Genetic implications of regional and temporal trends in ore fluid geochemistry of mississippi valley-type deposits in the Ozark region

Abstract

Fluids extracted from aqueous fluid inclusions in epigenetic gangue and ore minerals record the migration of huge volumes of highly saline fluids throughout the stratigraphic section of the Ozark region. The extracted fluids share many similarities regionally, but there are significant temporal differences which define two geochemically distinct end-member ore-forming fluids that we refer to as the Viburnum Trend main stage or Viburnum Trend type and the Tri-State type.Viburnum Trend-type fluids are enriched in potassium and are associated only with deposits close to the basal Lamotte Sandstone. The main-stage octahedral galena ore of the Viburnum Trend and much of the Old Lead Belt ore is thought to be derived from this type of ore fluid. Galena deposited by Viburnum Trend-type fluids contains less radiogenic lead than galena deposited by Tri-State-type fluids. Sulfldes deposited by Viburnum Trend-type fluids also contain isotopically heavier sulfur and significant amounts of copper, cobalt, nickel, and silver.Tri-State-type fluids have a low potassium content when compared with Viburnum Trendtype fluids and are characteristic of deposits where ore-forming fluids migrated through large volumes of carbonate rock. These fluids are thought to have formed the ore deposits of the Tri-State, Northern Arkansas, and Central Missouri districts, the cubic galena-stage ore of the Viburnum Trend, and the many trace occurrences of sphalerite throughout the Ozark region. Galena deposited by Tri-State-type fluids has more radiogenic lead and the sulfides have isotopically lighter sulfur than sulfides deposited by Viburnum Trend-type fluids. A systematic south to north increase of potassium in the Tri-State-type fluids suggests that they migrated from a southerly source such as the Arkoma basin.Possible explanations for the origins of these two end-member fluids include: (1) a single parent brine evolved into two distinct fluids due to reactions with geochemically distinct aquifers during migration, (2) the two distinct fluids reflect normal fluid evolution within a single source basin of a bittern and of later halite dissolution, and (3) the Viburnum Trend and Tri-State-type brines migrated to southeast Missouri from two different source basins. Our data does not preclude any of these possibilities; however, the geochemical similarity of the Viburnum Trend end-member fluid to a bittern may be accounted for by water-rock modifications of the brine during migration. Other evidence strongly supports a southerly source for the ore-forming brines thus limiting possible sources for the Viburnum Trend-type fluid to the Arkoma and/or Black Warrior basins of the Ouachita foreland trough. Viburnum Trend-type fluid flow was probably funneled northward through basal sandstones within the Reelfoot rift and water-rock modifications occurring there may have resulted in its unique geochemistry.

Feldspar diagenesis in Cambrian clastic rocks of the southern Ozark Mountains and Reelfoot Rift, southeastern Missouri and northeastern Arkansas; implications for mississippi valley-type ore genesis

“…These high K/C1 values were attributed by Viets and Leach (1990) to a fluid migrating through the Lamotte Sandstone, a conclusion that is consistent with the isotopic data of Aleinikoff and others (1993).…”

“…Importantly, fluid-inclusion studies of early to main-stage mineralization in the Viburnum Trend (Crocetti and Holland, 1989;Viets and Leach, 1990) revealed exceptionally high K/Q ratios compared to other MVT ore-forming brines of the region (table F2) and even compared to paragenetically later parts of the Viburnum Trend mineralization. These high K/C1 values were attributed by Viets and Leach (1990) to a fluid migrating through the Lamotte Sandstone, a conclusion that is consistent with the isotopic data of Aleinikoff and others (1993).…”

“…We suspect that it is this potassium that led to elevated K/C1 values in main-stage mineralization of the Viburnum Trend. Clearly, other deep sedimentary basins, particularly the Arkoma basin south of the Ozark region, could have contributed K+ by similar processes of burial diagenesis, but the lower K/C1 values of fluids from ores closer to the Arkoma basin (Viets and Leach, 1990) argue against such a source. The proximity of the Viburnum Trend and the other lead ores surrounding the St. Francois Mountains to the Reelfoot Rift thus suggests a fluid-flow path linking the Reelfoot Rift to the lead belt area; this fluid-flow path uniquely contributed extra K+ as well as other ore constituents to the lead belt area.…”

“…Furthermore, brines that formed MVT deposits are known to be anomalously rich in K+ compared to typical upper crustal (that is oil-field) brines (Manor, 1979). Even among MVT brines, the fluids of the main-stage mineralization of the deposits in the southeastern Missouri lead belts are anomalously rich in this ion (Viets and Leach, 1990). Therefore, the ore fluids are good candidates for the source of K+ that favored authigenic potassium feldspar precipitation.…”

Replacement Dolomites and Ore Sulfides as Recorders of Multiple Fluids and Fluid Sources in the Southeast Missouri Mississippi Valley-Type District: Halogen-87Sr/86Sr- 18O- 34S Systematics in the Bonneterre Dolomite

“…In a reconnaissance study of noble gas and halogen analysis of fluid inclusions from several MVT districts, they noted that fluids responsible for deposition of Mainstage dolomite cement in the Viburnum Trend were different from those responsible for precipitation of Main-stage sphalerite. They interpreted these fluids to be one brine associated with overlying carbonate aquifers (Tri-State-type, Viets and Leach, 1990) and another brine associated with the underlying Lamotte Sandstone aquifer (Viburnum Trend-type, Viets et al, 1996), respectively. Waxing and waning of the influence of a brine that originated in overlying carbonates was thought to cause two alternating chemical regimes, one favoring dolomite precipitation and a second favoring sulfide precipitation (an acid-producing process that is not conducive to simultaneous precipitation of dolomite).…”