Ore-forming process and fluid development at Nanpo Au deposit, Luang Prabang-Loei Belt (NW Laos): Evidence from H-O-S-Pb isotopes and inclusions

Abstract

The Luang Prabang-Loei tectonic belt, as a significant polymetallic metallogenic belt within the Southeast Asian Tethys tectonic domain, hosts numerous gold deposits. However, the current understanding of the genetic types and mineralization processes of these gold deposits remains insufficient, which restricts the advancement of regional metallogenic theories. This study focuses on the newly discovered large-scale Nanpo gold deposit in this metallogenic belt. By systematically conducting analyses of ore deposit geological characteristics, fluid inclusion testing, and H-O-S-Pb isotope analyses, it aims to reveal the sources of ore-forming materials, clarify the properties and evolutionary patterns of ore-forming fluids, and further elucidate the genetic mechanism and the process of mineralization of the deposit. The orebodies of the Nanpo gold deposit typically have lenticular, stratoid, and veined shapes, which are clearly controlled by ductile shear zones. The ores are mainly of two types: quartz vein type and altered rock type. Based on the interpenetrating relationship of veins and the symbiotic assembly of minerals, the metallogenic process of the Nanpo gold deposit can be divided into three stages: (I) quartz-sericite-gold-poor pyrite stage, (II) quartz-polymetallic sulphide stage, and (III) quartz-carbonate mineral stage. With the help of studies on fluid inclusion petrography, microscopic thermometry and laser Raman spectroscopy, it can be found that there are three main types of fluid inclusions in the deposit, namely, NaCl-H₂O, NaCl-CO₂-H₂O and pure CO₂. In general terms, they are part of a NaCl - H₂O - CO₂ ± CH₄ ± N₂ low-temperature to intermediate-temperature/low-salinity fluid system. Using the isotopes S-Pb can be seen, the ore - forming materials come to a large extent from the homogenized lower crust - upper mantle. The H - O isotopes suggest that the ore - forming fluids were of metamorphic water which had an affinity to meteoric waters in the later stage. These evidences suggest that the Nanpo gold deposit is a typical orogenic gold deposit, and its formation is closely associated with the collisional orogeny in Indo - China block after the Paleo — Tethys closed. This study provides a theoretical foundation for the advancement of regional metallogenic theories and offers guidance for regional mineral exploration.