DIAMONDS — Mano River seeks African diamonds — Properties also deemed prospective for gold

Both Liberia and Sierra Leone have lived through bloody civil wars in the last decade, and are only now emerging from the strife. Soldiers from other West African countries keep the peace as the sides talk and disarm, both slowly.

But despite the area’s political problems, or perhaps because of them, Mano River Resources (MNO-V) is looking seriously at a set of properties in both countries, prospecting for both gold and diamonds. Mano’s strategy is to be in the country as stability returns and to make itself a big fish in what will be a new pond.

Over the coming field season, the company will carry out geophysical surveys and sampling in an attempt to identify kimberlite bodies.

The alluvial diamond deposits of western Liberia sit atop a relatively small Archean-aged block, and cluster around the boundaries between the granitic rocks and the broadly northeasterly striking greenstone “schist belts.” A few small kimberlites are known.

Mano has negotiated three exploration permits in the Archean terrane: one surrounding Lake Piso on the southwest coast, one in the Bea Mountains near the border with Sierra Leone, and a third in the Kpo Ranges in the country’s interior. These cover a corridor of “crustal-scale” shearing, parallel to the structural grain of the schist belts, and are roughly analogous to the large structural breaks of the Abitibi Belt in Ontario and Quebec. This long-term crustal weakening permits later intrusion by kimberlite bodies.

As well, the Archean rocks of southeastern Sierra Leone and western Liberia are profoundly folded and make up a thick crustal block. This thicker crust, where it allows kimberlites to intrude along zones of weakness, may tap extremely deep parts of the mantle where pressures are high and diamonds likely to form.

One of Mano’s chief areas of interest is the area around Weasua, about 120 km northeast of Monrovia, in Mano’s 2,072-sq.-km Kpo concession. There, local small-scale miners have long produced diamonds from alluvial deposits in creeks flowing into the Lofa River, and in the Lofa itself downstream from Weasua village.

The diamond distribution makes a neat circular pattern around the townsite, and then follows the banks of the Lofa for about 15 km downstream. Some of the diamonds may be in place, rather than alluvial, in the upper layers of a weathered kimberlite pipe or dyke.

A little to the west, in the Bea Mountains, artisanal miners have concentrated around two towns: Weaju and Israel. At Weaju, a schist belt is cut by several cross-faults, and at Israel the mining centres downstream from a swarm of kimberlite dykes.

The United Nations Development Program has already sampled some of the dykes and a kimberlite pipe at Mano Godua, but the sampling did not return significant grades. The hope is that the alluvial diamond has a yet-undiscovered source rock in the uplands.

Down-country, the streams and rivers that flow into Lake Piso are worked by small-scale miners as well. Lake Piso, separated from the Atlantic by a thin strip of coastal land near the city of Robertsport, is ringed by beach and river sands and gravels that Mano has marked out as possible sampling targets.

Limited mapping on the 1,545-sq.-km concession has shown a series of ultramafic and mafic dykes, which are also thought to lie beneath the lake bed.

In Sierra Leone, Mano has picked up three exploration licences, including one that covers the Kono region in the eastern part of the country near the frontiers with Guinea and Liberia. Kimberlites in the area near the town of Koidu have produced most of Sierra Leone’s diamonds in the past. Mano’s Njaiama-Nimikoro and Yengema East licences, both of which are just over 100 sq. km in area, are immediately west of the Koidu pipes.

Yengema East has a series of kimberlite dykes striking northeast, and a network of rivers carries diamondiferous sands and gravels. Satellite imagery of the licence area shows features similar to those around the productive pipes.

At Njaiama-Nimikoro, immediately to the south, the diamondiferous gravels are absent, but Mano has targeted the area as a potential highland source of the alluvial diamonds.

On the smaller Nimini licence, on Njaiama-Nimikoro’s western boundary, the northeastern fracture trend that hosts the kimberlite dykes on Yengema East intersects a north-south shear system on the edge of a schist belt. Mano, noting that several diamond-producing streams on the Yengema East licence drain from the west and not the south, believes there may be a hard-rock source on this property as well.