Magmatic initial and saturated water thresholds determine copper endowments: insights from apatite F-Cl-OH compositions
Description
Magmatic volatiles (H2O, F, Cl), especially water, are critical in the formation of porphyry copper deposit, for its significance as a carrier for metals. Magmatic water contents can be estimated using geochemical characteristics or directly measured using melt inclusions. However, accurately quantifying the water contents of deep ore-forming magma remain a challenge. Two critical factors are concerned in describing magmatic water evolution, initial water content, and water saturation threshold. Further constraints of these thresholds and volatile evolution path are needed to understand their roles in copper endowments. To address this problem, we used apatite and forward modelling methods to reconstruct magmatic water evolution histories. Samples investigated include granitoid rocks and apatite from highly copper-mineralized and barren localities. Generally, our research suggested that both ore-related and ore-barren magma systems are hydrous, the modeled magmatic water contents vary significantly among systems whether mineralized or not, and the major difference lies in the threshold of water saturation (6.0 wt.% for barren, and up to 10.0 wt.% for highly mineralized). Combined with whole rock geochemistry data, phenocrysts assemblage and modeling, we think the ore-related magma are stored at deeper depth with higher water solubility. In conclusion, we propose that the level of magmatic water saturation plays a crucial role in the formation of porphyry copper systems. Magma with mineralization potential not only has higher water solubility but also undergoes significant water enrichment upon saturation. The deeper storage depth and injection of hydrous mafic magma are likely contributing factors to the increased water solubility.