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Fluid flashing and mineral precipitation in intermediate sulfidation epithermal vein deposits: case studies from the Creede and Sunnyside deposits in southwest Colorado

Guzman, Mario A.
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Monecke, Thomas
Düzgün, H. Sebnem
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2024
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2024 - Mines Theses & Dissertations
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https://hdl.handle.net/11124/179224
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Abstract
This study examines the mineralization processes that resulted in the formation of world-class intermediate sulfidation epithermal vein deposits in the San Juan Volcanic Field in southwest Colorado. Research focused on the North Amethyst deposit in the 26.5 Ma Creede mining district and the ~16–13 Ma Sunnyside deposit. Petrographic analyses from the North Amethyst intermediate-sulfidation epithermal deposit in Creede, Colorado, revealed that ore minerals originally formed dendrites in the colloform banded quartz veins. The ore mineral dendrites are interpreted to record crystal growth at far-from-equilibrium conditions and appear to have formed in an originally non-crystalline silica matrix that has recrystallized to quartz. The textural evidence is consistent with the attainment of high degrees of metal and silica supersaturation during vein formation, possibly suggesting that mineral deposition accompanied flash vaporization of the hydrothermal liquids caused by sudden pressure drops. 40Ar/39Ar radiometric dating of vein adularia from the North Amethyst vein yielded a younger age than the Main Creede mining district to the south. Late-stage mineralization formed during more steady-state flow conditions. The occurrence of flashing at North Amethyst and possibly other parts of the district along with the new geochronological constraints contradicts the long-standing district scale mixing model and necessitated the development of a new hydrogeological model. At the Sunnyside intermediate-sulfidation epithermal deposit, six polymetallic veins stages were distinguished that are characterized by different ore mineral assemblages. The textural evidence, including the presence of ore mineral dendrites, suggests that mineral precipitation during the different vein stages also occurred at far-from-equilibrium conditions. Steady-state fluid flow prevailed during the waning stages of the hydrothermal activity. Petrographic analysis of deep drill core below the Sunnyside deposit revealed the presence of high-temperature quartz veins that contain heterogeneous silicate melt inclusions suggesting the presence of a nearby porphyritic intrusion. Sunnyside represents an example of an intermediate-sulfidation epithermal system that formed by flash vaporization directly above a porphyritic intrusion. The research highlights that mineral precipitation in the epithermal environment is most effective where flash vaporization of hydrothermal liquids occurs, adding to the growing evidence that this process of near instantaneous vaporization of large amounts of liquid is more common in the epithermal environment than previously recognized and likely an important process involved in the formation of intermediate-sulfidation epithermal deposits.
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