6 - Bull Pass Geology

Summary

The geology in and around Bull Pass is filled with an amazing assortment of complex and highly revealing relationships. This is the central sill filling point where magma ascending through the crust went on upward and outward to form this entire sill complex. Tens of thousands of cubic meters of magma came through this central receiving station. The surrounding rock suffered and recorded a great deal of highly revealing wear and tear. The granitic country rock is extensively melted to great distances and crustal granitic magmas began forming in the adjacent rock. In the area of the central feeding conduit there is a partly exposed ultramafic body with 20 wt. % MgO and all sorts of examples of anorthosite formation, including circular bundles of plagioclase stringers. These bundles reflect the filling dynamics where magma heavily laden with crystals was ascending and possibly draining back, time and time again. Similar structures are found in grain silos as the grain is drained. The large granite block in Orestes Valley is sinking into the Basement Sill here, indicating the possible existence of an overlying volcanic caldera system.