Petrophysical properties of the Goonumbla Volcanic Complex, NSW: Implications for magnetic and gravity signatures of porphyry Cu-Au mineralisation

Abstract

Petrophysical data are important for constraining the geophysical signatures of the Endeavour Cu-Au deposits within the Goonumbla Volcanic Complex (GVC). Susceptibilities vary systematically with lithology and, particularly, with alteration. Remanence tends to be subordinate to induced magnetisation in the GVC. Densities are predictably related to composition, Alteration effects on densities are generally minor, although rocks with particularly strong development of secondary magnetite, hematite or sulphides have higher densities. Modelling of magnetic and gravity profiles over the Goonumbla volcanic belt and the GVC, constrained by geological information and petrophysical properties, suggests that the GVC is underlain by a large zoned intrusion, representing the parent magma chamber, which has a substantial low density, weakly to moderately magnetic, core of alkali feldspar granite to monzonite composition, enclosed by marginal mafic monzonite and monzodiorite phases. The mafic roof zone and marginal phases of the GVC have high susceptibilities (>0.08 SI, >2.5 vol % magnetite). A prominent ridge of low density material occurs along the eastern margin of the mother intrusion. A zone of lower susceptibility occurs above the felsic ridge, probably representing magnetite-destructive alteration due to fluids emanating from the inferred underlying felsic intrusion. This zone appears to be related to the Endeavour lineament, which is thought to control the emplacement of many of the mineralising intrusions in the GVC. Magnetic signatures of deposits tend to be obscured by the heterogeneous magnetic environment, but reflect variably developed halos of enhanced magnetite content, associated with early potassic alteration, surrounding a core of reduced magnetite content, which represents the combined effect of felsic mineralising intrusives, mineralising phase (K-feldspar dominated) alteration and phyllic overprinting. Different signatures can be expected for lava-dominated wall rock sequences (weak to moderate annular high with well-developed central low) and volcaniclastic-dominated sequences (unimodal weak to moderate high).