What makes a hotrock with no iron?

[eureka77]

Posted in the MD forum and thought i link it here to get more anwsers.
http://forum.treasurenet.com/index.php/topic,327195.0.html

 

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[ivan salis]

certain minerals will mimic metal content * or it might have fine amounts of disssovled metal in it (depending on your area and the type it is.)-- they are called "hot rocks" for a reason -- the best way to get rid of them is to ground balance for them so the metal detector sees them as part of the naturally occuring background and not as metal -- however if your if your detector is a "preset" ground balance type * which does not have the "ground balance"option -- the only other cure is to drop your machines sen level / power out level till they do not ring up if in area heavily infested with them (there will be some loss of depth but its a trade off) --normally they ring up at the "iron" range .

 

[Jim Hemmingway]

Hi Eureka77,

I have encountered similar appearing rocks that react the same way to some of my units even when advancing the discrimination beyond the iron range.

Your photo indicates a highly iron mineralized rock. The reddish-brown to brown weathering on the rock surface represents oxidized non-conductive iron minerals. I can see why it might yield a strong positive signal under most field ground balance conditions at least when operating in an all-metal mode. As Ivan pointed out, a preset ground balance (normally set to ferrite) will, if anything, serve to enhance a positive hotrock response. Positive iron oxide hotrocks respond because their ground balance setting is lower than the ground balance you are using while searching. These hotrock types will normally discriminate within the iron range. Scrape a few crumbs from the surface and check those with a magnet. I believe you’ll see they are at least weakly magnetic.

There are exceptions that may have constituents other than just iron oxides. I suspect your rock contains low conductive iron sulfide but cannot be certain based on a photo. Have a good look at the myriad of brighter flecks I see in the photo and comment if they have a metallic “look” for example a pyrite “look” or luster. Try a few sharp whacks with a hammer and check it for a sulfur odor. Pyrrhotite, for example, will yield an unmistakable sulfur odor.

Low conductive iron sulfides usually discriminate within the iron range, but some larger specimens require partial foil discrimination. Larger pieces normally give a broad and even a raspy-edged response in discrimination that is easily distinguished from discrete but smooth conductive target signals. Low conductive sulfides will eventually weather to form iron oxides, for example, hematite or maghemite. Maghemite, a substance we are all familiar with in the form of metallic rust, is quite magnetic, and is normally associated with positive hotrock responses.

I can’t give you a more definitive answer as to why these rocks signal despite discrimination set above the iron level. An analogy would be to consider what happens when we are out coin hunting with iron to foil discrimination settings and get a smooth signal. We dig down to find there is no discrete target such as a coin or piece of conductive trash. In these instances we find rusty colored soil frequently containing bits of disseminated rusted iron. Possibly larger iron mineralized rocks, especially those containing low conductive sulfides, invoke similar metal detector responses.

Jim.

 

[Hoser John]

EZ & simple answer- conductive mineralization of the host rock by quite a number of conductive minerals John ie-copper-nickle etc etc

 

[Jim Hemmingway]

I agree that this rock may very well contain a myriad of electrically low conductive sulfides of copper, iron or whatever else might occur in the area. Many of these substances give similar low conductive responses to metal detectors, while other metal sulfides such as molybdenite or sphalerite give absolutely no response no matter how a metal detector is adjusted. An exception is niccolite (nickel arsenide). Other than in a disseminated form, niccolite will yield a relatively high conductive response, but niccolite is rare compared to the commonly encountered sulfides. I cannot speculate on pentlandite because I have had no opportunity to examine a good sample. Pentlandite is the primary source of nickel produced here in Ontario, but it is embedded within variably magnetic pyrrhotite as the ore source.

Electrically low conductive sulfides frequently continue to signal at ground balance settings well below a ground balance setting required to eliminate iron oxide generated hotrock responses. If we had this rock in hand, we could readily determine if it is a low conductive sulfide responsible for giving the signal.

I supplied a response based on what I can see in a photo of this rock. I see no evidence of copper or nickel minerals or their oxidations, but that is not to say they don’t exist within the rock. I don’t know about that because I can’t see it, and neither can anyone else looking at that photo. What I definitely do see is abundant iron oxidation on the surface of the rock, and have framed my reply accordingly. If a few crumbs of the red-brown surface material were lightly scraped-off, I think it likely these will be at least weakly magnetic. No guarantee, but most iron oxides have some measurable magnetism.

Jim.

 

[Lanny in AB]

I'd have to agree that this rock looks heavily oxidized--which usually denotes some type of rusting from some type of iron (magnetite [Fe++Fe+++2O4--Molecular Weight = 231.54 gm] and hematite [Fe2O3--Molecular Weight = 159.69 gm ] are two natural forms--one, magnetite is magnetic, and one, hematite may not be (depending on its associated minerals/mineral configurations) [certainly most often non-magnetic where I mine]. So, iron could be causing your detector to sound off. Having said that, and on a different note, I have found hot-rocks that have high concentrations of some type of lead alloy (not galena) that give my PI fits!

All the best,

Lanny