Geologic, geochemical, and isotopic studies of a carbonate- and siliciclastic-hosted Pb-Zn deposit at Lion Hill, Vermont

Bulletin 2145
By: , and 



Zn-, Pb-, Cu-, and Fe-bearing rocks of the Lion Hill area in western Vermont formed during the Early Cambrian by syngenetic sedimentary-exhalative and diagenetic replacement processes. Sphalerite, galena, chalcopyrite, pyrite, and, locally, magnetite form stratabound and broadly stratiform lenticular zones, -300 meters long and 25-50 meters thick, which are uneconomic at the present time. The lenses are structurally disrupted and metamorphosed to greenschist facies, probably due to the Taconic orogeny. Textural evidence suggests that mineralizing fluids permeated the sediments prior to lithification and that a dilatant fracture zone, possibly a feeder zone, contains some of the discordant veins at Lion Hill. The veins may have formed when the sediments were in a plastic, semiconsolidated state. The association of layered iron formation containing base-metal sulfide minerals provides possible lithologic evidence for syngenetic mineralization by submarine exhalative activity. Sand bars and tidal channels present in the sedimentary section could have acted as permeable pathways for movement of mineralizing fluids. The complex interlayering in the sedimentary sequence of carbonate and siliciclastic rock types having widely varying permeabilities created numerous fluid traps.

Homogenization temperatures of primary and secondary inclusions in vein sphalerite range from 152°C to 196°C; salinities range from 11.5 to 14.0 equivalent weight percent NaCl. δ34S values of sulfides from Lion Hill vary from -25.9 to +10.0 per mil, and fall within the expected range for sulfide produced from bacteriogenic reduction of sulfate with δ34S values of 25 to 30 per mil. In addition, some pyrite probably formed from sulfate in trapped pore fluid that resulted in heavier isotopic values characteristic of more closed-system behavior. Three sphalerite samples that have heavier sulfur isotopic values may reflect a change in the source of sulfur during a later episode of mineralization, perhaps a change to a deep-seated source. Lead isotopic compositions of galenas from mineralized zones at Lion Hill range from 18.351 to 18.632 for 206Pb/204Pb, from 15.546 to 15.618 for 207Pb/204Pb, and from 38.126 to 38.496 for 208Pb/204Pb. The lead isotopic compositions of galena from Lion Hill and fluid inclusion and sulfur isotopic values for the Lion Hill sulfides are more like those of Pb-Zn-Ag deposits of Ireland than those of MVT or Appalachian-type Zn deposits.

The prospect of an Irish-type sedimentary-exhalative origin for stratabound Pb-Zn deposits of the Paleozoic shelf of North America is of considerable importance to understanding the timing of mineralization relative to platform evolution and for evaluating the mineral resource potential of the region. Our study of the Lion Hill deposit indicates a potential for Irish-type Pb-Zn deposits in platform rocks of western Vermont; however, at Lion Hill they contain enrichments of Pb, Zn, and Cu rather than a Pb, Zn, and Ag association.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic, geochemical, and isotopic studies of a carbonate- and siliciclastic-hosted Pb-Zn deposit at Lion Hill, Vermont
Series title Bulletin
Series number 2145
DOI 10.3133/b2145
Year Published 1995
Language English
Publisher U.S. Government Printing Office
Publisher location Washington, D.C.
Contributing office(s) Eastern Mineral and Environmental Resources Science Center
Description iv, 31 p.
Country United States
State Vermont
Other Geospatial Lion Hill
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
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