NATURAL GOLD DENDRITES FROM HYDROTHERMAL Au-Ag DEPOSITS: CHARACTERISTICS AND COMPUTER SIMULATIONS

Abstract

Epithermal vein gold deposits in the western United States locally contain bonanza zones with extraordinary concentrations of gold in banded ores consisting of alternating gold-rich and barren silica bands. Gold-rich bands commonly consist of coalescing or isolated gold dendrites, which occur in a matrix of variably preserved colloidal silica. Most dendrites appear to have grown outward from near-vertical vein walls and have a rough radial symmetry. SEM images of dendrite surfaces suggest that they formed by the aggregation of spherical particles in the 10–100 nm range. Fractal dimension (Df) of true two-dimensional slices through the dendrites typically are in the 1.6–1.7 range, which compares favorably with the theoretical Df value of ~1.7 for 2-dimensional diffusion-limited colloid aggregation (DLA). Monte Carlo computer simulations best reproduce natural dendrite morphologies when particle randomness is large, sticking efficiency=1, and when particles are required to contact 3 adjacent particles on the dendrite as a prerequisite for aggregation. Thus, the physical transport and aggregation of gold colloids offers an attractive explanation for the origin of these enigmatic super high-grade gold ores.