Hydrothermal Fluid Origins in Mississippi Valley-Type Ore Districts: Combined Noble Gas (He, Ar; Kr) and Halogen (Cl, Br; I) Analysis

摘要

Samples were selected from three of the classic Mississippi Valley--type districts in the midcontinent area of North America. The Illinois-Kentucky fluorspar district belongs to the fluoritic subtype of Mississippi Valley-type districts, and the Ozark districts of the Viburnum Trend and Tri-State are representative of the Pb-rich and more typical Zn-rich subtypes of Mississippi Valley-type mineralization, respectively. Noble gas (Ar, Kr) and halogen, (Cl, Br; I) data have been obtained simultaneously by noble gas mass spectrometry of inclusion fluids released by in vacuo crushing of irradiated fluorite, quartz, carbonate, and sphalerite samples. Addition-ally, He analyses have been obtained from nonirradiated fluorite from the Illinois-Kentucky fluorspar district. Fluorite from the Illinois-Kentucky- fluorspar district has a maximum ~3He/~4He ratio of 0.35 Ra (where Ra = atmospheric ~3He/~4He ratio of 1.4 X 10~(-6)), higher than values typical of crustal fluids and confirming the presence of a minor mantle component, equal to less than 6 percent of the total Rein the fluids of the Illinois-Kentucky fluorspar district. The Br/Cl mol ratios of all the deposits are, with one exception, higher than the value of seawater (1.54 X 10~(-3)), ranging from 1.69 to 3.70 X 10~(-3), and indicating acquisition of salinity by the evaporation of seawater beyond the point of halite saturation. Late quartz from the Tri-State district contains a very minor component of halite dissolution water and has a Br/Cl mol ratio of 1.46 X 10~(-3). The I/Cl mol ratios are typical of oil field brines and are ubiquitously higher than what is attainable by the evaporation of seawater alone. I/Cl mol ratios are in the range of 6 to 270 X 10~(-6), indicating that the fluids have interacted with I-rich organic matter present in sedimentary rocks. In the Ozark districts, ~(40)Ar/~(36)Ar ratios van between 320 and 345 in the Tri-State and between 350 and 420 in the Viburnum Trend. In the Illinois-Kentucky fluorspar district ~(40)Ar/~(36)Ar ratios reach values of 1,200. The corresponding concentrations of ~(40)Ar_(excess) (~(40)Ar not attributable to radiogenic decay of ~(40)K or an atmospheric source) are similarly elevated in the main-stage fluorite mineralization of the Illinois-Kentucky fluorspar district (6.8-18.2 X 10~(-4) cm~3cm~(-3)H_2O) relative to the Viburnum Trend and Tri-State districts mostly 1.3-3.3 X 10~(-4) cm~3cm~(-3)H_2O and 0.3-1.4 X 10~(-4) cm~3cm~(-3)H_2O, respectively). Together the noble gas and halogen data indicate the existence of three brine provinces, (1) a regional Tri-State brine present throughout the Ozark region, (2) a Viburnum Trend brine, and, (3) an Illinois-Kentucky-brine. Furthermore, in the Viburnum Trend the halogen composition of main-stage dolomite is distinct from that of the main-stage sphalerite but identical to the regional hydrothermal dolomite seen in the Tri-State district. The data imply that mixing relationships in the Viburnum Trend arc highly complex but are compatible with regional models in which the Tri-State brine, present throughout the Ozark region, was sourced in the Arkoma basin.