Oxygen and Hydrogen Isotope Data on the Interaction of Meteoric Ground Waters with a Gabbro-Diorite Stock, San Juan Mountains, Colorado

Abstract

The mid-Tertiary Stony Mountain stock, about 1.3 km in diameter, intrudes horizontally bedded volcanic rocks 10 km NW of the Silverton caldera, in a zone of numerous radial and concentric fractures. The sequence of intrusion is (1) an outer, 150-m-thick, arcuate ring-dike of fine-grained diorite, (2) the main mass of medium-grained biotite gabbro and (3) a central plug, about 0.5 km across, of fine-grained diorite. These rocks are well-exposed over a vertical distance of more than 600 m. The range and average (in parentheses) of whole-rock δO¹⁸ values (SMOW) are: outer diorite, -1.1 to +5.8 per mil (+4.5); gabbro, +1.5 to +5.7 (+4.6) and central diorite, +1.3 to +2.7 (+2.3). The volcanic country rocks within 2 to 3 km of the stock are all highly depleted in 18, with an average whole-rock δO¹⁸ of about +1.5. The δD values of biotites from the gabbro range from -135 to -145 per mil, in striking contrast to biotites in "normal" igneous rocks, which typically have δD = -60 to -80. Thus, during cooling and crystallization, the Stony Mountain pluton underwent varying degrees of exchange with hot, circulating waters of meteoric origin. The calculated water/rock ratios in the hydrothermal system range from about 0.1 to 0.8 (in atom % oxygen). Generally: (A) The δO¹⁸ values in the stock increase with increasing elevation, probably because the heated waters were moving upward here; the higher-level rocks thus reacted with water that already had undergone considerable O¹⁸ exchange. Further, the overlying volcanics were apparently less permeable to H₂O than the lowermost unit in the pile, the San Juan Tuff. (B) δO¹⁸ values are lowest near intrusive contacts, because the convecting ground waters are hottest here. (C) The finest-grained rocks tend to be most depleted in δO¹⁸. (D) The greater the abundance of propylitic alteration minerals (sericite, carbonate, chlorite, epidote, etc.) the lower is the whole-rock ao11 value. (E) Coexisting minerals typically exhibit disequilibrium δO¹⁸ fractionations, indicating that the rock-forming minerals undergo δO¹⁸ exchange to varying degrees, quartz being the most resistant and feldspar the least. (F) Quartz in the hydrothermal veins has δO¹⁸ = -1.8 to +5.1; thus it also was deposited from meteoric waters.

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