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 plagioclase crystallization from hydrous calc-alkaline magmas. In normalized multi-element diagrams, all analysed rocks are characterized by enrichments in large ion lithophile elements and depletions in high field strength elements, and display typical features of subduction-related calc-alkaline magmas. We used igneous mineral compositions to constrain the conditions of crystallization and emplacement. Biotite compositions plot above the nickel–nickel oxide (NNO) buffer and close to oxygen fugacity values defined by the hematite–magnetite (HM) buffer, indicating oxidizing conditions during crystallization. Assuming a minimum crystallization temperature of 775°C, the oxygen (fO₂) and water (fH₂O) fugacities are estimated to be 10^−10.3 bars (∼ΔNNO+4) and ≤748 bars, respectively, during the crystallization of biotite phenocrysts. The temperature and pressure conditions, estimated from temperature–corrected Al-in-hornblende barometry and amphibole-plagioclase thermometry, suggest that the hornblende phenocrysts in Dalli rocks crystallized at around 780 ± 20°C and 3.8 ± 0.4 kbar. An alternative method using the calcic amphibole thermobarometer indicates that the Dalli magmas were, on average, characterized by an H₂O content of 4.3 wt.%, a relatively high oxygen fugacity of 10^−11.0 bars (ΔNNO+1.3), and a hornblende phenocryst crystallization temperature of 880 ± 68°C and pressure of 2.6 ± 1.7 kbar.

Keywords:

• porphyry Cu–Au deposit
• Dalli
• Central Iran
• Miocene
• diorite
• granodiorite
• hornblende
• biotite
• P–T conditions
• oxygen fugacity
• water fugacity

Acknowledgements

We thank Danielle Demaiffe and Olivier Féménias of the Université Libre de Bruxelles (ULB) for some of the major and trace element analyses; M. Khalili of the University of Isfahan and J. Morgan of the University of Oklahoma for electron microprobe analyses; C. Mayers for ⁴⁰Ar/³⁹Ar analyses; and Dorsa plc for financial support. We thank Julien Berger (ULB) for his helpful suggestions too. JPR acknowledges the support of a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada.