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6th International Archean Symposium
6th International Archean Symposium

Illuminating transcrustal upflow zones with implications for ore localization and reusage of crustal architecture

Oral

Talk Description

The integration of geological and geophysical (seismic, gravity, and magnetotelluric) datasets along a ~50 km transect through the world-class Noranda District in the Neoarchean Abitibi greenstone belt, Superior Province, Canada, identifies crustal-scale features and architecture that might explain the clustering and relative location of conventional and Au-rich volcanogenic massive sulfide (VMS) deposits. Furthermore, these same crustal-scale features may have played a role in focusing younger orogenic-style Au mineralization. The Abitibi greenstone belt is considered one of the most highly endowed greenstone belts in the world and its VMS endowment has in part been explained by its extensive juvenile crust that would facilitate high level magmatism, high heat flow, and extension-related structures. However, the processes and geological features responsible for the differential metal endowment and occurrence of deposit clusters within the Abitibi greenstone belt are less clear. The Noranda District is host to ~20 VMS, ~19 smaller orogenic Au, and minor synvolcanic intrusion-hosted Cu-Mo ±Au ±Ag mineralization, with a large proportion of the metal budget (17 VMS deposits accounting for 104 Mt of 130 Mt VMS ore) restricted to a smaller area coinciding with a magmatic centre characterized by the greatest combined surface area of synvolcanic plutons and intrusions, felsic volcanic rocks and the highest density of synvolcanic structures. A whole-of-crust integration of seismic, gravity, and magnetotelluric imaging with surface geology shows the subsurface in the endowed area is characterized by a deeper extending low seismic reflectivity zone, the largest low-density volume, and the presence of subvertical low resistivity (≤ 50 Ω·m) pipe-like features that connect to a relatively low resistivity (~50-100 Ω·m) middle-lower crust, itself potentially extending into the upper mantle. The results imply that clustering of ore deposits in the Noranda District was controlled by ancestral transcrustal structures that localized, optimized, and sustained magmatic and ore-forming processes. The largest and most Au-rich VMS deposits are located where three major synvolcanic faults coincide with one of the most prominent subvertical low resistivity corridors at the locus of the magmatic centre and an optimal location for a magmatic contribution of metal, in particularly Au, to the VMS system. The spatial association with overprinting ca. 30 m.y. younger orogenic Au deposits suggest that the primary crustal architecture responsible for focusing VMS deposits may have played a role in localizing later Au mineralization.

Speakers