第七届青年地学论坛

Reading the geochemical record of magma mixing and degassing processes in the formation of porphyry deposits is critical in order to fully understand and quantify the processes that lead to metal deposition. Here, the halogens and trace element compositions of apatite inclusions shielded within magmatic biotite, low-Al amphibole, feldspar, and groundmass in the pre-ore quartz diorite porphyry (QDP), the syn-ore quartz monzonite porphyry (QMP), and the mafic microgranular enclaves (MMEs) from the giant Pulang porphyry Cu-Mo-Au deposit were present to interpret its behaviors during the late magmatic-hydrothermal translation stage. The biotite-hosted apatite inclusions with 0.01 ~ 1.01 wt.% SO₃ and variable X_F/X_Cl (7.14 ~ 80.48) from the QMP were crystallized in an adakitic magma chamber, which was probably injected with a S-rich mafic melt (MMEs-hosted apatites with 0.14 ~ 0.37 wt.% SO₃). The broad range of X_F/X_Cl (12.30 ~ 964.35) from the amphibole-hosted apatite inclusions of the QMP, together with its lower SO₃ (0.01 ~ 0.65 wt.%) contents, are mostly ascribed to losses of volatiles triggered by the magma degassing. Whereas, those from the barren QDP systematically returns very low sulfur concentrations (< 0.05 wt.% SO₃) and limited X_F/X_Cl (22.36 ~ 160.85), indicating a lacking of the above magma mixing and degassing process. Such open magmatic processes are also contributed to the sharp ƒO₂ (ΔFMQ = +0.7 ~ +1.6) decrease of magmatic biotite, and the following ƒO₂ (ΔFMQ = +2.7 ~ +3.7) increase of low-Al amphibole at very shallow depth (< 4 km). The low-Al amphibole-hosted apatite of QMP show lower Sr/Y (0.61 ~ 0.81) and Mg (61 ~ 78 ppm) contents than those crystallized in biotite, plagioclase and groundmass, but conversely elevated Eu/Eu* (0.55 ~ 0.73) values, also indicating a fluid saturation and high ƒO₂ condition caused by the magma degassing. The remain apatite inclusions of the QMP exhibit consistent Sr/Y ratios (2.21 ~ 3.62) with decreasing Mg contents (119.06 ~ 528.79 ppm), suggesting that these inclusions are likely co-crystallized with biotite, earlier than the plagioclase crystallization. Whereas, none variations occur both in the Sr/Y ratios (0.82 ~ 0.97) and Mg contents (11.24 ~ 42.67 ppm) of most apatite inclusions in groundmass of the QDP, possibly indicating that these inclusions crystallize later than biotite, low-Al amphibole and plagioclase. Therefore, the magma mixing and degassing processes, together with the fraction of primary minerals, could both cause large variations in apatite trace elements. Overall, the mafic magma recharge model could generate the sulfur fluxing events in the biotite-hosted apatites from the QMP, and likely triggered the extensive magma degassing at the crystallization of low-Al amphibole in the Pulang porphyry Cu-Mo-Au deposit.
 

磷灰石,挥发分,微量元素,镁铁质岩浆补充,普朗斑岩矿床