McKinnons Gold Deposit
The McKinnons gold deposit, located 37km southwest of Cobar occurs within CCR's EL6402. Norgold Limited (and its precursor company, Geopeko Limited) conducted regional drainage geochemical surveys throughout the area. Prospecting work conducted during these surveys led to the discovery of the outcropping McKinnons deposit in December 1988. Norgold drillied 4920m of RC and 1070m of diamond core in evaluating the deposit but sold the deposit and associated exploration tenements to Burdekin Resources Limited (BRL). BRL completed a further 5743m of RC and 389m of diamond drilling based and based on all the drilling defined a resource of 2.7 million tonnes at 1.7 g/t Au using a 0.6 g/t Au cut off. A pit optimisation generated a reserve of 2.2 million tonnes at 1.91 g/t Au at a 0.7 g/t Au cut off. Mining commenced in February 1995 and was completed in December 1996. Milling of ore commenced in April 1995 and continued until April 2000 when the operation was shut down. Almost all ore mined and milled was oxide material.
Figure 1: McKinnons open cut - view to north
Geology
The geology of the deposit is described by Elliot (1998) and Forster and Seccombe (1999). Although the deposit did outcrop much of the area is covered by shallow residual and, in places, thicker transported soils. the deposit is located in the Lower Amphitheatre Group and diamond drilling has allowed for the definition of 5 different lithological units in the mine environment. The lowermost unit comprises of generally thin-bedded gray siltstone interbedded with weakly carbonaceous fine grained quartz sandstone and mudstone. the 100m thick Unit 2 consists of interbedded mudstone and fine-grained mudstone. Unit 3 consists of alternating dark gray siltstone and very fine-grained quartz sandstone. Unit 4, host to the McKinnons oxide deposit, is approximately 300m thick and consists of fine grained quartz sandstone interbedded with thin (3-15mm) siltstone beds. Unit 5 has only been defined north of the open pit and comprises a fine-grained dark gray massive limestone more than 20m thick.
In the mine area, bedding dips at 15-35 degrees towards the north and northwest and the units form a broad anticlinal structure with associated subsidiary folds along its axis. In the pit area the dominant structures are a set of four 20-50m wide steeply dipping fault zones of differing orientations. the 330 trending fault controls the long axis of the deposit with the 035 trend controlling the distribution of higher grade material. Offset on the faults appear minor and they contain varying amounts of quartz veining that in places generate stockworks grading to breccias.
Although most of the ore mined and treated at McKinnons was oxidised material, alteration in the primary zone consists of an extensive zone of carbonate alteration, an intermediate zone of pyrite and inner silica alteration zone. This alteration pattern is similar to that observed at Elura.
Figure 2: McKinnons geological section
Primary gold occurs as small grains locked veins, silica breccias and pyrite veinlets. Supergene gold occurs as tiny grains associated with goethite and clay-rich fault zones. Supergene gold contains almost no silver but in the primary zone silver assays of up to 800 g/t Ag have been recorded. Copper, lead and zinc have been leached by the oxidation process but a poorly developed supergene blanket, rich in these metals occurs at the redox interface.
The mineralised body in the primary zone is much narrower than the oxidised orebody and occurs within a 330 degrees trending shear zone. Primary mineralisation is dominantly refractory in nature with most of the gold being ultrafine grained and locked within pyrite. Pyrite is the dominant sulphode in the deposit but other suplhides present include arsenopyrite, low iron sphalerite, galena, chalcopyrite, tetrahedrite, covellite, and digenite.
The McKinnons gold deposit is a supergene enriched zone which developed during the Tertary above an Early to Middle Devonian, structurally controlled zinc-lead-gold system. The style and obvious structural control of the primary mineralisation is broadly similar to the other Cobar Basin deposits.
Operations
Following the completion of the feasibility study in 1994 a decision was taken to commence mining of the orebody by an open cut. To achieve economies of scale the mining was carried out at a much faster rate than the proposed mill could process the ore. Mining operations commenced in February 1995. Ore, mineralised waste, and mullock were stockpiled separately. A small (nominal 62tph - approximately 500,000tpa) CIP plant was constructed on site and ore treatment commenced in April 1995. Mining of the open pit was completed in December 1996 but milling of stockpiled ore was not completed until August 1998. During mining of the orebody, an 850,000 tonne stockpile of mineralised waste had been generated and batch sampling of this material indicated that grades were higher than had been predicted from grade control sampling in the pit. A decision was taken to mill this material and treatment continued until the operation was shut down in April 2000.
Mill recoveries in excess of 90% were achieved for run of the mine ore but fell to approximately 85% with the treatment of the lower grade (approximately 0.9 g/t Au) mineralised waste material. the last tonnage treated was low grade (0.65 g/t Au) and contained some unoxidised material and recoveries fell to 65%.
Total life of mine production was approximately 127,000 ounces of gold against the feasibility study of 129,000 ounces. Slightly lower grades and recoveries resulted in a small shortfall in ounces compared to the feasibility study. A similar number of ounces of silver were produced.
Production Reconciliation
| Feasibility Reserves - Whittle Pit Optimisation | 2.5mt @ 1.73 g/t Au for 135,000 oz |
| Feasibility Reserves - Final Pit Design | 2.4mt @ 1.75 g/t Au for 134,500 oz |
| Actual Tonnes Treated | 2.7mt @ 1.61 g/t Au for 138,200 oz |
CCR Program
CCR has completed a drilling program to test a series of parallel IP anomalies associated with anomalous soil and RAB geochemistry 1 to 2km to the north and along strike from the McKinnons open pit. Results were in general disappointing although a narrow intersection of Ag (2m @ 448 g/t from 121 to 123m in CCR140) and some untested IP response warrants further work. CCR has yet to assess the potential for extensions and repetitions of the McKinnons gold mineralisation, or the base metal intersections of up to 21.9% Zn and 10.9% Pb made in drilling below the open pit.
References
Elliot, S. M., Bywater, A. and Johnson, C., 1998. McKinnons Gold Deposit, Cobar In: Geology of Australian and Papua New Guinean Mineral Deposits (Eds. D A Berkman and D H McKenzie), (The Australian Institute of Mining and Metallurgy: Melbourne): pp 567-574
Forster, D. B., and Seccombe, P. K., 1999. Syntectonic Base metal Mineralisation with an Epithermal Gold Overprint: McKinnons Gold Deposit, Cobar, NSW, Australia. In: (Ed. G. Weber) Proceedings International Conference on Earth Science. Exploration around the Pacific Rim AIMM V4 pp 235-242.
