Apatite chemistry indicates that oxidized auriferous fluids along the world-class Boulder Lefroy – Golden Mile fault system were significantly different to fluids from global porphyry systems

Bath, Dr Adam, B.1, Walshe, John, L., Ireland, Tim., Cobeñas, Gisela., Sykora, Stephanie., Cernuschi Federico., Woodall, Katie., MacRae, Colin., Williams, Morgan., Schmitt, Leanne.

1CSIRO Mineral Resources, PO Box 1130, Bentley, Western Australia 6102, Australia, 2First Quantum Minerals Ltd, 1/24 Outram St, West Perth WA 6005, 3Lundin Mining, El Bosque Norte 500, piso 11, Of. 1102,  Las Condes, CP 7550092, Santiago, Chile

The Boulder Lefroy – Golden Mile (BL-GM) fault system contains >70 Moz of gold and accounts for more than a quarter of the known gold mineralization in the Archean Yilgarn Craton, Western Australia. Alteration along the BL-GM fault system shows a wide range of redox conditions (hematite/anhydrite- to pyrrhotite-stable), and this range in many cases can be linked to mineralization. Oxidized fluids have been linked to adakitic magmas or more generally, Au fluids linked with sub-arc mantle wedges. However, invariably in Archean Au systems there is a lack of evidence of causative intrusions at the time of oxidized alteration and mineralization, leading numerous authors to reject the possibility that magmas were a major source of auriferous fluids. Despite this, oxidized fluids derived from deeper concealed intrusions cannot be ruled out, particularly given the length and breadth of anhydrite alteration can be spatially significantly large (km-scale). Here we present mineral and isotopic datasets on oxidized fluids from the BL-GM fault system with the aim to view these datasets in comparison to those from porphyry systems. Porphyry Cu-Au systems are associated with oxidized magmatic fluids generated in arc environments, and here we test if apatite chemistry can be used to make any links between oxidized fluids in Archean Au systems and global porphyry systems. Close to 1000 apatite analyses were obtained on the electron microprobe. The apatite crystal formula is Ca5(PO4)3X, where X represents the channel volatile site that runs parallel to the crystallographic c-axis, occupied by OH, F, and Cl. Results from our dataset show that apatite from gold-bearing oxidized alteration assemblages in Archean Au system have variable amounts of F (2.5 to 4.1 wt. %), whereas Cl values remain low (<500ppm) over the wide range of F values. In contrast, apatite from porphyry Cu-Au deposits with anhydrite have a wide range of F values (2.2 to 4.8 wt. %) and show a significant linear increase in Cl (<300ppm to 1.7 wt. %) with decreasing F. The stark difference in the X-site substitution appears the main variance between the chemistry of apatite from the two environments. In hydrothermal Archean Au and porphyry Cu-Au systems, higher amounts of F tend to correspond to lower temperatures (based on biotite-apatite thermometer). Also, previous experimental studies show that relatively low Cl and F concentrations in acidic (H20-HCI) fluids result in high ratios of XClAp/XOHAp and XFAp/XOHAp in apatite, whereas much higher abundances of CI and F are required in basic fluids to achieve the same result. Data from previous studies also show that the dilution of aH2O with higher aCO2 results in apatites with higher ratios of XCIAp/XOHAp and XFAp/XOHAp from fluids with comparatively lower levels of Cl or F. Our data shows no evidence of these processes. The most plausible explanation for the comparatively low XCIAp/XOHAp apatites from Archean Au systems is that the fluids were comparatively low saline compared to those from oxidized alteration assemblages in porphyry Cu-Au systems. Furthermore, results of this study show that apatite from oxidized and reduced alteration assemblages of Archean Au systems across the Yilgarn Craton plot along the same trend on the Cl versus F diagram. One notable difference between the two populations is that those forming part of reduced alteration assemblages have lower F values (1 to 3.3 wt. %) and appear to be associated with hotter fluids.


Biography

Mineral system scientist with experience of mapping, characterising and exploring for Cu and Au mineral systems in a range of terranes. Experience developing and implementing new and novel techniques towards mapping hypogene alteration and exploring in Archean gold systems in various terranes of the Yilgarn Craton at the camp-scale.

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