Greenstone’s Oronorte on target

The Oronorte property, wholly owned by Greenstone Resources (TSE), an underground gold mining operation about 96 miles in a direct line north-northeast from here, went into production in November, 1990. There is little doubt that the first year’s minimum estimated output of 15,000 oz. gold will be met.

Oronorte’s proven and probable reserves total 123,000 tons grading 0.51 oz. gold per ton. The company estimates a “potential reserve” of 600,000 tons of a similar grade. The vein itself has been traced on surface for 9,800 ft., but intensively developed over only 1,150 ft. The estimated 600,000 tons is, therefore, a conservative figure.

Discovered in 1939 and worked intermittently on a small scale until 1975, the property was acquired by Greenstone in 1986. It was subjected to underground development and to diamond drilling and, on confirmation of estimated grades and tonnages, the decision was made to build a mill and take the mine into production. The mill turned over in November, 1990, and started shipping a high-grade flotation concentrate early this year. The company has a contract with Sumitomo, and 600-700 tons of concentrate are expected to be shipped to Japan every year. The grade of the sulphide concentrate will be in the range of 25-28 oz. gold.

The mine itself lies in a well-mineralized belt that has been exploited for gold since the days of the Spanish conquest. By far the greater part of the region’s output has been from placer deposits, because of the ease of working such deposits and because of the minimal amount of capital required. In fact placer mining remains one of the principal occupations of the area today with operations ranging from one-man shows to bulldozer and diesel shovel on land to bucket dredges on the wide, swift flowing Nechi River. Small quantities of alluvial platinum are still recovered from the once-prolific Colombian placer deposits, with the same Nechi River providing its quota.

Numerous vein deposits have been known in the area since historic times but shortage of capital and, until recently, poor access, have discouraged methodical exploitation.

The Oronorte vein is one of these vein deposits. It averages two feet in width, dips at about 40 degrees and is bounded by sharply defined walls. The vein filling is quartz carrying up to 5% sulphides and the bulk of the gold is carried with the sulphides. The wall rocks are hard, competent, free-standing gneisses and dioritic intrusives. Millhead grade is 0.5 oz. gold.

Earlier workings on the vein reached from surface to a depth of 200 ft. Greenstone’s development starts at a lower elevation and comprises an adit entry and four levels taken off an inclined winze at 130-ft. vertical intervals. It is a track mine and all rock breaking is by jackleg with air and electric slushers serving the stopes.

The ore is treated by a straightforward crush-grind-flotation circuit.