Preliminary Results of Reconnaissance Sampling for Diamond Indicator Minerals 1

Transcription

1 Preliminary Results of Reconnaissance Sampling for Diamond Indicator Minerals 1 F.J. Swanson and MR. Gent Swanson, F.J. and Gent, M.A. (1992): Preliminary results of reconnaissance sampling for diamond indicator minerals; in Summary of Investigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep Heavy mineral investigations in the southern region of the Saskatchewan Phanerozoic Basin reported by Simpson (1991) and Gent (1989, 1991, 1992), have detected significant indications of kimberlitic minerals, as well as rubies and sapphires. The current study continues the work reported by Gent and is designed to better define these anomalous occurrences. It also includes sampling in east-central and southeastern Saskatchewan on a reconnaissance basis. Te sting of glacio-fluvial till and bedrock samples collected in 1991 has revealed three discrete areas of abundant kimberlite indicator minerals. Significant variations in the quantity and character of the indicator minerals among individual samples within each area suggest that each area includes, or is in very close proximity to, two or more pipes. A number of corundum trends including rubies and sapphires have been identified and suggest that several igneous rock sources may be present in southern Saskatchewan. Furthermore, testing of nearsource samples indicates that effective use of heavy mineral sampling for diamond exploration must include both fine-grained and coarse-grained (0.710 to 2 mm) concentrates. Sampling of g!acio-fluvia! gravels along the southern margins of the exposed Precambrian Shield was conducted to document the distribution of shield-sourced heavy minerals. It is expected that significant variations in some minerals from different litho-tectonic regions of the shield will be reflected in the Phanerozoic Basin samples. An understanding of these variations and the source materials will assist in interpreting glacial transport directions of diamond indicator minerals encountered in the basin. 1. Sample Collection and Processing Methodology A total of 11 O samples, primarily of glacio-fluvial or glacial till origin were collected. Sites were selected for the type of material present and were strategically located to either follow-up a mineral train, or to representatively sample as large an area as possible. Sufficient sample material was dry screened on site to provide 22 litres of 5 mm material. Subsequently, the -5 mm material was wet screened into 1 mm, 1 to 2 mm, and 2 to 5 mm size fractions in order to increase the concentrating efficiency of the 'Diester' shaker table. There is a secondary sorting by grain size during tabling and the coarser heavies may be removed with the lighter fines. All light sample fractions were stored for future reference. The two table concentrates were dried, recombined, and subjected to heavy liquid separation using tetrabromoethane with a specific gravity of 2.9 g/cm 3. The light fractions were weighed and stored and the heavy fractions (>2.9 g/ cm 3 ) were then screened to mm and mm fractions to conform with the size restrictions required by the Magstream separator. The coarse fraction was weighed and examined under a binocular microscope for indicator minerals. After removing magnetic material, the fine fraction was subjected to Magstream separation at a density of 4.15 g/ cm 3. The resulting heavy fraction (>4.15 g/ cm 3 ) was weighed and stored and the (2.9 to 4.15 g/ cm 3 ) fraction was visually examined with a binocular microscope for non-opaque kimberlitic indicator minerals, corundum grains, or any other unusual mineral grains. The mm fraction was examined because the macrocrystic suite of minerals generally range in size from 1 to 3 mm and, if a sample was collected close to source, the indicator minerals present will have undergone very little transport or abrasion. By eliminating the examination of the coarser fractions it is possible that al! the indicator minerals in a sample could be eliminated and a significant exploration target missed. Grains selected during microscopic examination were analysed by microprobe at the Department of Geological Sciences, University of Saskatchewan, for Ti02. Cr20J, FeO, MgO, CaO, Si02, Al203, Na20, and MnO content. Mineral identification was based on microprobe analysis, the Dawson and Stephens (1975, 1977) garnet and pyroxene method of group classification, and a PCbased program developed at Saskatchewan Energy and Mines. Stoichiometric formulae and analyses of minerals given by Deer, Howie, and Zussman (1966) were used as a basis for the computer identification of 95 percent of the minerals microprobed. Garnet and pyroxene group classifications were conducted to identify the best statistical fit using discriminate analysis. A second best classification was calculated for those grains whose com- (I ) Saskatchewan Project A211 was funded In 1992 under tile Canada-Saskatchewan Partnership Agreement on Mineral Development Saskatchewan Geological Survey 199

2 positions did not match within one standard deviation of the mean of the first group classification. This method was found useful in identifying trends in the garnet and pyroxene compositions as indicated by their group classifications. It was found that discriminant analysis is not completely reliable and that most of the G-10 group garnets identified in this study might best be classified as G- 9 group garnets tending toward a G-10 group classification. 2. Colour Variations of Kimberlitic Garnets Garnets, which appeared to be of kimberlitic origin, were picked from the coarse fraction of concentrate under a binocular microscope, sorted by colour, and counted. Kimberlitic garnets are typically rounded or irregularly shaped and clear with no or few inclusions. They can be coated with a rind of kelyphite-an alteration product formed at the expense of the garnet during equilibration with the kimberlite melt. Secondary transport of these grains often removes this coating leaving the garnet etched or pitted. Sorting by colour was undertaken to determine any trends or patterns among sample sites within a particular mineral train and among clusters of anomalous sites. The technique is subjective, as some colours tend to grade into each other. This was particularly evident with pink, flesh, orange, red, and brown garnets. ~owever, wine-red and violet garnets appear to be d1st1nct groups. It is believed that these garnets can be subdivided accurately, on the basis of colour variations, into discrete populations that may locally be effective in identifying different diatreme sources from within a cluster. The ratios of violet to deep wine-red garnets were determined for all anomalous sample sites. Garnets of both colours were identified according to Dawson and Stephens (1975) garnet classification scheme as G-9 type garnets. The colour differences appear to be unrelated to the element compositions tested for, even though there is no gradation between them. Significant variations in colour ratios were found to occur between sample sites within an anomalous area. This may?e indicative of multiple diatreme sources each producing a distinct ratio of garnet colours. 3. Areas of Abundant Indicator Mineral Occurrences At the time this report was prepared, only 1736 grain analysis of an estimated 2700 had been received. The majority of these (1115 grains) are from the coarse ( mm) size fraction. Initial results indicate three areas of anomalous counts of mineral grains with kim berlitic affinities. The indicator minerals are predominantly, high chrome pyrope garnets, calcic pyrope-almandine garnets, magnesian almandine garnets, and chrome diopsides. Compilation of anomalous sample sites from this investigation with those from Simpson (1991) and Gent (1992) (Figure 1) shows the presence. of a ~ig~ific~nt, larger contiguous area of abundant k1mberhte tnd1cator minerals in the extreme south-central part of the province. This area is very irregular in shape with a slightly east-west elongation. Three separate clusterings of anomalous concentrations of kimberlite indicator minerals were identified from samples taken within this contiguous area. One clustering of anomalous samples is located in southwest Saskatchewan, east of the town of Val Marie, and centered in 3-10-W3. All six samples collected in this area contained indicator minerals. Garnets were found to be significantly more abundant in the coarse than in the fine fraction with up to 30 fine and 150 coarse grains being recovered from individual samples. This relationship suggests a local source. The second anomalous clustering is centered in W2 and is located just north of the town of Lisieux. Sampling of 11 sites in this area was undertaken to delimit an anomalous mineral train reported by Gent {1992). Of these samples, 10 have anomalous counts of kimberlitic indicator minerals. The large size of this mineral train and the types of material sampled suggests that a cluster of pipes may be present. The third clustering, located in the East Poplar area and centered in 2-26-W2, was also selected to follow up an anomalous sample reported by Gent (1992). A total of 7 samples were collected to better define the site and delineate any associated mineral train. Of these samples, 4 contained mineral grains of kimberlitic affinity. The clustering in this area appears to be of more limited areal extent. However, this may be due to the limited availability of sample material and suitable sample sites. The number of indicator minerals recovered is substantially lower in this area than in the above mentioned clusters. Whether this is a function of distance from source and dilution during transport, or of poor quality sample material, or a reflection of a finergrained source with a paucity of indicator minerals is unknown at this time. On a larger scale, a strong trend can be seen between the three anomalous clusters. The Val Marie area cluster has a high proportion of violet compared to red garnets (approximately 10 to 1 ). In the Llsieux area cluster, violet garnets also dominate but to a lesser degree _(approximately 2 to 1). In the East Poplar area wine-red garnets outnumber the violet garnets (approximately 2 to 1 ). Variations between the anomalous areas may be interpreted as representing separate clusters of diatremes and reflecting the paragenesis of the mantle sampled by the kimberlite melt. 4. Other Indicator Mineral Occurrences Orange, pink, and red-brown G-3 and G-5 type garnets that are of eclogitic paragenesis, as well as violet and wine-red G-9 type, high chrome pyrope garnets of peridotitic origin, are abundant in the anomalous areas. A total of six G-10 type garnets have been identified from five sample locations. The East Poplar area is the only one of the above mentioned clusters to produce a G-10 pyrope. Two G-10 grains were identified from a site northeast of Moose Jaw, in W2. One grain 200 Summary of Investigations 1992

3 56 :i pa IOO 00 -rll7 00 -t06 all.--l - U:,j 00 -L - 1'34 00 IID 00 -U!2 00 CJj 00 N 56 ~ La Ron11-55 ~ ' 5< 00 Hudson Bay t -'I- Saskatoon - a 0 Y or~ton a Figure 1 Compilation map for southern Saskatchewan showing the location and abundance of kimberlitic garnets recovered from samples tested for kimberlite indicator minerals. Results include those of Simpson (1991), Gent (1992), and this study. Open circle = O count; open square= 1 to 10 count; filled triangle = 10 to 50 count; filled diamond= > 50 count; cross= site with no analysis; dashed line =area of anomalous sample locations. Saskatchewan Geological Survey 201

4 was recovered from a site located in W3 that lies approximately 10 km north of the Maple Creek Structure, reported by Gent (1992). Two other unrelated sites one located in 20-7-W2 lying southwest of Melville, and ' the other near the town of Hodgeville in 14-8-W3 each produced a G 10 type garnet. These sub-calcic high chrome pyrope garnets are also of peridotitic paragenesis and are significant due to their close association with diamondiferous kimberlites in South Africa (Gurney, 1985, 1989). All ~hree areas with ano~~lous garnet counts yielded grains that have compositions corresponding to Dawson and Stephens (1977) classifications of pyroxenes from kimberlites and their xenoliths. The recovered grains c_onsist entirely of clinopyroxenes, the most common being one of the 'classic kimberlitic indicator minerals', the very bright emerald green, group 5, chrome diopside. Its relative abundance, as compared to other pyroxenes, is most likely related to its distinctive appearance and hence preferential picking. Other clinopyroxenes identified were; group-1 subcalcic diopside grains; group-2 diopside grains; group-3 Ti-Cr diopsid~ grains; group-4 low-cr diopside grains; and one gra,n of group-9 omphacite. Bedrock samples of the Cypress Hills Formation, Frenchman Formation, and Whitemud Formation and the Whitemud-Eastend Formation transition 2one 0 were collected from exposed sections near the town of Eas!end._!he Cypress H!lls Formation yielded two garnets, 1dent1f1ed as magnesran almandine G-5 type garnets (of possible kimberlitic affinity) and two pyroxene grains that tend toward group 5 chrome-diopside, but do not fit any of Dawson and Stephens (1977) classifications. 5. Corundum Occurrences A total of 7 grains from separate locations were identified as corundum. Each of the areas with anomalous kimberli~ic minerals yielded at least one grain, with the Val Mane area producing two grains from separate sites. Single grains of corundum were also identified from three locations; W3, W3, and from a sample of the Cypress Hills Formation taken near the town of Eastend in 6-22-W3. T~e locations and?ounts of these grains were compiled with corundum grains reported by Simpson (1991) and Gent (1992) to determine any trends or patterns. There appear to be sets of parallel individual elongated northwest-southeast-trending groupings (Figure 2). The most well defined cluster has a total of 13 grains from six sample sites and extends from W2 to 4-20-W2 lying directly on the Missouri Coteau (see A). Sample ' MS-82 reported by Simpson (1991). located to the north e~st of this grouping, in 9-22-W2, yielded 8 grains, the highest count from any individual sample site. The next most distinct grouping extends through the Lisieux area from 6-3-W3 to 1-29-W2 and contains 10 grains from 7 sites_ (see B). A third, more poorly defined cluster, extending from W3 to 9-12-W3, contains 9 corundum grains from 5 sample sites (see C). The Val Marie area, containing anomalous quantities of kimberlitic minerals, also produced a total of 4 corundum grains ~~r I 51 ~ -1 J t..,.. I., Regina -l -! l ~ I \ l I Ill! \ l -- ~E_s_te_v_a_n - -,- -,1 ' eil Figure 2 Cofr!pilation map for soutb 7m Saskatchewan showing areas of corundum occurrences, sample focat(ons, and number of corundum grams re_covered. Results,?elude those of Simpson (1991), Gent (1992), and this study. Cross =sample location; number= corundum gram count; dashed!me = areas of corundum populations. 202 Summary of Investigations 1992

5 from 4 sample sites that extend from 5-11-W3 to 2-1 O W3 (see 0). There appears to be some overlap between areas with high kimberlitic mineral counts and areas with corundum grains. It is unknown at this time whether there is any correlation between the two populations. However, it should be noted that both the ruby and sapphire varieties of corundum derived from xenoliths of corundum eclogite or crys1alline crustal rocks have been reported from several kimberlites (Dawson, 1980). The colour of corundum is dependant on trace quantities of particular oxides. Ti02 and FeO produce the blue sapphire, while Cr203 produces the red ruby variety. Most of the corundum grains recovered in this study, and those of Simpson (1991) and Gent (1992), are similar in composition, having slightly elevated amounts (generally < 1.0 percent) of Cr203 and FeO, but Ti02 was rarely detected or was present in very small amounts. Colours are predominantly lilac to purple and occasionally orange, blue, red, pink, black, and colourless. One well preserved hexagonal cornflower blue grain between 0.5 and 0.71 mm diameter that appeared to be of gem quality was observed during this study. The source for these grains is open to speculation. It is expected that some of the corundum grains recovered could have come from Shield rocks, but the distribution of recovered corundum also suggests that there may be several local sources. There is also the remote possibility that these corundum grains have been transported by sedimentary processes from the corundum-bearing regions of Montana. The Montana occurrences, particularly the Yoga dyke deposits, have been well documented. They produce primarily cornflower blue sapphires, while red or lilac varieties are rare. The stones are mostly tabular or plate-like with no prism development (Zeihen, 1987 and Dahy, 1991). Other occurrences of sapphires in Montana (placer) have predominantly irregular grains (Hearne, pers. comm. Gent, 1992). It seems unlikely that the high proportion of lilac and purple stones, or grains with well developed and preserved hexagonal prisms found in southern Saskatchewan, could have been selectively transported from Montana sources. 6. Discussion and Conclusions Results of the heavy mineral sampling of glacial overburden show that areas with anomalous quantities of minerals of kimberlitic affinity exist in south-central and south-western Saskatchewan. Two theories are considered for the sources of these grains: i} long distance sedimentary transport from the Central Montana Alkalic Province in late Tertiary time and subsequent reworking into the glacial overburden, and ii) local kimberlitic diatreme sources that have been emplaced in clusters, followed by secondary transport during glacial events. The latter seems more reasonable, as the variability in garnet colours, the large grain size of indicator minerals encountered, the surface morphology of the grains, and the presence of chrome-diopside grains (that do not survive long distance transport) suggest multiple local sources. Although Simpson (1991) reported a possible association with the Wood Mountain Formation {Miocene), anomalous sample sites do not appear to correlate spatially with any particular bedrock formation. Further study of Wood Mountain exposures, to determine the extent of any glacial reworking, is recommended. 7. References Dahy, J.P. (1991): Geology and igneous rocks of the Yogo sapphire deposit, Little Belt Mountains, Montana; in Baker, D.W. and Berg, RB. (tech. eds.), Guidebook of the Central Montana Alkalic Province Geology, Ore Deposits and Origin, Montana College Mineral Sc. Tech., Spec. Publ. 100, p Dawson, J.B. (1980): Kimberlites and Their Xenollths; Springer Verlag, 252p. Dawson, J.B. and Stephens, W.E. (1975): Statistical classification of garnets from kimberlites and associated xenoliths; J. Geol., v83, p ~~~ (1977): Statistical classification of garnets from kimberlites and associated xenoliths-addendum; J. Geol., vb4, p Deer, W.A., Howie, A.A., and Zussman, J. (1966): An lntroduc. lion to the Rock Forming Minerals; Longman, 528p. Gent, M.A. (1989): Regional Phanerozoic anomalies of Saskatchewan; in Summary of Investigations 1989, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 89-4, p ,..--~ (1991): Review of Industrial Minerals Investigations ; in Summary of Investigations 1991, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 91-4, p ~-- (1992): Diamonds and Precious gems of the Phanerozoic basin, Saskatchewan: Preliminary Investigations; Sask. Energy Mines, Open File Rep. 92-2, 67p. Gurney, J.J. (1985): A correlation between garnets and diamonds in kimberlites; in Glover, J.E. and Harris, P.G. (eds.), Kimberlite Occurrence and Origin: A Basis for Conceptual Models in Exploration, Dep. Geol. and Univ. Exten., Univ. W. Australia Publ. No. 8, p (1989): Diamonds; in Ross, J., Jaques, A.L., Ferguson, J., Green, D.H., O'Reilly, S.Y., Oanchin, R.V., and Janse, A.J.A. (eds.), Kimberlites and Related Rocks, Fourth International Kimberlite Conference Proceedings, Geel. Soc. Australia, SP 14, v2, p Simpson, M.A. (1991): Kimberlite indicator minerals in southwestern Saskatchewan; Sask. Resear. Counc., Rep. R E-91, 13p. Zeihen, L.G. (1987): The sapphire deposits of Montana; in Lawson. D.C. (compiler), Directory of Montana Mining Enterprises for 1986, Montana College Mineral Sci. Tech., Bull. 126, p Saskatchewan Geological Survey 203

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