Iron Oxide-Copper-Cobalt-Gold-Rare Earth Element Deposits of Southeast Missouri—From the Ore Deposit Scale to a Global Deposit Model

Science Center Objects

The project main objectives are to: 1) geologically, characterize the setting and origin of the iron-copper-cobalt-gold-rare earth element deposits, and advance the knowledge of rare earth element and Co potential within iron oxide-copper-gold (IOCG) deposits of southeast Missouri, and 2) geophysically delineate and characterize the subsurface Precambrian geology using existing ground and new (proposed) airborne geophysical data. Identifying new targets for exploration could result in the discovery of new deposits, which if delineated by drilling and determined to be commercially economic to mine, would increase the domestic resource for critical metals such as rare earth elements, and lessen U.S. dependence on foreign sources for these metals.

Science Issue and Relevance

St. Francois area geologic map

Simplified regional geologic map of Mesoproterozoic rocks in St. Francois Mountains terrane, showing distribution of volcanic and intrusive rocks, caldera boundaries, and mineral deposits and prospects. Modified from Kisvarsanyi (1981). From Day and others, 2016, doi:10.2113/econgeo.111.8.1825
(Public domain.)

mineral deposit schematic figure

Schematic figure showing principal characteristics and inferred setting of iron oxide ± apatite (IOA)- and iron oxide-copper-gold (IOCG)-type mineral deposits in Missouri iron metallogenic province (based on available drill core and surface information). From Day and others, 2016, doi:10.2113/econgeo.111.8.1825.
(Public domain.)

The geological framework and origin of the iron-copper-cobalt-gold-rare earth element (IOCG-REE) deposits in southeast Missouri are not well defined. In most areas, the geology surrounding the deposits is uncertain, owing to limited outcrops of the host Precambrian igneous rocks (St. Francois Mountains terrane) and a widespread cover, as much as about 450 meters thick, of Cambrian sedimentary rocks. As a result, the geometry, age, and petrology of buried plutons and subvolcanic intrusions in the St. Francois terrane--and potentially undiscovered metal deposits--are unknown, except where data are available from drill cores.

Identifying new targets for exploration could result in the discovery of new deposits, which if delineated by drilling and determined to be commercially economic to mine,

  • would increase the domestic resource for critical metals such as rare earth elements,
  • and lessen U.S. dependence on foreign sources for these metals.

Methodology to Address Issue

This project has two main objectives:

  1. Geologically, characterize the setting and origin of the iron-copper-cobalt-gold-rare earth element deposits, and advance the knowledge of rare earth element and Co potential within iron oxide-copper-gold (IOCG) deposits of southeast Missouri. An improved understanding of the distribution, age, and origin of these deposits, and their genetically related pluton(s), will provide a valuable database for new industry exploration in the region and future mineral resource assessments.
  2. Geophysically delineate and characterize the subsurface Precambrian geology using existing ground and new (proposed) airborne geophysical data. Develop a petrophysical database that contributes to mapping controls on rocks and structures that host high contents of IOCG-rare earth mineralization.

The geologic and geophysical components will address a regional area that includes known concealed deposits at Pea Ridge, Bourbon, Camel's Hump, Boss Bixby, and Kratz Spring. Depths to these deposits vary from 325 to 415 meters below the topographic surface.

The St. Francois Mountains terrane likely has the highest potential for undiscovered large rare earth element deposits in the conterminous United States. This terrane is geologically analogous to iron-copper-gold-rare earth element-uranium deposit and similar (but smaller) deposits that have been discovered there in recent years. All of these deposits in the Gawler Craton (south Australia) occur within granite and rhyolite, beneath hundreds of meters of flat-lying sedimentary rock, and each was discovered by airborne geophysics (Skirrow et al., 2002). Geological and geophysical techniques used successfully in the Gawler Craton, by the Australian Geological Survey Organisation and the Geological Survey of South Australia, will be evaluated by this project, and where relevant, applied to the St. Francois Mountains terrane.

The impact of this project will be significant in greatly improved understanding of iron-copper-cobalt-gold-rare earth element deposits in southeast Missouri. Data generated by the project will significantly advance the potential for new discoveries in the study area, including likely applications to other buried Mesoproterozoic terranes in the Midcontinent region.

Reference

Skirrow, R.G., Bastrakov, E., Raymond, O.L., Davidson, G., and Heithersay, P., 2002, The geological framework, distribution and controls of Fe-Oxide Copper-Gold mineralisation in the Gawler Craton, South Australia -Part II: Alteration and mineralisation, in Porter, T.M., ed., Hydrothermal iron oxide copper-gold & related deposits: A global perspective: Adelaide, PGC Publishing, v. 2, p 33-47.

Location and geologic map in MO

(A). Map showing location of concealed and exposed Mesoproterozoic rocks in the St. Francois Mountains terrane (after Pratt et al., 1979), iron oxide and other metal deposits and prospects, and locations of new airborne magnetic and gravity gradiometry surveys. (B) Interpreted geology within airborne geophysical survey areas (after Kisvarsanyi, 1981)
(Public domain.)

Geophysical, Spectral, and Petrophysical Characterization: Since the vast majority of the St. Francois Mountain terrane is concealed beneath Paleozoic rocks and Quaternary deposits, delineation of buried structures, rock types, and IOA and IOCG ore systems must rely on potential field (aeromagnetic and gravity) geophysical data. Characterization of the host rocks, ore minerals, and ore-formation mineral assemblages are key to understanding the chemistry of ore forming systems.

We plan to use modern geophysical data processing techniques, spectral imaging, and petrophysical characterization to refine our understanding of the St. Francois Mountain terrane. Our initial efforts were to compile and interpret existing magnetic and ground gravity data, and we determined the existing magnetic and gravity data available were insufficient to distinguish important features at deposit scale. We acquired new magnetic, gravity gradiometry, and radiometric data (2014: magnetic and gravity gradiometry; 2015: magnetic and gamma ray) and used modern geophysical data processing techniques, which provided new insights into our understanding of the geology of the Precambrian basement. We also are examining and spectrally imaging our archive of several hundered, hand, core, and rock samples with CoreScan's new technique. Results will provide a unique library of compositional parameters and mineral maps related to an iron-oxide mineralized terrane in the U.S. Any newly acquired data from exposed parts of the St. Francois Mountains will aid in mapping surface variations in surface element chemistry and surface and subsurface magnetic signatures related to iron-oxide related alteration types. Models using recently developed in-house 3-D software of shallow deposits will be calculated to define the deposits' geometry and depth extent.

Our results will be used in other project activities to help refine the regional geologic framework, to relate the regional geophysical characteristics to new geochronological data of subsurface plutons, volcanic unites, and mineralized rock, and related the detailed mineralogy to the petrophysics (susceptibility and density) and regional geochemistry across the study area.

Precambrian basement map

Map showing Precambrian basement provinces of the United States and location of St. Francois Mountains terrane in southeast Missouri. Neodymium (Nd) line separates basement rocks with Nd model ages greater than (northwest side) and less than (southeast side) 1.6 Ga. Modified from Bickford et al. (2015).
​​​​​​​(Public domain.)

Regional Geologic Framework Studies: The Missouri iron-oxide deposits are members of the iron-oxide-copper-cobalt-gold rare earth element (IOCG-REE) deposit type, which have proven to be an important type for associated rare earth element (REE) mineral resources critical for modern industrial applications. Understanding the framework and ore genesis of the Missouri deposits hinges on understanding the buried Precambrian basement geology. The study area is predominantly concealed beneath a thick sequence Paleozoic rocks, Quaternary deposits, and dense vegetation. Recent work by the USGS, in conjunction with the Missouri Department of Natural Resources Division of Geology and Land Survey, laid the groundwork for understanding the regional geologic framework and ore genesis of the deposits. Integration of recently acquired data and interpretations is required to define the regional geologic and tectonic setting.

Our objective is to develop an updated geologic framework for the iron-oxide-apatite (IOA) and (IOCG) ore deposits by producting a regional scale integrated geophysical/geologic regional map and a modern geologic map of the Ironton, MO area. The Ironton area includes most of the outcropping ore deposits in the St. Francois Mountains, is relatively well exposed, and contains the complex volcanic geology and caldera structures that controlled the emplacement of the IOAand IOCG mineralizing systems. The new regional digital compilation will incorporate recent geologic mapping, geochronology, drill core data, and results of the latest USGS geophysical surveys.

Another goal is to provide a continental-scale framework for metallogeny and genesis of A-type 1.4 Gagranites along the southern margin of Laurentia. IOCG deposits in southeast Missouri are temporally and spatially associated with ~1.4 Ga igneous rocks; similar rocks exposed throughout southern Laurentia could host similar deposits.

Geochronology of St. Francois Mountains Terrane and Associated Ore Deposits: Our objective is to determine the ages of formation of the deposits in the St. Francois Mountains terrane. Our initial work focused on the age and origin of Pea Ridge deposit rare earth element mineralization. New age data from this regional study of plutonic rocks will provide a temporal framework for derivation of improved tectonic and petrologic models for the Eastern Granite-Rhyolite province and the iron deposits. The results are critical for interpreting the results of potential field geophysics studies and isotope tracer studies. Important dating targets include:

  1. volcanic rocks to definitively define the relationship between plutonism and volcanism;
  2. granitic rocks beneath the Missouri Gravity Low, identified by geophysics but never dated;
  3. rare Proterozoic metasedimentary rocks;
  4. regional granitic rocks, and;
  5. apatite and monazite in magnetite-apatite ore deposits.

We will utilize both U-Pb zircon sensitive high-resolution ion microprobe (SHRIMP) dating and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb analysis of magnetite deposits.

SEM micrographs

Scanning electron micrographs (all backscattered electron images) of Pea Ridge REE-rich breccia pipes. From Ayuso and others, 2016, doi:10.2113/econgeo.111.8.1935.
(Public domain.)

mineralogy pair plot

Δ-δ plot for quartz-magnetite pairs in rocks from Pea Ridge deposit (Δ18Oquartz-magnetite = δ18Oquartz − δ18Omagnetite). Data for individual minerals form linear trends that are nominally consistent with isotopic equilibration under closed-system conditions of originally hydrothermal or igneous mineral pairs. Abbreviations: Amp = amphibole, Mag = magnetite, Qz = quartz. From Johnson and others, 2016, doi:10.2113/econgeo.111.8.2017.
(Public domain.)

Radiogenic Isotopes and Geochemistry: Our task is focused on obtaining modern radiogenic isotope data (Nd-Sr-Pb) and trace element geochemistry to provide definitive information on source, age, and tectonic setting of magmas associated with rare earth element mineralization in southeast Missouri. The new geochemical information will help to evaluate the regional distribution and origin of rare earth element mineralized occurrences and their host rocks in order to distinguish between sources and settings associated with fertile or barren igneous rocks. The radiogenic isotope data will constrain the types of geochemical processes controlling the evolution of rare earth element mineralizing systems during magma transfer through the crust and the origin of hydrothermal fluids. New results will help to identify economically important targets that resemble the large Fe-rare earth element Pea Ridge deposit. These radiogenic isotope data are critical to understanding the relationship between basement terranes and the complexly mineralized units.

Our initial efforts were focused on the Pea Ridge iron-oxide-apatite (IOA) deposit. Our current phase of research using radiogenic isotopes will be to broaden the effort from the Pea Ridge IOA deposit to include major igneous units from across the St. Francois Mountains terrane with the goal of fully understanding the tectonic setting as well as why the terrane hosts IOA and IOCG deposits and other 1.4 Ga igneous terranes across Laurentia appear to be barren. Is the IOA/IOCG ore formation in Missouri a singular event and if so, what is unique about the crustal architecture that led to mineral deposit formation?

Stable Isotope Applications: We address basic ore genesis questions at the regional scale using stable isotope methods, using both published and new data on regional patterns in oxygen, hydrogen, and sulfur isotopes. We will tie in stable isotope data to the iron oxide-copper-cobalt-gold rare earth element deposits of the region to help constrain the origin of metals and fluids.

  1. What were the sources of the ore-forming fluids?
  2. To what extent were fluid-rock reactions important in establishing the isotope and chemical properties of the fluids?
  3. Did the fluids access both magmatic and surficial solute reservoirs through space and time?
fluid inclusions

Photomicrographs of secondary fluid inclusions in apatite (A) and milky quartz (B–H) from Pea Ridge. A.–C. Saline liquid-rich inclusions. D. Hypersaline inclusion with halite, calcite, and hematite. E. Vapor-rich inclusion. F. Hypersaline inclusion with CO2. G. Vapor-rich inclusion with CO2. H. Inclusion assemblage with variable phase ratios. Similar inclusions are present in each alteration zone and within fragments in the REE-rich breccia pipes. From Hofstra and others, 2016, doi:10.2113/econgeo.111.8.1985.
(Public domain.)

Ore, Mineral, and Fluid Geochemistry: In order to advance genetic and exploration models of iron oxide-apatite (IOA) and iron oxide-copper-cobalt-gold (IOCG) deposits that may host rare earth elements, additional knowledge is needed of the physical conditions (pressure-temperature), source(s) of ore fluid components, and processes that produce the minerals and mass transfer observed in these deposits. Our work initially focused on the Pea Ridge IOA rare earth element deposit in southeast Missouri. To facilitate comparisons, the next step is to collect similar data sets on other deposits in southeast Missouri. Resulting data, interpretations, and chemical models will be used to identify the key factors that produced each deposit type and help characterize the nature of the hydrothermal fluids and processes responsible for ore formation in the Missouri metallogenic province. This information will be used in the development of a USGS IOA-IOCG deposit model.

Data Management: Several data streams will be developed as an outgrowth of this project's research. This task will develop a project database containing archived and new geophysical, petrophysical, and geochemical data sets derived from all phases of the project. The compilation of project-generated data on a single user-friendly platform will facilitate the sharing of data, modeling, interpretation, and publication. The main datasets developed from this project will be released to the public.

USGS Global IOGC Model Development: Our objectives are to bring together all of the research done on the southeast Missouri IOA and IOCGdeposits into one concise occurrence model relevant to Missouri and southern Laurentia. We will also update the USGS grade and tonnage models for IOA deposits and develop a robust data set and grade tonnage model for IOCG deposits worldwide. An integrated global USGS IOA/IOCG ore deposit model will be developed that pulls together the research done in Missouri with research from ore deposits around the world into a concise description of the deposit type and grade and tonnage information to help in future mineral resource assessments.

geologic map

Simplified geologic map of the Great Bear magmatic zone and locations of main deposits, prospects, and showings, and prospective iron oxide and alkali-altered systems. Modified from Corriveau et al. (2016). From Slack and others, 2016, doi:10.2113/econgeo.111.8.1803.
(Public domain.)

Completed Activities - Setting and Origin of Iron Oxide-Copper-Cobalt-Gold-Rare Earth Element Deposits of Southeast Missouri: In our previous phase of this project, we focused on the Pea Ridge iron oxide-apatite deposit and its rare earth element resources. While many activities continue on with an emphasis of expanding efforts to a regional scale, some activities were intended to provide the needed data to determine current project efforts.

Geochemistry of Ores and Altered Wall Rocks: The task's major goals were to characterize the inorganic geochemical composition of ores and altered wall rocks in the mineral deposits, and document the basic mineral textures and paragenesis. Methods used included standard observations including hand lens, binocular microscope, and polarizing microscope (both transmitted and reflected light). Findings provided critical information on the nature of the ores and wall rocks of the mineral deposits, and provided the foundation for future laboratory studies of the project.

Mineralogy and Mineral Chemistry: Knowledge of the mineralogy and mineral chemistry of various mineral phases in the iron-oxide-copper-cobalt-gold-rare earths deposits of southeast Missouri is incomplete. Without a detailed characterization, certain paragenetic and petrogenetic aspects of this deposit will remain uncertain. We worked to characterize the mineralogy and mineral chemistry of samples from the iron-copper-cobalt-gold-rare earths deposits of southeast Missouri. Electron microprobe wavelength-dispersive spectroscopy and scanning electron microscope (SEM) petrography and energy-dispersive spectroscopy were used to characterize, identify, and determine the composition of the constituent phases.

Evolution of Mineralization and Alteration: The consensus among many workers is that the iron-copper-cobalt-gold-rare earths deposits of southeast Missouri belong to the iron oxide-copper-gold (IOCG) family of mineral deposits (e.g., Kisvarsanyi and Kisvarsanyi, 1989; Seeger, 2000). However, the ages of these deposits, relative contributions of magmatic and meteoric fluids during mineralization, and sources of the contained metals, remain uncertain. We studied modeling of fluid-rock reactions in ore zones and altered wall rocks which is hoped to provide a mineralizing system aspect to the deposit studies.

References

  • Kisvarsanyi, G., and Kisvarsanyi, E.B., 1989, Precambrian geology and ore deposits of the southeast Missouri iron metallogenic province: Society of Economic Geologists, Field Trip Guidebook Series, v. 4, p. 1-40.
  • Seeger, C.M., 2000, Southeast Missouri iron metallogenic province: Characteristics and chemistry, in Porter, T.M., ed., Hydrothermal iron oxide copper-gold and related deposits: A global perspective: Adelaide, PGC Publishing, v. 1, p. 237-248.