The tectonic Neoarchean evolution that resulted in refolded folds and overprinting planar fabrics affected one of the world’s giant gold provinces, the Lake Victoria Goldfield in which lies the Sukumaland Greenstone Belt (SGB; Henkel et al., 2016), Tanzania Craton, and was of major importance in focusing gold mineralisation. This multiphase and/or progressive deformation history led to a complex structural pattern and heterogenous gold distributions as expressed in the two greenstone-hosted gold deposits of Nyanzaga and Kilimani located less than a kilometre apart, in the heart of the SGB. We conducted an extensive field work at the deposit scale and a detailed petro-structural-textural study at core to microscopic scales on sulphide and silicate minerals, in order to determine the controls of deformation and hydrothermal events on gold mineralisation. Early D1 shortening under ductile conditions resulted in kilometric asymmetrical folds showing various geometries and orientations, from open to isoclinal and cylindrical to sheath geometries, with northwest- to northeast-oriented axial planes and moderately plunging to the northwest through to northeast. A discrete S1 cleavage is expressed within the finest sedimentary layers, parallel to the D1-related fold axial plane. Strong carbonatisation and silicification developed alongside dolomite-quartz veins, either crustiform when parallel to the bedding or comb when perpendicular to it, typical of syn-folding veins (Druguet, 2019). The second stage consists of progressive deformation with subhorizontal north shortening and a progressive shift from ductile D2a to brittle D2b events. The D2a stage is characterised by east- to east-northeast-striking folds with subvertical plunge that refolded the D1-related folds, attested by the major variations in their geometries and orientations. A discrete S2a cleavage overprinted the S1 fabric. Disseminated pyrite grains crosscut the D1-related veins and are affected by the D2a-related folds. The two-stage folding led to overturned anticlines with northwest-oriented subvertical limbs at the Kilimani deposit, and a subvertical east limb shifting from northwest- to northeast-oriented at the Nyanzaga deposit. Finally, major subvertical faults developed during the D2b stage, preferentially along the vertical lithological contacts that constitute weak planes favourable for fracturing, focusing the ore-bearing fluid(s) and depositing gold. It resulted in mineralised northwest-trending dextral strike-slip faults in both deposits, and highly mineralised north-oriented dilatational jogs constrained by the strike-slip faults at the Nyanzaga gold deposit where the stratigraphy was north-south re-oriented during the D2a-related folding. Jogs are expressed by electrum- and As-Sb-Pb-Cu-Ni-rich sulphide-bearing N-S normal faults and extensional veins. Therefore, if Kilimani and Nyanzaga deposits show similar volcano-sedimentary sequences and underwent the same history of deformation and hydrothermal alteration, the geometry of their folded lithologies did not allow similar fracturing and faulting, explaining the higher mineralisation rate of the Nyanzaga deposit.

Reference(s)

Druguet, E 2019, Deciphering the presence of axial-planar veins in tectonites: Geoscience Frontiers, v. 10, p. 2101-2115. Henkel J, Poulsen KH, Sharp T, Spora P, 2016, Lake Victoria Goldfields: Episodes, v. 39, p 135-154.