The black skarns of Pine Canyon, Piute County, Utah

A small micromonzonite pluton intruding Miocene extrusives of the Marysvale volcanic field of southern Utah has undergone a sequence of explosive brec ciation, syenite veining, rheomorphism, calcium metasomatism and zeolitizat ion. Skarn-like occurrences of magnetite and Al-spinel-rich feldspathoidal rocks within the outcrop are of two main types: (1) an olivine-bearing vari ety with occasional REE-rich minerals (zirconolite) occurs as veins and sea ms in desilicated latite screens and xenoliths.; (2) an aluminous, corundum - and hibonite-bearing variety with accessory zirconolite, perovskite and a rmalcolite occurs mainly as fillings to the micromonzonite breccias. The ch emically disparate types are interpreted as having initiated as propylitic, and argillic/alunitic, alterations of latite country rock in the early hyd rothermal system of the pluton. Subsequent intrusion of the micromonzonite to higher levels has incorporated these products into the igneous complex, in which they have experienced varying degrees of nephelinization, pyrometa morphism, rheomorphism and veining.