The significance of 1.4 Ga A-type granitoids (granitic rock) in the geologic evolution of the conterminous United States has been well established since Anderson (1983) compiled and synthesized available data pertinent to these rocks. A-type granites, as originally defined by Loiselle and Wones (1979), are iron-enriched granitoids that occur in an anorogenic (free from mountain-making disturbance), within-continent setting. The importance of A-type granitoid magmatism is underscored by the large-volume magmatic events these rocks depict as well as their broad geographic distribution and association with mineral deposits. It is not well known whether 1.4 Ga magmatism is related to secular (slow, steady) evolution of terrestrial tectonic processes, a special case of crustal melting of thin young crust, and/or differentiation of mafic crust. Similarly, the characteristics of 1.4 Ga intrusive rocks known to be associated with rare earth element mineral deposits that distinguish them from similar rocks that lack these types of mineral deposits are not well constrained. Consequently, seeking answers to these questions and developing the associated knowledge is critical to the Mineral Resource Program and their efforts to conduct more accurate mineral resource assessments.
We intend to evaluate the petrology and metallogeny of rare earth element-mineralized 1.4 Ga intrusive systems at Mountain Pass and in the St. Francois Mountains relative to those of other, apparently unmineralized 1.4 Ga intrusive systems in the conterminous U.S. The principal objective is to fully characterize, using existing data, the petrochemical and isotopic attributes as well as the large-scale tectonic setting of particularly well-studied 1.4 Ga intrusions of the conterminous U.S. and, for comparative purposes, synthesize information for the full suite of these 1.4 Ga magmatic systems. We also plan to investigate the petrochemical, petrographic, and isotopic character of particular 1.4 Ga intrusions known to have temporal and genetic associations with rare earth element deposits (especially those at Mountain Pass and in the St. Francois Mountains) and to compare and contrast their attributes with those of apparently barren systems.
Anderson, J.L., 1983, Proterozoic anorogenic granite plutonism of North America, in Medaris, L.G., Jr., Byers, C.W., Mickelson, D.M., and Shanks, W.C., eds., Proterozoic geology—Selected papers from an international Proterozoic symposium: Geological Society of America Memoir 161, p. 133–154.
Loiselle, M.C., and Wones, D.R., 1979, Characteristics and origin of anorogenic granites: Geological Society of America Abstracts with Programs, 11 (7), p. 68.
Basic geochemical parameters will be established for well-studied 1.4 Ga intrusive bodies. We plan to characterize how their geochemistry is distinct relative to that of similar, but mineralized, systems. We will draw compositional distinctions between those 1.4 Ga igneous systems with associated mineral deposits and those without. Our investigations will also include detailed evaluations of particular, especially well-characterized systems in order to establish the geochemical evolution of associated magma reservoirs and whether particular magma evolution pathways were more conducive to formation of mineralizing systems than others.
We will use available radiogenic isotope data to evaluate the spatial-temporal secular variation of the broadly 1.4 Ga magmatism in the conterminous U.S. and to evaluate whether magmatism associated with specific times or spatial domains was more conducive to REE mineral deposit formation than others. Radiogenic tracer data will also be applied to determine whether magma derived from a particular source or containing distinct contaminants is more likely to be associated with mineralization. We also plan to incorporate unpublished USGS data from two critical regions (the Northern Rocky Mountains and the Southwest U.S.), where excellent exposures of 1.4 Ga intrusions in their country rock context are plentiful, to provide vital direct observations about tectonic setting for the best-characterized intrusions.
A critical component of this investigation includes evaluating the region-scale tectonic/petrologic framework in which each of these magmatic/mineralizing systems developed. Concepts already advanced by the Basement Tectonic Framework of the U.S. Project and the resulting basement domain map (Lund and others, 2015) pertaining to the geologic structures and processes responsible for growth of North America provide the essential framework within which to consider and interpret 1.4 Ga magmatism and associated mineralization within the conterminous U.S. The age and isotopic characteristics of the basement province from which various 1.4 Ga magmas were produced and whether certain basement provinces enhance potential for associated REE mineral deposits will be determined by application of available radiogenic data.
du Bray, E.A., Holm-Denoma, C.S., San Juan, C.A., Lund, Karen, Premo, W.R., and DeWitt, Ed, 2015, Geochemical, modal, and geochronologic data for 1.4 Ga A-type granitoid intrusions of the conterminous United States: U.S. Geological Survey Data Series 942, 19 p., http://dx.doi.org/10.3133/ds942.
Edward du Bray
Central Mineral and Environmental Resources Science Center