Summary:
Mazowe Mine is a merger of several formerly independent mines of which the old Jumbo mine was the single most important producer. Host rocks to the mineralization are the Jumbo granodiorite stock and the slightly earlier, comagmatic Jumbo quartz-feldspar porphyry, both intrusive into mainly metadacites and metasediments of the Passaford and Mount Hampden Formations. The ore bodies are found in subparallel, planar shear zones striking nearly E±W and dipping 25±45°N. The major ore bodies, formerly independent mines, are known (from S to N) as the Carn Brae, Birthday, Nucleus, Jumbo, Connaught, Bojum, Bucks, SOS, and Flowing Bowl reefs, which are separated on outcrop by between 100 and 300 m and can be followed for about 500 to 1500 m along strike. The major sections of the E±W striking reefs are hosted in the Jumbo granodiorite stock, but also extend eastwards into the Jumbo porphyry and greenstones. The reefs are exposed by underground mining which presently extends down to the 28th level (about 900 m below the shaft collar). The reefs have average widths of 20 cm but commonly split into mineralized stringer zones some 30±60 cm wide which may join again or cross to adjoining reefs.
The ores are dominated by pyrite, which is often massive, quartz and rarer carbonate, which together form veins usually a few centimetres wide. Gold is mainly associated with the pyrite, and to a lesser degree with quartz. Contacts of the veins with the granodiorite or porphyry country rocks are sharp, and little sign of hydrothermal alteration is obvious in the adjoining rocks. Thin sericite selvages may be developed, and up to 1 cm wide scheelite veinlets, lenses or pockets locally accompany the reefs on both their hanging and footwall contacts.
Mineralised zones are up to 1m in width, have average grades of 4 to 5g/t, and mostly dip between 10° and 60° to the north.
Pyrite is the dominant sulfide (>95% of the ore minerals). It occurs in the form of massive and layered ores, of which both fine- (grain diameters<50 mm) and coarse grained (mm-size) varieties are present. The pyrite grains are compact, homogeneous, and idiomorphic to hypidiomorphic in shape. They usually form aggregates that are locally fractured. Pyrrhotite, sphalerite, chalcopyrite, galena, arsenopyrite and cobaltite are present in minor quantities in the ores. Gold recoveries of >90% are achieved by a combination of gravity separation and cyanide leaching, indicating that gold is present in a particulate form. Gold occurs as roundish inclusions (5-30 mm in diameter) in pyrite or in rarer instances in gangue, and as thin films along grain contacts or cracks in pyrite. Occasionally, gold also forms large (up to 200 mm in diameter), irregular grains in pyrite or gangue. Silver contents of the gold, analyzed by microprobe, range between 7 and 20 wt% Ag (Tomschi 1987; n = 121) in gold grains of different samples collected throughout the mine. The silver contents of gold vary in an unsystematic manner with little correlation with depth or from one reef to the other.