Stable isotope geochemistry of a porphyry-style hydrothermal system, West Silverton District, San Juan Mountains, Colorado, U.S.A.

Abstract

A porphyritic monzonite (25.1 Ma) intrudes Tertiary volcanic rocks of pre- and post-San Juan Caldera age along the western margin of the Silverton Caldera and the northern side of the Sultan Mountain monzonite stock (25.9 Ma), within an area of strong sericitic to argillic alteration (25.0 Ma). These alteration facies, which are genetically related to the monzonite, are characterized by quartz-MoS 2 mineralization (Type 1), are surrounded by pyritic and propylitic facies alteration within which quartz/ base-metal sulfide (Type 2), quartz pyrite (Type 3), and barren quartz (Type 4) veining occurs. Whole-rock H- and O-isotope compositions are variable throughout the district (δD= − 148 to −72%; δ 18O= − 3.5 to +9.3%). The propylitized country rocks are strongly depleted in 18O and D but more highly altered samples have “normal” δD and δ 18O values. Coarse-grained sericites that also exhibit a very large range of δD values to −60%) with respect to biotites and clay-sized sericites that also exhibit a very large range of δD values (−137 to −98% and −139 to −84%, respectively). Type 1 veins have a very restricted range of 18O/ 16O ratios (+7.6 to 9.3%), whereas veins of Type 2 and Types 3 and 4 have very large, overlapping, δ 18O ranges of −2.7 to +12.0% and −2.8 to +9.0%, respectively. Type 1 veinlets are characterized by halite-bearing and vapour-rich types, whereas liquid-rich types (excluding halite-bearing types) are dominant in quartz and sphalerite of vein Types 2, 3, and 4. Homogenization and salinity data indicate higher temperatures and much higher salinities of quartz fluids in the central alteration zones (∼350°C, ∼40 ≡wt%NaCl) ompared with those of the peripheral pyritic and propylitic (∼60°C, ≦5 ≡wt%NaCl). The δD values for quartz fluid inclusion waters are quite variable: Type 1 (−125 to −79% 0); Type 2 (−130 to −67% 0); and Types 3 and 4 (−128 to −98% 0). A “magmatic” fluid component is indicated by the calculated composition of sericitic fluids. Of the quartz fluids, those associated with MoS 2-mineralization lie closest to the field of “Primary Magmatic Water”; those associated with the other vein types lie within a large field which ranges from the composition of nearly unexchanged, local meteoric water to compositions reflecting extensive hydrothermal exchange and mixing.