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GSC Prairies Reconnaissance till indicator mineral grains – microprobe analysis

Type: lab data

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GSC 1991 Prairies Reconnaissance survey till indicator mineral grains.

3457 grains were picked as potential indicator minerals. About 70% of the grains are from the 0.25-2.0 mm fraction, the remainder were recovered from the 63-250 μm fraction of five samples in order to test the viability of using this fraction.  The grains were mounted in four 25 mm epoxy cylindrical mounts.  Coordinates of points to be analyzed were entered using an optical microscope.  One point per grain was analyzed, with care to avoid inclusions or fractures.  A second point was analyzed for one grain in every 20, in order to assess analytical variability, considered primarily a function of grain heterogeneity.  Occasional apparent gross errors between duplicates are attributed to inclusions.  Tests indicated that the somewhat unusual operating conditions used for these analyses (200 nA, 20 kv) was acceptable.  Background counts were included in the routine for each element, twice for sodium.  Detection limits were about 0.01 wt. % for SiO2, Al2O3, CaO, FeO, MgO, Cr2O3, TiO2, Na2O, and K2O, 0.02 wt. % for MnO, NiO, ZnO and CoO, 0.04 wt. % for Nb2O5, and 0.05 wt. % for V2O3.  Tentative mineral identifications are based on sorting of weight % oxides.

Electron microprobe analysis of background grains, using the same methods as described for picked potential indicator minerals, was also carried out.  A total of 1378 grains, in two 25 mm araldite cylindrical mounts, were selected on the basis of the following preliminary visual identifications: 160 pink garnets, 160 orange garnets, 160 clinopyroxenes, 80 hornblendes, 80 orthopyroxenes, 50 epidotes, 10 barites, 20 rutiles, 160 black opaques, 80 grey ilmenites, 80 magnetites, 40 hematites, 10 goethites, 10 tourmalines, 10 apatites and gahnites, 10 leucoxenes, 40 staurolites, 40 sphenes, 30 weakly paramagnetic garnets, 30 weakly paramagnetic clinopyroxenes, 28 weakly paramagnetic black opaques, 30 paramagnetic garnets, 30 paramagnetic clinopyroxenes, and 30 paramagnetic black opaques.

96 0.25-2.0 mm grains were classified as kimberlite indicator minerals.  Thirteen grains, 7 orange, 6 purple, and 1 red, were classified as peridotitic garnet (including megacrysts) on the basis of anomalous levels of both Mg and Cr.  Using a simplified classification procedure based on Dawson and Stephens (1975) and Fipke (1989), these grains classified as 1 G10, 1 G11, 6 G9, 4 G2 and 1 G1.  Seventeen grains, all orange, were classified as eclogitic garnet on the basis of anomalous levels of Mg, Ti and Ca.  These grains were classified as 2 G6 and 15 G3.  A total of 62 chrome diopsides, defined as pyroxenes with >7.5% CaO, <2.7% Na2O, <6.0% FeO, and >0.5% Cr2O3, are present.  Four occurrences of magnesian ilmenite exceed 6% MgO.  No analyzed chromites exceed Cr2O3 values typical of diamond inclusion chromites (e.g.>62.5%, Gurney and Moore, 1993).  Two chromites, however, from sites 3001 and 3010, exceed 60% Cr2O3.

[N.B. detection limits reported in the original spreadsheets are 0.01% for all oxides.  Values that lie between the detection limits listed above and 0.01% should be treated cautiously.]

[Description taken from GSC Open File 2685].

Sample Classification (Analytical)

IndexSample CountSurveySample TypePrep Lab MaterialControl RefUnknown SourceDuplicates count
915  Ultra-low density till and soil sampling survey, Manitoba, Saskatchewan and Alberta, 1991. (more) Till Grain Mount: 0.063 – 0.250 mm No  No  915 
4154  Ultra-low density till and soil sampling survey, Manitoba, Saskatchewan and Alberta, 1991. (more) Till Grain Mount: 0.25 – 2.00 mm (2) No  No  4154 

Analytical Package Description

Laboratory: GSC: Microprobe Laboratory, Geological Survey of Canada, Ottawa

Name: GSC electron microprobe (15 oxides) (more)

Geological Survey of Canada electron microprobe. Indicator minerals are probed for 15 major and minor element oxides.

Analytical Suites:

1  GSC electron microprobe (15 oxides)

Analytical Methods:

IndexColumn NameSuite OrderMethod OrderSuite RepeatQuantityUnitsDetermination Limit
1  Al2O3    Al2O3  percent 0.01  
2  CaO    CaO  percent 0.01  
3  CoO    CoO  percent 0.01  
4  Cr2O3    Cr2O3  percent 0.01  
5  FeO    FeO  percent 0.01  
6  K2O    K2O  percent 0.01  
7  MgO    MgO  percent 0.01  
8  MnO    MnO  percent 0.01  
9  Na2O    Na2O  percent 0.01  
10  Nb2O5  10    Nb2O5  percent 0.01  
11  NiO  11    NiO  percent 0.01  
12  SiO2  12    SiO2  percent 0.01  
13  TiO2  13    TiO2  percent 0.01  
14  V2O3  14    V2O3  percent 0.01  
15  ZnO  15    ZnO  percent 0.01  
16  Total  16    Total Oxides  percent  
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