Tracing the source of the Mineralization Fluid,and metamorphism of the Nampala Gold Deposit,southern Mali: Constraints from stable (oxygen and hydrogen) isotopes and Raman Spectroscopy

Abstract

The Nampala gold deposit is a sub-world-class occurrence in Mali's Birimian Terrain. Although numerous detailed investigations have been conducted on Mali's gold deposits, the debate on the origin of the mineralizing fluid remains unresolved. To address that knowledge gap this study presents oxygen, and hydrogen isotope data for auriferous quartz veins, along with Raman spectroscopy analysis of graphitic schist, to investigate the genesis and evolution of the mineralizing fluid of the Nampala gold deposit and the associated metamorphic grade. The isotopic signatures of quartz veins and their fluid inclusions reveal δD values ranging from −48 to 0.3‰ (mean −25.98‰, n = 15) and δ¹⁸O values from 15.23 to 16.57‰ (mean 16.14‰, n = 15). The narrow δ¹⁸O range of auriferous quartz veins indicates a common hydrothermal fluid source, while the high δD values suggest meteoric water influence. The Nampala gold deposit likely formed from metamorphic fluids derived from nearby metasedimentary rocks during regional metamorphism, with Au deposition driven by fluid pressure drops and interaction between meteoric and metamorphic fluids with wall rocks. Metamorphism studies indicate greenschist facies, with temperatures ranging from 358 °C to 486 °C (avg. 416 °C ± 50 °C). The gold-enriched fluid at Nampala precipitated gold through metasomatism, driven by interaction with surrounding rocks and decreases in temperature and pressure.