Geology and Mineralization of the Great Excelsior Mine, Whatcom County, Washington

The Great Excelsior Mine is located in the North Cascades of northwest Washington, approximately 6 miles (10 km) east of the town of Glacier. The deposit consists of Ag with subordinate Au and is hosted within felsic volcanic breccias, tuffaceous siltstones and graywackes, and felsic tuffs belonging to the Middle Jurassic Wells Creek Volcanics. The deposit is located approximately 1000 feet (305 m) below the contact between the Wells Creek Volcanics and the overlying Nooksack Group metasediments. Both units comprise the lowermost structural-metamorphic unit of the western North Cascades System. The Wells Creek Volcanics have been informally subdivided into four mappable units. From oldest to youngest they are; 1) the Lower Volcanic Unit, 2) the Sedimentary Unit, 3) the Siliceous Tuff Unit, and 4) the Upper Volcanic Unit. Combined, these units represent a minimum of 4000 feet (1220 m) of intermediate volcanics along with intercalations of marine shales, siltstones, volcanic graywackes, and shaley tuffs. The contact between the Wells Creek Volcanics and the overlying Nooksack Group appears to be gradational. The base of the Wells Creek Volcanics is not exposed in the study area. The Wells Creek Volcanics and Nooksack Group have been subjected to regional low-grade metamorphism which produced mineral assemblages common to the prehnite-pumpellyite metamorphic facies. The Wells Creek Volcanics are structurally dominated by a large, open, upright, north trending anticline. At least three vertically separated mineralized zones (based on a 1.5 oz/ton Ag cutoff) have been identified on the property. These occur in gently to steeply dipping tabular zon es which are commonly subparallel to the stratigraphy. The mineralization is characterized by microscopic argentite, tetrahedrite, polybasite?, and electrum replacing pyrite and locally calcite. The three zones combined represent 2.5 million tons of mineralization which average 4.67 oz/ton Ag and 0.047 oz/ton Au. The Great Excelsior deposit shares several features common to typical Kuroko-type deposits. These features include: 1) the mineralization is associated with felsic volcanics, 2) it was deposited in a submarine environment, 3) fluid temperature and composition are similar to those of Kuroko fluids, and 4) it was deposited in an island arc tectonic setting. The major differences between them are the Excelsior deposit lacks massive stratiform mineralization and commercial base metal concentrations. The absence of massive stratiform mineralization can be explained by either: 1) extensive boiling of the hydrothermal solution resulting in subsurface deposition of metals, or 2) by dispersal of a low density hydrothermal plume by current action above the sea floor. The absence of commercial base metal concentrations can be explained by one or more of the following: 1) the base metals have been removed by erosion, 2) the hydrothermal solution was devoid of base metals, or 3) the base metals were flushed through the system and were dispersed. Thermochemical calculations suggest that the metals were transported primarily as chloride complexes and petrographic and field observations indicate precipitation took place in response to boiling and/or reaction with host rocks.