Element ratios in nickel sulphide exploration: vectoring towards ore environments

ELSEVIER

Journal of Geochemical Exploration 67 (1999) 145–165 www.elsevier.com/locate/jgeoexp

Element ratios in nickel sulphide exploration: vectoring towards ore environments Nigel W. Brand * WMC Leinster Nickel Operations, P.O. Box 22, Leinster, WA 6437, Australia Accepted 2 September 1999

Abstract The use of element ratios in geochemical exploration can help to distinguish mineralised from barren geological complexes, predict deposit types, and assist in estimating and evaluating dispersion of ore indicator elements. Exploration at Kambalda Nickel Operations, Western Australia, has focused on developing a repetitive robust geochemical method that can vector towards Ni–Cu–PGE sulphide deposits. One method has been developed so as to utilise element ratios in a manner which are compatible with the genetic geological exploration model for komatiite-associated massive Ni sulphide deposits. A particularly successful element ratio is expressed as [(Ni=Cr) ð (Cu=Zn)]. The [(Ni=Cr) ð (Cu=Zn)] ratio is effective in vectoring towards channellized komatiitic environments. This paper presents key examples that demonstrate the power of the [(Ni=Cr) ð (Cu=Zn)] ratio in fresh komatiite rock, its intensely weathered equivalent, and in surface materials. Analytical techniques employing weaker acid attacks (HCl) and finer size fractions from surface materials enhance ratio contrasts. Exploration implications are discussed. Komatiites associated with Ni sulphides are low in Cr compared to barren komatiites. Low Cr values persist along the flow from Ni sulphide mineralisation in channellized komatiites. Mineralised basal komatiites flows are enriched in Ni compared to barren flows. Thus, utilising a Ni : Cr ratio, potentially mineralised flows (high Ni, low Cr) can be distinguished from barren flows (low Ni, high Cr) with Ni : Cr ratio values typically >1. This Ni : Cr signature can be traced down-plunge, on the basal komatiite, for >14 km and through the komatiitic stratigraphy. In addition, this signature is, in part, preserved through the weathered komatiite and in surface material. It has successfully identified new Ni sulphide occurrences and highlighted significant areas of exploration interest. Sulphide-bearing sedimentary horizons of variable thickness occur at the base of, and within, the komatiitic successions. These meta-sedimentary rocks typically have three times the Zn concentration (1500 ppm) to that of Cu (500 ppm). However, because nickel sulphides have a greater concentration of Cu than Zn, the Cu : Zn ratio can assist in discriminating Ni sulphide-bearing zones from those with no Ni sulphides. Combining the Ni : Cr and Cu : Zn ratios has the effect of reducing the influence of particular lithologies (e.g. sulphidic meta-sedimentary rocks) and aids in the delineation of Ni sulphide mineralisation. Multiplying the ratios enhances the contrast of element ratios related to mineralisation and subdues the effect of lithologies. Utilisation of the [(Ni=Cr) ð (Cu=Zn)] ratio increases contrast between background and mineralisation, and thus the ability to vector towards basal komatiite channellized environments through the distinctive behaviour of Ni, Cr, Cu and Zn.  1999 Elsevier Science B.V. All rights reserved. Keywords: nickel sulphide; mineral exploration; element ratio; mineralised komatiite; barren komatiite; Kambalda

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1. Introduction: element ratios and nickel exploration The use of element ratios in mineral exploration can assist in identifying ore-associated environments and deposit styles, especially those of a magmatic origin such as Ni sulphides. Interpretations of element ratios in fresh and weathered rock environments associated with komatiitic lithologies in the Norseman–Wiluna greenstone belt in Western Australia, have led to the effective targeting of Ni sulphide mineralisation, in particular: (1) discrimination of mineralised from barren komatiites; (2) vectoring towards Ni sulphide ore environments; (3) determination of plunge direction of a Ni sulphide ore shoot; and (4) estimation and evaluation of dispersion of ore indicator elements. Documented applications of elemental ratios for routine, cost effective, Ni exploration in highly weathered terrains are rare. In the regolith terrains with overburden removed, or ‘stripped’ terrains, of Pioneer Dome, Western Australia, Cox (1975) noted that Ni : Cr values >1.0–1.5 in soils were “related to Ni sulphide mineralisation” and values <1.0 were related to “barren areas”. A similar conclusion was reached by McNeil (1980) who showed from a number of isolated examples that an increasing Ni : Cr ratio was indicative of increasing Ni sulphide potential in fresh rock within the Yilgarn Craton. At a regional scale, Barnes and Brand (1996) and Brand (1997) showed that Ni and Cr concentrations and Ni : Cr ratios from channellized komatiite sequences (i.e. potential Ni sulphide deposit hosts) have distinctly different signatures than those of barren dunite sheets and ponded lava flows (Fig. 1). In this paper the use of Ni, Cr, Cu and Zn ratios are presented from several case study areas, as shown in Fig. 2. Their effectiveness in targeting for and vectoring to komatiite-hosted Ni sulphide mineralisation is demonstrated. Case studies from the Norseman–Wiluna greenstone belt, Western Australia, show the effectiveness of these ratios as lithogeochemical prospecting tools for komatiite-hosted

Fig. 2. Location map of case study sites discussed in this paper.

Ni–Cu–PGE deposits in the fresh rock, regolith and surface environments. These element ratios assist in the delineation of prospective areas in highly altered and deformed terrains where primary textures, assemblages and relationships are unclear.

2. Geological model: a brief description Geological understanding of komatiite-hosted Ni– Cu–PGE sulphide deposits has significantly evolved since the discovery of Kambalda in 1966. In excess

Fig. 1. (A) Regional scale distribution of whole-rock Ni–Cr values in the Norseman–Wiluna belt, Western Australia (Brand, 1997). Compiled from Naldrett and Turner, 1977; Donaldson, 1983; Dowling and Hill, 1990; Gole and Hill, 1990. (B) Schematic summary of whole-rock Ni–Cr trends, based on the data presented in (A).

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of 4400 drill holes have delineated mineralisation on the Kambalda Dome. Numerous geological models have evolved from exploration and subsequent mining of the Kambalda Ni ore-bodies. Over the past decade, there has been increased recognition of the importance of dynamic volcanological processes, and the accumulation of Ni sulphide in channellized environments (e.g. Lesher et al., 1984; Huppert and Sparks, 1985; Barnes et al., 1988; Lesher, 1989; Hill et al., 1989, 1995). A brief overview of the genetic model based on work by Gresham and Loftus-Hills (1981) is given below and an idealised stratigraphic section shown in Fig. 3. A detailed review of the

model developments is beyond the scope of this paper and the reader is referred to Hudson (1990) and references therein. Magmatic Ni–Cu–PGE sulphide deposits form by segregation of an immiscible sulphide melt from a silicate host magma following magma mixing, rapid cooling, differentiation and=or contamination. At Kambalda, komatiite magmas formed channellized flows. These are represented by linear belts at least 14 km in length that contain individual Ni sulphide bodies over 3 km long, but less than 300 m wide, and less than 5 m thick. Sulphides typically occur at the base of the lowermost channellized flow

Fig. 3. Idealised schematic stratigraphic section of the Kambalda Dome depicting lithologies, ore and non-ore environments, and geochemical trends.

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unit in ‘contact’ ore positions (80% of reserves), or at the base of the next one or two flow units in hangingwall positions. These sulphides are typically stratified with massive (>80% sulphide) pyrrhotite– pentlandite layers at the base overlain by matrix (40–80% sulphide) and disseminated (<40% sulphide) layers. The sulphides contain minor Cu, Co, Cr and Zn and trace levels of As, Pb, Bi, Ag, Au and the platinum group elements (Pt, Pd, Os, Ir, Rh, Ru). The chemical contrast between the underlying basalt and overlying komatiite lithologies (Fig. 3) facilitates the use of element ratios to vector towards channellized environments. However, once a prospective mineralised channel is located, no element or element ratio can vector towards mineralisation (i.e. up-channel or down-channel). This is probably because continuous lava flows remove any prospective geochemical signatures (Hill et al., 1995), and basal topography controls settling of the sulphide bedload (Lesher, 1989). Along strike from the ore position, sulphidic cherts and shale sequences separate the footwall basalt from the barren komatiite lavas. These sedimentary rocks are enriched in Zn relative to Cu (Bavington, 1981), whereas mineralised komatiites are enriched in Cu relative to Zn. This allows discrimination between the sedimentary rocks and mineralised komatiites by using Cu : Zn ratios. Using the Gresham and Loftus-Hills (1981) model as a framework, the chemical and mineralogical signatures of komatiites can be used to identify prospective stratigraphy and ore environments within the primary environment and its weathered equivalents.

3. Methodology 3.1. Kambalda Dome Data for the Kambalda Dome basal komatiite study were obtained from the Kambalda drill data base. This data base contains information on all bore holes drilled on the Kambalda Dome during the past 30 years, their location, logged lithologies, and geochemical results from diamond core samples. All komatiite lithologies are routinely quarter core sampled selecting 1 to 3 m core lengths per sample. Various sample preparation techniques, analytical meth-

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ods and instrument finishes have been used to assay Kambalda drill cores over time. The predominant method has been a mixed acid digestion with an AAS, and more recently an ICP–OES finish. These results have been used for annual ore reserve calculations at Kambalda. Thus, for this project, it has been assumed that there has been no variation in laboratory procedures which will dramatically affect the overall results. This assumption is valid as a mixed acid digestion has always been the primary analytical method at Kambalda; however, with modernisation, methods of instrument finishes have changed. To calculate the average geochemistry of the basal ultramafic units a thickness of 30 m was used. This 30 m represents the maximum thickness of non-mineralised ultramafic flows and a minimum thickness of mineralised ultramafic flows on the Kambalda Dome (Lesher, 1983). To characterise the non-ore (lithogeochemical) part of the ultramafic flow, any Ni values of >0.4 wt%, as well as the corresponding Cu, Cr and Zn values, were excluded from the calculations. The value of 0.4 wt% Ni represents approximately 1% pentlandite in an ultramafic rock, and has been shown empirically to identify all mineralised positions at Kambalda. A total of 22,500 samples were utilised in this basal komatiite study. The upper filter of 0.4 wt% Ni was applied so a lithogeochemical ratio signature could be obtained for the ultramafic rocks. The use of ratio values alone is not definitive and must be viewed along with the absolute concentrations of individual elements. Where necessary, a filter could be applied to exclude non-ultramafic lithologies. However, the choice of filter values will depend on the sample medium (i.e. soil versus weathered rock versus fresh rock) and the objective of the study. It is noted that for elements such as Cr, the mixed acid digestion method is only a partial digest, the consequences of this are discussed in Section 6. Ratios will vary depending on the geological system, sample medium, and digestion method. However, empirically the Ni : Cr ratio has identified Ni sulphide mineralisation in geological settings over a wide geographical area, even when the base metal signature was very subtle. Other than the 0.4 wt% Ni filter applied to the Kambalda Dome data set, no filter has been applied to any other data set presented in this paper.

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3.2. Mt Keith and Cawse

4.1. Nickel and chromium signatures

and Ni : Cr ratio and a low Cr concentration (Barnes and Brand, 1996). This reflects a primary mineralogy dominated by olivine and=or its metamorphically altered equivalent. In fractionated non-ore environments Cr increases in concentration whereas MgO, Ni and Ni : Cr ratios decrease away from the potentially mineralised channellized environment. This reflects a decrease of olivine and an increase in orthopyroxene and chromite. Within the channellized ore environment, elevated Ni and low Cr values are observed in the lower flow unit of the ultramafic pile (Fig. 4a). Nickel concentrations exceed those of Cr, so the Ni : Cr ratio is >1. Higher in the ultramafic pile, Ni decreases as Cr increases. In the non-ore environments (Fig. 4b) Ni values are generally lower and Cr concentrations higher in the lower MgO komatiite lavas. Where there is a thin high MgO basal unit, Ni is elevated and Cr lower, so signatures are similar to those of the ore environment. However, the absolute Cr concentrations exceed those of Ni, and the Ni–Cr difference is subdued and lacks consistency with a Ni : Cr ratio generally <1. Using an upper 0.4 wt% Ni filter and associated Cr concentrations in the lower 30 m of the basal ultramafic stratigraphy, Brand and Williams (1993) showed that a Ni : Cr ratio of >1.8 could identify the position of all known mineral trends on the Kambalda Dome (Fig. 5). This Ni : Cr ratio represents the non-ore lithogeochemical signature of the mineralised komatiite unit. Areas with Ni : Cr ratios >1.8 (reflecting local concentrations due to a well mineralised field) show a strong NNW orientation and are inferred to extend for over 14 km. This suggests a geochemical fingerprint with a primary volcanological control, which is consistent with the Gresham and Loftus-Hills model (see Fig. 3). At Lunnon, the Ni mineralisation (Fig. 6A) extends for over 5 km. The distribution of Ni : Cr ratios for the non-ore portion of the basal unit overlying the Ni sulphide mineralisation shows a pronounced left-hand en-echelon trend. This trend is identical to individual ore surfaces within the Lunnon ore body (Fig. 6B).

4.1.1. Nickel and chromium signatures in fresh rock Within channellized environments (i.e. environments with the potential to host Ni sulphides) the komatiite sequence has a relatively high MgO, Ni

4.1.2. Nickel and chromium signatures in weathered rock As was shown by fresh rock samples, there are distinct primary lithogeochemical signature differ-

Thirty nine (39) regolith samples from percussion drill-hole MKNC10 (Mt Keith) and 39 samples from diamond drill-hole CWD005 (Cawse) were collected between March and November 1994 and analysed for Ni, Cu, Cr and Zn by mixed acid digestion (including HF) with an ICP–MS finish. The samples were taken at 1-m intervals and were selected to represent in situ regolith materials characterising saprock, saprolite, Fe-saprolite and collapsed Fe-saprolite. 3.3. Widgiemoortha: 132N — Moore region A number of 1029 soil samples, collected on a 100 ð 40 m grid, at a depth of 15 cm, were sieved to 1 mm and analysed for Ni, Cu, Cr and Zn using a mixed acid digest (including HF) and an AAS finish. An additional thirteen bulk soils were sampled over the 132N deposit, at a depth of 15 cm, sieved and the individual fractions analysed the by the same method. All surface sampling was conducted in 1992. 3.4. Talbot Island Sixteen samples were selected from a surface rock chip survey on the basis of being either mineralised komatiite (>0.4 wt% Ni) or barren komatiite (0.15–0.3 wt% Ni). These samples were then processed by three types of acid attack: hydrochloric acid, mixed acid and mixed digest with HF. This was followed by an AAS finish to evaluate geochemical characteristics that discriminate mineralised from non-mineralised environments.

4. Elemental and element ratio signatures associated with Ni–Cu–PGE komatiite hosted deposits

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Fig. 4. (a) Drill trace showing the distribution of Ni and Cr from a mineralised komatiite sequence (channellized facies) on the Kambalda Dome. Data from diamond drill hole KD6033 representing 250 m of komatiite stratigraphy from the footwall mafic contact and utilising 84 samples (black shows Ni > Cr concentration, grey shows Cr > Ni concentrations). (b) Drill trace showing the distribution of Ni and Cr from a barren komatiite sequence on the Kambalda Dome. Data from diamond drill hole KD440 representing 250 m of komatiite stratigraphy from the footwall mafic contact and utilising 96 samples (black shows Ni > Cr concentrations, grey shows Cr > Ni concentrations).

ences between mineralised and barren ultramafic flows. These differences are preserved and in some incidences enhanced during weathering. Within the erosional and residual surface environments, use of Ni : Cr ratios, in association with their absolute values, can aid in distinguishing lithotypes and vectoring towards Ni sulphide ore environments in the regolith. 4.1.3. Ni and Cr in the regolith environment At the regional scale, Ni–Cr data from the regolith generally support the observations made by Barnes and Brand (1996), with channellized sequences having Ni : Cr ratios greater than dunite sheet and ponded lava sequences (Fig. 7). This occurs despite the fact that Ni is not considered an

effective ore pathfinder due to its high solubility and numerous host phases within the regolith (Brand, 1997). Mean Cr values are <4000 ppm from channellized komatiitic sequences and >4000 ppm from dunite sheet and ponded lava sequences. However, in the regolith the absolute enrichment of Ni effectively increases Ni : Cr ratios above unity, due to association of Ni with Mg-bearing minerals below, and Fe oxides above the Mg-discontinuity 1 (Fig. 7). In talc–carbonate altered lithologies, the Ni : Cr ratio remains similar to that of the protolith because of the 1

Mg-discontinuity marks the horizon at which all Mg-bearing minerals (primary and secondary) are lost from weathered ultramafic rock within the in-situ regolith.

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Fig. 5. Kambalda Dome basal ultramafic lithogeochemistry: idealised Ni : Cr ratio distribution for the lower 30 m of the komatiite basal unit identifying all known ore shoots. This diagram is a summary of 22500 data points sourced from over 4400 drill holes and excludes Ni values >0.4 wt%.

limited ability of talc to take up or release Ni during weathering (Springer, 1974).

4.1.4. Ni and Cr in the surface environment Within the Widgiemoortha region, the 132N Ni sulphide deposit has a gossan exposed in an erosion regime with a mafic footwall and hangingwall.

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Fig. 6. Lunnon Shoot — (A) Idealised Ni ore surfaces distribution. (B) Trend of elevated Ni : Cr ratios within the basement komatiite unit and excluding Ni values >0.4 wt%.

Profiled geochemistry (518,800mN) derived from the 2 mm fraction of routine surface soil samples, which were analysed using a mixed acid digest (including HF), shows that ultramafic rocks are characterised by an average Ni : Cr ratio of 0.62, mafic rocks by a ratio of 0.28, and the 132N mineralisation by a ratio of 2.62 (Fig. 8A, Table 1). The contrast between Ni sulphide mineralisation (Ni : Cr ratio D 2.62) and background values from ultramafic rocks (0.62) at 132N is up to seven-fold. 4.2. Copper and zinc signatures 4.2.1. Copper and zinc in fresh rock Sedimentary horizons in the non-ore environment occur at the base of and within the ultramafic sequences at Kambalda (Fig. 3). They have mean Zn and Cu concentrations of 1500 and 480 ppm, respectively (Bavington, 1981). In comparison, komatiites

typically have <60 ppm of both Zn and Cu, whereas contact sulphide ores have Zn values <100 ppm and Cu values >400 ppm. Thus the Cu : Zn ratio can aid in discriminating between ore and non-ore sulphides, even in the presence of moderately elevated Ni values (i.e. 0.4 wt% Ni) in the sedimentary horizons. Fig. 9 shows a Cu, Zn and Cu : Zn profile through the komatiite stratigraphy containing sediment horizons on the Kambalda Dome. 4.2.2. Copper and zinc in weathered environments 4.2.2.1 Copper and zinc in the regolith environment. Within the regolith, Cu and Zn tend to residually accumulate within the profile. Oxidation of chalcopyrite generates submicron copper nuggets spatially associated with secondary Fe-oxides; however, the effect of weathering on silicate-hosted Cu is uncertain (Brand, 1997). Zinc within the komatiite protolith is predominantly associated with chromite

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Fig. 7. Ni : Cr ratio through the regolith profile over a mineralised channellized adcumulate (Mt Keith) and a barren dunite sheet flow (Cawse).

(Challis et al., 1995). This Zn–chromite=chrome– magnetite association persists through the regolith to the surface. In addition, Zn is known to be associated with secondary Fe oxides within the profile (Brand, 1997). The elevated Cu : Zn ratio in mineralised channellized environments is maintained, due to the immobility of sulphide-derived Cu; thus the Cu : Zn ratio is generally maintained through the regolith because both elements residually accumulate in the profile (Fig. 10). 4.2.2.2 Copper and zinc in the surface environment. The surface expression of Ni sulphide mineralisation is characterised by high Cu and low Zn (Cochrane, 1973; Joyce and Clema, 1974; Bull and Mazzuc-

chelli, 1975; Moeskops, 1977; Butt, 1979; Roberts and Travis, 1986; Taufen and Brenner, 1987). This indicates that the Cu–Zn relationship observed in the primary environment is preserved during weathering through the regolith and onto the surface. Thus the Cu : Zn ratio can assist in distinguishing between mineralised and lithological Ni anomalies. The Cu : Zn ratio is also a powerful tool in characterising lithotypes, as indicated by the distinction between mafic and ultramafic rocks for the 132N area (Fig. 8b, Table 1). The Cu : Zn ratio can: (1) highlight high concentrations of Cu, derived from oxidation of Ni sulphide mineralisation (concentrations of Cu are 3 to 15 times greater than Zn in ore (Lesher, 1989)); (2) distinguish Cu associated with

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Fig. 8. Geochemical profiles over the 132N Ni sulphide deposit, Widgiemoortha: (A) Ni : Cr geochemical traverse, (B) Cu : Zn geochemical traverse, (C) [(Ni=Cr) ð (Cu=Zn)] geochemical traverse. Data sourced from a regional 100 ð 40 m soil survey as described in Section 3.

sulphidic sedimentary rocks in which concentrations of Zn are typically 3 times those of Cu (Bavington, 1981); and (3) subdue the influence of Cu derived

from weathered silicates (concentrations of Zn are typically 20 ppm greater than those of Cu in fractionated komatiites (Ross and Hopkins, 1975).

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Table 1 Nickel, Cr, Cu, Zn and element ratios from the in the 132N region, Widgiemoortha

1 mm size fraction of surface soil samples taken over mafic and ultramafic lithologies Mean

Maximum

Minimum

Std. dev.

Range

Ni

Ore zone Ultramafic rock Mafic rock

1750 543 151

3650 1040 210

770 210 110

1645 195 27

2880 830 100

Cr

Ore zone Ultramafic rock Mafic rock

593 1031 526

780 1980 660

490 420 280

161 411 129

290 1560 380

Ni : Cr

Ore zone Ultramafic rock Mafic rock

Cu

Ore zone Ultramafic rock Mafic rock

208 98 94

355 345 120

130 45 80

127 63 12

225 300 40

Zn

Ore zone Ultramafic rock Mafic rock

100 249 66

130 710 140

80 60 50

26 178 24

50 650 90

Cu : Zn

Ore zone Ultramafic rock Mafic rock

2.32 0.54 1.58

4.44 1.92 2.20

1.08 0.21 0.61

1.84 0.43 0.48

3.36 1.70 1.59

(Ni=Cr) ð (Cu=Zn)

Ore zone Ultramafic rock Mafic rock

8.26 0.29 0.47

20.77 1.10 1.00

1.75 0.10 0.21

10.83 0.23 0.19

19.01 1.01 0.79

2.63 0.56 0.30

4.68 0.97 0.50

1.57 0.23 0.22

1.78 0.19 0.08

3.11 0.74 0.28

Number of ultramafic rock samples (19), mafic rock (15) and gossan zone samples (3). Although the number of gossanous samples is statistically invalid, they have been included for comparison. Data selection described in Section 3. All element values in ppm.

Table 2 Mean, minimum and maximum Ni, Cu, Cr, Zn values and the Kambalda [(Ni=Cr) ð (Cu=Zn)] element ratios associated with different komatiite environment in the basal ultramafic unit Kambalda Dome (after Brand and Williams, 1993) Element

Mineralised

Barren

mafic

sediment

mafic

sediment

Ni

2112 617–3925

1878 744–3342

1503 520–2887

1334 570–2909

Cu

92 10–235

108 27–225

66 10–150

66 14–150

Cr

739 100–1350

858 371–1345

834 57–1678

1018 364–1657

Zn

83 10–162

91 33–161

106 36–231

116 16–283

Ni=Cr ð Cu=Zn

2.96 0.52–7.04

2.40 0.78–6.78

1.75 0.29–3.79

1.38 0.50–3.45

The calculations were derived from 22500 drill core samples as described in Section 3. All element values in ppm, maximum basal ultramafic thickness used is 30 m, Ni values >0.4 wt% excluded from calculations, statistical outliers removed. Italic D concentration range.

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Fig. 9. Drill trace showing the distribution of Ni, Cu, Zn and Cu : Zn ratio through a komatiite sequence containing sedimentary horizons on the Kambalda Dome. Data from diamond drill hole KD6042A representing 250 m of komatiite stratigraphy from the footwall mafic contact and utilising 124 samples.

5. Combined ratios (Ni=Cr) ð (Cu=Zn) 5.1. Combined ratios in fresh rock Brand and Williams (1993) subdivided drill holes according to a mineralised (i.e. drill holes which have >0.4 wt% Ni on the stratigraphic basal contact) or barren komatiite environment, and also distinguished their associated footwall type (i.e. mafic or sediment). They demonstrated that mean Ni and Cu

values from drill holes within the basal ultramafic unit, when combined, show an increase from the non-mineralised to the mineralised environment, in contrast to mean Cr and Zn values for drill holes which decrease (Fig. 11A,B). The mean values for drill holes, subdivided according to both ore environment and footwall type, are given in Table 2. Combining these trends into a Ni, Cr, Cu and Zn ratio (Eq. 1) suppresses the influence of certain lithologies (e.g. black shales). The combined

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Fig. 10. Cu : Zn ratio through the regolith profile over a mineralised channellized adcumulate (Mt Keith) and a barren dunite sheet flow (Cawse).

ratio has been shown to both delineate mineralised environments and detect Ni sulphide mineralisation either directly or through vectoring. .Ni=Cr/ ð .Cu=Zn/

‘Kambalda ratio’

(1)

By multiplying the two ratios together, the positive contrast of the element ratios related to mineralisation is enhanced and the effects of different lithotypes are subdued due to low contrast (Table 2). The combined ratio increases the contrast between Ni sulphide mineralisation and lithotypes. The effectiveness of the ‘Kambalda ratio’ in vec-

toring towards and identifying prospective zones of Ni sulphide mineralisation is shown schematically in Fig. 11C. This ratio has a greater contrast than individual elements in identifying channellized environments. 5.2. Combined ratios — weathered environment 5.2.1. Regolith environment Brodie-Hall (1975) estimated that 85% of the regions of the Yilgarn Craton that have potential to host Ni sulphide mineralisation are covered by

Fig. 11. Mean element and element ratio data from the lower 30 m of the komatiite basal unit showing trends away from the trough associated with ore environment and footwall characteristics: (A) Ni and Cr trend, (B) Cu and Zn trend, (C) [(Ni=Cr) ð (Cu=Zn)] or ‘Kambalda ratio’ trend.

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Fig. 12. Mean response of element and ‘Kambalda ratio’ data within the lower saprolite at Moore, vectoring towards a mineralised channellized environment. Note that the contrast of individual elements does not emphasise this mineralised zone. Data are from 120 aircore drill samples selected within the saprolite and analysed using a mixed acid digest and a AAS finish (data from Brand, 1997).

strongly leached or transported material. Conventional surface geochemical sampling over concealed Ni mineralisation shows a lack of surface response. Base metal results either display little contrast or have concentrations below detection limits. Assessment of regolith geochemistry using base metal ratios has been successful at vectoring towards Ni sulphide mineralisation. The Moore Ni sulphide deposit in the Widgiemoortha region (Fig. 2) provides an example of this. Elevated base metal results from an aircore drill traverse shows that the saprolite zone is the most responsive zone in the regolith. Mean Ni, Cu, Cr and Zn values from the saprolite were calculated for each drill hole. Although they all reach a maximum away from the mineralisation, the combined element ratios show a peak approximately over the Ni sulphide mineralisation (Fig. 12).

5.2.2. Surface environment The Kambalda ratio at the 132N gossan position has a peak to background contrast >40 (Table 1). It is markedly accentuated relative to the individual elements (Ni, Cu, Cr and Zn) and element ratios Ni : Cr and Cu : Zn (Fig. 8C). Presentation of the Kambalda ratio in plan view (Fig. 13) gives discreet element ratio anomalies with their peaks coincident with the gossan position. Lower ratio values depict halos dispersing away from mineralisation. The halos effectively act as vectors to mineralisation (within the Widgiemooltha region), identifying basal contacts and locating gossans that overlie potential Ni mineralisation. Other prospects in the area show similar relationships, but the element ratio response is typically less and relates because of local regolith conditions and the exposure levels of the gossans.

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Fig. 13. Regional Kambala ratio values over the 132N region, Widgiemooltha, WA. Data selection described in Section 3.

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Fig. 14. Comparison of Cr concentrations extracted from rock chip samples representing eight mineralised komatiites (>0.4 wt%) and eight barren komatiites (0.15–0.25 wt% Ni). Digestion methods are presented from the weakest method, hydrochloric acid, through to mixed acid digest to the strongest mixed acid digest with HF.

6. Enhancement of ratio contrast 6.1. Choice of analytical method The choice of analytical method is important in enhancing ratio contrast. As dramatically shown in

Fig. 14, Cr concentrations from mineralised komatiites are substantially less than those from barren komatiites. This study showed that stronger acids (e.g., mixed acid with HF), as would be expected, extract a much higher percentage of the metals present than weaker acids (e.g. HCl). The study showed that dif-

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Fig. 15. Kambala ratio values calculated for five size fractions from soils collected over 132N, Widgiemooltha, WA. Data selection described in Section 3.

ferent acids extracted similar concentrations of Ni, but significantly different concentrations of other elements, like Cr. Weaker acids, like HCl, have been found to be the best for optimising contrast, because less Cr is extracted relative to Ni, when compared to stronger acids (Fig. 14). 6.2. Mesh size Ratio contrast can be enhanced and anomalies further refined by using finer soil fractions (>400 µm). However, as demonstrated in the subcropping saprolite regolith environment of 132N, all soil fractions adequately detected the surface expression of 132N mineralisation and would enable successful vectoring (Fig. 15), due to the nature of the regolith.

7. Conclusions This paper has demonstrated that Ni : Cr and Cu : Zn ratios are effective in identifying and vec-

toring towards potential Ni sulphide mineralisation and host lithotypes. In summary: (1) Ni : Cr ratios, when combined with Cu : Zn ratios (‘Kambalda ratio’ [Ni=Cr] ð [Cu=Zn]), provide a powerful vector to locate channellized komatiitic environments and Ni sulphide mineralisation. (2) The ‘Kambalda ratio’ can be effectively applied in deeply weathered terrains and in erosional=residual surface environments to vector towards prospective mineralisation. (3) Ratios within the regolith can assist in discriminating barren from mineralised sequences on a regional and project scale. (4) The inclusion of a Cu : Zn ratio has the advantage of highlighting Cu derived from Ni sulphide whilst subduing Cu derived from sulphidic sedimentary rocks and silicates. (5) Ni : Cr ratios on a regional scale have the ability to predict whether greenstone terrains are prospective. (6) Ratio contrast can be enhanced using weaker digestion methods and finer sample size fractions in weathered material.

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It is concluded that the Kambalda ratio is effective in vectoring towards channellized komatiitic environments in the Norseman–Wiluna greenstone belt. Like all ratios, the absolute concentrations of individual elements must also be noted and, where necessary, a filter should be applied to exclude values derived from exotic material or non-ultramafic lithologies.

Acknowledgements This project developed from discussions within the Kambalda Exploration Department during the early 1990s, headed by David Miller. The support, direction and encouragement of the exploration group are duly acknowledged, in particular Rohan Williams and Richard Evans. Adrian McArther and David Gray are thanked for their constructive comments on the manuscript. Owen Lavin and Eric Grunsky are acknowledged for their reviews of the paper and for providing much appreciated feedback that has enhanced the quality of this paper. Jenny Gifford is thanked for assisting in the drafting of figures.

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