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Magmatic Cl-H2O contents, fluid extraction and porphyry fertility: Evidence from zircon and its apatite inclusions

Wenting Huang1,3, Madeleine C.S. Humphreys2, David J. Colby2, Xiaoping Xia3, and Huaying Liang3

  • Affiliations: 1School of Jewelry, City Polytechnic of Shenzhen, Shenzhen, China. 2Department of Earth Sciences, Durham University, Science Labs, Stockton Road, Durham, UK. 3Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China. 

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 249

  • Programme No: 1.1.35

  • Theme 1 > Session 1

Abstract

Formation of economically significant porphyry deposits requires vast amounts of magmatic volatiles (H2O, Cl, and S). However, the conditions enhancing efficient trans-crustal delivery of volatile fluxes remain unclear. In this work, we investigate the water contents of zircon and volatile compositions of zircon-hosted apatite of the co-sourced pre-ore (ca. 110 Ma), syn-ore (ca. 103Ma), and post-ore (ca. 97 Ma) intrusions in the Zijinshan porphyry-epithermal ore field (ZOF) in South China. Although geochemically similar, zircon-hosted apatite inclusions from the syn-ore porphyry yielded highly variable Cl (0.14-2.94 wt.%) and F (0.04-3.37 wt.%) contents, whereas the pre-ore and post-ore apatite populations show restricted halogen compositions (0.62-2.70 wt.% Cl, and 0.41-1.56 wt.% F). Numerical modeling of apatite evolutionary trends suggests that the syn-ore magma had the highest initial melt H2O (5.08 wt.%) and Cl (2035 ppm) contents, reaching volatile saturation at ~215MPa and exsolving fluids with a salinity of ~7.0 wt.% NaCl eq., consistent with zircon data. In comparison, pre-ore and post-ore magmas had lower initial H2O (3.6-3.9 wt. %) and Cl (890-1370 ppm) contents, saturating at ~80-166 MPa with less saline fluid (2.3-4.7 wt.% NaCl eq.). We suggest that the syn-ore saturated conditions optimize Cl-Cu extraction efficiency, promoting the formation of connected ore-fluid networks and fluxes that are favourable for porphyry mineralization. Given the lack of direct syn-ore mafic magma input, we propose that fluids derived from underplating mafic magmas serve as the dominant volatile accumulation mechanism for porphyry mineralization in the ZOF.