LINKING LAMPROPHYRE DIKES AND HYDROTHERMAL GOLD DEPOSITS IN GREAT FALLS PARK, MARYLAND AND VIRGINIA

Several igneous dikes were identified over 50 years ago that cut through the Potomac River along the border of Maryland and Virginia within Great Falls Park (GWMP and CHOH National Historical Monuments). Originally identified as lamprophyre dikes, these differ from other mafic rocks (such as basalt or diabase) due to their high-alkali and volatile contents, reflected by an abundance of biotite and/or amphiboles. Lamprophyre formation can occur in a variety of different tectonic settings and through a variety of petrogenetic processes which was explored in a prior study to help identify the specific geochemistry and petrologic origin of the Great Falls features. This project seeks to build upon this initial geochemical research and ultimately determine if there is a linkage between the Great Falls lamprophyres and the local orogenic gold deposits found in the immediate area. Trace element geochemistry displayed a negative Nb/Ta anomaly suggesting a relation to a subduction zone, confirming their origin of these mafic dikes to be from a volcanic arc setting. There are various instances around the world where lamprophyre dikes are found proximally to hydrothermal orogenic gold deposits (e.g., Tintina District, Alaska, New South Wales, Australia,), however, a definitive linkage between the two has yet to be formally defined. It has been suggested that since gold mineralization and lamprophyres both occur in shared structural environments, such as fault lines and shear zones, that the relationship between the two features is largely indirect (Muller & Groves, 2019). Other studies have argued that this occurrence is not coincidental, citing significantly higher gold abundance in lamprophyres compared to the average igneous rock, suggesting that lamprophyre dikes originate from characteristically deep, gold-enriched source areas, or that their composition resembles a hydrothermal fluid enough so that it allows for them to retain gold as they elevate from depth (Rock & Groves, 1988). This project will examine the trace element composition of the hydrothermal host quartz veins and conduct fluid inclusion analyses to provide a better understanding of the conditions under which the gold deposit formed, and most critically, identify if there is relation between the two.