Isotopic studies of authigenic sulfides, silicates, and carbonates, and calcite and pyrite veinlets in the Creede Formation, San Juan Mountains, Southwest Colorado

Abstract

Sulfur isotope analysis of authigenic pyrite in the Creede Formation documents its precipitation by the reaction between iron in the volcaniclastic sediments and H 2 S formed through bacteriogenic reduction of sulfate added to the lake during and immediately following repeated volcanic eruptions during sedimentation. Pyrite veinlets in the underlying Snowshoe Mountain Tuff were formed by the percolation of H 2 S-bearing pore waters into fractures in the tuff. Conventional analyses of bulk samples of authigenic pyrite range from -20.4‰ to 34.5‰, essentially equivalent to the range of -30‰ to 40‰ determined using SHRIMP microprobe techniques. Conventional analyses of bulk samples of pyrite from veinlets in the Snowshoe Mountain Tuff range from -3.5‰ to 17.6‰, much more limited than the ranges of -23‰ to 111‰ and -15.6‰ to 67.0‰ determined by SHRIMP and laser ablation microbeam techniques, respectively. The extreme range of δ 3 4 S for the veinlets is interpreted to be the result of continued fractionation of the already 3 4 S-depleted pore waters. Oxygen isotope analysis of authigenic smectite, kaolinite, and K-feldspar together with fluid-inclusion temperatures and oxygen isotope analysis of calcite coexisting with kaolinite indicate that the smectites formed early during burial diagenesis, in accord with the petrographic observations. The 4 0 Ar/ 3 9 Ar dating of K-feldspar, concordance of K-feldspar, kaolinite, and calcite δ 1 8 O values, and fluid-inclusion temperatures in calcite, indicate that the sediments at core hole CCM-1 were subjected to a hydrothermal event at 17.6 Ma. The minerals formed from oxygen-shifted meteoric waters with δ 1 8 O values of ∼-9‰. Smectites at CCM-1 at least partially exchanged with these waters. Carbon and oxygen isotope analysis of authigenic calcites in the Creede Formation show that they formed over a wide range of temperatures from fluids having a wide range of isotopic composition, presumably over an extended period of time. Some of the cements apparently formed very late from unexchanged meteoric water. Concretions and possibly some cements at CCM-1 appear to have exchanged with the 17.6 Ma oxygen-shifted hydrothermal fluids. Such exchange is consistent with evidence that lacustrine carbonates at CCM-1 exchanged with low 1 8 O waters, whereas those at CCM-2 underwent little, if any, exchange. The δ 1 3 C-δ 1 8 O values for calcite veinlets in the Creede Formation are similar to those for authigenic calcites. Fluid-inclusion temperatures and δ 1 8 O values indicate that some were deposited during the 17.6 Ma hydrothermal event and others from unexchanged meteoric water at a later date. The isotope studies confirm that part of the model of Rye et al., proposing that the barites in the southern end of the Creede Mining District were formed by mixing of the Creede hydrothermal system with Lake Creede pore or lake waters. The silicate and carbonate isotope data indicate that the pores of the Creede Formation were occupied by at least three isotopically distinct waters since the time of deposition. The original pore fluids probably shifted to lower δ 1 8 O values during burial diagenesis as a result of the hydrolysis of the volcanic glass to form smectites and other hydrous silicates. During or prior to a 17.6 Ma hydrothermal event in the vicinity of CCM-1, the Creede Formation was flushed with oxygen-shifted meteoric water, possibly related to the breaching of the east side of the caldera wall sometime between 20 and 22 Ma.