Volatile variations in magmas related to porphyry Cu-Au deposits: Insights from amphibole geochemistry, Duolong district, central Tibet
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Ore-forming fluid exsolution in a shallow magma chamber is a critical step in the formation of porphyry Cu ± Mo ± Au deposits but one for which direct evidence is rarely found. Here, Cl abundance, major-trace element content and H isotope analysis of amphibole in diorite and barren granodiorite porphyry samples from the Duolong porphyry-epithermal Cu-Au district are presented in order to reveal processes associated with fluid exsolution and magma evolution. Low-Al Mg-hornblende formed in ore-bearing diorite at ∼750–860 °C and ∼80–200 MPa. On primitive mantle-normalized diagrams, these low-Al amphibole grains have slightly convex upward REE patterns with distinctly negative Eu anomalies and negative anomalies in Pb, Sr, Eu, Zr, Hf, and Ti anomalies, suggesting crystallization from the same arc magma after plagioclase and magnetite crystallization. A large variation in estimated melt H2O (∼6 to 3 wt%) and Cl content (0.09–0.38 wt%), as well as low δD values (−103 to −113‰) indicate that the magma underwent large-scale fluid exsolution, contributing to the formation of Duolong Cu-Au mineralization. Additionally, amphibole from a barren granodiorite porphyry shows two distinct populations, distinguished by their Al content. This intrusion has low-Al amphibole (Mg-hornblende) which formed at similar conditions to low-Al amphiboles in the diorite (∼790–870 °C and ∼100–230 MPa), whereas high-Al amphibole (tschermakite) formed at ∼880–970 °C and ∼210–400 MPa. Some high-Al amphibole phenocrysts have slightly convex REE patterns with no negative Eu anomalies, a depletion in Nb, Zr, and Hf, and positive Sr, Ba, and Pb anomalies, likely consistent with amphibole crystallization from more mafic basaltic-andesitic melts. Low-Al amphibole in the granodiorite porphyry shows different compositional trends compared to those in the diorite, suggesting that they crystallized from different magmas. Considering the evidence for mafic magma replenishment suggested by the positive correlation between AlIV and Mg# values in high-Al amphibole, the low-Al amphibole in the barren granodiorite porphyry likely crystallized from a new hybrid magma with mafic to intermediate-felsic magma compositions. Meanwhile, smaller variations in Cl content (0.08–0.24 wt%) in low-Al amphibole was dominantly controlled by magma evolution (shown by variable Mg#) rather than fluid exsolution. Thus, the low δD values (−102 to −122‰) were likely inherited from evolved dioritic magmas that underwent fluid exsolution. Importantly, the Cl compositional variation of amphibole has a potential application as a powerful tool to identify ore-bearing and barren intrusions within porphyry Cu ± Mo ± Au deposits.
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