Analytical Sample Bundle Metadata
GSC/TSD -1995-Contwoyto Lake 76E (south) Till: Picked Grain Probe Checks
Type: Interpreted data
Download analytical data for this bundle in one of three formats, depending on the desired convention for treating values below the lower detection limit:
Appendix 3 of GSC Open File 3386 lists the HMC indicator mineral grains that were picked by I and M Morrison and summarises how many were picked and microprobed. Appendix 5 lists the microprobe analyses and includes a column assigning each grain to a mineral species. The link between the ID of a picked grain and the corresponding microprobe analysis is not reported, making it impossible to positively assign an analysis to a particular picked grain. But by maximizing the correspondence between the provisional picked grain identities and the confirmed identities, we can make a reasonable guess at how many picked grains were subsequently confirmed by microprobe analysis.
A total of 132 grains were probed. Detailed assignments are:
- Purple garnet – 6 probed: 5 pyrope, 1 almandine
- Orange garnet – 26 probed: 25 almandine, 1 staurolite
- Uncertain garnet – 14 probed: 3 pyrope, 10 almandine, 1 staurolite
- Chrome diopside – 29 probed: 23 low-chrome diopside, 5 chrome diopside, 1 zoisite
- Ilmenite/chromite – 49 probed: 34 ilmenite, 1 spessartine, 1 staurolite, 2 apatite, 3 rutile, 6 leucoxene, 1 tourmaline, 1 plagioclase; 10 of the misidentifications came from a single sample: 94BCW2779
- Olivine – 2 probed: 2 grossular
- Corundum – 1 probed: 1 grossular
- Gahnite – 5 probed: 4 apatite, 1 spinel
Full details of the analytical procedures (HMC preparation, picking procedures, microprobe analysis) are presented elsewhere in other analytical bundles/packages/suites.
Sample Classification (Analytical)
Analytical Package Description
Laboratory: No laboratory was required
Name: Reconciliation of picked, probed and confirmed indicator mineral grain counts (Morrison/GSC, 1995) (more)
Kimberlite indicator mineral grain counts reported by I and M Morrison are presented alongside the number of grains analysed by microprobe, and the mineral identifications assigned based on the microprobe analysis.
Indicator mineral identification typically involves an initial picking phase, using a binocular microscope, followed by electron microprobe analysis to confirm the identification and gain additional data on the mineral chemistry. The picked grains are often referred to as “potential” indicator minerals, prior to confirmation by microprobe. In order to evaluate the data quality, it is useful to know how many of the picked grains were analysed by microprobe, and how many were confirmed to correspond to what was picked.
In general, reports do not explicitly include this information, but it may be possible to deduce it, subject to the following caveats:
- The list of grains that were searched for by the picking laboratory should be known. Grains may be searched for, but not found, and hence not reported. This is commonly the case for diamond – always looked for, but very rarely found. Ideally, detailed criteria for the searching should be known. For example, when searching for garnets, what are the criteria for discriminating based on colour (e.g. did the lab distinguish between purple and red garnets?). In a typical kimberlite indicator mineral picking, only six or seven different kinds of grains will be picked (peridotitic (purple) garnet, eclogitic (orange) garnet, chrome diopside (green), ilmenite, chromite, forsterite, diamond). Other kinds of indicator mineral picking may yield a much greater variety of picked grains.
- The picked grains are usually mounted in epoxy on 25 mm diameter lucite disks, with several hundred grains on a single disk. A “map” is created by the mounting laboratory, referencing the original grain ID with its position on the mount. The map is needed to link the picked grain to the probe analysis. In practice, these maps are very rarely published, making it impossible to unequivocally associate a specific picked grain with a probe analysis.
- The microprobe analysis returns a “complete” analysis of major oxides in the grain, which should sum to almost 100% for most minerals. It is then possible to make a good guess about the identity of the mineral. It is sometimes difficult to make a positive identification because (a) many minerals form complex solid solutions, and (b) some components (e.g. carbonate, phosphate) cannot be analysed by electron microprobe.
- Classification of mineral identities may vary according to the requirements of the study. For example, what are the chromium concentrations that separate low-chrome-diopside from chrome diopside from high-chrome diopside? Should low-chrome diopside be considered a KIM?
Peculiarities of this package include:
- Ilmenite and chromite were picked as a single entity, not two distinct entities.
- Garnets were picked that were not clearly eclogitic or peridotitic
- Gahnite and corundum were both picked, but neither of these minerals is indicative of kimberlite.
|1||Reconciliation of picked, probed and confirmed indicator mineral grain counts (Morrison, 1995). ( Picked grains )|
|2||Reconciliation of picked, probed and confirmed indicator mineral grain counts (Morrison, 1995). ( Probed grains )|
|3||Reconciliation of picked, probed and confirmed indicator mineral grain counts (Morrison, 1995). ( Confirmed grains )|
|Index||Column Name||Suite Order||Method Order||Suite Repeat||Quantity||Units||Determination Limit|
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