Chemical fingerprinting of hydrothermal zircons: an example from the Gidginbung high sulphidation Au-Ag-(Cu) deposit, New South Wales, Australia

Hydrothermal zircons occur as an integral component of the alteration and mineralization assemblage in the Gidginbung high sulphidation Au-Ag-(Cu) deposit in the Lachlan Fold Belt, Australia. Their hydrothermal origin is demonstrated by (1) their paragenetic relationships to ore and mineral assemblages within the orebodies, (2) the higher concentration of zircon within the orebody relative to that of the host rocks, (3) the presence of ore and alteration minerals as solid mineral inclusions within the zircons, (4) similar fluid inclusion compositions in these zircons and the associated ore and gangue minerals, and (5) their trace element composition and zonation as measured by the CSIRO-GEMOC nuclear microprobe. The trace element chemistry and mineral inclusion assemblage of these hydrothermal zircons differ markedly from those in coeval intrusive, volcanic and sedimentary lithologies. This chemical fingerprinting permits the hydrothermal character of the zircons and their association with mineralization to be demonstrated unequivocally, thereby enabling the mineralizing event to be dated with confidence and precision at 436.4 +/- 3.1 Ma. Isotopic dating of hydrothermal zircons may have particular application where related ore deposits have been overprinted by later magmatic, hydrothermal. metamorphic, deformation and/or crustal weathering processes that disturb isotopic systems in other less stable minerals. This account shows that uncritical acceptance of U-Pb zircon ages as dating primary igneous events is unwise.