THE NATURE OF LOW-T K-METASOMATISM AT CREEDE, COLORADO

Wallrocks of the Creede epithermal Ag-Pb-Zn deposit (25.1 Ma) underwent extensive pre-mineralization low-T K-metasomatism. The nature and timing of this alteration in the Creede district are debated: early studies linked this metasomatism to resurgence of the Bachelor caldera (~27.5 Ma), but K-metasomatism of later Creede caldera fill (~26.7 Ma) suggests it may only slightly predate mineralization. To better characterize this alteration, we have examined the macro- and grain-scale trace element and oxygen isotope features of: 1) four traverses sampling wallrocks away from the Amethyst vein; 2) a short traverse across an unmineralized alteration front at Nelson Creek, and 3) five drill cores of paleochannel fill in the wall of the Creede caldera. In each suite, feldspar phenocrysts were replaced by K-feldspar (Or_>97) ± I/S ± kaolinite. Samples are enriched in K, Rb, and Cs and depleted Na, Sr, and Ca. Fe₂O₃/FeO increases with K/Na. Replacement K-feldspar is rich in Rb (~1000 ppm) and poor in Sr (15-500 ppm) relative to igneous feldspars.

Minor but significant differences exist among the suites. K-metasomatism affected both tuff cobbles and their conglomerate matrix throughout all paleochannel cores. Although late, celsian–rich feldspar replacement is common in the paleochannel rocks, it is rare in the other suites. Celsian may represent spent mineralizing fluids that drained from the ore system into the paleochannel. Samples from the paleochannel, like some samples from Amethyst wallrocks, are rich in Pb and Zn, consistent with interactions with mineralizing fluids. Wallrocks from the northern Amethyst mine workings contain coexisting adularia and albite (the latter is absent from the other suites), possibly the result of higher alteration temperatures. Rocks and replacement feldspar alike from the mine workings and the creek traverse have d¹⁸ O values of 4-6‰ that anticorrelate with whole-rock K/Na, whereas initial results from paleochannel rocks show alteration feldspar and groundmass with d¹⁸ O of 6-8‰. These rocks have undergone multiple metasomatic events; our observations raise the possibility of there having been multiple episodes of K-metasomatism in the Creede district.