Orogenic auriferous mineralisation in structurally-complex orebodies of Blanket Mine, Gwanda Greenstone Belt, Zimbabwe: New insights into the structural framework and evolution of orebody geometries
Tect was approached in early 2020 by Caledonia Mining Corporation to conduct a thorough review of the historical underground data and workings of Blanket Gold Mine in order to create a fully-constrained baseline structural and auriferous grade shell model of their underground workings, for drillhole planning and ore resource extension. The primary aim was to provide insights into the increasing structural complexity of orebody geometry, continuity and predictability at increasing depths.
Blanket Gold Mine consists of several auriferous oreshoots, deposits and prospects situated within the NNW-portions of the W-NW-trending, arcuate Archaean (2.7 Ga) Gwanda Greenstone Belt, southwestern Zimbabwe. Blanket has produced in excess of 1.5 million ounces of gold since 1904. The Gwanda Greenstone Belt is dominated by greenschist-amphibolite facies ultramafic/mafic to felsic volcanics with intercalated sedimentary units and banded-iron formation (BIF) horizons, exhibiting a typical synclinal, keel-shaped geometry surrounded by several large felsic granitoid plutons.
The deposit-scale mineralisation is centralized along a prominent NW- to NNW-trending structural zone, commonly referred to as the Northwest Deformation Zone (NWDZ), that refers to a first- to second-order, deep-seated thrust zone which has been transposed to a steeply southwest-dipping to sub-vertical, brittle-ductile shear zone (SZ) corridor. The tectonically-early (D1) NWDZ has been subjected to reactivation during subsequent deformation episodes and exhibits a complex overprinting relationship with the other structural features.
Blanket Gold Mine principally consists of several discrete auriferous orebodies, including Lima, Eroica, Sheet, ARM, ARS, Blanket and Jethro-Feudal, which are associated with pervasive chlorite-biotite-carbonate-sericite±quartz wall-rock alteration, particularly well-developed within high-strain varieties of schistose and sheared wall-rock. There are two main modes of mineralisation: disseminated sulphide replacement (DSR)-type and quartz reef-type mineralisation. DSR-type mineralisation is considered to be the primary mode, consisting of fabric-concordant auriferous sulphides related to early (D1?) shearing, occurring as fine-grained disseminations and lodes to foliation-parallel sulphide stringers. Quartz reef-type mineralisation, such as the Blanket Quartz Reef (BQR), is a secondary mode of mineralisation, confined to discrete, silicified thrust/shear zones.
Tect conducted the validation, 3D georeferencing, digitization and interpretation of ∼80 years historical underground mapping and drillhole data, followed by the structural analysis of mine- to license-scale features, in order to elucidate the structural controls and favourable geometries that facilitated preferential fluid throughput and concentrated economic mineralisation, as well as later offsetting structures. Steeply dipping to sub-vertical DSR orebodies have locally been upgraded by a second phase of auriferous mineralisation associated with oblique, moderately to steeply-dipping silicified fault zones (BQR). These auriferous orebodies, in turn show 15m to 200m of displacement by later faulting/deformation events. This structural interpretation and mineralisation model provides a robust framework to foreshadow structural complexities at depth and to delineate prospective fault blocks for on-going exploration and resource extensions.