Increasing sulfur and chlorine contents in ore-forming magmas: The key to Pulang porphyry Cu-Au formation, SW China

Abstract

The giant Pulang porphyry Cu-Au district (446.8 million tonnes at 0.52% Cu and 0.18 g/t Au) is located in the southern Yidun terrane of the Sanjiang (Three-river) region, southwest China. It has six phases of porphyry intrusions: premineralized fine-grained quartz diorite (FQD) and coarse-grained quartz diorite porphyry (CQD), the main synmineralized quartz monzonite porphyry (QMP), and late synmineralized granodiorite porphyry (GP), diorite porphyry (DP), and aplite. New zircon U-Pb ages for the premineralized FQD and late synmineralized GP, DP, and aplite are 226.3 ± 2.6 Ma (MSWD = 1.9), 216.4 ± 1.0 Ma (MSWD = 2.0), 216.0 ± 1.1 Ma (MSWD = 1.7), and 216.0 ± 1.5 Ma (MSWD = 1.7), respectively. All Pulang porphyry rocks have high whole-rock Sr/Y ratios (37.8–184) with minor or no negative Eu anomalies (0.73–1.04), and are characterized by high large-ion-lithophile-element (LILE: Th, U, Ba, Rb) abundances and low high-field-strength-element (HFSE: Nb, Ta, Ti, Zr, Hf).Zircons from premineralized FQD and synmineralized GP, DP, and aplite rocks have consistently high EuN/EuN* ratios (mostly > 0.4). The magmatic oxygen fugacity calculated from zircon compositions, show that the FQD (ΔFMQ = 1.59 ± 0.43), GP (ΔFMQ = 1.52 ± 0.18), DP (ΔFMQ = 1.52 ± 0.30), and aplite (ΔFMQ = 1.75 ± 0.60) were all derived from moderately oxidized magmas. Magmatic water contents estimated from amphibole compositions and the extensive presence of amphibole phenocrysts suggest that H2O contents are >4 wt%. Together with the published results for premineralized CQD and main synmineralized QMP, it is concluded the causative magmas for the six porphyry phases at Pulang were all relatively oxidized and hydrous.The difference between premineralized FQD and CQD and synmineralized QMP, GP, and DP are reflected in magmatic chlorine and sulfur contents. To minimize the effect of hydrothermal alteration, the apatite microphenocrysts enclosed by least altered phenocrysts were selected for analyses. The Cl contents of magmatic apatite microphenocrysts from the synmineralized QMP (avg = 0.340 ± 0.192, n = 26), GP (avg = 0.317 ± 0.151, n = 21), and DP (avg = 0.179 ± 0.112, n = 18) are significantly higher than those from the premineralized FQD (avg = 0.046 ± 0.032, n = 9) and CQD (avg = 0.043 ± 0.034, n = 13). Similarly, the SO3 contents of igneous apatite microphenocrysts from the synmineralized QMP (avg = 0.259 ± 0.156, n = 25), GP (avg = 0.203 ± 0.163, n = 20), and DP (avg = 0.306 ± 0.173, n = 18) are higher than those from the premineralized FQD (avg = 0.181 ± 0.136, n = 10) and CQD (avg = 0.085 ± 0.068, n = 6). The S-Cl contents of synmineralized melts estimated from apatite S-Cl compositions are higher than those of premineralized magmas, suggesting that the elevated magmatic S-Cl contents in the synmineralized porphyries might have been important for Cu-Au mineralization. This was likely due to S-Cl rich mafic magma recharge, supported by the presence of abundant mafic magmatic enclaves in the synmineralized QMP.