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Proactive disclosure Print version   | |  EXTECH: Exploration Science and Technology Initiative EXTECH 1: A multidisciplinary approach to massive sulphide research in the Rusty Lake-Snow Lake greenstone belts, Manitoba
EXTECH 1: A multidisciplinary approach to massive sulphide research in the Rusty Lake-Snow Lake greenstone belts, Manitoba Geological Survey of Canada Bulletin 426, 1996 Edited by G.F. Bonham-Carter, A.G. Galley, and G.E.M. Hall Available from the GSC Bookstore
EXTECH was initiated in 1989 to promote the development of new approaches to base metal exploration. Canadian reserves of copper, lead, and zinc had decreased substantially during the 1980s and, in the absence of significant new discoveries, it seemed likely that production from some of Canada's established mining camps would begin to decline before the turn of the century. The decrease in reserves in large part reflected reduced expenditures on base metal exploration and a shift of the industry's priority to gold exploration. Many mining camps had been well explored using existing methods and the general consensus was that new discoveries would most likely come from innovative concepts and technologies. EXTECH represents an integrated, multidisciplinary approach to mineral exploration research. It encourages synergies among specialists in the various fiends of geology, geophysics: and geochemistry by focusing their efforts on understanding the occurrence of a specific deposit type in a specific mining camp. The nominal goal is to develop an integrated deposit model - that is, a model that incorporates not only the geological attributes but also the geophysical and geochemical signatures of the deposit type. In working towards this goal, the regional geoscicnce knowledge base is enhanced and specific technologies are developed. This volume describes the results of the first EXTECH project which was implemented in the Snow Lake and Rusty Lake belts of Manitoba. This area was selected for a number of reasons. From the socio-economic standpoint, it represented a highly productive base metal district, which urgently required the discovery of new reserves to forestall mine closures, and the resulting loss of employment. From the scientific standpoint, it possessed the right combination of abundant outcrop, complex glacial stratigraphy, and well documented mineral occurrences for the multidisciplinary studies that were envisioned. From the operational stand- point, the project would build upon the excellent working relationships which already existed among the Geological Survey of Canada, the Manitoba Geological Services Branch, and the exploration companies active in the area. This was in many respects a pilot project with both strengths and weaknesses. It was strongly multidisciplinary and benefitted from a significant degree of integration in both the planning and execution of the research. With the close integration of the efforts of the federal and provincial geological surveys, each side brought its particular expertise to bear on the problem. Although EXTECH 1 perhaps fell short of achieving all its goals, we believe that this volume contains many insights which could stimulate new approaches to massive sulphide exploration and contribute to the discovery of J.M. Duke
1 BHP Minerals Canada Ltd., 33 Yonge St., Suite 610, Toronto, Ontario M5E 1G4 The Exploration Science and Technology (EXTECH) program was initiated in 1989 in response to the significant decline in Canadian base metal reserves that had occurred in the preceding decade. The objective was to improve concepts and technologies applicable to exploration in established base metal camps; one means of achieving this was to foster a closer integration of the traditional disciplines of geology, geochemistry, and geophysics. Accordingly, a multidisciplinary team of geoscientists from the Geological Survey of Canada (GSC) and the Manitoba Geological Services Branch (MGSB) was assembled to undertake a comprehensive study of the Snow Lake and Rusty Lake volcanogenic massive sulphide districts of Manitoba. The selection of these two Paleoproterozoic base metal mining districts for the first EXTECH project reflects the fact that the over 220 million tonnes of massive sulphide ore extracted from the two belts combined represents more contained base metal per square kilometre than any other tectonic province in Canada, and is the richest Paleoproterozoic domain in the world (Fig. 1).
Figure 1
The identification of the controls responsible for the development of these base-metal-rich environments is clearly measurable. More specifically, the selection of the Snow Lake and Rusty Lake districts for the first EXTECH program was based on:
The choice of the Snow Lake and Rusty Lake mining districts for this EXTECH I program was also influenced by the concurrent initiation of LITHOPROBE and NATMAP projects in the area. This would allow the EXTECH l participants to take advantage of expertise that would not otherwise be available. The Rusty Lake district was included as it was a unique opportunity to define the depositional environment of Canada's fourth (tied with the Flin Flon Main mine) largest volcanogenic massive sulphide deposit. The GSC has been involved in the Snow Lake camp since the 1970s, and the federal and provincial surveys had already started a cooperative geological mapping program centered on the base metal deposits. The five-year project started in April l 989 in both the Snow Lake and Rusty Lake districts. The program had 13 projects divided into four main components:
The geological studies formed a base, or starting point, from which other projects were initiated (Fig. 2). The other focal point for the group was the GIS project through which all of the data collected were channelled for digital integration and regional- scale modelling.
Figure 2: A schematic illustration of the multi-disciplinary nature of the EXTECH I program. The following papers are divided between the two sub-programs. The three projects completed in the Rusty Lake region involved bedrock geology, mineral deposit, and surficial geochemical studies. The remaining 13 papers describe research that took place in the Snow Lake mining camp, and include papers on bedrock geology, surficial geochemistry and associated methodologies, and geophysics. The volume includes a list of all publications related to the EXTECH I program. Rusty Lake subprogram Whereas most Precambrian massive sulphide districts are developed through a number of massive sulphide deposits, the Rusty Lake greenstone belt, to date, contains a single, 64 million tonne VMS deposit known as Ruttan. The other exception is, of course, the Archean Kidd-Munro belt, which hosts only the giant 145 million tonne Kidd Creek deposit. The Ruttan deposit is the fourth largest volcanogenic massive sulphide deposit in Canada, after Brunswick No.12, Home, and Kidd Creek. A deposit of this size usually lies at the far end of the deposit distribution curve in most massive sulphide camps, suggesting that other, albeit smaller, massive sulphide deposits should be present within the Rusty Lake belt. The main objective of the Ruttan bedrock mapping and deposit modelling project was to define the type of tectonic and depositional environment in which the Ruttan deposit formed, thereby fingerprinting a favourable volcanic domain for use as an exploration model in the remainder of the Rusty Lake belt. The first paper of the volume by Doreen Ames is the documentation of the pathogenesis and depositional environment for the Ruttan deposit based on 1:5000 scale bedrock mapping of the Ruttan structural block. The detailed mapping resulted in a description of the proximal alteration facies about the deposit in light of pretectonic reconstruction, and the definition of the Ruttan horizon 900 m northeast of the deposit, where it is associated with a distinctive feldspar-rich alteration comparable to that found on the immediate periphery of several other VMS deposits of various ages. The most significant finding was the fact that the majority of the footwall strata are structurally emplaced from an arc terrane type (back arc) traditionally thought of as barren of economic VMS deposits. In light of the recent spate of lithogeochemical studies that have been used to define optimum VMS-hosting terranes, this study demonstrates that care must be taken in using strictly chemical discriminates in evaluating various structural blocks within a greenstone belt for VMS potential. The second paper by Ames and Taylor gives a detailed description of the composition, morphology, and associated alteration of the 8.2 million tonne West Anomaly, the most recently developed lens in the Ruttan deposit. This study gives us another look at the characteristics of a giant massive sulphide deposit. That the deposit contains marginal metal grades does not detract from the fact that it is essential to understand the physical parameters necessary for the creation and preservation of such a giant, the nature of the metal-bearing fluids, and the reasons for the low base metal content. Like most giant massive sulphide deposited Ames and Taylor have shown that it formed in a basin unfilled with volcaniclastic material overlain by a thin rhyolite sequence that hosts the deposit itself. Whereas Kidd Creek, Flin Flon, and parts of the Horne deposit arc covered by altered and mineralized volcaniclastic material, it appears that the Ruttan deposit formed near the top of a volcaniclastic sequence. It may be that the low base metal content is not a primary characteristic of the deposit, but rather a function of over-exposure at the seawater-rock interface where late-stage fluids flushed" the mound of base metals. The integration of exploration and environmental technology is becoming increasingly apparent as companies realize that the expertise gained by geoscientists for comprehending the hydrology and geochemistry of hydrothermal systems is essential in understanding lower temperature fluid reactions that concern environmental studies. An example is the third paper which documents a multi-parameter surficial geochemical study of the Ruttan tailings drainage system by Shilts et al. Using sonar profiling both as a guide to sediment collection strategy and as a tool in the interpretation of element anomalies, sediment (surface and core) and water samples were collected in Ruttan, Brehaut, Rusty, and Alto lakes. Anodic stripping voltametry was used for on-site analysis of waters down to low ppb levels of Zn, Cu, and Pb. At present, liming appears to control metal migration effectively so that, for example, concentrations of metals in Rusty Lake, downstream from Brehaut Lake, are equivalent to background for that lithology. A thorough understanding of the extreme variation in chemistry of the sediments, especially in Brehaut Lake, was predicated on information derived from application of sonar profiling, phase-selective leaching, and scanning electron microscopy, in addition to "conventional" chemical analyses. Snow Lake subprogram The Snow Lake mining district is considered ideal for a multi-disciplinary study, as it is the source of at least 35 million tonnes of massive sulphide hosted by a well-exposed, homoclinal sequence of volcanic strata. The strata are intruded by two large subvolcanic intrusive complexes, with both extrusive and intrusive suites affected by broad areas of synvolcanic hydrothermal alteration. The presence of abundant lakes and a complex Quaternary stratigraphy of variable thickness is ideal for surficial geological and geochemical studies, and the outcropping of several subeconomic sulphide occurrences is suitable for geophysical studies. The first paper of this subprogam by Bailes and Galley is a description of the depositional environments of the area's seven past-producing VMS deposits. The study is based on detailed 1:5000 scale mapping of the Chisel Lake-Lost Lake-Ghost Lake and Anderson Lake-Stall Lake deposit-hosting strata associated subvolcanic intrusions. The result is first of all a detailed physical and petrochemical description that, coupled with work by Stern et al. (in press), defines the evolution of an oceanic island arc sequence from the formation of a protoarc through more evolved island arc cycles to rifting of the arc and creation of new seafloor. Questions are raised as to the possibility of bedding parallel tectonic discontinuities within the sequence that are still being addressed. The mapping program also defined the internal zonation within an intrusive complex hypothesized to be responsible for the generation of the Cu-rich deposits. This is important for allowing explorationists to evaluate the possibility of discovering VMS deposits quickly by recognizing certain characteristics of the subvolcanic suites. Lastly, three spatially distinct, regional-scale hydrothermal systems were defined, only two of which are associated with known massive sulphide deposits. The third zone may represent an immature phase of a broad-scale hydrothermal system. The second part of the geology project was a study of the Chisel Lake-Chisel North Zn-Cu-Au-Ag-Pb massive sulphide mineralization (Galley et al., 1993). The results were a detailed mineralogical and lithogeochemical description of a carbonate-hosted VMS deposit, a type whose occurrence is so far restricted to other Paleoproterozoic camps such as Bergslagen (Sweden). Ladysmith-Rhinelander (Wisconsin) and the VMS deposits of the Sudbury structure. The characteristics of the Chisel Lake orebodies and associated alteration suggest shallow-water (<1000 m) deposition. The recognition of shallow-water massive sulphide deposits is important as there appears to be a correlation between water depth of emplacement and the precious metal content of VMS systems, indicating that this deposit subtype may represent a transition between classic VMS and epithermal deposits. The surficial geochemical program was specifically designed to develop new sampling and analytical methods utilizing media such as glacial till, humus, and vegetation. The glacial till project (by Kaszycki et al.) was carried out in a region that is not traditionally thought to be "friendly" to surficial geochemical exploration, due to the patchy nature of the glacial deposits and the presence of Quaternary lacustrine clay beds. The sampling program was established only after a thorough examination of the Quaternary stratigraphy and identification of three distinct till sheets. The success of the techniques used are reviewed by Kaszycki et al. in describing the three phases to the geochemical program.
In surficial geochemical exploration for base metal deposits, the methods employed are only useful if an anomaly can be quantitatively assessed as a vector towards mineralization. Though the most common analytical method employed in geochemical exploration is based on determination of the "total" or near total element (e.g. dissolution of sediment or soil by aqua regia), it is usually the "labile" (free, noncrystalline) form of the element which is pertinent to the surface expression of a mineral deposit. Thus the geochemical pattern can be obscured by using a near total decomposition where that portion of an element bound in a silicate or sulphide phase is included. Renewed interest in the application of selective leaches was stimulated amongst the Canadian mining industry when, in 1990, major companies funded the demonstration of Russian methodologies designed to locate deeply buried mineral deposits. Two of the six methods presented by the team from Rudgeophyzika, St. Petersburg, are essentially selective leaches to extract metals bound in surface "scavenging" phases, that is, (a) in the humic and fulvic organic components of humus, and (b) in the amorphous Fe oxyhydroxide and Mn oxide "phase" of soil or till (Antropova et al., l 992). The Analytical Method Development Section of the GSC, under Hall's leadership, refined these two methods and went on to develop a sequential leach designed to identify and quantify four more phases which would be of use in the interpretation of surface geochemical anomalies. Three papers by Hall et al. describe these extractions, together with their advantages and limitations, and provide data on the precision and accuracy to be expected, using samples from the Snow Lake area and Canadian international reference standards. The cost-effective selective leaches developed were then applied in a study by Kaszycki and Hall to categorize surficial anomalies in the Chisel Lake area. The result was the differentiation between geochemical anomalies in humus, soil, and till associated with glacial dispersion, hydromorphic/biogenic demobilization, and anthropogenic contamination. Completing the spectrum of surficial geochemical studies from glacial till through humus to vegetation is the study by Fedikow and Dunn on the sensitivity of northern tree species to the presence of deeply buried massive sulphide deposits. The premise is that certain elements will be transported towards the ground surface and incorporated into the living tissue of plants with a greatly extensive root structure. Their choice of a study area overlies the Chisel North Zn-Cu deposit, which lies 600 to 625 m below the ground surface. This careful study involved sampling of bark and twigs of three different species for a number of elements known to be anomalous within the massive sulphide mineralization. Ash normalization was used to reduce effects of wind-blown contamination. The result was definition of a multi-element trace element anomaly directly over the deposit. A detailed lake sediment and water survey carried out by Frisks and McCurdy resulted in the collection of samples at a density of 1 site per 4.3 km . In addition, short cores were collected from some lakes to aid in the interpretation of near-surface anomalies. Several anomalous base metal values were recognized in lakes directly south of known base metal occurrences and deposits indicating that bedrock mineralization has a strong influence over metal concentrations in the lakes. In most instances the near-surface anomalies were matched in the short cores by anomalies at depth, showing that the elevated values were caused by natural mineralized sources not by mining contamination. In only three lakes, anomalies were caused by pollution from tailings. Other anomalies were recognized in lakes some distance from known mineralization, suggesting the presence of possibly untested bedrock base metal anomalies. Four geophysical studies took place under the EXTECH l banner involving a broad range of data acquisition and analysis, from spectrometry through electromagnetics to magnetics. The spectrometry study by Shives et al can be considered a cross between geophysics and geochemistry as it involves the measurement of rock chemical properties In conjunction with VLF and total field and gradiometric magnetics. The objectives of the study were:
Reevaluation of airborne data resulted in the conclusion that the line spacing normally used for regional surveys was too coarse to define meaningful differences in either the volcanic stratigraphy or contained hydrothermal alteration zones. The airborne data were useful in differentiating subvolcanic from synkinematic plutons, a first order criterion for identifying potential VMS-hosting terranes. The ground surveys were successful in distinguishing among felsic volcanic formations, as well as identifying coeval extrusive-intrusive felsic suites. This is useful in correlating subvolcanic intrusive phases with potential VMS-hosting rhyolite complexes. This study shows that although bedrock geologists and explorationists have long used airborne magnetic data to assist in bedrock mapping, existing airborne spectrometry data have been under utilized. The use of a hand-held spectrometer is also shown to be useful in differentiating and comparing felsic rock suites, a tool that is particularly useful in highly deformed terranes. The next two geophysical studies on electromagnetic methods and electrical rock properties are interconnected, as the second was initiated due to the results of the first. The Sinha and Palacky study involved the re-evaluation of standard EM methodology on massive sulphide deposits in highly metamorphosed and deformed terraces. The study examined the problem of identifying the Snow Lake massive sulphide mineralization at detection levels expected for the size and composition of the orebodies. Four subeconomic occurrences (including the l3 million tonne Linda deposit) were tested using ground mufti-frequency electromagnetic (HLEM), magnetic total field and gradiometer surveys. It became evident that the calculated conductivity values were very low in relation to similar sulphide mineralization in the Archean Superior Province. Sinha and Palacky made recommendations on how to increase the sensitivity of ground EM surveys over Snow Lake-type tisanes, and also initiated a project to study the electrical properties of the Snow Lake massive sulphide deposits. The electrical properties study by Katsube et at. analyzed specimens from a variety of types of Snow Lake base metal mineralization. The result showed an exponentially higher resistivity in comparison to other known massive sulphide camps, which was thought to be caused by sulphide grain separation during amphibology facies metamorphism. The fourth study involved the development of porthole geophysical instrumentation. Killeen et at. describes the development of an oriented, 3-component porthole magnetometer. This instrument produces a continuous record of dip and azimuth of the borehole, and carries out a vector magnetic survey while traversing the porthole. The vector magnetics will allow for the detection of magnetically anomalous bodies in the vicinity of the drillhole. This is useful for two reasons: a) an anomalous magnetic field will result in the distortion of azimuth data as they are normally recorded in a drillhole, and b) it can detect the presence of Cu-rich mineralization which is commonly associated with massive pyrrhotite and disseminated to massive magnetite. Even if the drillhole is out of range of a magnetic field set up by a massive sulphide body, the associated metamorphosed alteration zones commonly contain disseminated magnetite and pyrrhotite. The changes in intensity of magnetization within the various alteration facies can therefore be used to direct drilling towards a potential orebody. GIS data integration and modelling Perhaps the most innovative aspect of the EXTECH II program was the work by Wright and Bonham-Carter demonstrating the power of Geographic Information Systems (GlS) for building a multi-layered exploration data set and combining the data to map mineral potential. Some of the early work on the EXTECH I data was published previously (e.g. Reddy et al., 1991; Reddy and Bonham-Carter, 1991; Reddy et a1., 1992). GIS is a marvelous "electronic light table" for overlaying maps and searching for coincident anomalies, and for examining characteristics of individual localities by interactive query. It is also being increasingly recognized as a tool for decision support, and exploration decisions can be optimized by making use of all the data sets available by means of statistical and expert system models for data integration. Wright and Bonham-Carter show such models can be implemented, and produce a series of VMS potential (or favourability) maps. These maps show that all the known deposits occur in areas of elevated potential, as expected, and indicate several favourable zones where no deposits have been discovered. Recently, the Photo Lake deposit was discovered within one of these "hot spots", after the GIS study was complete. The discovery was made using airborne EM, without the benefit of the GIS map. Nevertheless, the GIS result bears out the value of the integrated approach, and the methods descried in Wright and Bonham-Carter's paper are directly applicable to mineral exploration elsewhere. GIS as a focal point for an integrated project The GIS study was a focal point through which the results of the various projects were integrated along with pre-existing data sets. The geological and mineral deposit research formed a starting point from which the other projects could define the various characteristics of a base metal camp that they would hope to identify and model geochemically and geophysicist. The GIS group further directed data collection by pointing out gaps in the pre-existing data sets, and by identifying the relative importance of individual data layers as predictors of the known mineral deposits. From this starting point the projects and subprojects then can be arranged as shown in Figure 2. The collection of data sets and requirement for specialist information by the GIS group gave the project a constant focus. GIS-based integration models The success of the GIS work comes not simply from the application of a computational "black box", but from the careful development of a deposit model (and the subsequent exploration model) that guides the selection of data sets and the manipulation of these data sets to extract the spatial evidence that is critical for prediction. Wright and Bonham-Carter show that the statistical approach (such as weights of evidence, or logistic regression) is invaluable because it allows the spatial association between each data layer and the known deposits to be calculated objectively. On the other hand, the expert system approach (such as fuzzy logic or Dempster Shafer) is not bound by the known deposits, but makes use of exploration experience for assigning weights (in the form of "membership" of "belief" functions) to each data layer, and for formulating an "inference network" that attempts to mimic the thought processes of an experienced geologist in combining the data sources together. The result of applying a variety of different models, with different assumptions, is to produce a series of integrated "favourability" maps, and the degree of the differences between the maps is a measure of the sensitivity of the results to changes in assumptions and changes in the parameters. It is interesting that the three models applied here gave remarkably similar results, showing the robustness of the approach. Not only did the maps predict that the as yet undiscovered Photo Lake was in a highly favourable zone, but also that (1) the various data sources could be ranked according to predictive capability, and (2) both weights of evidence and Dempster Shafer methods allow maps showing uncertainty of prediction to be produced. These methods are valuable not just for target selection, but for formalizing an exploration model-This brings together the experts in different data types and leaves an "audit trail" so that others can repeat the modelling calculations and obtain the same answer. It also facilitates the explanation and justification of a particular decision to managers. It seems likely that GIS integration models will have a growing importance in the "toolbox" of the exploration industry. EXTECH 1: multi-disciplinary or integrated? At the completion of a project such as this, one must define objectively its successes and failures. One management objective was to involve the exploration industry in the design of the project to a greater extent than had been typical of earlier survey projects. It was hoped that an appropriate balance would lie achieved between what the researchers believed to be directions in which science can improve exploration strategy and the industry's pragmatic assessment of what scientific results could realistically be applied to exploration problems. We believe that the project was only partly successful in this respect accuse of some hesitancy to bridge the gap on the part of geoscientists in nth government and industry at the outset. In the end, however, the project served to build mutual trust and understanding with the result that more recent cc-operative projects such as the EXTECH II project in the Bathurst district of New Brunswick are achieving the desired degree of interaction. As the GSC's first multidisciplinary mining camp study, EXTECH I was successful in acquiring critical new exploration data, and in developing several new methodologies. It was the first time that GSC and MGSB had attempted l:5000 scale mapping of a mining camp, and the result was a much otter understanding of the complexities of volcanogenic massive sulphide environments, of the hydrothermal systems spatially associated with these deposits, and the role of sub- volcanic intrusive complexes in generating these hydrothermal systems. It becomes evident though, that 1:20 000 scale mapping is the optimum scale for defining regional- scale relationships in a massive sulphide camp. Installed till sampling resulted in the identification of meaningful multi- element discriminates, new analytical techniques (including speciation studies), and a chance to develop new discriminants centered on the mineralogical and chemical characteristics of metamorphosed hydrothermal alteration zones. Identification of deeply buried VMS deposits through vegetation geochemistry gives further proof of the release of tracer elements from deposits during deformation and metamorphism, and may develop into a powerful tool to be used in conjunction with phase-selective leaching and deep penetrating geophysics. Methodologies for identifying various sources of lake water and sediment anomalies, and for increasing our understanding of how metals can be cyclicly isolated and dispersed in these media will allow explorationists to better assess geochemical trace element anomalies, and help government and industry better forecast environmental impact from both natural and anthropogenic metal sources. Although airborne spectrometry was not shown to the a useful tool for identifying large-scale alteration systems, it can tie used to identify K-poor subvolcanic intrusive complexes, whereas ground measurements can assist in correlating these intrusions with their extrusive equivalents. The results of the HI-EM and electrical properties studies have alerted the exploration industry to the need of adjusting their methodologies for more sensitive detection of metamorphosed massive sulphide deposits. This multi-disciplinary study can be considered integrated from two aspects. Firstly, the interaction of the group through planning sessions, semi-annual progress meetings, and ad-hoc discussions improved our collective knowledge as to the strengths and limitations of the individual disciplines. The gathering of the individual investigators into one field camp during the summer season was a further impetus to inter-group consulting and discussions. Politically these federal-provincial field camps helped cement cooperation between government surveys and minimized duplication. Secondly, the GIS study demonstrated how diverse data sources can tie integrated, with the aid of a well-structured exploration model, to support the decision-making process. The project came up short of being truly integrated, because the manuscripts within this volume are not products of cc-operative authorities between disciplines, but rather a collection of papers whose common thread is that they took place in the same mining camps. Another missing factor was our inability to take otter advantage of company data, especially in the field of airborne geophysics, and our failure in not formally involving individuals from the exploration industry in the research, experimentation, and documentation. This restricted our ability to evaluate the exploration methodologies used, and to evaluate the potential of such information as airborne EM data in further refining the GIS-generated favourability maps. In the end we believe that our successes far outweighed the failures, and that EXTECH I not only generated new data, methodologies, and concepts, but also acted as a testing ground for a new generation of cc-operative government-industry research geared to the needs of the mineral industry. Acknowledgments The EXTECH I program was funded by the Geological Survey of Canada under the 1989-1994 Rusty Lake-Snow Lake Mining Camps Canada-Manitoba Exploration Science and Technology Initiative (EXTECH I), and by the Canada-Manitoba Partnership Agreement on Mineral Development (1990- 1995).
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