OPERATION FOCUS
formation layers. The ultramafic-mafic unit is in turn overlain to the west by the mafic unit, a thick sequence of tholeiitic and pillow basalts. Within this unit a prominent shear zone up to 50 m wide runs the length of and is the locus of all orebodies on the Blanket property. The sequence is completed by an andesitic unit which caps the stratigraphic sequence.
Blanket mine is part of the group of mines that make up the North Western Mining Camp otherwise also called the Sabiwa group of mines. What is today referred to as Blanket is a cluster of mines extending from Jethro in the south, through Blanket itself, Feudal, AR South, AR Main, Sheet, Eroica and Lima in the north. These ore shoots occur in the Blanket shear zone, a low angle transgressive shear characterised by the presence of biotite relative to the massive amphibolites forming the country rocks.
A regional sub horizontal dolerite sill intruded the above sequence and is emplaced about 500 m below surface. The sill does not cause a significant displacement and although it truncates all the ore shoots, the mineralised shoots continue undisturbed below the sill.
Active mining at the Blanket mine takes place over a 3 km strike that includes eight discrete ore shoots. Mineralisation occurs in near vertical shoots aligned along an approximately northsouth axis. The ore shoots vary in shape from the tabular to lensoidal quartz reefs to the massive to pipe-like disseminated sulphide reefs( DSR).
Two main types of mineralisation have been recognised; disseminated sulphide replacement reefs and quartz-filled reefs and shears.
The disseminated sulphide replacement mineralisation comprises the bulk of the ore shoots. Typically these zones have a silicified core with fine sprays of disseminated arsenopyrite hosting the best grades. Disseminated sulphide replacement orebodies range up to 50 m in width with a strike between 60 and 90 m. Free-milling gold constitutes up to 50 % of the total metal content with the remainder occluded within the arsenopyrite.
Quartz-filled shear zones form the second type of mineralisation. Two quartz shears are mined, the Blanket Quartz Reef and the Eroica Reef. These reefs tend to have long strikes but are not uniformly mineralised although continuous pay shoots of over 100 m on strike are not uncommon. The Quartz Reef at Blanket has a surface strike of some 500 m, but economic mineralisation is restricted to three 90 m shoots which were defined on surface by the early workers. Grade fluctuations are more extreme in the quartz reefs than in the disseminated type reefs but on average these shears have higher grades and are used as a‘ sweetener’ of ore to the mill.
The current Blanket mining area has eight ore shoots in the producing section of the mine. The majority of the mine production is sourced at present from the AR Main and AR South orebodies with a lesser contribution from the Blanket, Eroica and Lima reefs. AR Main and AR South are massive orebodies up to 30 m wide and are ideally suited to the long-hole open stope mining method, while the remainder of the Blanket orebodies are tabular and better suited to underhand stoping methods.
Current production is about 1,500 t / d of ore, following the installation of a new ball mill( manufactured by Nimr and Chapman in Bulawayo) in late 2016, using both long-hole open stoping and underhand stoping methods.
The mine operates five GHH Aardvark LF 4.2 LHDs and recently acquired two Fermel Liberator 14 t low profile dump trucks. Broken ore is trammed along the 22 level rail system by battery locos and the ore cars self-tip onto one of three grizzlies above the ore bins which are located between 22 level and the 765 m level crushing station. The minus 300 mm rock is held in three underground storage bins; payable ore and waste ore are held in separate storage bins and handled accordingly. Ore is gravity fed from level crushing station conveyor which discharges the ore onto a vibrating grizzly feeder which discharges the oversize into a 30 x 20 Telsmith jaw crusher.
The underground crushing station ensures that all the ROM ore is reduced to minus 150 mm in size providing for the optimisation and greater efficiency of the automated skip loading and hoisting operations. This allows mining and hoisting activity to continue without interruption. Blanket No 4 Shaft has been equipped with the first automated loading system in Zimbabwe which sequentially fills the two 6 t ore skips which are hoisted to surface. The use of this state of the art automation reduces the risk of ore loading accidents and injuries, reduces manpower costs, minimises spillage, reduces skip loading times, increases hoisting capacity, ensures precise ore tonnage accountability, and enhances winder efficiency while lowering loading and hoisting costs. The structures for this automated system were designed, manufactured and installed by Kenvist SA. The automation was designed, supplied and installed by Festo in July 2010.
The double compartment No 4 Shaft is Blanket ' s main shaft for hoisting ore to surface from the loading stations at 510 and 789 m below surface, and it has a proven hoisting capacity of 110 t / h from 789 m. The Jethro and Eroica Shafts and the No 5 and No 6 Winzes are used for transporting personnel and materials underground, and the No 2 and Lima Shafts are also used for hoisting ore to surface.
The entire underground and surface operations of the Blanket mine, except for the Lima Shaft, including the surface compressors and the No 4 Shaft Winder can be operated by the 10,000 kVA standby diesel powered generating sets.
The skips automatically tip ore hoisted to surface into the Shaft Bins on the No 4 Shaft headgear. Ore is gravity fed from the shaft bins onto the No 1 belt which conveys the ore over the automated belt scale and to vibrating screens and 14 x 24 Telsmith jaw crushers. This crushing circuit reduces the ore to minus 50 mm and it is then deposited by the No 2 belt stockpile conveyor onto the coarse ore stockpile which has a live capacity of approximately 2,000 t of material. Ore from the coarse ore stockpile is then fed onto the triple-deck vibrating screen with the oversize being crushed to minus 12 mm by one of two 38H Telsmith Gyrasphere crushers.
The 12 mm ore is then fed into the 600 t Mill Bin which feeds two( of the three installed) 1.8 x 3.6 m rod mills where it is milled down to approximately 70 % passing 75 micron, before being passed through two 30” continuous Knelson Concentrators where approximately 49 % of total gold production is recovered. The Knelson tails are pumped through cyclones and into a 3.66 x 4.9 m x 750 kW regrind ball mill. As part of the No 4 Shaft Expansion Project, the capacity of the secondary crushers was increased to over 2,000 t / d and the capacity of the rod mills was increased to 1,800 t / d.
Slurry from the regrind mill is pumped into a CIL plant consisting of eight, 600 m 3 leach tanks equipped with 45 kW agitators where leaching at 50 % solids and simultaneous adsorption of dissolved gold onto activated carbon takes place. The CIL plant has a nameplate capacity of 3,800 t / d of milled ore. Elution of the gold from the loaded carbon and electro winning is done on site. Gold is deposited onto steel wool cathodes, the loaded cathodes are aciddigested and the resultant gold solids are smelted in an induction furnace to produce gold bullion of approximately 90 % purity, after which the bullion is sold as required by Zimbabwean law to Fidelity.
Overall gold recovery rates have been increased from 85 % at the time of acquisition by Caledonia to over 94 % as a result of the redesign of more efficient CIL agitators and the installation of an automated liquid sodium cyanide facility which allows for multiple stage cyanide dosing and monitoring of the CIL. The PSA( Pressure Swing Adsorption) oxygen generator has been re-commissioned and the controlled sparging of oxygen into the CIL has resulted in an increase in leach recoveries to approximately 94 %.
14 International Mining | JANUARY 2018