Back to Ganoksin | FAQ | Contact

How are agates formed?


#1

I am reading the Firefly book on agates and I have a large bucket of
Fraser River agates here.

Agates are usually found as “floats”. Does anyone know of any agate
deposit mined in bedrock which would confirm theories on the origin
of loose agates in river rock?

The subject may be an interesting project for a Chilliwack museum
display as agate gathering is a local hobby.


#2

Peter,

Perhaps you can start by browsing through Roger Pabian’s Agate
Lexicon. You’ll find any number of descriptions of agate deposits
there.

http://snr.unl.edu/data/geologysoils/agates/agatelexicon.asp

If this link doesn’t carry through to the list, google: Agate
Lexicon Pabian It will be right at the top of the list.

Carol


#3

Peter, Most agates are deposited by groundwater seeping through rock
where it can pick up silicon dioxide and then deposit it in a void or
crack. (The jury is still out on “thunder eggs”) Volcanic ash is a
good source of silicon dioxide. In the case of some sedimentary
agates the source seems to have been ash raining down on the sea from
volcanic eruptions.

On doing an online search I come across this site for on
the Fraser river geology.

http://geoscape.nrcan.gc.ca/fortfraser/bedrock_e.php

Scrolling down the page I come to Mafic rocks:

“Andesite and basalt form layered sequences up to 700 m thick. These
rocks are dark coloured, heavy, and massive or full of bubbles
(vesicles) and formed from volcanic flows similar to those occurring
today at Hawaii. The rocks date to four periods: 280-247, 230-190,
70-47, and 27-11 million years ago. Some have conspicuous cooling
joints (columnar joints) or contain fragments of sandstone or red
basalt. Volcanic rocks are chemically reactive, and streams draining
them have elevated concentrations of important nutrients such as
phosphate. Some of the younger mafic volcanic rocks contain opal and
agate. Quarries at Fraser Lake west of Fort Fraser and near Stellako
supply crushed rock for railway and roadbed fill.”

It sounds like in your case the agates are formed in the bubble
holes in the basalt flows. This can be called amygdaloidal basalt.

When the basalt weathers the agates do not decompose as fast as the
host rock does and the agates end up in the stream and river
drainage. Sometimes you can find the source rock for the agates and
find them still in place…and difficult to get out…other times
they will be found by digging in the soil formed by the decomposed
rock. Sometimes the source rock is completely eroded away. I would
suggest contacting your local rock club to locate a source where they
are still in place. Another plus to this avenue is that older
rockhounds often have rock from sources that are no longer accessable
to the general public.

Rose Alene McArthur…down in Idaho in the Columbia River basalts


#4

Agates are extremely durable, which leads to their being found in
river cobbles in many places, the end result of millenia, or even
thousands of millennia, of erosive processes that may remove
thousands of feet of surface. Yellowstone River or Montana agates are
examples. I have picked up river nodules in several places in
Wyoming.

And yes, agates and their kin (chalcedony and jasper nodules) may
also be found still in place, as cavity fillings in extrusive rock,
in such places as Brazil and Uruguay, also Mexico. I personally have
dug agates closer to home from a much smaller exposure than the ones
in South America in a road cut near Berkeley, California. They are
both loose in the soil and still embedded in a basalt flow, and range
in size from a few millimeters to 7 or 8 cm across.

There are many web sites that discuss agates, their formation, and
specific sites where they have been mined, in some cases for
centuries. Agate deposits in Germany and Poland are well described
and illustrated. Just search and you will find answers to your
questions.

Dick Davies


#5

Around here (Arizona) several locales offer agate in bedrock. The
Arizona Agate Mine in the Tonto National Forest is an example. Most
of the “float” we find comes from volcanic rock. The agate forms in
cavities and is released when the softer volcanic stuff wears away.

RC


#6

Thank you for this helpful Rose and also Carol for her
reply specifically about agate mines.

Though agates are plentiful in the Fraser R. there are no agate
mines here.

Gold in the rivers is readily traced to bedrock sources (even
platinum around Princeton). Nobody has yet traced these agates to
their bedrock source. We read at

that “In Canada, the Thunder Bay is one of the three places where
Agate is found in veins and it is the only agate mine in Canada”.

The Firefly book has superb pictures and it is by Pabian et al. They
write on page 7 that “Some authorities see agate as a vraiety of
chalcedony, others as a near synonym”.Chesterman and Lowe in the
Audubon " Field Guide to North American Guide Rocks and Minerals"
list agate as a type of chalcedony, ie "…variegated, banded"
chalcedony.

I just epoxied 40 lbs of beautifully banded river stone from the
Chilliwack River. The bands of white to orange to yellow alternate
with black though black predominates. They are opaque and the largest
one is about 5 lbs. The project is to set them in a concrete garden
bench and they are flat enough on one side without cutting. These
bands are curved and swirled and they go right through the stones
from top to bottom. It seems likely this is "variegated, banded"
chalcedony but it does not look like the Fraser agates which are
translucent.

Do the Columbia River river stones lead you to chalcedony
outcroppings upstream?

Some of our river stones (jasper etc) are so attractive that I
imagine a hardrock mine yielding quantities would be quite
profitable. River stones seem to come in small quantities and may be
localized. For example I found some great looking stones of mottled
green and red chalcedony but after picking a couple of hundred lbs
they have just about run out. A hardock mine of these would be an
excellent find. An interesting challenge for future prospectors…


#7

Oh, that old Columbia River has such a turbulent past that it
probably does not happen on the main river. Higher on the subsidiary
canyons that may be the case. Although one mineable deposit of jasper
that comes to mind was simply found when a highway road cut was made.
On up around Redmond, Oregon creek gravel is a good indicator and
float will lead you to deposits…very well prospected and protected
at this point. Float is rock that has worked its way downhill from
the original deposit, not something that is floating down the river.
You might look for a book written some years ago by John Sinkankas on
field prospecting. Amazon has some real bargains in used books. The
main Columbia River has had some mighty big floods come down it. The
most recent, geologically speaking, were the Missoula floods. Huge
ice melt lakes in Montana were held back by lobes of ice in Northern
Idaho and those ice dams broke loose more than once. Huge rushes of
water, ice and rock rushed over the landscape scouring off the
topsoil, backing up in narrow places and dumping sediments, rushing
and tearing through canyons and backing up subsidiary canyons for
miles. There are big boulders that were carried in ice one hundred
miles up the subsidiary Willamette river. The mother rock for those
is a couple of hundred miles away. So river gravels are very well
traveled.

Rose Alene


#8

Thank you Rose Alene for the wonderful explanation about Agate
formation along the mighty Columbia. I wonder where the agates that
are so plentiful at Agate Beach, Oregon originated.

Alma


#9

the primary agate source is volcanic rock. Typically they form in
vugs (gas bubbles) in basalts and similar composition lavas and as
hydrothermal deposits in more acid igneous rocks suck as crazy lace
agates. Their formation requires an availablility of water and
permeability of the rock. The agate is normally formed by a sol gel
process at temperatures around 500-600 deg c but can be as low as 100
deg c. This puts silica into a solution which solidifies to a gel
then a solid as the temp and pressure drop. The silica then forms
microcrystals and as this takes a long time or many heating/cooling
cycles you get the banding in the agate. Agate is slightly porous and
that is why you can dye it quite easily. Crazy lace agate forms on
the walls of fissues in hot water or steam vents associated with
volcanic activity that produce commercially important mineralisation
as well. The agates are harder than the surrounding rock and less
prone to chemical and physical weathering so they weather out and can
be collected. In the UK they are found mostly in the late tertiary
volcanic rocks of Scotland, the same strata as Fingal’s Cave and the
Giants Causeway in N Ireland. They are normally collected as pebbles
in the fields where the soil is weathered basalt in the potato
growing area in Ayrshire. Our local source of crazy lace agate is the
tin bearing rocks of Cornwall, associated with the granite batholiths
there and the edge of Dartmoor. Agates and geodes are mined from the
bedrock in Mexico, Brazil and Uruguay. You can see agate nodules in
the rock that makes up Edinburgh castle if you look hard enough.

Nick Royall


#10

Hello Peter

I live in Winfield B.C. and have picked hundreds of agates off the
Fraser river over the last 50 years---- there are many places in
B.C. where one can dig agates out of bedrock----both seams and
nodules do not get any ideas about getting a profitable mining
operation going as most areas are just hobby collectable—you need
worldwide recognition(read American) of a particularly unique
variety or pattern to make any kind of serious money out of a per/lb
bulk operation—remember to create a market you need a continuous
supply of high quality material and usually only a maximum of 5% out
of any dig is premium grade. Having said that I have dug or picked
some really nice ones in this province----suggest you join a rock
club in your area and converse with some of the experienced people
you will meet

David Barclay C&D Gemcraft


#11

My main interest was for the Chilliwack Museum since our city motto
is “City in the Mountains”. They gave me permission to organize a
historical geology subgroup.

Agate hunting is a popular local sport. I met a fisheries officer
recently on the Fraser and he was watching for agates as he did his
fisheries work. In our half hour walk and chat he found three and one
was split, showing the interior banding.

I told him I was looking for other agate-like “nodules”. In
particular I found some very nice caramel coloured chalcedony.
Because of the size and shape these nodules are ideal for the pond
project I am working on.

I think a showcase of rounded river stones (including granite and
other speckled stones and jade) and a showcase of local fossils would
be of general interest.

As for economics, my guess is that if diamond blade technology keeps
improving, the cutting of chalcedony may become profitable, allowing
dimension stone claims in the bedrock up river to become mines. The
Fraser Canyon seems most likely.


#12

Find a copy of “Agates and Jaspers” by Ron Gibbs ISBN
978-0-578-00555-3 Everything you want to know plus great pictures.

Larry Ostler V&O Lapidary