Abstract

The Pliocene Silver Creek porphyry Mo deposit at Rico, southwest Colorado, is the youngest known porphyry Mo deposit in the Colorado mineral belt. Associated sulfate replacement deposits, epithermal Pb-Zn-Ag vein and replacement deposits, and Recent hot springs also occur. We investigated the S, C, Sr, and Pb sources at the various levels of this well-preserved hydrothermal system by isotopic studies of the mineralization, and of sedimentary and igneous rocks in the area.Hydrothermal sulfide delta 34 S values are tightly grouped at ca. 0 per mil (x = 0.3 + or - 2.0ppm, 1Sigma ; n = 107) and are substantially lower than those of the hydrothermal sulfates (x = 15.2 + or - 1.6ppm, 1Sigma ; n = 14). Pliocene hydrothermal carbonate delta 13 C values (-7.9 to +1.1; x = -4.9 + or - 2.7ppm, 1Sigma ; n = 16) are similar to those for the Pennsylvanian Hermosa Formation carbonate (delta 13 C = -6.7 to +2.6ppm; x = -3.0 + or - 3.2, 1Sigma ; n = 5), which hosts most of the base and precious metal mineralization, whereas Recent hot spring tufas have somewhat higher values (delta 13 C = 1.8-5.5ppm; x = 3.7 + or - 1.3ppm, 1Sigma ; n = 5). Pliocene hydrothermal carbonates have delta 18 O values between -5.1 and 12.3 per mil (x = 5.0 + or - 6.3ppm, 1Sigma ; n = 16). The tufas have higher delta 18 O values (x = 15.4 + or - 2.6ppm, 1Sigma ; n = 5).Hydrothermal carbonates (including tufa) and sulfate have initial 87 Sr/ 86 Sr (sub (i)) values between 0.70700 and 0.71168. Pennsylvanian sedimentary rocks in the Rico district had 87 Sr/ 86 Sr ratios between 0.70877 and 0.71694 at the time of mineralization. Fresh and altered Pliocene monzonite intrusions at Rico have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70581 and 0.70563, respectively, whereas variably altered rholite dikes have initial 87 Sr/ 86 Sr (sub (4)) values of 0.70987, 0.71574, and 0.71716. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 206 Pb/ 204 Pb ratios of molybdenite, pyrite, and galena taken together are 18.27 to 19.65, 15.55 to 15.70, and 37.75 to 38.62. Pliocene lamprophyre and rhyolite dikes at Rico have equivalent Pb isotope ratios of 18.26, 15.48, and 37.81 and 18.79, 15.65, and 38.02, respectively. These Pb isotope data are typical of Tertiary volcanic rocks and associated sulfides in southwest Colorado.Our delta 34 S data indicate a magmatic S source for the porphyry Mo and epithermal base metal sulfide mineralization. Most of the sulfide Pb is also of magmatic origin. In contrast, hydrothermal sulfate replacement deposits peripheral to the porphyry Mo deposit, carbonate gangue from the base metal deposits, and the Recent hot spring tufa derived their respective S, C, and Sr from local sedimentary rocks. Our delta 18 O data indicate that the hydrothermal carbonates were deposited from 18 O-shifted meteoric waters. Our comprehensive isotopic data set presents a similar genetic picture to that of other molybdenite deposits in the mineral belt, in that the porphyry Mo mineralization was dominated by magmatic components, whereas the peripheral base metal mineralization contains both magmatic and upper crustal components. These important genetic similarities are independent of fundamental differences in tectonic setting.

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