MINERAL EXPLORATION — CASE STUDIES — Mustang tests PGE potential at East Bull Lake

Magmatic deposits of platinum group elements (PGE) are among the world’s rarest mineral repositories, but, with the aid of academic and government research, Mustang Gold (MUST-C) is confident it can beat the odds at its East Bull Lake property in northern Ontario.

Over the past few months, the Toronto-based junior has inked deals with private vendors to earn 100% interests in 392 contiguous claims covering more than 90% (or 61 sq. km) of the intrusion. To earn its interest, the company must pay a total of $125,000 in cash and issue 200,000 shares over the next three years.

Intermittent exploration at East Bull Lake dates back several decades, but most of the current geological understanding is based on a 4-year field and laboratory investigation completed in 1994. The study was spearheaded by geologists at Laurentian University and reported in several open file reports of the Ontario Geological Survey.

Situated 80 km west of Sudbury, East Bull Lake is part of the Huronian-Nipissing magmatic belt, a 200-km-long arcuate zone of coeval, early Proterozoic-aged dyke swarms, mafic intrusions and related volcanics extending from Elliot Lake to Sudbury. Magmatism is believed to have been triggered by a long-lived (300-million-year) continental rifting event that was centred beneath the Sudbury Basin and spread along the southern margin of the Super Province.

The East Bull Lake Instrusion (EBLI) is 22 km long, a maximum of 5 km wide and, on average, 1 km thick. It is a gabbroic-anorthositic lopolith consisting of three complex, but distinct, cumulate units, each of which contains two or more sub-zones. Underlying the cumulates is a feeder zone of magmatically brecciated fragments of Archean granitic and tonalitic country rock intruded by irregularly shaped veins and dykes of mafic and ultramafic composition.

Within the intrusion, layers strike parallel to the northern and southern margins, dip inwards, and are commonly cut by brittle to ductile shear zones and faults. Regional-scale metamorphism and deformation have cooked the rocks to upper greenschist facies and formed mineral foliations in the cumulate rocks.

Petrologic and geochemical studies indicate that the EBLI formed as a result of fractional crystallization of multiple injections of aluminum-rich tholeiitic basaltic liquid, a petrogenesis indicative of most igneous complexes hosting known PGE deposits. Although observed at several stratigraphic layers, mineralization appears to be concentrated near the base and consists of three types:

* contact sulphide mineralization — disseminated to clotty magmatic pyrrhotite, chalcopyrite, pyrite and minor pentlandite enriched in, or proximal to, palladium-bismuth-tellurium compounds and sperrylite (this occurs predominantly in the feeder and basal cumulate unit, both of which outcrop along the northern and southern margins of the intrusion); * disseminated sulphide mineralization — similar to the contact-type but occurring higher in the stratigraphic sequence and generally containing low tenors of platinum and palladium; and

* structurally controlled sulphide mineralization — disseminated to massive sulphide minerals remobilized from contact-mineralized zones where cut by deformation zones.

While Mustang’s exploration is still at a preliminary stage, President Robin Dunbar says the earlier work serves as a beacon to areas potentially rich in platinum and palladium.

“There are a lot of places across northern Ontario where you can find PGE values,” Dunbar tells The Northern Miner, “but the real problem is piecing together a cohesive geological picture to identify the right [mineral] horizon in the intrusion . . . which we already have.”

Project Geologist Ken Lapierre notes that the intrusion has never really been tackled from an exploration point of view. As such, it remains virgin territory. “The OGS basically laid it out for us, but what we have is an academically studied area,” says Lapierre. “For example, they didn’t do IP [induced polarization].” This is all about to change, as crews are preparing to carry out not only IP surveys but line-cutting, mapping and sampling. Any conductors outlined beneath areas of surface mineralization are targeted for drilling and possibly bore-hole geophysics.

The first phase of exploration, completed this past summer, has already provided ample targets for drilling in the fall. Seven conductors were detected in a 1.6-km-long-by-800-metre-wide area known as Moon Lake. One of these anomalies coincides with a laterally continuous zone of mineralization extending 1.5 km in length and up to 125 metres in width.

Initial samples assayed up to 6 grams combined platinum and palladium per tonne. The highest grades reported from the previous studies were 11.9 combined platinum and palladium, 0.93% copper, 0.74 nickel and minor gold.

Geologically, the Moon Lake target is part of a rotated, uplifted block of the basal cumulate and feeder zones, and thus has potential for the more favorable contact mineralization. While this area received plenty of attention in the previous studies and was thus easily chosen as a starting point, it serves as an example of the type of problem that may potentially arise in other areas of the intrusion.

“It is actually very boring-looking rock, with only up to 2% sulphides,” notes Lapierre. “And where you have sulphides, you get up to 10 grams PGE, which can drop to 2 to 4 grams in barren areas.”

Another issue is the size of the property, which Lapierre says can only been explored by slowly chipping away it. “Our mandate is simply to concentrate and isolate specific targets, and thus effectively carry out a solid job in a smaller area.”

The only other company active in the area is Freewest Resources (FWR-T), which owns two small blocks along the southern margin of the intrusion to the east and west of Moon Lake. Exploration is focused on the eastern Folson Lake property, where a new showing was recently uncovered.

Samples from a 25-metre-long-by-15-metre-wide section of the intrusion returned up to 9.4 grams combined platinum and palladium, 8.3 grams silver, 0.3 gram gold, 1.79% copper and 0.66% nickel.

The showing is near one that was outlined previously and which returned 3.2 grams palladium, 1.4 grams platinum, 3.4 grams silver and 0.2 gram gold.

As at Moon Lake, mineralization is hosted by the basal cumulate rocks.

Another 2-km section of this zone has yet to be explored.

Line-cutting, mapping, and magnetic and IP surveys are scheduled; trenching and drilling will follow.

Freewest is seeking a joint-venture partner for its Bull Lake property, west of Moon Lake. Grab samples collected during staking returned 2.2 grams combined platinum and palladium, 59.6 grams silver and 0.2 gram gold.