THRUST-CONTROLLED FORMATION OF THE GIANT HUGO DUMMETT CU-AU PORPHYRY DEPOSIT, OYU TOLGOI, MONGOLIA

The Oyu Tolgoi District features a 6.5 km long belt of porphyry Cu deposits centered on Late Devonian monzodiorite to quartz monzodiorite intrusions. The largest and highest grade of these is the Hugo Dummett deposit, which contains a resource estimated at 582 million tonnes grading 1.89% Cu and 0.41 g/t Au, and continues to grow during step-out drilling. This deposit has several aspects that are unusual both within the Oyu Tolgoi district, and for Cu-Au porphyry deposits in general. Much of the resource lies within an up to 200 m thick elongate tabular, gently north-plunging high grade core. This core zone extends for over 3.5 km, and has a moderately east dipping intermediate axis that can exceed 700 metres. The high grade core is characterized by intense sheeted to stockwork quartz + sulfide veins, which can form over 90% of the rock volume. Grades are exceptionally high, with some drillholes intersecting true thicknesses of over 150 m grading >3% Cu and > 1 gm/t Au.

Dips in post-mineral Carboniferous strata overlying the deposit indicate that the high grade core formed as a gently-dipping body, localized along intrusive contacts of the quartz monzodiorite. Veins within the core formed with subhorizontal preferred orientations. Mineralization was synchronous with thrust faulting, as indicated by the presence of: 1) strata-parallel faults that contain tectonized quartz+sulfide vein fragments and displace mineralized zones, but are cut by late-mineral dykes; 2) late-mineral dykes that intrude along thrust planes, and consist of mineralized intrusive-matrix tectonic breccias adjacent to faults; and 3) fault-related recumbent folds in slightly pre-mineral cover rocks, that are cut by late-mineral dykes.

An interpreted thrust setting for the Hugo Dummett deposit is consistent with the sill-like, originally subhorizontal syn-mineral intrusive contacts and sheeted vein networks that characterize the deposit. Horizontal compressional stresses would have inhibited vertical migration of fluids, facilitating repetitive hydraulic fracturing just beneath the hydrostatic/lithostatic boundary. These conditions contributed to concentration of metals within the extremely vein-rich, high-grade deposit core.