Geological evolution of the world class Gejiu Ore District, SW China: information from magmatism, mineralization and microanalysis studies

Abstract

This study targets the magmatic rocks and Sn polymetallic ores in the Gejiu district, Yunnan Province, with the aim of resolving problems related to geodynamic setting, ore-forming processes and genesis of the mineralization in the study area. Systematic new SHRIMP/LA-ICP-MS U-Pb zircon dating results indicate that all the intrusive rocks in the Gejiu district are broadly coeval, ranging from 78 Ma to 85 Ma. Elemental and isotopic geochemistry of these rocks reveal that the granites in the Gejiu district experienced different degrees of fractionation, and they were mainly sourced from crustal materials with minor input from mantle-derived magma. The Jiasha gabbro derived from the partial melting of the ithospheric mantle with some contamination of crustal materials, and the mafic microgranular enclaves represent the results of magma mingling and mixing between the crustal- and mantle-derived magmas. Both alkaline rocks and mafic dykes were sourced from lithospheric mantle but experienced different evolutionary processes. Based on all of these concepts, I conclude that extensive interaction between mantle and crustal materials occurred in Gejiu district during Late Cretaceous. Metal zonation away from granitic intrusions, both in horizontal and vertical directions, are clearly developed in the Gejiu Sn polymetallic ore district, commonly from W-Be-Bi±Mo±Sn zone closest to the granites, out to Cu-Sn zone and then to a Pb-Zn zone. New data indicate that the homogenization temperature and salinity of the ore-forming fluid display continuous variations in different mineralization stages, which is a typical characteristic of the granite-related hydrothermal deposits. Sulfur isotopes of various sulfides, and hydrogen and oxygen isotopic compositions of different quartz generations exhibit obvious differences, which further supports varying conditions for mineralisation. Moreover, application of different geochronological methods on different minerals, which include mica Ar-Ar, molybdenum Re-Os and LA-ICP-MS cassiterite U-Pb dating, reveal that all the ore styles in the Gejiu district formed in a short time period, and mineralization and magmatism ages are contemporaneous. Above all, the large scale metallic accumulation and mineralization in Gejiu district are of hydrothermal origin. Iron isotope variations in ores from Gaosong deposit show primary sulfide ores enriched in lightest isotopes, while δ⁵⁷Fe values of oxidized ores and gossan ores becoming heavier progressively. Moreover, the range of δ⁵⁷Fe values of primary ores and oxidized ores are larger than that of gossan samples. Combined with the mineral association, this Fe isotope features suggest that Fe isotope compositions are affected by mineralogy, and it is the primary mechanism of the large Fe isotopic variations in the primary ores. On the whole, the systematic variations of Fe isotope composition show that Fe isotope studies have potential to constrain metal sources, ore genesis and evolutionary of hydrothermal deposit. Furthermore, CL textural results indicate distance between samples and granites may be one of the key factors controlling the cassiterite size and internal micro-texture variations. Fe, Ti and W are the most abundant elements in cassiterite, and variation of Zr/Hf ratios maybe caused by various tourmaline and fluorite contents. Cassiterite geochemical and textural variations may reflect the temperature, oxygen fugacity and redox station of ore forming fluids experienced significant changes during the tin mineralization process.