LEAD-ISOTOPE COMPOSITIONS AS GUIDES TO EARLY GOLD MINERALIZATION - THE NORTH AMETHYST VEIN SYSTEM, CREEDE DISTRICT, COLORADO

The North Amethyst vein system, which is hosted by similar to 27 Ma Ca rpenter Ridge Tuff and similar to 26 Ma Nelson Mountain Tuff has two m ineral associations separated by brecciation and sedimentation in the veins. The early association consists of quartz, rhodonite, hematite, magnetite, electrum (Au0.3-0.5Ag0.7-0.5), and Mn carbonate, Au-Ag sulf ide, Ag sulfosalt, and base metal sulfide minerals. The later mineral association cuts the Mn- and Au-bearing assemblages and consists of qu artz, calcite, sericite, chlorite, hematite, adularia, fluorite, base metal sulfides, and Ag-bearing tetrahedrite. Our detailed studies show that the Pb isotope compositions of paragenetically early galenas ass ociated with Au-rich mineralization in the North Amethyst vein system are relatively unradiogenic (Pb-206/Pb-204: 18.826-18.881, Pb-207/Pb-2 04: 15.588-15.602, and Pb-208/Pb-204: 37.790-37.926) compared to Pb is otope compositions of galenas formed later at about 25 Ma during Ag an d base metal mineralization (Pb-206/Pb-204: 19.041-19.115, Pb-207/Pb-2 04: 15.627-15.672, and Pb-208/Pb-204: 37.829-38.057) New Pb isotope da ta for the central and southern parts of the Creede district, which ar e located 5 to 7 km south of the North Amethyst area, agree with the r esults of a regional study by Doe et al. (1979) that included five sam ples from the main part of the Creede district. Galenas and adularia f rom the central and southern Creede district and galenas from the Bond holder district, 7 km north of the North Amethyst area, are similar to the Pb isotope compositions of galenas formed later in the North Amet hyst area during Ag and base metal mineralization. Galenas from the Al pha-Corsair vein, which was mined for Ag prior to 1910, are isotopical ly similar to galenas associated with North Amethyst Au-stage minerali zation. This isotopic similarity suggests that unexplored segments of the Alpha-Corsair structure may have the same mineralogy as the North Amethyst Au stage; thus, the Alpha-Corsair structure has the potential for high gold contents. Pb isotope compositions from the late stage o f the North Amethyst vein system and from the Bondholder and central a nd southern Creede mining districts are more radiogenic than the host volcanic rocks of the central cluster of the San Juan volcanic field. Our Ph isotope results indicate that early Au mineralization of the No rth Amethyst area may represent the product of an older and relatively local hydrothermal system distinct from that of the younger base meta l and Ag mineralization found throughout the region. Fluids that depos ited Au minerals may have derived their Pb isotope composition by a gr eater degree of interaction with shallow, relatively less radiogenic v olcanic wall rocks. The younger, base metal and Ag-rich mineralization that overprints the Au mineralization in the North Amethyst area clea rly has a more radiogenic isotopic signature, which implies that the l ater mineralization derived a greater component of its Pb from Protero zoic source rocks, or sediments derived from them. Paragenetically ear ly sulfide-rich vein assemblages have the least radiogenic galenas and generally also have the highest Au contents. Thus, identification of paragenetically early vein assemblages with relatively unradiogenic Pb isotope compositions similar to those of the North Amethyst area prov ides an additional exploration tool for Au in the central San Juan Mou ntains area.