Quartz fluid inclusions and trace elements in the Lailisigaoer porphyry Mo deposit, Western Tianshan, Xinjiang: Fluid evolution and metallogenic mechanism in magmatic-hydrothermal system

Abstract

The Lailisigaoer deposit is a typical porphyry Mo deposit located in the late Paleozoic Boluokenu metallogenic belt of Western Tianshan, NW China. This study aims to understand molybdenum mineralization and the dynamics of magmatic-hydrothermal fluid within the porphyry system, focusing on fluid inclusion, cathodoluminescence textures, and trace elemental composition in various generations of quartz. Six vein types in the Lailisigaoer deposit are classified as comb quartz veins with unidirectional solidification texture (UST, QI), actinolite/biotite ± quartz veinlets (M-type, QII), quartz-chalcopyrite vein (A-type, QIII), quartz-molybdenite vein (B-type, QIV), quartz-pyrite vein (D-type, QV) and quartz-carbonate vein (E-type, QVI). Micro thermometry data indicate decreasing homogenization temperatures in primary fluid inclusions from UST to E-type veins. The magmatic-hydrothermal fluids associated with UST and M-, A-, B-, D-, E-type veins underwent a transition from high temperature (342–495℃), high-pressure (∼1.25 kb), and variable salinity (6.7–41.1 wt%NaCl) in the H2O-CO2-NaCl system to low-temperature (142–238℃), low-pressure (∼0.25 kb), and low-salinity (1.6–5.1 wt%NaCl) in the H2O-NaCl system. This evolution is reflected in the proportion of quartz with oscillatory zones decreasing and irregular quartz increasing, indicating a shift from turbulent to stable fluid conditions. Furthermore, a gradual reduction in Ti concentration (from 79 ppm in UST to 1.4 ppm in E-type quartz) suggests a decrease in fluid temperature during the magmatic-hydrothermal evolution. Mineralization primarily occurs in A- and B-type veins, with molybdenum precipitation attributed to fluid immiscibility in a medium–high temperature (240–368 ℃) and high pressure (∼0.95 kb) environment. The A-type veins (mean 31 ppm in Ti concentrations) consist of equigranular (dark-core) (QIII-1) and patchy inhomogeneous CL-gray quartz (QIII-2). The B-type veins (mean 24 ppm in Ti concentrations) are characterized by equigranular (bright-core) inhomogeneous quartz (QIV-1) with growth zones that are cemented by later mosaic homogeneous CL-gray quartz (QIV-2). Based on the quartz-Ti thermometer, the mineralized veins (A- and B-type) were confined to be formed at depths of 2.8–3.6 km, with molybdenum mineralization occurring at 3.4–3.6 km. Molybdenum mineralization initiates during the QIV-1 crystallization period in magmatic-hydrothermal fluids and terminates in the QV-2 generation.