Abstract

Polymetallic vein ores of the Lake City district, in the western San Juan Mountains, Colorado, have produced approximately $12 million in Ag, Au, Pb, Zn, and minor Cu since 1874. Veins cut intermediate to silicic Tertiary volcanic rocks in an arcuate belt around the north, northeast, and east sides of the 22.5-m.y.-old Lake City caldera. The veins, typically 0.5 to 1.5 m wide and as much as 2,000 in long, fill steeply dipping (> or = 60 degrees ) radial, transverse, and concentric fractures and faults outside the ring fault of the Lake City caldera and within the moat area of the older (28 m.y.) Uncompahgre caldera. Evidence of open-space filling (banding, crustification, vugs) is widespread. Multiple periods of mineral deposition in the veins are documented by compound structures in which ore and gangue minerals were repeatedly brecciated and rehealed by other minerals. Wall-rock alteration characteristically is quartz-sericite-pyrite, grading laterally outward into rocks affected by regional preore propylitization.Two distinct mineral suites have been recognized in the veins, an older quartz-base metal assemblage (QBMA) and a younger barite-precious metal assemblage (BPMA). The quartz-base metal assemblage forms veins in fractures and faults exclusively on the north side of the Lake City caldera, where vein minerals include: pyrite; unzoned green or yellow sphalerite (< or = 0.8 mole percent FeS); galena; chalcopyrite; sparse tetrahedrite-tennantite; rare arsenopyrite, hematite, and gold in a gangue of quartz; generally minor rhodochrosite; and traces of pyroxmangite, calcite, sericite, and fluorite. At the Ute-Ulay mine, the largest producer of the district, this assemblage has been divided into four stages (from oldest to youngest): Ia, early quartz-pyrite; Ib, banded quartz-sulfide ores; Ic, massive rhodochrosite; and Id, late quartz-carbonate-fluorite. On a district-wide scale, the quartz-base metal assemblage shows a concentric mineral zonation outward from the ring fault, from an inner quartz-chalcopyrite zone, through a middle hybrid zone, to an outer carbonate-tetrahedrite zone. Mineral zoning is best seen laterally along the 1-km length of the Ute-Hidden Treasure vein system at the Ute-Ulay mine, from quartz and sulfides at the southwest end nearest the ring fault, to massive rhodochrosite, sericite, and tetrahedrite at the northeast end.The barite-precious metal assemblage, younger than the quartz-base metal assemblage on the basis of crosscutting textures and patterns of hydrothermal leaching, fills radial fractures and faults on the northeastern and eastern sides of the caldera. Barite-precious metal assemblage ore minerals include: pyrite, Ag-rich tetrahedrite, galena, distinctively banded red-brown sphalerite (< or = 3 mole percent FeS). chalcopyrite, numerous Ag-Cu-Pb sulfo-salts, minor arsenopyrite, and rare hematite and magnetite; pitchblende, electrum. and Au-Ag tellurides are found locally. Gangue minerals are diagnostic barite, quartz (commonly as banded jasperoid), minor rhodochrosite, and rare sericite and clays. Major paragenetic divisions are: stage IIa, barite-sulfosalt ores; stage IIb, Au-Ag telluride ores; and stage IIc, late barite-chalcedony. A vertical zoning pattern is especially prominent, with barite, tetrahedrite, and tellurides more abundant at higher elevations.Fluid inclusions were examined in minerals from many veins throughout the district. All observed inclusions are simple liquid-vapor types without immiscible liquid CO 2 or daughter minerals. Detailed heating and freezing studies on inclusions in quartz, sphalerite, and fluorite from the quartz-base metal assemblage document a general time-space decrease in filling temperatures (260 degrees -185 degrees C) and salinities ( approximately 4--0.1 equiv. wt. percent NaCl) along the Ute-Hidden Treasure vein system. Quartz and sphalerite from the barite-precious metal assemblage uniquely contain primary liquid-rich and vapor-rich inclusions that at the Ute-Ulay mine yield temperatures of 206 degrees to 385 degrees C and salinities of 1.3 to 12.4 equiv. wt. percent NaCl. The inclusion data suggest boiling of barite-precious metal assemblage fluids occurs at approximately 250 degrees C, a depth of 450 m, and a hydrostatic pressure of approximately 40 bars. Throttling or irreversible adiabatic expansion of the fluid may have taken place at constrictions in the vein, where the barite-precious metal assemblage crosscuts and fills fractures within the older quartz-base metal assemblage.Evaluation of the environments of ore deposition suggests that the quartz-base metal assemblage and the barite-precious metal assemblage formed under generally similar geochemical conditions. Mineral stability relationships and the Fe contents of sphalerites limit the conditions of the quartz-base metal assemblage formation and the principal part of the barite-precious metal assemblage formation to the sulfate-dominant part of the pyrite stability field, with Sigma S = 0.01 m and pH = 4.5 to 5.7 (sericite stable), at 250 degrees C. Minor excursions from this environment took place during formation of the barite-precious metal assemblage, including early jasperoid formation on the acid side of the kaolinite-muscovite boundary, and main-stage ores deposited locally near the pyrite-hematite-magnetite triple point. Major differences in mineralogy between the quartz- and barite-rich assemblages, together with the similarity of the geochemical environments, suggest differences in the original bulk composition of the hydrothermal fluids which deposited them.Integrated field and laboratory studies suggest that the two assemblages formed from two separate hydrothermal systems generated independently during the Lake City caldera cycle. Fractures hosting the quartz-base metal assemblage were initially opened during resurgent doming of the Uncompahgre-San Juan-Silverton calderas 27.5 m.y. ago and were reactivated and mineralized probably contemporaneously with the emplacement of granite porphyry bodies intruded during resurgence of the Lake City caldera, 22.5 m.y. ago. The Red Mountain plug dome, emplaced along the eastern ring fault zone late in the caldera cycle, initiated a separate hydrothermal system that deposited the younger barite-precious metal assemblage. The Lake City caldera is unique among ash-flow calderas of the San Juan Mountains, for it possesses alteration and mineralization that are closely related both in time and space to the caldera cycle itself.

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