Core shack PORPHYRY SILLS: The Colomac Property

Colomac, located 137 miles north of Yellowknife, N.W.T., contains proven and probable reserves in Zone 2 of 16,027,000 tons averaging 0.064 oz gold per ton in a fractured and altered quartz-feldspar porphyry sill. The orebody strikes north and is steeply east-dipping with a strike length of 3,000 ft, an average width of 160 ft and is open to depth. Colomac is owned 60% by Neptune Resources and 40% by Johnsby Mines. Gold was first discovered in the Indin Lake area by Leta Mining in the 1940s. Early efforts to develop Colomac as an underground mine failed because of its over-all low grade. In the late 1960s, Discovery Mines and Hydra amalgamated their claims within Johnsby Mines to form the Colomac property.

In 1974, Cominco, and in 1981, Newmont Mining did further exploration and metallurgical work on the property. Neptune, with previous experience in large tonnage, low-grade deposits, optioned the property in 1986 and has earned a 60% interest from Johnsby Mines. The 1987 programs consisted of over 29,000 ft of drilling in Zone 2 to prove up ore reserves and extract 1,500 tons of ore. This material was vat-leached for 35 days with a gold recovery of 80%.

The Colomac property lies in the Slave Structural Province within the irregularly-shaped Archean Indin Lake Greenstone belt. The property is underlain by north-striking, steeply dipping, intermediate and minor felsic volcanic rocks and sedimentary rocks, all of which are intruded by Proterozoic-age diabase dikes. North-trending, multi-phase diorite to quartz-diorite sills intrude the sequence. Parallel to the more mafic sills are near-vertical auriferous quartz-albite porphyry sills. Although they are sills, they are known in field terminology as the “Colomac and Goldcrest dikes.” The two sills are fractured, medium-grained, weather-ed grey-white to pink, and composed of albite,quartz and lesser amounts of chlorite,biotite, hornblende, about 3% pyrrhotite and, locally, have small blue quartz eyes. The Colomac dike, which is four miles long and varies in width from 30 ft to 200 ft, has four zones of economic-grade gold mineralization along its length, in addition to the Zone 2 orebody. Alteration of the Colomac dike near zones of gold mineralization consists of chloritiza tion, silicification and carbonatization.

Located 1,000 ft to the west, the Goldcrest dike is 3,300 ft long, varies from 65 ft to 230 ft in width and contains auriferous zones.

In Zone 2, gold occurs in the fine fractures in the quartz-albite porphyry and along the selvages of the quartz veins. Although the quartz veins have minor tourmaline and trace arsenopyrite, they are generally barren of gold. The gold is free-milling and averages between 100 and 300 microns in size.

The Colomac deposit will likely be developed by open-pit mining and vat leaching. The Zone 2 pit is designed to extend 4,000 ft along surface and will be developed to a depth of 660 ft. The planned production sequence is as follows:

Recovery of the gold starts with primary crushing. Cyanide reagent is added in a semi-autogenous mill and the resulting slurry of sand-size particles and reagent is pumped to a lined vat 200x500x60 ft, constructed with waste rock and designed to con form to the natural topography.

After further treatment in the vat with water and cyanide, the pregnant solution is decanted to the carbon adsorption circuit, where the gold is removed from solution and trans ported to the bullion furnace. The barren solution is pumped back into the vat.

Vat-leaching will continue through out the winter months with solutions flowing upwards from the bottom of the vat. Ice and snow will act as a sur face insulator. When extraction of the gold is complete, lower grade material dumped on top will be heap leached, which will permit continued gold recovery. The vat will remain as a per manent tailings area.

The cutoff grade for Zone 2 of the Colomac deposit is 0.03 oz gold per ton. Mining, expected to begin in mid-1989 at a rate of 10,000 tons per day and at a stripping ratio of 3.4:1, will result in 44,000 tons of rock (ore and waste) being blasted and moved on a daily basis. Mining of the Colomac property is planned for 350 days per year. Pay-back period is estimated at 1 1/2 years. Geologist Pamela Phillips is project co-ordinator for Greenstone Resources of Toronto. She would like to thank Neptune Resources for the information, specifically Ross Burns, vice-president, development, and geologist Nicki Davison.