Fluid properties and origins of the Lannigou Carlin-type gold deposit, SW China: Evidence from SHRIMP oxygen isotopes and LA-ICP-MS trace element compositions of hydrothermal quartz

Abstract

Oxygen isotope and trace elements characteristics of ore-related quartz from Lannigou gold deposit, a typical fault-control second largest Carlin-type Au deposit in SW China, provide valuable information about the prop- erties and origins of the ore-forming fluids. Previous works has focused on vein quartz instead of ore-related jasperoid-like quartz grain due to hard separation. Based upon different cathodoluminescence (CL) textural patterns and intensities observed within the quartz, four generations of quartz from this deposit were identified: Quartz I is sedimentary detrital quartz; Quartz II is syn-mineralization jasperoid-like quartz intergrown with and/or containing fine (< 10 μm) As-bearing pyrite and arsenopyrite; Quartz III is late-mineralization veinlet quartz with a small of medium-coarse-grained (about 50–100 μm) As-bearing pyrite; and Quartz IV is the post- mineralization drusy thick-veined quartz. Analyses by sensitive high-resolution ion microprobe (SHRIMP) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) were used to determine O isotope and trace elements compositions of quartz formed in different stages. The trace elemental (e.g., Al, Li, Ge, Ti) variation of quartz from Quartz II to IV reflect the effects of fluid saturation in CO2 as the result of decarbonization reactions, followed by a progressive fall in CO2, whereas the fluid pH and intensity of argillic alteration increase, pro- gressively. The range of δ18OVSMOW values in Quartz II (12.1–24.8‰) is wide but narrows in Quartz III (24.1–27.8‰) and IV (24.3–26.9‰). The estimated fluid δ18O values of early- or main-stage (calculated from Quartz II) is 3.2–15.9‰, the late-stage (calculated from Quartz III) is 12.5–16.2‰, and the post-stage (calcu- lated from Quartz IV) is 9.0–11.5‰. It is suggested that the ore-forming fluids of Lannigou gold deposit reflect a mixed source: a low O isotope end-member that was magmatic- or basement related, whereas the high O isotope end-member may have been fluid interacted with wall-rock, whose influence strengthened during the miner- alization episode. The narrow δ18OVSMOW ranges of Quartz III and IV also suggest that any meteoric water influence on the late hydrothermal system was weak.