GERMANIUM: A NOVEL PATHFINDER FOR SN-W IN THE WESTERN ST.FRANCOIS TERRANE OF MISSOURI

Rift-related rhyolite ignimbrites, lavas, and airfall units in the Eminence-Van Buren area of the western St. Francois Terrane (SFT) attain a thickness > 7 km and a volume > 360 km3. USGS quadrangle mapping of these caldera-related 1470 ± 2.7 Ma rocks is supported by petrographic studies and 111 new whole rock chemical analyses for 11 major and 47 trace elements. Unexpectedly, 6 of these analyses recorded Ge values 10-63X its crustal Clarke (1 ppm) with Ge/Ga = 0.6-1.2. Correlation analysis in this sample set revealed high Ge correlations (r > +.90) with W, Sn, As, Ga, Tl, Mo, Bi, V, U, Th, Ta, Nb, Hf, Co, Cr, Ca, and Mg. Slightly lower correlations exist between Ge, Zn, and Sb. This combination of siderophile and chalcophile elements points to greisen mineralization and suggests that our samples record pneumatolytic Sn-W occurrences. Such deposits are unknown in the western SFT but a notable greisen deposit occurs in the eastern SFT at the Silver Mine locality where cassiterite, wolframite, Ag-tetrahedrite, and base metal sulfides have been mined sporadically since the late 19th century. Abundance reversals such as Ge > Zn, Ga > Zn, and Ge > Sn are not compatible with the primary mineralogy of such deposits and indicate element redistribution. Ge and Ga usually follow Si-Al into weathering products under supergene conditions. However, in a post-caldera environment Ge and Ga were probably disconnected from normal chalcophile partners by geothermal circulation related to dome intrusion and fumaroles. The areas surrounding the anomalous sample sites should be systematically sampled and evaluated for Sn-W potential.