TECTONIC EMPLACEMENT OF PGE BEARING CU-NI SULFIDE DEPOSITS

Platinum group element (PGE) bearing Cu-Ni sulfide deposits have two stages of emplacements. The first is hot and magmatic and the second cold and tectonic. The host rocks of these deposits are either mafic or ultramafic. When carrying up to a few percentages of Fe-monosulfide melt, the magma is simply too heavy for a direct intrusion into upper continental crust. A second stage of tectonic uplifting is, therefore, required to bring these deposits up to near the surface for mining.

Intrusion of alkali olivine basaltic or tholeiitic magmas carrying various amount of Fe-monosulfide is modeled according to Pratt’s scheme of isostatic compensation and the structure of the present crust. The level of isostatic compensation is set at 120 km depth. Density of magma is corrected for pressure variation with depth. Based on our calculation, a tholeiitic basaltic magma carrying 2% Fe-monosulfide could only reach a maximum height of 11 km below the surface and an alkali olivine basaltic magma carrying 5 % Fe-monosulfide could only reach a maximum height of 21 km below the surface. Using Mona Loa as a height calibration standard, the magma of this composition could only carry 0.15% of Fe-monosulfide and it is not economic under current mining conditions. The modeling suggests that a previously formed economic grade PGE bearing Cu-Ni sulfide deposit must be emplaced in the lower continental or upper oceanic crust, and lifted by either a major tectonic thrusting event or significant crustal thinning and upwelling of mantle material such as meteorite impact or thermal uplift.

All the major PGE bearing Cu-Ni sulfide deposits appear to have gone through this second stage of tectonic uplifting. The Sudbury and the Voisey's Bay deposits in Canada have been uplifted by Grenville Orogeny. The Noril'sk deposit in Russia was on an ascending plume at Permo-Triassic transition. The Jinchuan deposit in China has been on the uplifting course since the Indian subplate collided with the Eurasian plate.