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Radiogenic Pb Enrichment of Mississippi Valley-Type Metallic Ore Deposits, Southern Ozarks

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Radiogenic Pb Enrichment of Mississippi Valley-Type Metallic Ore Deposits, Southern Ozarks geosciences Article Radiogenic Pb Enrichment of Mississippi Valley-Type Metallic Ore Deposits, Southern Ozarks: Constraints Based on Geochemical Studies of Source Rocks and Their Diagenetic History Jonathan Chick 1, Sydney E. McKim 2, Adriana Potra 1,* , Walter L. Manger 1 and John R. Samuelsen 3 1 Department of Geosciences, University of Arkansas, Fayetteville, AR 72701, USA; [email protected] (J.C.); [email protected] (W.L.M.) 2 Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA; [email protected] 3 Arkansas Archeological Survey, University of Arkansas, Fayetteville, AR 72704, USA; [email protected] * Correspondence: [email protected] Abstract: Southern Ozark Mississippi Valley-type ores are enriched in radiogenic Pb, with isotopic signatures suggesting that metals were supplied by two end-member components. While the less radiogenic component appears to be derived from various shale and sandstone units, the source of the more radiogenic component has not yet been identified. Analyses of cherts from the Early Ordovician Cotter Dolomite and tripolitic chert from the Early Mississippian Boone Formation contain highly radiogenic Pb, with isotopic ratios comparable to those of ores. However, most 208 204 207 204 206 204 Citation: Chick, J.; McKim, S.E.; samples have lower Pb/ Pb and Pb/ Pb for a given Pb/ Pb compared to ores. These Potra, A.; Manger, W.L.; Samuelsen, relationships demonstrate that the enriched Pb isotopic values of the ore array cannot be related to the J.R. Radiogenic Pb Enrichment of host and regional lithologies sampled, suggesting that the source of high ratios may lay further afield. Mississippi Valley-Type Metallic Ore The slope of the linear trend defined by the Pb isotope ratios of ores corresponds to an age of about Deposits, Southern Ozarks: 1.19 Ga. Therefore, an alternative for the linear array is the involvement of Precambrian basement in Constraints Based on Geochemical supplying ore Pb. Rare earth element patterns show that diagenetic processes involving the action Studies of Source Rocks and Their of groundwater and hydrothermal fluids affected the sampled lithologies to various degrees, with Diagenetic History. Geosciences 2021, Cotter Dolomite having experienced the highest degree of alteration. 11, 172. https://doi.org/10.3390/ geosciences11040172 Keywords: Mississippi Valley-type ores; Ozark region; tripolitic chert; Pb isotopes; rare earth elements Academic Editors: Suzanne Golding and Jesus Martinez-Frias Received: 29 December 2020 1. Introduction Accepted: 7 April 2021 It has been hypothesized that Ordovician, Mississippian, and younger carbonate Published: 10 April 2021 and clastic formations found within the Ouachita Uplift of west-central Arkansas, and northward onto the Ozark Dome, experienced interaction with hydrothermal fluids due Publisher’s Note: MDPI stays neutral to tectonic forces that caused the Ouachita Orogeny [1–3]. Geochemical studies indicate with regard to jurisdictional claims in that the Mississippi Valley-type (MVT) ores were produced from a mixing and cooling of published maps and institutional affil- these sedimentary, basinal brines by associated meteoric or connate waters [4]. Previous iations. investigations of the mid-continent MVT Pb and Zn mineralization (Figure1) have been conducted, resulting in a large amount of geological and geochemical data, as well as numerous theories as to the source of the MVT ore metals and mineralizing fluids [5–10]. In the Ozark region, the linear trend defined by the Pb isotope compositions of the MVT Copyright: © 2021 by the authors. ores suggest their formation involved a mixing of metals from two end-member compo- Licensee MDPI, Basel, Switzerland. nents [7–10]. One end-member must have been highly radiogenic with Pb isotope ratios This article is an open access article equal or higher than the highest measured value in the ores. The other end-member distributed under the terms and must have been less radiogenic, with Pb isotope ratios equal or lower than the lowest conditions of the Creative Commons measured value. Extensive geochemical studies of rocks from the southern Ozark region Attribution (CC BY) license (https:// and the Ouachita Mountains (Figure1) have been carried out in order to constrain the creativecommons.org/licenses/by/ two end-member components in the ores from the Northern Arkansas and the Tri-State 4.0/). Geosciences 2021, 11, 172. https://doi.org/10.3390/geosciences11040172 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 23 been less radiogenic, with Pb isotope ratios equal or lower than the lowest measured value. Extensive geochemical studies of rocks from the southern Ozark region and the Ouachita Mountains (Figure 1) have been carried out in order to constrain the two end- member components in the ores from the Northern Arkansas and the Tri-State districts: the Devonian and Mississippian shales of the Ozark Plateaus [9], the Ordovician and Car- boniferous shales and sandstones of the Ouachita Mountains [10], the various cherts from the Devonian-Lower Mississippian Arkansas Novaculite [11,12], and from the Lower Mis- sissippian Boone Formation (the lower Boone penecontemporaneous chert and the upper Boone tripolitic and diagenetic chert) [13,14]. While the less radiogenic component has been generally attributed to various shale and sandstone units [9,10], the more radiogenic Geosciences 2021, 11, 172 2 of 22 component has not yet been identified. Whole-rock Pb isotope analyses conducted for this study may help better constrain the more radiogenic component that contributed Pb to the ores. Rare earth element (REE) analyses have also been carried out on these whole- districts:rock samples the Devonian to shed light and on Mississippian the depositional shales environment of the Ozark and Plateaus the diagenetic [9], the Ordovician processes andthat Carboniferousmight have affected shales them. and sandstones of the Ouachita Mountains [10], the various chertsLead from isotopes the Devonian-Lower are an excellent Mississippian tracer of metal Arkansas sources in Novaculite hydrothermal [11,12 systems.], and from Un- theder Lowermedium Mississippian (mesothermal, Boone around Formation 200– (the400 lower°C) to Boone high penecontemporaneoustemperature (hypothermal, chert andaround the 400 upper–600 Boone °C) conditions, tripolitic like and those diagenetic encountered chert) [in13 hydrothermal,14]. While the environments, less radiogenic Pb componentis soluble [15]. has beenUnder generally relatively attributed low temperature to various environments, shale and sandstone Pb may units be complexed [9,10], the morewith organic radiogenic matter component [15]. Although has not yetmetal been-bisulfide identified. and Whole-rockorganometallic Pb isotopecomplexes analyses have conductedbeen suggested, for this only study metal may-chloride help better complexes constrain are the considered more radiogenic likely base component metal trans- that contributedporting species Pb to for the MVT ores. ore Rare fluids earth [16]. element A threshold (REE) analyses of approximately have also been10 wt. carried % chlorinity out on thesein sedimentary whole-rock brines samples has to been shed inferred light on thefor depositionalthe presence environmentof Pb (and Zn) and in the sedimentary diagenetic processesfluids at MVT that mightore-forming have affected conditions them. [17]. FigureFigure 1.1. MapMap ofof thethe UnitedUnited StatesStates midcontinentmidcontinent regionregion showing showing the the location location of of sedimentary sedimentary basins, ba- shalesins, shale plays, plays, and majorand major MVT MVT mining mining districts districts and and deposits deposits (1 through (1 through 12): 12 (1)): Northern(1) Northern Arkansas, Ar- (2)kansas, Tri-State (2) ,Tri (3)-State, Burkesville (3) Burkesville deposit, deposit, (4) Old ( Lead4) Old Belt, Lead (5) Belt, Southeast (5) Southeast Missouri Missouri Barite, Barite, (6) Central (6) Central Missouri Barite, (7) Viburnum Trend, (8) Upper Mississippi Valley, (9) Illinois–Kentucky Missouri Barite, (7) Viburnum Trend, (8) Upper Mississippi Valley, (9) Illinois–Kentucky Fluorspar, Fluorspar, (10) Central Tennessee, (11) Central Kentucky, and (12) Eastern Tennessee. Modified (10) Central Tennessee, (11) Central Kentucky, and (12) Eastern Tennessee. Modified from [4,9]. For from [4,9]. For drawing this basemap: https://www.eia.gov/maps/maps.htm (accessed on 29 De- drawingcember 2020 this) basemap:. https://www.eia.gov/maps/maps.htm (accessed on 29 December 2020). Lead isotopes are an excellent tracer of metal sources in hydrothermal systems. Under medium (mesothermal, around 200–400 ◦C) to high temperature (hypothermal, around 400–600 ◦C) conditions, like those encountered in hydrothermal environments, Pb is soluble [15]. Under relatively low temperature environments, Pb may be complexed with organic matter [15]. Although metal-bisulfide and organometallic complexes have been suggested, only metal-chloride complexes are considered likely base metal transporting species for MVT ore fluids [16]. A threshold of approximately 10 wt. % chlorinity in sedimentary brines has been inferred for the presence of Pb (and Zn) in sedimentary fluids at MVT ore-forming conditions [17]. Lead has a similar geochemical behavior to Cu and Zn in hydrothermal fluids [18] and it occurs in the same paragenetic stages as Zn, Cu, Ag, and Cd [19,20]. Therefore, Pb isotopes can be used to constrain the source of associated metals such
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