I ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLO.GISTS VOL. 81 MARCH-APRIL, 1986 No.2 Hydrologic Constraints on the Genesis of the Upper Mississippi Valley Mineral District from Illinois Basin Brines CRAIG M. BETHKE Hydrogeology Program, Department 0/ Geology, 245 Natural History Building, University 0/ Illinois. Urbana. Illi"ms 61801 Abstract Mississippi Valley-type deposits of the Upper Mississippi Valley mineral district probably formed during a period of regional ground-water flow across the Illinois basin initiated by uplift. of the Pascola arch in post-Early Permian and pre-Late Cretaceous time. Numerical modeling of this inferred paleohydrologic regime shows that district, temperatures attained by this process depend on flow rates through the basin, heat flow along flow paths, and presence of structures to cause convergence and upwelling of fluids. Predicted flow rates and timing of mineralization agree with previous estimates. Modeling results also offer explanations of banding in district mineraliza~on and district silicification patterns. Modeling of ground-water flow due to sediment compaction during basin subsidence, however, shows that this process was not responsible for mineralization. Fluids displaced from the deep basin by compaction­ driven flow moved too slowly to avoid conductive cooling to the surface before reaching the district. Episodic dewatering events are unlikely to have occurred, because the basin did not develop significant overpressures during subsidence. Results of the compaction-driven flow modeling probably also apply. to the Michigan and Forest City basins. Study results suggest that exploration strategies for Mississippi Valley-type deposits should account for tectonic histories of basin margins distant from targets. ' Three variants of the basinal brine theory currently Introduction exist. The "stratifugic" flui~ variant holds that deep ORES of the Upper Mississippi Valley, or Wisconsin­ fluids move toward basin margins as a result of sedi­ Illinois, mineral district are well-studied examples of ment compaction during basin evolution (Noble, Mississippi Valley-type ore deposits. Mississippi Val­ 1963; Jack~on and Beales, 1967; Dozy, 1970). In a ley-type deposits contain epigenetic, hydrothermal second, episodic dewatering variant, sediment com­ ores, both. as open-space-filling and replacement paction in basins causes high excess pore pressures, mineralizatio~. The ores are commonly found in shal­ such as those observed in the U. S. Gulf Coast (Dick­ lowly buried rocks on margins of sedimentary basins inson, 1953), .which drive sudden bursts of deep (Anderson and Macqueen, 1982). Fluid inclusions brines toward basin margins (Sharp, 1978; Cathles from these deposits typically give temperature esti­ and Smith, 1983). A third variant, suggested nearly mates of 100° to 150°C and are filled with predom­ a century ago (Daubree, 1887; Cox,. 1911; Siebenthal, inantly Na-Ca-CI brines containing> 15 wt percent 1915) and recently revived and revised by Garven salts (Roedder, 1979a, p. 95). Mississippi Valley-type. and Freeze (1984a and b) and Garven (1985), holds ore-f()rming fluids differ significantly in chemical and that gravity-driven ground-water flow due to topo­ isotopic coinposition from fluids which form other graphic differences across basins carries warQl fluids types of hydrothermal deposits (Roedder, 1979b), but from de~p strata into shallow sediments. Evaluating many workers have compared these fluids to modern each variant is important to understanding relation­ oil-field brines (White, 1958; Hall and Friedman, ships of parent basins to mineralization and refining 1963; Giordano, 1978; T~ylor, 1979; Sverjensky, exploration strategies for Mississippi Valley-type. de- 1984). This similarity is partly responsible (Ohle, posits. , , 1980) for the present popularity of the basinal brine This paper uses numerical modeling techniques to theory of Mississippi Valley-type genesis, suggested study possible hydrologic mechanisms of forming the by White (1958). According to this theory, warin Upper Mississippi Valley district from brines derived sedimentary brines from nearby basins carry dissolved in the Illinois basin. The Illinois basin is the most metals and possibly sulfur from depth to baSin mar­ plausible source of ore-forming fluids because of its gins, forming ore deposits. proximity to the district, its depth, and the presence 0361-0128/86/502/233-17$2.50 233 234 CRAIG M. BETHKE of mineralization within the basin which seems ge­ Upper Mississippi Valley Mineral District netically related to the Upper M~ssissippi Valley dis­ The Upper Missi~sippi Valley mineral district (Fig. trict. Modeling of compaction-driven ground-water 1) is a widespread ore district covering 8,000 km2 in flow shows that the stratifugic fluid variant is not vi­ athin «1 km thick) veneer of Cambrian through Si­ able because fluids move slowly and cool by conduc­ lurian strata overlying the Wisconsin arch in Wiscon­ tion long before reaching host rocks at the basin mar:­ sin and Illinois (Heyl et al., 1959; ,Heyl, 1968). The gin. Calculations also show that, due to slow burial district contains primarily zinc and lead ores, mos~ly rates and low shale content, the Illinois basin was not deposited as open-space fillings, in pitches and flats, overpressured during its c!lmpaction. This result in­ tectonic and solution breccias~ veins, joints, and vugs. dicates that episodic dewatering events are unlikely Heyl et al. (1959) identified silicified and~inerali~ed to have occurred. Other possible fluid sources, such veins and fractures extending into upper Cambnan as the Michigan and Forest City basins, are tectoni­ sandstones, the stratigraphically lowest sediments in cally similar to the Illinois bliSin and also seem unlikely the Illinois basin, as the "mineralized roots" of the to have formed the Upper Mississippi Valley district ore district. by compaction flow. Noneco~omic mineralization occurs over a much Models of gravity-driven ground-water flow, how­ larger~ ioo,ooo km2 area in Wisconsin, Illinois, Iowa, ever, demonstrate that this process, possibly resulting and Minnesota. The crystal habit, paragenesis, and from tectonic events in the southern basin, is capable trace element and isotopic composition of much of of having introduced warm fluids to the Upper Mis­ this mineralization is similar to Upper Mississippi sissippi Valley district. Gravity-driven flow due to to- Valley ores (Heyl and West, 1982; Garvin ~t al., o: pographic differences. across basins is the preferred 1985). Epigeneti.c sphalerite also occurs in Pennsyl­ variant of the basinal brine theory in the Illinois basin, vanian coals and Mississippian-Pennsylvanian con­ and by inference in other intracratonic environments. cretions on the western shelf of the Illinois basin. This result has important implications for Mississippi Cobb (1981, p. ·181-186) interprets these occur­ Valley-type exploration. rences to be genetically related to district o~es. Although located in shallow sediments, the district was extensively heated during mineralization. Fluid inclusions from or,e minerals give estimates of tem­ peratures during deposiqon of 75° to 220°C ~New­ ,.-~ house, 1933; Bailey and Cameron, 1951; McLlmans~ 1977). These values agree with temperatures of 52 MN r "- WI to 227°C calculated from sulfur isotope compositions \ J'-", ....... .... of coprecipitated galena and sphalerite (McLimans, , 1977, p. 109....:131). Measurements of fluid inclusions --~,--, '--------~, from sphalerite in coal give estimates of 75° to 113°C, \ "OUTLYING " and vitrinite reflectance of host coals also suggests \ I heating to 70° to 110°C (Cobb, 1981). Fluid inclu­ \ , , \ sions in diagenetic cements from shallow «450 m \ \ deep) Mississippian strata in ~estern lI,inois give es­ \, ----,\. timates of 85° to 109°C (Smith et al., 1984). , IA , I McLimans et al. (1980) show that coloJ: banding i~ I I district sphalerites forms a distinct paragenetic stra­ " MINERALIZATION ,/ t )-- tigraphy which can be traced for many kilometers in \ I "II' IL some cases. They argue that paragenetic uniformity \ . 1/ and textures of ore minerals are consistent with a slow, \ 1 district-wide mineralizing process in which metals and , t sulfur were transported by the same fluid. Assuming that. the zinc distribution in carbonate host rocks is due to diffusion from ore-forming flui~, Lavery and Barnes (1971) estimate thatmineralizatio~ occurred over, a period 'of about 0.2 to 0.25 m.y. Giz~ et al. (1981) note that the thermal maturity of organic mat­ ter associated with Upper Mississippi Valley m~ner­ alization is consistent with these interpretations of FIG.!' Location of Upper Mississippi Valley mineral district, the degree and duration of heating in the district. showing mining district, area of outlying mineralization, and oc­ The age of mineralization in the district is poorly currences of hydrothermal sphalerite in Pennsylv~an coals (~er Cobb, 1981; Heyl and West, 1982). Shaded areas s~ow lo.cations constrained (Heyl, 1968, p. 442), but the most prob­ of Pennsylvanian rocks of the Illinois and Forest City baslD~. able interval for ore emplacement is Pennsylvanian GENESIS OF UPPER MISSISSIPPI VALLEY DISTRICT 235 to Cretaceous (McLimans, 1977, p. 24; A. Heyl, pers. into the Pennsylvanian. Six major unconformities oc­ commun.). This interval agrees with oxygen isotope cur within this interval (Willman et a1., 1975). In post­ and fission-track dating studies of basement rocks