REE-rich hydrothermal zircons in footwall chloritic hydrothermal haloes (Iberian Pyrite belt, Spain): a textural and chemical interpretation

Category Petrology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author PASCUAL, Emilio۱; TOSCANO, Manuel۲; DONAIRE, Teodosio۱
Holding Date 07 October 2008

Footwall chloritic haloes in some VMS deposits in the Iberian Pyrite Belt are characterized by an appreciable degree of mobility of some elements supposed to be immobile, such as Zr, Hf, and REE (Almodóvar et al., 1995). Apart from geochemical evidence, this significant mobility results in some areas in hydrothermal zircon overgrowths, as well as in nucleation of minute hydrothermal zircon grains. Textural evidence for hydrothermal crystallisation of zircon includes the occurrence of inclusions of typically hydrothermal minerals, such as sericite, pyrite and galena, as well as precipitation of numerous, tiny zircon crystals trapped as inclusions in hydrothermal chlorite (Pascual et al., 1997). Also, it has been previously suggested that ion-probe dating of zircon in these altered rocks may represent a way to the direct dating of VMS deposits (Nesbitt et al., 1999), instead of indirect dating based upon the closeness between volcanic and hydrothermal events.
LA-ICPMS study of zircons in the Aznalcóllar area (IPB, Spain) has yielded novel data indicating a contrasting composition of igneous and hydrothermal zircon grains. In the studied area, hydrothermal zircons are distinguished from igneous ones at least by several major chemical features:
- Higher REE content (both of LREE and HREE), up to two orders of magnitude.
- Very high content in intermediate/heavy REE with TbN/YbN ratios ≥1, resulting in REE diagrams characteristically convex upwards.
- No positive Ce anomaly.
- No negative Eu anomaly.
In spite of these differences, a still remaining major problem in assessing interpretation of hydrothermal zircon dates has been the difficulty to distinguish individual grains of igneous and hydrothermal zircons, apart from cases in which inclusions of hydrothermal minerals occurred. We present here a new method in distinguishing these two populations by conventional EMP study. Accordingly, typical hydrothermal zircons showing the above features are generally smaller, in places exhibiting skeletal-like or irregular growth, and very often exhibit (PO4)-3 - rich zones. These latter do NOT correspond to phosphate inclusions, as they still contain large amounts of zirconium. We interpret them to correspond to precipitation of impure zircon, in which some lattice domains have been substituted by P-rich submicroscopic domains. EMP maps also show that P-richer, Zr-depleted zircons also contain higher amounts of Th and U. We conclude that dates of hydrothermal events, based on specific detection of hydrothermal zircons, can be obtained by combined EMP and SHRIMP study.
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Work supported by the Spanish project BTE2003-04354