cinder cones

The Cinder Cone Caper

In my role as an exploration geologist, I examined many properties submitted to my employer in the hope that they would be of interest for development.  Some of those properties turned out to be scams, especially gold and platinum prospects.

I remember one in particular.  A small company claimed it had discovered gold in one of the basaltic cinder cones near Flagstaff, Arizona.  I was assigned to go take a look.

spCinder cones are relatively small, usually less than 1,000 feet tall, and form within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity.  Cinder cones are probably one of the last places you would expect to find gold.

When I arrived on the property, I spoke to the owner and asked him where he had found gold.  I dutifully collected samples there and at other spots on the property.  There was no visible sign of gold, but that’s not unusual.

The first red flag appeared when the owner offered to have his on-site assayer analyze the samples for me.  So, just to humor him, I split some of the samples and offered a portion for assay by the owner’s chemist.  Not surprisingly, the chemist reported gold in all the samples.

When I got back to Tucson, I again split the samples and gave a portion to our in-house assayer and sent the rest to an independent assayer.  Again not surprisingly neither our chemist nor the independent assayer found any gold.

Conclusion: the submitter’s chemist was either incompetent, or a crook.  He could have just faked the numbers, but there are many ways to “salt” the sample.  The most common are dropping a gold chloride solution on the sample before assay or adding a little gold dust to the sample.

One of the most blatant ways of salting mine workings is the shotgun method.  I once examined an adit that had blebs of gold on its walls, real gold.  The pattern suggested that gold dust was put into a shotgun shell and fired at the walls.  And that owner could not explain why the original miners did not notice and recover the gold along the adit walls themselves.

Caveat emptor.

See also:

Yellowstone Super Volcano

Young Volcanic Fields of Arizona

The dual nature of Hawaiian volcanos

Old mines of the Tucson Mountains

Surprising Structure of the Copper Deposits near Green Valley, Arizona