Textural Evidence of Episodic Introduction of Metallic Nanoparticles into Bonanza Epithermal Ores

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 (Ag 2 Se). 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.

Formation and Aggregation of Gold (Electrum) Nanoparticles in Epithermal Ores

“…The "fractal" electrum dendrites (discussed below) have been observed in many Tertiary bonanza epithermal ores in northern Nevada, and have been interpreted to be evidence of nanoparticle nucleation and aggregation in ore formation [35,36]. More recently, similar textures and genetic interpretations have been made from ores from the southeastern USA [37] and Bulgaria [38]. Finally, disseminated electrum nanoparticles have recently been discovered in the epithermal Round Mountain (Nevada) deposit and have been proposed to be precursors for coarser electrum crystals there [39,40].…”

“…It is clear that nanoparticle formation and aggregation were significant processes in the formation of Tertiary epithermal ores in Nevada, Eastern Europe, and apparently, even at least one (Cambrian) epithermal deposit of the Carolina Slate Belt of the SE USA [37]. Although, the presence of electrum nanoparticles in the ores does not imply that gold cannot also be precipitated directly from solution as well.…”

“…Other examples of Se-rich LS epithermal ores include Silver City, Idaho; Republic, Washington; and Hishikari, Kushikino; and, Koryu deposits of Japan [45]. In the highest-grade middle Miocene ores, dendritic textures of Au-Ag minerals are common and apparently formed by the aggregation of nanoparticles of electrum, Ag-Se-S phases, and less commonly chalcopyrite [37]. Lead-, osmium-, copper-, and sulfur-isotope studies have been conducted specifically on bonanza ore minerals in the middle Miocene ores, and the results indicate a mafic-magmatic source for the ore minerals [45,48,49].…”

“…The formation of opal as opposed to quartz or chalcedony is additional evidence of the far-from-equilibrium chemical conditions during bonanza ore formation [12]. Fractal dendrites formed by nanoparticle aggregation in bonanza epithermal ores were first recognized for electrum [12], then Ag-Se-S phases [35,36], and more recently chalcopyrite [37,49]. Finally, other textural features of bonanza ores have added further insights about the potential physical transport and also deposition of metallic nanoparticles [36].…”

Textural Characteristics of Noncrystalline Silica in Sinters and Quartz Veins: Implications for the Formation of Bonanza Veins in Low-Sulfidation Epithermal Deposits

“…Electrum is less abundant and typically occurs as inclusions in naumannite. Pyrargyrite locally forms part of the naumannite dendrites [44][45][46]. …”