Central Mineral and Environmental Resources Science Center
Project Chief: Karen Lund
Associate Project Chief: Christopher Holm-Denoma
Large regional (national) resource evaluations can be based on near-surface geology and geologic history and/or on affinity and architecture of the underlying crust. The nature and history of underlying basement is most applicable to evaluating metallogenic provinces (geographic area characterized by a particular assemblage of mineral deposits) and the regional-scale mineralizing processes that operate at lithospheric scales (crust and upper mantle), notably iron oxide-copper-gold (IOGC) and rift-related systems. There are no existing maps of basement affinity and architecture for the entire U.S., so crustal characteristics were generally unavailable for use in previous USGS Mineral Resources Program national mineral resource assessments.
The project's first objective is the development of a U.S. basement terrane map based on basement tectonic processes, the first of its kind for the U.S. The second objective is to investigate 1.7 and 1.4-Ga A-type granites (formed unrelated to collisional tectonics and/or under anhydrous conditions) in relation to the process by which cratons (stable part of continent, typically forms central mass) are formed from juvenile crust and mineralization events.
The task will finalize a basement map of the U.S. (GIS layer) that encompasses the entire U.S. (excluding Hawaii), including from the Archean core cratons to the fringing geologic provinces and terranes (fault-bounded areas/regions) as well as the Alaskan collage of Mesozoic-Tertiary terranes. This involves defining the criteria to identify basement blocks and the various crust-forming tectonic processes that modified the Earth's crust through geologic time. It also involves defining the characteristics relevant to mineral forming processes. The dataset will include crustal affinity, the age of formation and the style and timing of accretion or growth events for each crustal component to allow time-slice views of the continental growth and assembly. Use of a basement map and crustal perspective in assessments potentially provides the Mineral Resources Program with new approaches for evaluating the Nation's mineral resources.
The goals of this task are to (1) characterize the juvenile crust and associated sediments, and (2) determine if juvenile crust was cratonized in-situ during subsequent anorogenic, alkalic (A-type) granite/rhyolite magmatism and to determine if this was a major mineral deposit-generating event. The approaches are to identify and evaluate available trace- and major-element geochemical, isotopic, and geochronologic data for the basement and granitoids/rhyolites. The evaluation of existing data will be used to better characterize the affinity of the basement into which they were emplaced, to interpret the tectonic setting and process of granite magma genesis, and to compare the data for the different geologic settings. A secondary outcome of this research will be methods development for single-crystal, in-situ high-resolution elemental and isotopic analyses using laser ablation coupled with high resolution ICP-MS.