The geology and geochemistry of the Naringla Monzodiorite, and the mineralization and alteration of the associated Porphyry Copper Deposits, Yeoval, Central-Western N.S.W.

A study of the petrology, geochemistry and field relationships of sedimentary, volcanic and plutonic rocks from the Yeoval district has revealed a complex and varied geological history. Volcanics derived from the nearby Molong Volcanic Rise were deposited with marine sediments in the Middle to Late Silurian. They were then deformed during the Bowning Orogeny. Intrusion of the Naringla Monzodiorite occurred in the Early Devonian. This is a heterogeneous body ranging in composition from pyroxenite cumulates to microgranodiorite and dacite porphyry. The latter two rock types are Co-genetic prior to pyroxene formed by the host to disseminated porphyry copper style mineralisation. basaltic andesites were erupted above the monzodiorite body final crystallization of the monzodiorite. The pyroxenites, bearing basaltic andesites and pyroxene crystal fractionation of augite from bearing monzodiorites a primitive magma of basaltic andesite composition. The chemical variations in the Naringla Monzodiorite (apart from the low-Si rocks) is thought to have been derived by chemical fractionation of hornblende, plagioclase and possibly biotite, although magma mixing and restite unmixing cannot be ruled out as formation processes. Hydrothermal alteration associated with the porpyritic intrusions has produced propylitic mineral assemblages in most of the rocks of the Naringla Monzodiorite. Alteration is most intense around the copper deposits, and occasional albitic alteration is developed. Alteration is of a weak and patch nature, and lateration zones are generally not developed. A small body of gabbro that outcrops adjacent to the Naringla Monzodiorite is probably unrelated due to chemical inconsistencies between the two units. The Ohley Adamellite intruded during the Middle Devonian, and contact metamorphosed the gabbro and a body of quartz-hornblende-diorite. The intrusion of rhyolite dykes into the Naringla Monzodiorite may have been related to the emplacement of the adamellite.The final geological event in the area was the emplacement of small basic dykes along east-west joints in the Naringla Monzodiorite. dykes are of probable Tertiary age.