Metallogenic potential of porphyries in Tulasu basin, Northwest Tianshan: Insight from magma nature and crustal thickness

Abstract

Tulasu basin in Northwest Tianshan hosts a lot of Late Paleozoic epithermal deposits, such as Axi, Jingxi-Yelmand, and Tawuerbieke deposits. Epithermal deposits are considered as telescoped systems of porphyry deposits. However, porphyry deposits are not discovered in Tulasu basin, though granitoid porphyries are widespread in this region. Tulasu pluton complex, as one of the largest intrusions located in northeast part of Tulasu basin, is composed of the Late Devonian quartz diorite porphyry and pyroxene diorite porphyry, and the Late Carboniferous granodiorite porphyry and granite porphyry. The metallogenic potential of each rock type is evaluated in terms of magma nature and crustal thickness of source. The magma nature including magmatic oxygen fugacity and water content is estimated based on trace elements compositions of zircons, amphibole hygro-barometer and ilmenite-magnetite mineral pairs. The mean Ce4+/Ce3+ ratio, magmatic oxygen fugacity and water content of the Late Devonian quartz diorite porphyry are 291, ΔFMQ = 1.0–1.8, ΔFMQ = −0.2–1.5 and 0.9–2.2 wt%, respectively, while those of pyroxene diorite porphyry are 96, ΔFMQ = 2.6–3.3 and 0.2–1.3 wt%. The Late Carboniferous granodiorite porphyry has a mean Ce4+/Ce3+ ratio of 138. The crustal thicknesses in the Tulasu area are estimated to be 30–35 km at the Late Devonian and ~40 km at the Late Carboniferous, based on whole rock geochemistry of the Late Devonian dioritic rocks and the Late Carboniferous granitic rocks. Considering magmas associated with typical porphyry deposits which have high oxygen fugacity of ΔFMQ = +1.5 to +3 and water content (>4.5 wt%), and are derived from crustal thickness of >30 km, we think both dioritic and granitic rocks of Tulasu porphyries have favorable conditions of oxidation state and crustal thickness for porphyry deposit formation. However, the magmatic water contents of these rocks are relatively low. Therefore, the Tulasu porphyries have low potential in forming porphyry Cu deposits.