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Placer Mining Geology in BC

A Little Bit is All You Need

Knowing a little geology is relevant to placer mining and prospecting for gold in BC. Knowing a lot more about geology doesn't really help much.

This page is about physical geology - particularly about valleys. It is not about identifying rocks and minerals.

Note: The word stream is used for everything from a small creek to huge river.

Old Gold

In BC, a good deal of placer gold was originally deposited more than a million years ago. It tends to settle though loose material in the stream bed and much of it has been reworked - moved and redeposited - one or more times. There is often much more fine gold in a stream valley than coarse gold and nuggets.

There is an old saying that has a lot of truth in it...

"Gold is where you find it".

Bedrock

"Bedrock" is essentially rock that a placer miner assumes to be a "floor" with no placer gold in it (other than maybe gold in cracks).

A hard layer of clay, called "hardpan" or "false bedrock" can have placer gold under it - it is just harder digging.

Stream Deposits

Streams pick up material and move it downstream. Practically any stream can pick up and carry fine silt. A faster currents can pick up sand, pebbles, gravel. Even if a stream can't pick up heavier pebbles and gravel, enough current will roll them along the bottom and maybe pick them up briefly. This is why river gravel is rounded - corners get broken off as pieces bump each other.

The Faster the Current...

Fast current can move heavy gravel and larger pieces. Sometimes, if you are standing beside a deep creek during spring run off when the water is really moving, you can hear boulders clicking together as they move downstream.

Most movement of material happens when the stream is high and fast, with little material moved the rest of the year.

Deposits Where the Current Slows

When the speed of the water slows down, it can no longer carry and move heavier material - it is deposited on the bottom of the stream. This generally happens:

These are the kinds of places where gold is most likely to be found.

Gold settling through loose material

Gold is very dense - heavy for its size - much denser than rock. When it is possible, gold will work its way down through the material under a stream. Often, much of the gold in a stream is on or close to bedrock. This kind of settling may have happened a million years ago and the material above may have become cemented together.

In the 1800s, it was common to dig shafts and mine underground. Underground placer mining is uncommon now because of modern regulations as well as physical and financial risks.

Benches

"Benches" are fairly level places on the bottom and sides of a valley that are above the present level of the stream - at a level where the stream flowed in the past. This is obvious in some valleys, where the stream is in a channel several feet below rest of the valley bottom. But benches can also be found dozens or hundreds of feet above stream level. They can be partially or totally buried.

Benches that have not been thoroughly explored are prime prospecting sites - in the gold rushes in the 1800s, it was very difficult to work deposits much above stream level.

Rich gold deposits on small buried benches are probably the best prospect for individual miners. They can be worked at low cost by hand mining and probably without obtaining permits (if a small enough water pump is used).

Abandoned Channels

Abandoned Channels are places where a stream used to flow in the past. Land slides, stream erosion and other factors can change where a streams flows.

Abandoned channels can contain great placer gold deposits. They are often the best prospects for larger operations involving machine digging (which requires a permit). This kind of mining can involve digging up and moving large amounts of material to reach gold deposits. When the mining is done, reclamation is required - the pits must be filled in.

The Shape of Valleys

Flat Bottoms

The bottom of a valley can be fairly flat for one or both of a couple of reasons:

Erosion can leave benches at different heights above the creek, scattered around and along a more or less flat bottomed valley. Gold deposited by the stream in one place may be way off to the side of the current location of the stream.

U and V Shaped Valley Bottoms

The cross section of a valley - its shape looking up or down stream - can sometimes tell you something about its geological history.

A V-shape cross section generally means no glaciers have moved down the valley.

A U-shaped cross section in a large enough valley can be the result of it being reamed out by a glacier. This will generally remove benches on the valley wall up to how high the ice was.


If You Have Had Enough Geology...
The rest of this page is provided
mostly for its interest value


Meandering Streams

A meandering stream is one that winds back and forth on a relatively flat valley bottom, generally where the angle down the valley is moderate.

Moving water has momentum like a moving car - it tends to keep moving in the same direction. Where a stream bends, the fastest part of the stream goes to the outside of the bend. Streams often erode their banks in these places - part way around a bend on the outside.

Around the bend on the inside, the water slows down and (some of the) material that is being moved by the stream is deposited. This is one of the most important places to check a stream for gold.

Because bends can cause erosion on one side of the stream and deposits on the other, the stream channel can slowly change the shape of the bend and move the stream sideways. Over long periods, a meandering stream can flow over most or all of a valley bottom, even though the stream channel is relatively small at any one time.

If a loop develops in a meandering stream, during high water part of the stream may take a short-cut. In some cases, the short-cut can become the channel by erosion - sometimes a massive amount of erosion in a matter of days or even hours. Thus the village of Kaskaskia, Illinois now lies on the West side of the Mississippi, despite the fact that the river has always defined the Western border of Illinois.

Ice Ages

There have been a number of ice ages in the past few million years, with cooling and warming periods in which glaciers and ice sheets advance and retreat. Ice sheets can be 3 or 4 kilometers thick. (Actually, a geologist would say that we are now in a warming period of an ice age that began about 2.5 million years ago.)

Valley (Alpine) Glaciers

Valley Glaciers form when great amounts of snow fall in the the high-elevation, upstream end of a valley, and the weight of the snow turns it into ice. If this process continues, the lower end of the glacier slowly flows down the valley.

When these glaciers advance, they ream out valleys and move rock and gold around.

Ice Sheets Over Parts of BC

During ice ages, parts of BC (including the Cariboo) were covered by ice sheets that were relatively static - they moved in and just sat there. This prevented valley glaciers from reaming out valleys in some areas.

Glacial Melt-Water

When glaciers and ice sheets melt, tremendous amounts of water REALLY move rock and gold around. This can create or enlarge valleys in some places, and partially fill valleys with clay, sand, boulders, etc. in other places.

The extremely rich deposits in Williams Creek in Barkerville in the Cariboo area were found 80 feet below the level of the creek. The depth to bedrock under Lightning Creek in the Cariboo is 200 feet in some places.

The photo at the top right of this page shows a "Chasm Created by Glacial Runoff near Clinton, British Columbia" (the photo caption by Getty Images).

Glacial Till

Salmon glacier Glacial Till is material that has been deposited directly by glaciers. It rarely contains placer gold worth mining.

In a valley that was reamed out by a glacier, till is the material that was moved. It is usually a random mixture of clay, silt, sand, gravel and boulders. In some cases, the clay in till is a greyish or bluish color. The gravel and larger pieces are usually angular rather than rounded.

In the photo of Salmon glacier (according to the caption by Getty Images), you can see material along the sides that the glacier has picked up as it moved down the valley.

Good Knowledge of Geology Not Required

Being able to identify rocks and minerals is occasionally useful for placer miners. For example:
In the Cariboo, the richest deposits around Barkerville - reached by underground mining deep under streams - were found on or near the top of rotten blue shale.

Generally, however, this kind of knowledge of geology is not particularly useful for placer miners (although it is very important for hard rock prospecting).

So...

Placer gold really is "where you find it".


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