Pseudobrookite From the Nine Mile Pluton, Marathon County, Wisconsin: An Occurrence From A Plutonic Environment

Abstract

Abstract Pseudobrookite has been found in vuggy episyenite surrounding a thin vein in granite of the Nine Mile pluton, the youngest and most silicic of four plutons comprising the Wausau Complex. The pseudobrookite occurs as black to dark grey prismatic crystals up to approximately 1.5 mm in length, clustered in radiating sheaves or as single crystals. Electron microprobe analyses show no significant components other than Ti, Fe, and minor amounts of Mn, Nb, and Ta. Major associated minerals are K-feldspar, biotite, and quartz, and accessory ilmenite, rutile, anatase, sphalerite, chalcopyrite, pyrite, arsenopyrite or marcasite replaced by goethite, and molybdenite, cassiterite, fluorapatite, fluorite, monazite, zircon, rhabdophane-(Ce), and bastnasite-(Ce). Small sprays of secondary gypsum crystals are also present. The cassiterite crystals commonly contain inclusions of fluorapatite and monazite-(Ce). This association suggests that the pseudobrookite formed in a high-temperature volatile- and Ti-rich environment (Parodi et al . 1989). Conditions were likely close to a vapor phase environment, consistent with the typical pseudobrookite environments. The ambient pressure was likely somewhat higher, although it is believed that the Nine Mile granite was intruded at a shallow level in the crust (Myers et al . 1984). Hence the conditions of formation of the Nine Mile pseudobrookite probably bore at least some similarities to the more typical volcanic volatile-rich, high-temperature paragenesis in lithophysae of felsic volcanic rocks. The key to the preservation of pseudobrookite from this plutonic environment was the rapid evacuation of all fluid phases during a pressure quench episode.