Syn-orogenic magmatism, mid-crust exhumation, and placer gold deposition: the Anaconda metamorphic core complex of western Montana

Abstract

Since their initial discovery in the late 1960's, metamorphic core complexes have remained of high interest in tectonics research. Early uncertainty regarding the mechanics of slip along low-angle normal (detachment) faults is now accompanied by controversy surrounding the relationship between magmatism and large-magnitude extension. As deeply exhumed geologic structures that record lithospheric thermomechanical processes, investigating core complex formation is crucial to understanding how the mid-crust behaves in extensional tectonic settings. In some regions, the exhumation of these structures is also linked to the formation of economically valuable mineral deposits, making them of notable societal importance. This thesis is a two-part investigation of core complex evolution that addresses the concepts above, formatted with introductory and concluding chapters that bound two main chapters prepared for publication. Chapter two consists of a study that tests the utility of using detrital zircon (DZ) U-Pb geochronology and DZ unmixing models to determine the source of placer gold. New zircon U-Pb (n=1,058) and Lu-Hf (n=61) isotopic data are presented from four placer deposit samples extracted from the Pioneer District of western Montana. Geochronology and DZ unmixing modeling suggest that gold from the placer deposits was derived from vein and skarn lode sources in northern footwall of the Anaconda metamorphic core complex (AMCC). Our data offers the first DZ-based support for previous interpretations that the Late Cretaceous Royal Stock pluton precipitated gold along its contact with overlying Proterozoic through Mesozoic supracrustal rock, and was subsequently weathered, transported, and deposited in the AMCC supradetachment basin during the Late Oligocene-Early Miocene. Chapter three consists of an integrated geologic, geochronologic, thermochronologic, and isotopic investigation of the AMCC footwall. Results suggest that the AMCC is an example of a core complex that was primed for large-magnitude extension through crustal thickening and voluminous magmatism. It is proposed that buckling of the Farallon slab, marked by the onset of 'ignimbrite flare up' volcanism, was responsible for the initiation of AMCC extension. Furthermore, a compilation of MCC cooling ages and ages of Cenozoic volcanics across the western USA suggest that removal of the Farallon Plate was a primary driver of Cordilleran core complex formation.