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Overview

Precambrian Basement map
Study area location, showing Precambrian Basement. Map from Kisvarsanyi, E.B., 1981.

Introduction:

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.

Shaded Relief/Magnetic Anomaly Map
Shaded Relief/Magnetic Anomaly Map from Dempsey and Meuschke, 1951a and 1951b.

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. See the USGS media advisory on the geophysical survey.

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.

image showing correlation of Missouri iron-oxide deposits
Image showing correlation of Missouri iron-oxide deposits, from Day and others, 1989.

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.

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.

References

Dempsey, W. J., and Meuschke, J. L., 1951a, Total intensity aeromagnetic map of Berryman quadrangle, Missouri: U.S. Geological Survey Geophysical Investigations Map GP-77, scale 1:31,680.

Dempsey, W. J., and Meuschke, J. L., 1951b, Total intensity aeromagnetic map of Sullivan quadrangle and part of Union quadrangle, Missouri: U.S. Geological Survey Geophysical Investigations Map GP-78, scale 1:31,680.

Kisvarsanyi, E.B., 1981, Geology of the Precambrian St. Francois terrane, southeastern Missouri: Missouri Department of Natural Resources Report of Investigations 64, 58 p.

Day, W.C., Kisvarsanyi, E.B., Nuelle, L.M., Marikos, M.A., and Seeger, C.M., 1989, New data on the origin of the Pea Ridge iron-apatite deposit, southeast Missouri---Implications for Olympic Dam-type deposits: Geological Society of America Abstracts with Programs, v. 21, no. 6, p. A132.

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.

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Geophysical Characterization

gravity map of project area
Left, a Bouguer gravity anomaly map of the study area; right, an index map of gravity station locations.
magnetic map of project area
Left, a magnetic anomaly map showing compilation of older magnetic surveys; right, an index map of airborne magnetic survey coverage over study area.

There are three geophysical efforts of the project:

  1. Compiling and interpreting existing magnetic and gravity data to determine overall basement architecture. The quality and resolution of the existing airborne magnetic and ground gravity data are only adequate to establish where large basement features occur and can be used to impart refinement on the existing basement map.
  2. Making petrophysical measurements on an inventory of several hundred drill core, surface, and underground mine samples collected by the USGS over the last decades. These samples are being statistically analyzed to determine relationships between petrophysical properties and alteration assemblages, ore mineralogy and chemistry. The petrophysical data will be also be used as input to a 3-D geophysical model of the Pea Ridge iron-ore rare earth element deposit.
  3. Fly a modern high resolution magnetic and gravity gradiometry survey to cover the Pea Ridge, Kratz Spring, and the Bourbon iron deposits. (​Digital flight line data are available at http://dx.doi.org/10.5066/F78P5XM4). The existing magnetic and gravity data are insufficient to distinguish important features at deposit scale. The goal is to cover a number of the iron deposits so that a geologic framework can be established.

Project News Story: Aerial survey to look for valuable minerals in southeastern Missouri, Véronique LaCapra, St. Louis Public Radio, posted January 8, 2016.

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Regional Geology

The Missouri iron-oxide deposits are members of the iron-oxide gold-copper (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. Recent work by the USGS, in conjunction with the Missouri Department of Natural Resources Division of Geology and Land Survey, has laid the groundwork for understanding the regional geologic framework and ore genesis of the deposits. USGS research conducted from 1989 to1994 has shown that the deposits are both hosted in and genetically related to the Mesoproterozoic rhyolitic and granitic rocks of the St. Francois Mountains of southeastern Missouri.

Understanding both

  • the regional framework geology, and
  • the character of the known mineral deposits

... is critical in the development of a process-oriented ore deposit model transferable to other similar terranes worldwide to support future mineral resource assessments for undiscovered IOCG and REE deposits worldwide.

A preliminary model for the ore deposits has been proposed, linking their genesis to late-stage magmatic processes related to igneous rocks present in the host terrane.

Our preliminary fluid inclusion, stable isotope, and geochemical data point to the source ore fluids were the result of immiscibility of an iron-oxide-gold-copper-rare earth element-rich phase sourced from high-iron trachytes common in the late stages of the regional volcanism.

The focus of this task is to build upon past efforts by the USGS and other partners --

  1. to provide the geologic framework of the regional host rocks and their petrogenesis in context with the mineral deposits,
  2. to help characterize the known deposits and occurrences, and
  3. to further investigate the immiscible liquid model for the ore deposits.

This research will be fully integrated into the other project activities and will form the foundation for associated geophysical, isotopic, mineralogical, and geochemical studies.

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Geochemistry of Ores and Altered Wall Rocks

The major goals of this task are to

  • characterize the inorganic geochemical composition of ores and altered wall rocks in the mineral deposits, and
  • document the basic mineral textures and paragenesis.

This task will provide critical information on the nature of the ores and wall rocks of the mineral deposits, and provide the foundation for other laboratory studies of the project.

Methods used include standard observations using

  • hand lens,
  • binocular microscope, and
  • polarizing microscope (both transmitted and reflected light).

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SHRIMP Uranium-Lead Geochronology

We will research the age and origin of Pea Ridge Rare Earth Element mineralization. Currently, the age of the Pea Ridge Rare Earth Element deposits is thought to be about 1.4 Ga, based on TIMS analysis of one xenotime grain. However, previous geochemical analyses of xenotime indicates multiple compositions, suggesting several episodes of xenotime formation. The previous TIMS age data probably represent a mixture of these age components. We will use SHRIMP spot analysis of xenotime in situ (to preserve textural context(s) of ore deposition) to decipher the complex history of the Pea Ridge deposit.

Our objective here is to determine the ages of formation of uraniferous rare-earth-element-bearing phosphate minerals (xenotime and monazite) that occur at Pea Ridge. We will obtain ore samples from existing cores for uranium-lead geochronology. In addition, we intend to collect outcrop samples of Mesoproterozoic plutonic rocks of the St. Francois Mountains that host the rare earth element deposits or that are related to the Mesoproterozoic magmatic activity of the region. We will date ore samples from core, in situ (in polished thin section). We will extract zircon from the plutonic rock samples. To better understand the genesis of the rare earth element minerals, we will also analyze dated zones for trace elements.

Images of mineral grains separated from REE-bearing breccia pipes: Hand-picked grains - A, B, and C below:
Monazite and apatite grains
A. Composite grains of monazite (white) and apatite (black)
Monazite grains with few xenotime grains
B. Granular grains of mostly monazite; few darker gray crystallites are xenotime
Adamantine yellow monazite grains
C. Adamantine yellow monazite
Polished mineral grains in epoxy mount for SHRIMP U-Pb geochronology, below:
backscattered electron images of grain sample
Images above: BSE (backscattered electron) images of large, reddish granular grain from sample 2275-15 ("hard" breccia pipe). A. High contrast/low brightness image showing showing faint zoning in monazite (gray). B. Low contrast/high brightness image image showing crystallintes of monazite (white) and xenotime (gray)
Backscattered electron image of pale yellow granular grains from sample PR-33C
Image above: BSE (backscattered electron) image of pale yellow granular grains from sample PR-33C ("soft" breccia pipe). Both large grains are composed almost entirely of monazite.
Backscattered electron image of pale yellow granular grains from sample PR-33C showing A. faint zoning, and B. crystallites of monizite. BSE (backscattered electron) image of pale yellow granular grains from sample PR-33C
Images above: BSE (backscattered electron) images of two varieties of monazite (large orangish granular; small yellow adamantine) from sample PR-182 ("hard" breccia pipe). A. High contrast/low brightness image showing faint zoning in monazite (gray); bright white grains are thorite. B. Low contrast/high brightness image showing crystallites of monazite (light gray), xenotime (medium gray), and thorite (white). C. Two grains of yellow adamantine monazite showing very faint zoning.

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Igneous Geochemistry and Radiogenic Isotopes

We will address the origin and evolution of igneous plutons and volcanic rocks in the St. Francois Mountains terrane. Our principal objectives are to fully characterize the whole-rock geochemistry (major, trace, and rare earth element) and radiogenic isotope (Sr, Nd, and Pb) signature of intrusive and volcanic rocks in the study area.

We will collect samples for analysis from outcrops and drill cores, and from previous collections of other workers (e.g., underground workings of mines). Analyses we will perform include:

  1. major, trace, and rare earth element geochemistry or whole rocks by modern techniques, mainly via XRF (major elements) and ICP-MS (trace elements and rare earth elements); and
  2. Sr, Nd, and Pb isotopes on whole rocks and mineral separates, done by thermal ionization mass spectometry.

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Stable Isotope Geochemistry

We will provide stable isotope data that can help constrain the origin of metals and fluids in the mineral deposits being studied.

Our principal objectives are to document the carbon, oxygen, hydrogen, and sulfur isotope composition of ore and gangue minerals in the deposits, and those in adjacent altered wall rocks.

The stable isotope data will be acquired by analysis of whole rocks and mineral separates using gas-source mass spectrometry.

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Fluid Inclusion Studies

How did the IOCG and related deposits in the St. Francois Mountains form? Specifically, what were the pressure-temperature conditions and depth of ore formation, chemical composition of ore fluids, and sources of ore fluid components that produced IOCG (e.g. Boss-Bixby) and related iron oxide and rare earth element (e.g. Pea Ridge) deposits? The initial focus will be on the Pea Ridge deposit.

The objectives of this task are to use fluid inclusion data collected from ore and gangue minerals to ascertain the presence or absence of basinal or crustal vs. magmatic or mantle fluids and their relative importance to ore formation as well as advance understanding of the P-T-X conditions and processes of mineral precipitation. Melt inclusions in coeval intrusive and extrusive igneous rocks may also be characterized. Ultimately, such data may elucidate the controls on iron oxide versus copper-gold versus rare earth element mineralization in the district and their relation to the geotectonic framework of the region.

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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 will work 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 will be used to characterize, identify, and determine the composition of the constituent phases.

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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 will study modeling of fluid-rock reactions in ore zones and altered wall rocks, which will 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.

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Products and References

Journal Articles and Reports

Day, W.C., and Granitto, Matthew, 2014, Geologic field notes and geochemical analyses of outcrop and drill core from Mesoproterozoic rocks and iron-oxide deposits and prospects of southeast Missouri: U.S. Geological Survey Open-File Report 2014–1053, 7 p., http://pubs.usgs.gov/of/2014/1053/.

Data Releases

Day, W.C., Granitto, Matthew, Slack, J.F., and Ayuso, R.A., 2016, Geochemical Database for Iron Oxide-Copper-Cobalt-Gold-Rare Earth Element Deposits of Southeast Missouri: U.S. Geological Survey data release, http://dx.doi.org/10.5066/F7P26W67.

McCafferty, A.E., 2016, Helicopter magnetic and gravity gradiometry survey over the Pea Ridge iron mine and surrounding area, southeast Missouri, 2014: U.S. Geological Survey data release, http://dx.doi.org/10.5066/F78P5XM4.

Abstracts of Posters and Presentations

Aleinikoff, J., Neymark, L., Selby, D., Day, W., Slack, J., Holm-Denoma, C., Pillers, R., Seeger, C., and Samson, I., 2015, U-Pb and Re-Os geochronology of REE-rich breccia pipes and magnetite ore from the Mesoproterozoic Pea Ridge Fe-REE-Au deposit, St. Francois Mountains, Missouri, USA: Society for Economic Geologists 2015 Conference, Hobart, Australia, 27-30 September 2015. View Aleinikoff SEG abstract. [PDF file, 36 KB]

Aleinikoff, J.N., Selby, D., Day, W.C., Slack, J.F., Pillers, R.M., Seeger, C., and Samson, I.M., 2013, U-Pb and Re-Os geochronology of REE-rich breccia pipes from the Mesoproterozoic Pea Ridge Fe-REE-Au deposit, St. Francois Mountains, Missouri: Geological Society of America Abstracts with Programs, v. 45, p. 499. View Aleinikoff abstract.

Day, W.C., Aleinikoff, J., Seeger, C.M., and Slack, J.F., 2015, Regional geologic setting of the Mesoproterozoic iron oxide ore systems of southeast Missouri, USA: Society for Economic Geologists 2015 Conference, Hobart, Australia, 27-30 September 2015. View Day SEG abstract. [PDF file, 19 KB]

Hofstra, A.H., Meighan, C.J., and Emsbo, P., 2013, Microthermometric and ion ratio analyses of fluid inclusions from the Pea Ridge iron oxide-apatite (IOA) and Boss-Bixby iron oxide copper-gold (IOCG) deposits support a magmatic-hydrothermal origin: Geological Society of America Abstracts with Programs, v. 45, p. 499. View Hofstra 2013 GSA abstract.

Ives, B.T., Metzger, A., Mickus, K.L., and McCafferty, A.E., 2013, Regional gravity survey to aid in determining the REE potential of the Pea Ridge mine and the surrounding area, southeast Missouri: Geological Society of America Abstracts with Programs, v. 45, p. 276. View Ives 2013 GSA abstract.

Ives, B.T., Mickus, K.L., and McCafferty, A.E., 2014, Regional gravity survey investigating poorly studied areas of the northwest St. Francois terrane, southeast Missouri: Geological Society of America Abstracts with Programs, v. 46, no. 1, p. 6. View Ives 2014 GSA abstract.

Ives, B.T., Mickus, K.L., McCafferty, A.E., Seeger, C., and Starkey, M., 2014, Analyzing the Paleozoic basement structure and lithologies of the northwest St. Francois terrane, Missouri using gravity data to investigate possible mineral deposits of economic interest: Geological Society of America Abstracts with Programs, Vol. 46, No. 6, p. 308. View Ives 2014 GSA poster abstract.

Johnson, C.A., Day, W.C., and Rye, R.O., 2013, Stable isotope geochemistry of the Pea Ridge iron oxide-rare earth element deposit, St. Francois Mountains, southeastern Missouri: Temperature of ore formation, source of hydrothermal fluid, and source of sulfur: Geological Society of America Abstracts with Programs, v. 45, p. 499. View Johnson abstract.

McCafferty, A.E., 2014, Insights into concealed iron oxide-rare earth element deposits from new airborne geophysical data, southeast Missouri: Geological Society of America Abstracts with Programs, v. 46, No. 4, p. 15. View McCafferty 2014 abstract.

McCafferty, A.E., Day, W.C., Slack, J.F., McDougal, R.R., and Driscoll, R.L., 2013, Geophysical setting of iron oxide-copper-cobalt-gold-rare earth element deposits of southeast Missouri: Geological Society of America Abstracts with Programs, Vol. 45, No. 7, p. 537. View McCafferty 2013 abstract.

McCafferty, A.E., and Phillips, J.D., 2015, Shallow to Deep Crustal Controls on Localization of Mesoproterozoic Iron Oxide Copper-Gold-Rare Earth Element Deposits in Southeast Missouri (USA): Evidence from Gravity and Magnetic Data: Society for Economic Geologists 2015 Conference, Hobart, Australia, 27-30 September 2015. View McCafferty SEG 2015 abstract. [PDF file, 119 KB]

Meighan, C.J., Hofstra, A.H., Marsh, E.e., Lowers, H.A., Koenig, A.E., and Hitzman, M.W., 2015, Zr-in-rutile and Ti-in-quartz thermometry support a metasomatic origin for the Pea Ridge iron oxide-apatite-REE deposit, southeast Missouri, USA: Society for Economic Geologists 2015 Conference, Hobart, Australia, 27-30 September 2015. View Meighan SEG abstract. [PDF file, 32 KB]

Mercer, C.N., Watts, K.E., Meighan, C.J., and Bennett, M.M., 2015, Mineral and melt inclusion constraints on the petrogenesis of regional magmas and magnetite ore from the Pea Ridge (IOA-REE) and Boss Bixby (IOCG) deposits, USA: Society for Economic Geologists 2015 Conference, Hobart, Australia, 27-30 September 2015. View Mercer SEG abstract. [PDF file, 20 KB]

Neymark, L., Aleinikoff, J., Holm-Denoma, C., Pietruszka, A., Pillers, R., and Moscati, R., 2015, High spatial resolution SHRIMP and LA-ICPMS U-Pb geochronology of Pea Ridge Fe-REE-Au deposit, USA: Goldschmidt Conference, Prague, Czech Republic, 16-21 August 2015. View Neymark Goldschmidt abstract.

Slack, J.F., Day, W.C., McCafferty, A.E., Seeger, C., and Nold, J.L., 2013, A new look at iron oxide-apatite and iron oxide-copper-gold-rare earth element deposits in the Mesoproterozoic St. Francois Mountains terrane of southeast Missouri: Geological Society of America Abstracts with Programs, v. 45, p. 498. View Slack abstract.

Slack, J.F., McCafferty, A.E., Aleinikoff, J.N., Ayuso, R.A., Belkin, H.E., Cosca, M.A., Day, W.C., Hofstra, A.H., Johnson, C.A., Meighan, C.A., Neymark, L.A., Nold, J.L., Seeger, C.M., and Selby, D., 2014, Mesoproterozoic iron oxide-apatite and iron oxide-copper-gold-rare earth element deposits of southeast Missouri, USA: Geology, geophysics, and exploration potential: Society of Economic Geologists, Conference Proceedings, 27-30 September 2014, Keystone, Colorado, USA. [PDF file, 143 KB]

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