Rare earth element geochemistry of the Sierra Blanca laccoliths, Texas.

摘要

The Sierra Blanca area covers about 950 square Miles (2460.5 square km) in southern Hudspeth County, Texas. It includes the Finlay Mountains, Triple Hill, and Sierra Blanca 15-minute quadrangles and the parts of the Fort Quitman and McNary quadrangles that are in the United States northeast of the Rio Grande (Albritton and Smith 1965).Laccoliths exposed at Sierra Blanca are known as the Sierra Blanca peaks, located just west of the town also named Sierra Blanca, approximately 85 miles (135 km) southeast of El Paso, Texas and North of Interstate highway 10, in the Trans-Pecos magmatic province. The five laccoliths are, clockwise from the west (Figure 1) Round Top (RT), Little Round Top (LRT), Little Blanca (LB), Triple Hill (TH), and Sierra Blanca (SB). Round Top laccolith, which is the focus of this investigation, is a mushroom shaped shallow intrusion exposed at the northwest edge of the Sierra Blanca area. Round Top lies in an ancient structural boundary known as the Texas Lineament. This boundary is composed by a rigid structural platform and a mobile geosynclinal belt. The platform nowadays forms part of the basin and range province while the mobile belt forms part of the Sierra Madre Oriental in Mexico (Albritton and Smith, 1965).;The laccoliths are contemporaneous to the adjacent Quitman Caldera (~35Ma) (Shannon and Goodell, 1986) and originated from the same thermal event, but it is suggested that they have a different rare earth element (REE) enriched source (Goodell, Mahar, personal communication 2013). This area has been has been studied for economic reasons in the past. Since the discovery of the enrichment in REEs, uranium, and beryllium (Albritton and Smith, 1965) Round Top has been the target of few exploration activities to determine the potential economic value of the deposit. In the 1800s, geochemists observed a pattern in the ocurrence and crustal abundance of the REEs. REEs with even atomic numbers were more common in nature. Additionally, REEs with lower atomic numbers (La, Ce, Pr, Nd, Pm, Sm,Eu, and Gd) were noted to be coomon ionic constituents in REE mineral ores and, in general, ocurred in greater abundance than REEs with higher atomic numbers (Tb, Dy, Ho, Er, Tm, Yb, and Lu) (US Environmental Agency, 2012). The former elements La, Ce, Pr, Nd, Pm, Sm,Eu, and Gd are referred as the light REEs and the later Tb, Dy, Ho, Er, Tm, Yb, and Lu are the heavy REEs. In Early 1960s, REEs were divided into three groups: Lights or cerium group, from lanthanum (La) through samarium (Sm); Middle or terbium group, from europium (Eu) through dysprosium (Dy); Heavy or yttrium group, from holmium (Ho) through lutetium (Lu) and including yttrium (Y) (Kramers, 1961). Early 1990s, REEs were grouped as: LREE or cerium group, from lanthanum (La) through gadolinium (Gd); and HREE or yttrium group, from terbium (Tb) through lutetium (Lu), including yttrium (Jackson and Christiansen, 1993). In mid 1990s, the REEs grouping was proposed as: Lights group, from lanthanum (La) through neodymium (Nd); Middle group, from samarium (Sm) through dysprosium (Dy); and Heavy group, from holmium (Ho) through lutetium (Lu) and including yttrium (Sabot and Maestro, 1995). However, Round Top is enriched especially in heavy REEs and Yttrium, accounting for more than 60% of total rare earth elements enrichment making it a unique deposit. Most deposits around the world are primarily enriched in light REEs with lower concentration of heavy REEs. Round Top is potentially a low grade high tonnage deposit and possible supply of heavy REEs the western hemisphere.