A new frontier in exploration geochemistry is the development of techniques that detect orebodies located at depth. The propylitic halo is the most extensive footprint of porphyry systems and may be the only exposed part of a buried ore system, so is at the heart of such research. Key trace elements within propylitic minerals can vary systematically with distance from known orebodies, therefore, can be used as vectors to ore. Additionally, certain trace element signatures within the same minerals are unique to mineralised systems and are useful fertility indicators.
Porphyry systems form in active tectonic settings so alteration can be overprinted by later events, particularly in ‘older’ camps such as the Palaeozoic Oyu Tolgoi deposits of Mongolia. In such complex terranes, successful exploration relies on distinguishing porphyry-related assemblages from later overprinting events.
The porphyry alteration halo at Oyu Tolgoi is zoned, with potassic and sericitic alteration in the core hosting the orebodies and flanked by the innermost subzone of the propylitic halo, characterised by pyrite-K-feldspar-epidote. Beyond this is the epidote subzone, characterised by epidote- albite-calcite- titanite-andradite. The outermost chlorite subzone is truncated by faults and rarely observed.
New LA-ICP-MS U-Pb dating of propylitic titanite identified hydrothermal events occurring at approximately 370 Ma, 340-320 Ma and 290 Ma. These dates broadly match known magmatic events: porphyry mineralisation in the late Devonian; the intrusion of granodiorite plutons in the Carboniferous; and the emplacement of the giant Khanbogd Granite in the Permian. The distribution of these dated samples suggests that post-mineralisation alteration overprinted significant portions of the porphyry halo.
Using the samples with dated alteration, it has been possible to develop a geochemical classification system that can distinguish between porphyry-related epidote, and later non-porphyry epidote. The results have been integrated to create a workflow for exploration that has identified prospective targets across the wider district.