Highrock Island Main Vein
Gold mineralization in quartz veins occurs on Highrock Island. The “Main vein” crosscuts mafic volcanic rocks where the volcanics have been intruded by granitic rocks and diabase dykes. Drilling shows that the vein is sometimes within the diabase dyke, sometimes at the contact of the dyke and granitic rocks, and sometimes within the volcanics. Quartz stringers and en echelon veins, most less than one metre long, occur on both sides of the Main vein.
The gold is distributed throughout the quartz either as small particles, or in fractures in the quartz, or may be contained in sulphides minerals. The gold mineralizing event is a late-stage phenomenon, and occurred when the metamorphism and tectonism of the belt were essentially finished.
The gold mineralization on Highrock Island and surrounding area is coarse gold, irregularly distributed in the quartz veins. This means that while one sample may return a very high assay, the sample next to it may contain no gold, a phenomenon known as the “nugget effect”.
Surface sampling of the 200 metre exposed length of the Main vein up to 1985 gave an average gold value of 0.669 oz/t over an average width of 0.71 metres. In 1986, the vein was extensively channel sampled every 1.5 metres along strike. The resulting 195 samples gave an average gold grade for the surface Main vein of 0.39 oz/t over an average width of 1.86 metres. This width included not only the vein, but some of the rock on both walls.
Samples from the walls graded very high, up to 3 oz. Au/ton. Four bulk samples, each weighing approximately 800 lbs, yielded an average of 0.625 oz. Au/ton over an average width of 1.5 metres. The Main vein has been investigated underground by means of a two-compartment vertical shaft 70 metres deep, with a station at the 37-metre level and a 38-metre long drift at the 67 metre level.
Mapping on the 37-metre level indicated a quartz Main vein of up to 1.2 metres wide, with numerous quartz stringers up to10 cm wide in the east wall. The combination of vein and stringers gave the zone a width of 3.7 metres. On the 67 metre level. the Main vein was mapped and chip sampled every 1.5 m along its length, yielding an average grade being 0.081 oz. Au/ton. The vein width varied from thin quartz stringers at its north end to a 0.9 metre-wide vein at the south end.
Manitoba Industry, Economic Development and Mines has published a “reserve” estimate for the Main vein of 404,000 tons grading 0.26 oz. Au/ton. This figure is not in compliance with the requirements of National Policy 43-101.
N. Ralph Newson, P.Eng., P.Geo.