Mineralogy, Paragenesis, and Mineral Zoning of the West Fork Mine, Viburnum Trend, Southeast Missouri


ASARCO's West Fork mine, the newest mine in the Viburnum Trend, began production in 1985 and reached full production in 1988. Although significant mineralogical and paragenetic studies have been performed on the other ore deposits of the Viburnum Trend, no attempt has been made to correlate these studies with mineral zoning patterns. The distinct mineral zoning present at the West Fork mine provides a unique opportunity to relate mineral paragenetic sequence to mineral zoning.The present study concentrated on the main ore horizon (M bed) which is composed of four north-south-trending linear metal zones which together consist of an inner zone of colloform iron and zinc sulfides, bordered outward by zones of disseminated iron and zinc sulfides and fringed by outermost zones of galena that also overprint the inner zone. Principal metallic sulfides in the ores at the West Fork mine are PbS (galena), ZnS (sphalerite and wurtzite), and FeS2 (pyrite and marcasite). Paragenetic sequence and metal zoning show that the ore-forming fluids migrated initially through the inner zone where ZnS was deposited and subsequently traveled outward depositing FeS2 followed by PbS. The presence of rapidly deposited, colloform ZnS and FeS2 and bladed galena in the central Fe-Zn zone indicates that ore fluids which deposited the sulfides in the central zone were more saturated with respect to metal sulfides. Outer layers of the colloform ZnS bodies in the Fe-Zn zone are characterized by an abundant complex intergrowth of sphalerite and wurtzite (schalenblende). X-ray analysis indicates that the wurtzite within the schalenblende inverted to sphalerite, thereby producing pseudomorphous sphalerite after wurtzite. Because wurtzite is the sulfur-deficient form of ZnS, it can be postulated that sulfur fugacities were low at the end of the main stage of zinc sulfide deposition. Subsequent increase in sulfur fugacity during the main stage of iron deposition caused inversion of wurtzite to sphalerite. Interlayered pyrite and marcasite are evidence that the ore fluids during the main stage of deposition were oscillating above and below a pH of 5. Minerals subsequently deposited in the outer zones were precipitated more slowly from fluids that were relatively depleted in metals. The distinct differences in ore character between the metal zones may well be a result of multiple fluids. Separate fluids may have been channelized (each fluid representing a zone), or may have mixed through time. The results of this study are not compatible with ore deposition from a single fluid source.


Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Fluid Source; Massive Sulphide; Mineralization; Paragenesis; Zonation; USA, Missouri, Viburnum Trends, West Fork Mine

Geographic Coverage

Southeast Missouri

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Document Type

Article - Journal

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© 1992 Society of Economic Geologists, Inc., All rights reserved.

Publication Date

01 Feb 1992