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GAC Special Paper
Geological Association of Canada
GAC Special Paper

Geological Association of Canada

0072-1042

GAC Special Paper/Journal GAC Special Paper
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    Introduction to the geology, physiography, and glacial history of the Canadian Cordillera in British Columbia and Yukon

    Adrian S. HickinBrent C. WardAlain PlouffeJoAnne Nelson...
    25页
    查看更多>>摘要：The Canadian Cordillera hosts a variety of mineral deposits that reflect a protracted and complex geological history. Combining physiography and geological heritage, the Canadian Cordillera is commonly described in terms of five morphogeological belts (from west to east): Insular, Coast, Intermontane, Omineca, and Foreland. Within these belts are a number of exotic and pericratonic terranes that were accreted to the margin of Ancestral North America. Several times during the Quaternary, the Canadian Cordillera was covered by continental ice maBes referred to as the Cordilleran Ice Sheet. As ice sheets formed and flowed, they altered the landscape and deposited diverse sediments. The evolution of the ice sheets commonly resulted in a range of ice-flow directions at any given location, a critical consideration for mineral explorers. Armed with an understanding of surface proceBes and an appreciation of the growth, decay and flow histories of Pleistocene glaciers, geochemical and mineralogical anomalies detected in stream, lake, and glaciogenic sediments can be effectively traced back to bedrock sources, thus enhancing the potential for new mineral discoveries.
      原文链接:
    • NSTL

    Sampling and interpreting stream, lake, and glacial sediments for mineral exploration in the Canadian Cordillera, a review

    Adrian S. HickinAlain Plouffe
    25页
    查看更多>>摘要：The Canadian Cordillera is well endowed with mineral deposits and holds significant potential for new discoveries. Surficial media sampling has evolved as an effective mineral exploration approach and has had many succeBes. Given that the next generation of mineral deposits in the Canadian Cordillera will likely be concealed by sediments, surficial media sampling is a critical tool for exploring large regions, rapidly and cost effectively. Stream sediment, lake sediment, and till are routinely sampled for mineral exploration in the Canadian Cordillera. In this review, we highlight the application of geochemistry and mineralogy of stream, lake, and glacial sediments and discuB sampling strategies and interpretive concepts based on recent developments in using indicator minerals that can identify specific deposit, alteration and sources of geochemical anomalies.
      原文链接:
    • NSTL

    A review of analytical methods for regional geochemical survey (RGS) programs in the Canadian Cordillera

    Ray LettAlexei S. Rukhlov
    56页
    查看更多>>摘要：Commonly sampled media for regional geochemical survey (RGS) programs in Canada are stream and lake sediments and waters, and till deposits. During the last 60 years in the Canadian Cordillera of British Columbia and Yukon, surveys have advanced by sampling more diverse media such as moB mat sediment, stream heavy mineral concentrates, vegetation, and using increasingly sophisticated analytical methods. In the 1950s, stream-sediment samples collected by mining companies as part of regional geochemical surveys were typically analyzed for a small number of elements by colorimetric techniques. Later Canadian government-managed regional geochemical surveys used other sample types (e.g., lake sediment and water), initially to locate strategic metals (e.g., uranium) and later to detect other metals. For these surveys sample densities range from one sample per 5-10 km2 for till and vegetation to one sample per 10-15 km~2 for stream and lacustrine sediments. Samples from these surveys were analyzed for a greater number of elements by a combination of colorimetric, fluorimetric and acid digestion-atomic absorption techniques. A greater appreciation of Cordilleran glacial geology and ice-flow patterns led to till geochemistry being succeBfully used to detect new sources of bedrock mineralization concealed beneath locally transported surficial cover. At the same time as new sample media were being applied to regional geochemical surveys, new analytical methods were being developed, such as instrumental neutron activation (INAA), aqua regia digestion followed by multi-element analysis by inductively coupled plasma emiBion spectroscopy (ICP-ES) and, later, maB spectroscopy (ICP-MS). Over time, the development of systematic quality control schemes for sampling and sample analysis and new standard reference materials by the Geological Survey of Canada improved the reliability of regional survey geochemical data. Routinely sampled as part of regional geochemical survey (RGS) programs in the Canadian Cordillera are stream and lacustrine sediments and waters, vegetation, and till deposits. Since 2000, heavy mineral (e.g., >SG 3.2) concentrates recovered from the <2 mm grain-size fraction have been prepared from many stream-sediment and till samples collected in British Columbia. After microscopic examination, these concentrates are analyzed by methods such as scanning electron microscopy (SEM) combined with Energy Dispersive X-ray Spectroscopy (EDS) to identify and count gold, platinum, sulphide, oxide, and silicate grains. Different deposit types contributing to the mineral suite in a sample can be distinguished from different indicator mineral aBemblages such as diamond indicator minerals, magmatic/metamorphosed maBive sulphide indicator minerals or porphyry Cu indicator minerals. Other micro-analytical methods such as in situ high-precision isotope analysis and laser ablation followed by ICP-MS can also be applied to better identify mineralized source rocks of heavy mineral concentrates. Combining heavy mineral studies with the results of sediment and till sample geochemistry has added a new dimension to applying regional survey data to explore for new mineral resources in the Cordillera.
      原文链接:
    • NSTL

    Origin and character of till and other diamictons and their applicability to mineral prospecting

    Olav B. LianAdrian S. HickinBritish Columbia Geological Survey, Ministry of Energy, Mines and Petroleum Resource
    19页
    查看更多>>摘要：The use of near-surface sediment sampling for mineral exploration, or drift prospecting, is a well-established technique in the Canadian Cordillera. To be useful, a geochemical or mineralogical signal detected in a sample must be from media with a decipherable transport history that enables the geochemical or mineralogical anomaly to be traced back to a primary bedrock source. Subglacial till, the sediment transported by, and deposited directly from, ice at the base of a glacier, with minimal sorting by water, is an excellent media for drift prospecting. It is commonly a first derivative of bedrock, with a transport path that is typically linear and predicable, even in cases with multiple ice-flow directions. Distinguishing subglacial till from other glacial and non-glacial deposits may, however, be challenging for mineral explorers. To this end, we review the origin and nature of till deposits, and those of other origins that may resemble till, in order to aBist in identifying the distinctive characteristics that can be used to interpret genesis. Consistently recognizing and sampling subglacial till will simplify interpretation and ultimately increase the potential for new mineral deposit discoveries.
      原文链接:
    • NSTL

    Porphyry Cu indicator minerals in till: A method to discover buried mineralization

    A. PlouffeT. Ferbey
    31页
    查看更多>>摘要：Porphyry Cu indicator minerals (PCIM) can be used to detect bedrock mineralization covered by glacial sediments. PCIM recovered from unconsolidated, transported sediments like tills are traced back to their bedrock source by reconstructing their transport history using ice-flow indicators like striations, drumlins, and crag and tails. We provide an overview of PCIM," and potential PCIM, that occur in bedrock and have been recovered from tills in central British Columbia. We present their spatial distribution and abundance in till in the region of three porphyry Cu mines (Gibraltar, Mount Polley and Highland Valley Copper) and one prospect (Woodjam) and claBify them into two simple categories. Group 1 minerals (e.g., chalcopyrite, gold, epidote, jarosite) can be directly linked to porphyry Cu mineralization or aBociated alteration, based on their spatial distribution and abundance in till. Group 1 PCIM are more abundant in till near mineralization and decrease in abundance with increasing distance down-ice from the source. In contrast, the spatial distribution and abundance of Group 2 minerals (e.g., tourmaline, zircon, rutile) may not be obviously linked to porphyry Cu mineralization and such links may be better established by mineral chemistry. The two-group claBification of PCIM is site specific and dependant on the ore and alteration mineralogy in each deposit. Future research on PCIM should be prioritized on resistate minerals that can survive glacial erosion and comminution, are stable to para-stable in the oxidizing surficial environment, and are abundant enough to be routinely detected in unconsolidated sediment samples.
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    • NSTL

    Magnetite as an indicator mineral in porphyry Cu+-Au+-Mo deposits of British Columbia, Canada

    D. CanilL. PisiakT. Lacourse
    14页
    查看更多>>摘要：Glacial overburden can overlie large areas of Mesozoic intrusive igneous rocks in the Canadian Cordillera that potentially host porphyry Cu-Au mineralization. Magnetite is an acceBory mineral in many such deposits, and once eroded from these sources by glacial action and transport, may serve as a useful indicator mineral for prospecting in glaciated terrain. Previous work shows that magnetite from ore deposits has a trace element signature that is unique from that of a common acceBory mineral in igneous or metamorphic rocks. To this end we review the geochemical characteristics of magnetite of hydrothermal origin from porphyry deposits in British Columbia. We observe much heterogeneity at the grain and deposit scale for many trace elements (Mo, Cu, Sn, Mn, V, Sc, Ni) in magnetite from five porphyry deposits. Hydrothermal magnetite is consistently low in elements that are temperature-sensitive in the spinel structure (Ti, Al, Mg), and is rich in Sn and sometimes Mo, relative to Sc. A linear discriminant analysis (LDA) of magnetite compositions compiled from worldwide porphyry deposits and intrusive igneous rocks (n=481) permits a rigorous definition of the chemical signature of hydrothermal magnetite from such systems in terms of its Mg, Al, Ti, V, Mn, Co, and Ni contents.We review an application of magnetite composition as a vector to locate porphyry deposits in glaciated terrain, using 20 subglacial till samples collected in a 900 km2 area surrounding the Mount Polley porphyry Cu-Au deposit, in south-central British Columbia. Twenty trace elements were measured by laser ablation inductively coupled plasma maB spectrometry (LA-ICP-MS) in 50 magnetite grains from each till sample. Application of our LDA models to magnetite populations in the till samples shows that anomalous concentrations of hydrothermal magnetite grains are detected in till up to 2.5 km west-southwest and 4 km northwest of the Mount Polley deposit. The dispersal direction of these grains is consistent with the ice-flow history of the region. The use of LDA to identify hydrothermal magnetite has a strong potential to be an effective indicator in exploration for buried porphyry systems jn British Columbia and elsewhere. Further tests of this exploration method should be performed around other known sources of porphyry Cu+-Au+-Mo mineralization with different ice-flow histories. In addition, improvements to the LDA models we developed for hydrothermal magnetite identification will be poBible with more analyses from other porphyry deposits.
      原文链接:
    • NSTL

    The direct indicator mineral concept and QEMSCAN~R applied to exploration for carbonatite and carbonatite-related ore deposits

    G.J. SimandlD.A.R. MackayX. Ma
    16页
    查看更多>>摘要：Indicator minerals have long been used in diamond exploration and the approach has been expanded to search for other commodities. Orientation surveys using stream sediment samples collected from the drainages of three carbonatite-related deposits in the Canadian Cordillera (Aley,Lonnie,Wicheeda) establish that a number of carbonatite indicator minerals contain elevated concentrations of high field strength elements (HFSE),including rare earth elements (REE) and Nb. These elements are detectable by traditional geochemical or portable X-ray fluorescence (XRF) analyses of stream-sediment samples even if only a few indicator mineral grains are present. We refer to these minerals,which are rich in carbonatite pathfinder elements,as direct carbonatite indicator minerals (e.g.,REE-fluorocabonates,REE phosphates,pyrochlore supergroup minerals,and columbite-iron). These direct indicator minerals can also be effectively identified by QEMSCAN~R (Quantitative Evaluation of Minerals by SCANning electron microscopy),a fully automated micro-analysis system that is widely available at commercial laboratories. There is a strong link between high concentrations of direct carbonatite indicator minerals and high concentrations of HFSE in stream sediments draining known carbonatites and related deposits. This relationship is the basis of the direct indicator mineral concept; only samples identified as geochemically anomalous by traditional laboratory or portable XRF analyses need to be examined for carbonatite indicator minerals by QEMSCAN*. Advantages of QEMSCAN~R over traditional indicator mineral proceBing and analysis include the capability to: 1) identify minerals that are too small for hand picking; 2) provide automated,simultaneous indicator mineral grain counts and weight % equivalents; 3) perform detailed grain-size analysis on a mineral-by-mineral basis; 4) quantitatively aBeB abundances of free and polymineralic grains; and 5) identify mineral species within polymineralic grains. Because all samples are first geochemically analysed for carbonatite pathfinder elements,only those with concentrations above background need to be evaluated for carbonatite indicator minerals,thereby reducing costs. The direct indicator mineral concept in combination with QEMSCAN~R analysis is an effective mineral exploration approach because it combines traditional geochemical analysis with the interpretative power of modem indicator mineral surveys. This approach may have exploration applications for deposits other than those that are carbonatite-hosted.
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    • NSTL

    A novel approach using detrital apatite and till geochemistry to identify covered mineralization in the TREK area of the Nechako Plateau,British Columbia

    Mao MaoAlexei S. RukhlovStephen M. Rowins
    53页
    查看更多>>摘要：Resistant indicator minerals (RIM) have been used in mineral exploration in recent years to help identify and locate buried deposits because their compositions reflect their source rocks and,in some cases,specific types of mineral deposits. Apatite is a potential RIM because,in addition to its resistance to physical and chemical weathering,it is widespread in most rocks and mineral deposits. Our previous work discriminated the major types of mainly magmatic-hydrothermal mineral deposits in terms of linear discriminant functions using the Mg,V,Mn,Sr,Y,La,Ce,Eu,Dy,Tb,Pb,Th and U contents of apatites. Step-wise discrimination diagrams permit subdividing apatites by origin. We have analyzed detrital apatite grains recovered from till in ten study areas from the glaciated and underexplored Nechako Plateau of central British Columbia by electron probe microanalysis and laser ablation-inductively coupled plasma maB spectrometry. To aid interpreting these apatite data,we also used till geochemistry,and detrital chalcopyrite and gold grain counts. A total of 342 apatite grains (344 analyses) were claBified as aBociated with mineralization,whereas 41 apatite grains were claBified as derived from barren rocks,and two grains were claBified as 'unknown'. Mineralization-related apatite grains were claBified as alkalic porphyry Cu-Au (80),porphyry Cu-Mo-Au (28),porphyry Mo (72),porphyry-related Cu-Au breccia (16),W skarn (112),orogenic Au (26),orogenic Ni-Cu (7),and Kiruna-type 10A (3) deposit-types. Detrital apatite grains in till down-ice from developed mineral prospects in brownfield areas were correctly identified by the discriminant method. Detrital apatites also helped to generate many new exploration targets in greenfield areas lacking known mineralization or hosting only minor mineral occurrences. These findings validate the apatite discriminant method and demonstrate its usefulneB as a practical exploration tool in graBroots programs.
      原文链接:
    • NSTL