Abstract

Bonanza-vein gold ore from the Sleeper deposit, Nevada, was deposited at a depth of less than 500 m and exhibits a characteristic texture of alternating colloform bands of fine-grained quartz and bands of opaline silica containing abundant electrum. Textural evidence indicates that the quartz bands recrystallized from a gel-like amorphous precursor, whereas the gold-rich bands were deposited originally as amorphous precipitate formed by the coagulation of colloidal silica and gold particles. Gold and silica precipitated initially as colloidal particles at deeper levels in the hydrothermal system and were mechanically transported upward by the hydraulic action of the ore-forming fluids. Coagulation of the colloidal particles may have been caused by solution cooling or boiling, and catalyzed by hydrated polycationic Al3+ aqueous species. Transport of gold as colloidal particles in epithermal systems indicates that, at least in some instances, higher gold content is possible than in true solution. A gold-enriched ore fluid appears necessary to explain the origin of the bonanza-grade ore at the Sleeper mine, and the colloidal gold transport process locally might have played a role in the genesis of the more-common disseminated epithermal precious-metal deposits.

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