The tectonic evolution and fluid characteristics of gold-quartz veins at the Globe and Phoenix Mine, Zimbabwe

The Globe and Phoenix Mine in central Zimbabwe has been developed down to a vertical depth of 1460m and, prior to effective cessation of underground working in 1977, produced 120t of gold at an average recovery grade of 27.6g/t. The deposit contains a large number of veins with a variety of orientations and is located at and around the contact between granitic gneisses of the Rhodesdale batholith and altered rocks of the Kwekwe Ultramafic Complex. Wide zones of ductile deformation are concentrated within the ultramafic rocks and occur both along the E-dipping contact and along NNW-striking zones. Where the two intersect, major alteration to magnesite-rock grading outward to talc-schists occurs. Early deformation involved ∼E-W compression with veins generated along both the gneiss-ultramafic contact and ENE-WSW-striking, subvertical fractures. These preceded the main mineralising event which involved NE-SW-directed compression, with subsequent veining along both NE- and SW-dipping reverse-sense fractures and major veins (the Phoenix Main and Parallel Reefs) related to an oblique reverse-dextral-sense reactivation of the gneiss-ultramafic contact. A further series of veins which steepen progressively down-dip formed along curved stress trajectories late in the evolution of the deposit. In all cases, veining is restricted to gneisses and carbonatised ultramafics, with lodes terminating rapidly on entering talc-schist, indicating that carbonatisation pre-dated vein development. Within the composite lodes, early, deformed, shear-parallel vein increments are clearly post-dated by later units. In the ultramafics, mutually cross-cutting age relationships between steeply dipping shear-parallel vein increments and horizontal extensional veins indicate the operation of stress cycling in the overall formation of the composite orebodies. Shear-parallel increments formed where fluid pressures rose to supralithostatic values sufficient to cause rupture, increased dilatancy, and subsequent rapid fluid infiltration.