Structural geometry of orogenic gold deposits: Implications for exploration of world-class and giant deposits

• Published in: Di xue qian yuan. Vol. 9; no. 4; pp. 1163 - 1177
• Main Authors: Groves, David I., Santosh, M., Goldfarb, Richard J., Zhang, Liang
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier B.V 01.07.2018; Elsevier Science Ltd; State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China%State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China; Centre for Tectonics Resources and Exploration,Dept. of Earth Sciences,University of Adelaide,SA 5005,Australia; Division of Interdisciplinary Science,Faculty of Science,Kochi University,Kochi 780-8520,Japan%State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China; Orebusters Pty Ltd,Gwelup 6018,Western Australia,Australia
• Subjects: Activation; Angles (geometry); Belts; Contact stresses; Deformation mechanisms; Deposition; Deposits; Ductile-brittle transition; Exploration; Exploration criteria; Fluid infiltration; Fluid pathways; Forming; Geological faults; Geometry; Gold; Metamorphism (geology); Orogenic gold deposits; Orogeny; Parameter robustness; Plate tectonics; Rocks; Strain; Structural geometry; Subduction (geology); Tectonic history; Volcanic activity; Volcanoes; Structural geometry; Orogenic gold deposits; Fluid pathways; Exploration criteria; Tectonic history
• ISSN: 1674-9871; 2588-9192
• DOI: 10.1016/j.gsf.2018.01.006

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 ∼10°–25° district-scale jogs in these crustal-scale faults. These jogs are commonly the site of arrays of ∼70° 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 ∼30° 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. [Display omitted] •Structural geometry of gold deposits inherited from earlier tectonic features.•Boundaries between rigid granitoids and ductile greenstone are common sites for gold deposition.•Deeper level mineralization focused along strain gradients between compressional zones.•World-class orogenic gold deposits located in strain gradients adjacent to triple-point junctions.•District-scale structural geometry provides critical information for exploration targeting.