THE CRYO-GENIE PEGMATITE: A COMPLEXLY ZONED, LCT PEGMATITE IN THE PENINSULAR RANGES BATHOLITH, SAN DIEGO COUNTY, CALIFORNIA

The Cryo-Genie pegmatite is located in the eastern zone of the Peninsular Ranges batholith, San Diego County, California. The immediate host rocks appear to be a combination of medium-grained biotite granite and scattered bits of sillimanite-grade metasedimentary rocks, both of which are enclosed by a zoned La Posta-type pluton. The dike exploited by the Cryo-Genie mine has not been dated but geologic evidence places it between 98 and 89 Ma. The pegmatite ranges in thickness from about 1 meter to 3 meters and has a north-plunging synformal pattern. Since 2001, 4 major and numerous smaller pockets have produced elbaite tourmaline crystals not seen from this region in nearly 100 years. One specimen is more than 34 centimeters in length and weighs more than 2 kilograms. All the larger elbaite crystals exhibit a distinctly flared prismatic habit and are generally doubly terminated. Most of the crystals are pink in color starting from the larger flat termination and but typically taper to a smaller, green pinacoidal termination. One pocket contained aquamarine beryl with elbaite tourmaline inclusions. Other minerals found include fluorapatite, cassiterite, ferrotapiolite, ferrocolumbite, stibiotantalite, lollingite and paakkonenite, a rare arsenic- bearing stibnite. This pegmatite, as well as all pegmatites in the province, are complex zoned. The hanging wall consists of a finer grained granitic rock that contains schorl crystals less than 2 cms in diameter and up to 5 cms in length. In places, these slightly tapered crystals radiated from pocket zones. The footwall consists of alternating bands of tourmaline- and garnet-bearing granite in roughly parallel bands (line rock) up to 5 cms in thickness. The core zone, where pockets occur, is typically between these two zones and contains schorl greater than 1 inch in diameter. Pockets were commonly associated with monoclinal rolls in the line rock. Pockets greater than one meter by one meter exhibited zonation with tourmalines at the bottom of the pocket and a large zone of homogeneous red clay devoid of any crystalline material at the top.