Geochemical and petrographic evidence for fluid sources and ...

GEOCHEMICAL AND PETROGRAPHIC EVIDENCE FOR FLUID SOURCES AND PATHWAYS DURING DOLOMITIZATION AND LEAD-ZINC MINERALIZATION IN SOUTHEAST MISSOURI: A REVIEW Jay M. Gregg Department of Geology and Geophysics University of Missouri-Rolla Rolla, MO 65401 and Kevin L. Shelton Stable Isotope Geology and Geochemistry Group Department of Geology University of Missouri-Columbia Columbia, MO 65211 ABSTRACT: Regional geological studies in southeastern Missouri have made a significant contribution to knowledge of the origin of Mississippi Valley-type mineralization and the late diagenetic history of sedimentary rocks throughout this part of the Midcontinent. In addition to mineral exploration, investigations such as these provide insights into basinal processes such as migration and distribution of petroleum, dolomitization, and porosity modification of carbonate petroleum reservoir rocks. Cathodoluminescent microstratigraphies of epigenetic dolomite cements indicate that mineralizing basinal fluids altered carbonate rocks over a much larger area than was affected by sulfide mineralization. Trace element distributions in southeast Missouri also indicate that sedimentary rocks, over a large region, were altered by basinal fluids from both a southern (Arkoma Basin) and a northern or northeastern (possibly the Illinois Basin) source. Fluid inclusions in sphalerites and dolomites throughout southern Missouri and northern Arkansas indicate that mineralizing basinal brines were very saline (-16 wt % equiv. NaCI] and warm (_80° to 150°C). The lack of discernible thermal gradients in fluid inclusion temperatures from regionally extensive epigenetic dolomites and the range of fluid inclusion salinities indicate that more than one basinal fluid was involved in dolomitization and associated Pb-Zn ore deposition. Carbon and oxygen isotope compositions of dolomites show that basinal fluids evolved in chemical compositions with time and indicate interactions of fluids from at least two sources. Studies of strontium compositions of dolomite cements south and east of the Viburnum Trend subdistrict provide further evidence of an Arkoma-Ouachita source of one of these fluids. Lead isotope compositions of galenas, however, indicate multiple sources: I) the Illinois Basin, 2) the Arkoma Basin, 3) and the underlying local granitic basement. Distinct northern and southern basinal sources are postulated for sulfur based on sulfur isotope data from galenas. Taken as a whole, available geochemical and petrographic evidence indicate that dolomitization and Pb-Zn mineralization in southeast Missouri were the result of a more complex, multiplebasin fluid interaction than previously recognized. INTRODUCTION Mississippi Valley-type (MVT) mineralization is observed commonly in cratonic carbonate rocks throughout the world (Anderson and MacQueen, 1982; Sverjensky, 1986). These deposits

Carbonates and Evaporites.

v. 4, no. 2, 1989, p. 153-175

are characterized by the occurrence of epigenetic galena, sphalerite, pyrite, chalcopyrite, and other metal sulfides in carbonate rocks (and less commonly in sandstones). Gangue minerals in these deposits may include barite, fluorite, and epigenetic dolomite and calcite cements. Petroleum is also associated commonly with MVT mineralization (Fowler ,1933; Anderson and

154

JAY M. GREGG AND KEVIN L. SHELTON

Macqueen, 1982; Sverje nsk y, 1984; Mazzullo, 1986). MVT ore deposits and occurrences are usually distributed on shelf areas surrounding sedimentary basins, and are believed to have precipitated within a few hundred meters to a kilometer of the surface at temperatures of 75° to 140°C (Anderson and Macqueen, 1982; Cathles and Smith, 1983). Recent studies (e.g. Bethke et al., 1988; Gregg and Shelton, 1989) indicate that MVT mineralization is the result of a complex interrelationship of regional tectonics, which affect the timing and pathways of sedimentary basin dewatering, and local subsurface geochemical conditions. Thus, the study of MVT mineralization can provide important information about the timing and pathways of basin fluid migration (including petroleum) and burial diagenesis of carbonate reservoir rocks, in addition to the more obvious application to exploration for base metals in sedimentary provinces. In this paper we review the literature on those aspects of the geochemistry and petrology of the southeast Missouri MVT district which relate to the regional hydrologic events that produced the ore deposits on this part of the midcontinent. These studies are pertinent to other geological provinces where similar late diagenetic events may have modified sedimentary rocks during burial diagenesis and affected the timing and pathways of hydrocarbon migration. GEOLOGICAL SETTING The southeast Missouri mineral district occurs within and around the St. Francois Mountains of the Ozark Uplift which, in turn, is part of the stable cratonic interior of North America. The Ozark Uplift is bounded on the northeast by the Illinois Basin, on the northwest by the Forest Ci ty Basin, and on the south by the Arkoma Basin and Ouachita Mountains (Fig. I). The geologic history of the region was discussed by Thacker and Anderson (1977). The lead-zinccopper sulfide ore bodies of southeast Missouri are hosted primarily by the Upper Cambrian Bonneterre Dolomite, with some mineralization also occurring in the underlying Upper Cambrian Lamotte Sandstone and overlying Upper Cambrian Davis Formation (Snyder and Gerdemann, 1968). The Bonneterre Dolomite was deposited adjacent to the St. Francois Mountains (composed

ARKOMA

BASIN

OUACHITA MOUNTAINS

N

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Figure 1.-- Regional tectonic map showing relationships of MVT districts and the Ozark Uplift to surrounding sedimentary basins. The igneous St. Francois Mts. is the hachured area between the Old Lead Belt and the Viburnum Trend. The inset west of the St. Francois Mt s. covers the approximate area shown in figure 5. Modified from Gerdemann and Gregg ( 1986).

of Precambrian igneous rocks) which was an area of high relief, forming islands in the late Cambrian sea. Carbonate lithologies in the Bonneterre include dolomitized cryptalgalaminates (planar stromatolites) and partially dolomitized lagoonal mudstones occupying a restricted platform "back reef" facies in the nearshore and inter-island area (Figs. 2 and 3). Digitate algal stromatolite bioherms and associated oolitic and skeletal grainstones and wackestones were deposited along the platform edge and are dolomitized pervasively. Initial dolomitization of the platform facies of the Bonneterre may have occurred soon after deposition, either by mixed marine and meteoric water or by seawater modified by evaporation (Gerdemann and Gregg, 1986). Further dolomitization

FLUID SOURCES AND PATHWAYS, SE MISSOURI

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and neomorphism of preex istmg dolomite occurred in the subsurface during the period of sulfide mineralization (Gerdemann and Gregg, 1986; Gregg and Shelton, in editorial review). Offshore, the Bonneterre grades into a deeper-water, open shelf facies composed of interbedded limestones and silty green shales (Gregg, 1988). This offshore facies continues upward into the Davis Formation which forms a transgressive unit overlying the plaform facies of

the Bonneterre. The Bonneterre is conformably underlain by the Lamotte Sandstone (quartzurenites and arkoses) except near Precambrian highs, where it sits directly on Precambrian basement. Ore-grade mineralization in the Bonneterre is restricted largely to the dolomitized platform edge facies, particularly the algal bioherms and associated grainstones. The association of particular dolo mote lithologies with mineralization, and the recognition of local structural control on are


156

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bodies, has been used as an exploration guide in southeast Missouri (Ohle and Brown, 1954; Snyder and Gerdemann, 1968; Gerdemann and Myers, 1972).

HYDROGEOLOGY OF THE MINERALIZING FLUIDS Early workers in the southeast Missouri mineral district believed that mineralizing fluids originated deep in the underlying Precambrian Igneous rocks (Ohle and Brown, 1954). Later, Snyder and Gerdemann (1972) postulated that mineralizing fluids originated in adjacent sedimentary basins. This theory has been supported by many subsequent studies which show that the mineralizing fluids were metal-bearing, chloriderich basinal brines, in many respects similar to some oil-field brines (Sverje nsk y, 1984). Geochemical studies of basinal brines show that they have the characteristics of complex mixtures of meteoric and connate waters, modified by rockwater interactions in the deep subsurface (Clayton et al., 1966; White, 1974; Carpenter et al., 1974). are precipitation occurred at temperatures generally between 75° and 140°C (Anderson and Macqueen, 1982). The mode of metal transport and precipitation, and the source of sulfur for MVT deposits, however, are still being debated (Jackson and Beales, 1967; Sverjensky, 1981).

Petrographic Evidence for Fluid Pathways Hagni and Trancynger (1977) interpreted zoning patterns of ore minerals in the Bonneterre Dolomite to indicate an upward movement of mineralizing fluid out of the underlying Lamotte Sandstone. A generalization of data on asymmetry of mineral growth patterns in the Viburnum Trend ore bodies has been interpreted to indicate important northerly and southerly fluid flow pathways, during mineralization, in addition to the vertical component of flow (Fass and Hagni, 1979; Fass, 1980). This view appears to be supported by mineral zonation and distribution of trace occurrences of metals in mines of the Viburnum Trend (Hagni, 1983). A distinctive cathodoluminescent (CL) microstratigraphy was observed in ore-associated gangue dolomite cement in all of the Viburnum Trend ore bodies by Voss and Hag n i (1985). Individual CL zones in the gangue dolomite cement have been correlated with distinct periods of sulfide mineralization and collapse brecciation in the mineral district (Voss et al. this issue). Complex (non-rhombic) banding in the cathodoluminescent zonation of the southeast Missouri district may be due to the high metal content of the saline brines that precipitated the dolomite (Gregg and Hagni, 1987).


The distinctive CL micostratigraphy in open-space-filling, dolomite cement has been traced throughout southern Missouri at the Lamotte-Bonneterre contact (Gregg, 1985; Farr, this issue; Voss et al ., this issue) and into northern Arkansas (Rowan, 1986). Voss et al. (this issue) observed a decreasing number of alternating luminescent and non-luminescent bands, in their zone 3 dolomite, from south to north in the Viburnum Trend and from the back reef westward into the offshore facies. Farr (this issue) reasons that such a decrease in banding would occur in the down-flow direction of the precipitating fluid. This suggests a southeasterly to northwesterly fluid flow during the time of zone 3 dolomite precipitation. We have observed the characteristic dolomite cement stratigraphy in rocks from the back reef facies of the Bonneterre (Gregg and Shelton, in prep.) from the Fredericktown and Old Lead Belt subdistricts (Fig. 2), and as high in the stratigraphic section as the Lower Ordovician Gasconade Dolomite. These same dolomite occurrences are associated with traces of sphalerite and galena. These observations, as well as those of the other workers discussed above, indicate that mineralizing fluids affected a much larger area than that covered by ore-grade mineralization. Hydrologic Models for Fluid Flow

Several mechanisms have been suggested to explain the movement of mineralizing fluids from sedimentary basins to adjacent platforms. Snyder and Gerdemann (1968, p. 357) proposed that brine movement out of the neighboring Forest City and Illinois Basins "... was probably initiated by uplift and erosion of the Ozark Dome, accompanying circumferential faulting, that upset hydrodynamic stability." They believed that water expelled during initial compaction of shales predated mineralization and was unlikely to have precipitated the ore deposits. Geopressured sections of upper Paleozoic rocks in the Ouachita basin are cited as possible sources of mineralizing fluids by Sharp (1978). Cathles and Smith (1983) state that such fluids may have been released episodically over a period of several million years. The resulting rapid migration of warm mineralizing fluid from deep sedimentary basins would explain the anomously high temperatures, determined by fluid inclusion thermometry, associated with MVT mineralization (Roedder, 1977;

157

Hagni, 1983). A drawback to this model, termed "lateral secretion", is that only a limited amount of connate water is available in sedimentary basins unless they have a recharge mechanism. The question then emerges: was enough water available in the Arkoma-Ouachita Basin for the large-scale mineralizing events, including regional epigenetic dolomitization, in southern Missouri? Bethke et al . (I988) calculated that only minor geopressured zones, that might have expelled fluids, formed in the Arkoma Basin during sedimentation. Oliver (I986) proposed that continental collision in the Ouachita region resulted in lateral migration of fluid. It is unlikely, however, that these fluids were responsible for ore formation because flow rates would have been too slow to allow for heat transport to the site of mineralization (Bethke, personal communication, 1988; Bethke et al., 1988). A numerical computer model presented by Garven and Freeze (1984) shows how fluid may flow upward out of a deep foreland basin, and onto adjoining shelf areas. This flow is driven by a hydrostatic head established in meteoric recharge areas in mountains on the opposite side of the basin (see also Bethke et al., 1988). A similar model was proposed by Bethke (1986) to explain fluid flow from the intracratonic Illinois Basin to the upper Mississippi Valley district. Computer simulations show that fluid migration by gravity flow is rapid enough for brines to have moved from deep sedimentary basins to the site of MVT mineralization without significant cooling (Garven and Freeze, 1984; Bethke et al., 1988). The nearly unlimited volume of water available through such a gravity flow mechanism, by meteoric recharge, makes this a more attractive explanation for the large-scale regional events associated with southeast Missouri mineralization, than does fluid flow by lateral migration from geopressured zones. Timing of Fluid Flow

Timing of fluid flow in southeast Missouri is thought to be Late Pennsylvanian to Early Permian in age. Studies of iron-bearing minerals in the southeast Missouri MVT district indicate a Pennsylvanian to Early Permian paleomagnetic pole during mineralization (Wu and Beales, 1981; Wisnioweki et al., 1983). Potassium-argon analysis of clay minerals associated with galena in

158


the Lamotte Sandstone (Rothbard, 1982) and authigenic K-feldspar in the Bonneterre Dolomite (Hearn et al ., 1986) are consistent with a Late Paleozoic fluid migration event. A late Paleozoic age for Pb-Zn mineralization in southeast Missouri coincides with regional tectonic disturbances, including uplift of the Ouachita Mountains. The Ouachita region (among others) may have provided a recharge area for meteoric waters for a postulated gravitydriven fluid flow system out of nearby basins (Bethke et al .. 1988). Fluid inclusion and K-Ar studies of Ba-rich adularia and quartz veins in the Ouachita Mountains by Shelton et al. (1986a) indicate that post-collisional hydrothermal systems of Early Permian age may have a genetic tie to expulsion of basinal brines from the Arkoma Basin and the subsequent formation of MVT PbZn-Ba deposits. Bethke (1986) suggested that uplift of the Pascola Arch during the Ouachita orogeny may have permitted Illinois Basin fluids to flow northward and deposit Pb-Zn ores in the Upper Mississippi Valley district. Uplift of other such features may have permitted fluid contributions from multiple basins surrounding the southeast Missouri Pb-Zn district during the late Paleozoic. TRACE AND MINOR ELEMENT STUDIES

A number of studies have addressed the problem of distribution and pathways of mineralizing fluids in southeast Missouri using geochemical methods. Insoluble residue chemistries from lower Paleozoic carbonates from throughout southern Missouri were examined by Erickson et al. (1983). They found enrichments in Pb, Zn, Cu , Ni , Co, Mo , and Ag in insoluble residues within and near areas of known mineralization, in the vicinity of the St. Francois Mts. A second area of enrichment was observed in south-central Missouri near the Arkansas border. They suggested that these areas of metal enrichment were markers left by migrating mineralizing brines. More recent studies by Erickson et al. (1989) have identified specific multiple basin sources (the Arkoma and the llIinois Basin-Reelfoot Rift system) for the lead-mineralizing brines. Graf (1983; 1984) and Graf and Alcott (1986) examined rare earth element (REE) patterns in mineralized and unmineralized zones of the Bonneterre Dolomite. They found that REE patterns of apparently recrystallized host dolomite

differed significantly from those of unaltered dolomite, and that epigenetic open space-filling calcites had low La/Sm values both within and outside of ore bodies in the Viburnum Trend. The REE patterns were believed to have been imparted by interaction of the host rock with circulating ore solutions depleted in light REE. Graf (1984, p. 320-321) further asserted that, "When good data exists regarding the partitioning and solution behavior of REE ... [they] might serve as useful tracers of solution flow." Major, minor, and trace element contents from both carbonate and insoluble residue constituents of the Bonneterre Dolomite were studied by Viets et al., (1983), in samples collected from 6 cores in an east-west section across the Viburnum Trend. Decreasing contents of Fe, Mn, Pb, Zn, and Cd were observed in the carbonate component of the samples with decreasing vertical distance away from the Lamotte Sandstone. This pattern was interpreted to result from partitioning of the metals into dolomite as the fluid moved upward from the Lamotte Sandstone. High metal-sulfide contents were observed in insoluble residue fractions from the middle Bonneterre, corresponding to the relative location of ore-grade mineralization in nearby ore bodies. We systematically sampled a core that penetrated the Bon neterre in the Viburnum Trend, at approximately 3 m intervals, from the underlying Lamotte Sandstone to the overlying Davis Formation. Individual crystals of replacement dolomite were analyzed using an electron microprobe, and the data are presented in Table 1. Both Fe and Mn contents of the samples decrease away from the Lamotte Sandstone (Fig. 4; Table I). This trend, and a possible increase in Sr content of replacement dolomite were also observed by Buelter (1985). These data are in agreement with Viets et al. (1983), and are interpreted to indicate that fluids moved upward from the Lamotte Sandstone aquifer. We did not observe a corresponding decrease in Fe and Mn contents of dolomite cement with vertical distance from the Lamotte. This may be because the total amount of dolomite cement precipitated over the relatively small vertical distance (85 m) was not enough to significantly alter the composition of the fluid. An alternative interpretation for these data is that increasingly higher energy depositional environments upward in the section, from open shelf facies, lime mudstones and wackestones to

159


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Figure 4.-- FeCO3 and MnCO 3 content of the Bonneterre Dolomite in the Viburnum Trend relative to distance above the Lamotte Sandstone contact. platform edge grainstones, resulted in increasingly more oxidizing conditions, and therefore less Fe 2 + and Mn 2 + available for incorporation into the dolomite. A concurrent increase in Sr 2 + with decreasing Fe and Mn would argue against this alternate hypothesis. Available Sr data (Buelter, 1985) are inconclusive on this point. A zone of enrichment in Mn, Fe, Zn and to a lesser extent Na, K, and As exists in shales of the Davis Formation overlying the Buick Mine ore body in the Viburnum Trend (Panno et 01., 1988). Alteration of the shales is believed to have been caused by upwelling mineralizing fluids from the underlying Bonneterre. These fluids then moved laterally at the BonneterreDavis contact and up into the Davis along fracture permeability created by solution collapse of underlying dolomite during the ore-forming process. Panno et al. (1988) suggested that Fe and Mn enrichments in the Davis shales may serve as an exploration guide to underlying breccia-hosted ore bodies. They also provide evidence that the distribution of arsenic may be useful as an indicator of fluid flow pathways in southeast Missouri and elsewhere. This study, in conjunction with the work of Viets et al. (1983), Buelter (1985), and our own data (Table I, Fig. 4) appear to support the view of a strong vertical component to fluid movement in the southeast Missouri district. The fluid (or fluids) altered not only the platform carbonates of the Bonneterre,

but the overlying Davis formation as well. The immediate source of fluid would have been the underlying Lamotte Sandstone. A reconnaissance study of trace element distributions in the Bonneterre dolomite in southeast Missouri was made by Buelter (1985) (also Buelter and Guillemette, 1988). He observed an apparent regional south-to-north decrease in Fe and Mn (from 1.47 wt % Fe and 0.30 wt % Mn, in the south, to 0.23 wt % Fe and 0.03 wt % Mn, in the northwest) and a corresponding increase in Sr (from 8 ppm to >100 ppm), in the basal dolomite of the Bonneterre (Gregg, 1985), west of the Viburnum Trend. Applying trace element partitioning theory (Veizer, 1983), he interpreted these data to reflect fluid flow northward from the Arkoma Basin during mineralization. Low Fe and Mn values in the back reef facies of the Bonneterre were attributed to dilution of basinal fluids by meteoric waters. Citing the studies of Buelter (1985) and Buelter and Guillemette (1988) as a starting point, Gregg and Shelton (1989) analyzed samples of replacement dolomite taken at the LamotteBonneterre contact in 35 drill cores covering an area of more than 25,000 km 2 west of the St. Francois Mountains. They found distinct regional trends in Fe, Mn, and Sr contents of the dolomite from values of 3.47 wt % Fe, 0.38 wt % Mn, and 27 ppm Sr, in the south, to 0.82 wt % Fe, 0.12 wt % Mn,and 54 ppm Sr, in the north. In and near

160


TABLE 1 COMPOSITION OF REPLACEMENT DOLOMITE RELATIVE TO DISTANCE ABOVE THE LAMOTTE-BONNETERRE CONTACT SAMPLE DISTANCE (Meters) 1

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MOLE PERCENT CARBONATE FeC03 MgC03 MnC03

50.03 49.70 50.52 50.71 49.79 49.75 50.21 49.62 49.96 50.40 50.99 50.11 50.18 50.37 50.66 50.25 50.58 49.71 49.89 50.98 49.75 49.76 49.65 50.31 50.51 49.59 50.82 51.09

the Viburnum Trend lead-zinc subdistrict they found that the distributions of Fe, Mn, and Sr are reversed, from values of 0.50 wt % Fe, 0.10 wt % Mn, and 105 ppm Sr , in the south to 3.15 wt % Fe, 0.55 wt % Mn, and 40 ppm Sr, in the north. Fe and Mn contents of gangue dolomite cement in the Viburnum Trend ore bodies show a similar south-to-north enrichment ranging from lAI mole % FeCO s and 0.11 mole % MnCO s in the south to 2.34 mole % FeCO s and 0.23 mole %

45.37 47.16 48.69 44.30 44.70 48.51 48.65 48.71 47.95 47.90 46.64 49.16 47.95 48.95 49.02 48.42 48.41 49.64 49.21 47.98 49.15 49.44 49.78 49.14 49.14 49.95 49.04 48.73

3.80 2.61 0.58 4.17 4.60 1.20 0.71 1.12 1.35 1.15 1.62 0.56 1.45 0.44 0.19 1.02 0.76 0.42 0.68 0.78 0.83 0.62 0.39 0.42 0.28 0.36 0.08 0.11

0.80 0.53 0.21 0.82 0.90 0.53 0.43 0.54 0.74 0.55 0.75 0.17 0.42 0.23 0.13 0.31 0.25 0.22 0.22 0.26 0.27 0.17 0.18 0.31 0.06 0.10 0.06 0.06

MnCOs in the north. This latter enrichment was also observed by Ramage (1984; also Ramage and Holland, 1984). Based on fractionation of minor and trace elements between fluids and dolomites, the data of Gregg and Shelton (1989) are interpreted to indicate a regional south-to-north flow of water from the Arkoma Basin, through the Lamotte Sandstone (Fig. 5). A second fluid flow system, with a northern or northeastern source, precipitated the late phases of gangue dolomite

FLUID SOURCES AND PATHWAYS. SE MISSOURI

·.8

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Figure 5.-- Contour map showing the distribution of Fe in dolomite at the Lamotte-Bonneterre contact (for location see inset in Fig. 1). A dashed line separates a regional area dominated by decreasing Fe from south to north from the Viburnum Trend area, dominated by increasing Fe from south to north. These respective areas correspond generally to sedimentary rocks of the offshore, open shelf facies and nearshore, platform facies. Wider lines with arrows illustrate hypothesized fluid-flow paths of two basinal fluids, one from the south (Arkoma Basin) and one from the northeast (Illinois Basin). After Gregg and Shelton ( 1989).

cement in the Viburnum Trend and may have been active during the earlier precipitation of dolomite at the Lamotte-Bonneterre contact (Gregg and Shelton, 1989). FLUID INCLUSION STUDIES

It is generally accepted that dolomitehosted MVT lead-zinc deposits of southeast

161

Missouri formed from basinal brines (see Sverjensky, 1986). This belief is based principally on studies of fluid inclusions in ore and gangue minerals (see Leach and Rowan, 1986, and references therein). However, there is much less agreement on the sources, flowpaths, and timing of fluid migration, and virtually every other aspect of ore genesis. Early investigations of fluid inclusions in the Bonneterre focussed on their occurrence in sphalerites from the Viburnum Trend (Roedder, 1977). These studies concluded that mineralization formed from extremely saline brines (>16 wt % equivalent NaCl) at temperatures between 94° and 137°C. However, sphalerite from pyritesphalerite-calcite veins in the Precambrian basement beneath Pb -Zn mines trapped hotter (148° to 152°C), more dilute fluids (4.2 to 5.6 wt % equivalent NaCl). Roedder (1977) interpreted these inclusions to indicate an earlier period of zinc deposition from fluids unrelated to the main, later period of ore fluid migration and mineralization. Alternately, these data have been reinterpreted by Bauer (1989) to indicate the presence of a second fluid during ore mineralization. Subsequent studies of widespread minor and trace occurrences of sphalerite in central and western Missouri and other midwestern localities (Leach, 1979; Coveney and Goebel, 1983; Coveney et al., 1987) have demonstrated homogenization temperatures and salinities similar to those observed in fluid inclusions in adjacent mining districts. These results indicate that mineralizing brines were in thermal equilibrium with an enormous volume of rock throughout the midcontinent of the United States, in addition to major lead-zinc mineral districts. Studies by Leach (1979), Viets et al. (1984) and Leach and Rowan (1986; also Leach, 1979) have proposed that many MVT lead-zinc occurrences of the Ozark region of Missouri and Arkansas share a common genetic link with Ouachita tectonism. Hot, subsurface brines are thought to have migrated onto the southern flank of the North American craton in response to deformation of the Arkoma Basin and uplift of the Ouachita Mountains following the late Paleozoic collision of the Llanorian and North American plates (Shelton et al., 1986a). Leach and Rowan (1986) and Bethke et al. (1988) have emphasized that an apparent south-to-north decrease in mean homogenization temperatures of fluid inclusions in sphalerite

162


(from the northern Arkansas district through the Tri-state and Viburnum Trend districts to the central Missouri district), and remarkably uniform salinities, in conjunction with a systematic shift from higher to lower Na/K ratios of fluid inclusions (Viets et al., 1984) indicated a single south-to-north migration of fluids from the Arkoma Basin. Bethke et al. (1988) went so far as to propose a thermal gradient of 0.09°Cjkm based on these data. Studies by Hagni (1983); however, have documented that temperature variations within and between various periods of sphalerite and quartz mineralization in the Viburnum Trend district were as great or greater than variation between districts. The results of Hagni's studies call into question the utility of comparing homogenization temperature ranges, or mean homogenization temperatures, of major lead-zinc districts in definitively reconstructing fluid migration directions. The model of a single south-to-north movement of basinal fluids in southeast Missouri is further questioned because it is based mainly on studies limited to inclusions in sphalerite, which represent only a portion of the mineralization history of a particular district. Furthermore, sphalerite occurrences are restricted geographically and are not continuous. The major sphalerite occurrences (in ore districts) display a wide vertical stratigraphic range (Cambrian to Mississippian age rocks), yet interpretation of their fluid inclusion temperatures has failed to account for possible temperature variations vertically within the stratigraphic section. Fluid inclusion studies of dolomites in the host Bonneterre show more promise as a guide to reconstructing fluid migrational pathways in southeast Missouri because the dolomites are geographically continuous, they occur at one stratigraphic level, and record a more complete history of fluid movement and fluid-rock interaction. Bauer (I989) showed that there is no discernible temperature gradient regionally in fluid inclusion homogenization temperature data from pre-ore replacement dolomite and syn- and post-ore dolomite cements. These results demonstrate that a simple steadystate model of fluid flow out of the Arkoma Basin cannot alone account for dolomitization and associated Pb-Zn mineralization in southeast Missouri. More recent studies of fluid inclusions have suggested that Pb-Zn mineralization in southeast Missouri is likely the result of multiple

fluid interactions. Studies of fluid inclusion gas geochemistry (Hofstra et al., 1989) found anomalously high CO 2 contents (up to 10 mole %) and small amounts (generally