Concepts and Revised Models for Phanerozoic Orogenic Gold Deposits

Existing published models for orogenic gold deposits (OGDs) do not adequately describe or explain most deposits of Phanerozoic age, and there are numerous reasons why Phanerozoic OGDs might differ significantly from older deposits. We subdivide Phanerozoic OGDs into four main subtypes, based on a number of descriptive criteria, including tectonic setting, lithological siting, and characteristics of the mineralization in each subtype. The four subtypes are: 1) crustal scale fault associated (CSF) subtype, 2) sediment-hosted orogenic gold (SHOG) subtype, 3) forearc (FA) subtype, and 4) syn- and late tectonic dispersed (SLTD) subtype. Lead isotopic studies suggest that Pb and other metals in all but the FA subtype were likely derived from relatively small source reservoirs in the middle or upper crust. OGDs formed in large, lithologically and structurally homogeneous regions will tend to be of the same subtype; however, in geologically complex orogenic belts it is common to find two or more subtypes that formed at approximately the same time. Based on the synthesis of global OGDs of Phanerozoic age districts containing CSF or SHOG subtype deposits appear to have the best potential for hosting multiple large deposits. FA subtype deposits form in a relatively uncommon tectonic setting (accretionary forearc, possibly overlying a subducting spreading ridge) and are likely to be rare. SLTD subtype OGDs are the most common, but most are small and uneconomic, although they commonly generate substantial alluvial gold deposits.

Thermochemical anomalies in the upper mantle control Gakkel Ridge accretion

Despite progress in understanding seafloor accretion at ultraslow spreading ridges, the ultimate driving force is still unknown. Here we use 40Ar/39Ar isotopic dating of mid-ocean ridge basalts recovered at variable distances from the axis of the Gakkel Ridge to provide new constraints on the spatial and temporal distribution of volcanic eruptions at various sections of an ultraslow spreading ridge. Our age data show that magmatic-dominated sections of the Gakkel Ridge spread at a steady rate of ~11.1 ± 0.9 mm/yr whereas amagmatic sections have a more widely distributed melt supply yielding ambiguous spreading rate information. These variations in spreading rate and crustal accretion correlate with locations of hotter thermochemical anomalies in the asthenosphere beneath the ridge. We conclude therefore that seafloor generation in ultra-slow spreading centres broadly reflects the distribution of thermochemical anomalies in the upper mantle.

Ridge Jumps and Mantle Exhumation in Back-Arc Basins

Back-arc basins in continental settings can develop into oceanic basins, when extension lasts long enough to break up the continental lithosphere and allow mantle melting that generates new oceanic crust. Often, the basement of these basins is not only composed of oceanic crust, but also of exhumed mantle, fragments of continental crust, intrusive magmatic bodies, and a complex mid-ocean ridge system characterised by distinct relocations of the spreading centre. To better understand the dynamics that lead to these characteristic structures in back-arc basins, we performed 2D numerical models of continental extension with asymmetric and time-dependent boundary conditions that simulate episodic trench retreat. We find that, in all models, episodic extension leads to rift and/or ridge jumps. In our parameter space, the length of the jump ranges between 1 and 65 km and the timing necessary to produce a new spreading ridge varies between 0.4 and 7 Myr. With the shortest duration of the first extensional phase, we observe a strong asymmetry in the margins of the basin, with the margin further from trench being characterised by outcropping lithospheric mantle and a long section of thinned continental crust. In other cases, ridge jump creates two consecutive oceanic basins, leaving a continental fragment and exhumed mantle in between the two basins. Finally, when the first extensional phase is long enough to form a well-developed oceanic basin (>35 km long), we observe a very short intra-oceanic ridge jump. Our models are able to reproduce many of the structures observed in back-arc basins today, showing that the transient nature of trench retreat that leads to episodes of fast and slow extension is the cause of ridge jumps, mantle exhumation, and continental fragments formation.

Geophysical Studies in the Southwest Pacific : Primarily Studies of Crustal Structure between New Zealand and Antarctica

<p>Geophysical data - primarily magnetic field measurements, bathymetry, and seismicity data - are presented for the area between New Zealand and Antarctica from approximately 145[degrees]W to 155[degrees]E. The data are used to determine the structure of the Pacific-Antarctic boundary, the oceanic part of the Pacific plate and the area of intersection of the Indian, Pacific and Antarctic plates. In the southwest Pacific basin the magnetic anomalies are very clear and an extensive pattern of anomaly lineations with some offsets is mapped. The magnetic anomalies show that the uniform Pacific basin area formed between about 83 and 63 mybp. The Pacific-Antarctic boundary is shown to differ either side of about 175[degrees]W. To the east it is a relatively uniform aseismic spreading ridge, offset by some transform faults. West of 175[degrees]W, to 161[degrees]E, the boundary consists of a seismically active zone of disturbed bathymetry and magnetic anomalies striking about N.70[degrees]W. The zone, the Pacific-Antarctic fracture zone, probably consists of several fractures striking about N45[degrees]W. The area between the Pacific-Antarctic boundary and the southwest Pacific basin represents the interval 10 to -55 mybp, and only in the east are anomaly lineations clear. The Indian-Antarctic Pacific triple junction is near 61.5[degrees]S, 161[degrees]E and is a stable ridge-fault-fault junction; the Indian-Antarctic boundary being the ridge. Plate tectonics is applied to the area and the structure is shown to fit, and be explained by a different rotation pole for each of the major intervals indicated by the structure, i.e. 0-10 mybp, 10-63 mybp and 63-80 mybp. The poles, with rotation rates deduced from the magnetic anomalies, are used to reconstruct the position of New Zealand relative to Antarctica at 80 mybp. The two continents probably started to separate at close to 83 mybp. The times of the major changes of structure and plate movement in the area are shown to coincide with major plate movement changes in the southwest Pacific area and in the rest of the world. A new method for determining poles of rotation, based only on epicentre locations is presented, The method is applied to independently determine the Indian-Pacific, Pacific-Antarctic and Indian-Antarctic poles. The poles should form a consistent. set and they do. The method yields effectively instantaneous poles, is quantitative, and is applicable to most plate boundaries. Earthquake magnitude-frequency relationship b values for the plate boundaries in the area are determined. Comparisons with results from elsewhere indicate an association of high b with high temperature and conversely. Several factors which have previously been suggested as determining b value are shown to not be determinants. A revised and extended magnetic reversal time scale based on model studies of the southwest Pacific basin anomalies is presented. Other model studies indicate that a magnetized layer thickness of at least 2 km is probable. Variations of anomaly amplitudes are studied. A detailed study of the application of numerical correlation techniques to magnetic anomalies is presented. It is concluded that horizontal scale variations and discontinuities in profiles can be critical. Methods for over-coming some of the problems, and for determining quantitative error estimates, are. given. The methods, and conclusions, are applicable to any correlation problem.</p>

Geophysical Studies in the Southwest Pacific : Primarily Studies of Crustal Structure between New Zealand and Antarctica

<p>Geophysical data - primarily magnetic field measurements, bathymetry, and seismicity data - are presented for the area between New Zealand and Antarctica from approximately 145[degrees]W to 155[degrees]E. The data are used to determine the structure of the Pacific-Antarctic boundary, the oceanic part of the Pacific plate and the area of intersection of the Indian, Pacific and Antarctic plates. In the southwest Pacific basin the magnetic anomalies are very clear and an extensive pattern of anomaly lineations with some offsets is mapped. The magnetic anomalies show that the uniform Pacific basin area formed between about 83 and 63 mybp. The Pacific-Antarctic boundary is shown to differ either side of about 175[degrees]W. To the east it is a relatively uniform aseismic spreading ridge, offset by some transform faults. West of 175[degrees]W, to 161[degrees]E, the boundary consists of a seismically active zone of disturbed bathymetry and magnetic anomalies striking about N.70[degrees]W. The zone, the Pacific-Antarctic fracture zone, probably consists of several fractures striking about N45[degrees]W. The area between the Pacific-Antarctic boundary and the southwest Pacific basin represents the interval 10 to -55 mybp, and only in the east are anomaly lineations clear. The Indian-Antarctic Pacific triple junction is near 61.5[degrees]S, 161[degrees]E and is a stable ridge-fault-fault junction; the Indian-Antarctic boundary being the ridge. Plate tectonics is applied to the area and the structure is shown to fit, and be explained by a different rotation pole for each of the major intervals indicated by the structure, i.e. 0-10 mybp, 10-63 mybp and 63-80 mybp. The poles, with rotation rates deduced from the magnetic anomalies, are used to reconstruct the position of New Zealand relative to Antarctica at 80 mybp. The two continents probably started to separate at close to 83 mybp. The times of the major changes of structure and plate movement in the area are shown to coincide with major plate movement changes in the southwest Pacific area and in the rest of the world. A new method for determining poles of rotation, based only on epicentre locations is presented, The method is applied to independently determine the Indian-Pacific, Pacific-Antarctic and Indian-Antarctic poles. The poles should form a consistent. set and they do. The method yields effectively instantaneous poles, is quantitative, and is applicable to most plate boundaries. Earthquake magnitude-frequency relationship b values for the plate boundaries in the area are determined. Comparisons with results from elsewhere indicate an association of high b with high temperature and conversely. Several factors which have previously been suggested as determining b value are shown to not be determinants. A revised and extended magnetic reversal time scale based on model studies of the southwest Pacific basin anomalies is presented. Other model studies indicate that a magnetized layer thickness of at least 2 km is probable. Variations of anomaly amplitudes are studied. A detailed study of the application of numerical correlation techniques to magnetic anomalies is presented. It is concluded that horizontal scale variations and discontinuities in profiles can be critical. Methods for over-coming some of the problems, and for determining quantitative error estimates, are. given. The methods, and conclusions, are applicable to any correlation problem.</p>

The TsuJal Amphibious Seismic Network: A Passive-Source Seismic Experiment in Western Mexico

The geodynamic complexity in the western Mexican margin is controlled by the multiple interactions between the Rivera, Pacific, Cocos, and North American plates, as evidenced by a high seismicity rate, most of whose hypocenters are poorly located. To mitigate this uncertainty with the aim of improving these hypocentral locations, we undertook the TsuJal Project, a passive seafloor seismic project conducted from April to November 2016. In addition to the Jalisco Seismic Network, 10 LCHEAPO 2000 Ocean Bottom Seismometers (OBSs) were deployed by the BO El Puma in a seafloor array from the Islas Marías Archipelago (Nayarit) to the offshore contact between the states of Colima and Michoacan. We located 445 earthquakes in four or more OBSs within the deployed array. Most of these earthquakes occurred in the contact region of the Rivera, Pacific, and Cocos plates, and a first analysis suggests the existence of three seismogenic zones (West, Center, and East) along the Rivera Transform fault that can be correlated with its morphological expression throughout the three seismogenic zones. The seismicity estimates that the Moho discontinuity is located at 10 km depth and supports earlier works regarding the West zone earthquake distribution. Subcrustal seismicity in the Central zone suggests that the Intra-Transform Spreading Basin domain is an ultra-low spreading ridge. A seismic swarm occurred during May and June 2016 between the eastern tip of the Paleo-Rivera Transform fault and the northern tip of the East Pacific Rise-Pacific Cocos Segment, illuminating some unidentified tectonic feature.

Improved Seismic Monitoring with OBS Deployment in the Arctic: A Pilot Study from Offshore Western Svalbard

The mid-ocean ridge system is the main source of earthquakes within the Arctic region. The earthquakes are recorded on the permanent land-based stations in the region, although smaller earthquakes remain undetected. In this study, we make use of three Ocean Bottom Seismographs (OBSs) that were deployed offshore western Svalbard, along the spreading ridges. The OBS arrival times were used to relocate the regional seismicity using a Bayesian approach, which resulted in a significant improvement with tighter clustering around the spreading ridge. We also extended the regional magnitude scales for the northern Atlantic region for OBSs by computing site correction terms. Besides location and magnitude improvement, the OBS network was able to detect hundreds of earthquakes, mostly with magnitude below Mw=3, including a swarm activity at the Molloy Deep. Our offshore observations provide further evidence of a low velocity anomaly offshore Svalbard, at the northern tip of Knipovich ridge, that was previously seen in full waveform inversion. We conclude that even a single permanent OBS near the ridge would make a significant difference to earthquake catalogs and their interpretation.

An intermittent detachment faulting system with a large sulfide deposit revealed by multi-scale magnetic surveys

Magmatic and tectonic processes can contribute to discontinuous crustal accretion and play an important role in hydrothermal circulation at ultraslow-spreading ridges, however, it is difficult to accurately describe the processes without an age framework to constrain crustal evolution. Here we report on a multi-scale magnetic survey that provides constraints on the fine-scale evolution of a detachment faulting system that hosts hydrothermal activity at 49.7°E on the Southwest Indian Ridge. Reconstruction of the multi-stage detachment faulting history shows a previous episode of detachment faulting took place 0.76~1.48 My BP, while the present fault has been active for the past ~0.33 My and is just in the prime of life. This fault sustains hydrothermal circulation that has the potential for developing a large sulfide deposit. High resolution multiscale magnetics allows us to constrain the relative balance between periods of detachment faulting and magmatism to better describe accretionary processes on an ultraslow spreading ridge.