Science of the Comstock - Earth Science
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Earth Science Topics:
Introduction
Mapping
Local Geology
Regional Geology
Local Geology
Introduction
The Comstock has several key geologic elements that control its geometry and position. These elements include faulting, brecciation, and boiling and magma emplacement. These elements combined in unique ways to form the Comstock.
Faulting
The Comstock Lode occurs along an ancient north trending normal fault within a thick sequence of volcanic rocks. Deposits of high-grade gold and silver ore occur in the fault trace and in the volcanic rock immediately above the fault in what is know as the "hanging wall." The image below is a schematic section that shows the relationship of the hanging wall to the foot wall; note the line marked MB, which stands for marker bed. If a marker bed exists across a normal fault, the amount of vertical displacement can be estimated. For this section, imagine you are looking north at Virginia City; Mt. Davidson would be the mountain to the left, and Virginia City would be built mostly on the hanging wall surface.
The Comstock Lode was formed in a fault zone that was invaded by hot thermal water, which transported gold and silver and was heated an underlying magma chamber. The fault zone acted like a plumbing system that allowed fluids to be transported upwards. As those fluids came near the surface, the great pressure that they had been under eased; this caused the fluids to boil and explode into steam like when the top comes off of a pressure cooker. The metal bearing waters destabilized and the metals precipitated, or came out of solution, to form the deposits that miners later extracted.
Breccia
The ore deposits occurred in breccia. Breccia is simply broken rock. The act of boiling shattered the rock and created breccia that had distinctive and different characteristics from the top to the bottom of the breccia. Where boiling was subtle deep in the deposit breccia was not well developed, where boiling was intense nearer the surface, breccia was more milled or tumbled, and hence the individual fragments were more rounded. Follow this link (breccia.JPG ) to view a diagram of breccia in such an environment and note the changes from the bottom to the top.
Another place in the world where a similar looking breccia deposit may be forming is at Yellowstone's Old Faithful Geyser. The geyser's steam is a result of water boiling underground. The fault above the Comstock at the time of active mineralization probably looked like Old Faithful. Importantly, the zone of boiling influenced the position of ore formation. If one were to mine too deep, there would be no chance for finding ore, because the rocks would be below the breccia and the zone of boiling. Several expensive mining ventures tapped into the fault at levels that were too deep, and they found no ore. The above schematic diagram shows the relationship of the boiling zone to the breccia formation and the underlying intrusive source rock. Note the surface hot springs and geysers.
On the surface, the Comstock has a map expression of being long and narrow like a snake, because of the control that the fault had on the ascending fluids. Those fluids reacted with the volcanic rocks in the fault and altered them to clay while adding quartz and other minerals. Overall, the material within the fault and in the ore zones was soft and difficult to mine.
Faulting Causes Changes
After mineralization, later movement occurred along the fault. This further complicated the ore character and the mining conditions by creating "soft broken ground". More importantly, this faulting changed the geological character of the land. To the east in the hanging wall of the fault the land was down dropped; and to the west the land was raised. Note the changes in the schematic diagram below compared to the one shown above. Mt. Davidson is cored is a topographic high. Virginia City, on the other hand, is in a topographic low and rests on the hanging wall of the mineralizing fault. To clarify the images, magma chambers solidify into intrusive rock. The transformation is from liquid to solid. It is during this transformation that fluids are driven from the magma and mineralization occurs. The intrusive rock preserved on top of Mt. Davidson is instrumental to the mineralization seen at the Comstock. In addition to being a possible source of metals and sulfur, the intrusion provided heat for circulation of hot water, which may have dissolved metals from surrounding rocks. Without the intrusive the Comstock would not have formed and the existing fault zone would not have become mineralized.
The Rock
This image is of the intrusive rock at the top of Mt. Davidson. A film canister top is used for scale. The rock is composed of at least two different groups of minerals, dark and light. Most notable is that the dark minerals are in hydrous (water-bearing) minerals, biotite and hornblende. These minerals in abundance indicate that the magma origionally was rich in water. Having lots of water in the magma was important to development of the Comstock ore deposits.
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