Summary:
The How Gold Mine (HGM) is located in the Umzingwane Formation of the Bulawayo Greenstone Belt (BGB). The lithological units characteristic of the Umzingwane Formation include clastic metasediments, fine-grained tuffaceous rocks, banded shales and siltstones, ferruginous cherts or Banded Iron Formation (BIF), and rhyodacites and andesitic lavas. This assemblage has been subjected to metamorphism of lower greenschist facies.
The local geology around HGM consists of a sequence talc-chlorite schist, laminated black shale, silicate facies BIF, tuffaceous units, and siltstone from southwest to northeast with the occurrence of felsic porphyry intrusions and mafic dykes.
Deposit type and geology
The geological and structural information seen in the open pits provides some insight into the structural controls that are believed to be responsible for the localisation of mineralisation at the HGM. The main control is believed to be an extensional duplex, with a long axis oriented along the direction of extension (BMC 2023b).
Previous models suggesting that the ore was stratabound within felsites and was syngenetic in nature have now been disregarded due to the transgressive nature of the mineralisation. The felsites are in fact, propylitised tuffs. Prominent shears are evident on both the hangingwall and footwall contacts of the ore channel, and these represent the principal shears of the duplex system (BMC 2023b).
A new theory being tested suggests that the hangingwall contact of the dacite controlled mineralisation in the main orebodies on both strike and dip. This theory is evident when a composite plan of the mine is viewed. All development that continued into the hangingwall contact of the dacite, both north and south, does not contain significant mineralisation when compared to the main orebodies. However, there are some orebodies in the top section of the mine that are hosted in dacite. It appears that these orebodies are located at the central inflection point of the hangingwall dacite contact. This phenomenon is observed above the 16 Level. This theory is significant in terms of defining future exploration targets within the dacite hangingwall contact envelope and within the dacite itself and suggests the possibility of another “trouser leg” orebody plunging to the south (BMC 2023b).
Mineralisation
Orebodies are generally elliptical in plan, strike to the north, and dip steeply at approximately 80° to the west, with a steep northerly plunge. The major orebodies are the 300N, 180N, 350N, 400N, and 10S, which have a combined strike length of approximately 500 m. A waste or low-grade parting separates the 300N and 180N orebodies, which becomes narrower at depth, and below the 20 Level is approximately 5 to 10 m in width.
Where this occurs, the two orebodies are combined into a single domain for resource evaluation purposes, between the 20 Level and 24 Level, with a maximum width of approximately 55 m. This change in orebody dimensions necessitated a change in sampling technique and sludge drilling was replaced by the drilling of deeper diamond drill holes (known as profile drilling) to reach the limits of the mineralisation (BMC 2023b).
Between the 24 Level and 26 Level, the waste parting between the 300N and 180N was left intact. This was as a result of recommendations made by a rock mechanics consultant engaged by BMC in mid-2012 and resource evaluation was conducted accordingly. However, between the 26 Level and 28 Level, the 300N and 180N orebodies merge and are being mined as a single orebody (BMC 2023b).
Current information from deep diamond drilling indicates that the orebodies gradually taper in width with depth and become shallower in dip i.e. approximately 60°. The deposit shows significant hydrothermal alteration is frequently associated with sulphide mineralisation. Strong carbonation, silicification, and in extreme cases propylitisation occur together with sulphides. Pyrite is the dominant sulphide, with significantly less chalcopyrite and occasional pyrrhotite present. Chalcocite and arsenopyrite have also been reported. Ore microscopy has shown that gold is associated with a late generation of pyrite and chalcocite. The location of gold mineralisation is either in fractures within the sulphides or on sulphide grain surfaces (BMC 2023b).
Mineralisation at the HGM is thought to have been formed by hydrothermal solutions migrating along structurally controlled channels, predominantly caused by an extensional duplex, with its long axis parallel to the direction of extension. Recent underground exploration drilling between the 30 Level and 32 Level, has identified three new orebodies, namely 320N, 330N and 360N, which are situated in the footwall of 300N and are hosted within the dacite, a completely different environment when compared to the other orebodies. However, current evaluation work focuses on establishing the orebody strike extensions between the 28 Level and 30 Level, and down-dip extension below the 30 Level. This discovery brings in the dacite environment as a prospective future exploration target (BMC 2023b).
The mineralised zones at the HGM have sharp grade boundaries, that in most cases can be defined with confidence. The Mineral Resource envelopes (domain wireframes) are delineated using grade-based cut-offs (soft boundaries) based on pay limits (BMC 2023b).