Cr-diopside (Clinopyroxene) as a Kimberlite Indicator Mineral for Diamond Exploration in Glaciated Terrains
David Quirt 1
Quirt, D.H. (2004): Cr-diopside (clinopyroxene) as a kimberlite indicator mineral for diamond exploration in glaciated terrains; in Summary of Investigations 2004, Volume 2, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004- 4.2, CD-ROM, Paper A-10, 14p.
Abstract For diamond exploration in glaciated terrains, Kimberlite Indicator Minerals (KIMs) are those minerals, contained in glacial deposits, that are characteristic of kimberlite, the dominant host for diamonds. As KIMs are many times more numerous in the kimberlite host rock than are diamonds, they are an important pathfinder for kimberlites. Typical KIMs include garnet, pyroxene, Cr-spinel, Mg-ilmenite, and olivine. Interpretation of the chemistry of KIM grains, such as clinopyroxene, allows evaluation of the kimberlitic affinities of the grains and delineation of possible glacial dispersion trains suggestive of potentially diamondiferous kimberlite sources. Pyroxenes from both peridotitic and eclogitic mantle sources can be KIMs, with clinopyroxene (cpx) being a common groundmass mineral in kimberlite.
There have been two historical interpretative problems, however, with the use of cpx as a KIM. Firstly, the colour criteria used in picking KIM cpx (e.g., Cr-diopside) appear to vary widely between individual microscopists; and secondly, problems can arise from the presence of non-kimberlitic (alkalic gabbro, basalt, komatiite, syenite, carbonatite, ultrapotassic volcanic) Cr-diopsides in the glacial deposit sampled.
Given the wide range of cpx host environments, determination of suitable cpx compositions for use in KIM evaluation and pyroxene thermobarometry is important. The chemical data should be constrained to compositions similar to those determined for kimberlitic cpx and diamond inclusion (DI) cpx worldwide. Kimberlitic cpx from peridotitic sources are typically diopsidic or, to a lesser extent, calcic/non-ferrous augitic on a Wo-En-Fs diagram; as are many cpx grains of crustal origin, most eclogitic (low-Cr) DI cpx, and many ‘eclogitic’ non-DI cpx. Consequently, the data must be screened for peridotitic cpx, permitting evaluation of only those grains having the selected criteria: diopsidic to calcic/non-ferrous augitic compositions (27.5%
1. Introduction Nearly all diamond mines recover diamonds from an ultramafic volcanic rock called kimberlite, typically containing many fragments of mantle peridotite and eclogite. The diamonds, which are xenocrysts, crystallized in the upper mantle and were brought to the surface by very deep-seated volcanic eruptions that formed the kimberlites. The peridotitic rock fragments, or xenoliths, are dominantly composed of garnet, olivine, and ortho- and/or clinopyroxene; eclogitic xenoliths consist of orange (Fe, Ti, Mg, Ca) almandine garnet, green clinopyroxene, and hornblende. Over the past 15 years, kimberlite pipes have been discovered in the Precambrian Shield of west-central
1 Saskatchewan Research Council, 15 Innovation Boulevard, Saskatoon, SK S7N 2X8; E-mail: [email protected].
Saskatchewan Geological Survey 1 Summary of Investigations 2004, Volume 2 and northern Canada – thus northern Saskatchewan and Alberta, Quebec, Ontario, Nunavut, and the Northwest Territories (NWT) have become prime diamond exploration areas. In glaciated areas, exploration for diamond-bearing kimberlite is undertaken mainly by two methods: airborne geophysics and drift prospecting. Drift prospecting involves sampling glacial deposits to identify economically significant components and trace them up ice to their bedrock source (DiLabio and Coker, 1989). It is routinely used in the Precambrian Shield areas of Canada because the dominant surficial material is glacial till and many ore deposits have been found using this technique, including those containing diamonds, gold, copper, zinc, uranium, and rare earth elements. An important component of a drift prospecting survey is extraction and identification of indicator minerals. An indicator mineral is a mineral characteristic or representative of a given host rock or mineral deposit. Examples of indicator minerals used in mineral exploration (cf. Averill, 2001) in Saskatchewan include clay minerals (uranium), gold grains (gold), and heavy minerals (base metals, gold, kimberlite/diamond). The heavy mineral (HM) component of sediment consists of all clastic grains with specific gravities greater than about 2.9. Many indicator minerals are part of the HM fraction, and as such this fraction is examined to determine the distribution and dispersion of HMs related to various types of mineralization, such as gold, base metals (volcanogenic massive sulphides, skarn, and magmatic Ni-Cu), and diamond. Kimberlite Indicator Minerals (KIMs), sometimes called Diamond Indicator Minerals (DIMs), are important in diamond exploration as diamonds are present in only trace amounts in only some kimberlites. Kimberlites contain other minerals, however, that are both specifically characteristic of kimberlite and much more abundant than diamond. These KIMs, which include garnet, Mg-ilmenite, chromite, clinopyroxene (cpx) (Cr-diopside), and olivine (Figure 1), are used to identify the presence of kimberlite and are thus useful in diamond exploration and diamond potential evaluations.
In glaciated terrain, kimberlite exposed at surface has commonly been eroded and indicator minerals released. In a KIM drift prospecting survey, the sample material commonly obtained for analysis is the lowermost, or basal, till unit overlying bedrock. If KIMs are found in this material, their bedrock source usually lies in the up-ice direction. This exploration method was the primary tool used in the discovery of the Lac de Gras (NWT) kimberlites which are currently being exploited by Canada’s two operating diamond mines, Ekati and Diavik.
2. Kimberlite Indicator Minerals Heavy minerals, including KIMs, are separated from bulk till samples in support of kimberlite drift prospecting programs. Typically 10 to 25 kg till samples are processed for heavy mineral separation, KIM identification and extraction (picking), and diamond extraction and description. The sample size is reduced through non-destructive disaggregation, washing, and screening to obtain the <2 mm size-fraction (-10 mesh), which is further reduced by use of a shaker table to produce a rough heavy mineral concentrate. The concentrate then undergoes a heavy media separation procedure during which the HMs, including the KIMs, pass through a dense media. This separation procedure uses heavy liquids (MI: methylene iodide; TBE: tetrabromoethane) to separate the higher-density minerals of interest. Alternatively, a Magstream separator can be used to eliminate the need for toxic/dangerous heavy liquids. The KIMs and/or KIM intergrowths are variably magnetic, so the sample is further concentrated through the use of magnetic separation to separate the paramagnetic fractions from the non-magnetic fraction. The final HM concentrates are sieved to produce four weighed size-fractions (<0.25 mm, 0.25 to 0.5 mm, 0.5 to 1 mm, and 1 to 2 mm) within each magnetic fraction obtained. KIMs are identified, hand-picked (extracted), and sorted from selected HM concentrate size-fractions using optical binocular microscopy by highly trained mineral sorters. Quality control procedures commonly include spiking of samples with marker grains and/or the re-examination of some samples, depending on the background mineralogy and difficulty. Figure 1 - Kimberlite Indicator Minerals. From top left- clockwise: picroilmenite (Mg-rich ilmenite); eclogitic Fe- Following grain picking, further analysis includes Mg-Ca almandine G3 garnets; peridotitic chrome pyrope spreadsheet counting statistics and/or mineral chemical G9/G10 garnets; chromites; chrome diopsides; Ti-Cr-Mg pyrope G1/G2 garnets; and olivines in the centre. analysis. Selected grains are prepared for electron
Saskatchewan Geological Survey 2 Summary of Investigations 2004, Volume 2 microprobe major-element chemical analysis. Interpretation of the KIM grain mineral chemistry is completed to evaluate kimberlitic affinities and to document and describe possible dispersion trains suggestive of potentially diamondiferous kimberlite sources. Use of the mineral chemical data allows KIM grain classification and end- member grouping of the grains (e.g., garnet and pyroxene; Dawson and Stephens, 1975, 1976 and Stephens and Dawson, 1977, respectively) in aid of petrogenetic and metallogenetic interpretations. Illustrative plots of these data allow comparisons of KIM grain chemistry to be made with established fields representing mantle lithologies and/or diamond inclusion chemistry. Comparisons of till geochemical data are commonly made with the mantle and/or diamond inclusion indicator mineral results. a) Pyroxenes Pyroxenes from both peridotitic and eclogitic mantle sources can be KIMs and cpx is a common groundmass mineral in kimberlite, however, there are two problems with the use of cpx as a KIM. Firstly, the colour criteria used for picking KIM cpx (e.g., Cr-diopside), which is linked to chromium content, appear to vary between individual microscopists, creating a sample processing problem. Secondly, non-kimberlitic Cr-diopsides in the sediment sample can create a data interpretation problem. This latter problem becomes important when using Cr- bearing cpx as a KIM because it also occurs in many other crustal/mantle lithologies besides kimberlite, such as alkalic gabbro, basalt, komatiite, syenite, carbonatite, and ultrapotassic volcanics. In addition, Cr-bearing cpx occurs in several mantle lithologies formed under various P-T-X conditions, resulting in mineral chemical data overlaps on traditional bivariate discriminant plots. The pyroxenes are a group of inosilicate (chain-structure) minerals whose basic chemical unit is the single-chained silica tetrahedron (SiO4). They form a chemical series of monoclinic (clinopyroxene) and orthorhombic (orthopyroxene) minerals with the general formula of (M2)(M1)T2O6, where the T-site is a tetrahedral site occupied primarily by silicon (Si4+) and aluminum (Al3+) (Table 1). The M1-site is a six-fold octahedral site that typically contains Mg, Fe2+, and/or Al, but can also can contain Ti, Mn, and Cr. The coordination number of the M2-site varies from six-fold (orthopyroxene) to eight-fold (clinopyroxene) and the coordination geometry of this site varies with its coordination number. The site is octahedral in orthopyroxene and is roughly cubic in clinopyroxene. The M2-site occupancy also varies with the coordination number with orthopyroxene having the site occupied primarily by Mg and Fe2+ with minor Ca and with clinopyroxene having the site occupied primarily by Ca with lesser Mg and 2+ Fe . These site occupancies result in the general pyroxene chemical formula of (Ca,Mg,Fe)Si2O6. There is a very wide range of chemical substitutions for Ca, Mg, and Fe, including Al, Na, Cr, Ti, and Mn, as many cations have similar sizes and valencies, with the substitutions being limited by the principles of coordination and charge balance (Table 1).
The compositions of most pyroxenes can be represented in the Ca-Mg-Fe ternary system plot of wollastonite- (CaSiO3)-enstatite (MgSiO3)-clinoferrosilite (FeSiO3), the Wo-En-Fs end-member diagram (Figure 2). Most natural pyroxene compositions fall into the lower half of this triangular system, within the ‘pyroxene quadrilateral’, and are expressed in terms of mole percentages of the Wo-En-Fs end-member components. b) Clinopyroxene (Cr-Diopside) In general, kimberlites contain abundant clinopyroxene, and kimberlite is one of the few rock types to commonly contain very Cr-rich diopsidic cpx. So the primary picking criteria for cpx KIMs is based on the pale green to apple green to emerald green colours of Cr-diopside. These colour shades are reflective of the Cr content.
Table 1 - Pyroxene end-members with site occupancies for the general formula (M2)(M1)T2O6 . Cation Site M2 M1 T2 O6 Name Crystal System
Mg Mg Si2 O6 enstatite orthorhombic 2+ 2+ Fe Fe Si2 O6 ferrosilite orthorhombic
Ca Mg Si2 O6 diopside monoclinic 2+ Ca Fe Si2 O6 hedenbergite monoclinic Ca Mn Si2 O6 johannsenite monoclinic Na Al Si2 O6 jadeite monoclinic 3+ Na Fe Si2 O6 aegirine monoclinic Na Cr Si2 O6 kosmochlor monoclinic Ca Al Al, Si O6 Ca-Tschermaks monoclinic Mg Al Al, Si O6 Mg-Tschermaks monoclinic
Saskatchewan Geological Survey 3 Summary of Investigations 2004, Volume 2 Wo 3. Clinopyroxene Mineral Chemistry and Data Evaluation [Ca2Si2O6] There have been several versions of cpx KIM data analysis that have been designed to deal with the data interpretation problem caused by the possible presence of non-kimberlitic Cr-diopsides in the KIM grain suite (e.g., Morris et al., 2002; Crabtree, 2003; Quirt and Maki-Scott, 2003). All have made use of the characteristic that the entries of Fe, Al, Na, Ca, and Cr pyroxene quadrilateral into the cpx structure are strongly affected by the P-T- 50% diopside hedenbergite X conditions present during crystallization. For example, the jadeite (Na-Al) and kosmochlor (Na-Cr)
augite end-member compositions tend to be favoured by high P and are typical of mantle-derived cpx. Similarly, the proportions of Fe-bearing pyroxene end-member cpx, such as hedenbergite (Fe-Ca) and aegirine (Fe-Na), pigeonite tend to be favoured by lower P, or ‘crustal’, conditions. 5% (clino)enstatite (clino)ferrosilite The 10 cpx discriminant classes (C1 to C10; Table 2) obtained by Stephens and Dawson (1977) are based on En Fs these trends. [Mg Si O ] [Fe Si O ] 2 2 6 2 2 6 The determination of suitable cpx compositions for use Figure 2 - Wo-En-Fs end-member diagram for the Ca-Mg- in KIM data evaluations is very important given the Fe clinopyroxenes. Note that orthopyroxenes contain less wide range of potential cpx host environments. The than 5 mol% Wo. focus of this work was to constrain the examined data to compositions similar to those determined for kimberlitic cpx and diamond inclusion (DI) cpx worldwide. As seen in Figure 3A, kimberlitic cpx from peridotitic sources plot mostly in the diopside field on a Wo-En-Fs diagram, and to a lesser extent into the calcic and low-iron part of the augite field, as do most of the eclogitic (low-Cr) DI cpx and a lesser proportion of the ‘eclogitic’ non-DI cpx. Many cpx grains of crustal origin, however, also have similar compositions (Figure 3B).
In KIM data evaluations, the cpx mineral chemical data are thus first screened to include those grains plotting in the ‘KIM field’ centred on the Wo-En-Fs diopside field. The cut-off values used are: (27.5%
Table 2 - Stephens and Dawson (1977) discriminant function cpx classes. Mean Oxide Value (wt%) Class Name Cr2O3 Al2O3 TiO2 FeO MgO CaO Na2O C1 subcalcic diopside 0.43 2.51 0.31 5.17 20.71 13.80 1.58 C2 diopside 0.71 2.69 0.26 4.16 16.94 18.44 1.78 C3 Ti-Cr diopside 1.02 3.86 0.80 2.61 15.99 19.51 1.94 C4 low-Cr diopside 0.09 3.19 0.50 5.86 16.88 17.55 1.85 C5 Cr diopside 1.45 2.50 0.09 2.02 16.80 20.66 1.68 C6 ureyitic diopside 2.99 3.14 0.27 2.37 15.19 17.94 3.11 C7 high-ureyitic diopside 11.80 3.14 0.19 1.68 19.27 10.60 7.07 C8 jadeitic diopside 0.10 7.61 0.44 6.10 11.54 14.52 4.50 C9 omphacite 0.15 11.34 0.27 3.29 10.35 14.62 5.09 C10 diopsidic jadeite 0.02 16.87 0.22 2.42 6.36 10.19 7.64
Saskatchewan Geological Survey 4 Summary of Investigations 2004, Volume 2
Wo
[Ca2Si2O6] GSC OF-2124 peridotitic DI and non-DI cpx "eclogitic" non-DI cpx eclogitic DI-cpx
"eclogitic" non-DI cpx Wo KIM field [Ca Si O ] peridotitic DI diopside hedenbergite 2 2 6 and non-DI cpx
augite
eclogitic DI cpx
pigeonite 'non-kimberlitic' indicator grains clinoenstatite clinoferrosilite from till
En Fs diopside hedenbergite [M g Si O ] [Fe Si O ] KIM field 2 2 6 2 2 6 KIM grains augite (A) from kimberlite
pigeonite
clinoenstatite clinoferrosilite
En Fs [Mg Si O ] [Fe Si O ] 2 2 6 2 2 6 (B)
Figure 3 - (A) Wo-En-Fs composition ranges for KIM Ca-Mg-Fe clinopyroxenes (data and grain groupings from Fipke et al., 1989); (B) Wo-En-Fs composition ranges for Ca-Mg-Fe clinopyroxenes from a kimberlite in Alberta and from eastern Ontario glacial till derived from non-kimberlitic Canadian Shield lithologies (Quirt and Maki-Scott, 2003).
5 10
4 8 Cr-diopside field for kimberlite xenoliths and xenocrysts (Morris et al., 2002) 3 6 (wt%) 3 O (wt%) O 2 2 Diamond Inclusion 4 2 Na Cr field
1 2
0 0 (A) 10 12 14 16 18 20 22 24 26 0 0.2 0.4 0.6 0.8 1 CaO (wt%) (B) Ca/(Ca+Mg) (cation proportion)
Figure 4 - Clinopyroxene mineral chemistry binary plots. (A) CaO versus Cr2O3 (field from Fipke et al., 1989) and (B) Na2O versus Ca/(Ca+Mg).
Saskatchewan Geological Survey 5 Summary of Investigations 2004, Volume 2 To demonstrate the utility of diopsidic cpx as kimberlite discriminators (i.e., as KIMs), ternary diagrams of cpx atomic cation proportions were constructed. Fe-Cr-Na and Fe-Cr-Ca plots (Figure 5) show the control on the composition of the mantle-derived kimberlite cpx by the jadeite-kosmochlor (Na-Cr-Al) end-members relative to more Fe-rich crustal grains present in till derived from the Canadian Shield. These series of plots show that there are antipathic Fe-Cr and Fe-Na trends that can be used to discriminate between ‘deeper mantle-derived’ grains and ‘more crustal’ grains. The Al-Cr-Na plot (Figure 6) also shows the jadeite-kosmochlor cation substitution trends related to P-T conditions of formation (Morris et al., 2002), but it does not show as great a separation of kimberlite cpx from ‘crustal’ cpx as observed on the Fe-Ca-Na, and Al-Ca-Na plots. For example, Figure 6 shows that while the ‘85% field for kimberlite xenoliths and xenocrysts’ of Morris et al. (2002) does represent most grains present in the Alberta kimberlite, it only poorly differentiates these kimberlitic grains from non-kimberlite grains obtained from glacial till (Quirt and Maki-Scott, 2003). Four-dimensional plots (tetrahedra) better display the above compositional trends, for example, Cr-Na-Ca-Fe and Cr-Na-Ca-Al. These data can be represented in a trigonal-bipyramidal data space with a Cr-Na-Ca ternary girdle and Fe and Al apices (Figure 7). This pair of tetrahedral data spaces is best illustrated using Piper-style ternary plot layouts.
Cr
5Cr
grains from kimberlite
'non-kimberlitic' indicator grains from till
strong crustal signature Fe2+ (A) Na grains from kimberlite
'non-kimberlitic' indicator grains from till
strong crustal signature 2+ 2Fe (B) Ca/2
Figure 5 - Clinopyroxene ternary cation plots with empirical separator lines between kimberlitic cpx and ‘crustal’ cpx. Cpx grain sources as for Figure 3B. (A) Fe2+-Cr-Na cation plot and (B) Fe2+-Cr-Ca cation plot.
Saskatchewan Geological Survey 6 Summary of Investigations 2004, Volume 2 Cr Fe
Cr-diopside field for kimberlite xenoliths and xenocrysts (Morris et al., 2002)
grains from Na kimberlite Cr Ca 'non-kimberlitic' indicator grains from till
strong crustal signature Al Na Figure 6 - Clinopyroxene Al-Cr-Na ternary cation plot with Cr-diopside ‘85% field for kimberlite xenoliths and xenocrysts’ of Morris et al. (2002). Cpx grain sources as for Al Figure 3B. Figure 7 - Four-dimensional plots (tetrahedra) of the Cr-Na- Ca-Fe and Cr-Na-Ca-Al data space. 4. Case Studies a) Comparison of Kimberlite Cpx Data with Non-kimberlitic Cpx from Canadian Shield Glacial Till The mineral chemical data from two cpx grain suites – one from an Alberta kimberlite and one from glacial till overlying the Canadian Shield of eastern Ontario – were evaluated. All grains examined from these cpx grain suites are peridotitic in composition and nearly all passed the data screening tests. The kimberlite cpx are relatively high in Cr (>0.8% Cr2O3; average 1.70%), however, the cpx from the till tend to be very low in Cr (average 0.39% Cr2O3). The kimberlite cpx grains are classified as C5 cpx with lesser C6 and C2 cpx, and with no C4 cpx present. In contrast, the cpx from the till are strongly classified as C2 cpx with lesser C5 and C4 cpx, and with no C6 cpx present. These differences are also apparent in the respective mineral chemical data (Figures 8A and 8B). The
0.200 0.200 5 Cr-diop side field for kimberlite xeno liths and xenocrysts (Morris et al., 2002) kimberlite cpx 0.160 0.160 4
'non-kimberlitic' cpx )
+ 0.120 from till 0.120 3 % 3 l t A w
3+ ( grains from n total Al O o kimberlite i 2 t Aliv a a 'non-kimberlitic' c
0.080 0.080 N 2 indicator grains from till
0.040 0.040 1
0.000 0.000 0 (A) 0.000 0.100 0.200 0.300 0.400 0.500 (B) 0.3 0.4 0.5 0.6 0.7 cation Fe2+ Ca/(Ca+Mg) Figure 8 - Illustrative mineral chemistry binary plots for clinopyroxene. The dashed purple line is the empirical separator 3+ 2 between kimberlitic cpx and ‘crustal’ cpx. (A) cation Al versus cation Fe and (B) Na2O versus cation Ca/(Ca+Mg).
Saskatchewan Geological Survey 7 Summary of Investigations 2004, Volume 2 5 kimberlitic cpx are relatively Fe-poor (<0.12 Fe per cation Na = 0.25 formula unit) and are sodic (0.5 to 3.0% Na2O) compared to the cpx from the till (>0.10 Fe per formula unit; <1.5% Na2O). The Ca/(Ca+Mg) ratio values from 4 kimberlite cpx (<~0.5) are significantly lower than those from the cpx from till (>~0.5). GSC OF-2124 2/2003 eclogitic cpx (non-DI) b) Kimberlite Cpx (GSC Open File OF-2124) 3 eclogitic DI cpx
3 peridotitic cpx (non-DI)
O peridotitic DI cpx This compilation of kimberlite indicator mineral 2 r compositions, assembled by the Geological Survey of C Canada (Fipke et al., 1989), separates the cpx suite into 2 grains from peridotitic (high-Cr: >0.50% Cr2O3) and eclogitic (low-Cr) parageneses, with DI and non-DI grains from both parageneses. However, the group of ‘eclogitic’ non-DI grains, while placed into the 1 eclogitic group, may contain peridotitic grains with Cr O = 0.50% relatively low amounts of Cr (Figure 9), as these grains 2 3 cluster at the low-Cr (<0.5% Cr2O3) and low-Na (<0.1 cation Na) intersection of the peridotitic and eclogitic 0 trends on this diagram. The peridotitic grains, both DI 0 0.2 0.4 0.6 0.8 and non-DI, are dominantly C5 cpx with lesser cation Na quantities of C6 and C2 cpx. In great contrast, the Figure 9 - KIM compositions: Cr2O3 versus cation Na eclogitic DI grains are classified as C8, C9, and C10 mineral chemistry binary plot for clinopyroxene (data from cpx. The ‘eclogitic’ non-DI grains are dominantly C4 Fipke et al., 1989). and C3 cpx.
The Na-Ca-Cr-Fe diagram (Figure 10A) shows that the peridotitic, mantle-derived kimberlite cpx can be well discriminated from the dominantly crustal cpx found in the Canadian Shield till and from the eclogitic cpx, in Fe Cr Fe 2+ 2Fe2+ 5Cr Cr Fe e r 8Cr u t a n ig s l a s t t s r o u r n c g g c n r u o r s t t a s l s ig n (B) a t u r e Al Cr Al Ca/2 strong crustal Na 2Al 5Cr Cr Al Ca/4 s ignat ure 2Na 8Cr Cr-diopside field for kimberlite Ca/2 4Na e Ca Na r e xenoliths and xenocrysts u r t u a t (Morris et al., 2002) n a ig s n l a g t i s s s u t r l r c a o g t s n s n o t r u r g t r o s c c n r g u g n c s t o r a r u l t s s s t a ig l n s a ig t u n GSC OF-2124 r a e t u peridotitic non-DI r e peridotitic DI eclogitic non-DI Ca/6 strong crustal eclogitic DI Na 2Fe2+ Ca/4 signature 2Na Ca 4Na Na Ca/4 strong crustal signature Fe
(A)
4Al
Al Figure 10 - Clinopyroxene illustrative mineral chemistry ternary plots with empirical field separator lines (data from Fipke et al., 1989). (A) Na-Ca-Cr-Fe diagram: the eclogitic DI grains (highlighted by red ellipse) show distinct compositional differences relative to the ‘eclogitic’ non-DI grains which display chemical characteristics similar to those of the ‘crustal’ cpx. (B) Na-Ca-Cr-Al diagram: as for the Na-Ca-Cr-Fe diagram, plus the low-Cr eclogitic grains do not plot near the Cr- diopside ‘85% field for kimberlite xenoliths and xenocrysts’ of Morris et al. (2002).
Saskatchewan Geological Survey 8 Summary of Investigations 2004, Volume 2 particular the Fe-Cr-Na and Fe-Ca-Na plots, as indicated by the empirical separator lines. The Na-Ca-Cr-Al diagram (Figure 10B) also provides reasonable separation of the two cpx populations, as indicated by the empirical separator lines, but the populations are not as well separated as on the Na-Ca-Cr-Fe diagram. In addition, the kimberlite Cr- diopside field of Morris et al. (2002) does not discriminate between these cpx populations, as both populations overlap within this field. In Na-Ca-Cr-Fe space (Figure 10A), the peridotitic cpx show trends toward higher Cr and Na contents and lower Fe and Ca contents. The eclogitic non-DI cpx display trends similar to those for ‘crustal’ cpx (higher Fe, Ca; lower Cr, Na), while the screened eclogitic DI cpx plot in distinct and tight groupings (higher Na, lower Ca, very low Cr). In Na-Ca-Cr-Al space (Figure 10B), the peridotitic and eclogitic cpx show trends similar to the Na-Ca-Cr-Fe data. On the Al-Ca-Na and Al-Cr-Na plots, the peridotitic cpx data overlap typical non-kimberlite compositions and ~50% of the grains do not fall into the Morris et al. (2000) field, and the eclogitic cpx data do not fall near the field.
Many of the peridotitic cpx compositions fit well into the Morris et al. (2002) Cr-diopside field on a Na2O versus Ca/(Ca+Mg) plot (Figure 11). However, the eclogitic DI cpx display a very wide range of Na2O contents (0.2 to 5 note: diagram does not appear Cr-diopside field for kimberlite to be applicable to xenoliths and xenocrysts (Morris et al., 2002) screened eclogitic cpx compositions either 4
GSC OF-2124 screened data peridotitic non-DI cpx peridotitic DI cpx ) 3 eclogitic non-DI cpx % t eclogitic DI cpx w
( O 2
a
N 2 10 note: diagram is not applicable to unscreened eclogitic DI cpx compositions 1
8 GSC OF-2124 unscreened data peridotitic cpx eclogitic cpx 0 ) 6 0.3 0.4 0.5 0.6 0.7 % t Ca/(Ca+Mg) w (
O 2
a Cr-diopside field for kimberlite N 4 xenoliths and xenocrysts (Morris et al., 2002)
2
0
0 0.2 0.4 0.6 0.8 1 Ca/(Ca+Mg)
Figure 11 - KIM compositions: Na2O versus Ca/(Ca+Mg) mineral chemistry binary plot for clinopyroxene (data from Fipke et al., 1989).
Saskatchewan Geological Survey 9 Summary of Investigations 2004, Volume 2 9.3%) with a restricted range of Ca/(Ca+Mg) of 0.40 to 0.55. The Morris et al. (2002) field does not apply to unscreened eclogitic DI cpx or to screened ‘eclogitic’ non-DI cpx compositions because of unsuitable Na and Ca contents. c) The Prairie Kimberlite Study (GSC Open Files OF-2745 and -2875) The Prairie Kimberlite Indicator Mineral till survey of 1991 and 1992 (Thorleifson and Garrett, 1993; Thorleifson et al., 1994) was stimulated by the diamond discoveries in central Saskatchewan announced in 1988. The objective of the work was to provide maps of the regional variability in indicator mineral frequency and soil geochemistry. The KIM maps were meant to be used as a reference to which detailed studies can be compared, to provide an enhanced understanding of indicator mineral transport history, and to test for previously unrecognized mineral occurrences. The KIMs were recovered from 1526 till samples collected across Manitoba, Saskatchewan, and Alberta. The data were regionally subdivided into grains from UTM Zone 13 (Saskatchewan) and UTM Zone 14 (Manitoba) to see if possible kimberlite-derived grains could be distinguished in the UTM Zone 13 suite. Cr-diopsides are abundant throughout the till sequences sampled. Almost all grains examined from this cpx grain suite are peridotitic (Cr-rich) in composition and nearly all grains passed the data screening tests. The cpx grains are dominantly classified as C2 and C5 cpx with very minor quantities of C4 and C1 cpx. They generally contain 2+ moderate amounts of cation Fe (0.08 to 0.20). The Na2O contents are generally low and vary somewhat with the Ca/(Ca+Mg) ratio in that most grains are relatively calcic (ratio >~0.5) and contain up to ~1% Na2O, while the less calcic grains contain lower Na2O contents (<0.5%). These characteristics suggest a non-kimberlite source for the grains (see Figure 8). Dominantly ‘crustal’ trends are indicated in Na-Ca-Cr-Fe data space (Figure 12A), particularly on the Fe-Cr-Na and Fe-Ca-Na plots. In Na-Ca-Cr-Al space (Figure 12B), a strongly ‘crustal’ trend is evident on the Al-Ca-Na plot.
F e Cr Fe
e 2+ 5Cr Cr non-kimberlitic 2+ r 2Fe Fe u t 8Cr a n ig s l a t s s non-kimberlitic u t r r c o n g g kimberlitic n o c r r t u kimberlitic s s t a l s ig n (B) a t u r kimberlitic e Al Cr Al Ca/2 Na 2Al 5Cr Al non-kimberlitic Cr e Ca/4 r strong crustal signature 2Na Cr-diopside field for kimberlite u 8Cr t Ca/2 xenoliths and xenocrysts a Ca 4Na Na n (Morris et al., 2002) ig kimberlitic s l a t kimberlitic s u s r t r c o s g n t r n g o o n r c g t r s u non-kimberlitic c s r kimberlitic t u kimberlitic and a s l t non-kimberlitic s a non-kimberlitic i l compositions overlap g s n i a g t n u a r t e u r Prairie Kimberlite Study OF-2875 e strong crustal (n=1084) Ca/6 non-kimberlitic UTM 13 (n=581) s ignat ure Na 2+ Ca/4 2Fe UTM 14 (n=503) 2Na Ca Ca/4 4Na Na
non-kimberlitic Fe s t r o n kimberlitic g c r u (A) s t a l s ig n a t u r e
4Al
Al Figure 12 - Clinopyroxene illustrative mineral chemistry ternary plots. (A) Na-Ca-Cr-Fe diagram: dominantly ‘crustal’ trends are indicated. (B) Na-Ca-Cr-Al diagram: a strong ‘crustal’ trend is shown on the Ca-Na-Al plot (Thorleifson et al., 1994).
Saskatchewan Geological Survey 10 Summary of Investigations 2004, Volume 2 The Cr-diopside grains obtained from the Saskatchewan and Manitoba parts of the Prairie Kimberlite Study do not show good potential as kimberlite indicators (<20 grains out of 1084). This is paralleled by the probable Thompson Nickel Belt, Manitoba source for most of these grains (Averill, 2001; Quirt and Maki-Scott, 2003), as is well- depicted in Figure 13. Averill used the Cr-diopside grain distribution as a base metal (Ni-Cu) indicator of the Thompson Ni-bearing pyroxenites/peridotites. These Ni-Cu indicator cpx generally contain <1.25% Cr2O3 and are typically of a paler green colour than KIM cpx (>1.25% Cr2O3), but Averill notes that there is considerable overlap in both Cr2O3 and colour, thus demonstrating the need for alternative KIM cpx criteria from cpx mineral chemical data.
Thompson Nickel Belt
glacial dispersion
Figure 13 - Distribution of Cr-diopside grains in surface till in Manitoba (after Averill, 2001) extending southwest 400 km down ice from the Thompson nickel belt.
Saskatchewan Geological Survey 11 Summary of Investigations 2004, Volume 2 d) Clinopyroxenes from High-pressure (HP) HP-cpx Terranes peridotitic kimberlite eclogitic kimberlite DI In recent years, diamonds have been recovered from 'eclogitic' kimberlite non-DI non-traditional non-cratonic terrains. A high-pressure 15 HP-cpx (tremolite schist) HP-cpx (kimberlite) (HP) and subduction-related model for formation of 14 cation Na = 0.25 (J = 0.5) HP-cpx (eclogite/ultramafic) these diamonds has been proposed (e.g., Barron et al., 13 2001; Dobrzhinetskaya et al., 2003). Mineral chemical data for cpx from several HP rock suites, including 12 kimberlite, eclogite-ultramafic complex, and 11 eclogite/ultramafic serpentinite (e.g., Sobolev et al., 1975; Liu et al., 1998; 10 Tsujimori and Liou, 2004, respectively; Figure 14) ) 9 % t
show that the Na and Cr contents are very high. The w 8 (
3 kimberlite grains fall into the C6, C7, and C9 cpx classes: ureyitic 7 O 2
diopside, high-ureyitic diopside, and omphacite. Thus, r 6 identification of possible HP cpx in a KIM grain suite C is important in interpreting cpx data for diamond 5 exploration. 4 3 tremolite schist 2
5. Summary 1 Cr2O3 = 0.50% Some observations and conclusions from this 0 preliminary evaluation of clinopyroxene compositional 0 0.2 0.4 0.6 0.8 cation Na = 0.125 cation Na data include: (J = 0.25)
Figure 14 - HP clinopyroxene compositions: Cr2O3 versus 1) Peridotitic kimberlite cpx (non-DI and DI) are cation Na mineral chemistry binary plot. typically classified as C5, C6, and lesser C2, but not C4. The non-kimberlitic ‘crustal’ cpx examined are typically C2 and C5, but not C6. 2) Peridotitic kimberlite cpx typically display relatively elevated Cr and Na Cation proportions at the expense of Ca, and low Fe2+ cation contents and proportions. 3) Eclogitic (low-Cr) DI cpx are typically classified as C8 with lesser C9, C10, and C4 – very different than the peridotitic cpx classifications and the non-kimberlitic ‘crustal’ cpx classifications. 4) The eclogitic DI cpx compositions plot in distinct, tight clusters and display elevated Na, lower Ca, and very low Cr cation proportions. On a Fe-Ca-Na cation plot, these eclogitic DI cpx display similar proportions to those for the peridotitic DI and non-DI cpx. 5) ‘Eclogitic’ (low-Cr) kimberlite non-DI cpx are typically classified as C4 and C3, with lesser C1 and C2, and these cpx show trends to higher Fe2+ cation proportions at the expense of Cr and Na. 6) The eclogitic kimberlite/mantle cpx pose a problem in discrimination due to their wide range of compositions which are similar to ‘crustal’ cpx compositions in many ways, although they display elevated Na, lower Ca, and very low Cr cation proportions.
7) While ‘common usage’ suggests that higher Cr2O3 contents in peridotitic cpx indicate a greater likelihood of derivation from kimberlite, two points show that this criteria should be used with caution: a) the average Cr2O3 value for peridotitic non-DI cpx in the KIM database (Fipke et al., 1989) is 1.61% Cr2O3 (maximum of 4.43%) and is 2.09% Cr2O3 for peridotitic DI cpx (maximum 7.58%); and b) many non-kimberlitic HP diopsidic cpx also contain extreme contents of Cr2O3 (e.g., up to 10%; Liu et al., 1998). 8) The Fe-Cr-Na, Fe-Ca-Na, and Al-Ca-Na ternary data plots appear to be the ‘more useful’ plots, as they show the jadeite (Na-Al) and kosmochlor (Na-Cr) cation substitution trends as controls on the composition of the mantle-derived kimberlite cpx relative to more Fe-rich crustal pyroxenes, such as hedenbergite (Fe-Ca) and aegirine (Fe-Na). The antipathic Fe-Cr and Fe-Na trends can be used to discriminate between ‘deeper mantle- derived’ grains and the ‘crustal’ grains and provide suitable discrimination between kimberlitic and non- kimberlitic peridotitic cpx, although minor compositional overlaps are present on each plot. Use of a variety of data plots is recommended to strengthen the genetic and diamond/kimberlite exploration interpretations of a suite of cpx KIM chemical data. 9) Although, the Cr-diopside ‘85% field for kimberlite xenoliths and xenocrysts’ of Morris et al. (2002) on the Al- Cr-Na diagram is applicable to certain kimberlitic cpx suites, it may only be of limited use in cpx KIM data evaluations as a significant proportion of ‘crustal’ grain compositions also fall into this field. Additionally, it only applies to data-screened Cr-rich peridotitic cpx and is not applicable to low-Cr eclogitic cpx.
Saskatchewan Geological Survey 12 Summary of Investigations 2004, Volume 2 10) Identification of possible HP cpx in a KIM grain suite is important relative to interpretations of cpx data for traditional and non-traditional diamond exploration. Are any differences revealed in the Prairie Kimberlite Study peridotitic cpx suites from UTM Zone 13 and UTM Zone 14? Yes, but the differences are only minor: 1) Circa 15 to 20 grains from UTM Zone 13 (Saskatchewan) display compositional trends that are consistent with a kimberlitic origin. These grains are distinguished best on the Fe-Cr-Na, Fe-Ca-Na, and Al-Ca-Na plots. Only several grains from UTM Zone 14 (Manitoba) show these features. 2) A very low proportion (<2%) of the grains in this indicator mineral suite appear to be prospective as KIMs, however, these few grains are worth further consideration in diamond exploration.
6. Acknowledgments The author wishes to thank Bernard Gartner of the SRC Geoanalytical Laboratory for encouraging the author to continue the process of advanced KIM data analysis and evaluation. The original manuscript was improved thanks to reviews by Dr. Harvey Thorleifson (Minnesota Geological Survey) and Cristiana Mircea (SRC).
7. References Averill, S.A. (2001): The application of heavy indicator mineralogy in mineral exploration; in McClenaghan, M.B., Bobrowsky, P.T., Hall, G.E.M., and Cook, S.J. (eds.), Drift Exploration in Glaciated Terrain, Geol. Soc., London, Spec. Publ., v185, p69-181.
Barron, B.J., Barron, L.M., and Duncan, G. (2001): Garnets, Diamonds, Diatremes and Subduction at Bingara, in the New England Orogen of New South Wales, Australia; A presentation to the Sydney Mineral Exploration Discussion Group, Sydney, Australia, November 29, http://www.smedg.org.au/nova01.html (accessed May 2004).
Crabtree, D.C. (2003): An overview of the Ontario Geological Survey’s KIM data base: Interpretation of chromite and Cr-diopside data from regional surveys; in Thorleifson, H. and McClenaghan, M.B., (orgs.), Indicator Minerals in Mineral Exploration, Proceedings of a Short Course, March 8, Prospect. Develop. Assoc. Can., p35-43.
Dawson, J.B. and Stephens, W.E. (1975): Statistical classification of garnets from kimberlites and associated xenoliths; J. Geol., v83, p589-607.
______(1976): Statistical classification of garnets from kimberlites and associated xenoliths - Addendum; J. Geol., v84, p495-496.
DiLabio, R.N.W. and Coker W.B. (eds.) (1989): Drift Prospecting; Geol. Surv. Can., Pap. 89-20, 169p.
Dobrzhinetskaya, L., Renfro, A.P., and Green, H.W. (2003): Microdiamonds from ultra-high pressure terranes: From discovery to synthesis in a laboratory; Geol. Soc. Amer., Abstr. with Prog., v35, no6, p633.
Fipke, C.E., Gurney, J.J., Moore, R.O., and Nassichuk, W.W. (1989): The development of advanced technology to distinguish between diamondiferous and barren diatremes; Geol. Surv. Can., Open File 2124, 1175p. Liu, X., Zhou, H., Ma, Z., and Chang, L. (1998): Chrome-rich clinopyroxene in orthopyroxenite from Maowu, Dabie Mountains, central China: A second record and its implications for petrogenesis; The Island Arc, v7, no1-2, p135. Morris, T.F., Sage, R.P., Ayer, J.A., and Crabtree, D.C. (2002): A study of clinopyroxene composition: Implications for kimberlite exploration; Geochem. Explor. Analy., v2, no4, p321-331. Quirt, D.H. and Maki-Scott, P. (2003): Gold grain and indicator mineral services: Sample processing to data presentation; in Thorleifson, L.H. and McClenaghan, M.B. (orgs.), Indicator Minerals in Mineral Exploration, Proceedings of a short course, March 8, Prospect. Develop. Assoc. Can., p17-21. Sobolev, V.S., Sobolev, N.V., and Lavrent'ev, Y.G. (1975): Chrome-rich clinopyroxenes from the kimberlites of Yakutia; Neues Jahrbuch Mineralogie Abhandlung, v123, no2, p213-218.
Saskatchewan Geological Survey 13 Summary of Investigations 2004, Volume 2 Stephens, W.E. and Dawson, J.B. (1977): Statistical comparison between pyroxenes from kimberlites and their associated xenoliths; J. Geol., v85, p433-449. Thorleifson, L.H. and Garrett, R.G. (1993): Prairie kimberlite study-till matrix geochemistry and preliminary indicator mineral data; Geol. Surv. Can., Open File 2745, 14p and one diskette. Thorleifson, L.H., Garrett, R.G., and Matile, G. (1994): Prairie kimberlite study indicator mineral geochemistry; Geol. Surv. Can., Open File 2875, 13p and one diskette. Tsujimori, T. and Liou, J.G. (2004): Coexisting chromian omphacite and diopside in ultramafic schist from the Chugoku Mountains, SW Japan: The Cr effect on the omphacite-diopside immiscibility gap; Amer. Mineral., v89, p7-14.
Saskatchewan Geological Survey 14 Summary of Investigations 2004, Volume 2