Analytical Sample Bundle Metadata

1995

GSC/TSD -1995-Contwoyto Lake 76E (south) Till: Picked Grain Probe Checks

Type: Interpreted data



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Download analytical data for this bundle in one of three formats, depending on the desired convention for treating values below the lower detection limit:

Bundle Description

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)

IndexSample CountSurveySample TypePrep Lab MaterialControl RefUnknown SourceDuplicates count
27  Till sampling survey, Contwoyto Lake area, NTS 76E, Northwest Territories and Nunavut, 1994. (more) Till HMC separation (Canamera/DIP) No  No 

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.

 

Analytical Suites:

IndexName
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 )

Analytical Methods:

IndexColumn NameSuite OrderMethod OrderSuite RepeatQuantityUnitsDetermination Limit
1  GP_1    KIM GP  count  
2  G__1    Grt  count  
3  GO_1    KIM GO  count  
4  IC_1    KIM IM/CR  count  
5  DC_1    KIM DC  count  
6  CD_1    Crn  count  
7  GH_1    Ghn  count  
8  OL_1    Ol  count  
9  GP_2    KIM GP  count  
10  G__2    Grt  count  
11  GO_2    KIM GO  count  
12  IC_2    KIM IM/CR  count  
13  DC_2    KIM DC  count  
14  CD_2    Crn  count  
15  GH_2    Ghn  count  
16  OL_2    Ol  count  
17  GP_3    KIM GP  count  
18  G__3    Grt  count  
19  GO_3    KIM GO  count  
20  IC_3    KIM IM/CR  count  
21  DC_3    KIM DC  count  
22  CD_3    Crn  count  
23  GH_3    Ghn  count  
24  OL_3    Ol  count  
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