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 1010.3 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 1011.0 bars (NNO+1.3), and a hornblende phenocryst crystallization temperature of 880±68°C and pressure of 2.61.7 kbar.
Original language | English |
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Pages (from-to) | 158-184 |
Number of pages | 27 |
Journal | International Geology Review |
Volume | 55 |
Issue number | 2 |
DOIs | |
Publication status | Published - 20 Jan 2013 |
Funding
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.
Funders | Funder number |
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Dorsa plc | |
University of Oklahoma | |
Natural Sciences and Engineering Research Council of Canada | |
University of Isfahan | |
Université libre de Bruxelles |
Keywords
- biotite
- Central Iran
- Dalli
- diorite
- granodiorite
- hornblende
- Miocene
- oxygen fugacity
- P-T conditions
- porphyry Cu-Au deposit
- water fugacity