Les elements du groupe du Platine dans le dyke de Mequillon ceinture de Cape-Smith Nouveau-Quebec

Abstract

Nickel-copper sulphides rich in platinum-group elements (PGE) occur associated with the komatiitic basalts of the Chukotat Group in the Ungava trough. These komatiitic basalts are fed by sills and dykes which cut the underlying Povungnituk continental basalts and sediments. The Mequillon dyke is an example of this type of feeder which contains Ni-Cu sulphides rich in PGE.The dyke is 120 m thick with a dip of 20°. It is asymetrically, concentrically zoned, the margins consist of melagabbro containing a fine-grained mat of chlorite, actinolite and plagioclase, the interior consists of poikilitic olivine cumulates partly altered to amphibole and serpentine. Chromite and sulphides are present as accessory phases throughout, but sulphides are concentrated towards the base of the dyke.The MgO content of the marginal rocks (15-18%), their flat REE patterns and their near chrondritic ratios for Zr/Y=3.5. Al2O3/TiO2=20, Sc/Y=3.6 indicate that the magma was a komatiitic basalt similar to the olivine pheric basalts of the Chukotat group. The 534 S values which cover the range 0 to 5 and Se/S ratios 200-350* 10^ which indicate that the sulphur was mantle derived and not derived by contamination from the enclosing sediments.It is possible to numerically model the major oxides and compatible trace elements of the rocks in the center of the intrusion by considering them to be a mixture of initial liquid with a composition similar to the margin rocks, 15 to 35% olivine, 0 to 25% orthopyroxene and 0 to 10% sulphides. However these models produce rocks too poor in the incompatible elements e.g. Zr, Sm and Sc. To model these elements it necessary to assume that the trapped liquid component is fractionated and has lost approximately 30% olivine. A mass balance calculation indicates that the system was opensince the sum the rocks within the dyke is more MgO and FeO rich than the margins. The fact that the sum of rocks within the intrusion is more mafic than the margin rocks implies that some fractionated liquid has been expelled from system. The rocks with highest Pd and Pt contents (1.5 ppm) are the sulphide-rich rocks towards the base of the intrusion. The amount of sulphide present in the dyke (average 1.5%) is too great to have formed from the volume of magma present, this provides further evidence that the system was open. The collection of the sulphides at the base of the dyke is attributed to the geometry of the intrusion and the sulphide segregation in the conduit. The composition of the sulphides are unusual for sulphides associated with a komatiitic basalt; the Pd/Ir and Cu/Ir ratios are higher and the Ni/Cu ratios are lower than normal, suggesting that the sulphides formed in equilibrium with a more fractionated liquid than a komatiic basalt.

A review of sulfur to selenium ratios in magmatic nickel–copper and platinum-group element deposits

“…In the Delta horizon, the S/Se ratios of the massive sulfides from the Delta 8 and 9 deposits (3944 and 2546, respectively) are typical of mantle values. In addition, the S/Se ratios (geomean of 4174) and the δ 34 S values (4.1 to 5.1‰) of the Méquillon Dyke contrast with those of the surrounding country rocks (S/Se = 13,781, δ 34 S = 0.2 to 3.0‰), indicating that the S is mantle derived (Tremblay, 1990). This dispersion in the S/Se ratios of the Cape Smith ores suggests that camp deposit scale variation occurs in the S contamination derived from the country rocks.…”