Tourmaline as an indicator of hydrothermal fluid sources at the Cadia East alkalic porphyry Au-Cu deposit and comparisons with IOCG and other porphyry deposits

The Cadia East deposit is the largest of four porphyry Au-Cu deposits that comprise the Cadia district which is located ~220 km west of Sydney in NSW, Australia. Porphyry Au-Cu and related skarn mineralisation at Cadia is hosted by middle to late Ordovician subaqueous volcanic and volcaniclastic rocks that were deposited in an ancient intra-oceanic island arc named the Macquarie Arc (Percival and Glen, 2007). Mineralisation is associated with alkalic monzonite and quartz monzonite intrusions that were emplaced between the latest Ordovician and the early Silurian after collision of the Macquarie Arc with the eastern margin of Gondwana (Glen et al., 2007).

Mineralisation at Cadia East is centred on a swarm of NW trending monzonite and quartz monzonite dykes that define an ore zone 2.5 km in length and 0.8 km in width, and that extends vertically for over 1.5 km. Hydrothermal alteration is zoned from an early sodic-calcic/calc-potassic assemblage (plagioclase + actinolite + tourmaline + K-feldspar + epidote) at greatest depth (2,000 to 1,500 m) that transitions vertically and laterally into a pervasive potassic assemblage (biotite + magnetite + K-feldspar). A second potassic alteration overprints the earlier assemblages and occurs principally as K-feldspar + biotite alteration envelopes around sheeted quartz-sulfide veins which are associated with Au-Cu mineralisation. Inner propylitic alteration is actinolite-bearing, a common feature of alkalic porphyry deposits (Holliday and Cooke, 2007), and is located peripherally to the calc-potassic and potassic alteration zones. Alteration in the shallow part of the Cadia East deposit comprises a 200 to 300 m thick zone of pervasive assemblage of K-feldspar + plagioclase + calcite + pyrite + apatite ± tourmaline that overprints all earlier alteration stages. This shallow potassic-sodic alteration is an atypical feature of alteration in porphyry deposits but is voluminous and laterally extensive (>2 km) at Cadia East.

Hydrothermal tourmaline is highly sensitive to its chemical environment of formation and it can therefore provide an excellent chemical and isotopic record of the hydrothermal fluids from which it precipitated (e.g., van Hinsberg et al., 2011). Early hydrothermal tourmaline at Cadia East is associated with pervasive calc-potassic alteration within the high-grade ore zone. Tourmaline also occurs in late-stage breccias and veins which are temporally associated with an extensive zone (>200 m thick, 1 km wide and several kilometres in length) of pervasive feldspar-rich alteration that caps the deposit. The total range in the boron isotope composition (δ11B) of tourmaline ranges between -5.2 and +7.7‰ and the calculated boron isotope composition of fluids in equilibrium with tourmaline range between -1.1 and +10.4‰. Although relatively high calculated δ11B values of fluids have been attributed to seawater or basinal brine sources (e.g., Xavier et al., 2008), a combination of oxygen, hydrogen and strontium isotope analysis of tourmaline indicate that external fluids, including Ordovician-Silurian seawater and meteoric water, were precluded from the hydrothermal system at Cadia East. Instead, the isotopic composition of tourmaline was controlled by hydrothermal fluids that were derived from alkalic intrusions that may have boiled locally. The range in the δ11B composition of tourmaline at Cadia East is comparable to that documented in porphyry Cu and iron-oxide-copper-gold systems in the central Andes (e.g., Wittenbrink et al., 2009; Tornos et al., 2012) and therefore, suggests a similar magmatic-hydrothermal origin. A magmatic origin for the laterally extensive zone of shallow potassic-sodic alteration at Cadia East suggests that this alteration may be analogous to the extensive zones of advanced argillic alteration that forms lithocaps in the upper parts of calc-alkaline porphyry deposits (e.g., Sillitoe, 1995).