Genesis of the Xiaolonghe quartz vein type Sn deposit, SW China: Insights from cathodoluminescence textures and trace elements of quartz, fluid inclusions, and oxygen isotopes

Abstract

Quartz textures revealed by scanning electron microscope cathodoluminescence (SEM–CL) images suggest that quartz in the Xiaolonghe Sn deposit formed in at least three fluid episodes. Quartz (Q1) in the pre-ore silicification zone has homogeneous texture, high CL intensity and high Ti content (19–53 ppm). Fluid inclusions hosted in Q1 are vapor-rich (V1-type) and homogenized as supercritical fluid at ∼570 °C. Quartz (Q2) in the syn-ore topaz-cassiterite-quartz veins has lower CL intensity and is characterized by euhedral growth zones with lower Ti content (1.3–12 ppm). Fluid inclusions hosted in Q2, topaz and cassiterite are liquid-rich fluid inclusions of moderate salinity (∼15 wt% NaCl equiv) (L1-type) which homogenized at ∼423 °C. The post-ore quartz (Q3) closely associated with sericitization of topaz generally crosscuts Q2, and has dark CL intensity and much lower Ti concentration (<1.5 ppm). In Q3, the vapor-rich (V2-type) fluid inclusions coexist with the halite-bearing (S-type) fluid inclusions, which homogenized at ∼269 °C, 0.05 kbars.The upper limits of T–P conditions of pre-ore and syn-ore stage are constrained by the intersection of NaCl–H2O fluid inclusion isochores and Ti-in-quartz isopleths, and the lower limits are based on microthermometry results of fluid inclusions. Combined with the δ18Ofluid value (8.3‰) of Q1, it indicates that the pre-ore supercritical fluid was directly exsolved from a granitic magma at T = 570–600 °C and P = 0.70–0.77 kbars, corresponding to a maximum depth of 2.8 km under lithostatic pressure. The syn-ore subcritical fluid (8.0‰ > δ18Ofluid > 6.3‰) mixed with some meteoric water was under 423–450 °C, 0.32–0.45 kbars. The transition from the pre-ore supercritical fluid to the syn-ore subcritical fluid indicates boiling condition (probably at 512 °C, 0.62 kbars), which was likely caused by hydraulic fracturing under high pressure. Both the fluid boiling and fluid mixing contribute to the cassiterite precipitation in Xiaolonghe Sn deposit. In the post-ore stage, the coexisting S-type and V2-type fluid inclusions in Q3 indicates a second fluid boiling at 269 °C, 0.05 kbars, which was probably caused by decompression due to rapid denudation of the overlain strata. The low salinity (5.5 wt% NaCl equiv) L2-type fluid inclusions hosted in fluorite homogenized at 199 °C and <0.02 kbars, indicating that fluorite veins probably formed in open fractures. The δ18Ofluid results from sericite (5.1–5.0‰), quartz (−1.3 to −2.0‰), and chlorite (−7.9‰) suggest that increasing meteoric water was involved after second fluid boiling.