Intensive parameters of a sulfide and aluminosilicate-bearing granite, Hancock County, Maine

Abstract

In Hancock County, Maine, a small mineralized fine-grained granite lies at the southeast portion of the felsic Lucerne pluton, near the contact with the Blue Hill pluton. The Cambro-Ordovician, chlorite-rich Ellsworth schist occurs to the north and south of the younger fine-grained granite and is the host to many sulfide deposits within the region. Within the study area the fine-grained granite and the Ellsworth schist have been contact metamorphosed by the Devonian age Lucerne. The fine-grained granite is a quartz-rich, leucocratic, two-mica, two-feldspar granitoid. It is marked by the presence of 1) sulfides (pyrrhotite, pyrite, loellingite, arsenopyrite, chalcopyrite, and sphalerite) 2) volatile-rich phases such as muscovite, tourmaline, apatite, and chlorite, and 3) high-temperature anhydrous phases such as andalusite and fibrolite. Feldspars have been partially altered to muscovite, and biotite has partially altered to chlorite. Sulfides and tourmaline appear to have formed late in the crystallization sequence.

Prior to the intrusion of the Lucerne the fine-grained granite probably cooled to a maximum temperature of about 600°C, and crystallized to form feldspar, biotite, quartz, and muscovite. Andalusite and sillimanite probably formed when the Lucerne intruded; at about 650-725°C and about 1-2 kilobars. Quartz, muscovite, tourmaline and sulfides probably formed during subsequent cooling. Feldspar composition indicate reequilibration with a fluid at about 400°C. Dehydration reactions within the Ellsworth schist probably resulted in the release of metal bearing fluids from the Ellsworth schist and redeposition into the adjacent fine-grained granite.