Marathon, Ont. — Guiding Marathon PGM’s (MAR-T, MRPGF-O) exploration effort at the Coldwell intrusive complex is a classical magmatic model for platinum group elements and copper, plus a sophisticated induced-polarization technique to detect disseminated sulphides.
“It’s not one single intrusion, it’s probably hundreds of intrusions,” says David Good, Marathon’s vice-president, exploration, and as a result, the mineralized magmatic phases are broken up by weakly mineralized or unmineralized ones. But in general, the zoning of the mineralization — pyrrhotite and chalcopyrite at the bottom of the intrusion, grading to a chalcopyrite-dominated sulphide phase nearer the top, matches a magmatic emplacement model. Further up in the section, the platinum group elements break away from copper, occurring in sulphides or native form in the gabbro.
It contrasts with the apparent hydrothermal origin of platinum group mineralization in the North Range of the Sudbury intrusive complex, where Wallbridge Mining (WM-T, WLBMF-O) is finding substantial wall rock alteration around copper and platinum group element occurrences.
There is no sign of alteration in the Marathon mineralized zones, and often enough very little sign of mineralization either: “I found two blebs of sulphide,” says Good, pointing at some mineralized core. “When you’re logging, you have no way of knowing you’re in high grade.”
The sheer volume of mineralized rock, also, argues for a magmatic origin. Good has come to the view that areas where pre-existing rock forces magma pulses to slow down are places where denser sulphides will settle out of the magma, forming massive or semi-massive sulphide zones and scavenging copper and platinum group elements from the still-molten silicates.
Another help has come from induced-polarization and resistivity surveying; Marathon uses chargeability to map sulphides and resistivity to map rock type. The surveys, by Oakville, Ont.-based Insight Geophysics, use Insight’s own array design, which facilitates a three-dimensional inversion of the results.
“It’s giving us the confidence to step out 500 metres instead of 100,” says Good.
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