|
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 adjacent image 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.
|
|
Faulting
|
|
