Kevin L. Shelton

Dolomitization and Dolomite Neomorphism in the Back Reef Facies of the Bonneterre and Davis Formations (Cambrian), Southeastern Missouri

The back reef facies of the Bonneterre and Davis Formations consists of dolomitized cryptalgalaminates and partially dolomitized peloid mudstones. These subfacies were deposited on tidal flats and in shallow lagoons that existed in the St. Francois Mountains of southeast Missouri during late Cambrian time. The back reef facies is bounded on the seaward side by dolomitized algal bioherms and grainstone banks which host the southeast Missouri Mississippi Valley-type (MVT) lead-zinc-copper ore deposits. Cryptalgalaminates are replaced by both fine crystalline, planar dolomite and coarse crystalline, nonplanar dolomite. Peloid mudstones are partially replaced, but mainly by coarse crystalline, nonplanar dolomite. Planar dolomite, replacing cryptalgalaminates, shows gradational transitions into coarse crystalline, nonplanar dolomite. The transitions are accompanied by progressive increase in average crystal size and an increased skewness toward the coarse size fraction. Planar dolomite is enriched in 18 O relative to stratigraphically equivalent marine limestones. Coarse crystalline, nonplanar dolomite, which replaces both cryptalgalaminates and peloid mudstones, has delta 13 C and delta 18 O values transitional between those of planar dolomite and late dolomite cements associated with MVT mineralization. Fluid inclusion analysis of nonplanar dolomite and associated dolomite cements indicates precipitation at elevated temperatures (60 to 114 degrees C) by saline, basinal fluids. These data indicate that the back reef facies was subjected to a complex diagenetic history involving both early and late diagenetic dolomitization and dolomite neomorphism. Fine crystalline, planar dolomite was formed during early diagenetic dolomitization of the cryptalgalaminate subfacies by a fluid approaching seawater composition. Coarse crystalline, nonplanar dolomite was formed as a result of neomorphism of pro-existing planar dolomite and dolomitization of peloid mudstones, after burial, accompanying MVT mineralization in southeast Missouri.

Fluid-inclusion studies of regionally extensive epigenetic dolomites, Bonneterre Dolomite (Cambrian), southeast Missouri: Evidence of multiple fluids during dolomitization and lead-zinc mineralization

Sources of basinal fluids that precipitated the dolomite-hosted Mississippi Valley-type orebodies of southeast Missouri historically have been a subject of debate. This study presents microthermometric data for fluid inclusions in the regionally extensive epigenetic dolomite at the base of the ore-hosting Bonneterre Dolomite and in gangue dolomite of the Viburnum Trend Pb-Zn district. Samples of epigenetic dolomite cover an area of more than 25,000 km 2 west of the St. Francois Mountains and permit determination of regional variations of temperature and composition of the mineralizing fluids and possible fluid interactions. Homogenization temperature-ice-melting relationships (temperature-salinity) among these inclusions document at least two end-member fluid components: a warmer, less saline fluid (120 to 187 °C; 5 wt% equiv. NaCl) and a cooler, more saline fluid (60 to 80 °C; >30 wt% equiv. NaCl). Intermediate temperatures and fluid compositions indicate that the end-member fluids likely mixed as they flowed through the region. Mixing was not confined to a stationary front but occurred throughout the study area. Comparison of homogenization temperatures to distance from possible basinal fluid sources indicates no discernible temperature gradient over the >25,000 km 2 study area. The data are interpreted to indicate multiple basinal-fluid interactions coeval with dolomitization and associated Pb-Zn ore formation.

Cyclic variations of sulfur isotopes in Cambrian stratabound Ni-Mo- ( PGE-Au) ores of southern China

Abstract Cyclic variations of δ 34 S values over a range of at least 48‰ in pyrite nodules from stratabound Ni-Mo-PGE(Au) ores of southern China are attributed to biogenic reduction of seawater sulfate in an anoxic, phosphogenic, and metallogenic basin. Cyclic introduction and mixing of normal seawater into typically stagnant basin waters led to extreme variations in δ 34 S values of aqueous sulfide species present at different times. Intermittent venting of metal-laden hydrothermal fluids into such a bacteriogenic sulfide-rich environment resulted in precipitation of metal sulfides as pseudomorphous replacements of organic debris and as sulfide sediments that record large δ 34 S CDT variations from −26 to +22‰. Apatite and silica dominated the replacement of the organic debris when metals were not being introduced into the basin. The combination of abundant organic debris, localized topographic basins for accumulation of the debris, bacterial production of sulfide species, and introduction of metal-bearing hydrothermal fluids provided the environment necessary to form these unusually rich Ni-Mo ores.

Early ontogeny of Eutrephoceras compared to Recent Nautilus and Mesozoic ammonites: evidence from shell morphology and light stable isotopes

Observations on the morphology of the early whorls of Eutrephoceras dekayi (Morton), a widespread Cretaceous nautilid, are supplemented with oxygen and carbon isotopic analyses (δ 18 O and δ 13 C) of the early septa of five well-preserved specimens to help identify the point of hatching on the shell. Septa 4 and 5 are more closely spaced than preceding septa and probably correspond in time of formation with a constriction or first broken aperture on the outer shell one-third whorl forward of the fourth septum. In modern Nautilus , morphologic, isotopic, and observational data suggest that similar features mark hatching. Between the fourth and fifth septa in E. dekayi , δ 18 O values show a shift of variable magnitude from heavy to lighter values followed by a return to heavier values over the next one to three septa. This isotopic shift is compatible with a hatching interpretation and may be explained as the result of kinetic and equilibrium effects on emergence from an egg capsule. Eutrephoceras dekayi hatched at about 9 mm in diameter, one-third the hatching size of modern Nautilus. Like Nautilus, E. dekayi probably produced few young, all of which were active swimmers at hatching. In contrast, Mesozoic ammonoids produced numerous offspring ranging from 0.5 to 1.5 mm in diameter which may have spent some time in the plankton. These differences in life history may correlate with differences in ecologic specialization, environmental tolerance, and habitat between ammonoids and nautilids and may have contributed to their disparate rates of evolution during the Mesozoic.

Minor- and trace-element distributions in the Bonneterre Dolomite (Cambrian), southeast Missouri: Evidence for possible multiple-basin fluid sources and pathways during lead-zinc mineralization

The sources and flow paths of basinal fluids that precipitated the dolomite-hosted Mississippi Valley-type orebodies of southeast Missouri have historically been a subject of debate. This study presents geochemical data for epigenetic dolomites of the Bonneterre Dolomite directly above the Lamotte Sandstone aquifer and from the Viburnum Trend orebodies. Samples of epigenetic replacement dolomite were collected at the Bonneterre Dolomite-Lamotte Sandstone contact from 35 drill cores covering an area of more than 25,000 km 2 west of the St. Francois Mountains. There are distinct aerial trends in Fe, Mn, and Sr contents of the dolomite from values of 3.47 wt%, 0.38 wt%, and 27 ppm in the south to 0.82 wt%, 0.12 wt%, and 54 ppm in the north, respectively. In and near the Viburnum Trend lead-zinc subdistrict, the distributions of Fe, Mn, and Sr are reversed, from values of 0.50 wt%, 0.10 wt%, and 105 ppm in the south to 3.15 wt%, 0.55 wt%, and 40 ppm in the north, respectively. Fe and Mn contents of gangue dolomite cement in the Viburnum Trend orebodies show a similar south-to-north enrichment, ranging from 1.41 mole% FeCO 3 and 0.11 mole% MnCO 3 in the south to 2.34 mole% and 0.23 mole% in the north, respectively. On the basis of fractionation of minor and trace elements between fluids and dolomites, the data are interpreted to indicate a regional south-to-north flow of water from the Arkoma Basin through the Lamotte Sandstone. A second fluid-flow system, with a northern source (possibly the Illinois Basin), precipitated the late phases of gangue dolomite cement in the Viburnum Trend and may have been active during the earlier precipitation of the dolomite at the Lamotte-Bonneterre contact. The inferred fluid-flow paths suggest a more complex, multiple-basin fluid involvement than previously suggested for the Mississippi Valley-type Pb-Zn mineralization of southeast Missouri.

Dolomitization and Dolomite Neomorphism: Trenton and Black River Limestones (Middle Ordovician) Northern Indiana, U.S.A.

ABSTRACT The Trenton and Black River Limestones are dolomitized extensively along the axis of the Kankakee Arch in Indiana, with the proportion of dolomite decreasing to the south and southeast of the arch. Planar and nonplanar dolomite replacement textures and rhombic (type 1) and saddle (type 2) void-filling dolomite cements are present. Three stages of dolomitization, involving different fluids, are inferred on the basis of petrographic and geochemical characteristics of the dolomites. Nonferroan planar dolomite has relatively high 18O values (-1.8 to -6.1o/oo PDB) and has 87Sr/86Sr ratios (0.70833 to 0.70856) that overlap those of Middle Ordovician seawater. Petrography, geochemistry, and the geometry of the dolomitized body suggest that the planar dolomite was formed in Middle and Late Ordovician seawater during the deposition of the overlying Maquoketa Shale. Ferroan planar and nonplanar dolomite occurs in the upper few meters of the Trenton Limestone, confined to areas underlain by planar dolomite. This dolomite contains patches of nonferroan dolomite with cathodoluminescence (CL) characteristics similar to underlying planar dolomite. Ferroan dolomite has relatively low 18O values (-5.1 to -7.3o/oo PDB) and has slightly radiogenic 87Sr/86Sr ratios (0.70915 to 0.70969) similar to those obtained for the overlying Maquoketa Shale. These data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying Maquoketa Shale during burial. The absence of ferroan dolomite at the Trenton-Maquoketa contact, in areas where the earlier-formed nonferroan planar dolomite also is absent, indicates that the fluid expelled from the overlying shale did not contain enough Mg2+ to dolomitize limestone. Type 1 dolomite cement has isotopic compositions similar to those of the ferroan dolomite, suggesting that it also formed from shale-derived burial fluids. CL growth zoning patterns in these cements suggest that diagenetic fluids moved stratigraphically downward and toward the southeast along the axis of the Kankakee Arch. Type 2 saddle dolomite cements precipitated late; their low 18O values (-6.0 to -7.0o/oo PDB) are similar to those of the type 1 dolomite cement. However, fluid-inclusion data indicate that the saddle dolomite was precipitated from more saline, basinal fluids and at higher temperatures (94° to 143°C) than the type 1 cements (80° to 104°C). A trend of decreasing fluid-inclusion homogenization temperatures and salinities from the Michigan Basin to the axis of Kankakee Arch suggests that these fluids emerged from the Michigan Basin after precipitation of type 1 cement.

Basin-Wide Diagenetic Patterns: Integrated Petrologic, Geochemical, and Hydrologic Considerations

This volume contains papers, many of which were presented at the SEPM Research Conference entitled Basin-Wide Diagenetic Patterns: Integrated Petrologic, Geochemical, and Hydrologic Considerations which was convened May 21 to 25, 1994 at Lake Ozark, Missouri, U.S.A. Some of the issues addressed at this conference and in this volume include: factors governing the temporal evolution of hydrodynamic systems, the origin and evolution, and spatial distribution of paleoflow conduits and their diagenetic products in sedimentary basins, the nature of subsurface fluid-rock interactions, temporal and spatial distribution of the geochemistry of basinal fluids, and factors controlling heat flow in sedimentary basins.

Sulfur isotope evidence for penetration of MVT fluids into igneous basement rocks, southeast Missouri, USA

Previous studies of galena and sphalerite from Paleozoic MVT deposits in the Viburnum Trend, southeast Missouri documented large variations in δ34S values throughout the ore-forming event. The present study of Cu-Fe-sulfides reveals a similar δ34S variation that reflects two end-member sulfur reservoirs whose relative importance varied both temporally and spatially. More 34S-enriched sulfides (δ34S approaching 25‰) indicate introduction of sulfur from basinal sedimentary sources, whereas more 32S-enriched sulfides (δ34S < 5‰) may reflect fluids moving through underlying granitic basement. Two areas containing Precambrian, igneous-hosted FeCu mineralization in southeast Missouri (West and Central Domes of Boss-Bixby) were investigated to elucidate their relationship to Cu-rich MVT orebodies hosted nearby within the overlying Cambrian Bonneterre Dolomite. Mineralization at Boss-Bixby is composed of an early phase of iron oxide deposition followed by Cu-Fe-sulfides. The Central Dome is faulted and its mineralization is more fracture-controlled than the typically podiform ores of the West Dome. The δ34S values of West Dome sulfides are 0.9 to 6.5‰ and pyrite-chalcopyrite indicate a temperature of 525° ± 50 °C. These data indicate an igneous source of sulfur during Precambrian ore deposition. In contrast, δ34S values of Central Dome sulfides are 9.4 to 20.0‰ and pyrite-chalcopyrite indicate temperatures of 275° ± 50 °C. Similar δ34S values are obtained for chalcopyrite from the overlying MVT deposits. We speculate that deeply circulating, basin-derived MVT fluids mobilized sulfur and copper from the underlying igneous basement and redeposited them in overlying Curich MVT orebodies, as well as overprinting earlier Precambrian sulfides of the Central Dome with a later, Paleozoic MVT sulfur isotope signature. Many models for MVT fluid circulation in the Midcontinent region of North America assume that igneous basement rocks are an impermeable boundary, but in southeast Missouri, evidence exists for structurally controlled MVT fluid movement > 600 m vertically through underlying Precambrian igneous rocks. Such basement involvement has been suggested for other carbonate-hosted base-metal districts (e.g. Irish base metal deposits) and should be considered an integral part of the ore-forming process in southeast Missouri.

Middle Miocene paleotemperature anomalies within the Franciscan Complex of northern California: Thermo-tectonic responses near the Mendocino triple junction

This study documents three localities in the Franciscan accretionary complex of northern California, now adjacent to the San Andreas fault, that were overprinted thermally between 13.9 and 12.2 Ma: Point Delgada–Shelter Cove (King Range terrane); Bolinas Ridge (San Bruno Mountain terrane); and Mount San Bruno (San Bruno Mountain terrane). Vein assemblages of quartz, carbonate, sulfide minerals, and adularia were precipitated locally in highly fractured wall rock. Vitrinite reflectance (R m ) values and illite crystallinity decrease away from the zones of metalliferous veins, where peak wall-rock temperatures, as determined from R m , were as high as 315 °C. The δ 18 O values of quartz and calcite indicate that two separate types of fluid contributed to vein precipitation. Higher δ 18 O fluids produced widespread quartz and calcite veins that are typical of the regional paleothermal regime. The widespread veins are by-products of heat conduction and diffuse fluid flow during zeolite and prehnite-pumpellyite–grade metamorphism, and we interpret their paleofluids to have evolved through dehydration reactions and/or extensive isotopic exchange with accreted Franciscan rocks. Lower δ 18 O fluids, in contrast, evolved from relatively high temperature exchange between seawater (or meteoric water) and basaltic and/or sedimentary host rocks; focused flow of those fluids resulted in local deposition of the metalliferous veins. Heat sources for the three paleothermal anomalies remain uncertain and may have been unrelated to one another. Higher temperature metalliferous fluids in the King Range terrane could have advected either from a site of ridge-trench interaction north of the Mendocino fracture zone or from a “slabless window” in the wake of the northward migrating Mendocino triple junction. A separate paradox involves the amount of Quaternary offset of Franciscan basement rocks near Shelter Cove by on-land faults that some regard as the main active trace of the San Andreas plate boundary. Contouring of vitrinite reflectance values to the north of an area affected by a.d. 1906 surface rupture indicates that the maximum dextral offset within the interior of the King Range terrane is only 2.5 km. If this fault extends inland, and if it has been accommodating most of the strike-slip component of San Andreas offset at a rate of 3–4 cm/yr, then its activity began only 83–62 ka. This interpretation would also mean that a longer term trace of the San Andreas fault must be nearby, either offshore or along the northeast boundary of the King Range terrane. An offshore fault trace would be consistent with peak heating of King Range strata north of the Mendocino triple junction. Conversely, shifting the fault to the east would be compatible with a slabless window heat source and long-distance northward translation of the King Range terrane after peak heating.

GEOCHEMICAL AND PETROGRAPHIC EVIDENCE FOR FLUID SOURCES AND PATHWAYS DURING DOLOMITIZATION AND LEAD-ZINC MINERALIZATION IN SOUTHEAST MISSOURI: A REVIEW

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. NaCl] 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: 1) 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, multiple-basin fluid interaction than previously recognized.