GOLDEN OLDIES: ASPECTS OF ANCIENT (PRE-MESOZOIC) CU-AU PORPHYRY SYSTEMS WITH SPECIFIC CANADIAN CORDILLERAN EXAMPLES

Gold-rich porphyry copper deposits are important contributors to global gold and copper production. Individual porphyry deposits normally contain several hundred tonnes of gold, however some “super-giant porphyry systems” contain greater than 1200 tonnes of gold (e.g. Grasberg, Irian Jaya; Oyu Tolgoi, Mongolia; Pebble, Alaska; KSM, British Colombia; Cadia District, Australia. Average grades are generally low at 0.5 g/t Au but can reach values greater than 2.0 g/t Au. The overall low grades are offset by the economies of scale.

Au-Cu porphyry deposits are generally more abundant in the modern geologic record (post Mesozoic) and are commonly situated around recently active volcanic arcs in the South American and the SW Pacific portions of the “rim of fire”. However, several of the ”super-giant” Au-Cu porphyry deposits are within arcs of Pre-Mesozoic accreted terranes of the North American Cordillera, Lachlan Fold Belt (LFB) of Australia and the Central Asian Orogenic Belt (CAOB) (here termed “Ancient Porphyries”) ².

The Canadian Cordillera is well endowed with several well preserved examples of Ancient Au-Cu and Cu-Mo porphyry systems of varying size and grade, which are, and will be, important to the British Colombia’s economic future.

The processes linking formation and preservation of Ancient porphyry systems in convergent arc settings show common themes in a variety of terranes. In particular, association with late basins, stress regime, depth and age of emplacement, and climate are all important fundamental controls on Au-Cu porphyry preservation ^1,2.

¹Tosdal, R.M., and Richards, J.P., 2001

²Wurst, A.T., 2010