Where Gold Comes From (The Mother Lode) - 8/13/00
High Grade Mine - Hatcher Pass, Alaska
The owner of the High Grade lode mine in Hatcher Pass stopped
by the store. He was interested in buying a metal detector to aid in hardrock
prospecting. I’ve done a bit of this before, and realize how hard it can
be for a beginner to get results. I offered to meet him at the mine last
Sunday to demonstrate what I could do on his mine dump. If he liked what
he saw, great. If not, he would save the price of a detector, and I would
still get a fun trip out of town.
I took my to the mine under gathering clouds. When chasing
tiny gold enclosed in quartz a high frequency detector like the White's
Goldmaster with a small coil is the way to go. The High Grade is up behind
the Independence and Gold Cord mines at Hatcher Pass. The High Grade was
named because it literally produced some very high-grade ore. The veins
were narrow, however, and the mine only produced a limited amount of ore.
The dump had so much
iron trash in it that I found it easier to simply pick up quartz samples
and wave them over the detector search coil. The ore in Hatcher Pass is relatively
clean quartz with free gold. The gold does tend to associate with pyrite,
so any quartz with reddish staining or obvious sulphides is worth extra attention. The pyrite here is non-conductive, and so will be ignored by
the detector. Many very rich samples, such as those pictured, have little
or no pyrite; so all quartz should be detected. I have seen gold in perfectly
pure, white quartz in the area.
Gold Ore from Fern Mine, Hatcher Pass
In less than an hour I found eight pieces of quartz that
gave obvious signals. Some had visible gold, while others were giving signals
from gold totally enclosed within the quartz. Unfortunately, rain began
to fall, and then it REALLY began to fall. I was not prepared for rain,
but we had proven the point… the detector worked very well on the ore. I
headed on home.
I left the ore with the owner. BLM is putting the squeeze
on him, so I figure he needs all the samples he can get. The ore pictured
is from the Fern Mine, also in Hatcher Pass. I obtained it from a geologist
that worked the mine years ago. It is shot through with gold, and is a good
example of what might be found by a lucky prospector in the Hatcher Pass
area. It exhibits black streaking that is often associated with the better
ore in the area.
An excellent source of
additional information is Hatcher Pass
Gold by Ron Wendt. Ron tells about all the mines in the area, and
includes many maps and photos. The book is out of print but can be found
Be aware that most of the hardrock mines in Hatcher Pass
are patented properties. In other words, they are no longer just mining
claims, but actually are private property. Few of the mines in the area
are ''abandoned'' and permission should be sought from the mine owners to
sample the mine dumps. Use extreme caution around the old mines, as many
tunnels, shafts, and old structures present a hazard to the unwary.
How was the gold deposited at Hatcher Pass? What follows is a simplified
view of gold deposition. In reality
is this is all theory, and entire books are devoted to the many theories
of how gold deposits form. So what I am presenting below is a layman's view
of a commonly accepted theory... not a "fact". Still, the theory
works well enough to be used to predict where gold occurs.
Gold most commonly occurs in quartz veins. The quartz and gold were deposited
within crevices and fractures in rock far below the earth by circulating
hot water. You will see the term "hydrothermal" a lot. hydro =
water + thermal = hot. Most gold was formed by hydrothermal processes. Note
that most quartz veins do not contain gold, so quartz alone means little.
So we need two things... rock with crevices and fractures, and a source
of hot, mineral laden water. The classic gold deposit is the hardrock mine
area at Hatcher Pass north of Anchorage. A large mass of molten rock, in
this case granite, rose towards the surface from far below. This kind of
activity tends to result in a pattern of fractures or faults in the surrounding
rock as this molten mass forces it's way upward.
When this mass of molten rock cools, it shrinks, and more fractures form
within this rock as it cools. What finally results is a "granitic intrusive",
another term you will see often when reading about gold deposits. The hardrock
deposits at Hatcher Pass are a were formed around a granitic intrusive.
As the molten rock cools, water seeping down from the surface reaches
the zone of newly introduced minerals and heat. Water that is extremely
hot and under pressure can dissolve many minerals that we think of as insoluble,
especially when some of the dissolved minerals cause the solution to become
even more corrosive.
The water, now mineral-laden and hot, rises back towards the surface.
As it circulates through the crevices and faults in the rock it deposits
many of these minerals. Much of the mineralization is simply because the
solution is cooling, and so can no longer keep the minerals in solution.
Often, in the case of rich mineral deposits, the solution comes into contact
with another type of mineral that causes a chemical reaction. The classic
mineral in this case is limestone. Many of these solutions are acidic, and
when they come into contact with limestone, the acidity is neutralized,
and the mineral drop out of solution. Many very rich mineral deposits have
been found where limestone comes into contact with other rock types.
In the case of Hatcher Pass, the deposit follows the classic example.
There was a rounded mass of granite far underground. Fractures formed in
the top of the granite, and in the other rocks immediately above and around
the granite. Hot water solutions deposited quartz and gold in these fractures.
Ages of erosion exposed the top of the granite and the fractures to the
surface. Erosion released the gold from the veins and deposited some of
it in the streams and rivers in the valley. The rest remained in the hardrock
veins, to later be discovered and mined.
Very common in this scenario also, is the concept that the gold veins
have a limited depth. The gold veins tend to occur just above and within
the upper layer of the granitic mass. As erosion (or mining) extends downwards
below a certain level, the gold deposits tend to thin out and disappear.
A situation arises where areas that have extensive gold in the streams often
have little in the rock (it all eroded out) and areas where the stream deposits
are poor will often be associated with very rich hardrock mines (most of
the gold is still in the rock). Again, a generality.
Granitic intrusives are common along
major fault lines. Maps can readily be had of faults and their related intrusives,
and it is no surprise these tend to coincide with many of the major gold
regions of the world.
Volcanoes are another process where
by molten rock rises to the surface, surrounding rocks are fractured, and
circulating waters deposits minerals, including gold, in these fractures.
The oldest hardrock mine in Alaska is the Apollo Mine on Shumagin Island
in the Aleutians, near Sand Point. This and other gold deposits in the Aleutians
and the Alaska Range are volcanic in origin. The Aleutians are a "volcanic
chain", a long string of islands that are actually volcanoes.
There are many other types of gold deposits, and many variations on these
types. The best reference I have seen on the subject is "The Geochemistry
of Gold and its Deposits" by R. W. Boyle, (1979) Geological Survey
of Canada, Bulletin 280, 584 pages.
~ Steve Herschbach
Copyright © 2000 Herschbach Enterprises