Abstract
Tertiary low sulfidation (LS) epithermal deposits in the western USA often show evidence of the former presence of nanoparticle-sized precious-metal and silica phases in the highest grade (bonanza) ores. Here, nanoparticles are defined to have a size less than ~10−7 m. The ore-mineral textures that formed from aggregation of nanoparticles (or colloids) observed to date in these ores include electrum and naumannite (Ag2Se). Here it is proposed that chalcopyrite also forms nanoparticles in these ores, but sulfide nanoparticles apparently have significantly different physical (surface) properties than the precious-metal phases, and thus exhibit different mineral textures (e.g., no textural evidence of previous chalcopyrite nanoparticles). Textures described here show that nanoparticles of precious-metal phases and silica were episodically and often repeatedly deposited to form the banded bonanza veins typical of many western USA epithermal deposits. Chalcopyrite is the most abundant metal-sulfide mineral in these bonanza ores, and it was also deposited episodically as well, and it appears to replace earlier formed naumannite dendrites. However, this apparent “replacement” texture may just be the result of naumannite dendrite limbs trapping chalcopyrite nanoparticles that later recrystallized to the apparent replacement texture. The episodic and repetitive nature of the metal-depositing events may record periodic “degassing” of magma chambers at depth, where metals are repeatedly delivered to the shallow epithermal environment by “vapor-phase” metal (loid) transport.
Highlights
The “paragenetic sequence” diagram has been the hallmark of ore-textural investigations for more than a century, and it has proven useful to illustrate time relationships in ore-forming events
Assuming that both metal “alloys” and sulfide nanoparticles form in the epithermal deposits discussed here, and their deposition leads to the textures documented here, an important question is where do they form? Saunders [1,2]
The term “nanoparticle” is proposed here with respect to ores and hydrothermal fluids, as opposed to colloids that are more of a low-temperature laboratory construct
Summary
The “paragenetic sequence” diagram has been the hallmark of ore-textural investigations for more than a century, and it has proven useful to illustrate time relationships in ore-forming events. Ag-Se(±S) phases; (3) sulfide and sulfosalts in intimate association with precious- metal minerals at the hand-sample scale are typically much less abundant than electrum+naumannite; and (4) they all have some textural evidence of silica and Au-Ag phases being transported and deposited in the bonanza ore setting as small particles [1,2,3,4]. The Sleeper deposit, and later Saunders et al [3] proposed that naumannite (Ag2Se) formed colloids in bonanza Se-rich epithermal deposits of western USA. Saunders et al [4] proposed two different textures that support nanoparticle formation in epithermal ores: (1) evidence of physical transport of nanoparticles by ore-forming fluids, which Saunders et al [4] “sluice box” textures; and (2) a specific colloid aggregation texture which we call “fractal dendrites”. Perhaps the surface properties of the nanoparticles and/or colloids is precisely what keeps them entrained in the upward moving fluids and brings them to the point of deposition that perhaps result in the textures described below
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