The increasing global demand for critical minerals to support energy and technological advancement has accelerated exploration and research efforts for these essential resources. Since 2019, the United States Geological Survey (USGS) Earth Mapping Resources Initiative (EMRI) has worked to modernize geologic mapping of the Nation to better understand its critical mineral resources. To further this initiative, the USGS has flown a series of high-resolution airborne magnetic and radiometric surveys over large areas of the southern Midcontinent. The surveys cover known critical mineral deposits and areas with the potential to host additional critical minerals based on the presence of one or more overlapping mineral systems. One aspect of EMRI emphasizes close collaboration between the USGS and the Association of American State Geologists, as well as other government and industry partners to leverage geophysical, geological, and geochemical expertise on both regional and local scales. The EMRI high-resolution airborne survey data provide new insights into the geophysical framework of the southern Midcontinent and its critical mineral endowment. Additionally, discoveries made from the data have directed new studies for critical mineral exploration.

The inversion of airborne gamma-ray spectrometry (AGRS) data has been studied and demonstrated to be a good alternative to the standard processing approaches. This study explores an inversion-based processing of radiometric data collected as part of an airborne geophysical survey over the Hicks Dome, IL area. Hicks Dome is located within the Illinois-Kentucky Fluorspar District (IKFD). The IKFD includes a large, mineralized area of carbonate-hosted fluorite deposits classified as Mississippi Valley-type (MVT) mineralization as well as fluorite that is associated with Permian age alkalic rocks. Hicks Dome, which is an igneous intrusive complex, has potential for several critical minerals including fluorite ± rare earth elements (REEs), barite, titanium, niobium, and beryllium. The REEs occur primarily in phosphate minerals including fluorapatite, monazite and xenotime. Other REE-bearing fluorcarbonate minerals of the bastnaesite group including parasite and/or synchysite are present in Hicks Dome samples. Accessory minerals like monazite typically contain small amounts of thorium, which can be measured by the AGRS surveys. Thorium can then be used as a proxy to map the potential for REEs in the right geologic environments. The investigation of radioelement concentrations from the high-resolution airborne radiometric survey has yielded insightful results. For example, the relative concentration of equivalent thorium (eTh) to potassium (K) is highlighted by the eTh^2/K ratio and identifies rocks and soils with low potassium levels. These findings not only elucidate eTh anomalies associated with the Hicks Dome complex but also underscore the importance of considering elemental interrelationships for a comprehensive understanding of the geological landscape. Noteworthy anomalies in uranium and thorium, both near the dome and to the north, provide valuable insights into the geological intricacies and mineralogical variations within the surveyed area. This study summarizes the AGRS thorium map as a good mapping tool for setting the stage for further exploration and a deeper understanding of the geological nuances and potential REE mineralization in the Hicks Dome vicinity.

Extensional faults are key components of foreland basin systems. They form within the upper crust in response to flexure of the lithosphere and accommodate subsidence within the foredeep and forebulge depozones. Such faults are excellent proxies for orogenic system evolution and control the distribution of natural resources and hazards. However, the spatiotemporal evolution of flexural extension has not been documented previously at a regional scale, thereby limiting our understanding of underlying geodynamic controls. Here, we resolve late Paleozoic flexural extension in the northern Arkoma basin and southern Ozark dome, USA. We synthesize a large database of previous mapping, existing research, subsurface data, and geophysical data into 3D geologic and 2D kinematic models. Mesh surfaces representing several key horizons from the Carboniferous Period (ca. 335-306 Ma) were constructed. These surfaces were built from oil and gas well tops (n = -10,000) and surface geologic map contacts using an advanced kriging method. The mesh surfaces are offset by a complex 3D fault network, allowing detailed analysis of along-strike and down-dip variations in fault displacement. Analysis of the 3D model reveals a regular and repeated fault segmentation pattern wherein E-W striking, left- and forelandstepping en echelon normal faults are segmented by inherited NE striking basement faults. Maximum vertical separation along the E-W normal faults is generally focused between the inherited NE-trending faults. This suggests that the inherited basement faults delocalized extensional strain during late Paleozoic normal faulting. Maximum vertical separation and fault localization may correlate to areas with high-amplitude positive magnetic anomalies interpreted as Mesoproterozoic granitic rocks. Speculative covariance of magnetic anomalies and fault displacements implies that the relatively strong basement granite concentrated stress, leading to localized faulting within the relatively thin sedimentary cover. Lastly, we show that flexural extension migrated southeast to northwest from the Chesterian-Morrowan (ca. 335-319 Ma) to the Desmoinesian (ca. 306 Ma). The migratory flexural extension may be explained by diachronous loading during Pangean assembly, or by synchronous loading but variable load compensation due to inherent factors.

The U.S. Geological Survey (USGS) has established robust collaborations with domestic state and international geological surveys to provide geophysical and other types of earth science data that act to underpin critical mineral research efforts across the United States, Canada, and Australia. The Earth Mapping Resource Initiative (EMRI) is a national-scale collaborative effort with state geological surveys to improve geophysical and geological data to advance our understanding of the United States' critical mineral endowment. The Critical Mineral Mapping Initiative (CMMI) is a tri-national collaboration with the federal geological surveys of Canada and Australia to conduct research that will aid in identifying new areas with potential for critical mineral deposits across all three countries. This study describes the important interplay between the EMRI and CMMI and how each act in a complementary fashion to advance critical mineral research. We present examples that illustrate how magnetic anomaly data are used to define critical mineral prospectivity for Mississippi Valley-type (MVT) Zn-Pb mineral systems and illustrate how CMMI magnetic derivative maps were considered into USGS' EMRI efforts to acquire modern high-resolution airborne geophysical data over a large area within the US Midcontinent.

The southern Midcontinent of the United States is the nexus for several modern airborne geophysical surveys that cover more than a dozen known or prospective critical mineral systems. The effort to upgrade antiquated airborne geophysical data to modern standards began in 2014 over a portion of the Missouri iron metallogenic province and continues today as part of the congressionally mandated Earth Mapping Resource Initiative (Earth MRI). Airborne data for eight adjoining magnetic and radiometric surveys have been collected or are in progress. Once complete, the surveys will form the largest contiguous coverage of high-resolution airborne magnetic and radiometric data in the US to date. These data will form the backbone of the geophysical contribution to mapping critical mineral systems. Collaboration with the State Geological Surveys of Arkansas, Missouri, Illinois, and Kentucky as part of the Earth MRI have identified 15 focus areas that either host or are prospective for critical mineral deposits. The surveys cover areas where 16 critical minerals have been identified including Aluminum (Al), Antimony (Sb), Barite (BaSO4), Cobalt (Co), Fluorite (F), Gallium (Ga), Lithium (Li), Manganese (Mn), Niobium (Nb), Rare Earth Elements (REEs), Strontium (Sr), Tantalum (Ta), Thorium (Th), Titanium (Ti), Vanadium (V) and Zinc (Zn). Some of these commodities are being actively mined, while others have a history of production or have been identified via exploration and research. This presentation will highlight the state of the modern airborne surveys along with a complementary effort to map prospectivity for Mississippi Valley Type Pb-Zn mineralization that incorporates several different geophysical data layers. The prospectivity effort is part of the tri-national Critical Mineral Mapping Initiative (CMMI), a joint effort between the national geoscience surveys of the United States (USGS), Canada (Canadian Geological Survey), and Australia (Geoscience Australia).

The North Australian Zinc Belt is the largest zinc-lead province in the world, containing three of the ten largest known individual deposits (HYC, Hilton-George Fisher, and Mount Isa). The Northern Cordillera in North America is the second largest zinc-lead province, containing a further two of the world’s top ten deposits (Red Dog and Howards Pass). Despite this world-class endowment, exploration in both mineral provinces during the past 2 decades has not been particularly successful, yielding only two significant discoveries (Teena, Australia, and Boundary, Canada). One of the most important aspects of exploration is to choose mineral provinces and districts within geological belts that have the greatest potential for discovery. Here, we present results from these two zinc belts that highlight previously unused datasets for area selection and targeting. Lead isotope mapping using analyses of mineralized material has identified gradients in μ (238U/204Pb) that coincide closely with many major deposits. Locations of these deposits also coincide with a gradient in the depth of the lithosphere-asthenosphere boundary determined from calibrated surface wave tomography models converted to temperature. Furthermore, gradients in upward-continued gravity anomalies and a step in Moho depth correspond to a pre-existing major crustal boundary in both zinc belts. A spatial association of deposits with a linear mid- to lower-crustal resistivity anomaly from magnetotelluric data is also observed in the North Australian Zinc Belt. The change from thicker to thinner lithosphere is interpreted to localize prospective basins for zinc-lead mineralization and to control the gradient in lead isotope and geophysical data. These data, when combined with data indicative of paleoenvironment and changes in plate motion at the time of mineralization, provide new exploration criteria that can be used to identify prospective mineralized basins and define the most favorable parts of these basins.

Demand for critical raw materials is expected to accelerate over the next few decades due to continued population growth and the shifting consumption patterns of the global economy. Sedimentary basins are important sources for critical raw materials and new discoveries of sediment-hosted Mississippi Valley-type (MVT) and/or clastic-dominated (CD) Zn-Pb deposits are likely required to mitigate future supply chain disruptions for Zn, Pb, Ag, Cd, Ga, Ge, Sb, and In. Herein we integrate public geoscience datasets using a discrete global grid to system to model the mineral potential for MVT and CD deposits across Canada, the United States of America, and Australia. Statistical analysis of the model results demonstrates that surface-wave tomography and derivative products from satellite gravity datasets can be used to map the most favourable paleo-tectonic settings of MVT and CD deposits inboard of orogenic belts and at the rifted edges of cratonic lithosphere, respectively. Basin development at pre-existing crustal boundaries was likely important for maintaining the low geothermal-gradients that are favourable for metal transport and generating the crustal fluid pathways that were reactivated during ore-formation, as suggested by the statistical association of both sediment-hosted mineral deposit types with the edges of upward-continued gravity and long-wavelength magnetic anomalies. Multivariate statistical analysis demonstrates that the most prospective combination of these geophysical datasets varies for each geological region and deposit type. We further demonstrate that maximum and minimum geological ages, coupled with Phanerozoic paleogeographic reconstructions, represent mappable proxies for the availability of oxidized, brine-generating regions that are the most likely source of ore-forming fluids (e.g., low-to mid-latitude carbonate platforms and evaporites). Ore deposition was likely controlled by interaction between oxidized, low-temperature brines and sulfidic and/or carbonaceous rocks, which, in some cases, can be mapped at the exposed surface or identified using the available rock descriptions. Baseline weights-of-evidence models are based on regional geophysics and are the least impacted by missing surface information but yield relatively poor results, as demonstrated by the low area-under-the-curve (AUC) for the spatially independent test set on the success-rate plot (AUC = 0.787 for MVT and AUC = 0.870 for CD). Model performance can be improved by: (1) using advanced methods that were trained and validated during a series of semi-automated machine learning competitions; and/or (2) incorporating geological and geophysical datasets that are proxies for each component of the mineral system. The best-performing gradient boosting machine models yield higher AUC for the test set (AUC = 0.983 for MVT and AUC = 0.991 for CD) and reduce the search space by >94%. The model results highlight the potential benefits of mapping sediment-hosted mineral systems at continental scale to improve mineral exploration targeting for critical raw materials.

First posted July 14, 2022 For additional information, contact: Mineral Resources Program CoordinatorMineral Resources ProgramU.S. Geological Survey913 National CenterReston, VA 20192 The Earth Mapping Resources Initiative (Earth MRI) is conducted in phases to identify areas for acquiring new geologic framework data to identify potential domestic resources of the 35 mineral materials designated as critical minerals for the United States. This report describes the data sources and summary results for 13 critical minerals evaluated in the conterminous United States and Puerto Rico during phase 3 of the study (antimony, barite, beryllium, chromium, fluorspar, hafnium, helium, magnesium, manganese, potash, uranium, vanadium, and zirconium). Phases 1 and 2 of the Earth MRI addressed aluminum, cobalt, graphite, lithium, niobium, platinum-group elements (PGEs), rare earth elements (REEs), tantalum, tin, titanium, and tungsten. Critical minerals in Alaska are covered in a separate report. No focus areas for phase 3 critical minerals are delineated for Hawaii.The geologic, geochemical, topographic, and geophysical mapping provided by the Earth MRI documents geologic features that reflect the extent of individual mineral systems and provides information about critical mineral deposits that may not have been previously considered. The mineral-systems approach links critical mineral commodities to deposit types that represent the manifestations of large mineral systems.Each of the 13 critical mineral commodities for phase 3 of the Earth MRI is discussed in terms of its importance to the Nation’s economy, modes of occurrence, mineral systems, and deposit types, and is accompanied by maps and tables listing examples of focus areas in the conterminous United States and Puerto Rico. Examples of important mineral systems for this group of 13 critical minerals include basin brine path systems for barite and fluorspar, Carlin-type systems and Coeur d’Alene systems for antimony, chemical weathering and volcanogenic seafloor systems for manganese, Climax-type systems for beryllium, mafic magmatic systems for chromium, marine evaporite systems for potash and magnesium, meteoric recharge systems for uranium, petroleum systems for helium, and placer systems for zirconium and hafnium.

First posted September 18, 2020 For additional information, contact: Coordinator, Mineral Resources ProgramU.S. Geological Survey913 National CenterReston, VA 20192 In response to a need for information on potential domestic sources of critical minerals, the Earth Mapping Resources Initiative (Earth MRI) was established to identify and prioritize areas for acquisition of new geologic mapping, geophysical data, and elevation data to improve our knowledge of the geologic framework of the United States. Phase 1 of Earth MRI concentrated on those geologic terranes favorable for hosting the rare earth elements (REEs). Phase 2 continued to address the REEs and also identified focus areas for potential domestic sources of 10 more of the 35 critical minerals on the U.S. critical minerals list (aluminum, cobalt, graphite, lithium, niobium, platinum-group elements, tantalum, tin, titanium, tungsten). This report describes the methodology, data sources, and summary results for mineral systems that host these 11 critical minerals in the conterminous United States, Hawaii, and Puerto Rico; Alaska is covered in a separate report. The mineral systems framework adopted for this study links critical mineral commodities to families of genetically related mineral deposit types. The mineral systems approach is an efficient approach, providing a simultaneous evaluation of geologic terranes through aggregation of genetically related mineral deposit types that are much larger than individual ore deposits. Geologic, geochemical, topographic, and geophysical mapping provided by Earth MRI will document geologic features that reflect the extent of individual mineral systems and provide information about critical mineral deposits that may not have been recognized previously.Each critical mineral commodity is discussed in terms of importance to the Nation's economy, modes of occurrence, mineral systems, and deposit types along with maps and tables listing examples of focus areas for each critical mineral. Important mineral systems for these critical minerals include chemical weathering systems for aluminum (bauxite); placer systems for titanium and REEs; metamorphic systems for graphite; mafic magmatic systems for platinum-group elements and cobalt; lacustrine evaporite and porphyry tin systems for lithium; and copper-molybdenum-gold (Cu-Mo-Au) systems for tungsten. REEs occur in many different mineral systems. Focus areas were developed by scientists from the U.S. Geological Survey in collaboration with scientists from State geological surveys and other institutions. This first national-scale compilation of focus areas represents an initial step in addressing the Nation's critical mineral needs by screening areas for acquisition of new data to provide the geologic framework necessary for identifying domestic sources of critical minerals.

Several types of critical mineral-bearing ore deposits in the southern Midcontinent region of the U.S. are hosted in Mesoproterozoic igneous rocks largely concealed beneath Paleozoic cover. Discerning the architecture of igneous intrusions and volcanic centers in the crust is fundamental to understanding the geologic evolution of this vast region and its mineral resources. To advance the understanding of the geologic framework beneath the southeast Missouri iron metallogenic province, we invert continental-scale magnetic and gravity anomaly data to three-dimensional (3D) physical property models. The regional models image altered and mineralized igneous rocks near the Proterozoic basement surface and underlying intrusive complexes that extend down to the Moho. At shallow crustal levels, our models confirm that iron oxide-apatite +/- rare earth element (IOA +/- REE) deposits and iron oxide-copper-gold +/- cobalt (IOCG) deposits occur within or near the edges of large low density/low susceptibility early Mesoproterozoic (ca 1.4 Ga) silicic calderas and (ca 1.3 Ga) granitic plutons. Previous isotopic and geochemical studies conclude that the iron deposits and their volcanic host rocks originated from mantle-derived and crustal melts that erupted during regional extension. Extension was associated with thermal event(s) that produced the large-scale silicic magmatism related to the ca 1.45 Ga Eastern Granite Rhyolite Province (EGRP) and the 1.35 Ga Southern Granite Rhyolite Province (SGRP). We postulate that early in the evolution of the EGRP, several trans-crustal magmatic plumbing systems developed that are evident in the 3D models. The southeast Missouri metallogenic province is underlain by one such magmatic system that is expressed as a northwest-trending similar to 50 km-wide by 200 km-long elongate track of high susceptibility at deep crustal levels. The high susceptibility corridor splays upward through the crust to the Proterozoic surface where the iron deposits are the epigenetic manifestation of this magmatic event. Our findings confirm that the iron deposits, with no distinct connection at the surface, are connected to one large magmatic system at depth. We propose that other similar susceptibility tracks, which are present along the top of the mantle, mark additional feeder zones that allowed magma to ascend to the main eruptive centers that produced the Granite Rhyolite Provinces. The early Mesoproterozoic extensional tectonic framework established crustal-scale pathways that controlled the distribution of subsequent magmatic activity, including the ca 1.4 Ga calderas and underlying intrusions, ca 1.3 Ga silicic plutons and Phanerozoic alkaline intrusions. If these interpretations are correct, our study has identified large areas that are prospective for critical mineral-bearing ore deposits and, importantly, suggests that the Mesoproterozoic architecture may have influenced subsequent magmatism and hydrothermal activity in the southern Midcontinent of the U.S.

3D inversion models of magnetic, gravity, and magnetotelluric (MT) data reveal evidence for concealed critical metal mineral systems at regional scales in the US southern Midcontinent. The region contains several igneous-related critical-element-bearing ore deposits of different types and ages that have been sourced from ore-forming fluids originating from the upper mantle to shallow crustal levels. These include the Mesoproterozoic IOCG-Co and IOA-REE deposits, which are part of the southeast Missouri iron metallogenic province. A depth slice along the top of the mantle from the 3D models reveals feeder zones that allowed development of large ca 1.4 Ga magmatic systems that host mineralization in the upper crust. Shallow crust to upper mantle conductivity sources dip under the iron province and may reflect a potential sulfur source for IOCG mineralization. Our findings indicate that the iron deposits show an unambiguous connection at depth by means of an inferred magmatic system that is traceable to the mantle and that different IOA, IOA REE and IOCG deposits may be located in distinct parts of the same magmatic system. Several trans-crustal to trans-lithospheric magmatic plumbing systems are revealed and show how the Mesoproterozoic architecture may have influenced subsequent carbonatite and peralkaline igneous activity.

Results from three-dimensional (3-D) density and magnetic susceptibility models provide tectonic-scale geophysical constraints on crustal architecture underlying the Mesoproterozoic iron province in southeast Missouri, USA. The iron province contains one iron oxide-copper gold (IOCG) and several iron oxide-apatite (IOA) +/- rare earth element (IOA +/- REE) deposits within 1.48 to 1.44 Ga rhyolites and intermediate to mafic composition igneous rocks of the mostly concealed St. Francois Mountains terrane. The geophysical models cover over a 900 x 900 km area; susceptibility distributions are calculated within a volume defined by the Cambrian erosion surface and the Moho. Density distributions are constrained by these surfaces, but extend from sea level to 50 km depth. The models show that the iron deposits occur along the edges or in the center of caldera structures underlain by partly coincident dense and magnetic domains interpreted as intermediate to mafic composition intrusions in the middle and lower crust. Several such calderas are associated with known iron deposits, whereas others highlight the potential for new areas of IOCG and IOA REE mineralization. A major crustal boundary, demarcated by Nd isotopic data, coincides with abrupt density and susceptibility boundaries that extend to the mantle.

Many discoveries of large deposits in the United States, including the Carlin, Mountain Pass, Henderson, Red Dog, and Eagle deposits, were based in part on U.S. Geological Survey (USGS) geologic, geochemical, or geophysical data. The Mineral Resources Program contributes to exploration success by (1) providing geologic, geochemical, and geophysical data; (2) developing mineral deposit models that target permissive areas; and (3) developing techniques that are used by industry in exploration. Basic geoscience data, particularly USGS geological maps, form the foundation for mineral exploration. Development and refinement of deposit models impact how exploration is focused. Two models that significantly changed based on USGS research include orogenic gold and sediment-hosted Pb-Zn. The new data from these models have changed exploration strategies. Indirect contributions by the USGS include development of techniques, such as the initiation of fluid inclusion research in the 1950s, and the development of geochemical and geophysical techniques that play a major role in exploration programs. The USGS also has contributed directly to exploration through collaborative studies with industry in well-known districts in the United States.

The Pea Ridge deposit is in a 1.48-1.44 Ga volcano-plutonic terrane consisting of rift-related, depleted mantle-derived, tholeiitic, mostly concealed basalt to andesite and voluminous subalkaline ferroan dacite to rhyolite composition igneous rocks. Eruption of the rhyolite host at similar to 1473 Ma was followed by faulting and formation of the magnetite-apatite deposit (210 Mt, 47-55% Fe) at similar to 1471 Ma. Th-rich REE breccia pipes (0.2 Mt, 12% REE oxides) formed at similar to 1465 Ma. Nd, Pb, and He in IOA and REE mineralization were derived from a mantle source. Ore and gangue minerals precipitated from fluids containing magmatic H, O, C, S, Fe, Cl and Br. The IOA deposit formed at similar to 1.5 km depth under lithostatic to hydrostatic conditions at similar to 350-750 degrees C by cooling and decompression of ascending fluids enriched in Fe, Mg, Ca, P and REE that were derived from an intermediate composition intrusion. REE breccia pipes formed under vaporstatic conditions from similar to 400-200 degrees C hypersaline brine derived from a fractionated alkalic (?) intrusion that dissolved and replaced apatite with monazite, xenotime, and thorite. The results show that IOA deposits and REE breccia pipes can be produced by the discharge of fluids from successive intrusions in an underlying igneous complex.

A statistical modeling approach was applied to airborne geophysical data to locate areas favorable for gold mineralization in Rio das Velhas Greenstone Belt, Quadrilatero Ferrifero (QF). A positive spatial coincidence exists between banded iron formation (BIF) host-rocks, structures, and mineralogy that are indicative of gold mineralization. Quantitative relationships between airborne geophysical and geological data with known gold occurrences were determined and combined to produce predictive models to explore for Archean oxide-BIF-hosted deposits. Results map areas within exposed and subsurface rocks favorable for gold mineralization. Selected target areas for gold mineralization were checked in the field and provide validation that encourages future ground follow-up in the Rio das Velhas Greenstone Belt. The approach used in this study has shown great capability for linking geological and geophysical data and illustrates an approach that may significantly increase the odds of exploration success in the area.