Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Geochemical Evidence for Rare-Earth Element Mobilization during Kaolin Diagenesis Michael C. Cheshire,*,† David L. Bish,‡ John F. Cahill,§ Vilmos Kertesz,§ and Andrew G. Stack† †
Geochemistry and Interfacial Sciences Group, Chemical Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee 37831-6110, United States ‡ Indiana University, 1001 East 10th Street, Bloomington, Indiana 47405, United States § Mass Spectrometry and Laser Spectroscopy Group, Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6131, United States S Supporting Information *
ABSTRACT: This study investigates how saprolization influences inherent rare-earth element (REE) source rock signatures and how depositional environment(s) and diagenetic reactions ultimately impact the REE signature within sedimentary kaolin bodies. Rare-earth element geochemistry signatures are particularly useful for tracking element sources and mobility and are, therefore, powerful tools in the investigation of clay mineral formation and diagenesis. Rareearth element and bulk chemical compositions were determined using discrete chemical analyses and chemical imaging. Saprolitic materials show an enrichment in the light and heavy REEs, compared with the parent material, with enhanced Ce/Eu anomalies. Light REEs within sedimentary kaolins are associated with phosphate mineralogy and have experienced variable degrees of diagenetic fractionation and mobilization. Cretaceous kaolins display more light REE mobility compared with Tertiary kaolins, which show very little REE fractionation. Degrees of REE fractionation are driven primarily by differences in sedimentary kaolin physical properties and the presence of organic acids in groundwater. Unfortunately, the provenance of the Georgia kaolins could not be determined based solely on the trace-element and REE compositions because fractionations during saprolization and diagenesis mask much of the inherent provenance signatures. Finally, implications for the REEs as an economic deposit and their beneficiation are discussed. KEYWORDS: Kaolinite, Rare-Earth Element, Saprolite, Florencite, Fractionation, Georgia
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INTRODUCTION
In addition to the geological significance, REEs are of national importance for many countries. Various governments have classified REE, along with other metals (e.g., yttrium, indium, tellurium, cobalt, gallium), as “critical materials” due to their applications within clean-energy technologies (e.g., permanent magnets, batteries, photovoltaics, and phosphors).13 The United States has initiated the Critical Materials Institute to address some domestic supply issues of critical materials. This CMI effort is partly accomplished through the evaluation nontraditional REE sources for both REE abundances and beneficiation potential, especially in areas where preexisting mining and processing infrastructures are present. Saprolization. Chemical weathering of felsic materials, via a process known as saprolization, serves as the primary process
Clay mineral formation from a granitic source is generally initiated via a series of aqueous geochemical processes that drive extensive remineralization of granitic rocks, leaching soluble components, and causing the redistribution of remaining cations. Trace and rare-earth element (REE) geochemistries have been extensively used in studies of fluid−rock interactions in general and specifically in the investigation of clay mineral formation and diagenesis.1−6 Although REEs are relatively insoluble, many studies2,7−9 have shown that REEs can be mobilized and can fractionate during the formation, deposition, and diagenesis of sediments and rocks, thereby providing a valuable geochemical tool for diagenetic studies of highly weathered materials. REEs have unique fractionation patterns that are related to source rock chemistry, water/rock ratios, saprolization processes, fluid− mineral interactions, and fluctuations in oxidation/reduction potential (Eh) and pH during transport and sedimentation.2,3,5,10−12 © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
November 2, 2017 March 5, 2018 March 6, 2018 March 6, 2018 DOI: 10.1021/acsearthspacechem.7b00124 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry
REEs to mineral surfaces, such as the commercial ion-sorbed clays in China. Many of the REE fractionation patterns generated during saprolization are mimicked in diagenetic processes because diagenetic reactions are similar to saprolization processes. This paper focuses on the REE characteristics of suspected sources for the sedimentary kaolins in middle Georgia, U.S.A., with the application of REE signatures to clarify the provenance and diagenetic reactions related to sedimentary kaolin deposits found in middle Georgia.
producing source materials for sedimentary kaolin deposits. During saprolization, REEs are concentrated as a result of the mass loss associated with leaching of more soluble elements with some REEs redistributed (via precipitation) into authigenic mineral phases. With a sufficiently high water/rock ratio under acidic conditions, mobilization of REEs can occur during mineral dissolution.2,7−9 Unfortunately, the mobility and fractionation of REEs during the saprolization process are highly variable, and no uniform, well-defined REE signatures characteristic of saprolite sequences have been identified.14 The variability in REE distribution as a result of saprolization is due to variable solution chemistry (pH, Eh, ionic strength), water/ rock ratio, and parent rock composition. In a granitic source, such as the source of the majority of the kaolin deposits in Georgia, the major REE-bearing minerals are carbonates, phosphates, feldspars, zircon, and titanite.14−16 Each of these minerals can have a different REE composition due to their different crystal structures and variable REE partition coefficients.17 Existing data suggest that carbonate (e.g., bastnaesite/parisite) and phosphate (e.g., apatite, monazite, and xenotime) minerals constitute the major reservoir for the REEs in the parent rock.1,16 Dissolution of these minerals during saprolization liberates Ca2+, REEs, and PO43− into the Al-rich residuum, likely contributing to the crystallization of both crandallite (CaAl3(PO4)2(OH)5·H2O) and florencite ((La,Ce,Nd)Al3(PO4)2(OH)6).1 HREE concentrations are generally controlled by zircon, mica/illite, ilmenite, and/or garnet.5,17 Zircon and garnet are resistant to weathering; therefore, these two minerals tend to develop higher concentrations, and subsequently higher HREEs, within saprolite compared with parent materials. Overall, REE concentrations tend to progressively increase concentrations with continual saprolization due to the loss of the other more mobile constituents. Sedimentary Kaolin Diagenesis. Extensive weathering of sedimentary kaolin typically alters many of the geochemical signatures that are commonly used for interpretations of depositional history and diagenesis, contributing to ambiguity in interpreting their mode(s) of formation. 15,18 REE compositions of sediments have been used for crustal evolution and provenance studies because source-rock REE signatures are often preserved in fresh sediments.10,19,20 However, as much of the original signatures can be modified by diagenetic alteration, many studies have investigated the use of REEs as geochemical tracers for diagenetic processes and conditions.1−3,8,9,20,21 Diagenetic fractionation of individual REEs is related to REE solubility and the details of REE-bearing mineral phases along with other factors, e.g., water/rock ratio, pore-water chemistry, and temperature. For REEs to undergo transport or fractionation, REE-bearing minerals must first undergo partial or total dissolution, thereby liberating structural REEs. However, if surface-adsorbed REEs are present, even a slight change in pore-water chemistry or water/rock ratio is sufficient to desorb any loosely bound metals. High water/rock ratios and/or acidities, in addition to time, are necessary for the dissolution and transport processes associated with REE fractionation due to the insoluble nature of REE-bearing minerals. Organic acids have also been shown to facilitate REE mobility by enhanced phosphate mineral dissolution, lowering the pore-water pH (∼4−5), and stabilizing dissolved REEs via ligand formation.3,9,22,23 Changes in the subsurface pH and Eh can also destabilize any REE−organic acid complexes, causing crystallization of authigenic minerals and/or adsorption of
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SAMPLE SITES Saprolites and their corresponding parent rocks were collected from Hancock and McDuffie Counties in Georgia, U.S.A. One site was located at the Tudor primary kaolin mine operated by Advanced Primary Minerals (now known as Southeastern Primary Minerals, LLC.). The parent rock associated with the Tudor saprolite (“Tudor”) is a light gray to pink biotitemuscovite granitic gneiss that is part of the metamorphic complex of McDuffie County. The saprolite forming on the granitic gneiss has a lenticular geometry with a maximum thickness of ∼6.6 m over a 0.162 km2 area (Coughlan, personal communication, 2010). The second saprolite sequence studied came from two discrete sites located within the Sparta granite complex in Hancock County. One site was part of the U.S. Aggregates (“USA”) mine, and the second site was in a railroad cut in Sparta, Georgia, U.S.A. (“SPG”). The Sparta granite is an equigranular, medium- to fine-grained biotite granite with a variable saprolite thickness (4.6−6.1 m). A 0.3−1.5 m thick soil horizon was situated on top of this saprolite. Sedimentary kaolin samples were collected from the kaolin mining district and inner Piedmont of Georgia, U.S.A. Sedimentary kaolins were collected from open-pit mines and drill-cores located in Wilkinson, Washington, Glascock, McDuffie, and Jefferson Counties. Sedimentary kaolin units focused within this investigation come from the Late Cretaceous Buffalo Creek Kaolin Member (Gaillard Formation of Oconee Group), Paleocene Marion Member (Huber Formation of Oconee Group), and Eocene Jeffersonville Member (Huber Formation of Oconee Group).
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ANALYTICAL METHODS REE and bulk chemical analyses were performed at Activation Laboratories (Ancaster, Ontario, Canada) using instrumental neutron activation analysis (INAA) and X-ray fluorescence (XRF). Samples were dried at 105 °C to a constant weight prior to all chemical analyses. Loss on ignition (LOI) was determined by measuring the weight loss after samples were heated to 1,050 °C for 2 h. The saprolites were analyzed as bulk, >45 μm, and
