Petrology and geochemistry of calc-alkaline volcanic and subvolcanic rocks, Dalli porphyry copper–gold deposit, Markazi Province, Iran

Abstract

Early Miocene igneous rocks associated with the Dalli porphyry ore body are exposed within the Urumieh-Dokhtar Magmatic Arc (UDMA). The Dalli porphyry Cu–Au deposit is hosted by subduction-related subvolcanic plutons with chemical composition from diorite to granodiorite, which intruded andesitic and dacitic volcanic rocks and a variety of sedimentary sequences. 40Ar/39Ar age data indicate a minimum emplacement age of ∼21 million years for a potasically altered porphyritic diorite that hosts the porphyry system. The deposit has a proven reserve of 8 million tonnes of rock containing 0.75 g/t Au and 0.5% Cu. Chondrite-normalized rare earth element (REE) patterns for the subvolcanic rocks are characterized by light REE enrichments [(La/Sm) n = 2.57–6.40] and flat to gently upward-sloping profiles from middle to heavy REEs [(Dy/Yb) n = 0.99–2.78; (Gd/Yb) n = 1.37–3.54], with no significant Eu anomalies. These characteristics are generated by the fractionation of amphibole and the suppression of plagioclas...