Quartz Mountain: GEOLOGY
Regional Geology
The general geologic setting of south central Oregon consists of a composite Cenozoic volcanic field that lies at the northwest end of the Great Basin in a zone that is transitional from Basin and Range extensional tectonism on the south to the High Lava Plains, Columbia River Plateau and the Cascade Range on the north and west. In this transitional zone, north trending Basin and Range extensional faulting decreases progressively northward across a series of northwest trending, right lateral fault zones in central and eastern Oregon. The area is also characterized by northerly trending fault block mountain ranges with intervening basins and volcanic highlands that are commonly capped by basalt flows. Mid-Tertiary to Quaternary volcanic rocks form the bedrock exposures throughout the fault block mountain ranges of the region with Pleistocene to Holocene lacustrine and alluvial deposits and tuffaceous sediments in the basins.
Property Geology
The Quartz Mountain gold and mercury district is located in a Mid-Tertiary volcanic highland within this northern transitional zone of the Great Basin. This highland area is characterized by a bimodal volcanic suite consisting of basalt flows and tuff with incestuous rhyolitic dome complexes. One of the primary structural features of the district is a wide (4.8 km) northwest trending structural zone that interrupts the thick sequence of basalt flows. Intruded into this structural zone are a number of rhyolite domes and related flows, breccias, and tephra deposits. It has been postulated that these rhyolitic dome complexes and their associated mineralization systems were localized by the intersection of the northwest trending structural zone and several cross cutting normal faults.
Hydrothermal alteration in the district is characterized by acid leaching of the host rocks and subsequent precipitation of quartz in the gold zones. Acid alteration as manifested by advanced argillic alteration is more typical of the upper mercury-bearing part of the dome complexes. The remaining alteration consists of an overprint of silica alteration on argillized rhyolite and propylitically altered basaltic country rock. Fault zones and veins within the system are numerous and contain textural evidence of episodic boiling within fractures.
Rhyolitic domes make up the central features of the volcanic stratigraphy in the area and are characterized by glassy tops. These dome complexes typically have contacts that taper downward into circular intrusive vents. The vents are usually less than 30 to 50 metres in diameter and are thought to have locally controlled the emplacement of several eruptive units. Textures of the rhyolite domes are variable, recording both intrusive and extrusive events, no doubt related to their shallow depth of emplacement in the crust.
Volcanic country rocks generally have flat lying to moderate dips and form flow units that are typically 15 to 60 metres in thickness and up to 915 metres in strike length. The generally well-ordered volcanic stratigraphy contains primarily heterolithic tuff and various breccia units. Units mapped as heterolithic tuff or breccia may include proximal ejecta derived from eruptions of the silicic dome rocks, poorly sorted detritus from other sources or a combination of both. These aprons of heterolithic tuff beds around the dome complexes thin outward to interfinger with the regional basalt flows.
The basalt units at Quartz Mountain are fine to medium grained, high-alumina basalts. These rock units are generally older than both the heterolithic tuff and breccia or the rhyolite dome complexes, however, as part of the bimodal volcanic suite, basalt eruptions are comagmatic with these rocks and locally are intercalated and post date the dome complexes. Basalt units can be vesiculated, scoriaceous or dense porphyritic rocks. Distinct flow units have been recognized in the basalts marked by brecciated and vesiculated flow tops and bases with a densely welded central portion of porphyritic basalt.
There are two distinct deposits at Quartz Mountain, namely Crone Hill and Quartz Butte.
Geology of Crone Hill
The volcanic stratigraphy at Crone Hill is complex in detail, but it can be generalized as a rhyolite dome complex resting on basalt with an apron of heterolithic lapilli tuff. The Crone Hill rhyolite dome complex is made up of at least four separate and nested dome-like intrusions surrounded by debris aprons. Silica and clay alteration are extensive and intensive throughout Crone Hill and are usually concentrated along fractures and faults. The dome-like geometry of Crone Hill reflects the original topography created by the emplacement of the dome complex. On the top of this dome complex mercury is localized, whereas gold is distributed around the fringe of the dome.
Gold mineralization has been observed in all lithologies on the fringe of dome complex. In the central part of the dome, adjacent to the inferred vent of the rhyolite dome, gold is distributed over a vertical thickness of 150 metres. In this area, lithology seems to have no control on the distribution of gold, which is most likely controlled by fractures developed in proximity to the inferred vent. On the margins of the resource area, the gold zone thins to 10 metres and is believed to be controlled by primary permeability in the volcanic stratigraphy, such as flow tops and coarse clastic units.
Geology of Quartz Butte
Quartz Butte is a discrete rhyolite dome with a well-defined neck or feeder system and a central depression. The Quartz Butte dome was intruded into basalt units and hot spring sinter deposits were developed in a central depression of the dome complex. In general, the rhyolite at Quartz Butte is more silica-rich than at Crone Hill, probably due to both original composition and hydrothermal alteration. Mercury is found in the sinter deposits above the rhyolite dome and gold is concentrated in fractures, faults and to a lesser degree in permeable volcanic rock adjacent to the dome.
Gold concentrations at Quartz Butte are distributed throughout the rhyolite dome and on the margins of the dome. The most continuous areas of gold are located in an elevation range of 1650 metres and 1738 metres. Below these elevations, the distribution of gold values tends to be more erratic. These isolated zones are believed to be fracture zones that may have been the source for gold-bearing hydrothermal fluids.
