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Proactive disclosure Print version   | |  Diamonds
Diamond exploration in glaciated terrain differs from precious or base metal exploration in that it uses indicator minerals and boulders, instead of till geochemistry, to detect glacial dispersal from a kimberlite. Kimberlites are small (few hundred meters across), circular point sources. They are relatively soft rocks that have been preferentially eroded by preglacial weathering and glacial scouring to deeper levels than the surrounding bedrock surface and as a consequence are covered by lakes or thick glacial sediments. Recent discoveries of kimberlite on the Canadian Prairies and in the Northwest Territories have sparked unprecedented levels of diamond exploration in the glaciated Shield terrane of Canada and Finland. Several minerals are useful indicators of kimberlite, and to a certain extent, in evaluation of the diamond potential of kimberlite. These minerals survive glacial transport, are far more abundant in kimberlite than diamond and are visually and chemically distinct. Cr-pyrope, eclogitic garnet, Cr-diopside, Mg-ilmenite, Cr-spinel, and olivine are the most commonly used kimberlite indicator minerals, although in rare cases, diamond is abundant enough to be its own indicator. Kimberlite indicator minerals are recovered from the medium to very coarse sand-sized fraction of glacial sediments, and analyzed by electron microprobe to determine concentrations of major oxides. Kimberlites typically contain large concentrations of garnets from peridotitic rocks and a few from eclogitic rocks. Peridotitic garnets are subdivided on the basis of Ca content into wherlitic (high Ca), lherzolitic and harzburgitic (low Ca) affinities. Most garnet inclusions in diamonds are from low-Ca harzburgite and thus these garnets are sought in diamond exploration. Other chemical criteria include Na2O levels in eclogitic and MgO and Cr2O3 concentrations in ilmenites to determine probability of diamond preservation. Cr-spinel with >60% Cr2O3 and >12% MgO are judged to have a diamond inclusion composition and diopsides with >0.5% Cr2O3 are classified as Cr-diopside. Indicator minerals are most abundant in the 0.25 to 0.5 mm (medium sand) size fraction of glacial sediments. In contrast to unglaciated regions, all kimberlite indicator minerals survive long distance glacial transport and the relative abundances of each mineral in a till sample is a function of the primary mineralogy of individual kimberlite pipes. Surface features and morphology of indicator minerals may provide clues as to the distance and nature (glacial versus fluvial) of their transport. Recently published results of regional indicator mineral distributions in various parts of Canada (Lac de Gras, north-central Manitoba, the Prairies, Kirkland Lake) have allowed exploration companies to place their local, more detailed survey results into the broader, regional context. In the Lac de Gras area, where Canada's first diamond mine will go into production in 1998, regional indicator mineral surveys have been completed over an area of 30,000 km2. A large train of pyrope and Cr-diopside, 50 km wide and 100 km long, trends northwest from the western part of the Lac de Gras field of approximately 150 kimberlite pipes, parallel to the direction of the main phase of Late Wisconsinan ice. The east half of the kimberlite field does not have an indicator mineral signature in the till. Pyrope is the most abundant indicator mineral, followed in decreasing abundance by Cr-diopside, Cr-spinel and Mg-ilmenite. In this area, till samples that contain more than 5 sand-sized pyrope grains in a 10 kg sample are considered anomalous. Dispersal trains from individual kimberlites pipes within the Lac de Gras field are typically narrow (100's m), sharp-edged linear ribbons that extend several 10's of km down-ice and reflect the relative abundance of indicator minerals in individual pipes. On the Canadian Prairie, results from a regional till survey indicate that elevated pyrope concentrations in western and southwestern Saskatchewan are derived from kimberlites in central Saskatchewan and reworked from Tertiary gravels. In the Kirkland Lake kimberlite field of northeastern Ontario, eskers contain pyrope and kimberlite boulders several km down-ice from kimberlites. Detailed glacial dispersal studies completed around the Kirkland Lake kimberlites revealed that each kimberlite pipe has a different indicator mineral signature.
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