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Off Sixmile Canyon Road between Virginia City and Dayton
Sugarloaf, the barren monolith protruding from Sixmile Canyon, is a steep-sided plug of intrusive andesite which has cut slightly older flows of the same formation. A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when lava hardens within a vent on an active volcano. Over time, the volcanic cone made up of less resistant rocks wears away, leaving behind the solidified volcanic cone producing a distinctive landform. Sugarloaf intruded older andesite flows about 12 million years ago, about the same time that the Comstock Lode was formed. The ground mass of Sugarloaf is a very light gray, with a faint tinge of blue, and the well-formed minerals are white feldspar with black hornblende and a little dark mica. On one side of Sugarloaf, you can see old rock walls, the remnants of one of the fourteen mills built in Sixmile Canyon to reduce ore from the Comstock Lode.
Sugarloaf and the Comstock Lode are part of the Virginia Range. The Virginia Range is a northeast-trending mountain block that lies between the drainages of the Truckee and Carson Rivers. The Virginia Range is composed of a thick section of Oligocene and Miocene volcanic rocks, that if we could see then all neatly stacked up, would form a pile about two miles deep. These volcanic rocks are mainly andesite and rhyolite, but ash-flow tuff, basalt, and mud-flow breccia (lahar) are also present. Shallow lakes formed in time intervals between some of the volcanic flows and, in places, sediments deposited in these lakes can be found interspersed with the volcanic rocks. The Tertiary rocks cover older (Mesozoic) granitic and metamorphic rocks similar to those you can now see in the Carson Range and Sierra Nevada to the west. Rocks forming the Virginia Range have been cut by northeast- and northwest-trending faults, then tilted to the form the mountain block that we see today. Although there is evidence of faults as old as 23 million years in the Reno area, major fault-bounded basins did not begin to form until about 12 million years ago. The present form of the Carson and Virginia Ranges developed over the past 3 million years, with the faulting and associated earthquakes continuing today.
These mountains host some of the most mineralized areas in the state, and silver and gold have been mined from several mining districts located along the length of the Virginia Range. The largest and most famous of these is the Comstock district, which is centered around Virginia City on the eastern slope of the range, but alteration associated with an earlier stage of the Comstock mineralization has bleached and stained rocks to various shades of red as far away as the foot of Geiger Grade on the west flank of the range.
The Comstock Lode is the north-northeast-trending group of veins that were mined between Gold Hill and the north end of Virginia City--the silver-rich mines bracketed between two 1859 discoveries. The discovery ore, found in a hanging-wall split of the main Comstock Lode, was crushed and weathered, and consisted of rusty-looking quartz, some gold, and a lot of heavy, blue-black material that turned out to be rich silver sulfide(the mineral acanthite). The precious-metal-bearing veins of the Comstock Lode occur in and along a north-northeast-trending, east-dipping fault (the Comstock fault) that bounds the southeast face of Mount Davidson, the mountain looming above Virginia City. The bonanza ores, formed 14 million years ago,consisted of quartz and a little calcite along with sphalerite, galena, chalcopyrite, pyrite, and lesser amounts of acanthite, argentite, and gold. The silver was contained mostly in the minerals acanthite and argentite (two different crystal forms ofsilver sulfide), although both native (metallic)silver and gold were present at the surface. During its main production period from 1860 to 1880, the Constock Lode produced more precious metals than all the rest of the United States. Before it finally came to a rest in 1984, 192 million ounces of silver and 8.2 million ounces of gold were dug from a roughly 3-mile-long stretch of ground along the base of Mount Davidson. Valued at over $405 million at the time of production, this bonanza would be worth about $4.5 billion at today's(2005) metal prices.
Question to be answered in order to log this EarthCache site: Standing in the pullout at the coordinates, facing Northeast looking at Sugarloaf, how many large outcroppings of rock can you see at the base of the main dome that have been separated by erosion from the dome?
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