DENDRITIC ORE MINERALS IN THE BUCKSKIN NATIONAL AND FIRE CREEK BONANZA-STYLE LOW-SULFIDATION EPITHERMAL DEPOSITS, NEVADA

Bonanza-style vein ores from low-sulfidation epithermal deposits such as the Buckskin National and Fire Creek deposits in northern Nevada contain ore minerals that show dendritic habits. Thin section inspection revealed that the dendritic ore minerals only occur in specific colloform bands within the veins. Most commonly, the ore minerals are intergrown with mosaic quartz or quartz forming small euhedral grains. Relict 2-5 µm large silica microspheres can be recognized in some of the ore-bearing colloform bands. The ore minerals were never observed to occur in colloform bands comprised of chalcedonic quartz.

To study the shape of the ore minerals in three dimensions, several vein samples from both deposits were treated in concentrated hydrofluoric acid. After several days, all quartz was dissolved and the fragile grayish brown to dark gray dendritic ore minerals could be recovered. Scanning electron microscopy showed that naumannite and sulfur-rich naumannite are the most common ore minerals in the obtained concentrates. The dendritic grains range from 0.5 to 1 mm by 1 to 5 mm in size. They show multi-branching tree-like forms that have a fractal pattern. The surfaces of the dendrites are characterized by the presence of small pits that have negative crystal shapes that are up to 10 µm in size, resembling the small quartz crystals identified in thin section.

The dendritic ore minerals in the bonanza-style veins from the Buckskin National and Fire Creek deposits are interpreted to have formed during brief periods of intense boiling or flashing of the hydrothermal fluids. Violent vaporization of the liquid may have resulted in the formation of colloidal particles allowing the growth of the dendrites. The occurrence of silica microspheres, which originally may have been composed of opal-A, is consistent with rapid deposition of silica during phase separation.