Mineral chemistry

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)

During LA-ICP-MS analyses, a laser is used to ablate a small pit on the surface of the mineral or material being measured. The ablated material forms an aerosol of condensed particles which is transported by a gas stream, usually He, to the ICP instrument. The ICP breaks down the particles into ions. These ions are then introduced to the mass spectrometer where they are separated into isotopes, and sequentially counted by a detector. Concentration is determined by calibrating detector counts with elements of known concentration in the sample and within bracketing reference materials. The reference materials are also used periodically during the analysis sequence to correct for any instrument drift over time.

Which elements are measured?

From 20 to 40 elements are routinely determined during each spot analysis, depending on the application. Specific element lists can be provided on request. The preparation of samples for analyses can be either mineral separates mounted in an epoxy resin button or 25mm round mounts made from rock samples for in situ mineral analyses. Thin sections are not recommended for analyses by this method due to the laser drilling through the sample into the glass slide.

The application of trace-element mineral chemistry to exploration

In a single mineral type, measurable trace element variation has been observed between the distal and proximal parts of the alteration halo in fertile mineralising systems. Such studies are dominantly available for porphyry systems (Cooke et al., 2014; Wilkinson et al., 2017; Uribe- Mogollon and Haher., 2020) but similar changes in mineral chemistry would be expected in other hydrothermal ore systems where alteration cells exceed the visible alteration distribution (e.g., using pyrite chemistry for exploration for SEDEX deposits; Mukherjee and Large, 2019) or where similar alteration systems are associated with both fertile and infertile systems.

One example from Cooke et al. (2014) describes the changes in epidote chemistry with distance from a porphyry deposit and how these changes can be used as a vectoring tool outside the pyrite halo. The importance of this new tool is that the pyrite halo around a porphyry system is easily identified using traditional geophysical and geochemical exploration tools, but the more distal “green alteration” zone has been much less useful as a regional exploration tool (Cooke et al., 2014). The ability to identify if epidote is part of a distal alteration assemblage around a fertile porphyry system, and in which direction the porphyry is expected, allows for more wide spaced sampling at early stages of exploration. This is particularly important where large parts of a porphyry system may be hidden under post-mineralisation cover and exposed portions of the system can be used to vector towards exploration targets.

Uribe-Mogollon and Maher (2020) have similarly identified variations in white mica associated with early phyllic alteration from mineralised and unmineralised to weakly mineralised porphyry systems. One tool used to quantify changes in trace-element composition of the white mica was LA-ICP-MS. The authors were able to characterise the trace-element changes in the white micas with fertility which could be used during exploration to discriminate between porphyry systems. Other mineral chemistry changes that have been identified in porphyry systems include zircon (Ballard et al., 2002), apatite (Bouzari et al., 2016), plagioclase (Williamson, et al., 2016), and magnetite (Dupuis and Beaudoin, 2011).

References & further reading

Ballard, J.R., Palin, J.M. & Campbell, I.H. 2002. Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: Application to porphyry copper deposits of northern Chile. Contributions to Mineralogy and Petrology, 144, 347–364, https://doi.org/10.1007/s00410-002- 0402-5

Bouzari, F., Hart, C.J.R., Bissig, T., Barker, S., 2016. Hydrothermal Alteration Revealed by Apatite Luminescence and Chemistry: A Potential Indicator Mineral for Exploring Covered Porphyry Copper Deposits. Economic Geology. Vol. 111. Pp. 1397-1410.

Cooke, D.R., Agnew, P., Hollings, P., Baker, M., Chang, Z., Wilkinson, J.J., Ahmed, A., White, N.C., Zhang, L., Thompson, J., Gemmel, J.B., Danyushevsky, L., and Chen, H., 2020. Recent advances in the application of mineral chemistry to exploration for porphyry copper-gold- molybdenum deposits: detecting the geochemical fingerprints and footprints of hypogene mineralization and alteration. Geochemistry: Exploration, Environment, Analysis. https://doi.org/10.1144/geochem2019-039

Cooke, D.R., Baker, M., Hollings, P., Sweet, G., Chang, Z., Danyushevsky, L., Gilbert, S., Zhou, T., White, N.C., Gemmell, B.J., and Inglis, S. 2014. Chapter 7: New Advances in Detecting the Distal Geochemical Footprints of Porphyry Systems- Epidote Mineral Chemistry as a Tool for Vectoring and Fertility Assessments. Society of Economic Geologists, Inc. Special Publication 18, pp. 127-152.

Dupuis, C. & Beaudoin, G. 2011. Discriminant diagrams for iron oxide trace element fingerprinting of mineral deposit types. Mineralium Deposita, 46, 319–335, https://doi.org/10.1007/s00126-011-0334-y

Hannington, M.D., Kjarsgaard, I.M., Galley, A.G., and Taylor, B., 2003. Mineral-chemical studies of metamorphosed hydrothermal alteration in the Kristineberg volcanogenic massive sulfide district, Sweden. Mineralium Deposita, issue 38, pp423-442.

Mukherjee I, Large R., 2019. Application of pyrite trace element chemistry to exploration for SEDEX style Zn-Pb deposits: McArthur Basin, Northern Territory, Australia. Ore Geology Reviews, Volume 81, Pages 1249-1270

Uribe-Mogollon, C. and Maher, K., 2020. White Mica Geochemistry: Discriminating Between Barren and Mineralized Porphyry Systems. Economic Geology. Society of Economic Geologists, Inc. vol. 115, no. 2, pp. 325-354.

Wilkinson, J.J., Baker, M., Cooke, D.R., Wilkinson, C.C. & Inglis, S. 2017. Exploration targeting in porphyry Cu systems using propylitic mineral chemistry: a case study of the El Teniente deposit, Chile. In Mineral Resources to Discover: Society of Geology Applied to Ore Deposits, 14th Biennial Conference Proceedings, Quebec, 3, 1112–1114.

Williamson, B.J., Herrington, R.J., and Morris, A. 2016. Porphyry copper enrichment linked to excess aluminium in plagioclase. Nature Geoscience; Advanced online publication. Macmillan Publishers Limited. DOI: 10.1038/NGEO2651 Repeat as necessary