Copper concentrations in silicate mineral phases of the Boggy plain zoned intrusion

Jung Woo Park and Ian H. Campbell

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Understanding copper geochemistry in an evolving magmatic system is important because it may give the information about the relationship between copper mineralization and the magma and the variation of physical and geochemical condition, e.g. oxidation state and sulfur and water contents, of the magma.

Copper is a distinctively chalcophile element and therefore it strongly partitions into sulfide melts if the magma is S-saturated. However, depending on the physical and chemical condition of magma, dissolved copper in the magma as Cu⁺¹ or Cu⁺² can be trapped in a silicate mineral as an intrinsic element. Cu⁺² is likely to be substituted for Fe⁺² or Mn⁺² in Fe-Mg silicates and Cu⁺¹ for Na⁺¹ in feldspar or hornblende because of their similar charge and radius (Stanton, 1994). Ewart et al. (1973) obtained 11-53 ppm copper in pyroxenes and 0-27 ppm in plagioclases of the Tonga-Kermadec volcanic island chain and found the copper levels in the plagioclases have positive correlation with whole-rock copper concentration. Wedepohl (1974) lists mean copper contents of 9.4-18, 9-17 and 2-45 ppm in amphibole, biotite and plagioclase of granitic rocks respectively.

Copper concentrations in silicate minerals were analyzed by LA-ICPMS in order to document variations in copper contents in the selected minerals during differentiation of the Boggy Plain magma, and their relationship with whole rock copper concentration.

Copper concentration in plagioclase are slightly higher in the mafic rocks of FeO^t+MgO>14.5 per cent, and copper contents of the northern gabbro which contains the highest whole rock copper concentration is distinctively enriched in copper compared with the felsic rocks of Boggy Plain zoned pluton (Figure 1). Except for several anomalously high copper values, the majority of them of plagioclases from outer/inner granodiorite and adamellite (FeO^t+MgO<14.5 %) are lower in order of 2-5 than them of mafic rocks. The mean copper contents of intermediate-felsic rocks range from 0.2 to 1 ppm showing a flat trend along with further magma differentiation. The Average values of copper concentration in plagioclase of Boggy Plain zoned pluton (0.18-2.2 ppm) is significantly less than that of the El Abra-Pajonal suite intrusion (0.4-69.1 ppm; Dianne 2008). This difference between the two suites can be partly explained by higher whole rock copper contents (12.5-5493 ppm) of the El Abra-Pajonal suite intrusion.

In order to estimate the amount of copper hosted by silicates phase, copper concentrations in other major silicate minerals are measured and mass balanced. This analysis yields silicate selective copper abundances ranging from 0.5 to 7.3 percent of whole-rock copper abundance. It indicates that the subtraction of copper by silicate crystallization is insufficient for the observed decrease of whole-rock copper in Boggy Plain rocks of FeO^t+MgO<14.5 per cent and the additional mechanism is required, e.g. the formation of Cu-bearing sulfide or Cu loss in vapor phase.

 

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Ewart F, Bryan W, Gill, J. (1973) Mineralogy and geochemistry of the younger volcanic islands of Tonga, SW Pacific. Journal of Petrology 14: 429-465
Dianne LV (2008) The geology, geochemistry and geochronology of the El Abra mine, Chile, and the adjacent Pajonal-El Abra suite of intrusions, Unpublished Ph.D. thesis, The Australian National University, 777 p.
Stanton RL (1994) Copper. In Ore elements in arc lavas: New York, Oxford University Press, p. 53-74.
Wedepohl, K.H. (1974). Copper. In Handbook of geochemistry (ed. K. H. Wedepohl). Springer-Verlag, Berlin.