|
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 adjacent 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.
|
|
Breccia
|
|
