Geochemical mapping has been undertaken in many parts of the world and at various scales as part of mineral exploration programs over the last 60 years. The designers of geochemical mapping programs are faced with a wide, and growing, variety of sampling, analytical and data processing options. Optimising the options has been largely based on orientation studies but a more systematic approach based on evolving understanding of regolith-landscape evolution and geochemical processes is required to detect deposits with subtle surface expression. In essence we need to address the question “What influence is the mineralogical needle likely to have on the regional regolith haystack?”.
David Cohen is Head of the School of BEES at UNSW and has over 25 years in exploration geochemical research. He is a past president of the Association of Applied Geochemists and co-authored the chapter on Exploration Geochemistry in the recently published Elsevier Treatise on Geochemistry devoted to the Geochemistry of Mineral Deposits. He has wielded everything from chainsaws to portable XRFs in the process.
This talk is based on work carried out in the Geological Survey of NSW by Cam Quinn, Ian Percival and Dick Glen, originally as a contribution to pmd*CRC.
It asks the question whether Ordovician basaltic to andesitic volcanic (and related sedimentary) rocks, and importantly mineralised porphyries, of the Lachlan Orogen in the central west of New South Wales really did form in an intraoceanic island arc, above a west-dipping subduction zone between the Gondwana plate in the west and the paleo-Pacific plate in the east, as has been widely accepted.
The talk will draw on new field and conodont data that question the arc interpretation, and instead support a setting for the formation of these rocks (and critically, the emplacement of porphyries) well behind the Ordovician plate boundary.
If this new model is correct, then the porphyry copper-gold deposits at Cadia Valley, Northparkes and elsewhere are not arc-related, thereby opening up new avenues for mineral exploration in backarc settings.
Rugby Club, 5:30 for 6:00, Thursday 30th May, 2013