Core Shack THE MOBRUN DEPOSIT

Regionally, the Mobrun deposit lies within the Archean Blake River Group of the Abitibi Greenstone Belt. In the area of the deposit, the Blake River Group consists of an east-west striking, steeply north-dipping, north- facing stratigraphic sequence of mafic and felsic volcanic rocks and mixed intermediate volcaniclastic rocks. All of the rocks are intruded by granite and northeast-striking diabase dykes and have been metamorphosed to the upper greenschist facies.

In the vicinity of the Mobrun deposit, the rocks strike east-west and dip about 85 degrees to the north. Rock types, from north to south, are rhyolite, felsic ash tuff, felsic ash to lapilli tuff, diorite and brecciated rhyolite. The massive sulphide orebody lies at the contact of the felsic ash to lapilli tuff and the brecciated rhyolite. The diorite occurs along the contact of the massive sulphide and the brecciated rhyolite and is sub-parallel to the orebody except in the deeper part of the deposit where it crosscuts the orebody.

The Mobrun deposit consists of four massive sulphide lenses which are termed the Main lens (which outcrops at surface), and the Nos 850, 870, and 930 lenses. These lenses are each offset from each other by, on average, 20 m and are located progressively deeper towards the southeast. A fifth massive sulphide lens, the No 1100 lens, was discovered in March, 1988, by down -hole pulse electromagnetic geophysical surveying in three drill holes. This lens lies 240 m southeast of the main Mobrun orebody. It has a strike length of 300 m, averages 25 m in thickness and is known to be in excess of 300 m in vertical dimension. It is open in all directions, except to surface. The main Mobrun orebody is 400 m in strike length and averages 15 m in width. It extends to 200 m in depth and outcrops at surface where it has been mined by open pit.

Mineralization in the orebody is predominantly pyrite with lesser chalcopyrite and sphalerite. The No 930 lens has a minor amount of associated magnetite and the No 1100 lens contains pyrrhotite. Footwall alteration in the main, No 850 and No 870 lenses consists of pervasive sericitization and also chloritization of the chalcopyrite stringer zones. In the footwall of the No 930 lens, silicification and chloritization of the rhyolite predominate, whereas in the 1100 lens, chloritization is widespread with lesser silicification and sericitization. In all of the lenses, sulphide stringer mineralization is associated with abundant chlorite. Hangingwall alteration consists predominantly of sericitization.

Production from the Mobrun deposit started in July, 1987, with open- pit mining at a rate of 1,000 tonnes per day. In October, 1987, production from underground started at a rate of 700 tonnes per day and was increased to 1,100 tonnes in March, 1988. A 10-month suspension of commercial production from September, 1988, to July, 1989, has enabled the deepening of the shaft to allow access to the Nos 850, 870 and 930 lenses and the construction of a mill on-site. At the time of writing, drilling was under way on the No 1100 lens. Re-opening of the mine and the start-up of the mill are planned for July, 1989. Pamela Phillips is a geologist and project co-ordinator for Greenstone Resources in Toronto. She would like to thank Audrey Resources for its assistance — in particular, Pierre-Jean Lafleur, chief geologist for Audrey Resources, and Michel Bouchard, vice- president and manager of exploration.

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