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摘要
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. Ar-40/Ar-39 age data indicate a minimum emplacement age of similar to 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 degrees C, the oxygen (fO(2)) and water (fH(2)O) fugacities are estimated to be 10(-10.3) bars (similar to Delta NNO+4) and %26lt;= 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 degrees 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 (Delta NNO+1.3), and a hornblende phenocryst crystallization temperature of 880 +/- 68 degrees C and pressure of 2.6 +/- 1.7 kbar.
- 出版日期2013
