第七届青年地学论坛

Porphyry Cu deposits are formed by Cu- and volatile (e.g., Cl, S)-rich fluids exsolved from underlying magma chambers. Intuitively, higher magmatic Cl and S contents likely indicate higher ore potentiality. However, the Cl contents of magmatic apatite, the major Cl-bearing mineral phase, from syn-ore intrusions are highly variable (from < 0.1 wt % to > 2 wt %), yet the causes of this paradox remain obscure. Here we compile existing geochemical data of magmatic apatite and amphibole phenocrysts from 25 porphyry Cu deposits worldwide to calculate magmatic physical-chemical conditions such as water contents and magma chamber depth. We find that the porphyry Cu deposits with greater magma chamber depth have systematically higher magmatic H₂O contents and apatite Cl but lower F contents than those of shallower deposits. These correlations are best explained by early fluid exsolution and Cl loss that predate apatite crystallization in shallower porphyry Cu systems. This is supported by the common occurrence of primary fluid inclusions in apatite from shallower systems. Post-subduction porphyry Cu deposits normally have shallower magma chambers, which is attributed to their formation in relatively extensional tectonic regimes. The results demonstrate that the magma chamber depth exerted important control on the timing of fluid exsolution and accompanied Cl loss. In contrast, high and constant apatite SO₃ content among deposit is minimally affected by fluid exsolution possibly due to buffering of early-saturated sulfate in oxidized and S-rich magmas, and therefore might be used as better potential fertility indicator than Cl.
 

斑岩型矿床,挥发分,岩浆房