Big Lake Gives Metalcorp a Big Surprise

SITE VISIT

Thunder Bay, Ont. — A mineral prospect recently discovered on the north shore of Lake Superior by Metalcorp (MTC-V, MTLCF-O) illustrates how nature has a surprise or two for everyone, and also shows how, even in a “mature” exploration area like North America, significant new metallogenic provinces can still unfold.

Metalcorp has been exploring the Big Lake property, about 10 km south of Marathon, since late 2004. The rocks are mainly Archean volcanic and plutonic rocks of the Hemlo volcanic belt, but younger Proterozoic mafic rocks of the Southern Province intrude them, notably near Marathon where the large Coldwell intrusive complex hosts copper, nickel and platinum group element mineralization.

The project at Big Lake had its beginnings when Metalcorp’s chief operating officer, Aubrey Eveleigh, was looking at magnetic maps of the Lake Superior area. Big red spots are not unusual on mag maps of the mid-continent rift zone, but one, about 14 km long extending on to land south of Marathon, caught Eveleigh’s eye. It was a magnetic ringer for the Yellow Dog peridotite in the northern peninsula of Michigan, where Kennecott, part of Rio Tinto (RTP-N, RIO-L, RIO-A), discovered the Eagle nickel-copper deposit, now awaiting a go-ahead for development. The magnetic anomaly, long but only about 200 to 350 metres wide, skirted the northern boundary of the national park at Pukaskwa, and was open for staking.

Government lake-sediment geochemistry reinforced the impression, with lake sediments in the area showing high concentrations of copper, nickel, cobalt, and a few trace elements typical of magmatic, ultramafic-hosted mineralization — chromium, silver and gold.

In 2004, Metalcorp started with airborne electromagnetic surveys to find conductive bodies: massive mineralization would be the most obvious, and lucrative, thing to find. The maps came back spotted with conductors, which later ground geophysics confirmed up close and showed to be coincident with magnetic anomalies as well.

Early drill holes on the better anomalies — and more accessible ones, an important consideration when trying to drill in a swampy area — intersected mainly disseminated mineralization plus some stringer and semi-massive sulphides. The intersections had the kind of modest grades that go right by the market, but show the explorationist that the prospecting model is sound.

At the same time on another anomaly, Metalcorp prospector Jim Walsh was having lunch on an outcrop when a little green secondary copper staining caught his eye. Schooled in platinum-palladium exploration, Walsh recognized the subtle hint and knocked off a sample. It kicked, and so did the drill hole that followed it, establishing the J4 platinum-palladium reef.

Then the surprise came; hole 7 in the first phase of drilling that tested an electromagnetic anomaly catalogued as “BL14.” It intersected disseminated and stringer copper and zinc sulphides in the footwall of the ultramafic intrusion, something that didn’t fit the magmatic copper-nickel picture. Down-hole electromagnetic surveys showed a nearby conductor, and two more holes drilled in the 2005 program intersected the same kind of mineralization. It was hosted in altered and brecciated mafic volcanic rocks, not the peridotite that hosted the copper, nickel and platinum group elements — the volcanics belonged to the Archean-aged Hemlo volcanic sequence, older than the ultramafics, and sat structurally below the peridotite sills.

The stringer sulphides, and the chlorite, talc, anthophyllite, cordierite and brown mica in the altered wall rock, all suggested this was the zone of stockwork mineralization usually found in the footwall of volcanic-hosted massive sulphide deposits.

Trace element chemistry of the wall rocks showed high chromium and nickel values that probably mean the old volcanic sequence is a komatiitic basalt. That, in turn, implies a very hot volcanic pile at the time the deposit was formed, a condition that can favour higher metal content generally and higher copper and gold content in particular.

A second airborne electromagnetic survey, on lines oblique to the earlier survey, identified more conductors, and in February 2006 a drill hole cut true massive chalcopyrite, sphalerite and pyrrhotite. A 4-metre interval ran 7.5% copper, 2.2% zinc, 138 grams silver and 9.2 grams gold per tonne. Clearly, the new target was different from the exploration model. James Franklin, a consulting geologist who made a name for himself at the Geological Survey of Canada studying volcanogenic massive sulphides, looked at the Big Lake cores and concluded the same thing: looking for magmatic nickel and copper, Metalcorp had found a seafloor-exhalative deposit of copper and zinc sulphides instead.

By the summer, the zone was known to be 250 metres long and it had been followed about 150 metres downdip. Down-hole geophysics through that fall showed conductors extending 350 metres east of the drilled intersections.

Drill holes through the fall consistently intersected the zone, mostly over widths of 1 to 2 metres but often widening to 5 metres, with grades mostly 1% to 2% copper and 0.5% to 1.5% zinc, with relatively high silver and gold credits — silver grades were usually 20 to 70 grams per tonne, and gold grades were consistently 0.1 to 0.8 gram, with a few above a gram.

The structure was looking simple, too: a moderately north-dipping sequence, immediately below the ultramafic body, with a strike direction roughly northeast. The sequence appeared to be overturned, because a cherty siltstone unit, symptomatic of a quiet period after the exhalative event, sat structurally below the mineralization.

The past winter’s program concentrated drilling on swampy ground northeast of the prospect, to take advantage of freezing for winter access. Metalcorp put 17 holes down on the prospect during the winter program — altogether 4,000 metres of drilling — and plans to put together a preliminary resource figure from the exercise. The mineralization has been intersected as deep as 250 metres down — about 300 to 350 metres downdip — and is still known over a strike length of 250 metres.

Meanwhile, a second machine was put to work on other conductors, with another 2,500 metres, in seven holes, drilled in that program. Two holes tested targets along strike from the BL14 prospect. Both holes — one collared 400 metres west of the prospect and one 200 metres east — intersected the mineralized horizon, encountering the alteration in the volcanics and stringer-style sulphide mineralization. To the west, the stringer mineralization consisted of both chalcopyrite and sphalerite; to the east, only chalcopyrite.

Most of the samples from the winter program were still in the lab at mid-April.

Even as Metalcorp drills the discovery that didn’t fit, the property shows potential for deposits that do. The two platinum group element reefs, J4 and J5, are each about 1.6 km long and have been intersected at 250 metres vertical depth, with surprisingly consistent platinum and palladium grades of 0.5 to 0.8 gram per tonne for each metal (and a platinum/palladium ratio very near 1).

The A2 zone, a little more than 1 km north of the BL14 prospect, has returned low copper and nickel grades and disseminated mineralization in the younger gabbroic rocks, typical of the halo zones around massive to semi-massive magmatic sulphide deposits. Locally, that mineralization coalesces into narrow massive sulphide layers with higher grades; the best to come out of the A2 zone so far was a 6.2-metre intersection that averaged 0.4% nickel and 0.4% copper, with two massive zones, one grading 1.7% nickel and 0.2% copper over 0.3 metre and one 0.8% nickel and 1% copper over 0.4 metre.