Description
The Deer Lakes piemontite occurs in a narrow 2.35 kilometer long by 25 meter wide zone located within the Jurassic-Triassic rocks of the volcanic series of Purple Lake. It comprises a portion of the Mt. Morrison metavolcanic and metasedimentary roof pendant in the eastern Sierra Nevada. The lithology consists of a polymetamorphosed pyroclastic series deposited as a sequence of predominantly crystal and crystal lithic tuffs. Composition of the country rock of the Deer Lakes piemontite zone is rhyolite to keratophyre with a matrix composed of microgranular quartz, feldspar, biotite, and/or muscovite. The piemontite zone is composed of quartz, albite, oligoclase, piemontite, epidote, tremolite, garnet, muscovite, phlogopite, chlorite, hematite, and calcite. Epidote is common throughout the volcanic series of Purple Lake; however, in the Deer Lakes area piemontite is developed in place of, or coexisting with, epidote. Major element composition and conditions of increased oxygen fugacity are the critical factors in the formation of piemontite. Total manganese content is high for both the piemontite-bearing and non-piemontite-bearing samples. Conditions of locally high fO2 favored development of piemontite rather than epidote due to the higher oxidation potential of Mn+3 over Fe+3. The source and control of fO2 is provided by high silica content and the mineral assemblage of hematite-phlogopite. Early regional low grade facies metamorphism was overprinted and partially obliterated by subsequent thermal metamorphism during emplacement of the Sierra Nevada batholith. High magnesium content and high fO2 inhibits the development of a straightforward characteristic metamorphic mineral assemblage placing the piemontite zone in a thermal metamorphic facies transitional between albite epidote hornfels and hornblende hornfels. High magnesium content increases the thermal stability of tremolitic amphibole, consequently preventing formation of hornblende at temperatures required for the development of hornblende hornfels facies. High fO2 also facilitates the preferential partitioning of Mg into the tremolitic amphibole structure with resultant hornblende development delayed.
