Petrogenesis and metallogenic implications of volcanic rocks from the Lawu basin, eastern Tibet: Insights into the intracontinental Eocene-Oligocene porphyry copper systems

Abstract

Generally, porphyry Cu deposits are associated with the comagmatic porphyry (or subvolcanic)-volcanic systems of high magmatic H2O-fO2 conditions. The volcanic rocks, as the counterpart of the porphyries, thus can provide some significant insights into the fertility of the porphyries to some extent. For this reason, we have used the Lawu volcanic rocks and spatial-temporal closely-related porphyries in the newly discovered porphyry Cu prospects (e.g., Seli, Zongguo, Mamupu) in the southern segment of the Yulong intracontinental porphyry Cu belt to illustrate the relationship between the porphyries and volcanic rocks, petrogenesis of the volcanic rocks, and then to evaluate the ore potential of the porphyries based on the magmatic H2O-fO2 conditions.Both the Lawu volcanic rocks and Seli-Zonguo-Mamupu porphyries are shoshonitic and metaluminous, and have similar REE patterns, and Sr-Nd-Hf isotopic compositions, which suggest a comagmatic relation between the volcanic rocks and porphyries. An episodic magmatism model is proposed to explain the slightly younger age (~36–35 Ma) and less evolved nature of volcanic rocks than the porphyries. The Lawu volcanic rocks of mainly intermediate composition (SiO2 = 54.25–64.68 wt%) have high K2O (4.75–5.94 wt%) and high K2O/Na2O ratios (1.69–2.00), broadly similar to the coeval Yulong fertile granitic porphyries and the Nangqian mafic lavas. The (87Sr/86Sr)i and εNd(t) values, uniform zircon εHf(t) and δ18O values, and lack of inherited zircons of the Lawu volcanic rocks don’t support their formation by mixing between the mantle-derived Nangqian mafic lavas and crust-derived Yulong felsic porphyries or assimilation and fractional crystallization (AFC) of mafic magmas. They are characterized by high Ba/Th, Ba/La and listric-shaped normalized rare earth element profile with significantly negative Nb-Ta-Ti anomalies, and have high initial 87Sr/86Sr ratios (0.7071–0.7079) and low εNd(t) values (−5.71 to −3.05), and low zircon εHf (t) (−1.53 to 4.09) and clearly high δ18O values (6.67–8.42‰), suggesting that, they were probably formed by fractional crystallization (FC) of mantle-derived mafic magmas and originated from mantle domains modified by significant amount of H2O-rich marine sediments of the Paleo-Tethyan oceanic slab.Magmatic H2O contents calculated from deepest-crystallized amphiboles indicate that, the Lawu volcanic rocks and Zongguo porphyries have initial magmatic H2O contents as high as the Yulong fertile porphyries and typical porphyry Cu systems worldwide (commonly >4 wt% H2O). Magmatic fO2 (ΔFMQ) of the Lawu volcanic rocks (0.6–1.3, ave. 0.9 ± 0.1) and the Zongguo porphyries (0.9–1.7, ave. 1.4 ± 0.2) are clearly lower than the fertile porphyries in the giant Yulong deposit (ΔFMQ = 1.6–3.3, ave. 2.3 ± 0.5) and typical porphyry Cu deposits in the world (commonly ΔFMQ > 2). The slightly lower magmatic H2O contents and slightly higher magmatic fO2 of the Zongguo porphyries than the Lawu volcanic rocks were ascribed to variable degassing during magmatic evolution. These suggest that, in spite of the high magmatic H2O contents, the comagmatic porphyries (at least the Zongguo porphyries) of the Lawu volcanic rocks in the southern segment of the Yulong porphyry Cu belt are unlikely to produce large-scale porphyry Cu mineralization like the giant Yulong deposit, due to the low magmatic fO2 conditions.