Contrasting scales of element mobility in metamorphic rocks near Harney Peak Granite, Black Hills, South Dakota

Abstract

Outcrop-scale and regional dispersion patterns of rare elements help establish origins of fluid components, mechanisms of fluid flow, and the scale of element mobility in metamorphic rocks near the Harney Peak Granite. Enrichment of granite-derived elements (Sn, B, W, Li, Cu, F, P) adjacent to a quartz vein 3.5 km north of the granite indicates that fracture-controlled flow was one mechanism for movement of magmatic fluids. Graphitization of wall rocks and CO2-CH4 fluid inclusions in vein quartz show that C was also transported by the vein-forming fluid. Mixing of H2O-rich magmatic fluid and CO2-CH4–bearing metamorphic fluid within a concealed intrusive below the vein is proposed, based on the observation of late-stage graphite in several exposed pegmatite bodies in the area. Other wall-rock alteration includes small relative gains of Rb, K, and Al toward the vein, and relative losses of Na, Sr, Ca, Si, Fe, Mg, Mn, and possibly Ti. Changes in P, K, Al, and Si, and reequilibration of whole-rock δ18O, are confined to a 50-cm-wide zone next to the vein. Additions of C, Sn, W, and Cu, and depletion of Ca, Na, and Sr reach 100–150 cm from the vein. Boron reaches a maximum 25–150 cm from the vein and drops to background levels between 150 and 250 cm. In contrast to these elements, Li is three times the regional background level throughout the outcrop (at least 600 cm from any known veins), and F and Rb remain at elevated levels. Comparing these results with a regional geochemical profile away from the Harney Peak Granite and published results on dispersion around rare-element pegmatites documents the extent of element redistribution on a broader scale. For example, Rb and F can be affected up to ∼100 m from local fluid sources, whereas Li appears to record interaction with granite-derived fluids in rocks that are 500 m or more from map-scale fluid sources. Thus regional dispersion patterns of Li and other rare elements provide a minimum estimate of the volume of mid-crustal rock that interacted with granite-derived fluids in the Harney Peak area.