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The Geology, Geochemistry and Geochronology of the El Abra Mine, Chile, and the adjacent Pajonal-El Abra suite of intrusions

Dianne L. Valente 1 , Ian H. Campbell 1 , Charlotte M. Allen 1

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

 

El Abra is a classic, yet simple porphyry copper deposit. World-class exposure of the complete suite of weakly altered, plutonic rocks directly associated with the ore-body provides a unique opportunity to examine the accepted paradigm of the relationship between igneous activity and copper porphyry formation.

Results of work completed to date indicate that the El Abra-Pajonal suite represents upper crustal magma chambers underpinned by a mid to lower crustal chamber which evolved over an 11 Ma period, between 45.1 and 34.5 Ma. Periodic injection a new pulses of magma into the upper crust from the lower crustal chamber occurred approximately every one Myr. Interpreted magmatic processes including assimilation, fractional crystallisation and magma mixing took place in the mid to lower crustal chamber. Zircon inheritance and field work shows crustal assimilation occurred however, as emplacement-aged zircon dO 18 data plot within mantle values, it is likely to be a minor process. The likely assimilate is hydrothermally-altered meta-igneous rocks, rather than meta-sediments.

Ti-in-zircon thermometry (Watson and Harrison, 2005) shows that the Pajonal-El Abra suite lies on a clearly defined cooling trend (Fig. 1a), initiated and then truncated by at least two major thermal events interpreted to be injection of mafic magma into the lower crustal chamber. The overall cooling trend is consistent with the dating data, suggesting relatively slow cooling in the interpreted lower crustal magma chamber is ~15ºC/Myr.

Interpretation of whole rock major element and trace element data, especially whole rock Sr/Y ratios (Fig. 1b), along with comparative emplacement-aged zircon Ce +4 /Ce +3 ratios (Ballard et al., 2002), reveals the El Abra-Pajonal suite can be broadly divided into a dry magma series (plagioclase/pyroxene dominated fractionation) and a wet magma series (amphibole dominated fractionation). As the Ti-in-zircon thermal data only shows one cooling trend, this implies that the interpreted mid-to deep crustal chamber is chemically stratified.

Temperature corrected, zircon Ce +4 /Ce +3 ratios clearly show that the wet magma series is more oxidised than the dry magma series, with the economic porphyries recording the highest Ce +4 /Ce +3 ratios. Comparing the Ti-in-zircon temperatures against corresponding Ce +4 /Ce +3 ratios for the same zircon (Fig. 1c), allows intrusions associated with mineralisation to be discriminated from barren intrusions. This observation indicates that with further development, a threshold value for temperature corrected, Ce +4 /Ce +3 ratios in zircons could be used to define rocks which may be prospective for copper porphyry style mineralisation.

Figure 1. A ): Age versus temperature plot for the El Abra-Pajonal suite based on emplacement-aged zircons. The suite clearly defines a single cooling trend, indicating the suite evolved from a single magma chamber at mid to lower crustal levels. B ): Whole rock Sr/Y ratio which clearly discriminates the intrusions associated with mineralization from barren intrusions. C ): Temperature versus Ce +4 /Ce +3 ratios from emplacement-aged zircons which shows dry series (barren) intrusive rocks are more reducing than wet series rocks, for intrusions of similar age and temperature (e.g. Clara granodiorite c.f. Apolo granite). The El Abra porphyry associated with the main mineralization event at El Abra, is also the most oxidized intrusion relative to the rest of the El Abra-Pajonal suite.

References: Ballard, J.R., Palin, J.M., and 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.

Watson, E.B., and Harrison, T.M., 2005, Zircon thermometer reveals minimum melting conditions on earliest Earth, Science , 308, 841-844.