Electronic Supplementary Materials
1.
Structural Measurements
Twenty seven drill holes were re-logged in detail to understand the structural controls on the mineralisation.
Only seven of these drill holes were oriented. On the oriented drill holes, measurements were taken using a core
orientation device (“rocket launcher”) allowing direct measurement of orientations of foliations and lineations.
For non-oriented drill holes, the angle between the core axis of the drill hole and the plane of the
structure/foliation was measured (alpha angle). This alpha angle allows the interpretation of variations of the
orientation of structures along the drill holes even if the actual orientation is not quantified. In total, 52 foliation
measurements, 25 lineation measurements and 121 alpha angles on foliations in un-oriented core were
measured. All the structural measurements taken during this study are tabulated in Electronic Supplementary
Materials Table 1 (ESM T1).
2.
Portable X-Ray Fluorescence Analyses (pXRF)
One hundred and seventy pXRF analyses were collected on 31 samples (average of 5 analyses per sample)
giving reliable results for concentrations of a range of major and trace elements (Si, Ca, K, S, Fe, Ti, Cr, Mn, Ni,
Cu, Zn, As, Zr, Rb and Sr). The analytical protocol followed is described in Le Vaillant et al. (2014). Analyses
were collected on unpolished (or slightly polished when needed due to oxidisation of samples) half core with the
Desktop X500 X-Ray fluorescence analyser. The protocol comprises analyses of a batch of 7 matrix matched
standards at the beginning of each analytical batch and the regular analysis of a monitor and a silica blank. The
internal standards used for the calibration of the data are composed of felsic volcanic and volcaniclastic rocks of
a similar composition to the Mount Keith Dacite. The detection limits for each element of interest and the
instrumental precision of the pXRF which were determined for the analyses collected on felsic volcanic rocks
are tabulated in Electronic Supplementary Material Table 2a and 2b (ESM T2a and T2b). All the results of
pXRF analyses are tabulated in Electronic Supplementary Material Table 2c (ESM T2c).
3.
Laboratory Analyses
One hundred and twenty six samples were analysed by Ultra Trace laboratories in Perth (Western Australia), for
major elements, trace elements and platinum group elements (PGE). The analytical protocol is as follows. Once
dried and sorted, samples are crushed with a steel jaw crusher, and pulverised in a vibrating disc pulveriser with
a steel bowl and puck. The main oxide concentrations are obtained by X-Ray Fluorescence (XRF) spectroscopy
on part of the samples previously cast using 12:22 flux to form a glass bead (SiO2, Al2O3, CaO, Fe2O3, K2O,
MgO, Na2O, P2O5, TiO2, MnO, BaO, V2O5, Cr2O3, Cu, Ni, Zn). The same pulverised samples are also
digested and refluxed with a mixture of acids (hydrofluoric, nitric, hydrochloric and perchloric acids). This
method approaches a total digest for most mineral phases; however, a few refractory minerals such as zircon and
chromite are not completely attacked, although recoveries of better than 90% are indicated for most samples
(unpublished CSIRO data). The obtained solutions are analysed by both Inductively Coupled Plasma (ICP)
Mass Spectrometry (La, Ce, Cs, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ga, Y, Nb, Pb, Th, Hf, Rb,
Sn, Sr, Ta, U, Zr) and ICP Optical Spectrometry (ICP-OES) (Fe, Co, Cr, K, Mn, P, Sc, Ti, V) to obtain trace
element concentrations. For 82 of these 126 samples, platinum group elements (PGE) concentrations were
determined by ICP-MS after a nickel-sulphide fire assay pre-concentration step. Analytical protocols and quality
control data are reported by Fiorentini et al (2010). All the results of laboratory analyses are tabulated in
Electronic Supplementary Material Table 3 (ESM T3).
4.
Comparison between Laboratory XRF and pXRF analyses
Figure 1 (ESM Fig1) shows the results of the comparison between laboratory analyses and multiple pXRF
analyses on equivalent samples. Both pXRF and laboratory analyses concur on the composition of the Mount
Keith dacite and the Mount Keith komatiites. Small differences can still be observed between laboratory results
and pXRF analyses, such as elevated Ni, Cr and As in dacite for laboratory data compare to pXRF. These, in
average, more elevated values for laboratory analyses than for pXRF analyses are explained by sample
heterogeneity: in most of the dacite samples analysed for this study As and Ni are concentrated within nickel
sulfarsenides, and Cr is concentrated in chromite grains, which are very sparsely distributed within the rock.
Laboratory analyses will produce a result representative of a larger mass of rock, therefore taking in account the
heterogeneous distribution of the nickel sulfarsenides and chromite grains at sample scale. All of the results of
this comparison study are tabulated in Electronic Supplementary Material Table 4 (ESM T4).
5.
Tornado Maps
Samples analysed by pXRF and containing analyses highlighting anomalous concentrations in nickel were
mapped using micro X-Ray Fluorescence (μ-XRF). This technique allows the production of elemental maps
which are used to locate the source of the anomalous nickel concentrations and identify possible associations
with other elements. The instrument that was used to collect the data is a Bruker M4 TORNADO™ equipped
with a rhodium target X-ray tube operating at 50 kV and 500 nA and an XFlash® silicon drift X-Ray detector.
Analyses were then quantified using the Bruker ESPRIT software. Maps were created using a 25 μm spot size
on a 25 μm raster with dwell times 10-15 ms per pixel. Maps presented in the figures are represented as unquantified background-corrected peak height data for Kα peaks for each element.
6.
High-Resolution X-Ray Computed Tomography (HRXCT)
Sample MKTD23-303C was scanned in 3D to observe the distribution of sulphides and nickel sulfarsenides
within the sample. The sample was analysed using an XRADIA XRM High-resolution three-dimensional X-Ray
microscope system at the Australian Resources Research Centre (ARRC, Perth, Australia). The analytical
technique and data processing protocol is described in detail in Godel (2013). Parameters used for the analysis
of MKTD23-303C were a voltage of 110 kV, a power of 10 W and a pixel size of 5.96 μm. 2000 projections
were collected per 360° rotation of the sample.
7.
Electron Microprobe analyses
Electron microprobe analyses were carried out on the Cameca SX50 instrument at CSIRO in Perth. Microprobe
data were collected by wavelength dispersive spectrometry at an accelerating voltage of 15 Kv, beam current of
30 nA and counting times of 60 s per element (30 s peak 15 s background on each side).
8.
Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)

Laser Ablation ICP-MS (LA-ICP-MS)
The LA-ICP-MS instrument used for this study consists of an Agilent 7700x ICP-MS coupled with a Resonetics
RESolution M50-E, 193 nm Excimer (5ns Pulse Width) laser recently equipped with the S155 Dual Volume
Sample Cell. The laser lines (raster mode) were drilled across the mineral for 60s after measuring a gas blank
for 20s. Ablation was carried out using a helium carrier gas and was mixed with argon before entering the ICPMS. Compositions of various nickel sulfarsenides, mainly gersdorffite (NiAsS) grains, in the following isotopes
were determined: 34S, 57Fe, 59Co, 61Ni, 65Cu, 66,68Zn, 75As, 82Se, 95Mo, 101Ru, 103Rh, 105,108Pd, 109Ag, 111Cd, 118Sn,
121
Sb, 128Te, 185Re, 189Os, 193Ir, 195Pt, 197Au, 208Pb and 209Bi. Obtained detection limits are tabulated in Electronic
Supplementary Material Table 5a (ESM T5a).
Depending on the size of the mineral analysed, the size of the laser beam varied from 15 μm to 44 μm, with a
laser frequency of 15 Hz, a power of 5 mJ/pulse and a displacement speed of 4 μm/s. Laser lines (raster mode)
were drilled across minerals for 60 s after measuring a gas blank for 20 s. Each time beam sized was changed, a
new batch of standards and monitors was analysed with the appropriate beam size.
All nickel sulfarsenide grains analysed with LA-ICP-MS were analysed by electron microprobe before,
especially in order to know their composition in nickel, sulphur and iron. Electron microprobe analyses were
carried out on the Cameca SX50 instrument at CSIRO in Perth. Microprobe data were collected by wavelength
dispersive spectrometry at an accelerating voltage of 15 Kv, beam current of 30 nA and counting times of 60
seconds per element (30 s peak 15 s background on each side).
34
S was used as an internal standard to calculate the concentration of trace elements in the nickel sulfarsenide
grains. The calibration and quality control were carried out by using Laflamme-Po727, Mass-1, Mass-3, UMSS1
and JBMSS5 reference materials. These reference materials have been used in previous studies (Barnes et al.,
2008; Dare et al., 2010; Dare et al., 2011; Godel et al., 2012). A nickel blank was analysed at the beginning and
end of each analytical session to monitor and correct nickel interferences. Results obtained with the various
monitors, analysed as unknown samples for each run to check data quality, are shown in Electronic
Supplementary Material Table 5b (ESM T5b).
All the laser ablation ICP-MS data was reduced using the Iolite software by subtracting gas background from
each isotope analysed. Sulphur, As, Ni, Si and Ca were monitored to ensure that the mineral measured were
nickel sulfarsenides. The presence of accessory mineral Inclusions of accessory minerals within the nickel
sulfarsenides was monitored, and excluded from the time resolved spectrum before calculating the average
signal; their signals were not taken into account during the integration leading to the calculation of trace
elements concentrations in the mineral of interest.

LA-ICP-MS interferences
There is a possible interference on determining 101Ru in nickel arsenide which must be evaluated. This
interference is caused by argide (40Ar) interference with 61Ni on 101Ru. In order to correct for this interference, a
nickel blank was analysed at each run. Using the results obtained for 101Ru with this nickel blank, we could
account for the amount of 101Ru produced for 1% of Ni present in the mineral phase analysed. All the results
obtained were then corrected before calculation of trace elements concentrations. All the results obtained with
LA-ICP-MS are tabulated in Electronic Supplementary Material Table 5c (ESM T5c).
9.
3D Visualisation
All the geochemical results obtained with laboratory and pXRF analyses were combined with geological data,
and with BHP assay database of the Sarah’s Find area, and plotted spatially using Leapfrog® and GOCAD® 3D
modelling software. A model of the ore shells was produced with Leapfrog ® using a spheroidal interpolation of
nickel values above 2 wt% and incorporating a structural trend (orientation of the basal contact). A model of the
distribution of arsenic concentrations over the whole system was also produced with Leapfrog® using a
spheroidal interpolation associated with a structural trend defined using measurements of the main foliation
observed within the footwall dacite (dip direction of 154 and 25° dip). Interpolation of arsenic concentrations
was done using the ordinary kriging geostatistical method (Gaussian process regression), with the following
parameters: a nugget effect of 0.5, a resolution of 5 m for the ore shells and 10 m for the arsenic model, an
accuracy of 0.05, and a constant drift. Kriging is an interpolation technique in which the surrounding measured
values are weighted to derive a predicted value for an unmeasured location. Weights are based on the distance
between the measured points, the predicted locations, and the overall spatial arrangement among the measured
points. Kriging is based on regionalized variable theory, and is a technique which was named after the engineer
Danie G. Krige (1919-).
10. References
Barnes S-J, Prichard H, Cox R, Fisher P, Godel B (2008) The location of the chalcophile and siderophile
elements in platinum-group element ore deposits (a textural, microbeam and whole rock geochemical
study): Implications for the formation of the deposits Chemical Geology 248:295-317
Barnes SJ, Van-Kranendonk MJ (2014) Archean andesites in the east Yilgarn craton, Australia: Products of
plume-crust interaction? Lithosphere 6:80-92
Dare SAS, Dare SAS, Barnes S-J, Prichard H (2010) The distribution of platinum group elements (PGE) and
other chalcophile elements among sulfides from the Creighton Ni–Cu–PGE sulfide deposit, Sudbury,
Canada, and the origin of palladium in pentlandite Mineralium Deposita 45:765-793
Dare SAS, Dare SAS, Barnes S-J, Prichard H, Fisher P (2011) Chalcophile and platinum-group element (PGE)
concentrations in the sulfide minerals from the McCreedy East deposit, Sudbury, Canada, and the
origin of PGE in pyrite Mineralium Deposita 46:381-407
Fiorentini ML, Barnes SJ, Maier WD, Burnham OM, Heggie G (2010) Global Variability in the Platinum-group
Element Contents of Komatiites Journal of Petrology 52:83-112 doi:DOI: 10.1093/petrology/egq074
Godel B (2013) High-Resolution X-Ray Computed Tomography and Its Application to Ore Deposits: From
Data Acquisition to Quantitative Three-Dimensional Measurements with Case Studies from Ni-CuPGE Deposits Economic geology and the bulletin of the Society of Economic Geologists 108:20052019
Godel B, Gondzalez-Alvarez I, Barnes SJ, Barnes S-J, Parker P, Day J (2012) Sulfides and Sulfarsenides from
the Rosie Nickel Prospect, Duketon Greenstone Belt, Western Australia Economic Geology 107:275294 doi:DOI: 0361-0128/12/4012/275-20
Le Vaillant M, Barnes SJ, Fisher L, Fiorentini ML, Caruso S (2014) Use and calibration of portable X-Ray
fluorescence analysers: application to lithogeochemical exploration for komatiite-hosted nickel
sulphide deposits Geochemistry: Exploration, Environment, Analysis doi:DOI: 10.1144/geochem2012166
11. Figure Caption:
ESM Fig 1: Comparison of results obtained between laboratory XRF and portable XRF (pXRF) analyses for
komatiite and dacite samples. The central box is the middle 50% of the data from Q1 (bottom of the box which
represents the 25th percentile) to Q3 (top of the box which represents the 75th percentile). An outlier (circle) is
further than 1.5*(Q3-Q1) from the box. A far outlier (triangle) is further than 3.0*(Q3-Q1) from the box. The
Whiskers are the extreme values that are not outliers. The line in the middle of the box represents the median
and the black dot represents the mean
12. Table Captions:
ESM Table 1: Structural measurements collected at the Sarah’s Find deposit.
ESM Table 2a and 2b: Quality control on portable XRF data collected on felsic volcanic rocks. a) Detection
limits calculated for each element of interest, and b) instrumental precision obtained with the Desktop X500 XRay fluorescence analyser
ESM Table 2c: Results of portable XRF analyses from the Sarah’s Find deposit
ESM Table 3a: Results of laboratory geochemical analyses collected within the scope of the project.
ESM Table 3b: Calculated correlation coefficients using results from laboratory analyses of Mount Keith dacite
samples.
ESM Table 4: Results of the comparison study between laboratory XRF and portable XRF.
ESM Table 5a: Detection limits for LA-ICP-MS for various beam size for 109Ag, 101Ru, 103Rh, 105,108Pd, 189Os,
193
Ir, 195Pt, 197Au, 118Sn, 121Sb and 82Se.
ESM Table 5b: Results obtained for in house reference material used as monitors.
ESM Table 5c: Results of in-situ LA-ICP-MS analyses on Ni-Co sulfarsenides from the Sarah’s find deposit.