THE ELOISE DEPOSIT, CLONCURRY DISTRICT, QUEENSLAND: ISOTOPIC CONSTRAINTS ON THE GENESIS OF A PYRRHOTITE-RICH END-MEMBER OF THE CU-AU-FE-OXIDE CLASS

Radiogenic (⁴⁰Ar/³⁹Ar) and stable (O, H and S) isotope analyses of metamorphic and metasomatic minerals constrain the age, metasomatic evolution and genesis of the Eloise Cu-Au deposit (3.1Mt @ 5.5% Cu, 1.4g/t Au). Biotite from a pre- to syn-D₂ vein has an ⁴⁰Ar/³⁹Ar age of 1555 ± 4 Ma which is interpreted to coincide with a regional metamorphic event synchronous with D₂. Six later stages of alteration and mineralization are recognized, all of which post-date peak metamorphism and D₂. Stages I to III are volumetrically the most significant and comprise early albitization (I), quartz-hornblende-biotite veins and alteration (II), and Cu-Au mineralization (III). Stages IV, V and VI are localized post-ore events. Two Stage II hornblendes have ⁴⁰Ar/³⁹Ar ages of 1530 ± 3 Ma. Biotite from the same stage has an ⁴⁰Ar/³⁹Ar age of 1521 ± 3 Ma. Muscovite from a post-ore shear zone has an ⁴⁰Ar/³⁹Ar age of 1514 ± 3 Ma. These results provide a maximum (ca. 1530 Ma) and minimum (ca. 1514 Ma) age for the mineralizing event. However, the intimate relationship between Stage II mafic-silicate veins and alteration, and the Stage III Cu-Au suggests that the older age is likely to be closer to the age of mineralization. Quartz from Stages II, III and IV have d¹⁸O ranging between 10.1 and 11.9 ‰. d¹⁸O and dD for Stage II biotite is 5.6 and –84 ‰ respectively. Stage II hornblende has lower d¹⁸O (6.8 and 7.4 ‰) and dD (-88 and –90 ‰) than Stage III actinolite (8.0 and –84‰). d³⁴S for chalcopyrite, pyrrhotite and pyrite falls in a narrow range between 0.0 and 2.3 ‰. A distinct trend can be recognized with d³⁴S becoming progressively greater from south to north reflecting the zoned alteration system (magnetite-pyrite-rich in the south through to pyrrhotite-rich in the north). The stable isotope data, combined with high temperature and high salinity fluid inclusion data indicate a predominantly magmatic origin for the ore forming fluids, and the ⁴⁰Ar/³⁹Ar ages coincide with the timing of regional granite emplacement.