Abstract Earth’s mineral deposits show a non-uniform spatial distribution from the craton-scale, to the scale of individual mineral districts. Although this pattern of differential metal endowment is underpinned by lithospheric-scale processes the geological features that cause clustering of deposits remains enigmatic. The integration of geological and geophysical (seismic, gravity, and magnetotelluric) features has produced the first whole-of-crust image through an iconic Neoarchean volcanic complex and mineral district in the Abitibi Greenstone Belt, Superior Province, Canada. Observations indicate an asymmetry in surface geology, structure, and crustal architecture that defines deep transcrustal magmatic-hydrothermal upflow zones and the limits of the Noranda District ore system. Here, extreme volcanogenic massive sulfide (VMS) endowment is confined to a smaller area adjacent to an ancestral transcrustal structure interpreted to have localized and optimized magmatic and ore forming processes. Although lithospheric-scale evolutionary processes might act as the fundamental control on metal endowment, the new crustal reconstruction explains the clustering of deposits on both belt and district scales. The results highlight a strong magmatic control on metal and in particular Au endowment in VMS systems. Overprinting by clusters of ca. 30 Ma younger orogenic Au deposits suggest the ore systems accessed an upper lithospheric mantle enriched in Au and metals.
The subduction interface beneath the northern part of the Hikurangi subduction margin on the east coast of New Zealand's North Island is exceptionally shallow and cool compared with other margins where slow slip has been observed. Here we use magnetotelluric data to show that a marked decrease in the conductivity of the fore‐arc sediments coincides with the onset of seismicity at ∼10 km depth. Below the sediments, a dipping band of seismicity and intermediate conductivity at the subduction interface connects to a deeper more conductive zone above the down‐going plate. This deeper conductive zone is interpreted to be under‐plated sediments. These results and results from previous seismic tomography in the area suggest that the intermediate resistivity zone represents a region of upward fluid transport near the plate interface followed by fluid escape into the upper‐plate.
Erebus volcano, Antarctica, with its persistent phonolite lava lake, is a classic example of an evolved, CO2-rich rift volcano. Seismic studies provide limited images of the magmatic system. Here we show using magnetotelluric data that a steep, melt-related conduit of low electrical resistivity originating in the upper mantle undergoes pronounced lateral re-orientation in the deep crust before reaching shallower magmatic storage and the summit lava lake. The lateral turn represents a structural fault-valve controlling episodic flow of magma and CO2 vapour, which replenish and heat the high level phonolite differentiation zone. This magmatic valve lies within an inferred, east-west structural trend forming part of an accommodation zone across the southern termination of the Terror Rift, providing a dilatant magma pathway. Unlike H2O-rich subduction arc volcanoes, CO2-dominated Erebus geophysically shows continuous magmatic structure to shallow crustal depths of < 1 km, as the melt does not experience decompression-related volatile supersaturation and viscous stalling.
Abstract. The geometry of ancient faults at depth can only be mapped by high-resolution geophysical surveys such as seismic reflection profiling. Recent deep (35–48 km) reflection profiles acquired across the Archean southern Superior craton of North America provided such data with which to map in 3-D some major shear zones, many of which are associated with significant orogenic gold or VMS deposits. Most faults are (re)interpreted as thrusts; a few appear as sub-vertically aligned breaks in prominent reflectors. Sub-vertical faults possibly originated as syn-volcanic transform faults. Thrusting probably relates to the dominant phase of folding and horizontal shortening strain that occurred during the regional crustal deformation, mineralization and peak metamorphism at 2.72–2.66 Ga, associated with the Kenoran orogeny. Most deformation after this orogenic event resulted in modest lateral movement. Coincident magnetotelluric (MT) surveys indicate pervasive conductive minerals such as graphite/carbon and sulfides, exist within the mid-crust and in near-vertical channels within the more brittle and resistive upper (greenstone) crust. Many such channels, but not all, coincide with fault zones and mineral deposits. Palinspastic and paleomagnetic-based reconstructions suggest many faults had multiple periods of activity with changing vertical to horizontal offsets. Some faults appear paired, partitioning normal and oblique strains on vertical shear zones and dipping thrust zones, respectively.
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