GOLD AND RARE EARTH ELEMENTS (REE) DEPOSITS ASSOCIATED WITH GREAT PLAIN MARGIN DEPOSITS (ALKALINE-RELATED), SOUTHWESTERN UNITED STATES AND EASTERN MEXICO

Lindgren (1933) first noted that a belt of alkaline-igneous rocks extends from Alaska-British Columbia southward into NM, Trans-Pecos TX, and eastern Mexico. Along this belt, Au, REE, W, Nb, F, Zr, and other critical elements have been found and exploited from several types of mineral deposits. In NM these deposits were called Great Plain Margin (GPM) deposits, because this north-south belt of Eocene-Oligocene alkaline igneous rocks roughly coincides with crustal thickening along the margin between the Great Plains physiographic province with the Basin and Range (including the Rio Grande rift) and Rocky Mountains provinces, which extends into Trans-Pecos TX and eastern Mexico. Significant Au production in NM has come from deposits found within this belt. Since 1996, only minor exploration and development of these deposits in NM, TX, and eastern Mexico has occurred because of low commodity prices and environmental concerns. However, as the current demand for critical commodities like Au, REE, and Te has increased, new exploration programs have encouraged additional research on these deposits. Deposits of Th-REE-fluorite (±U, Nb) veins are found in the several GPM districts, but typically do not contain significant gold, although trace amounts of gold are found in most GPM districts. Gold-rich deposits in these districts tend to have moderate to low REE and anomalously high W and sporadic amounts of Te. The origin of these deposits is still not well understood, but compilation of new and past data, including new geologic mapping, age dates, and isotopic and chemical analyses of igneous rocks and associated mineral deposits, allows for a better understanding of the characterization and origin of these deposits. The diversity of igneous rocks and associated mineral deposits along this boundary suggests that this region is characterized by highly fractionated and differentiated, multiple pulses of magmas. Both upper mantle and lower crustal source rocks may be involved. Deep-seated fracture systems or crustal lineaments apparently channeled the magmas and hydrothermal fluids. Once magmas and metal-rich fluids reached shallow levels, the distribution and style of these intrusions, as well as the resulting associated mineral deposits were controlled by local structures and associated igneous rock compositions.