hydrogeologic modeling of the genesis of carbonate-hosted lead-zinc ores

Hydrogeology Journal (1999) 7 :108–126 Q Springer-Verlag

Hydrogeologic modeling of the genesis of carbonate-hostedlead-zinc oresGrant Garven 7 Martin S. AppoldVera I. Toptygina 7 Timothy J. Hazlett

Received, February 1998Revised, July 1998Accepted, September 1998

Grant Garven (Y)Centre for Ore Deposit Research, University of TasmaniaGPO Box 252–79, Hobart, Tasmania 7001, AustraliaFax: c61-3-6226-7662e-mail: [email protected]

Martin S. Appold 7 Vera I. Toptygina 7 Timothy J. HazlettDepartment of Earth and Planetary SciencesThe Johns Hopkins UniversityBaltimore, Maryland 21218, USA

Abstract Carbonate-hosted lead–zinc ore deposits inthe Mississippi Valley region of North America and inthe central midlands region of Ireland provide goodexamples where ancient groundwater migrationcontrolled ore formation deep within sedimentarybasins. Hydrogeologic and geochemical theories for oregenesis are explored in this paper with mathematicalmodels that allow for complex permeability fields intwo or three dimensions, hydrothermal flows in faultsystems, and coupled effects of geochemical reactions.The hydrogeologic framework of carbonate-hostedores is analyzed with the aim of developing a quantita-tive understanding of the necessary and sufficient pro-cesses required to form large ore deposits. Numericalsimulations of basin-scale hydrodynamics and ofdeposit-scale reactive flow are presented to demon-strate the processes controlling low-temperature Pb–Znore genesis in two world-class ore districts, in southeastMissouri, USA, and central Ireland.

The numerical models presented here provide atheoretical basis for the following observations: (1)topography-driven brine migration was the most effec-tive mechanism for forming the large ore districts of theMississippi Valley, such as the Viburnum Trend ofsoutheast Missouri, during the uplift of the Appala-chian–Ouachita mountain belt in late Paleozoic time;(2) three-dimensional flow fields were created by adolomite facies of the Viburnum Trend, which acted asa giant lens for focusing metal and heat in southeastMissouri to produce the largest known concentration oflead in the Earth’s crust; (3) ore-mineralizationpatterns were controlled locally by basement relief,permeability structure, and sandstone pinchouts,

because of their effects on cooling and fluid-flow ratesalong the Viburnum Trend; (4) both density-driven andtopography-driven fluid flow were important for oregenesis in the Irish midlands, where brines movednorthward away from the Variscan orogen, leaked intothe Hercynian basement, and discharged along normalfaults up into the sedimentary cover; and (5) mixedconvection within northeast–southwest fault planeselevated heat flow and flow rates that fed ore deposi-tion by fluid mixing, in some cases near the Carbonif-erous seafloor in Ireland.

Résumé Les dépôts de minerai de plomb-zinc dans lescarbonates de la région de la vallée du Mississippi enAmérique du Nord et dans les comtés du centre del’Irlande fournissent de bons exemples de lieux où, àdes époques géologiques passées, les mouvementsd’eau souterraine ont contrôlé la formation de mineraien profondeur dans des bassins sédimentaires. Dans cepapier, les théories hydrogéologiques et géochimiquesde la formation de minerais sont examinées au moyende modèles mathématiques qui prennent en compte deschamps de perméabilité complexes en deux et en troisdimensions, des écoulements hydrothermaux dans dessystèmes de failles et les effets couplés de réactionsgéochimiques. Le cadre hydrogéologique des mineraisdans les formations carbonatées est analysé dans le butde proposer une compréhension quantitative desprocessus nécessaires et suffisants requis pour formerdes dépôts de minerais importants. Des simulationsnumériques de l’hydrodynamique à l’échelle du bassinet de l’écoulement réactif à l’échelle du dépôt sontprésentés afin de montrer les processus contrôlant lagenèse du minerai Pb–Zn à basse température dansdeux districts miniers de référence mondiale, dans lesud-est du Missouri et en Irlande centrale.

Les modèles numériques présentés ici donnent unebase théorique aux observations suivantes: 1) la migra-tion de saumure pilotée par la topographie a été lemécanisme le plus évident pour la mise en place desvastes gisements de la vallée du Mississipi, tels que leViburnum Trend dans le sud-est du Missouri, au coursde la surrection de la chaîne de l’Appalache-Ouachitaau Paléozoïque terminal; 2) des champs d’écoulementstridimensionnels ont été créés par un faciès dolomit-ique du Viburnum Trend, qui se comporte comme une

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lentille gigantesque concentrant le métal et la chaleurdans le sud-est du Missouri pour produire la plus vasteconcentration connue de plomb dans la croûteterrestre; 3) l’organisation des minéralisations métalli-fères a été localement contrôlée par le relief du subs-tratum, la structure de la perméabilité et le biseautagedes grès, à cause de leurs effets sur le refroidissement etsur les flux de fluide le long de Viburnum Trend; 4) lesécoulements de fluides commandés à la fois par ladensité et par la topographie ont été importants dans lagenèse du minerai dans les comtés du centre del’Irlande, où des saumures se sont écoulées vers le nordà partir de la chaîne varisque, ont pénétré dans le sub-stratum hercynien et se sont écoulées le long de faillesnormales jusque dans la couverture sédimentaire; et 5)la convection de mélanges dans des plans de faillesNE–SW a élevé les flux thermique et hydrique qui ontalimenté la formation de minerai par mélange defluides, dans certains cas à proximité du fond marin auCarbonifère en Irlande.

Resumen Los yacimientos minerales de plomo y zincen rocas carbonatadas en el Valle del Mississippi(EE.UU.) y en las Central Midlands de Irlanda propor-cionan buenos ejemplos de cuencas sedimentariasdonde las aguas subterráneas controlaron en el pasadola formación de yacimientos de minerales a granprofundidad. En este artículo se estudian teorías hidro-geológicas e hidroquímicas de génesis de yacimientosmediante modelos matemáticos que permiten simularcampos de permeabilidad complejos en dos o tresdimensiones, flujo hidrotermal en las fracturas y efectosacoplados de reacciones geoquímicas. Se analiza elmarco hidrogeológico de los yacimientos en rocascarbonatadas, con el objetivo de desarrollar, demod 7Cuantitativo, el conocimiento de los procesos queson necesarios y suficientes para la formación degrandes yacimientos minerales. Se presentan simula-ciones numéricas de los procesos hidrodinámicos quetienen lugar a escala de cuenca y del flujo reactivo aescala de depósito para mostrar los procesos quecontrolan la génesis de los yacimientos de Pb-Zn abajas temperaturas en las zonas antes mencionadas deEstados Unidos e Irlanda.

Los modelos numéricos proporcionan una baseteórica para entender las siguientes observaciones: 1) lamigración de salmueras por efectos topográficos fue elprincipal mecanismo que condujo a la formación de losgrandes yacimientos del Valle del Mississippi, como elViburnum Trend al sudeste de Missouri, formadodurante la orogenia de las Appalaches-Ouachita en elPaleozoico Superior; 2) la facies dolomítica delViburnum Trend originó un flujo tridimensional queactuó como un foco de concentración de metal y calorque dio lugar a la mayor concentración de plom Cono-cida en la Tierra; 3) la acumulación del mineral quedócontrolada localmente por el relieve del basamento, laestructura de permeabilidad y la presencia intercaladade areniscas, debido a sus efectos sobre el enfriamiento

y la variación de los caudales subterráneos; 4) tanto elflujo debido a las variaciones de densidad como a las detopografía fueron importantes para la génesis de yacim-ientos en las Midlands de Irlanda, donde las salmuerasse movieron en dirección norte, alejándose del orógenoVariscano, se infiltraron en el basamento Herciniano ydescargaron en la cubierta sedimentaria a lo largo defallas normales; y 5) la convección a lo largo de losplanos de fallas de dirección noreste-sudoeste produjoun aumento de los flujos de calor y agua subterránea, loque contribuyó a la formación de depósitos mineralesen algunas zonas de Irlanda.

Key words paleohydrology 7 numerical modeling 7hydrochemical modeling 7 hydrothermal ore deposits 7general hydrogeology

Introduction

Sedimentary basins around the world host majordeposits of lead, zinc, barium, fluorine, copper,uranium, gold, and other metals that appear to be asso-ciated with ancient hydrothermal flow systems. Manyof these ore deposits are epigenetic and stratabound,formed by the movement of hot, saline groundwaterand brines some time after deposition and lithificationof their permeable host rock. For example, carbonate-hosted ore deposits, such as those of the US MississippiValley/midcontinent region and of the Irish Carbonif-erous midlands, are known to have geochemical originsrelated to Paleozoic migrations of deep basinal brines.Despite this general understanding, geologists continueto debate the mechanisms of fluid flow and chemicaltheories for ore deposition. The coupled geochem-ical–hydrogeologic aspects of large-scale fluid migra-tion have begun to be developed and new mathematicalmodels built to quantify ore-forming processes (Garvenand Raffensperger 1997). These models permitcomparison and testing of conceptual theories of oregenesis that could aid and improve explorationventures in frontier basins.

The role of topography-driven groundwater flow hasseen the greatest amount of investigation in relation tothe origin of stratabound ores. Mississippi-Valley-type(MVT) ore formation was first analyzed by Garven andFreeze (1984a, 1984b), Garven (1985), Bethke (1986),and Garven et al. (1993) for the sedimentary basins ofNorth America. These early numerical studies consid-ered only the indirect coupling between fluid flow, heattransport, and aqueous mass transport. Calculations ofthe hydrothermal systems were restricted to two-dimensional profiles, and the mathematical models didnot take into account the effects of fractures, three-dimensional flow, geochemical effects of reactions onore genesis, or the effects of geochemical feedbacksthrough permeability changes on flow rates. Further-more, the occurrence of other ore districts in thrustbelts and rifts suggested additional fluid-drive mech-

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Figure 1 Conceptual modelfor topography-driven flowand stratabound-ore formationdeveloped for North Amer-ican sedimentary basins. Darkarrows depict general patternof fluid migration in basalaquifer and white arrowsdepict broad pattern ofseepage in overlying aquitard.(After Garven 1995)

anisms could be important under different tectonicsettings. The Pb–Zn–Ba ores in Ireland, for example,are closely affiliated with normal faults, suggesting animportant role of faults in extensional basins and fault-hosted, buoyancy-driven fluid convection (Russell1968, 1992, 1996). On the opposite side of the AtlanticOcean, carbonate-hosted Pb–Zn–Ba ores in NovaScotia are thought to have formed during hydrofrac-turing events in a subsiding evaporite basin (Raven-hurst and Zentilli 1987).

Geochemical mechanisms of metal transport and oremineralization depend on processes such as cooling,mixing of chemically distinct fluids, and fluid–rockreactions. Until recently, the hydrologic effects of tran-sient flow and coupled reactions were not well under-stood in MVT ore genesis (Appold 1998). Transportprocesses affecting mineralization also operate at avariety of scales that present additional challenges inhydrogeologic modeling; as shown in Figure 1, thesecan range from the scale of entire basin to a singledeposit. The role of faults is similarly not well under-stood, although good progress has been made in basinhydrogeology by incorporating fault structures explic-itly in the fluid flow mechanics (Wieck et al. 1995;Arnold et al. 1996; Hazlett 1997). In some cases faultsmay act as barriers or conduits for ore solutions or mayaffect the bulk permeability significantly (Garven1995). Faults may enhance vertical migration of orefluids or guide the migration of brines into adjacentbeds where mineralization occurs.

The purpose of this paper is to present some recentresearch developments in stratabound-ore hydrogeo-logy. Special reference is made to carbonate-hostedPb–Zn–Ba deposits in order to demonstrate the role ofgroundwater flow in geologic processes. The geologicsettings of these ore deposits are described, and somenew results are presented for systems in Missouri andIreland.

Geologic Setting of Pb–Zn Ores

The world’s largest Pb–Zn–Ba-F ore districts occur inthe Mississippi Valley region of the United States. Insoutheast Missouri, considerably more than 900 million

tons of ore exist in Upper Cambrian and Lower Ordo-vician dolomite strata that blanket the Precambrianrocks that form ridges and knobs of the St. FrancoisMountains and Ozark Dome; locations and geology areshown in Figure 2. Rich deposits of galena and sphal-erite are concentrated in a dolomitic reef facies of theCambrian Bonneterre Formation. Pinchouts of theunderlying Lamotte Sandstone (against the Precam-brian granite) and collapse brecciation trends in theBonneterre Formation appear to control ore-minerali-zation patterns (Anderson 1991). Fluid-inclusion dataindicate a range of ore-precipitation temperatures inthe Viburnum Trend. Temperatures range from90–120 7C, with a mean of 110 7C (Leach and Rowan1993). Salinity is as great as 35 weight % NaCl equiv-alent. Fluid-inclusion and trace-element data havebeen inferred by some to indicate multiple periods ofbrine mixing coeval with dolomitization and Pb–Zn oreformation (Shelton et al. 1992). Lead and sulfur isotopestudies by Goldhaber et al. (1995) implicate thePrecambrian granite as the source of lead and suggest acomplex history of fluid mixing during ore deposition.Plumlee et al. (1994) use geochemical reaction-pathmodeling to show how mixing of a metal-rich and asulfide-rich fluid might best account for the paragenesisof mineralization. The age of ore formation in south-east Missouri has not been established unambiguously,but most agree that ore mineralization was associatedwith the Alleghanian–Ouachita orogeny of Late Penn-sylvanian–Early Permian time (Sverjensky and Garven1992). Garven et al. (1993) model several hydrogeo-logic scenarios for regional flow and ore formation inthe US midcontinent basins. They conclude that brinesmigrated northward away from the Appala-chian–Ouachita orogenic belt through the Arkomaforeland basin. Transient flow systems, mostly drivenby topographic relief, transported base metals andelevated heat flow over the Ozark Dome. Ore forma-tion ceased after the topographic relief driving regionalflow dissipated and/or the source of sulfide or metal-bearing brine supply was exhausted.

In Ireland, smaller Zn–Pb–Ba ore bodies occur inplatform carbonates of the western European Carbon-iferous basin and proximal to faults that penetrate thelower Paleozoic metamorphic basement. These faults

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Figure 2 Geologic sectionA–A’ in southeast Missouri,USA, showing geology of theSoutheast Missouri OreDistrict. In the ViburnumTrend, several depositsformed in the reef facies ofthe Bonneterre Dolomite, andore-bearing solutions appearto have migrated up from theLamotte Sandstone. (AfterKaiser et al. 1987)

Figure 3 S–N section showinggeology of the Tynagh Zn–Pbore deposit, Ireland, about45 km north of Silvermines.Carbonate-hosted mineraliza-tion in Ireland commonlyoccurs in the hanging wall ofnormal faults and within thelowest permeable carbonateformation, as shown here.Ore-forming solutions arethought to have migrated upalong the fault from the OldRed Sandstone. (After Russell1992)

are thought to have acted as conduits for the transportof metal-bearing brines from deeper formations such asthe Old Red Sandstone and Silurian basement. Oresare concentrated in the more permeable limestone anddolomite layers immediately above the Old Red Sand-stone, commonly in the hanging walls of the normalfaults; relationships are shown in Figure 3. Faults frag-ment the Irish midlands region into a patchwork quiltof carbonate horsts and grabens, situated north of theVariscan–Hercynian orogen, which formed in Carbon-iferous time (Maynard et al. 1997).

Unlike the MVT Pb–Zn deposits, the Irish depositsare closely associated with faults, and sulfide minerali-zation lenses extend as much as 600 m away from zonesof maximum throw (Andrew 1993). The Irish ores arestratabound in lower Carboniferous carbonates, and

many deposits display large-scale stratiform morphol-ogies. In addition to the prominent structural control,fluid-inclusion data indicate much higher temperaturesfor mineralization, ranging from 50–280 7C, with amean about 190 7C (Hitzman and Large 1986).Although many features are similar to the MVTdeposits, the Irish metal suite contains more copper,silver, and iron with truly massive iron sulfides, prob-ably as a result of the higher temperature of formation.Some geologists interpret ore deposition by exhalationof fluids on the sea floor, whereas others interpretclassic epigenetic styles of precipitation and replace-ment in the subsurface. Several isotopic studies indicatemigration of metals from basement rocks (e.g., Samsonand Russell 1987; Everett et al. 1997). Two prominentgenetic mechanisms have been favored for the Irish

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ores: (1) free convection of fluids along NE–SW Cale-donian basement faults, which allowed for rapid trans-port of metal-bearing brines to the sea floor to precipi-tate sulfides and barite in early Carboniferous time(Russell 1978); and (2) northward migration of brinesfrom the Munster basin, driven either by compaction ortopographic relief, as the Variscan orogenic beltformed (Lydon 1986; Hitzman 1995).

Hydrogeologic Models for Stratabound-OreFormation

Groundwater flow plays an important role in manygeologic processes (Ingebritsen and Sanford 1998), andthis topic has matured as a science, particularly forhydrothermal systems such as ore deposits, where largesets of field observations are available for constraininghydrologic models. The role of fluids in the formationof ore deposits is reasonably well understood and manyolder articles review this topic (Cathles 1981; Garvenand Freeze 1984a; Sharp and Kyle 1988). Sedimentarybasins are subjected to several forces known to causelarge-scale fluid flow: topography associated with conti-nental landscapes, compaction associated with sedi-mentation or tectonic compression, and fluid-densitygradients associated with temperature and salinity areall important mechanisms that evolve during thegeologic history of a basin. Calculations by Bethke(1986), Ge and Garven (1992), and Garven et al. (1993)show that neither burial compaction nor tectonicsqueezing caused by thrusting can produce enoughfluid to generate large ore deposits far removed fromthe orogenic belt. General agreement exists that steadyflow from compaction cannot explain the thermalanomalies that appear to be associated with many MVTlead–zinc districts (Cathles and Smith 1983), but ’’spit-ting’’ expulsions during episodic flows might be favor-able for some mineralization events, according toCathles (1997).

New mathematical models of fluid flow and heattransport are now coupled with geochemical reactionsfor the purpose of quantifying the hydrogeology of oregenesis. Garven and Raffensperger (1997) provide acomprehensive review of ore-deposit hydrogeology andgeochemistry, and papers by Person et al. (1996) andRaffensperger (1996) describe in detail the mathemat-ical and numerical theory for modeling basin-scale,reactive-flow systems. The basic approach involvescasting the appropriate partial differential equationsgoverning fluid flow, heat transport, and geochemicalmass transport in geologic media into numericalexpressions for either fixed or deforming coordinates(compaction), for either steady or transient flow, andfor either thermodynamic equilibrium or kinetic-controlled reactions. Fluid properties, formation prop-erties, and chemical reactions create a strong couplingbetween the dependent variables of pressure, flowrates, temperature, and mass concentrations. The trans-

Figure 4 Midcontinent map of the USA showing locations ofMississippi-Valley-type (MVT) ore deposits. The deposits on theOzark Dome are thought to have formed from the migration ofdeep brines out of the Arkoma Basin during the late Paleozoicuplift. Location of hydrogeologic section model (Figure 8) indi-cated by line A–A’. (After Appold 1998)

port equations are solved with finite-difference orfinite-element techniques in order to provide coupledpredictions through time of fluid pressures andhydraulic heads, enthalpy or temperature, strain andstress, concentrations of aqueous species andcomplexes, and masses of minerals dissolved or precipi-tated in the flow system. Generally speaking, modelparameters such as permeability, solute dispersivity,and initial ore-component concentrations are not wellconstrained, and so sensitivity studies are undertakento explore the effects of parameters and boundaryconditions (Raffensperger and Garven 1995a, 1995b).

Southeast Missouri Ore DistrictThe Arkoma foreland basin and underlying basementare generally believed to have provided the source ofmetal-bearing brines that formed the large sulfide oredistricts in southeast Missouri on the Ozark Dome;locations are shown in Figure 4. Deep fluids weredriven across the foreland basin by topographic relief

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created by compression and uplift of the Appala-chian–Ouachita orogen in Permian time. Basinal brineswould have been driven mostly northward, withfocusing of flow, heat, and chemical mass within bothcarbonate and sandstone formations, which composeregional aquifers in the Cambrian–Ordovician sectionof the midcontinent region (Bethke and Marshak 1990;Garven et al. 1993). Metals were probably acquiredfrom clay and oxide surface reactions as fluids seepedacross thick mudstone units during deep burial. Thesame fluids would have interacted with the CambrianLamotte Sandstone and fractured Precambrian base-ment. Metal solubility and transport were significantlyenhanced by the formation of highly saline brines, mostof which were produced by evaporation and by dissolu-tion of bedded evaporites (Hanor 1994). Ore formationwas concentrated on the Ozark Dome because ofregional groundwater discharge, aquifer pinchouts, andfavorable conditions for geochemical deposition relatedto permeability, cooling, and fluid mixing. Garven et al.(1993) present two-dimensional finite-element modelsthat evaluate the effects of compaction, topography-driven flow, and tectonic thrusting for several sce-narios. They observed that topography-driven flowprovides the best hydrodynamic setting for ore genesis,although important issues on the origin of brines,effects of three-dimensional flows, and geochemicalmechanisms for ore deposition still need to be fullyresolved.

Three-dimensional modeling of the topography-driven flow system in southeast Missouri was initiatedwith the development of a new finite-element codecalled JHU3D (Garven and Toptygina 1993). This codesolves the coupled equations for variable-densitygroundwater flow in a nondeforming, heterogeneous,and anisotropic permeability region:

P=7rquprg (acnb)ihit

(1)

and heat transport due to conduction, dispersion, andconvection in porous media:

=7(E=T)P(rcq7=T)p[rcncrscs (1Pn)]iTit

(2)

where the Laplacian operator is =p1 iix

, iiy

, iiz2 and

r is fluid density, q the Darcy velocity vector, g thespecific weight of the fluid, a the bulk vertical compres-sibility, b the fluid compressibility, E the combinedthermal conductivity-dispersion tensor, T temperature,c specific-heat capacity of fluid phase, cs specific-heatcapacity of the solid phase, and n is porosity. Moreinformation about the geologic range of the hydraulicand thermal parameters is in Garven and Freeze(1984a). In Eqs (1) and (2), the Darcy velocity takes ona special form for variable-density flow:

qupPKmr (=hcrr =Z) (3)

Figure 5 a Three-dimensional mesh for modeling paleohydrol-ogy of the Arkoma basin–Ozark Dome region of southeastMissouri. The mesh is 500 km long, 200 km wide, and ranges inthickness from 8 km in the far south (left) end to about 2 km overthe Ozark Dome. Hidden within the basal aquifer is a large high-permeability lens. b Hydrostratigraphy in section view atYp100 km. Unit 1 is the basal Cambrian–Ordovician aquifer,Unit 2 comprises less permeable Ordovician carbonates andshale, Unit 3 is Permian shale, and Unit 4 is a high-permeabilitylens within the basal aquifer

where q is the Darcy velocity vector, K the hydraulic-conductivity tensor (assuming fresh water at 25 7C as areference state), h fresh-water hydraulic head, mr therelative viscosity, rr the relative density, and Z thevertical elevation above datum. Equations (1) and (2)are coupled through the fluid properties and by the waythe Darcy velocity appears as a variable in the secondterm of the heat equation. In some flow problems, it ispossible to uncouple the two equations and obtainexact analytical mathematical solutions for simplifiedsystems (Phillips 1991; Cathles 1997), but for most sedi-ment-hosted ore problems it is desirable to examinecoupled effects in geologically interesting domains withat least two- or three-dimensional fabrics.

Figure 5 displays a three-dimensional mesh repre-sentative of the Arkoma basin–Ozark Dome region ofsoutheast Missouri in Early Permian time. The meshwas created by extruding 3-D elements from a 2-D gridusing a graphically-interactive software package calledZeus3D. The finite-element mesh comprises 21 verticalnodal slices: each slice consists of 20 nodal rows and 50nodal columns. The distance between nodal slices

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represents 10 km. The mesh extends from south tonorth along the direction indicated by line A–A’(Figure 4), but only for 500 km, terminating at point Cnear the Missouri–Illinois boundary. The model basinis 200 km wide (east–west) and 8 km thick at thesouthern edge of the mesh. The landscape assumedhere slopes to the north away from the OuachitaMountain belt and rises slightly at the far northernedge of the grid. The hydrostratigraphy is representedby four material types: Unit 1 is a basal sand-stone–carbonate aquifer; Unit 2 is a less permeablesuccession of carbonates and shales; and Unit 3 is athick wedge of Upper Paleozoic mudstones. Hiddenwithin the basal aquifer bed is Unit 4, a high-permea-bility lens representing the Viburnum Trend of south-east Missouri. The lens is 20 km wide and about 100 kmlong, situated centrally within the basin and over thebasement arch. Salinity is assumed to increase as alinear function of depth such that the salt content of thefluid is about 15 weight % NaCl equivalent in the orehorizon. The temperature is assumed to be 20 7C at thewater table (top boundary, also at atmospheric pres-sure), and a uniform heat flow of 80 mW/m2 is assignedat the impermeable base of the mesh. The objective isto quantify the three-dimensional effects of the high-permeability lens on heat and fluid flow in the basinunder conditions of topography-driven brine migra-tion.

Numerical methods for three-dimensional ground-water flow are well established and reviewed elsewhere(Barry 1990; Ségol 1994). In the present study, thegoverning equations for variable-density flow and heattransport are solved numerically using a meshconstructed with triangular prisms (6 nodes). Fluidproperties are updated in an iterative fashion as thenumerical solution is marched through time. Forsteady-state calculations, the code iterates sequentiallybetween solving the flow equations and checking forconvergence in heads and temperatures at all nodes inthe grid. The slice successive-relaxation method ofHuyakorn et al. (1986) was tried at first but conver-gence was difficult for simulations where the contrast inpermeability between hydrostratigraphic unitsexceeded orders of magnitude. The numerical equa-tions were solved with the preconditioned conjugategradient and ORTHOMIN methods described andprogrammed in the ORTHOFEM software by Sudickyand McLaren (1992) and Mendoza et al. (1992). Thismethod appears to be very robust and was able to simu-late both topography-driven and density-driven flowsand to investigate a wide range of model parameters.Numerical results were verified against 2-D simulationsand with numerical output from well-known 3-D codessuch as MODFLOW and HST3D, published by the USGeological Survey (see Ingebritsen and Sanford 1998,p. 50).

A high-permeability lens such as the ViburnumTrend affects regional flow patterns and brine tempera-tures across the basin. Figures 6 and 7 show the steady-

state fluid-velocity and temperature pattern, respec-tively, sliced both vertically and horizontally throughthe 3-D mesh. For this simulation, K1p100, K2p20,K3p5, and K4p500 m/yr along the bedding. Theformations are assumed to be anisotropic such thatKxpKyp100Kz. Thermal conductivities were assignedto the solid framework as follows: l1p3.0, l2p2.5,l3p3.0, and l4p2.0 W/m 7C. Porosity varies as follows:n1p0.20, n2p0.15, n3p0.25, n4p0.10, and these areused to compute a weighted mean thermal conductivityfor each porous unit (Garven and Freeze 1984a).Under this scenario of topography-driven flow, deepfluids from the Arkoma basin mix with southward flowfrom northern Illinois to create a large discharge zoneover the Viburnum Trend where the topographic eleva-tions are the lowest, as shown in Figure 6a. Ground-water velocities show the focusing effect created by thelens, with Darcy flow rates approaching 4.3 m/yr in thedolomite Unit 3, as shown in Figure 6b. Temperatureranges from 20–160 7C.

The presence of the high-K lens creates a complexheat-flow pattern over the Ozark Dome, such that ahydrothermal anomaly develops in the discharge area,as shown in Figure 7b. High temperatures appear to beassociated with the ’’eyes’’ of the anomalies, where hotbrines discharge at the corners of the aquifer pinchout.This feature was first thought to represent numericaloscillations, but parametric sensitivity studies show thatit actually represents a physical hydrothermal dischargecreated by the permeability lens and geometry of thebasin topography. Three-dimensional patterns likethese could help explain the diverse temperatures indi-cated by the fluid-inclusion filling data in the SoutheastMissouri Ore District: the temperatures appear to be atleast 25 7C higher than simulations conducted alongsimilar 2-D profiles. These new calculations helpresolve the old debate regarding the apparent anoma-lous temperatures of MVT ore formation (see Inge-britsen and Sanford 1998, p. 140–141). Additionalthree-dimensional experiments of this type havemodeled the effects of basement topography and water-table configuration on flow patterns, and these resultsare planned to be presented in a later communication.

Realistic hydrogeologic models for ore mineraliza-tion and chemical mass transport at the district anddeposit scale require coupled calculations that simulatetransient, reactive flow fields. Fully coupled 3-D reac-tive models are beyond the scope of this paper, butincluded are 2-D results for steady flow systems. InFigure 8, the vertical section portrays the basin-scaleflow field and temperature map for a simulation acrossMissouri (along A–A’ in Figure 4). The hydrostratig-raphy is similar to that of the three-dimensional model.One can ’’zoom in’’ on the hydrothermal features alongthe Viburnum Trend and examine the effects of stratig-raphy and basement structure on the flow fields, asshown in Figure 9. Nearly horizontal flow mostly occursin the basal aquifer (Lamotte and Bonneterre Forma-tions), which is confined by the thin Davis Shale. Darcy

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Figure 6a,b Three-dimen-sional Darcy velocity field forsteady-state simulation ofsoutheast Missouri. aSouth–north vertical profilealong the central axis of thegrid with all vectors plotted atsame length to show flowdirection. b Plan map ofvelocity field zoomed on topof the regional discharge areanear Xp300 km. Vectors arescaled to show both directionand magnitude (the largestDarcy velocity is 4.3 m/yr)

flow rates of 1.4 m/yr are predicted for the basalaquifer, and this rate would ultimately control rates ofore mineralization for metals transported throughsoutheast Missouri.

Reactive flow simulations were carried out over thiszoomed section of the grid by applying boundaryconditions from the basin-scale simulation. Thesecalculations were done with the finite-element codeRST2D (Raffensperger 1996). It solves the combinedsets of advection–dispersion equations subject to chem-ical equilibrium between minerals and brine. Conserva-tion of mass for the chemical component requireswriting an advection–dispersion equation for allcomponents in the flow system (Raffensperger andGarven 1995b):

=7(nD=Ci)Pqu7=CipinCi

it(4)

where Ci is the total concentration (mol/L) of the i-thcomponent, and D is the hydrodynamic dispersiontensor. These equations are combined with additionalmass-balance expressions, which define the distributionof the total component among secondary species andminerals, and either kinetic-rate or mass-action expres-sions (assuming local equilibrium, as in this paper).RST2D uses a time-stepping loop in which the ground-water-flow and heat-transport equations are solvedfirst, followed by a predictor step, in which the set ofmass-transport equations is solved for each componentsequentially. After calculating the chemical speciationat the midpoint of the time step, a corrector scheme isused to solve for the concentration of all components atthe end of the time step, and the chemical speciation isrecalculated to distribute mass among aqueous speciesand mineral phases. Thermodynamic data are derivedfrom the SUPCRT92 programs reported in the work ofJohnson et al. (1992). For saline groundwater typically

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Figure 7a,b Three-dimensional isotherms (temperature in 7C)for steady-state simulation of southeast Missouri. a South–northvertical profile along the central axis of the grid. b Plan mapacross the entire basin, sliced at the elevation level Zp4500 m.The geothermal anomaly in the plan map develops because of theupward discharge of hot brines (140 7C) near the edge of the high-permeability lens

Figure 8a,b Continental-scale hydrogeologic simulation alongsection A–A’. Line of section is shown in Figure 4. a Hydrostrati-graphy. b Steady-state flow fields. Dashed lines are isotherms andsolid lines are stream-function flowlines. The dark line outlinesexpanded or zoomed model in Figure 9. (After Appold 1998)

encountered in sedimentary basins, RST2D uses amodified Debye–Hückel equation that provides a goodapproximation of activity coefficients for chargedaqueous species in NaCl solutions up to about 5.0 molal(Oelkers and Helgeson 1990). The thermodynamicmodels of Pitzer (1987) may be better for predictingactivity coefficients in highly concentrated solutions (upto 6.0 molal NaCl), but Pitzer’s method has not yetbeen implemented in RST2D. The chemical speciationprogram developed for RST2D was benchmarkedagainst other published geochemical codes with reason-ably good comparisons, given the differences in databases, activity-coefficient relations, and numerical algo-rithms. The advantage of RST2D is that it accuratelyincorporates effects of fluid advection, hydrodynamicdispersion, and diffusion subject to local chemicalequilibrium, processes that cannot be modeled withpopular reaction path codes such as EQ3/6 (Wolery1979), NETPATH (Plummer et al. 1994) orGeochemist’s Workbench (Bethke 1996). At present,the principal disadvantages of RST2D is that it doesnot consider 3-D flow, kinetics, or adsorption proc-esses.

The solution of the set of mass-transport equationsprovides two-dimensional maps of all primary and

secondary species concentrations and the masses ofminerals dissolved or precipitated over time. Changesin mineral volumes can be fed back into empiricalporosity-permeability correlations. This feature wasdisabled for the present simulation in order to simplifythe problem and isolate the effect of cooling a metal-sulfide-bearing brine as it migrates along the ViburnumTrend. The regional brine is modeled to have themajor-element concentrations similar to those used byPlumlee et al. (1994). Base metals and reduced sulfurare assumed to be transported in the same fluidthrough the sandstone aquifer at equilibrium concen-trations such that [Pb]p4 ppm, [Zn]p14 ppm,[S]p2.1!10–4 mol/L, log[O2]p–49.2, and pHp4.4.The Na–Ca–Cl brine is assumed to have an ionicstrength equivalent to 4.9 molality and saturated withrespect to dolomite, quartz, and muscovite. In all, 17chemical components defining 39 aqueous species and

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Figure 9a,b Regional flow model of the Arkoma basin, zoomed,for the cooling simulation. a Fluid flowlines or stream function atsteady state. Flow rates of about 3 m/yr develop in basal aquifer.b Isotherms at steady state. The region shown here is outlined inFigure 8

14 mineral phases were treated in the ’’cooling’’ simula-tion. Six days of CPU time were required on an IBMRS/6000–560 workstation for 2-D reactive flow simula-tions involving hundreds of time steps and hundreds oftriangular elements.

The fluid is assumed to remain saturated withrespect to galena and sphalerite within the basal aquifer(metal source region) in the foredeep of the Arkomabasin. Brines move updip, gradually cooling along theflow path. Figure 10 shows mineral concentration after100,000 yr of flow, and it indicates how significantenrichment of galena (PbS) ore develops in theViburnum Trend, where most of the cooling takesplace. Within the Trend, a second level of enrichmentdevelops over basement highs, not so much as a resultof cooling, but because of the way changes in ground-water velocity affect the rates of deposition due to thefocusing of fluid through the basal aquifer, as shown inFigure 11. These results imply that if the basinal brineswere saturated with respect to galena, then MVT oremineralization would be regionally concentrated insoutheast Missouri due to cooling along the flow path,and secondary enrichment could be attributed toincreased flow rates associated with basement highsthat serve to thin the basal aquifer and accelerate brinedischarge. High-permeability facies within carbonateformations, such as karstic channels or breccia zones,would be expected to have a similar effect in concen-

trating ore mineralization. Other geochemical modelsfor MVT ore deposition include aspects of sulfidereplacement, brine dilution, and fluid mixing, and someof these are evaluated in the hydrogeologic study byAppold (1998) for mineralization in southeastMissouri.

The problem of brine flushing is germane to the roleof topography as a driving mechanism for ore-fluidmigration in southeast Missouri. In fact, the sameproblem applies to other models for fluid migration,except for free convection, where the pore fluids arerecycled. At the low Pb concentrations assumed for thecooling simulation above, about 3 Myr of flow durationwould be required to account for the ore mass in theViburnum Trend in a topography-driven flow system.This constraint might preclude cooling as the solegeochemical mechanism, because brines would beflushed from the basin in about 1 Myr unless beddedevaporites provided additional NaCl (Deming andNunn 1991). Salt deposits do not exist in the Appala-chian–Ouachita basin today, yet perhaps they weredissolved hundreds of millions of years ago. Cathles(1993, 1997) reasons that many basins in NorthAmerica still contain connate brine that has not beenflushed and, therefore, he rightly questions the impor-tance of topography in ore genesis. It is argued herethat topography-driven flow fits most of the ore-genesispuzzle pieces together, but that the strongest fluxes inthese flow systems persisted for relatively short periodsof geologic time, perhaps less than 2 Myr; otherwise,the midcontinent basins would be flushed, as noted byCathles.

One novel way around the problem of brine flushingis to invoke free convection as the mechanism for fluidmigration. Deming (1992) conjectures the catastrophicrelease of heat from the deep basement by deforma-tion-induced free convection, and Spirakis and Heyl(1996) propose deformation-induced fracturing ofradioactive granites that permits localized convectionof brines. The greatest shortcoming of these models isthat exceptionally high permeability values arerequired at a regional scale to allow fluid convection;such values are several orders of magnitude abovemeasured permeability in fractured granite. Even ifthey did exist in the past under catastrophic circum-stances, severe mass-balance problems exist for metaltransport, because density-driven flow velocities wouldnot exceed the centimeter-per-year rate (Hazlett 1997).Radioactive granites could locally elevate heat flow,but the timing and spatial distribution of major MVTPb–Zn ore districts probably is not associated withintrusion of highly radioactive granites, except for asmall body in northern Illinois, which is of Precambrianage (Doe et al. 1983). But it is difficult to imagine howsuch a small body could account for the widespreadmineralization (10,000 km2) observed in the UpperMississippi Valley ore district.

Work by Hanor (1994, 1996) on the US Gulf Coastbasin suggests that brine development and Pb–Zn

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Figure 10 Distribution ofgalena (PbS) after 100,000 yrof reactive flow for thecooling simulation of theArkoma basin. Mineralconcentration units are mol/Lof bulk porous medium(bpm), as derived fromRST2D numerical model

Figure 11a,b Expanded viewof the reactive flow model ofthe Arkoma basin showing theeffects of cooling on galenamineralization after 100,000 yrof flow. a Distribution ofgalena, expanded fromFigure 10. Basement highs andsandstone pinchouts appear tocontrol the degree of minerali-zation in the ViburnumTrend. b Distribution oftemperature, with overlayfluid of velocity vectors.Largest vectors occur in theLamotte Sandstone aquifer

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Figure 12 Conceptual modelsfor ore genesis in the Carboni-ferous basin of Ireland. Bothtopography-driven flow anddensity-driven free convectionare implicated in the genesisof the Irish ores. Dark graytone is Hercynian basement;light gray tone includes OldRed Sandstone and Carbonif-erous limestone units. Arrowsdepict inferred patterns ofbrine migration in late Paleo-zoic time. (After Hazlett 1997)

mobilization develops early in basin history. Garven etal. (1993) adopt the same thinking as Hanor andpropose that brines formed in early Paleozoic timealong the continental margin, much like the Gulf Coasttoday. Some of these brines were driven away from theorogenic belts in late Paleozoic time toward the conti-nental interior to become the principal ore-formingsolutions. Fluid migration was a transient process,adapting to the regional dynamics of landscape erosion.Erosion diminished the hydraulic gradients shortlyafter the peak of tectonic uplift: subsurface flow rates inbasal aquifers declined from velocities of m/yr to cm/yror less. The reason that brines still exist in the remnantbasin foredeeps today is unknown, but perhaps theirexistence is related to dissipation of hydraulic gradientsby relatively rapid erosion of the landscape. Numericalmodels are only as appropriate as the initial andboundary conditions assumed for both the geochem-istry and hydrology, and so these numerical simulationscannot resolve the debate; however, they can showwhat is plausible in a hydrogeologic sense. Forexample, Appold (1998) numerically simulates thetransient history of fluid flow, heat transport, andsalinity in the Arkoma basin, based on the 2-D profilegiven in Figure 8. He observed that brines were indeedflushed from the foredeep after 2 Myr of flow,assuming the basin was tectonically uplifted at a rate of1 mm/yr basin-wide and that no supplement of NaCloccurred by dissolution of evaporite beds. These resultsimply that strong flow of the ore-forming brine musthave occurred in a time window of less than about1 Myr after initial subaerial uplift. The exact durationof mineralization in southeast Missouri is unknown.Ore mineralization is thought to be geologically rapid(0.5 Myr) in other MVT ore districts (Rowan and

Goldhaber 1995), so the issue of flushing in theViburnum Trend may not pose such a difficult problemafter all.

Irish Carboniferous Ore DistrictGenetic theories of fluid migration and ore formationin the Paleozoic carbonate strata of Ireland includeboth density-driven and topography-driven flowsystems; conceptual models for flow are illustrated inFigure 12. The occurrence of these ores is stronglycontrolled by basement faults. In one model, Lewis etal. (1995) proposes that seawater convected freelywithin the NE–SW planes of major faults, acquiringmetals at 10–15 km depths, and then discharged andreacted with reduced sulfur near the sea floor to formstratiform ore bodies in the rifted Irish midlands. Inanother conceptual model, Hitzman (1995) proposesthat basinal brines were driven northward out of theVariscan foreland basin by topography throughaquifers like the Old Red Sandstone. Epigenetic andstratabound ores formed in the Carboniferous strata ofthe Irish platform, where fluid discharge was focusedlocally by fault structures that acted as verticalconduits.

The role of density-driven convection is constrainedby fault permeability and crustal heat flow. Hazlett(1997) predicts free-convection flow rates of 0.8 m/yr todepths of 5 km, in cellular convection patterns,provided fault hydraulic conductivity exceeds 10 m/yr(permeabilityF30 md, or 3.3!10–14 m2) and the heatflow assigned to the base of 2-D grids exceeds 150 mW/m2 in the midlands, more than double the mean conti-nental heat flow. Temperatures approach 140 7C in thezones of upward flow, but large areas of the fault

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Figure 13a,b Basin-scale hydrogeologic simulation of the Irishbasin during the apex of the Variscan orogeny. a Distribution offluid flow lines assuming steady state. Elevated topographycauses deep flow in the Old Red Sandstone, leakage into theHercynian basement, and upward discharge along old NE–SWfault zones near Xp420 km. b Distribution of isotherms. SeeFigure 12 for the generalized geology along this section. (FromHazlett 1997)

Figure 14 a Hydrostratigraphy and b finite-element mesh for adeposit-scale reactive flow simulation of Lisheen, Ireland. (AfterHazlett 1997)

planes must subsequently be cooled in areas of down-ward flow, in order to achieve heat-balance in thesystem (Hazlett 1997).

Variscan-related topography-driven flow offersanother viable hydrogeologic model for Ireland. Onesouth–north late Paleozoic profile was discretized in afinite-element mesh with 20 rows and 140 columns ofquadrilaterals and modeled with RST2D; results areshown in Figure 13. The Old Red Sandstone is subdiv-ided into three hydrogeologic units such thatKp300 m/yr in the shallow beds, Kp30 m/yr at theintermediate depth, and Kp3 m/yr in the lowermostbeds. Underlying the Old Red Sandstone is meta-morphic basement, which is regionally fractured(Kp10 m/yr); major 1-km wide faults are conduits(Kp300 m/yr). Overlying the Old Red Sandstone arecarbonates (30 m/yr) and shale (3 m/yr). A uniformheat flow of 70 mW/m2 is assigned everywhere alongthe bottom of the mesh, temperature is 25 7C at the topsurface, and Kx/Kzp100 for all units. The topographyslopes from south to north at a gradient of about0.0057 m/m for about 350 km, and then it flattens at thebasin margin to represent an advancing orogenic belt.

Regional fluid flow is strongly focused by the upperpart of the Old Red Sandstone until it thins toward theedge of the Munster basin north of the Killarney-Mallow Fault, near xp300 km (Figure 13). The Hercy-nian basement is permeable enough to allow about140 m3/yr (per meter width) of brine to leak below the

sedimentary basin, where it eventually is focused byone of the conduit faults near xp425 km. Fluid veloci-ties reach 0.3 m/yr in the fault zone. Regional heat flowis elevated by the northward brine migration throughthe basin and basement, but the most noticeablegeothermal anomaly is created by discharge near thefracture zone. Temperatures of 200 7C occur at depthsless than 1200 m below the sea floor. Much higherdischarge temperatures could be achieved, but thisnumerical mesh does not allow for much finer resolu-tion of the fracture zone at this large scale, and theisothermal boundary condition on the mesh top wouldbe better represented with a radiative-type heat fluxcondition that would allow the discharge temperatureto increase toward hot-spring values. Besides the sub-200 7C temperatures, the model Variscan topographydoes not allow large brine flow rates in these steady-state simulations. Future modeling may require three-dimensional, transient simulations to investigate therole of combined or mixed convection where brines aredriven northward by Variscan topography, seepthrough the basement, and are focused into NE–SWfracture systems. There they convect more vigorouslydue to large density gradients and high permeability.Both Russell (1978) and Hitzman (1995) may bepartially correct, as neither driving hydrologic mecha-nism alone works extremely well in explainingcarbonate-hosted Pb–Zn ore genesis in Ireland.

Reactive flow experiments were developed for base-metal mineralization in fracture networks to test theefficiency of fluid mixing for ore precipitation at theformation scale. In Figure 14, a finite-element meshrepresents a 3-km long and 1-km thick geologic profileof a topography-driven flow system in the Irish

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midlands, where the stratigraphy (Figure 14a) has beenbroken by faults (assumed to be vertical for ease ofnumerical modeling). The rectangular mesh(Figure 14b) consists of 22 nodal rows and 69 nodalcolumns. A steady hydraulic-head gradient of 0.003 m/m is imposed across the profile from south to north(left to right), and the top and bottom edges of themesh are assumed to be impermeable to flow. Anisotherm and constant heat flow (70 mW/m2) areassigned to the top and bottom row of nodes, respec-tively. The hydrostratigraphy is similar to that of theZn–Pb–Ag deposit at Lisheen (Hitzman 1995): an oldmetamorphic basement is overlain by upper Paleozoicsandstone, carbonate, and shale, all of which are offsetby normal faults. The mesh is finer near the faults sothat bulk-permeability parameters can be assignedseparately to elements within the fault zone using aregular continuum approach, which is limited by theresolution of the mesh. True dual-porosity anddiscrete-fault hydrogeologic calculations are in Hazlett(1997). For the present simulation, the horizontalhydraulic conductivities K are as follows: Hercynianbasement, 0.3 m/yr; Old Red Sandstone, 300 m/yr;Ballysteen Limestone, 30 m/yr; Black Matrix Breccia,300 m/yr; Waulsortian Limestone, 30 m/yr; and theCross Patrick Shale, 1 m/yr. For these formations,Kxp10Kz at this scale. The three faults are treated asvertical conduits with Kxp300 and Kzp3000 m/yr.Thermal conductivities are the same values used beforefor these rock types, ranging from 2.0–3.0 W/m 7C, andporosity ranges from 0.05 (basement) to 0.25 (sand-stone). Solute transport across this flow field iscontrolled by advection and dispersion: all of theformations were assigned the same horizontal(εxp10 m) and vertical (εzp1 m) dispersivity (Garvenand Freeze 1984a).

The primary ore-bearing formation at Lisheen,Ireland, is the Black Matrix Breccia, which has beeninterpreted as a hydrothermally-altered dolomite at thebase of the Waulsortian Limestone (Hitzman et al.1992). The deposit contains sulfide zones 2–30 m thickthat form stratiform lenses and stratabound zones ofstockworks; these thicken on the hanging-wall (north)side of the normal faults. Ore solutions are thought tohave been guided up from the Hercynian basement andOld Red Sandstone along feeder faults to precipitateore in the lowermost ’’high-permeability’’ carbonatebed, in this case the Black Matrix Breccia. Secondarymineralization also occurs in permeable dolomite faciesof the Waulsortian Limestone. General agreementexists that basinal brines circulated within the Silu-rian–Ordovician basement rocks and that the Zn–Pbdeposits reflect mixing of two distinct fluids (Everett etal. 1997); perhaps one fluid was rich in base metal frommigrating through the basement and overlying sand-stone, and another fluid was rich in reduced sulfur.

In the model, south-to-north fluid flow is focused bythe Old Red Sandstone aquifer, where Darcy flow ratesof 1 m/yr carry metal-bearing brine into the ore district,

as shown in Figure 15a. Brine then encounters the firstfault (xp1100 m) and the flow field bifurcates; somebrine seeps down into the Old Red Sandstone andsome migrates up the fault to mix with fluid in theBlack Matrix Breccia. Fault displacement, fracturewidth, and the relative hydraulic conductivity of theargillaceous Ballysteen Limestone control how much ofthe aquifer flow is diverted upward into the BlackMatrix Breccia. The large contrast in permeabilitybetween the sediments and the faults provides for easeof vertical transport, although mostly horizontal flowoccurs in the formations. Discontinuities in formationproperties and flow patterns create a somewhatdisjointed temperature field, as shown in Figure 15b.

Reactive flow experiments help quantify the degreeof geochemical mixing associated with this hydrogeo-logic setting. Results are shown in Figure 16. For thisexperiment, it is assumed that two chemically distinctfluids are present in the two sedimentary aquifers, a Pb-bearing brine in the Old Red Sandstone and a H2S-bearing brine in the Black Matrix Breccia. The otherformations also contain a Na–Ca–Cl brine, are moder-ately acidic at pHp4.4, and are reduced, with log[O2]p–57.0 but with negligible aqueous base metals orsulfide. The assumed lead concentration is[Pb]p20 ppm in the Old Red Sandstone, and thereduced sulfur content in the Black Matrix Breccia ismaintained at [H2 S]p1.5!10–3 molal, prior to mixingduring flow. The solutions also are assumed to be inlocal chemical equilibrium initially with minerals suchas dolomite, calcite, quartz, feldspar, and muscovite. Atotal of 15 chemical components defines the geochem-ical flow problem. Aqueous complexing, speciation,and solute transport are numerically simulated withRST2D. Local chemical equilibrium is assumed asbefore, but now the fluids migrate and mix in thefaulted flow field. Dissolution and precipitation ofmineral phases is based on standard thermodynamicrules, as described in Wolery (1979) and Raffensperger(1996). Enough mineral mass is either dissolved orprecipitated per unit liter of porous meter to maintainlocal equilibrium at each time step.

Sulfate–sulfide–carbonate geochemistry in Missis-sippi-Valley-type lead–zinc deposits is reasonably wellunderstood (Sverjensky 1986; Anderson and Garven1987). Metal solubility is strongly controlled by salinityand temperature, and Pb, Zn, and other metals in solu-tion are transported as metal-chloride complexesbecause of the high ionic strength of Na–Ca–Cl brinesin basins. In this reactive flow simulation, PbCl4–2 is themain lead complex (Figure 16a). Mass-transport pro-cesses such as advection and hydrodynamic dispersionaffect the concentration pattern the most, with the bulkof the aqueous metal mass confined to formationsbelow the Black Matrix Breccia after 10,000 yr of flow.Some metal migrates up into the sulfide-bearinghorizon, but solute dispersion and galena precipitationserve to reduce aqueous lead concentrations because ofreactions such as:

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123


Figure 17a,b Distribution ofgalena mineralization for thedeposit-scale simulation ofLisheen, Ireland, showingeffects of mixing on ore depo-sition after a 10,000 yr and b50,000 yr of fluid flow

CaMg(CO3)2c2Pb2cc2 H2Sp(dolomite)

Ca2ccMg2cc2PbSc2CO2c2 H2O (5)(galena)

The precipitation of galena by the addition of thehydrogen-sulfide fluid near the mixing front releasesacid, which causes dissolution of dolomite such that onemole of dolomite is replaced for every two moles ofgalena precipitated. The same reaction mechanismapplies to sphalerite ZnS, the dominant sulfide ore atLisheen. Ore deposition therefore reduces the concen-tration of aqueous H2S (Figure 16b) near the mixingfront, causing even further dilution and displacement.Galena mineralization becomes concentrated in theBlack Matrix Breccia, as shown in Figure 17. This unitis assigned as a source bed of sulfide, although the massof ore produced is less than was expected. Feeder faultsplay an important role as a metal conduit for the BlackMatrix Breccia, but the hydrogeologic models do not

readily identify a superbly efficient mechanism for oredeposition via mixing. This limitation is probably dueto the way geochemical mixing is simulated here as aclassic problem in hydrodynamic dispersion or miscibledisplacement. In other words, a strong reduction ofmetal and sulfide concentration occurs where thesulfide minerals precipitate, and mass transport tendsto reduce the concentration gradients further by hydro-dynamic dispersion, as dictated in porous-mediatheory. In real hydrothermal environments, theplumbing system would be more complex and thehydrodynamics of mixing more complicated than thatpredicted by Fickian theory, as assumed in RST2D.Numerical schemes tend to smear out mixing fronts andthis also affects the wider patterns of mineralizationand reduces ore grade in the flow system. Despite theseshortcomings, however, the numerical experiments doremarkably well in predicting the general patterns ofmineralization and the likely time scales required forore formation.

124


Summary and Conclusions

1. The hydrodynamics associated with basin evolutioncreated favorable conditions for widespread tran-sient brine migration and sediment-hosted oregenesis in the Paleozoic basins of North Americaand Europe during periods of compressionaltectonism and continental uplift. Carbonate-hostedPb–Zn–Ba deposits are closely associated withregional aquifers of sandstone and carbonate thatfocused the transport of base metals from aquiferand aquitard beds in the foreland to the basinmargin, where the fluids cooled and mixed withother fluids in regional discharge areas. Continental-scale flow systems were strongest during periods ofmaximum relief soon after the apex of mountainbuilding, but this flow duration must have waneddue to erosion of the landscape after a few millionyears; otherwise, the resident basinal brines wouldhave been completely flushed and diluted byinvading meteoric groundwater.

2. In the Southeast Missouri Ore District, ore genesisaccompanied northward migration of brines drivenby uplift of the thick Arkoma basin after the Appal-achian–Ouachita orogeny in Permian time. Brinesmoved updip at rates of meters per year throughCambrian–Ordovician sandstone and carbonateaquifers toward the Viburnum Trend on the OzarkDome. Porous and permeable dolomite facies(Bonneterre Formation) of the 100-km long, 20-kmwide Viburnum Trend created a giant aquifer lensthat focused brines in a three-dimensionally-conver-gent flow system. Three-dimensional flow focusedmetal transport and elevated temperatures(Tp100–140 7C) in the large discharge area over theOzark Dome. Sandstone pinchouts and carbonatebreccia zones enhanced local fluid velocities, andthese factors controlled ore-mineralization grades atthe deposit scale. Reactive-flow simulations suggestthat cooling of a metal-sulfide-bearing brine explainssome of the geochemical features of these ores,although millions of years probably would havebeen required to form such large deposits at fluid-flow rates that probably did not last so long becauseof landscape erosion. Mixing of a sulfur-rich and ametal-rich fluid within the Viburnum Trend presentsa more attractive mechanism for ore deposition in ashorter period of time (Appold 1998).

3. In the Irish midlands district, ore genesis accompa-nied northward migration of brines driven by upliftof the western European Carboniferous basin duringadvancement of the Variscan thrust belt towardIreland in late Paleozoic time. Brines moved updipthrough the Old Red Sandstone and Carboniferouslimestone and dolomite, displacing resident brines inthe Hercynian basement in the heavily faultedmidlands. In many cases, these fluids convectedunder the influence of buoyancy within deep-seatedfracture zones connected to the leaky sedimentary

basin. Free convection (faults) and forced convec-tion (topography) probably worked in unison toelevate temperatures (Tp100–250 7C) in thedischarge areas. Fracture permeability allowed fordeeper metal-rich fluids, derived from reaction withthe basement and sandstone, to mix with sulfide-richfluids situated at shallow depths below the Carboni-ferous sea floor. Reactive flow simulations show onescenario where fault-controlled flow patterns werecrucial for fluid mixing and ore deposition in theBlack Matrix Breccia at Lisheen, albeit the fluidsmixed slowly.

4. The numerical methods presented here provide auseful tool for testing and comparing geochemicalmodels for sediment-hosted ore genesis. Newadvances have been highlighted to demonstrate theeffects of district-scale heterogeneity in permea-bility, the role of fault hydrogeology, and the scopeof reactive mass transport in the formation ofcarbonate-hosted ore. Unfortunately, many modelparameters and boundary conditions are not wellenough known in most paleohydrologic settings tomake accurate predictions in a true quantitativesense. Nevertheless, modeling has advanced to astage now where more detailed and complex simula-tions are being made that compare quite favorablywith field observations. Limitations in the theoryand applications still exist, but new frontiers areopen now for further study of groundwater and orehydrogeology.

Acknowledgments The first author is grateful to Ross Large forproviding facilities for writing this paper while on sabbatical leaveat the University of Tasmania. Constructive editing andthoughtful reviews were provided by Ward Sanford and BruceNesbitt, which greatly improved the manuscript. The authors arevery grateful also to Ed Sudicky for making available to us theUniversity of Waterloo software subroutines ORTHOFEM,which were used in building the finite-element program JHU3D.

The work reported here was supported by research grantsfrom the U.S. National Science Foundation (EAR-9117864 andEAR-9418011) and from the Petroleum Research Fund of theAmerican Chemical Society (ACS-PRF 30562-AC).

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