Textural characteristics of ore mineral dendrites in banded quartz veins from low-sulfidation epithermal deposits: implications for the formation of bonanza-type precious metal enrichment

Quartz veins in bonanza-type ore zones of low-sulfidation epithermal deposits frequently contain ore mineral dendrites. Gold and naumannite dendrites hosted by colloform silica bands from four deposits located in California and Nevada were studied to better understand the processes by which these delicate ore mineral aggregates are formed. High-magnification optical petrography revealed that the colloform bands hosting the ore mineral dendrites originally consisted of non-crystalline silica microspheres. Textural relationships suggest that the microspherical silica provided the structural framework for the delicate ore mineral aggregates to grow. The ore mineral dendrites either grew contemporaneously with the deposition of the microspherical silica along the vein walls or after their deposition within permeable gel-like layers of microspheres. Etching in hydrofluoric acid showed that the ore mineral dendrites exhibit complex surface morphologies. The surface morphology of the ore mineral dendrites and their textural relationships with the silica host were modified as a result of post-depositional maturation and recrystallization causing the conversion of the non-crystalline silica to quartz. It is proposed here that ore mineral dendrites formed in low-sulfidation epithermal veins during periods of two-phase flow associated with short-lived events of vigorous boiling or flashing, which caused supersaturation of silica in the liquid and the deposition of the ore minerals.