Potassic alteration of Precambrian amphibolites at Big Falls County park, Eau Claire County, Wisconsin
- Univ. of Wisconsin, Eau Claire, WI (United States). Dept. of Geology
The upper three meters of mafic Precambrian amphibolites (magnesium-hornblende with andesine (An[sub 35]), and minor biotite, quartz, sphene, pyrite, and apatite) near Eau Claire Wisconsin have been hydrothermally altered by diagenetic fluids flowing along the contact with the Cambrian Mount Simon Formation sandstone. Whole-rock XRF analyses of the altered material indicates almost complete leaching of Ca and Na (< 1 wt% Na[sub 2]O and CaO) and considerable enrichment of K by the diagenetic fluids ([approximately] 8 wt% K[sub 2]O). This alteration produced a gray blue/green clay (5 BG 5/2) that has previously been described in the field, based on color, as chloritic. Computer models of x-ray diffraction profiles indicates the clay is primarily illite/dioctahedral chlorite (I/C) interstratified clay with greater than 85% illite layers. Other minerals within the altered zone include discrete illite, potassium feldspar, celadonite, and also include minor but ubiquitous quartz, pyrite, siderite, lepidocrocite, anatase, apatite, and chloritized biotite. The I/C occurs as plates (possibly replacing precursor smectite), fibrous laths (neoformed) and vein fillings. Petrographic and SEM/EDX studies suggest that the altered zone is the product of cyclic changes in the chemistry of diagenetic fluids moving through the base of the Mount Simon. The location of this altered zone on the western edge of the Wisconsin Arch indicates probable shallow depths of burial. Production of such a highly illitic alteration zone from a mafic precursor indicates temperatures and/or fluid compositions outside of those found in other sedimentary sequences at similar shallow depths and remains problematical.
- OSTI ID:
- 7140958
- Report Number(s):
- CONF-9404217-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 26:5; Conference: 27. annual conference of the North-Central Section of the Geological Society of America (GSA), Kalamazoo, MI (United States), 28-29 Apr 1994; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
AMPHIBOLITES
DIAGENESIS
HYDROTHERMAL ALTERATION
CLAYS
CHEMICAL COMPOSITION
GEOLOGIC FORMATIONS
WISCONSIN
QUANTITATIVE CHEMICAL ANALYSIS
ROCK-FLUID INTERACTIONS
X-RAY FLUORESCENCE ANALYSIS
CHEMICAL ANALYSIS
DEVELOPED COUNTRIES
METAMORPHIC ROCKS
NONDESTRUCTIVE ANALYSIS
NORTH AMERICA
ROCKS
USA
X-RAY EMISSION ANALYSIS
580000* - Geosciences