Hydrothermal wallrock alteration is an important feature of the gold quartz veins of the Pioneer, and Bralorne Mines in the Bridge River district, British Columbia. In the principal host rocks, Bralorne soda diorite and Pioneer greenstone, the alteration is evidenced by bleaching and, to a limited extent, by the formation of a reddish-brown alteration material to a maximum distance of 2 or 3 feet from the vein walls. By microscopic means, the principal alteration minerals are determined to be chlorite, carbonate, sericite, pyrite, and arsenopyrite with some local biotite. In the minor host rocks, Bralorne soda granite and acid dykes, the hydrothermal alteration is not readily apparent to the naked eye. It consists essentially of sericitization although chlorite, biotite, carbonate and pyrite occur in minor quantities. The hydrothermal wallrock alteration in these four host rocks, has caused an increase in carbon dioxide, potash, sulphur and arsenic and a decrease in silica and soda. The above changes in the wallrock, together with the observed mineralogical nature of the veins, is typical of Lindgren's meso thermal mineral deposits. The high temperature minerals, biotite and tourmaline, are formed in minor amounts by mineralizing solutions introduced prior to the depostion of gold. An important feature of the Pioneer-Bralorne Gold Belt is the uniformity of the nature of quartz veins and of hydrothermally altered wallrock over a vertical range of over 3,000 feet. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate