Copper sulfide pseudomorphs after phytodetritus in Devonian sedimentary rocks of the Middle Timan region

Abstract

Copper sulfide pseudomorphs after phytodetritus in ore-bearing sedimentary rocks of the Middle Timan region were studied by ore microscopy, X-ray methods, electron scanning microscopy (microprobe), and chemical analysis. It was established that jurleite rather than chalcocite, as was previously assumed, is the major copper sulfide in pseudomorphs. Other copper sulfides detected in microscopic amounts are inferior to jurleite in abundance. Two types of jurleite—white and light blue—can be distinguished under an ore microscope. In addition to color, jurleite varieties differ in size of crystals and hardness. White jurleite occurs as large grains in superficial crusts and pith pseudomorphs or fills contraction fissures in plant tissue. Light blue jurleite segregations are combined with microgranular units and altered wood. Part of the light blue jurleite replaces the white variety. Light blue jurleite has a higher relief on a polished surface. The common crystallographic orientation of sulfide grains in pseudomorphs was noted for all modes of occurrence of jurleite. Jurleite grains of various shapes and different hardness may be detected in the same sample. In general, copper sulfides were formed in the following sequence: (i) filling of fissures and interstices in wood, (ii) formation of superficial crusts composed of white jurleite, (iii) joining of fissures by pseudomorphs after organic matter and partial replacement of white jurleite with the light blue variety, and (iv) secondary transformation of light blue jurleite with partial redistribution of its constituents between less frequent copper sulfides.