indicator minerals for magmatic sulfide mineralisation · magmatic sulfide deposits it has the...

AEGC 2019: From Data to Discovery – Perth, Australia 1

Indicator minerals for magmatic sulfide mineralisation Louise Schoneveld* Steve Barnes Margaux LeVaillant CSIRO Mineral Resources CSIRO Mineral Resources CSIRO Mineral Resources ARRC, Kensington WA 6151 ARRC, Kensington WA 6151 ARRC, Kensington WA 615 [email protected] [email protected] [email protected] Valentina Taranovic CSIRO Mineral Resources ARRC, Kensington WA 6151 [email protected]

INTRODUCTION

Small mafic-ultramafic intrusions are ubiquitous and are a common host to magmatic Ni-Cu-PGE deposits. Unfortunately, the likelihood of any one of these small, differentiated bodies to contain a significant proportion of sulfide is slim, with difference between mineralised and un-mineralised intrusions thus far not being reliably distinguishable by any remote sensing or sampling method. In this study we aim to determine a set of possible textural, mineralogical and chemical indications that an intrusion may be likely to contain sulfides. This will aid in a focused and precise exploration attempt with resources focussed on intrusions that show signs of sulfide interaction. It has been postulated that the ruthenium content of chromite may be an indicator of sulfide interaction (Locmelis et al., 2018). In this study we will test this hypothesis on a number of economically mineralised intrusions using LA-ICP-MS. Further, we will investigate the commonalities between these mineralised intrusions in comparison to their similarly located, seemingly un-mineralised counterparts.

The magmatic sulfide deposits investigated in this study are: • Rytky (Kotalahti belt), Finland • Ntaka Hill, Africa • Noril’sk-Talnakh, Russia • Aguablanca, Spain • Huangshanxi (Central Asian Orogenic Belt), China • Xarihamu, China • Nova-Bollinger, Australia

We found that both the clinopyroxene and orthopyroxene in the rocks that host the mineralisation have complex zonation patterns; notably in chromium, which is rarely found to exhibit sector zoning behaviour (Ubide et al., 2019). Thus far, the pyroxene investigated from un-mineralised intrusions show no such patterns.

METHOD Multiple experiments on the XFM beamline at the Australian Synchrotron, operated by ANSTO were carried out between 2015 and 2019. The common method used in these experiments is a pixel size of 4 * 4 μm with a movement speed of 5 mm/s and a dwell of 0.8 msec, though this method varied depending on target area and available time. The images were collected on a Maia detector using the Kirkpatrick Baez mirror microprobe end-station. This experiment used monochromatic 2 µm beam spot size at an energy of 18500 eV and a new method at 7050 eV; below the Fe-edge and focussing on the Cr-zoning. Equipped with the Maia 384 detector array, the XFM beamline can acquire data at 2 µm resolution from 384 detectors simultaneously over areas of several square centimetres with count rates of 4–10 M/s. These spectra are then processed by the GeoPIXE software into element concentrations represented as maps of quantified element concentrations based on standardless correction of raw count data (Kirkham et al., 2010; Ryan et al., 2014). The major element data obtained via the synchrotron XRF is in good agreement with the data obtained by energy dispersive spectroscopy (EDS) via scanning electron microscopy. Additional images were collected using the lab-based Maia Mapper system and the widely available Bruker Tornado desktop XRF mapper to compare the synchrotron results with more widely available technologies. The trace elements were collected using a Photonmachines, ATLex 300si-x Excite 193nm Excimer ArF laser with samples in a Helix-II sample cell. The He carrier gas was set at 0.6 L/min in both the cup and cell (1.2 L/min total) and the sample travels through 4m of tubing to a Meinhard mixing chamber where it is mixed with 0.7 L/min of Ar before being analysed in an Agilent 7700 ICP-MS. The plasma conditions

SUMMARY Small conduit or chonolith style intrusions dominated by olivine- and pyroxene-rich cumulates are well known to be favourable hosts to magmatic Ni-Cu-(PGE) sulfide mineralization. In many cases, such mineralization is closely associated with partially assimilated country rock xenoliths, volatile-enriched vari-textured or taxitic rocks and other evidence for assimilation of country rocks. We used the technique of microbeam XRF element mapping along with LA-ICP-MS to reveal other distinctive features and chemistries of a number of diverse mineralized small intrusions: transient saturation in Cr-rich spinel; complex zoning patterns of Cr in cumulus and poikilitic pyroxenes, and occasional development of dendritic growth textures in olivine. We further suggest that these features are indicative of dynamic assimilation of conduit wall rocks accompanied by rapid, disequilibrium fluctuations in silica content and redox state, and that these features may be if not diagnostic then at least indicative of Ni-Cu sulfide mineral potential in magmatic conduit systems. Key words: chromite, spinel, pyroxene, LA-ICP-MS, Synchrotron

Indicator minerals for magmatic sulfide mineralisation Schoneveld et al.


were optimized daily, to obtain highest counts with oxide production (248ThO/232Th) remaining below 0.4%. The spot size is generally 50 μm and line analysis are 50 μm circle with a movement speed of 10 μm/s and a laser repetition rate of 9 Hz. Blocks of standards were measured at the beginning and end of the run, as well as between every ~10 unknown analyses. 30 seconds of background was collected at the start of each line analysis before 260 shots of sample (~30 seconds). Data was reduced using Iolite software (Paton et al., 2011). The standards were chosen to be as similar as possible to the analyte and contain all of the elements of interest; with a choice of NIST610, USGS GSD-2g, USGS MASS-1, or UQAC FeS-1. Where necessary, the argide interference on the PGEs were corrected using the signals generate from ablating nickel, copper and cobalt metals.

RESULTS AND DISCUSSION Complex zoning of pyroxene The XRF images collected from the Australian Synchrotron were stacked in 3 colour (RGB) images to compare the relative concentrations of the elements. The colour scheme in Figure 1 shows chromium scaled logarithmically in red, with iron and calcium scaled linearly in green and blue respectively. This causes clinopyroxene [Ca(Mg,Fe)Si2O6] to be displayed in pink (Cr-rich) to purple (Cr-poor) while orthopyroxene [(Mg,Fe)SiO3] is orange (Cr-rich) to green/brown (Cr-poor) in colouration. Other key phases are olivine in dark green, (iron) sulphides in very bright green and plagioclase in dark blue. The zonation within both clinopyroxene and orthopyroxene are complex but can be separated into three distinct types; 1) abrupt zoning; change in concentration of chromium with a sharp boundary 2) sector zoning; hourglass style zonation 3) oscillatory zoning; zoning of small scale that are usually cyclic. Zoning of all three types can be present in a single grain ( Figure 1b). Although the following images were collected by synchrotron XRF mapping, the same zonation is visible with lab based Maia mapper technology and can be observed by widely available desktop XRF mapping tools. In some cases, high Cr contents are reflected in fine-scale lamellar exsolution of Cr-rich spinel, such that Cr zoning may be visible in thin section. Sector Zoning Within the core of the pyroxene it is common to observe hourglass shaped sector zoning ( Figure 1). This type of zonation is common in natural terrestrial (Barkov and Martin, 2015; Downes, 1974; Hollister and Gancarz, 1971; Leung, 1974; Welsch et al., 2016), lunar (Hargraves et al., 1970; Hollister et al., 1971) and experimental (Kouchi et al., 1983; Lofgren et al., 2006; Schoneveld, 2017) clinopyroxene but is rarely noted in orthopyroxene (Schwandt and McKay, 2006). These sector zones represent differences in chemistry of a crystal due to kinetic effects at the growth surface rather than changes in chemistry or conditions of the surrounding magma (Hollister and Gancarz, 1971). Though sector zoning is commonly observed as variations in Mg-Si and Ti-Al pairs (Ubide et al., 2019), chromium is the most notable tracer of the zoning patterns in these sample.

Figure 1. X-ray Fluorescence (XRF) images collected from the Australian Synchrotron of zoned pyroxenes in Ni-Cu magmatic sulfide deposits from around the world. Stacked false colour (RGB) image of logged chromium concentration in red, iron in green and calcium in blue. Clinopyroxene [Ca(Mg,Fe)Si2O6] is displayed in pink (Cr-rich) to purple (Cr-poor) while orthopyroxene [(Mg,Fe)SiO3] is orange (Cr-rich) to green/brown (Cr-poor) in colouration. Mineral abbreviations: cpx – clinopyroxene, ol – olivine, opx – orthopyroxene, sul – sulfide With clinopyroxene, the (010) crystal face exposes the M1 and T sites while on the (100) face either the M1 or the T are exposed (Hollister and Gancarz, 1971). This different



exposure of the vacant sites on different faces of the clinopyroxene causes the partition coefficients on each face to be different. This variation in chemistry on different growth faces is indicative of a crystallisation rate faster than the rate of lattice diffusion (Watson and Liang, 1995). Abrupt zoning The more common type of abrupt zonation is chromium rich cores which zone abruptly to chromium poor rims ( Figure 1), however the opposite can be true in the “reverse” zoned pyroxene such as those from Huangshanxi, China ( Figure 1c). This type of zoning is indicative of two stage growth. As the sector zoning tends to cease in the outer rims, this could indicate either; a change in growth rate of the clinopyroxene or a change in chemistry of the magma, where the budget of chromium has been exhausted or the redox state of chromium has changed to a less compatible speciation (i.e Cr2+ over Cr3+). Oscillatory zoning Additionally, there is oscillatory zoning visible when examined at high spatial resolution ( Figure 1). These oscillations in chemistry occur in both the cores and rims of both ortho- and clinopyroxene. There are two possible causes for the formation of oscillations of chemistry during crystal growth as outlined by Fowler and Shore (1996); 1) extrinsic causes such as changes in pressure (P), temperature (T) or magma composition (X) or 2) intrinsic causes such as the diffusion rate of cations in the melt immediately adjacent to the growing crystal. Fowler and Shore (1996) suggests that oscillatory zoning is favoured in silicates of “moderately rapid growth” i.e. growing between 10-11 and 10-13 m/s or 0.06-6 μm per week. This requires the pyroxene in Rytky ~10-1,000 years to grow the outer, oscillated rims of the pyroxene. Trace Elements in Chromite As chromite is a resistate mineral that occurs widely in magmatic sulfide deposits it has the potential to be a relevant indicator mineral. Locmelis et al. (2018) suggested that ruthenium content in chromite may be a reliable indicator of sulfide interaction. Due to the extremely high partition coefficient between sulfide and the platinum group elements, the chromite that forms from magma that has interacted with sulfide will have ruthenium values of <150 ppb. This concept will be thoroughly tested on the above deposits. Furthermore we will investigate the other trace element signatures in chromite that could indicate sulfide interactions. The trace elements in the chromite from the Noril’sk deposit have been thoroughly investigated and found to have two chemical populations; 1) chromites trapped with the pyroxene were formed early and have high chromium concentrations and 2) a second population associated with the sulfide that is enriched in iron toward more magnetite compositions. These major element variations are also paired with trace element variations, such as Co and Zn being more enriched in the more primitive chromite; trapped inside the pyroxene while Ni enrichment is associated with the more iron-rich spinel.

These population of chromite mark the evolving chemistry of the deposit during crystallisation. Comparing the similarities between Noril’sk and the other samples from magmatic sulfide deposits around the world will help to determine if there is an indicative change in chemistry when sulfides are present.

Figure 2. The major element composition of the Noril’sk chromite and their enclosing phases. Data collected from this study and from Barnes and Kunilov (2000)

CONCLUSIONS

This work gives plausible indicator minerals for magmatic sulfide deposits in the form of complexly zoned pyroxene and wide ranges in chromian spinel compositions.

ACKNOWLEDGEMENTS This research was undertaken on the X-ray fluorescence microscopy beamline at the Australian Synchrotron, part of ANSTO.

REFERENCES Barkov, A. Y., and Martin, R. F., 2015, Anomalous Cr-rich Zones in Sector-zoned Clinopyroxene Macrocrysts in Gabbro, Mont Royal, Montreal, Quebec, Canada: The Canadian Mineralogist, v. 53, no. 5, p. 895-910.

Barnes, S. J., and Kunilov, V. Y., 2000, Spinels and Mg Ilmenites from the Noril'sk 1 and Talnakh Intrusions and Other Mafic Rocks of the Siberian Flood Basalt Province: Economic Geology, v. 95, no. 8, p. 1701-1717.

Downes, M. J., 1974, Sector and oscillatory zoning in calcic augites from M. Etna, Sicily: Contributions to Mineralogy and Petrology, v. 47, no. 3, p. 187-196.

Fowler, A. D., and Shore, M., 1996, Oscillatory zoning in minerals; a common phenomenon: The Canadian Mineralogist, v. 34, no. 6, p. 1111-1126.



Hargraves, R. B., Hollister, L. S., and Otalora, G., 1970, Compositional zoning and its significance in pyroxenes from three coarse-grained lunar samples: Science, v. 167, no. 3918, p. 631-633.

Hollister, L. S., and Gancarz, A. J., 1971, Compositional sector-zoning in clinopyroxene from the Narce area, Italy: The American Mineralogist, v. 56, p. 959.

Hollister, L. S., Trzcienski, W. E., Jr., Hargraves, R. B., and Kulick, C. G., 1971, Petrogenetic significance of pyroxenes in two Apollo 12 samples: Geochimica et Cosmochimica Acta, no. 2, p. 529-557.

Kirkham, R., Dunn, P. A., Kuczewski, A. J., Siddons, D. P., Dodanwela, R., Moorhead, G., Ryan, C. G., De Geronimo, G., Beutenmuller, R., Pinelli, D., Pfeffer, M., Davey, P., Jensen, M., Paterson, D. J., De Jonge, M. D., Howard, D. L., Kusel, M., and McKinlay, J., 2010, The Maia Spectroscopy Detector System: Engineering for integrated pulse capture, low-latency scanning and real-time processing. Kouchi, A., Sugawara, Y., Kashima, K., and Sunagawa, I., 1983, Laboratory growth of sector zoned clinopyroxenes in the system CaMgSi2O6-CaTiAl2O6: Contributions to Mineralogy and Petrology, v. 83, no. 1, p. 177-184.

Leung, I. S., 1974, Sector-Zoned Titanaugites Morphology, Crystal Chemistry, and Growth: American Mineralogist, v. 59, no. 1, p. 127-138. Locmelis, M., Fiorentini, M. L., Barnes, S. J., Hanski, E. J., and Kobussen, A. F., 2018, Ruthenium in chromite as indicator for magmatic sulfide liquid equilibration in mafic-ultramafic systems: Ore Geology Reviews, v. 97, p. 152-170.

Lofgren, G. E., Huss, G. R., and Wasserburg, G. J., 2006,An experimental study of trace-element partitioning between Ti-Al-clinopyroxene and melt: Equilibrium and kinetic effects including sector zoning: American Mineralogist, v. 91, no. 10, p. 1596-1606.

Paton, C., Hellstrom, J., Paul, B., Woodhead, J., and Hergt, J., 2011, Iolite: Freeware for the visualisation and processing of mass spectrometric data: Journal of Analytical Atomic Spectrometry, v. 26, p. 2508-2518. Ryan, C., Siddons, P., Kirkham, R., Li, Z. Y., de Jonge, M. D., Paterson, D., Kuczewski, T., Howard, D., Dunn, P., Falkenberg, G., Boesenberg, U., De Geronimo, G., Fisher, L., Halfpenny, A., Lintern, M., Lombi, E., Dyl, K., Jensen, M., Moorhead, G., Cleverley, J., Hough, R., Godel, B., Barnes, S., James, S., Spiers, K., Alfeld, M., Wellenreuther, G., Vukmanovic, Z., and Borg, S., 2014, Maia X-ray fluorescence imaging: Capturing detail in complex natural samples, IOP Publishing, Institute of Physics, UK.

Schoneveld, L. E., 2017, Partitioning of trace elements between plagioclase, clinopyroxene and melt [Doctor of Philosophy: the Australian National University.

Schwandt, C. S., and McKay, G. A., 2006, Minor- and trace-element sector zoning in synthetic enstatite: American Mineralogist, v. 91, no. 10, p. 1607-1615.

Ubide, T., Mollo, S., Zhao, J.-x., Nazzari, M., and Scarlato, P., 2019, Sector-zoned clinopyroxene as a recorder of magma history, eruption triggers, and ascent rates: Geochimica et Cosmochimica Acta.

Watson, E. B., and Liang, Y., 1995, A simple model for sector zoning in slowly grown crystals: Implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks: American Mineralogist, v. 80, p. 1179-1187.

Welsch, B., Hammer, J., Baronnet, A., Jacob, S., Hellebrand, E., and Sinton, J., 2016, Clinopyroxene in postshield Haleakala ankaramite: 2. Texture, compositional zoning and supersaturation in the magma: Contributions to Mineralogy and Petrology, v. 171, no. 1, p. 1-19.

indicator minerals for magmatic sulfide mineralisation · magmatic sulfide deposits it has the...

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