A-1 Introduction 6 2 Location 6 3 Climate, drainage, and topography6 3 Culture and accessibility6 4 Mines and mineral exploration 6 4 Field and laboratory work 6 4 Coordinate system for locations6 5 Acknowledgments6 5
__________________________________________ Introduction. ______________________________________ Location and access______________ _______________ Topography and drainage.. _ _____________________ Present investigation. ___________________________ Acknowledgments-_____________________________ Previous geologic work_ _________________________ Geologic setting-___________________________________ Lower Precambrian me ta volcanic and gneissic rocks. ____ General features.. _ _____________________________ Mona Schist _ __________________________________ Distribution and thickness. __________________ Definition and subdivisions __________________ Metabasalt_ ___________________________ Amphibole schist-______________________ Description. _______________________ Origin of the layered structure. ______ Chloritic and actinolitic slates and schists. _ Description and origin.. _____________ Hematite-rich zones. ________________ Felsite ________________________________ Description. _______________________ Origin. ____-__-___-___-__________ Mafic meta-agglomerate _________________ Comparison of chemical analyses, norms, and modes. __________________________________ Metamorphism-____________________________ Compeau Creek Gneiss__-__-_-_____-____________ Definition, distribution, trends, and age_______ Description. _______________________________ Quartz veins and spatially associated silicified Origin. ____________________________________ Metamorphism. ____________________________ Middle Precambrian metasedimentary rocks Animikie Series. __________________________________________ Chocolay Group. _______________________________ Enchantment Lake Formation.. _ _____________ Definition, distribution, and thickness. Rock names and petrography. ___________ Lithologic distribution __________________ Lower part of formation. ____________ Middle part of formation ____________ Upper part of formation. ____________ Correlation and age relations. ____________ Origin. ________________________________ Mesnard Quartzite. _________________________ Definition, distribution, and thickness. Description. ___________________________ Origin and source of sediments. __________ Correlation and age relations. ____________ Kona Dolomite _ _ _________________________ Components and their general distribution-Upper limit and thickness. ______________
The Gossan Lead district is a 28-km-long, northeast-trending belt of discontinuous massive sulfide deposits in the Blue Ridge province of southwestern Virginia. The deposits, hosted by the Ashe Formation of late Proterozoic age, consist of strata-bound lenses and layers of massive pyrrhotite, minor chalcopyrite, sphalerite, and pyrite, and rare arsenopyrite and galena. Deposits were mined principally in the Iron Ridge and Betty Baker segments, respectively, at the southwestern and northeastern ends of the belt. Detailed mapping of the Gossan Howard, Huey, and Bumbarger pits in the Iron Ridge segment indicates that the deposits occur at one horizon and have been variously folded and brecciated after sulfide deposition. The Gossan Howard consists of a single, gently dipping lens of sulfide. The Huey deposit is complexly folded and locally contains tectonically thickened ore. The Bumbarger deposit is a lens as much as 40 m thick--the thickest known in the district. This deposit contains abundant coarse breccia fragments of wall rock around which the massive sulfide has flowed (during deformation and metamorphism), probably thickening the original deposit significantly. In the northeastern part of the district, drill holes intersect several sulfide layers that possibly are structurally repeated. The Ashe Formation in the district is a sequence of metasedimentary rocks and local conformable lenses of amphibolite and actinolite-chlorite schist. The metasedimentary rocks include metapelite, quartz-feldspar granofels (metagraywacke), and minor quartziteand carbonaceous schist, and are interpreted as marine turbidites. The amphibolites and other mafic rocks have chemical compositions similar to low Ti tholeiitic basalt, with a high Y/Nb (>10) and high average contents of Co (40 ppm), Cr (403 ppm), Ni (211 ppm), and V (247 ppm). Immobile trace element signatures (Ti-Y-Zr; Th-Hf-Ta; Ti-Cr) suggest a magmatic affinity with midocean ridge basalt (MORB); rare earth elements (REE) have low abundance levels (10)<-15)< chondrite), broadly fiat patterns [(La/Yb)N --0.7-1.1], and a slight depletion in the light elements similar to midocean ridge basalts. An amphibolite from a much higher stratigraphic level, south of the district, differs significantly from the mafic rocks closer to the sulfide zone in having the chemical signature of a transitional, slightly alkalic tholeiite with high TiO• (3.87 wt %), Fe•O3 (16.4 wt %), and P•O5 (0.56 wt %), low Y/Nb (3.3), and a highly fractionated rare earth element distribution [(La/Yb)N = 3.9] similar to continental basalt. Some silicate wall rocks of the deposits are mineralogically and chemically unusual, and differ substantially from the clastic metasediments of the Ashe Formation. Such rocks are composed mainly or wholly of plagioclase feldspar, biotite, chlorite, muscovite, or spessartinerich garnet. The unusual lithologies form local strata-bound lenses in the footwall and/or hanging wall of the deposits, typically within 10 m of massive sulfide. The plagioclase rocks (3.4-7.6 wt % Na•...
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