264 Economic Geology

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 101H-J

COMPOSITION OF FLUID INCLUSIONS FROM THE CAVE-IN-ROCK BEDDED-REPLACEMENT FLUORITE DEPOSITS IN THE ILLINOIS-KENTUCKY DISTRICT

Paper number: 264-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

The Illinois-Kentucky district is one of the principal districts of Mississippi Valley-type (MVT) mineralization in the North American mid-continent. Although the Illinois-Kentucky district is distinguished from other mid-continent MVT districts by its fluorite- instead of sulfide- or barite-dominated ore mineral assemblage, the Illinois-Kentucky deposits were probably precipitated from the same northward flowing sedimentary brines set in motion by Late Paleozoic Alleghanian-Ouachita tectonism that precipitated other mid-continent MVT mineralization, particularly in the Ozark Plateau. The primary objective of the current study was to characterize the chemical composition of the Illinois-Kentucky mineralizing fluids to enable a comparison to known Ozark MVT ore fluid compositions and to seek insights into the ore formation process. This characterization was carried out through in situ analysis of individual fluid inclusions using microthermometry, laser ablation-inductively coupled plasma-mass spectrometry, and Raman spectroscopy, and through bulk analysis of fluid inclusion leachates using ion chromatography.

Analyses indicate that the Illinois-Kentucky deposits precipitated from a mixture of fluids that had distinctly higher Ca/Na, Ca/Mg, K/Na, Sr/Na, and Ba/Na ratios but similar or slightly lower Mg/Na ratios compared to fluids from the Ozark MVT deposits. These differences may be attributable to the influence of ultramafic, rift-related igneous rocks in the Illinois-Kentucky district that are not present in any of the Ozark MVT districts. Though sulfide and barite mineral inclusions are present in much of the fluorite, a population of ore metal-rich fluid inclusions with Pb, Zn, Cu, and Fe concentrations as high as 1000’s of ppm was identified, likely indicating the invasion of a metal-rich brine during mineralization as documented for the Ozark MVT districts. Most fluid inclusions had high methane concentrations on the order of 100’s of ppm, suggesting the ores formed under reducing conditions at depths of several hundred to as high as 1500 m. Atomic F/Cl ratios in sphalerite-hosted fluid inclusions ranged between 0.0005 and 0.005. This means that for the average measured Ca concentration of 16,000 ppm, fluorite would have been super-saturated above a pH of 2.7 and 1.7, respectively.