Low magmatic Cl contents in giant porphyry Cu deposits caused by early fluid exsolution: A case study of the Yulong belt and implication for exploration

Abstract

High apatite Cl and S contents have previously been proposed to be effective in discriminating fertile porphyry Cu systems from infertile ones. However, apatite Cl contents could be very low in syn-mineralization intrusions of some giant porphyry Cu deposits (e.g., <0.1 wt% at Yerington, USA and Yulong, China). The reason for this abnormal phenomenon remains enigmatic, hindering further application of apatite as potential indicator mineral in porphyry Cu exploration.To address this outstanding issue, we conducted a comparative study between mineralized and coeval barren magmatic suites in the Yulong porphyry Cu belt in eastern Tibet, with emphasis laid on their contrasting apatite volatile contents. Apatites from pre- and syn-mineralization intrusions at the mineralized Yulong suite have similar SO3 contents (0.35 ± 0.26 wt% and 0.34 ± 0.23 wt%, respectively) with but lower Cl contents (0.41 ± 0.37 wt% and 0.08 ± 0.02 wt%, respectively) than the coeval barren suites (SO3: 0.20 ± 0.12 wt%–0.95 ± 0.26 wt%; Cl: 0.49 ± 0.08–1.17 ± 0.06 wt%). The Yulong suite displays a sudden and coupled drop of apatite XOH and XCl values and surge of XF/XCl ratios from pre- to syn-mineralization intrusions, likely indicating extensive fluid exsolution. However, this trend is not observed in the barren suites. Zircon grains from pre- and syn-mineralization intrusions at the mineralized Yulong suite have higher 10,000*(EuN/EuN*)/Y ratios and lower calculated Ti-in-zircon temperatures (691° ± 49 °C and 703° ± 48 °C, respectively) than the barren suites (770° ± 68 °C–790° ± 76 °C), suggesting higher magmatic water contents for Yulong. This is consistent with higher abundances of hydrous minerals such as biotite and amphibole in the Yulong suite. Amphibole phenocrysts from pre- and syn-mineralization intrusions at Yulong have significantly lower crystallizing pressures (2.1 ± 0.6 kbar and 1.9 ± 0.5 kbar, respectively) than the barren suites (4.6 ± 0.4 kbar to 4.7 ± 0.5 kbar). A similar trend is also shown by crystallizing pressures estimated from biotite phenocrysts from these barren and mineralized suites.We propose that the barren suites were more Cl-rich because their source magma chambers have greater depth but lower water contents, leading to limited fluid exsolution and Cl loss. In contrast, the shallowly-emplaced, more hydrous Yulong suite was originally Cl-rich but underwent early fluid exsolution and Cl loss before/during apatite crystallization. As such, apatite Cl contents may not always be good fertility indictor for porphyry Cu systems. We demonstrate that high apatite SO3 contents are less affected by fluid exsolution due to buffering of saturated anhydrite in oxidized and S-rich magmas, and may be used to aid exploration.