The below was excerpted from Digging Deeper into the Comstock, by the Nevada Bureau of Mines and Geology Educational Series E-48.
• Three distinct Miocene volcanic suites, one at 18.3–17.4 Ma, one at 15.8–15.2 Ma, and one at 14.9–14.2 Ma.
• Host rock, early to middle Miocene (15.8–15.2 Ma) Alta Formation andesite.
• Mount Davidson (above town, 15.3 Ma) is composed of Davidson Diorite (actually a granodiorite) which is correlative with age of Alta Formation; Davidson Diorite is in the footwall of the Lode for most of its length.
• Two types of mineralization are present:
Regionally most extensive is advanced argillic,
high-sulfidation mineralization (15.5–15.3 and 13.5–13.3 Ma). Lots of pyrite, silica ledges, and associated alunite, but very little gold or silver. Red rocks along Geiger Grade are evidence of this type mineralization.
More restricted Comstock quartz-adularia, low- sulfidation mineralization (14.2–14.1 Ma). Quartz, calcite, adularia ores with associated silver and gold. This is the Comstock ore-stage mineralization.
• Most of the known orebodies occur within the Comstock fault zone (at Virginia City and Gold Hill), or the Silver City fault zone (at Silver City). A few orebodies occur in hanging wall fractures of the Comstock fault zone, such as the famous Con Virginia bonanza.
• The Comstock fault zone can be traced on the surface by pits that can be seen on the slope to the west above town and by silicified zones and veins (narrow, light-colored ribs) that can be seen cropping out on the steep slope above the pits.
• The Comstock Lode is a stockwork zone of quartz veins. Individual veins commonly are 0.1 to one inch wide, and rarely exceed one foot. Near surface, the veins are composed of calcite, quartz, and adularia. Deeper veins are entirely quartz with adularia. The veins are usually banded. Ore minerals consist of argentite and electrum with some galena, chalcopyrite, and minor pyrite.
• The vertical extent of the orebodies within the Lode rarely exceeded 500 feet, and strike lengths rarely exceeded 1000 feet. Mining widths were up to 150 feet.
Virginia City, with its honeycomb of underground mine workings and many surface open pit operations is located high on the east side of the Virginia Range, which consists mostly of Oligocene to Miocene volcanic rocks that overlie Mesozoic metamorphic rocks and Cretaceous granodiorite. Andesitic to dacitic (intermediate) volcanic rocks – mostly flows and breccias – and associated intrusions, began to erupt about 18 million years ago and continued until about 8 million years ago with later rhyolitic and basaltic (bimodal) volcanism continuing to only a million years ago. About the same time the earlier volcanic rocks were being erupted, the Davidson Diorite was intruded, which now forms most of Mt. Davidson just west of Virginia City. As the volcanic system waned, hot water continued to percolate through fractures, altering and mineralizing the broken rock in what is called a hydrothermal system, much like that surfacing at Steamboat Hot Springs today. We will look for evidence of hydrothermal alteration of the rocks along the route and in the Comstock district, where the original minerals of the fresh, gray volcanic rocks have been leached away and new minerals formed, often lighter and brighter in color and softer than the original unaltered rock. Hydrothermal systems also deposited the gold and silver that led to the development of the mining district. Several widespread episodes of hydrothermal activity and precious metal mineralization have been identified by the many generations of geologists who have studied the rocks of the Virginia Range. Recent work (Nevada Bureau of Mines and Geology Map 165, Geologic map of the Virginia City quadrangle, 2009) documents several separate volcanoes and hydrothermal systems, including the famous Comstock Lode.
Virginia City’s mines produced a huge amount of silver as well as gold from the Comstock Lode mainly between 1859 and 1900, and mining along the trend continues to this day. The Comstock Lode is a set of roughly parallel quartz veins that extends for more than 13,000 feet along the east flank of the Virginia Range The vein was exposed at the surface in the area of the Kendall Cut above “A” Street, which we will visit next, and dipped or tilted down to the east at an angle steeper than the hillside, so the mines farther downhill from “A” street had to go progressively deeper and deeper to intersect the veins.You may notice that the size of the mine dumps increases downhill, indicative of the greater amount of material that had to be dug out to reach the orebodies. Refer to the “Comstock Facts” page at end of log for more detailed information.