Structural geometry of orogenic gold deposits: Implications for exploration of world class and giant deposits
Abstract With very few exceptions, orogenic gold deposits formed in subduction related tectonic settings in accretionary to collisional orogenic belts from Archean to Tertiary times. Their genesis, including metal and fluid source, fluid pathways, depositional mechanisms, and timing relative to regional structural and metamorphic events, continues to be controversial. However, there is now general agreement that these deposits formed from metamorphic fluids, either from metamorphism of intra basinal rock sequences or de volatilization of a subducted sediment wedge, during a change from a compressional to transpressional, less commonly transtensional, stress regime, prior to orogenic collapse. In the case of Archean and Paleoproterozoic deposits, the formation of orogenic gold deposits was one of the last events prior to cratonization. The late timing of orogenic gold deposits within the structural evolution of the host orogen implies that any earlier structures may be mineralized and that the current structural geometry of the gold deposits is equivalent to that at the time of their formation provided that there has been no significant post gold orogenic overprint. Within the host volcano sedimentary sequences at the province scale, world class orogenic gold deposits are most commonly located in second order structures adjacent to crustal scale faults and shear zones, representing the first order ore forming fluid pathways, and whose deep lithospheric connection is marked by lamprophyre intrusions which, however, have no direct genetic association with gold deposition. More specifically, the gold deposits are located adjacent to ~10deg 25deg district scale jogs in these crustal scale faults. These jogs are commonly the site of arrays of ~70deg cross faults that accommodate the bending of the more rigid components, for example volcanic rocks and intrusive sills, of the host belts. Rotation of blocks between these accommodation faults causes failure of more competent units and/or reactivation and dilation of pre existing structures, leading to deposit scale focussing of ore fluid and gold deposition. Anticlinal or antiformal fold hinges, particularly those of 'locked up' folds with ~30deg apical angles and overturned back limbs, represent sites of brittle ductile rock failure and provide one of the more robust parameters for location of orogenic gold deposits. In orogenic belts with abundant pre gold granitic intrusions, particularly Precambrian granite greenstone terranes, the boundaries between the rigid granitic bodies and more ductile greenstone sequences are commonly sites of heterogeneous stress and inhomogeneous strain. Thus, contacts between granitic intrusions and volcano sedimentary sequences are common sites of ore fluid infiltration and gold deposition. For orogenic gold deposits at deeper crustal levels, ore forming fluids are commonly focused along strain gradients between more compressional zones where volcano sedimentary sequences are thinned and relatively more extensional zones where they are thickened. World class orogenic gold deposits are commonly located in the deformed volcano sedimentary sequences in such strain gradients adjacent to triple point junctions defined by the granitic intrusions, or along the zones of assembly of micro blocks on a regional scale. These repetitive province to district scale geometrical patterns of structures within the orogenic belts are clearly critical parameters in geology based exploration targeting for orogenic gold deposits.