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Review of till geochemistry and indicator mineral methods for mineral exploration in glaciated terrain
Stream Sediment Indicator Mineral Signatures of the Casino Porphyry Cu-Au-Mo Deposit, Yukon, Canada
Hydrogeochemistry of porphyry-related solutes in ground and surface waters; an example from the Casino Cu–Au–Mo deposit, Yukon, Canada
Indicator mineral and till geochemical signatures of the Broken Hammer Cu–Ni–PGE–Au deposit, North Range, Sudbury Structure, Ontario, Canada
Indicator mineral and geochemical signatures associated with the Sisson W–Mo deposit, New Brunswick, Canada
Till geochemical signatures of volcanogenic massive sulphide deposits: an overview of Canadian examples
Indicator mineral and till geochemical dispersal patterns associated with the Ranch Lake kimberlite, Lac de Gras region, NWT, Canada
Front Matter
A Quaternary geological perspective on geochemical exploration in glaciated terrain
Abstract The application of Quaternary geology and glacial sedimentology is given as a broad guide for geochemical exploration in glaciated terrain. Predictive models of glacial dispersal provide an important basis for tailoring drift prospecting methods to suit regional variations in ice flow history and dynamics. The models relate compositional variations in glacial dispersal trains to ice flow direction, glacial history and subglacial processes. They are continually refined with reference to the geological and physical properties of the ice bed; new empirical field evidence constraining particle trajectories; and knowledge of subglacial processes affecting glacial erosion, transport and deposition. Transport at the ice bed leads to an exponencial decrease in indicator concentrations with increasing distance of glacial transport, whereas linear decrease is associated with englacial transport, and may be characteristic of ice streams. The partitioning of rock and mineral fragments through subglacial comminution leads to compositional differences among size fractions that can reflect intensity of subglacial process, distance of transport, and provenance; hence, the choice of size fraction is important to drift prospecting by geochemical methods.
Abstract Till is a favoured sample medium for locating mineral deposits in glaciated shield terrains of Canada and Fennoscandia because it best reflects the primary composition of the bedrock source area. In the sampling phase, an important and costly component of till surveys, sample density, sample depth and sample method must be chosen according to the needs of the exploration program. Surface till sampling methods in forested areas differ from those used in permafrost terrain. However, in both areas, concentrations of labile ore minerals and their products of decomposition can be detected in the fine fraction (< 2 mm) of weakly oxidized till. In thin drift-covered areas, till samples are collected by hand excavation or trenching at < 5 m depth. In areas of thicker drift, more expensive methods such as reverse circulation rotary drills, rotasonic drills and portable drills are used to collect till samples at depth and to determine lateral and vertical variations in till geochemistry. Laboratory methods are an essential part of till geochemical surveys. The choices of the size fraction and analytical methods are determined by the nature and composition of the expected bedrock target, and by costs.
Regional till geochemical surveys in the Canadian Cordillera: sample media, methods and anomaly evaluation
Abstract Basal tills have become a widely used regional geochemical sampling medium in recent years in the Canadian Cordillera. They reflect the primary composition of the source bedrock and contrast with B-horizon soil that can be developed on a variety of glacial and non-glacial surficial sediment types. Detailed sedimentological data are critical to collect and they are used to differentiate basal tills from other visually similar sediments including englacial and supraglacial tills, colluvial debris flow deposits, and very poorly sorted, glaciofluvial or glaciolacustrine sediments (e.g. diamictons or gravelly muds). The variable transport and depositional processes that form these different sediments make interpretation of geochemical data difficult. Deep (usually > 0.75 m) C-horizon sampling of basal till minimizēs the complicating effects of pedogenesis, weathering, surface washing and gravity remobilization of the tills. The latter processes, particularly pronounced in the wet, steep terrain, typical of much of the Canadian Cordillera, lead to depleted concentrations of heavy minerals (notably Au) and hydromorphic dispersion of mobile elements in the near surface sediments. Also, elements that are preferentially concentrated in the fine fraction can be selectively removed by surface waters. Offset sampling lines, oriented perpendicular to the dominant ice-flow direction, are most effective for detecting regional geochemical anomalies which are typically narrow and elongated parallel to ice-flow. Erratics trains and till anomalies are usually a few to several kilometres long and up to one or more kilometres wide. For some metals such as Au, anomalies are generally larger and more readily detected in till than in B-horizon soil. Surface till anomalies reflect up-ice sources and not the immediately underlying bedrock; down-ice displacements of > 500 m often occur in areas of thick till. Basal till anomalies usually can be traced to source along linear transport paths reflecting topographically controlled valley-glacier flow in mountainous areas and unidireccional ice-sheet flow in many plateau areas, chiefly representative of the last glacial event. Interpretations of till geochemical data are enhanced with a clear understanding of the surficial and bedrock geology, Quaternary stratigraphy, ice-flow history and down-ice dispersal characteristics around known mineral deposits.
The application of heavy indicator mineralogy in mineral exploration with emphasis on base metal indicators in glaciated metamorphic and plutonic terrains
Abstract Indicator mineralogy is used to explore for a wide variety of mineral commodities. The method utilizēs minerals which are sufficiently heavy to be readily concentrated in the laboratory, often colourful and possess other useful physical and chemical properties. The minerals also must be source specific. Some indicator minerals are true resistate minerals. The others, although less resistant, are stable in oxidized glacial drift and many non-glacial sediments. A few, such as gold grains, are silt sized but most are coarse grained. Grain size has a major impact on indicator mineral dispersal patterns in glacial drift. The coarse-grained indicator minerals are of two main types: (1) kimberlite indicator minerals (KIMs); and (2) metamorphosed or magmatic massive sulphide indicator minerals (MMSIMs). KIMs are enriched in Mg and Cr and most MMSIMs are enriched in Mg, Mn, Al or Cr. These indicator elements cannot be diagnosed geochemically in anomalous heavy mineral concentrates because the concentrates contain other, more plentiful non-indicator minerals containing the same elements in the same chemical form. Chalcopyrite is also a very useful MMSIM but the number of surviving grains in a dispersal train is too low for detection by selective geochemical analysis. MMSIMs are derived from three main types of base metal deposits and their associated alteration or reaction zones: (1) volcanosedimentary massive sulphides (encompassing volcanogenic, Sedex and Mississippi Valley subtypes) in medium to high grade regional metamorphic terrains; (2) skarn and greisen deposits; and (3) magmatic Ni–Cu sulphides. The variety of MMSIMs associated with Ni–Cu deposits is astonishing, apparently reflecting mineral hybridization related to assimilation of sulphurous sedimentary rocks by ultramafic magmas. Cr–diopside is one of the best indicators of fertile Ni–Cu environments although not necessarily of the actual Ni–Cu deposits. Heavy indicator mineralogy is much more sensitive than heavy mineral geochemical analysis and offers many exploration benefits in regional exploration programs including: (1) sampling efficiencies; (2) enlargement of both the bedrock target and dispersal train; (3) coverage of a wider range of mineral commodities; (4) undiminished sensitivity in areas of overabundant non-indicator heavy minerals; (5) visual evidence of points of origin of dispersal trains; and (6) indications of the economic potential of the source mineralization. It is most effective as a reconnaissance exploration tool and is particularly well suited for testing gneissic volcanosedimentary and plutonic terranes where base and precious metal deposits are highly modified and difficult to recognize by other prospecting methods.
Indicator mineral and geochemical methods for diamond exploration in glaciated terrain in Canada
Abstract This paper provides a summary of selected diamond exploration techniques used in the glaciated terrain of Canada, focusing on indicator mineral methods and till geochemistry but also including geochemistry of lake sediments and vegetation. Diamond exploration in Canada focuses on kimberlite, the primary host rock for diamonds in this country. Kimberlite is a mineralogically and chemically distinct point source which may yield discrete dispersal trains in glacial sediments. Understanding the ice flow history and depositional history of glacial sediments and identifying multiple till sheets in areas covered by thick glacial sediments are essential for successful sampling, interpretation and follow-up of indicator mineral and geochemical anomalies related to these rocks. Orientation studies over known kimberlites provide important information on the mineralogical and geochemical signatures of kimberlite, and the size fractions of glacial sediments that are best suited to indicator mineral and geochemical analysis. Kimberlite indicator minerals survive glacial transport over long distances and the relative abundance of each mineral in till is a function of the primary mineralogy of individual kimberlites. Indicator mineral distributions observed at a regional scale define the net effect of glacial dispersal, often along different ice flow directions. Local scale distributions define individual dispersal trains. The finer (0.25 to 0.5 mm) fraction of heavy mineral concentrates prepared from till samples is best suited for indicator mineral surveys. Till geochemistry is gaining popularity in diamond exploration because it is significantly cheaper than indicator mineral analysis and it can be performed quickly. Important kimberlite pathfinder elements that provide good contrast in till geochemical surveys include Ni, Cr, Ba, Co, Sr, Rb, Nb, Mg, Ta, Ca, Fe, K, Ti and REE, the relative importance of which will depend on kimberlite composition as well as that of the surrounding bedrock. Biogeochemical studies over kimberlites in Canada reveal geochemical signatures in vegetation despite the glacially transported substrate.
Abstract Lake sediment geochemistry has been used in Canada since the 1970s for mineral exploration and resource evaluation in glaciated regions of Appalachia, the Canadian Shield and the western Cordillera which are of low to moderate relief. Geochemical signatures of bedrock and mineralization within a lake’s catchment basin are commonly reflected in the chemical constituents of the organic-rich Holocene sediment which has been transported from source by a combination of mechanical and hydromorphic processes. Lake sediment geochemical surveys have been used successfully to discover base metal, Au, Mo, W, Sb, Sn, U, and REE mineralization. The scale of such surveys determines the size and density of lake sediment sampling. In regional surveys, large lithological targets such as greenstone belts or chemically distinctive intrusives can be identified by low density ( c . 1 per 10–15 km 2 ) sampling of relatively larger lakes. Smaller targets such as mineralized areas require higher density ( c. 1 per 4–5 km 2 ) sampling using a greater number of smaller lakes. Regional surveys are typically helicopter-borne, and employ a tubular grab sampler which permits rapid reconnaissance-scale coverage of large areas. Centre-basin profundal lake sediment, or gyttja, is an ideal sample medium because of its homogeneity and abundance of fine-grained organic matter, which complexes with many trace elements. Multi-element analytical techniques in common use include inductively coupled plasma emission spectrometry (ICP-ES), ICP mass spectrometry (ICP-MS) and instrumental neutron activation analysis (INAA). Reliablility of data is assessed by analysis of known standards, site duplicates and sample splits. PC statistical software packages facilitate the interpretation of geochemical data, and desktop GIS packages aid in further interpretation and generation of multi-layer maps.
Biogeochemical exploration methods in the Canadian Shield and Cordillera
Abstract This review article focuses on field methods in biogeochemical exploration and is based largely on the author’s experience 1 Much of this account and the Tables are abstracted from Dunn et al . 1993 and Dunn 1995 . Case histories can be found in these references and other texts such as Kovalevsky (1987). The present text focuses on methods, building upon the extensive pioneering studies of the late Professor Harry Warren and his colleagues at the University of British Columbia (e.g. Warren & Delavault 1950 ; Warren et al . 1968). . Consideration is given to reasons for applying biogeochemical methods as alternatives or supplements to other surficial sampling media that can be used in the exploration for mineral deposits in glaciated terrain. Extensive root systems can absorb metals from the substrate and integrate the geochemical signature of large volumes of sediment, groundwater and sometimes bedrock, thereby providing a more representative reflection of the chemical environment than that obtained from some other media. Sampling methods and precautions that should be taken are outlined. Variables that govern plant chemistry include the heterogeneity of composition among plant species and plant tissues, and the modifying effects of the seasons and contamination from external sources. Studies indicate that biogeochemical methods can provide a more proximal indication of concealed mineralization than the distal indications typical of till geochemistry programmes. Consequently, comparisons of till and biogeochemical data can help to define vectors toward mineralized sources such that the two methods are complementary.
Abstract Geographic Information Systems (GIS) provide the geologist with a powerful tool, when used in concert with statistical and geostatistical analysis, for archiving, manipulating, analysing and visualizing geochemical data. This paper uses geochemical (Zn, Cu) data obtained from various media (rock, lake sediments, till, soil and humus) over the Swayze greenstone belt in northern Ontario, to explore methods for analysing and visualizing geochemical data with a focus to mineral exploration applications. The behaviour of Zn and Cu in both bedrock and the surficial environment is studied using statistical and geostatistical techniques. Interpretation and uses of traditional statistics and dot plots are contrasted with interpolated geochemical maps as well as red-green-blue (RGB) ternary maps. Techniques for multimedia comparison and geochemical anomaly detection and screening are presented. The processing methods presented in this paper can be utilized and adapted by other geologists for exploring their own geochemical data. Many of the algorithms presented here are available within standard GIS software packages, or can be written easily using a GIS macro language.
Regional and local-scale gold grain and till geochemical signatures of lode Au deposits in the western Abitibi Greenstone Belt, central Canada
Abstract This paper is an overview of drift exploration methods for lode Au deposits in areas of thin and thick cover of glacial sediments within the Abitibi Greenstone Belt of central Canada. It summarizes a large volume of data produced by government regional surveys and case studies as well as that from industry-led gold exploration programs. Regional till surveys can be used as targeting mechanisms for further Au exploration. Anomalies are defined by a series of samples with elevated Au concentrations that lie along significant bedrock structures, occurring in clusters or as isolated samples in areas of low sample density. Thresholds between background and anomalous Au grain abundances or Au concentrations are variable and depend on location within the Abitibi Greenstone Belt. Case studies around known deposits provide examples of geochemical and mineralogical signatures of Au deposits that can be expected in till down-ice. These serve as sources of information on appropriate sampling methods and size fractions to analyse, and on ice flow patterns, local glacial stratigraphy and suitable till units for sampling. Two methods for measuring the Au content of till are commonly used: (1) a count of visible Au grains and (2) geochemical elemental analysis. Close to source, till contains thousands to hundreds of thousands ppb Au and several hundred Au grains. The Au grains vary from coarse sand to silt sizes and have pristine shapes. The presence of high Au concentrations in till indicates that the ore zones subcrop and that glacial processes have produced Au dispersal trains down-ice.
Application of composite glacial boulder geochemistry to exploration for unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan, Canada
Abstract Sampling glacially transported boulders is effective for mapping Subcrop clay alteration patterns associated with deeply-buried unconformity-type uranium deposits in the Athabasca Basin of northern Saskatchewan, Canada. The technique works well because the subcrop alteration haloes show significant geochemical contrast with background, because the glacial deposits are consistently distributed and rich in boulders, and the altered boulders are typically well-represented in the boulder population. Boulder sampling is preferred over some other techniques because it is rapid and inexpensive, and the use of a lithological sampling medium provides a direct measure of subcrop clay-mineralogy, in an area where outcrop exposure is very restricted. A large composite boulder sample data set, comprising c. 20000 samples from within the eastern part of the Athabasca Basin, has been compiled. Sampling and analytical techniques are described, and results are presented at both regional and semi-regional scales. The distributions show clear correlations with both basin-scale features, such as stratigraphical variation and regional alteration patterns, and with deposit-related features, such as hydrothermal illite, dravite and chlorite alteration. Composite boulder lithogeochemistry may be equally effective for mineral exploration in other glaciated regions with restricted bedrock exposure.
Abstract Glacier process models of till genesis cannot fully explain the properties of tills in Maritime Canada. A succession of local ice caps, called the Appalachian Ice Complex, developed during the last glaciation and were drained by ice streams into the submarine channels bordering the region. The migration of these centres produced areas with widely differing flow patterns, landform assemblages and deposits. Early regional phases of ice flow were characterized by wide, rapidly-flowing ice-streams that formed thick exotic, silty tills. In later phases, ice divides developed over highland areas underlain by metamorphic and igneous rocks, forming stony local tills. Terrain zones characterized by distinct transport histories and depositional sequences were produced by the interplay and migration of regional ice sheets and local ice caps. The interaction of local glaciers and previously deposited tills formed hybrid tills through two reworking processes: inheritance and overprinting. Inheritance is incorporation of till components and/or fabric into a younger till by erosion and entrainment of material from an older till. Overprinting is the injection or imprint of matrix, clasts or fabric on older tills by overriding ice. Glacial dispersal of rocks, minerals and trace elements in this complex glaciated terrain is controlled by the location of former ice divides. Simple unidirectional trains are preserved in relict terrains under divides. In ice marginal areas, reworking processes result in complex dispersal fans produced by smearing and widening of previously formed trains and fans. These dispersal fans can be modelled by vector addition of discrete flow events within each dispersal zone. A simple empirical model of glacier dispersal is presented using exponential decay and uptake algorithms, and incorporating the reworking of older till material.
Contrasting styles of glacial dispersal in Newfoundland and Labrador: methods and case studies
Abstract A review of practical approaches to drift exploration intended for use by exploration geologists working in drift covered areas is presented. The contrasting styles of glacial dispersal between Labrador, dominated by the effects of the Laurentide ice sheet, and the Island of Newfoundland, affected by small, coalescing ice caps at the glacial maximum and smaller topographically-controlled ice centres during deglaciation, are described. The effect has been to produce longer, ribbon-shaped dispersal trains in Labrador, except in the Labrador Trough near the centre of the Labrador sector of the Laurentide Ice Sheet, and shorter more diffuse dispersal patterns in Newfoundland.