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2. Mineral indicators of porphyry Cu fertility

Author

Loader, Matthew and Wilkinson, Jamie

Subjects
552
Abstract

'Fertile' magmas which generate metal-rich porphyry Cu deposits are thought to form by similar processes to normal 'barren' arc magmas which are unrelated to mineralisation. However, fertile magmas contain distinctive trace element signatures, which highlight the processes by which they achieved high ore-forming potential. These 'indicators of fertility' make potentially useful tools to be used in the exploration of porphyry systems and may be amplified in minerals within these rocks. Intrusive rocks were collected from the Resolution and Oyu Tolgoi porphyry Cu deposits. A combination of whole rock geochemistry, and trace element analyses of apatite and zircon were carried out, and these data were compared to literature data for barren rocks. Rocks from both systems indicators of fertility, some of which (especially Sr/Y) were found to be susceptible to feldspar-destructive hydrothermal alteration. The presence of these anomalies (e.g. La/Yb, Eu/Eu*, V/Sc) is indicative of garnet control on deep crustal fractionation, which may occur due to loaclised increases in melt H2O. Zircons from Oyu Tolgoi show that Eu/Eu* and Ce4+/Ce3+ ratios in zircon were strongly controlled by the co-crystallisation of titanite, limiting their utility as measures of redox. Ce/Ce* may provide a better measure of melt redox in zircon, but is difficult to measure. Eu/Eu* may still be useful for distinguishing barren and fertile systems. Igneous apatite has high Sr and Eu/Eu* compared to barren systems. Hydrothermal apatite from a number of porphyry systems were found to contain uniquely high Fe, Mn, Pb, Sr, and Eu/Eu*, which possibly relate to their precipitation from metal rich brines. The magnitude of Mn enrichment may also be sensitive to bulk rock Cu content. These results highlight the potential of apatite and zircon as mineral indicators of porphyry fertility, and recommendations are made for how they may be used in exploration.

Published
2018
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9. Mineral indicators of porphyry Cu fertility

Author

Loader, Matthew, Wilkinson, Jamie, and Rio Tinto Exploration

Abstract

‘Fertile’ magmas which generate metal-rich porphyry Cu deposits are thought to form by similar processes to normal ‘barren’ arc magmas which are unrelated to mineralisation. However, fertile magmas contain distinctive trace element signatures, which highlight the processes by which they achieved high ore-forming potential. These ‘indicators of fertility’ make potentially useful tools to be used in the exploration of porphyry systems and may be amplified in minerals within these rocks. Intrusive rocks were collected from the Resolution and Oyu Tolgoi porphyry Cu deposits. A combination of whole rock geochemistry, and trace element analyses of apatite and zircon were carried out, and these data were compared to literature data for barren rocks. Rocks from both systems indicators of fertility, some of which (especially Sr/Y) were found to be susceptible to feldspar-destructive hydrothermal alteration. The presence of these anomalies (e.g. La/Yb, Eu/Eu*, V/Sc) is indicative of garnet control on deep crustal fractionation, which may occur due to loaclised increases in melt H2O. Zircons from Oyu Tolgoi show that Eu/Eu* and Ce4+/Ce3+ ratios in zircon were strongly controlled by the co-crystallisation of titanite, limiting their utility as measures of redox. Ce/Ce* may provide a better measure of melt redox in zircon, but is difficult to measure. Eu/Eu* may still be useful for distinguishing barren and fertile systems. Igneous apatite has high Sr and Eu/Eu* compared to barren systems. Hydrothermal apatite from a number of porphyry systems were found to contain uniquely high Fe, Mn, Pb, Sr, and Eu/Eu*, which possibly relate to their precipitation from metal rich brines. The magnitude of Mn enrichment may also be sensitive to bulk rock Cu content. These results highlight the potential of apatite and zircon as mineral indicators of porphyry fertility, and recommendations are made for how they may be used in exploration. Open Access

Published
2017
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10. Multi-stage arc magma evolution recorded by apatite in volcanic rocks.

Author

Nathwani, Chetan L., Loader, Matthew A., Wilkinson, Jamie J., Buret, Yannick, Sievwright, Robert H., and Hollings, Pete

Subjects
*VOLCANIC ash, tuff, etc., *APATITE, *MAGMAS, *ORE deposits, *PLAGIOCLASE, *TRACE elements
Abstract

Protracted magma storage in the deep crust is a key stage in the formation of evolved, hydrous arc magmas that can result in explosive volcanism and the formation of economically valuable magmatic-hydrothermal ore deposits. High magmatic water content in the deep crust results in extensive amphibole ± garnet fractionation and the suppression of plagioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of central Chile. These rocks formed in a magmatic cycle that culminated in high-Sr/Y magmatism and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu*, and Mg in apatite to track discrete stages of arc magma evolution. We apply fractional crystallization modeling to show that early-crystallizing apatite can inherit a high-Sr/Y and high-Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modeling shows that crystallization of the in situ host-rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk-magma chemistry. Understanding this decoupling behavior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies. [ABSTRACT FROM AUTHOR]

Published
2020
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11. Abstracts from the 2017–2018 Mineral Deposits Studies Group meeting

Author

Santoro, L., primary, Yav, St. Tshipeng, additional, Pirard, E., additional, Kaniki, A., additional, Arfè, G., additional, Mondillo, N., additional, Boni, M., additional, Joachimski, M., additional, Balassone, G., additional, Mormone, A., additional, Cauceglia, A., additional, Robb, W., additional, Smith, T. L., additional, Currie, David, additional, Stuart, Finlay, additional, Faithfull, John, additional, Boyce, Adrian, additional, Chelle-Michou, C., additional, Cretella, S., additional, Scognamiglio, G., additional, Tarallo, M., additional, Putzolu, F., additional, Pirajno, F., additional, Aitbaeva, Saltanat, additional, Mizernaya, Marina, additional, Dyachkov, Boris, additional, Martin, Andrew J, additional, McDonald, Iain, additional, MacLeod, Christopher J, additional, McFall, Katie, additional, Prichard, Hazel M, additional, Jenkin, Gawen R T, additional, Kennedy, B., additional, McDonald, I., additional, Tanner, D., additional, Longridge, L., additional, Borst, A. M., additional, Finch, A. A., additional, Friis, H., additional, Horsburgh, N. J., additional, Gamaletsos, P. N., additional, Goettlicher, J., additional, Steininger, R., additional, Geraki, K., additional, Cloutier, Jonathan, additional, Piercey, Stephen J., additional, Allen, Connor, additional, Storey, Craig, additional, Darling, James, additional, Lasalle, Stephanie, additional, Dobrzanski, A., additional, Kirstein, L., additional, Walcott, R., additional, Butler, I., additional, Ngwenya, B., additional, Dobrzanski, Andrew, additional, Howard, Simon, additional, Troalen, Lore, additional, Davidson, Peter, additional, Walcott, Rachel, additional, Drummond, Drew, additional, Blakeman, Robert, additional, Ashton, John, additional, Marquis, Eva, additional, Goodenough, Kathryn, additional, Estrade, Guillaume, additional, Smith, Martin, additional, Zygouri, E., additional, Kilias, S. P., additional, Zack, T., additional, Pitcairn, I., additional, Chi Fru, E., additional, Nomikou, P., additional, Argyraki, A., additional, Ivarsson, M., additional, Finch, Adrian A., additional, Borst, Anouk M., additional, Hutchison, William, additional, Horsburgh, Nicola J., additional, Andersen, Tom, additional, Simonsen, Siri, additional, Waizy, Hamidullah, additional, Moles, Norman, additional, Hollis, Steven P., additional, Menuge, Julian F., additional, Doran, Aileen L., additional, Dennis, Paul, additional, Davidheiser-Kroll, Brett, additional, Marca, Alina, additional, Wilkinson, Jamie, additional, Güven, John, additional, Cooper, Mark R., additional, Stephen Daly, J., additional, Turner, Oakley, additional, McConnell, Brian, additional, Hughes, Hannah S. R., additional, Matusiak-Małek, Magdalena M., additional, Williamson, Ben, additional, Williams, James, additional, Dishaw, Guy, additional, Rees, Harri, additional, Key, Roger, additional, Bate, Simon, additional, Moore, Andy, additional, Tanner, Dominque, additional, Keith, Manuel, additional, Haase, Karsten M., additional, Smith, Daniel J., additional, Klemd, Reiner, additional, Schwarz-Schampera, Ulrich, additional, Bach, Wolfgang, additional, Walding, Sam J, additional, Jenkin, Gawen RT, additional, James, Daniel, additional, Clark, David, additional, Hart-Madigan, Lisa, additional, Armstrong, Robin, additional, Graham, Hugh, additional, Smith, Daniel J, additional, Abbott, Andrew P, additional, Holwell, David A, additional, Zygouri, Eva, additional, Harris, Robert C, additional, Stanley, Christopher J, additional, Grant, Hannah L.J., additional, Hannington, Mark D., additional, Petersen, Sven, additional, Frische, Matthias, additional, Zhang, Fei, additional, Williamson, Ben J., additional, Hughes, Hannah, additional, Smiles, Joshua, additional, Nathwani, Chetan, additional, Sievwright, Robert, additional, Loader, Matthew, additional, Blanks, Daryl E., additional, Holwell, David A., additional, Smith, W.D., additional, Darling, J.R., additional, Bullen, D.S., additional, Scrivener, R.C., additional, Boyce, Adrian J., additional, Broom-Fendley, Sam, additional, Brady, Aoife E, additional, Hudson-Edwards, Karen, additional, Hollis, Steve, additional, McClenaghan, Sean, additional, Doran, Aileen, additional, Fallon, Emily K., additional, Brooker, Richard, additional, and Scott, Thomas, additional

Published
2018
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