Fri 20th  February  12 – 1 pm  Rm 312.222

Mineralised Cu-Mo-Au porphyries were discovered in belts of Cambrian andesite in the Stavely region of western Victoria in the early 1990s.  Deep weathering complicated early geochemical exploration and only a small number of diamond holes to several hundred metres depth were drilled.  These encountered sub-economic grades in mostly propylitic alteration.  New work shows they only drilled into porphyry dykes emanating up into the host andesites, rather than intersecting the actual porphyry bodies where economic grades could exist in the potassic zone.  This lack of early success, combined with no historic artisanal mining and uncertainty over the geological context meant there was little appetite to persevere.

In the mid 2000s university wholerock geochemical research suggested that the plate tectonic setting had been an Andean-type convergent margin – very attractive for copper porphyries  – rather than a region of arc-continent collision as previously preferred.  The geochemistry recognised boninites that can only form in supra-subduction zone settings.  The andesite belts themselves and some associated granodiorite intrusions also possessed a geochemical subduction fingerprint with relative depletion of HFSE.  A government funded deep crustal seismic transect across the region in the late 2000s imaged a crustal geometry consistent with the Andean-type convergent margin model.  The faulted belts of andesite exposed at surface can be traced down into what appears to be a larger arc edifice buried in the subsurface.

This greater confidence for Andean-like mineralised porphyries has triggered a new round of exploration and research.   A major breakthrough in prospectivity was taking some of the mineralised porphyry drill core and doing wholerock geochemistry for comparison against the existing academic geochemistry.  This shows the porphyries are related to the granodiorite intrusions rather than the belts of andesite as previously thought.  This boosts prospectivity in two important ways: 1 The intrusions are younger than the steep dipping fault slices of andesite so that the mineral systems are upright for easy vectoring rather than being variably titled and/or partly dissected; 2 The porphyries can occur anywhere within the entire Stavely region rather than being narrowly restricted to a few fault slices of andesite.  A government stratigraphic drill program is being planned to better define the margins of the Stavely Geological Zone.

Like many mineralised porphyry systems there is complexity for explorers.  Current mapping shows the most obvious targets are where the porphyries intrude into the andesites with zone of phyllic overprinting causing demagnetisation haloes (historic Victor prospect).  Less obvious are porphyries intruding in siliclastic sandstone outside of the andesite belts still preserve the prograde potassic cap and can be magnetic highs (historic Junction prospect).  The intrusions occur in clusters with a multiphase intrusion history, some mineralised and some barren.  The clusters seem to be concentrated in late conjugate faults cutting across the paleo-arc trend to define corridors of enhanced prospectivity.  Current explorers are collecting gravity and IP to generate good exploration targets through the deep weathering and cover, whilst they raise money for the deep drilling (up to 1 km) that will finally test the economic viability of this very poorly known province of mineralised porphyries.