to be related entirely to overburden. A sharp,negative spike on the in-phase at .12E/10S should be re-checked. GRID 80-8 Magnetic Survey According to the map by Remick (1969) this grid should lie within a granite area.