Geochemical discrimination of hydrothermal and igneous zircon in the Iberian Pyrite Belt, Spain

Toscano, M., Pascual, E., Nesbitt, R.W., Almodóvar, G.R., Sáez, R. and Donaire, T. (2014) Geochemical discrimination of hydrothermal and igneous zircon in the Iberian Pyrite Belt, Spain. Ore Geology Reviews, 56, 301-311. (doi:10.1016/j.oregeorev.2013.06.007).

Abstract

Intensely altered, felsic volcanic/subvolcanic rocks located at VHMS footwalls in the Aznalcóllar Mining District, Iberian Pyrite Belt, Spain, contain both igneous and hydrothermal zircon grains according to textural evidence. In order to discriminate their respective features, we present a detailed description showing that these contrasting textural characteristics can be correlated with distinctly different chemical compositions. In comparison with igneous zircons, EMP study shows that hydrothermal zircon grains have lower SiO2 and ZrO2 contents, whereas they are enriched in P2O5, Al2O3, Y2O3 and CaO. On the other hand, LA–ICPMS study shows that the REE patterns of hydrothermal zircons strongly differ from those of igneous ones in that: a) hydrothermal zircons are highly enriched in REE; b) they display patterns with maximum values for intermediate REE, often showing TbN/YbN ratios > 1, and c) hydrothermal zircon grains do not display Ce or Eu anomalies, which are widespread in igneous zircons. We conclude that these chemical differences are consistent with zircon–xenotime solid-solution having reached here values close to a 10 mol%, which represent the maximum possible degree of xenotime substitution according to the available experimental data. The absence of Ce and Eu anomalies is explained according to the particular characteristics of the hydrothermal environment during intense footwall alteration, involving temperatures up to 350 °C and destruction of plagioclase or other Eu+ 2-hosting minerals. We suggest that discrimination of hydrothermal zircons may open new ways in dating hydrothermal zircons in VHMS and other mineralized areas related to intense hydrothermal alteration.

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