Hot Rocks

[mn90403]

My last trip to the desert was to an area where we are treated with many hot rocks.  I've dealt with them each time I go to the Mojave/Mohave Desert.  We picked up a few so that my son could give them to his classmates.  I brought 50 or so home and washed them and tested them with a magnet.  To my surprise there is a great difference in attraction to a super magnet.  Some give an indication of repulsion rather than attraction.

So, what exactly is a HOT ROCK?

This article gives me more information about hot rocks and metal detecting than I've ever read.  It makes some of my personal observations about ground, hot rocks, gold and meteorites come together.

 

Hot Rocks (metaldetectingworld.com) 

[Bishop]

that was a good read.
one thing i did not see in the article. maybe i missed it, is
how hot rocks and ironstone, even with discrimination can mask targets if they are near them or on top of them.
one of the areas i hunt, San Domingo. is littered with basalt hot rocks.
last year i found a small nugget on a hill side but the area was undetectable due to hot rocks.
i raked off the hot rocks and found 5 more nuggets. its a lot of work raking and i only do that on patches.
looking forward to see what Garrett Axiom can do with its discrimination.

[Steve Herschbach]

21 hours ago, mn90403 said:

So, what exactly is a HOT ROCK?

A hot rock or a cold rock are just rocks that fall outside the current ground balance setting. If a rock and the ground are the same, they will both balance out. Take the very same rock, put it on ground that is way different. Now you balance to the ground, the rock itself is out of balance. Rocks that give a positive response are usually called hot rocks. Those that give a negative or null response are cold rocks. But again, these rocks are only hot or cold by being different than the ground they reside in, and what varies is the amount of naturally occurring iron minerals in the ground and the rocks.

Some detectors like a Gold Bug 2 have an extremely narrow ground balance window, so anything even slightly different than the ground becomes a hot rock. PI detectors have built in ability and systems that allow them to ignore far wider ranges of soil and rock than VLF detectors, which is why so many prospectors prefer them. Hot rocks are lessa a problem with a PI.

Some rocks simply have too much iron response for most ground balance systems, so a fist sized piece of pure magnetite will normally overwhelm and signal on most detectors. These are the rocks that are strongly attracted to a magnet. I do not think there are any rocks that can be repelled by a magnet unless the rock itself has been magnetized, in which case it would attract or repel on one end or the other just like a magnet affects another magnet.

A special class of hot rocks are those that actually are conductive, in which case they are actually not classic hot rocks as described above, but targets. Best example is graphite. It’s conductive and so responds like a metal target. Some copper minerals are highly conductive, and are a big problem at some Arizona sites. Arsenopyrite is common in many gold ores (and not gold ores) and is also very conductive. The list goes on. These rocks will generally not ground balance out since they are conductive targets. Only a VLF type discrimination system will knock them out, and any gold that reads just like them.

The article you linked to is mixing and confusing magnetic and conductive responses. Obviously not written by a prospector. Download and read part 2 of this free book instead for the best write up on hot rocks I know of.

[Valens Legacy]

Thanks for the interesting reading as once again I learned something new. Will I remember this when I get a chance to go detecting I don't know.

[Beeper Bob]

Have had some of my best luck in the Dale district where the iron stone is. also have some nuggets surrounded with iron stone. I look at hot rocks as a indicator. There is a area north of Barstow that has a tumbled looking iron stone on the surface and about 2 feet down is a grey granite bedrock it is only about 15ac. and under claim. I have seen a coffee table full off large nuggets that have come off of this claim. I have searched for another with similar geology in the area but can not find anything like it. It is like a island amongst the large Joshua trees.

[paradiceau]

Hmmm, 

That sounds like good information, thank you Beeper Bob.

Thank you Steve.