Structural and timing constraints on molybdenum and tungsten mineralisation at Yea, Victoria

The molybdenite and scheelite mineralisation in the Native Dog Pluton at Monkey Gully near Yea is hosted within an I-type (post-orogenic) pluton, which shows extensive fractionation and magma mixing, and was emplaced in an extensional environment. The pluton comprises four principal rock types: tonalite, granodiorite, dacite and leucogranite. Emplacement of the pluton was in an extensional northwest to southeast paleostress field. Early extensional quartz veins, related to cooling, are overprinted by both dacitic dykes and late-stage quartz sheeted veins. The late-stage veins host the molybdenum and tungsten mineralisation in the deposit. 206Pb/238U zircon ages of 356 ±14 Ma and 375 ± 22 Ma place pluton formation and mineralisation at the onset of the Kanimblan Orogeny and later than other major molybdenum deposits in Victoria. Key factors governing the source for the granite and its associated mineralisation are: (1) the presence of a highly fractionated and sulfur-rich leucogranite; and (2) the pluton's location in a regional jog overlying the Selwyn basement block. Finally, a model is developed to explain the differences between this Melbourne Zone molybdenum and tungsten deposit compared with other metallogenic porphyry deposits.