Geochemical Analysis of Fluid Inclusions in Trace and Minor Mississippi Valley-type Zn-Pb Occurrences in the US Mid-continent: Implications for Crustal Metal Enrichment Processes

University Of Missouri-Columbia, Columbia MO

Investigators

Abstract

The Mississippi River watershed of the central U.S. hosts numerous deposits of lead and zinc ranging from the world-class Southeast Missouri and Tri-State (Oklahoma-Missouri-Kansas) mining districts to widely dispersed non-commercial traces of mineralization. Collectively these deposits are referred to as 'Mississippi Valley-type' (MVT) and occur as sulfide minerals that were precipitated in limestone and dolomite host rocks by highly saline groundwater (about 10 times more saline than seawater) at temperatures between 80 and 150° C. The largest deposits are in the Southeast Missouri district, which currently account for about 70% of primary lead production and about 6.5% of primary zinc production in the U.S. The Tri-State district was mined commercially from 1848 to 1970 and during its peak in the early 20th century accounted for as much as about 60% of primary zinc and 11% of primary lead production in the U.S. Recent research carried out by the principal investigators and their collaborators has revealed the chemical composition of the groundwater that precipitated the MVT ores in the Southeast Missouri and Tri-State districts and other formerly commercial deposits in northern Arkansas and central Missouri. This research showed that large MVT ore deposits in the central U.S. formed from groundwater with high concentrations of lead, high Ca/Na and K/Na ratios, low Ca/Mg ratios, and that was chemically reducing. The purpose of the present research project is to determine whether groundwater that formed trace and minor MVT mineralization had the same composition as the groundwater that formed large ore bodies. This information would be useful in determining the commercial potential of new MVT deposit discoveries and in determining the geologic processes responsible for concentrating metals into large ore bodies. The project will be carried out by analyzing fluid inclusions, tiny (< 0.1 mm in diameter) samples of ambient groundwater trapped in the precipitated MVT deposit minerals. The locations of numerous trace occurrences of MVT mineralization have been compiled from the published literature and will be visited and sampled by the principal investigators. Slices of the rock samples will be cut and polished to 0.05 to 0.1 mm thicknesses to make the fluid inclusions visible by transmitted light microscopy. Fluid inclusions will first be studied using microthermometry to determine their bulk salinity from their melting point depression and their trapping temperature from the point of phase homogenization. The presence of methane, carbon dioxide, sulfate, and hydrogen sulfide will be sought in the fluid inclusions using Raman spectroscopy and will constrain the redox potential of the mineralizing groundwater and depth of mineral deposit formation. The concentrations of elements like Li, Na, Mg, K, Ca, Mn, Fe, Cu, Zn, Sr, Ba, and Pb will be determined from laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and will be used to see if groundwater that formed the trace MVT occurrences has the same chemical signature as groundwater that formed the large MVT ore deposits. More specifically, the data acquired from this research will allow two fundamental hypotheses for the origin of MVT deposits to be tested: (1) That anomalously metal-rich, high Ca/Na and K/Na, and low Ca/Mg fluids are needed to form large MVT ore deposits, (2) that redox conditions at the trace and minor occurrences of MVT mineralization were locally too oxidizing to allow much of the sulfur, whether resident or delivered by invading fluid, to exist as sulfide rather than sulfate, which would inhibit sulfide mineral precipitation and deposit growth.