Birnin Gwari Goldfield

Geology and mineralisation
The host-rocks at Birnin Gwari are a series of N-S trending, tightly folded, graphitic and sericitic phyllites and ferruginous quartzites. A combination of field mapping with GPS and interpretation of very high-resolution Quickbird satellite imagery has enabled accurate mapping of the artisanal workings at the site. Twelve kilometres of workings have been identified in parallel veins located within a mineralised zone that is 8500m long and up to 500m wide. Gaps in the artisanal workings are where rivers cross the mineralised zone and thick laterite soils are developed.

Past goldfield activity
Gold mining at Tsoshon Birnin Gwari has been in progress for at least 100 years. There are very few records of gold production as most of it has been in the informal sector by artisanal miners. There was a brief period of more mechanised production from the ‘Colonial Mine’ during the 1930s1. The tailings dump from this operation assays at ca 3g/t gold and artisanal mining continues today. The artisanal workings at Birnin Gwari are extensive. Excavations vary from steep sided and unstable open cast workings to open deep trenches, shafts and adits. Workings extend down to the water table at 10 to 20m depth where the transition from oxide to sulphide ore forms the limit of the artisanal miners’ technology for successful extraction of gold from ore.

Recent work
When the shaft was originally sunk in the 1990s¹ a 30kg composite grab sample yielded an assay of 294g/t². Subsequent sampling programmes by Gabako in 2006³ and Willan in 2007⁴ yielded best values of 40g/t and 21g/t respectively from 40-50cm long channel samples.

Drilling near the exploration shaft at Birnin Gwari has so far produced typical results for a gold deposit with variable grain size and nuggety distribution. Assays are taken every metre from rock chips produced by the RC percussion drill rig. Average values over a 30m width are encouraging and would not preclude open pit extraction methods, especially as drilling with standard fire assay typically understates the overall gold content in such deposits⁵. Work is on-going to identify the best sampling and assay techniques to reveal the true potential value of the Birnin Gwari goldfield.