Kinds of Placer Deposits
A placer deposit
is a deposit that formed
in moving water where particles/pieces of gold and/or other
dense minerals have accumulated
because the water flow isn't sufficient to keep them moving.
In many cases, for a placer deposit to accumulate gold over time,
the water must be moving fast enough to flush out lighter sand.
One aspect is that if a piece of sand and a piece of gold have the
same weight, the piece of sand is much larger and is, therefore,
pushed much harder by the flowing water.
The Ice Age and the word "Ancient"
When describing placer deposits, "Ancient" means either...
- Preglacial - before the ice age of the last
2.6 million years
- Interglacial - a warmer period between
glacial periods - at least 110,000 years ago
For more information, see...
Streams, Gulch and Fan Placer Deposits
Placer deposits in streams are found where...
- the stream slows where it gets wider,deeper or less steep
- the gold enters a slower part of the stream
A "slower part of the stream" includes the classic placer targets...
- around the inside of bends
- behind boulders and bedrock obstructions
New sediment may create or add to gravel bars and
stream bank deposits.
is a narrow, steep-sided, steep valley. A gulch can
carry large amounts of water and sediment during a flood caused by
unusually rapid snow-melt or heavy rainfall.
In the Cariboo, Gulches with recent placer deposits - laid down
since the last glacial period ended 11,000 years ago - were easy
to find and mine, and were mostly depleted during the 1800s.
Ancient Gulch placer deposits can have thick overburden, although
in some places it has been at least partially removed by recent
Stevens Gulch and Beggs Gulch on Antler Creek
host ancient gulch placers deposits.
Alluvial Fans and Fan Deltas
An alluvial fan
is a fan or cone-shaped deposit of sediment
that forms where a stream flows onto a flatter area.
The water slows and drops much of its load.
At the Ballarat mine near Barkerville, gold-bearing alluvial fan
deposits overlie (and are therefore younger than)
the deeper pay gravels (described below).
Bench Placer Deposits
is a flat area above a stream.
It is what remains after
stream down-cutting and lateral (sideways) erosion
has removed part of what used to be the valley bottom or floodplain.
Old channels and old placer deposits can be buried on benches.
Large bench deposits are a primary target for
Small, buried benches in favorable locations
can host small but rich placer deposits that are targets
for hand-mining operations.
Ancient Channel Placer Deposits
Most ancient channels are deeply buried. Some placer mines are/were
located where more recent erosion has removed some of the overburden.
Some types of ancient channels include...
- Abandoned trunk valleys - partially buried
major valleys, which may be bedrock-walled. The Bullion Mine
near Likely is a large abandoned valley with bedrock walls
that contained sediments from at least two glacial periods -
most of the more than 3860 kilograms of gold that was recovered
came from the lowermost gravels sitting on bedrock.
- Abandoned High-Level Valleys - considerably
higher than modern streams, in some cases due to the overall
level of streams in the area being higher, before more recent
erosion. The Streicek mine, between Wells and the old gold rush
town of Stanley, is in a high-level buried valley.
- Buried Channels in Modern Valleys -
often far below the present level of the stream, often cut off by
more recent erosion and difficult to mine due to water problems
(and now, environmental problems), but potentially very rich.
The incredibly rich placer deposits found under Williams Creek
in Barkerville are an example.
Billy Barker's shaft produced some gold at a depth of 55 feet,
but the rich deposits were on and near bedrock, 80 feet below the
Another example - not as rich, but not as deep -
is the open-pit Ballarat Mine near Barkerville,
in which gold-bearing deposits partially filled an ancient channel
running adjacent to Williams Creek.
Most examples and some of the material on this page came from:
Stratigraphy And Geologic Setting of Gold Placers
in the Cariboo Mining District