Crustal architecture of a metallogenic belt and ophiolite belt: implications for mineral genesis and emplacement from 3-D electrical resistivity models (Bayankhongor area, Mongolia)

Abstract

Crustal architecture strongly influences the development and emplacement of mineral zones. In this study, we image the crustal structure beneath a metallogenic belt and its surroundings in the Bayankhongor area of central Mongolia. In this region, an ophiolite belt marks the location of an ancient suture zone, which is presently associated with a reactivated fault system. Nearby, metamorphic and volcanic belts host important mineralization zones and constitute a significant metallogenic belt that includes sources of copper and gold. However, the crustal structure of these features, and their relationships, are poorly studied. We analyze magnetotelluric data acquired across this region and generate three-dimensional electrical resistivity models of the crustal structure, which is found to be locally highly heterogeneous. Because the upper crust (< 25 km) is found to be generally highly resistive (> 1000 Ωm), low-resistivity (< 50 Ωm) features are conspicuous. Anomalous low-resistivity zones are congruent with the suture zone, and ophiolite belt, which is revealed to be a major crustal-scale feature. Furthermore, broadening low-resistivity zones located down-dip from the suture zone suggest that the narrow deformation zone observed at the surface transforms to a wide area in the deeper crust. Other low-resistivity anomalies are spatially associated with the surface expressions of known mineralization zones; thus, their links to deeper crustal structures are imaged. Considering the available evidence, we determine that, in both cases, the low resistivity can be explained by hydrothermal alteration along fossil fluid pathways. This illustrates the pivotal role that crustal fluids play in diverse geological processes, and highlights their inherent link in a unified system, which has implications for models of mineral genesis and emplacement. The results demonstrate that the crustal architecture—including the major crustal boundary—acts as a first‐order control on the location of the metallogenic belt.

The Bayankhongor Metal Belt (Mongolia): Constraints on Crustal Architecture and Implications for Mineral Emplacement from 3-D Electrical Resistivity Models

“…The electrical resistivity models show that some low-resistivity anomalies align with the suture zone, and that they extend to great depths. Thus the results show that the ancient suture zone, marked by the Bayankhongor Ophiolite Belt and associated with a paleo-ocean closure, is a deep-reaching structure (crustal or lithospheric scale) and a major crustal boundary [19]. The low-resistivity anomalies may be explained by hydrothermal alteration along fossil fluid pathways, which the MT method is sensitive enough to detect.…”

“…They were likely formed by fluid flow through the crust during metamorphism and indicate deeply-connected, ore-forming fluid pathways [3]. The observed low-resistivity signature may be explained by hydrothermal alteration along fossil Conference Proceedings 4 of 5 fluid pathways and associated sulfide mineralogy within the host complex related to ore emplacement [19].…”

“…The MT data were inverted with the MODEM inversion algorithm [17,18]. For details of the inversion procedure and model testing please refer to [19].…”

Investigating the Whole‐lithosphere Structure of a Mineral System — Pathways and Source of Ore‐forming Fluids Imaged with Magnetotelluric Modeling

“…Considering the available evidence, we favor a model that hypothesizes: 1) fluids were generated by devolatilization within a subducting slab during paleoocean closure, prior to the suturing of micro-continental blocks; 2) fluids migrated upwards along the plate boundary to the mantle wedge and hydrated the lithosphere; 3) later, enriched ore-forming fluids moved to the upper crust along pre-existing weaknesses and pathways, including crustal-scale suture zones between micro-continental blocks and shallow thrusts faults (see Goldfarb and Groves, 2015;Comeau et al, 2021).…”

“…We analyze data from magnetotelluric (MT) measurements across southern Mongolia (data described in Becken et al, 2021a, b) and explore three dimensional (3-D) models of the electrical resistivity structure throughout the whole lithosphere, from the upper crust to the asthenosphere (see Comeau et al, 2021; see also Kä ufl et al, 2020). This region has notable occurrences of gold and copper mineralization over an extended area and is located near the edge of a micro-continental block.…”