The lodes occur in Silurian metamorphic micaceous schists, intruded by granite, porphyry and diorite, and traversed by numerous quartz reefs, some of which are gold-bearing.
The upper valley of the Maranon has undeveloped gold-bearing lodes.
The most important minerals are lead and zinc, obtained in lodes in the forms of galena and calamine respectively.
Very rich lodes of the metal have been found in the Northampton, Murchison and Champion Bay districts, and also in the country to the south of these districts on the Irwin river.
Good lodes of stibnite (sulphide of antimony) have been found near Roebourne in Western Australia, but no attempt has yet been made to work them.