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M. Santosh - One of the best experts on this subject based on the ideXlab platform.
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a scale integrated exploration model for orogenic Gold Deposits based on a mineral system approach
Geoscience frontiers, 2020Co-Authors: David I Groves, M. Santosh, Liang ZhangAbstract:Abstract Concept-based orogenic Gold exploration requires a scale-integrated approach using a robust mineral system model. Most genetic hypotheses for orogenic Gold Deposits that involve near-surface or magmatic-hydrothermal fluids are now negated in terms of a global mineral system model. Plausible models involve metamorphic fluids, but the fluid source has been equivocal. Crustal metamorphic-fluid models are most widely-accepted but there are serious problems for Archean Deposits, and numerous Chinese provinces, including Jiaodong, where the only feasible fluid source is sub-crustal. If all orogenic Gold Deposits define a coherent mineral system, there are only two realistic sources of fluid and Gold, based on their syn-mineralization geodynamic settings. These are from devolatilization of a subducted oceanic slab with its overlying Gold-bearing sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a subduction event, particularly adjacent to craton margins. In this model, CO2 is generated during decarbonation and S and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. This orogenic Gold mineral system can be applied to conceptual exploration by first identifying the required settings at geodynamic to deposit scales. Within these settings, it is then possible to define the critical Gold mineralization processes in the system: fertility, architecture, and preservation. The geological parameters that define these processes, and the geological, geophysical and geochemical proxies and responses for these critical parameters can then be identified. At the geodynamic to province scales, critical processes include a tectonic thermal engine and deep, effective, fluid plumbing system driven by seismic swarms up lithosphere-scale faults in an oblique-slip regime during uplift late in the orogenic cycle of a convergent margin. At the district to deposit scale, the important processes are fluid focussing into regions of complex structural geometry adjacent to crustal-scale plumbing systems, with Gold deposition in trap sites involving complex conjugations of competent and/or reactive rock sequences and structural or lithological fluid caps. Critical indirect responses to defined parameters change from those generated by geophysics to those generated by geochemistry with reduction in scale of the mineral system-driven conceptual exploration.
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a holistic model for the origin of orogenic Gold Deposits and its implications for exploration
Mineralium Deposita, 2020Co-Authors: David I Groves, Jun Deng, M. Santosh, Qingfei Wang, Liqiang Yang, Liang ZhangAbstract:The term orogenic Gold Deposits has been widely accepted, but there has been continuing debate on their genesis. Early syn-sedimentary or syn-volcanic models and hydrothermal meteoric-fluid models are now invalid. Magmatic-hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships. The most plausible models involve metamorphic fluids, but the source of these fluids is equivocal. Intra-basin sources within deeper segments of the hosting supracrustal successions, the underlying continental crust, subducted oceanic lithosphere with its overlying sediment wedge, and metasomatized lithosphere are all potential sources. Several features of Precambrian orogenic Gold Deposits are inconsistent with derivation from a continental metamorphic-fluid source. These include the presence of hypozonal Deposits in amphibolite-facies domains, their anomalous multiple sulfur isotopic compositions, and problems of derivation of Gold-related elements from devolatilization of dominant basalts in the sequences. The Phanerozoic Deposits are largely described as hosted in greenschist-facies domains, consistent with supracrustal devolatilization models. A notable exception is the Jiaodong Gold Deposits of China, where ca. 120-Ma Gold Deposits are hosted in Precambrian crust that was metamorphosed over 2000 million years prior to Gold mineralization. Other Deposits in China are comparable to those in the Massif Central and elsewhere in France, in that they are hosted in amphibolite-facies domains or clearly post-date regional metamorphic events imposed on hosting supracrustal sequences. If all orogenic Gold Deposits have a common genesis, the only realistic source of fluid and Gold is from devolatilization of a subducted oceanic slab with its overlying Gold-bearing sulfide-rich sedimentary package, or the associated metasomatized mantle wedge, with CO2 released during decarbonation and S- and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. Although this model satisfies all geological, geochronological, isotopic, and geochemical constraints, and is consistent with limited computer-based modeling of fluid release from subduction zones, the precise mechanisms of fluid flux are model-driven and remain uncertain. From an exploration viewpoint, the model re-emphasizes the ubiquitous occurrence of orogenic Gold Deposits in subduction-related orogenic belts and importance of continental-scale lithosphere-tapping fault and shear zones to focus large volumes of auriferous fluid. It confirms the importance of the consistent spacing between world-class Deposits, broadly equivalent to the depth of the Moho, as derived from empirical observations.
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ancient deep roots for mesozoic world class Gold Deposits in the north china craton an integrated genetic perspective
Geoscience frontiers, 2020Co-Authors: Chengxue Yang, M. SantoshAbstract:Abstract The North China Craton (NCC) hosts some of the world-class Gold Deposits that formed more than 2 billion years after the major orogenic cycles and cratonization. The diverse models for the genesis of these Deposits remain equivocal, and mostly focused on the craton margin examples, although synchronous Deposits formed in the interior domains. Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major Gold Deposits in the NCC. In the Archean tectonic framework of the NCC, the locations of the major Gold Deposits fall within or adjacent to greenstone belts or the margins of micro-continents. In the Paleoproterozoic framework, they are markedly aligned along two major collisional sutures – the Trans North China Orogen and the Jiao-Liao-Ji Belt. Since the Mesozoic intrusions hosting these Deposits do not carry adequate signals for the source of Gold, we explore the deep roots based on available geophysical data. We show that the Gold Deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle, and that the mineralization is located above regions of high heat flow representing mantle upwelling. The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly enriched it. The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning. Based on these results, and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the Gold mineralization, we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction, underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC. The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs, leading to the major Gold metallogeny along craton margins as well as in the interior of the NCC.
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structural geometry of orogenic Gold Deposits implications for exploration of world class and giant Deposits
Geoscience frontiers, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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 ∼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.
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Structural geometry of orogenic Gold Deposits: Implications for exploration of world-class and giant Deposits
Elsevier, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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. Keywords: Structural geometry, Tectonic history, Fluid pathways, Orogenic Gold Deposits, Exploration criteri
David I Groves - One of the best experts on this subject based on the ideXlab platform.
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a scale integrated exploration model for orogenic Gold Deposits based on a mineral system approach
Geoscience frontiers, 2020Co-Authors: David I Groves, M. Santosh, Liang ZhangAbstract:Abstract Concept-based orogenic Gold exploration requires a scale-integrated approach using a robust mineral system model. Most genetic hypotheses for orogenic Gold Deposits that involve near-surface or magmatic-hydrothermal fluids are now negated in terms of a global mineral system model. Plausible models involve metamorphic fluids, but the fluid source has been equivocal. Crustal metamorphic-fluid models are most widely-accepted but there are serious problems for Archean Deposits, and numerous Chinese provinces, including Jiaodong, where the only feasible fluid source is sub-crustal. If all orogenic Gold Deposits define a coherent mineral system, there are only two realistic sources of fluid and Gold, based on their syn-mineralization geodynamic settings. These are from devolatilization of a subducted oceanic slab with its overlying Gold-bearing sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a subduction event, particularly adjacent to craton margins. In this model, CO2 is generated during decarbonation and S and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. This orogenic Gold mineral system can be applied to conceptual exploration by first identifying the required settings at geodynamic to deposit scales. Within these settings, it is then possible to define the critical Gold mineralization processes in the system: fertility, architecture, and preservation. The geological parameters that define these processes, and the geological, geophysical and geochemical proxies and responses for these critical parameters can then be identified. At the geodynamic to province scales, critical processes include a tectonic thermal engine and deep, effective, fluid plumbing system driven by seismic swarms up lithosphere-scale faults in an oblique-slip regime during uplift late in the orogenic cycle of a convergent margin. At the district to deposit scale, the important processes are fluid focussing into regions of complex structural geometry adjacent to crustal-scale plumbing systems, with Gold deposition in trap sites involving complex conjugations of competent and/or reactive rock sequences and structural or lithological fluid caps. Critical indirect responses to defined parameters change from those generated by geophysics to those generated by geochemistry with reduction in scale of the mineral system-driven conceptual exploration.
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a holistic model for the origin of orogenic Gold Deposits and its implications for exploration
Mineralium Deposita, 2020Co-Authors: David I Groves, Jun Deng, M. Santosh, Qingfei Wang, Liqiang Yang, Liang ZhangAbstract:The term orogenic Gold Deposits has been widely accepted, but there has been continuing debate on their genesis. Early syn-sedimentary or syn-volcanic models and hydrothermal meteoric-fluid models are now invalid. Magmatic-hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships. The most plausible models involve metamorphic fluids, but the source of these fluids is equivocal. Intra-basin sources within deeper segments of the hosting supracrustal successions, the underlying continental crust, subducted oceanic lithosphere with its overlying sediment wedge, and metasomatized lithosphere are all potential sources. Several features of Precambrian orogenic Gold Deposits are inconsistent with derivation from a continental metamorphic-fluid source. These include the presence of hypozonal Deposits in amphibolite-facies domains, their anomalous multiple sulfur isotopic compositions, and problems of derivation of Gold-related elements from devolatilization of dominant basalts in the sequences. The Phanerozoic Deposits are largely described as hosted in greenschist-facies domains, consistent with supracrustal devolatilization models. A notable exception is the Jiaodong Gold Deposits of China, where ca. 120-Ma Gold Deposits are hosted in Precambrian crust that was metamorphosed over 2000 million years prior to Gold mineralization. Other Deposits in China are comparable to those in the Massif Central and elsewhere in France, in that they are hosted in amphibolite-facies domains or clearly post-date regional metamorphic events imposed on hosting supracrustal sequences. If all orogenic Gold Deposits have a common genesis, the only realistic source of fluid and Gold is from devolatilization of a subducted oceanic slab with its overlying Gold-bearing sulfide-rich sedimentary package, or the associated metasomatized mantle wedge, with CO2 released during decarbonation and S- and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. Although this model satisfies all geological, geochronological, isotopic, and geochemical constraints, and is consistent with limited computer-based modeling of fluid release from subduction zones, the precise mechanisms of fluid flux are model-driven and remain uncertain. From an exploration viewpoint, the model re-emphasizes the ubiquitous occurrence of orogenic Gold Deposits in subduction-related orogenic belts and importance of continental-scale lithosphere-tapping fault and shear zones to focus large volumes of auriferous fluid. It confirms the importance of the consistent spacing between world-class Deposits, broadly equivalent to the depth of the Moho, as derived from empirical observations.
-
structural geometry of orogenic Gold Deposits implications for exploration of world class and giant Deposits
Geoscience frontiers, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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 ∼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.
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Structural geometry of orogenic Gold Deposits: Implications for exploration of world-class and giant Deposits
Elsevier, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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. Keywords: Structural geometry, Tectonic history, Fluid pathways, Orogenic Gold Deposits, Exploration criteri
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the giant jiaodong Gold province the key to a unified model for orogenic Gold Deposits
Geoscience frontiers, 2016Co-Authors: David I Groves, M. SantoshAbstract:Abstract Although the term orogenic Gold deposit has been widely accepted for all Gold-only lode-Gold Deposits, with the exception of Carlin-type Deposits and rare intrusion-related Gold systems, there has been continuing debate on their genesis. Early syngenetic models and hydrothermal models dominated by meteoric fluids are now clearly unacceptable. Magmatic-hydrothermal models fail to explain the genesis of orogenic Gold Deposits because of the lack of consistent spatially – associated granitic intrusions and inconsistent temporal relationships. The most plausible, and widely accepted, models involve metamorphic fluids, but the source of these fluids is hotly debated. Sources within deeper segments of the supracrustal successions hosting the Deposits, the underlying continental crust, and subducted oceanic lithosphere and its overlying sediment wedge all have their proponents. The orogenic Gold Deposits of the giant Jiaodong Gold province of China, in the delaminated North China Craton, contain ca. 120 Ma Gold Deposits in Precambrian crust that was metamorphosed over 2000 million years prior to Gold mineralization. The only realistic source of fluid and Gold is a subducted oceanic slab with its overlying sulfide-rich sedimentary package, or the associated mantle wedge. This could be viewed as an exception to a general metamorphic model where orogenic Gold has been derived during greenschist- to amphibolite-facies metamorphism of supracrustal rocks: basaltic rocks in the Precambrian and sedimentary rocks in the Phanerozoic. Alternatively, if a holistic view is taken, Jiaodong can be considered the key orogenic Gold province for a unified model in which Gold is derived from late-orogenic metamorphic devolatilization of stalled subduction slabs and oceanic sediments throughout Earth history. The latter model satisfies all geological, geochronological, isotopic and geochemical constraints but the precise mechanisms of auriferous fluid release, like many other subduction-related processes, are model-driven and remain uncertain.
Richard J Goldfarb - One of the best experts on this subject based on the ideXlab platform.
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structural geometry of orogenic Gold Deposits implications for exploration of world class and giant Deposits
Geoscience frontiers, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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 ∼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.
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Structural geometry of orogenic Gold Deposits: Implications for exploration of world-class and giant Deposits
Elsevier, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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. Keywords: Structural geometry, Tectonic history, Fluid pathways, Orogenic Gold Deposits, Exploration criteri
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paragenesis and geochemistry of ore minerals in the epizonal Gold Deposits of the yangshan Gold belt west qinling china
Mineralium Deposita, 2014Co-Authors: Jun Deng, Richard J Goldfarb, Liqiang Yang, Chuang Zhang, Erin E Marsh, Shibin Lei, Alan E Koenig, Heather A LowersAbstract:Six epizonal Gold Deposits in the 30-km-long Yangshan Gold belt, Gansu Province are estimated to contain more than 300 t of Gold at an average grade of 4.76 g/t and thus define one of China's largest Gold resources. Detailed paragenetic studies have recognized five stages of sulfide mineral precipitation in the Deposits of the belt. Syngenetic/diagenetic pyrite (Py0) has a framboidal or colloform texture and is disseminated in the metasedimentary host rocks. Early hydrothermal pyrite (Py1) in quartz veins is disseminated in metasedimentary rocks and dikes and also occurs as semi-massive pyrite aggregates or bedding-parallel pyrite bands in phyllite. The main ore stage pyrite (Py2) commonly overgrows Py1 and is typically associated with main ore stage arsenopyrite (Apy2). Late ore stage pyrite (Py3), arsenopyrite (Apy3), and stibnite occur in quartz ± calcite veins or are disseminated in country rocks. Post-ore stage pyrite (Py4) occurs in quartz ± calcite veins that cut all earlier formed mineralization. Electron probe microanalyses and laser ablation-inductively coupled plasma mass spectrometry analyses reveal that different generations of sulfides have characteristic of major and trace element patterns, which can be used as a proxy for the distinct hydrothermal events. Syngenetic/diagenetic pyrite has high concentrations of As, Au, Bi, Co, Cu, Mn, Ni, Pb, Sb, and Zn. The Py0 also retains a sedimentary Co/Ni ratio, which is distinct from hydrothermal ore-related pyrite. Early hydrothermal Py1 has high contents of Ag, As, Au, Bi, Cu, Fe, Sb, and V, and it reflects elevated levels of these elements in the earliest mineralizing metamorphic fluids. The main ore stage Py2 has a very high content of As (median value of 2.96 wt%) and Au (median value of 47.5 ppm) and slightly elevated Cu, but relatively low values for other trace elements. Arsenic in the main ore stage Py2 occurs in solid solution. Late ore stage Py3, formed coevally with stibnite, contains relatively high As (median value of 1.44 wt%), Au, Fe, Mn, Mo, Sb, and Zn and low Bi, Co, Ni, and Pb. The main ore stage Apy2, compared to late ore stage arsenopyrite, is relatively enriched in As, whereas the later Apy3 has high concentrations of S, Fe, and Sb, which is consistent with element patterns in associated main and late ore stage pyrite generations. Compared with pyrite from other stages, the post-ore stage Py4 has relatively low concentrations of Fe and S, whereas As remains elevated (2.05∼3.20 wt%), which could be interpreted by the substitution of As− for S in the pyrite structure. These results suggest that syngenetic/diagenetic pyrite is the main metal source for the Yangshan Gold Deposits where such pyrite was metamorphosed at depth below presently exposed levels. The ore-forming elements were concentrated into the hydrothermal fluids during metamorphic devolatilization, and subsequently, during extensive fluid–rock interaction at shallower levels, these elements were precipitated via widespread sulfidation during the main ore stage.
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the dilemma of the jiaodong Gold Deposits are they unique
Geoscience frontiers, 2014Co-Authors: Richard J Goldfarb, M. SantoshAbstract:Abstract The ca. 126–120 Ma Au Deposits of the Jiaodong Peninsula, eastern China, define the country's largest Gold province with an overall endowment estimated as >3000 t Au. The vein and disseminated ores are hosted by NE- to NNE-trending brittle normal faults that parallel the margins of ca. 165–150 Ma, deeply emplaced, lower crustal melt granites. The Deposits are sited along the faults for many tens of kilometers and the larger orebodies are associated with dilatational jogs. Country rocks to the granites are Precambrian high-grade metamorphic rocks located on both sides of a Triassic suture between the North and South China blocks. During early Mesozoic convergent deformation, the ore-hosting structures developed as ductile thrust faults that were subsequently reactivated during Early Cretaceous “Yanshanian” intracontinental extensional deformation and associated Gold formation. Classification of the Gold Deposits remains problematic. Many features resemble those typical of orogenic Au including the linear structural distribution of the Deposits, mineralization style, ore and alteration assemblages, and ore fluid chemistry. However, Phanerozoic orogenic Au Deposits are formed by prograde metamorphism of accreted oceanic rocks in Cordilleran-style orogens. The Jiaodong Deposits, in contrast, formed within two Precambrian blocks approximately 2 billion years after devolatilization of the country rocks, and thus require a model that involves alternative fluid and metal sources for the ores. A widespread suite of ca. 130–123 Ma granodiorites overlaps temporally with the ores, but shows a poor spatial association with the Deposits. Furthermore, the deposit distribution and mineralization style is atypical of ores formed from nearby magmas. The ore concentration requires fluid focusing during some type of sub-crustal thermal event, which could be broadly related to a combination of coeval lithospheric thinning, asthenospheric upwelling, paleo-Pacific plate subduction, and seismicity along the continental-scale Tan-Lu fault. Possible ore genesis scenarios include those where ore fluids were produced directly by the metamorphism of oceanic lithosphere and overlying sediment on the subducting paleo-Pacific slab, or by devolatilization of an enriched mantle wedge above the slab. Both the sulfur and Gold could be sourced from either the oceanic sediments or the serpentinized mantle. A better understanding of the architecture of the paleo-Pacific slab during Early Cretaceous below the eastern margin of China is essential to determination of the validity of possible models.
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lithospheric controls on the formation of provinces hosting giant orogenic Gold Deposits
Mineralium Deposita, 2006Co-Authors: Frank P Bierlein, David I Groves, Richard J Goldfarb, B DubeAbstract:Ages of giant Gold systems (>500 t Gold) cluster within well-defined periods of lithospheric growth at continental margins, and it is the orogen-scale processes during these mainly Late Archaean, Palaeoproterozoic and Phanerozoic times that ultimately determine Gold endowment of a province in an orogen. A critical factor for giant orogenic Gold provinces appears to be thickness of the subcontinental lithospheric mantle (SCLM) beneath a province at the time of Gold mineralisation, as giant Gold Deposits are much more likely to develop in orogens with subducted oceanic or thin continental lithosphere. A proxy for the latter is a short pre-mineralisation crustal history such that thick SCLM was not developed before Gold deposition. In constrast, orogens with protracted pre-mineralisation crustal histories are more likely to be characterised by a thick SCLM that is difficult to delaminate, and hence, such provinces will normally be poorly endowed. The nature of the lithosphere also influences the intrinsic Gold concentrations of potential source rocks, with back-arc basalts, transitional basalts and basanites enriched in Gold relative to other rock sequences. Thus, segments of orogens with thin lithosphere may enjoy the conjunction of giant-scale fluid flux through Gold-enriched sequences. Although the nature of the lithosphere plays the crucial role in dictating which orogenic Gold provinces will contain one or more giant Deposits, the precise siting of those giants depends on the critical conjunction of a number of province-scale factors. Such features control plumbing systems, traps and seals in tectonically and lithospherically suitable terranes within orogens.
Liang Zhang - One of the best experts on this subject based on the ideXlab platform.
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a scale integrated exploration model for orogenic Gold Deposits based on a mineral system approach
Geoscience frontiers, 2020Co-Authors: David I Groves, M. Santosh, Liang ZhangAbstract:Abstract Concept-based orogenic Gold exploration requires a scale-integrated approach using a robust mineral system model. Most genetic hypotheses for orogenic Gold Deposits that involve near-surface or magmatic-hydrothermal fluids are now negated in terms of a global mineral system model. Plausible models involve metamorphic fluids, but the fluid source has been equivocal. Crustal metamorphic-fluid models are most widely-accepted but there are serious problems for Archean Deposits, and numerous Chinese provinces, including Jiaodong, where the only feasible fluid source is sub-crustal. If all orogenic Gold Deposits define a coherent mineral system, there are only two realistic sources of fluid and Gold, based on their syn-mineralization geodynamic settings. These are from devolatilization of a subducted oceanic slab with its overlying Gold-bearing sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a subduction event, particularly adjacent to craton margins. In this model, CO2 is generated during decarbonation and S and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. This orogenic Gold mineral system can be applied to conceptual exploration by first identifying the required settings at geodynamic to deposit scales. Within these settings, it is then possible to define the critical Gold mineralization processes in the system: fertility, architecture, and preservation. The geological parameters that define these processes, and the geological, geophysical and geochemical proxies and responses for these critical parameters can then be identified. At the geodynamic to province scales, critical processes include a tectonic thermal engine and deep, effective, fluid plumbing system driven by seismic swarms up lithosphere-scale faults in an oblique-slip regime during uplift late in the orogenic cycle of a convergent margin. At the district to deposit scale, the important processes are fluid focussing into regions of complex structural geometry adjacent to crustal-scale plumbing systems, with Gold deposition in trap sites involving complex conjugations of competent and/or reactive rock sequences and structural or lithological fluid caps. Critical indirect responses to defined parameters change from those generated by geophysics to those generated by geochemistry with reduction in scale of the mineral system-driven conceptual exploration.
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a holistic model for the origin of orogenic Gold Deposits and its implications for exploration
Mineralium Deposita, 2020Co-Authors: David I Groves, Jun Deng, M. Santosh, Qingfei Wang, Liqiang Yang, Liang ZhangAbstract:The term orogenic Gold Deposits has been widely accepted, but there has been continuing debate on their genesis. Early syn-sedimentary or syn-volcanic models and hydrothermal meteoric-fluid models are now invalid. Magmatic-hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships. The most plausible models involve metamorphic fluids, but the source of these fluids is equivocal. Intra-basin sources within deeper segments of the hosting supracrustal successions, the underlying continental crust, subducted oceanic lithosphere with its overlying sediment wedge, and metasomatized lithosphere are all potential sources. Several features of Precambrian orogenic Gold Deposits are inconsistent with derivation from a continental metamorphic-fluid source. These include the presence of hypozonal Deposits in amphibolite-facies domains, their anomalous multiple sulfur isotopic compositions, and problems of derivation of Gold-related elements from devolatilization of dominant basalts in the sequences. The Phanerozoic Deposits are largely described as hosted in greenschist-facies domains, consistent with supracrustal devolatilization models. A notable exception is the Jiaodong Gold Deposits of China, where ca. 120-Ma Gold Deposits are hosted in Precambrian crust that was metamorphosed over 2000 million years prior to Gold mineralization. Other Deposits in China are comparable to those in the Massif Central and elsewhere in France, in that they are hosted in amphibolite-facies domains or clearly post-date regional metamorphic events imposed on hosting supracrustal sequences. If all orogenic Gold Deposits have a common genesis, the only realistic source of fluid and Gold is from devolatilization of a subducted oceanic slab with its overlying Gold-bearing sulfide-rich sedimentary package, or the associated metasomatized mantle wedge, with CO2 released during decarbonation and S- and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. Although this model satisfies all geological, geochronological, isotopic, and geochemical constraints, and is consistent with limited computer-based modeling of fluid release from subduction zones, the precise mechanisms of fluid flux are model-driven and remain uncertain. From an exploration viewpoint, the model re-emphasizes the ubiquitous occurrence of orogenic Gold Deposits in subduction-related orogenic belts and importance of continental-scale lithosphere-tapping fault and shear zones to focus large volumes of auriferous fluid. It confirms the importance of the consistent spacing between world-class Deposits, broadly equivalent to the depth of the Moho, as derived from empirical observations.
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structural geometry of orogenic Gold Deposits implications for exploration of world class and giant Deposits
Geoscience frontiers, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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 ∼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.
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Structural geometry of orogenic Gold Deposits: Implications for exploration of world-class and giant Deposits
Elsevier, 2018Co-Authors: David I Groves, Richard J Goldfarb, M. Santosh, Liang ZhangAbstract: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. Keywords: Structural geometry, Tectonic history, Fluid pathways, Orogenic Gold Deposits, Exploration criteri
R Powell - One of the best experts on this subject based on the ideXlab platform.
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formation of Gold Deposits a metamorphic devolatilization model
Journal of Metamorphic Geology, 2010Co-Authors: G N Phillips, R PowellAbstract:A metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many Goldfields such as those found in Archean greenstone belts, slate-belts and other Gold-only provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist-amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H2O and CO2 but also S and, in turn, Au. Elevated Gold in solution is achieved by complexing with reduced S, and by H2CO3 weak acid buffering near the optimal fluid pH for Gold solubility (the buffering is more important than being at the point of maximum Gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and/or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre-scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of Gold. Deposition of Gold from solution necessitates breakdown of the Gold-thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon-bearing host rocks and/or by sulphidation of wallrocks to generate iron-bearing sulphide and precipitated Gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite-bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major Goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite/biotite alteration where the host rock is of suitable bulk composition. The correlation of major Gold Deposits with rock type, even when the Gold is primarily in veins, argues for rock-dominated depositional systems, not fluid-dominated ones. As a consequence, a general role in Gold deposition for fluid mixing, temperature decrease and/or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at Gold-only Deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher-grade metamorphism and deformation, and/or (palaeo)-weathering may provide many of the most-obvious features of Goldfields including their mineralogy, geochemistry, geometry, small-scale timing features, geophysical response and even mesoscopic Gold distribution. © 2010 Blackwell Publishing Ltd.
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formation of Gold Deposits a metamorphic devolatilization model
Journal of Metamorphic Geology, 2010Co-Authors: G N Phillips, R PowellAbstract:A metamorphic devolatilization model can explain the enrichment, segregation, timing, distribution and character of many Goldfields such as those found in Archean greenstone belts, slate-belts and other Gold-only provinces. In this genetic model, hydrated and carbonated greenschist facies rocks, particularly metabasic rocks, are devolatilized primarily across the greenschist–amphibolite facies boundary in an orogenic setting. Devolatilization operates on the scale of individual mineral grains, extracting not just H2O and CO2 but also S and, in turn, Au. Elevated Gold in solution is achieved by complexing with reduced S, and by H2CO3 weak acid buffering near the optimal fluid pH for Gold solubility (the buffering is more important than being at the point of maximum Gold solubility). Low salinity ensures low base metal concentrations in the auriferous metamorphic fluid. Migration of this fluid upwards is via shear zones and/or into hydraulic fracture zones in rocks of low tensile strength. The geometry of the shear zones dictates the kilometre-scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of Gold. Deposition of Gold from solution necessitates breakdown of the Gold–thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon-bearing host rocks and/or by sulphidation of wallrocks to generate iron-bearing sulphide and precipitated Gold. As such, black slate, carbon seams, banded iron formation, tholeiitic basalt, magnetite-bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major Goldfields. Gold deposition is accompanied by carbonation, sulphidation and muscovite/biotite alteration where the host rock is of suitable bulk composition. The correlation of major Gold Deposits with rock type, even when the Gold is primarily in veins, argues for rock-dominated depositional systems, not fluid-dominated ones. As a consequence, a general role in Gold deposition for fluid mixing, temperature decrease and/or fluid pressure decrease and boiling is unlikely, although such effects may be involved locally. Several geological features that are recorded at Gold-only Deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process. Overprinting by higher-grade metamorphism and deformation, and/or (palaeo)-weathering may provide many of the most-obvious features of Goldfields including their mineralogy, geochemistry, geometry, small-scale timing features, geophysical response and even mesoscopic Gold distribution.
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formation of Gold Deposits review and evaluation of the continuum model
Earth-Science Reviews, 2009Co-Authors: Neil G Phillips, R PowellAbstract:Abstract The continuum model has been one of the more widely advocated genetic models for ‘Gold-only’ Deposits such as those found in Archaean greenstone belts. It postulates that hydrothermal Gold Deposits were formed throughout a 20–25 km vertical crustal profile, from temperatures above 700 °C to below 180 °C, and that the deposition occurred synchronous with the peak of metamorphism. The continuum model is reviewed at this stage because we believe that it does not successfully account for many aspects of Gold deposit formation. The most obvious shortcoming is in considering ore Deposits found in rocks of uppermost amphibolite and granulite facies domains where temperatures were appropriate for partial melting. The main chemical condition that favours melting is access to H2O from either aqueous fluid or the breakdown of hydrous minerals. A major Gold-forming hydrothermal event at the peak of high-grade metamorphism (as implied by the continuum model) is incompatible with partial melting of wallrocks: instead of forming a hydrothermal Gold deposit, an aqueous fluid introduced during partial melting would be consumed to produce further melt. Five Gold Deposits are documented from high metamorphic grade domains within four separate Archaean cratons; one is the type example used in the continuum model, the other four have been significant producers. Partial melting has been recorded in the wallrocks adjacent to Gold mineralisation at Big Bell, Hemlo, Challenger and Renco Gold Deposits, in the sulphide-rich ore itself at Challenger and Hemlo Deposits, and as cross-cutting dykes and migmatites at Griffins Find. For these five Deposits, and indeed in general, the continuum model does not easily account for aspects of the fluid source, fluid composition, Gold transport, and metal deposition. The evidence that has been used to support deposit formation at the peak of high-grade metamorphism, as required by the continuum model, is generally compatible with one or more alternative genetic models. Textural and structural siting of the Gold, high-variance mineral assemblages, proportion of various Gold host rocks, and relative element enrichments at high metamorphic grade can be accounted for by Gold formation at greenschist facies conditions followed by progress of the deposit to high metamorphic grade (metamorphic model). Moreover the type examples in the continuum model of Deposits in sub-greenschist facies domains appear to be incorrectly ascribed, and are likely to be in greenschist facies domains. The continuum model does not adequately explain Archaean Gold Deposits.