Open-File Report 2006-1259

Published 2006
Online Only

Metamorphic Geology

All of the volcanic, volcaniclastic, and intrusive rocks of the area, other than the late plutons and dikes, have undergone regional metamorphism to middle or upper greenschist facies, mainly expressed by saussuritization of plagioclase and general silicification. In addition, contact aureoles of hornfels as wide as 1,640 ft (500 m) have been recognized around the Lindley Farms Quartz Monzonite plutons (sectors J and N).

The mineralogy of the saussuritized volcanic rocks has depended on the composition of the original rock. Quartz and potassium feldspar appear little changed by metamorphism. Plagioclase, the most abundant mineral in all of the volcanic rocks, has been replaced by abundant epidote, chlorite, and, locally, by calcite in the more calcic rocks, such as andesites. Epidote-rich grain cores in larger plagioclase grains are palimpsests of original calcic zones. In more felsic rocks such as rhyolite, the plagioclase has been saussuritized to yield abundant muscovite and common epidote and very locally by some calcite and (or) chlorite. Leucoxene, sphene, and ilmenite are regular minor constituents associated with chlorite and epidote in the sites of former hornblende grains; little hornblende survived the metamorphism in the volcanic rocks.

Some of the effects of deuteric alteration are similar to the changes resulting from metamorphism and the two are sometimes hard to distinguish. Plagioclase, if of intermediate composition, has been altered to mostly albite and muscovite, but if calcic, to calcite and muscovite. Amphibole and biotite have been converted to mostly chlorite and epidote. Introduction of quartz is widespread and much has replaced matrix minerals and invaded the margins of plagioclase grains.

The sparse local development of amphibole minerals, such as acicular actinolite, suggests metamorphism near the upper limit of the greenschist facies, has taken place especially along the two major fault zones. Quartz gabbro plutons along the Snow Camp fault contain hornblende that we interpret to be metamorphic.

We think that within the intensely altered core zones, pyrophyllite, andalusite, and other aluminum silicate minerals have survived the subsequent regional metamorphism with little modification, exemplified by the dense fine-grained massive variety of pyrophyllite, or by no more than recrystallization, such as the formation of coarse, undeformed pyrophyllite rosettes. All of the andalusite was probably formed as a primary hydrothermal alteration, and much of the alunite as well (Milton and others, 1983; Klein and Schmidt, 1985; Schreyer, 1987; and Klein and Criss, 1988). The presence of these minerals in unmetamorphosed high-sulfidation deposits in other regions indicates that metamorphic conditions, especially deep burial, are not required for their formation.

In a significant study of mineral relations and the crystal forms of alunite in Carolina slate belt deposits, Schreyer (1987) concluded that premetamorphic alunite was preserved as vug fillings in undeformed "quartzites" (quartz granofels). The large, undeformed andalusite crystals observed in several deposits were also interpreted to be premetamorphic (Klein and Schmidt, 1985; Schreyer, 1987; and Klein and Criss, 1988).

 
 

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