HYDROUS CARBONATITE(?) VOLCANISM IN CENTRAL WYOMING

Lithologically consistent transgressive carbonate occurs at 29 localities throughout the (here defined) Sweetwater Rift. Host rocks vary in lithology and from Archean to Pliocene in age. The carbonate is cream-colored, dense to medium-grained xenomorphic granular, locally porphyritic, and variably rich in resorbed silica nodules, cognate and accidental xenoliths, and xenocrysts. Rhyolite, latite, and trachyte xenoliths and euhedral sanidine, oligoclase, and hornblende xenocrysts suggest associated alkalic cryptovolcanism. Fluid viscosity was high enough to produce isolation of nodules and xenoliths/crysts, flow structure, and sharp separation of distinct textures, but low enough to permit entry into tortuous microfractures. Dolomite, aragonite, wollastonite, tremolite, diaspore, zeolites, nontronite, illite, and chlorite are inconsistent accessories. Ba, Mn, Sr, Rb, P, V, Zn, Cr, Th, U, REE, Y, and Ni are anomalously abundant. The paragenetic cycle comprised phases of immiscible silica-gel separation, primary crystallization of silica, minor silicate, and carbonate, resorbtion of solids, recrystallization/replacement by carbonate, fracturing/brecciation, and hydrothermal veining by carbonate. These phases equate to magmatic, deuteric, and hydrothermal regimes. Up to five generations of silica and six of carbonate occurred in each cycle. Cycles were repeated several times at most localities. The sequence in silica nodules is jasperoid, alpha cristobalite, opal, chalcedony and coarse quartz. Successive generations of silica and carbonate were increasingly coarser-grained and punctuated by deformation. Carbonate bodies occur as impregnations/replacements within fault zones, stockworks in host-rock shatter-pipes, pipes, and small-mesa caps (flows?). Emplacement was hypabyssal to surficial at temperatures peaking in the jasperoid/alpha-cristobalite range. We interpret fluid evolution from carbonatite magma to hydrothermal carbonate brine, involving meteoric water.