Intiedougou takes shape

When Ottawa-based junior Orezone Resources (ORZ-T) took on a gold exploration property in southwestern Burkina Faso, it had no clear idea where to find gold. But on top of that, it didn’t even know where to look.

“There were photocopies of the old maps, with nothing to show where the old grid was,” according to geologist Christian Plouffe, who spoke to The Northern Miner on a recent visit to the African project. Without topographic control on the surveys, the only thing to do was repeat the soil survey, analyzing soil samples taken on a 50-metre grid pattern.

Fortunately, the new survey relocated the soil anomaly. Reverse-circulation (RC) drilling began in early 1997, and the company quickly outlined a promising gold zone 250 metres long and 45 metres wide.

Orezone is currently working on a program that includes 1,500 metres of RC drilling and 600 metres of diamond drilling. The most recent RC holes yielded grades of 0.1 to 2.6 grams per tonne, including 2.6 grams over 28 metres and 1.6 grams over 42 metres.

Three RC holes were advanced on a second target, about 300 metres southeast of the main prospect. The first intersected 8 metres grading 25.3 grams per tonne and 14 metres with 1 gram per tonne; the other two holes showed grades of 0.4 to 0.9 gram per tonne over multiple intervals ranging from 6 to 20 metres in length.

The Intiedougou prospect is at the southern tip of a small volcanic belt in the larger Hound greenstone belt, one of many Birimian (early Proterozoic) volcanic-sedimentary complexes in the West African Shield. The volcanic rocks, mainly andesites and rhyolites, are intruded by a late-stage felsic stock with multiple phases, including an irregular syenite body.

The gold mineralization shows up at the intrusive contact between the syenite and the andesite, usually in sulphide-rich stringers and disseminations. The most important alteration types are pyritization and carbonatization, both of which are widespread around the contact zone.

Strongly mineralized material will usually have about 3% pyrite and a gold grade of 1 to 7 grams per tonne; it is much better developed in the syenite than in the andesite.

Unlike many of the country’s gold deposits, which frequently occur on single vein or shear structures, the Intiedougou mineralization offers potential for a larger, lower-grade gold deposit centred on the intrusive body.

Orezone had originally looked at Intiedougou as a potential oxide-ore target, running a RC drill program over the known soil anomaly. The project expanded when early RC drill holes encountered long mineralized intersections — up to 26 metres grading 1.8 grams gold per tonne.

Subsequent diamond drill holes intersected comparable grades in the fresh rock, including a 62-metre length averaging 3 grams per tonne.

The principal challenge in drilling the mineralization has been to recognize the shape of the syenite body, which appears to have developed its irregular form by being injected along pre-existing zones of structural weakness. The mineralized zone itself appears to be about 25 metres wide and dips eastward, with the dip becoming progressively steeper with depth.

There is a main northeasterly-striking structural grain, roughly parallel to the stratigraphy, but no prominent break. On the contrary, the structures are mainly visible on aeromagnetic maps, particularly in the horizontal derivative of the total magnetic field.

The magnetic maps also show a subsidiary set of structures striking southeast, at nearly a right angle to the main structure.

The magnetic surveys have been useful in mapping the andesite, which (in its fresh state) carries 2-3% magnetite. Most of the property has relatively flat magnetic gradients, but in the south-central part surrounding the intrusion, the magnetic field is quite disturbed.

The disseminated and stringer sulphides make the mineralization a suitable target for induced-polarization (IP) surveys, but disseminated sulphides and magnetite in the surrounding andesite are also chargeable, decreasing the contrast between the gold-mineralized rock and the barren altered rock surrounding it. Still, combining the results of the IP surveys with the magnetic information provides an inferential picture of the subsurface. Said Orezone’s exploration manager, Jeffrey Ackert: “You can do a lot of deduction, because you’ve got your mag, resistivity and chargeability.” The syenite is electrically resistive, which marks it out from the andesite: “if [high resistivity] coincides with the geochem, you’ve got a great target,” said Ackert. Still, it is geochemistry that Orezone is leaning on.

“Soil geochemistry is the best suited for Africa,” stated Plouffe. “The simple gold geochem is working very fine.”

Fine enough, in fact, that the cornerstone of Orezone’s forthcoming exploration program is a US$150,000 reconnaissance-scale geochemical survey over the whole property. The survey will have a 500-metre grid pattern, with tighter (200-metre) coverage in the south-central part of the property around the intrusive rocks. The soil geochemistry should permit Orezone to carry out a rapid assessment of other areas on the property that have not been prospected up to now, and drop from its permit areas of low potential.

Surface mapping will also figure in the program, even though the tropical weathering (mainly 25 to 30 metres, but often deeper) has destroyed most of the surface outcrop. Weathering of the syenite and granite produces a light yellow saprolite, contrasting with the greenish buff color of saprolite derived from andesite. And bouldery rubble zones, which are not uncommon, often lead to the discovery of relatively fresh bedrock at shallow depths.

There is plenty of help available in Intiedougou village when hand stripping has to be done, and water washing will allow the stripped areas to be mapped for structure.

The structural information may be critical to Orezone’s success, allowing the geologists to piece together the pattern that controls the shape of the Intiedougou syenite.