MINERAL EXPLORATION — Zoning key in pegmatite exploration

As a company that made a habit of concentrating on Precambrian gold deposits and base metal sulphides, Avalon Ventures (AVL-V) is hitting the books to find out more about granitic pegmatites.

The company is exploring a property at Separation Rapids, north of Kenora, Ont., where a lithium-bearing pegmatite was discovered last year by an Ontario Geological Survey crew.

The pegmatite itself, which has been nicknamed “Big Whopper,” is a body measuring 1,200 metres long, with an eastern section up to 90 metres wide at surface, and a thin dyke-like western section 15 metres wide. It is bounded on the south by a fault and, in the north, is in intrusive contact with amphibolites; some parallel pegmatite dykes occur north of the main body.

Initial mapping suggests that the body is steeply dipping, and that its depth dimension may exceed its surface width.

Recent trench results from the eastern part revealed average lithium grades of 1.58% over a 58.9-metre width exposed at surface, and 2.02% over 6.9 metres.

Grades along the western dyke structure ranged from 1.56% to 1.78% lithium oxide over widths of 8.3 to 15.2 metres. The grades are remarkably consistent — of 42 channel samples in the five trenches, only four fell outside the 1% to 2% range.

The pegmatite body at Separation Rapids carries about 40% petalite, the lithium-aluminum silicate which makes the best lithium ore. It also carries 25% to 50% potassium feldspar, with the remainder made up of quartz and muscovite.

The high petalite content likely means that an essentially pure petalite concentrate (consisting of around 4.2% lithium oxide) will be easy to produce.

The pegmatite also carries rubidium, presumably hosted in the potassium feldspars. Limited sampling returned grades of 0.25% to 0.56% rubidium oxide — whole rock results similar in grade to some of the mineral concentrates found at operating mines. The unusually high rubidium content suggests the pegmatite is highly fractionated, and that several zones with different mineralogy may be present in the body.

Rare-metal production in Canada is restricted to two mines — the Niobec niobium mine at St. Honore, Que., owned by Teck (tek-t) and Cambior (cbj-t), and the Tanco mine in eastern Manitoba, owned by unlisted Tantalum Mining.

Another deposit, the Thor Lake beryllium occurrence in the Northwest Territories, is being bulk-sampled by Highwood Resources (hwd-t). Niobec extracts columbite ores from a carbonatite pipe, whereas the Tanco mine is built on the Bernic Lake pegmatite swarm, a suite of classical granitic pegmatites.

The Tanco pegmatite is in the Bird River greenstone belt, which extends as a narrow septum — in places, little more than a kilometre wide — into the Umfreville Lake area of northwestern Ontario between Kenora and Red Lake.

There, about 60 km north of Kenora, it widens and drapes over several granitic batholiths of the Winnipeg River intrusive belt.

In June 1996, a mapping crew led by Fred Breaks of the Ontario Geological Survey discovered a strongly deformed gneissic rock with an unusual feldspar-petalite mineralogy. Recognizing that it was a sheared pegmatite with plenty of lithium, Breaks announced the discovery in July, and local prospector Robert Fairservice staked the ground, with help from partner James Willis.

Avalon entered the picture in the autumn of 1996, when it was negotiating for an agreement on a gold prospect in northwestern Ontario. The company missed out on the property on which it was bidding, but Fairservice offered the Separation Rapids property instead. Their option agreement was signed in October, just in time for Avalon to run a small sampling program on the prospect.

Rare-earth pegmatites are commonly zoned, with an outer margin consisting chiefly of quartz, feldspar and muscovite, and with a highly siliceous core.

The intermediate zones are where the complex mineralogy develops, carrying such elements as lithium, beryllium, cesium, niobium, tantalum, tin and rubidium. The initial sampling at Separation Rapids does suggest that a zoning pattern exists.

This, in turn, might mean that the pegmatite has potential as a cesium resource. Sampling by the Ontario Geological Survey turned up wall rocks with up to 1.4% cesium, and a large zone to the east of the pegmatite with 0.1% cesium or better. Rock geochemistry is likely to form a pillar of future work at the prospect.

Avalon is planning a 5,000-metre program of diamond drilling to outline the resource, and a series of initial metallurgical tests. The budget for the work is $500,000.