| Tin mineralization in the Goodes Creek study area is stratiform in character and occurs in thin lenticular leucosomes in a 50 to 60 foot thick horizon of tin-anomalous (up to 117 ppm) biotite ({dollar}pm{dollar}hornblende {dollar}pm{dollar}garnet) gneiss. Leucosomes occurring structurally above and below this tin-enriched gneissic horizon are barren.; Mineralized and barren leucosomes are peraluminous, corundum normative, largely monzogranitic in composition, and contain variable amounts of quartz, oligoclase, K-feldspar, mica, Fe-Mg-tourmaline, almandine, and accessory materials.; The leucosomes are generally conformable with the enclosing gneisses, which, themselves, are variably enriched in Li (100-500 ppm) and interpreted to be metamorphosed slightly calcareous semi-pelitic and psammitic rocks deposited in a predominantly eugeoclinal environment. Tin-bearing leucosomes contain a mineral assemblage which includes cassiterite, nigerite, Fe-Mg-tourmaline, Mn-almandine, feldspars, quartz, minor epidote group minerals, and intergrowths of holmquistite, chlorite, muscovite, paragonite, and altered biotite. Cassiterite contains exsolved Nb-Ta phases characteristic of cassiterites from greisen and pegmatite environments.; Field, mineralogical, and chemical data support the derivation of the leucosomes from the gneisses by anatexis during a Barrovian-type sillimanite-grade (T = 710 {dollar}pm{dollar} 75 C, P = 5-8 Kb) metamorphic event that affected the Southern Appalachians during the Taconic Orogeny about 450 to 480 m.y. ago. Partial melting was accompanied by remobilization of Sn present as detrital cassiterite in the sedimentary protolith. Partitioning of Sn into a fluid phase was accompanied by enrichment of B, Li, Nb, Ta, Fe, Mg, Zn, K, Na, and Ca.; Alluvial cassiterite in the Hawkins Branch study area is characteristic of cassiterite from pegmatite and greisen environments. On the basis of a local stratiform tin occurrence in this area, and on cassiterite geochemistry, alluvial cassiterite is assigned to an anatectic leucosome source. |
