Soda Metasomatism in the East-Shasta Copper-Zinc District, Northern California

In this paper albitic volcanic, intrusive, and tuffaceous sedimentary rocks ranging from mafic to felsic in composition and from Middle Devonian to Late Jurassic in age are described. Most of the rocks contain albite as the only feldspar and fall into the petrographic classification of keratophyre and quartz keratophyre. Spilite, meta-andesite, metadacite, metabasalt, albite diabase, meta-quartz diorite, and albite granite are other petrographic types. Virtually all the rocks retain their primary structures and textures although they contain abundant chlorite, fine-grained white mica, actinolite, and other minerals of secondary origin. Fresh augite coexists with albite in a few specimens

Albite of demonstrably secondary origin occurs as partial replacement of plagioclase, and as veinlets, amygdule fillings and overgrowths. It is concluded from the abundance and widespread distribution of demonstrably secondary albite that virtually all the albite in the rocks of the area is secondary, formed principally if not entirely by soda metasomatism during a Late Jurassic orogeny.

The albitizing agents are believed to have been hot aqueous ftuids derived from one or any combination of three possible sources: (1) resurgent sea water, (2) a subjacent pluton of trondhjemite, and/or (3) hydrous minerals undergoing metamorphism to less hydrous types.