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Mineral systems can be thought of as a combination of several critical elements, including the whole-lithosphere architecture, favorable geodynamic/tectonic events, and fertility. Because they are driven by processes across various scales, exploration benefits from a scale-integrated approach. There are open questions regarding the source of ore-forming fluids, the depth of genesis, and their transportation through the upper crust to discrete emplacement locations. In this study, we investigate an Au-Cu metal belt located at the margin of an Archean-Paleoproterozoic microcontinent. We explore the geophysical signatures by analyzing three-dimensional models of the electrical resistivity and shear-wave velocity throughout the lithosphere. Directly beneath the metal belt, narrow, vertical, finger-like low-resistivity features are imaged within the resistive upper-middle crust and are connected to a large low-resistivity zone in the lower crust. A broad low-resistivity zone is imaged in the lithospheric mantle, which is well aligned with a zone of low shear-wave velocity, examined with a correlation analysis. In the upper-middle crust, the resistivity signatures give evidence for ancient pathways of fluids, constrained by a structure along a tectonic boundary. In the lower lithosphere, the resistivity and velocity signatures are interpreted to represent a fossil fluid source region. We propose that these signatures were caused by a combination of factors related to refertilization and metasomatism of the lithospheric mantle by long-lived subduction at the craton margin, possibly including iron enrichment, F-rich phlogopite, and metallic sulfides. The whole-lithosphere architecture controls the genesis, evolution, and transport of ore-forming fluids and thus the development of the mineral system., Geochemistry, Geophysics, Geosystems, 23 (8), ISSN:1525-2027

Intraplate processes, such as continental surface uplift and intraplate volcanism, are enigmatic and the underlying mechanisms responsible are not fully understood. Central Mongolia is an ideal natural laboratory for studying such processes because of its location in the continental interior far from tectonic plate boundaries, its high-elevation plateau, and its widespread, low-volume, basaltic volcanism. The processes responsible for developing this region remain largely unexplained — due in part to a lack of high-resolution geophysical studies — and thus are open questions. A recent project undertaken to map the crust and upper mantle structure of central Mongolia has collected a large magnetotelluric array (~700 km x ~450 km).

Additional file 1: Figure S1. Varying the starting model for model L4000. Panels (a), (b), (c), and (d) show the inversion model results when using an initial model that is a halfspace of 10, 100, 300, and 1,000 Ωm, respectively. Panels (e) and (f) use layered initial models, based on averaging the 2-D model of Comeau et al. (2018a) and from averaging 1-D models below each site, respectively. The model in (b) is the preferred model; it fits the data best. Note these models use a vertical increase factor of 1.2, and a horizontal covariance parameter of 0.5. Boxes highlight main features. Details for each model are (starting model RMS misfit / number of iterations / final RMS misfit): (a) 12.25 / 96 / 2.186, (b) 17.22 / 120 / 1.588, (c) 27.75 / 96 / 1.936, (d) 51.98 / 86 / 2.008, (e) 33.16 / 94 / 2.031, (f) 19.54 / 88 / 1.824. Figure S2. Varying the horizontal covariance parameter for model L4000. Panels (a), (b), (c), (d), and (e) show the inversion model results for a horizontal covariance parameter of 0.3, 0.4, 0.5, 0.6, and 0.7, respectively. Large values smooth the model. Note the effect of the covariance is over the number of cells rather than the physical cell size, see Robertson et al. (2020). The model in (c) is the preferred model; it fits the data best. Note these models use a vertical increase factor of 1.2 (38 vertical cells), as well as a starting model of a 100 Ωm halfspace. The vertical covariance parameter was fixed, as in (a). Details for each model are as follows (starting model RMS misfit / number of iterations / final model RMS misfit): (a) 17.22 / 75 / 2.247, (b) 17.22 / 105 / 1.968, (c) 17.22 / 120 / 1.588, (d) 17.22 / 114 / 1.903, (e) 17.22 / 93 / 2.498. Figure S3. Varying the data included for model L4000. The inversion model results shown in panel (a) include full impedance (off-diagonal and diagonal components) and tipper data, (b) full impedance data (off-diagonal and diagonal components), and (c) the off-diagonal components (Zxy, Zyx) of the impedance only. Note these models use a vertical increase factor of 1.2 (38 vertical cells), a starting model of a 100 Ωm halfspace, as well as a horizontal covariance parameter of 0.5. It is clear that the inclusion of the diagonal components influences the model and modifies the shape and strength of some structures. However, the locations of the main model features (boxes), and their interpretations, do not change. Details for each model are as follows (starting model RMS misfit / number of iterations / final model RMS misfit): (a) 17.22 / 120 / 1.588, (b) 18.45 / 95 / 1.757, (c) 21.92 / 78 / 1.227. Figure S4. Comparing the fit of six sites for model L4000 (apparent resistivity, ρapp, and phase, φ). Panels (a), (b), (c), (d), (e), and (f) correspond to sites 4343, 4335, 4300, 4295, 4290, and 4285 from the model (Figure S3c) that includes only off-diagonal components (column i) and the model (Figure S3b) that includes both off-diagonal and diagonal components (column ii). The corresponding diagonal components are shown in panels (g), (h), (i), (j), (k), and (l). The sites chosen are those above features of interest that appear to change between the two models (i.e., the fault/suture zone and the mineral belt). In both cases the fit of the model to the data is good. It is clear that the inclusion of the diagonal components influences the fit of the off-diagonal components. Figure S5. Error-normalized residuals of eight sites for model L4000. The residuals were computed as the absolute difference of the modelled data from the measured data for both apparent resistivity (top part) and phase (bottom part). Panels (a), (b), (c), (d), (e), (f), (g), and (h) correspond to sites 4377, 4343, 4335, 4300, 4295, 4290, 4285, and 4257 from the model (Figure S3c) that includes only off-diagonal components (column i) and the model (Figure S3b) that includes both off-diagonal and diagonal components (column ii). Note that sites 4377 and 4257 are at the ends of the model, not above the features of interest. The gray region indicates values

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 ( 1000 Ωm), low-resistivity (, Earth, Planets and Space, 73 (1), ISSN:1343-8832, ISSN:1880-5981

Earth and Planetary Science Letters, 487, ISSN:0012-821X, ISSN:1385-013X