The Modi Taung–Nankwe gold district, Slate belt, central Myanmar: mesothermal veins in a Mesozoic orogen

Abstract

The recently discovered quartz–gold veins comprising the Modi Taung–Nankwe gold district in central Myanmar are largely hosted by mudstones of the late Palaeozoic Slate belt. Mesozoic rocks of the Paunglaung–Mawchi zone separate the Slate belt from the Shan Scarp and Shan Plateau to the east. At Modi Taung 5 km of exploration adits indicate that the veins, within steeply dipping oblique reverse-slip shear zones, are displaced by movements along the shears, intruded by late Jurassic calc-alkaline dykes, and offset on conjugate cross-faults. Drill intercepts show that mineralization extends vertically for more than 500 m. Coarse visible gold, book-and-ribbon texture and stylolitic laminations, and trace metal values are consistent with mineralization at mesothermal depths of several kilometres in the brittle–ductile regime. Our field observations suggest that mineralization took place in the early Jurassic following collision of Myanmar on the passive western margin of a Greater Shan–Thai continental block with an oceanic arc on the overriding plate to the west. Collision generated intra-continental thrusting in and east of the Paunglaung–Mawchi zone, metamorphism of Plateau rocks thrust west beneath the Slate belt to form the Mogok Metamorphics, and ascent of mineralizing fluids expelled from dehydrating underthrust rocks. Reversal in orogenic polarity initiated late Jurassic eastward subduction of oceanic crust beneath Myanmar, and generation of a magmatic arc with dykes cutting the quartz–gold veins. Renewed east-directed thrusting translated the Slate belt over arc clastics, and probably resulted in exhumation of the Mogok Metamorphics and offsets of the veins on conjugate faults. Young K/Ar, Rb 87/Sr 86 and Ar 40/Ar 39 ages on the Mogok Metamorphic belt imply renewed Tertiary uplift and possibly intrusion of mid-Tertiary or Palaeogene granites.