Geology and geochemistry of the Deer Trail Pb-Zn-Ag-Au-Cu manto deposits, Marysvale District, west-central Utah

Abstract

The principle sulfide deposits exposed in the Deer Trail mine, Marysvale district, west-central Utah, form a semicontinuous group of elongated strata-bound Pb-Zn-Ag-Au-Cu bodies (mantos) that replaced as many as 15 carbonate beds in the lower part of the Permian Toroweap Formation. This formation is part of a thick section of Paleozoic and Mesozoic sedimentary rocks that underlies middle Tertiary volcanic rocks of the Marysvale volcanic field. The mantos are stacked along a prominent, steeply dipping axial vein and thin symmetrically away from it. Where unoxidized, individual mantos consist almost entirely of sulfides (pyrite-galenasphalerite-chalcopyrite), have sharp contacts with the country rocks, and range from <1 cm to 3 m in thickness. Hydrothermally altered rocks form indistinct envelopes around the veins and mantos. A K-Ar age of 14 m.y. on sericite from one of these altered envelopes indicates that the manto formed during the final stages of igneous activity in this area.In addition to the major metallic constituents Pb, Zn, Cu, Ag, and Au, the ores contain substantial quantities of Mo, As, Sb, Te, Se, Bi, and Hg. Average delta 34 S values for galena, sphalerite, chalcopyrite, and pyrite are -1.1, 0.6, 2.0, and 3.9 per mil, respectively, and a single galena sample had 206 Pb/ 204 Pb of 18.43, suggesting an igneous source for both the sulfur and the metals. Fluid inclusions in main-stage sphalerite homogenize at 250 degrees to 300 degrees C with salinities ranging from near zero to 17 to 22 equiv wt percent NaCl. The inclusions locally contain liquid CO 2 and show evidence of boiling. Temperatures based on delta 34 S sphaleritegalena values range from 245 degrees to 267 degrees C. The delta D (sub H 2 O) and delta 18 O (sub H 2 O) values of the hydrothermal fluids determined from isotopic analyses of quartz and inclusion fluids in fluorite, quartz, and sphalerite range from -131 to -75 per mil and -20.3 to -2.2 per mil, respectively, and correlate with inclusion salinities, temperatures of deposition, and position of the host mineral in the paragenesis. The deposit formed during the interaction of two types of heated aqueous fluids: a high-salinity, probably magmatic and a dilute meteoric fluid. Magmatic water was significant during main-stage base metal mineralization but was replaced almost entirely by meteoric water in the late stages of the paragenesis.One kilometer west of the mantos, an area of hydrothermally altered rock coincides with a domed area cut by radial fractures, suggesting a hidden igneous intrusion at depth. The Deer Trail deposits are part of a halo of metallic deposits that surrounds this altered and deformed core. Many geologic and geochemical relations at Deer Trail are similar to those in a number of the economically more important manto deposits in the western United States (Tintic, Park City, Leadville, Gilman, Pioche), which, by analogy, may have had a similar origin.