Mineralogical, fluid inclusion and stable isotope characteristics of Birimian orogenic gold mineralization at the Morila Mine, Mali, West Africa

Abstract

Gold mineralization at Morila is hosted in flat-lying meta-graywacke associated with a Birimian-aged Massigui schist belt in Mali. Three major events of deformation/metamorphism/fluid flow (D 1, D 2 and D 3) are recognized at Morila, with mineralization associated with various increments of deformation and fluid flow during D 2. Three distinct vein sets V 1–V 3, associated with D 1–D 3 respectively, have also been recognized. V 1 vein sets are thin, poorly defined, quartz–feldspar metamorphic segregations broadly parallel to the regional fabric. V 2 veins are generally boudinaged, foliation-parallel quartz–feldspar veins, or occur as felsic melt segregations in the form of quartz–K feldspar–muscovite–biotite blebs located essentially within the ore-zone. V 3 veins occur as sub-parallel to high angle biotite–arsenopyrite–gold veins that appear to be extensional in nature. The ore zone is characterized by compositionally distinct biotite (higher MgO and lower FeO) and more calcic plagioclase than otherwise texturally identical minerals in the footwall and hanging wall of the mineralized zone, suggesting that prograde metamorphic minerals were modified by post-metamorphic Mg- and Ca-rich mineralizing fluids. Gold forms discrete grains in the V 2 veins or in the adjacent host rock, and is closely associated with pyrrhotite and arsenopyrite. Gold also occurs with Sb–Bi ore minerals in rare microfractures. Numerous thin biotite–arsenopyrite–gold veinlets cut D 1 and D 2 fabrics, and often carry significant gold associated with kink bands in the D 2 fabric. Fluid inclusion and oxygen isotope study provide evidence that Morila fluids are intermediate salinity (12–16 wt.% NaCl equivalent), NaCl–CaCl 2–MgCl 2–H 2O–CH 4–N 2 fluids of magmatic origin (δ 18O water = 10.9 ± 0.3‰). The ore-forming fluid subsequently unmixed to form a compositional range of vapor-rich CH 4–N 2, dilute NaCl ± CH 4–N 2 aqueous and liquid-rich brine associated with the opening of a micro-fracture network and vein development. Quartz–biotite isotope geothermometry indicates that mineralization occurred between 350° to 450 °C. Gold mineralization at Morila was an integral part of coeval plutonism, deformation and regional metamorphism, while the modification of the ore fluid by unmixing was the most important factor associated with the concentration of economically viable gold grades at Morila.