What rock is this?

sjastro · #1 01-10-2010, 09:17 AM

Here is the background.

Did some metal detecting. The area examined is composed of sand, clay, scattered quartz and outcrops of granite. The area is geologically unstable, there is a faultline a few kilometres from the site and there were larva flows in "recent" history near the area.

There was no evidence of basaltic rock or volcanic soil in the area. A neodymium magnet is a good test for picking up hematite/magnetite in volcanic soils. None was found.

The metal detector picked up this rock partially buried in a clay sediment. The neodymium magnet showed it to be faintly magnetic.
The rock seemed to be completely out of place in the environment.

To me it looks like a conglomerate. Initially I thought the binding agent may have been hematite due to the red colour but the SG of the rock is only 2.6, hematite is around 5.8.

Any ideas what it might be?

Regards

Steven

PS Don't say it's a meteorite.

It would hard to explain the quartz in the material.

renormalised (Carl) · #2 01-10-2010, 09:56 AM

Looks like a very heavily weathered quartz/biotite porphyry. It's not a conglomerate....at least not one I've seen like that. The grains are too angular to have been transported by water and worn. Plus the presence of biotite mica would preclude that anyway (it quickly breaks up and dissolves away when transported). The red colouration is due to the weathering of the biotite and the feldspars in the matrix of the rock (both contain Fe...feldspars, especially plagioclase, have small amounts as inclusions, biotite is Fe mica and has quite a lot). The feldspars are the red material you see, the Fe is a patina on the surface of the crystals.

That you found it in clay suggests that this rock was weathered in situ, with little or no transportation. It's probably what's left of a rather large lump of granitic material that has weathered to clays, which can be quite common in many weathering regimes where you get alternating periods of wet and dry episodes.

I'm curious...do the clays you find this rock in follow any geological boundary or are comprised of linear features within the surrounding rocks?? The reason why I asked is because this could also be a rock they call a fault breccia (a cataclasite). The weathering to clays of these rocks commonly occurs along faults.

renormalised (Carl) · #3 01-10-2010, 10:20 AM

You know what, I'd like to get a thin section of that rock....one running across the rock and the other running length ways along its prevailing orientation. That way I could inspect the structure of the rock and see its contents more clearly. Might make identifying it a lot easier as well.

sjastro · #4 01-10-2010, 10:54 AM

Quote:

Originally Posted by renormalised

Looks like a very heavily weathered quartz/biotite porphyry. It's not a conglomerate....at least not one I've seen like that. The grains are too angular to have been transported by water and worn. Plus the presence of biotite mica would preclude that anyway (it quickly breaks up and dissolves away when transported). The red colouration is due to the weathering of the biotite and the feldspars in the matrix of the rock (both contain Fe...feldspars, especially plagioclase, have small amounts as inclusions, biotite is Fe mica and has quite a lot). The feldspars are the red material you see, the Fe is a patina on the surface of the crystals.

That you found it in clay suggests that this rock was weathered in situ, with little or no transportation. It's probably what's left of a rather large lump of granitic material that has weathered to clays, which can be quite common in many weathering regimes where you get alternating periods of wet and dry episodes.

I'm curious...do the clays you find this rock in follow any geological boundary or are comprised of linear features within the surrounding rocks?? The reason why I asked is because this could also be a rock they call a fault breccia (a cataclasite). The weathering to clays of these rocks commonly occurs along faults.

Thanks for that Carl.

The area is elevated above a lava plain but there are no definite geological boundaries such as vertical faces. I'm not sure how far the faultline extends. Very obvious faultline features occur about 3 km away. Whether the faultline extends into the area I don't know.
The area is heavily forrested so it's difficult to access the geology of the region. There are some isolated bare patches where a metal detector search could be conducted.

It's very unusual for a metal detector to go off under these conditions. I live in the same area where the lava plains occur where metal detectors don't react to Fe rich basaltic rock.

That puzzles me. I wonder if a cross section to the rock will provide some answers.

Regards

Steven

sjastro · #5 01-10-2010, 11:00 AM

Quote:

Originally Posted by renormalised

You know what, I'd like to get a thin section of that rock....one running across the rock and the other running length ways along its prevailing orientation. That way I could inspect the structure of the rock and see its contents more clearly. Might make identifying it a lot easier as well.

Oh no! When I had the suspect meteorite rocks checked for Ni, I had to almost sell my soul to the analytical lab in the negotiations. They can section and photograph the rocks but I don't want to go through that again.....

renormalised (Carl) · #6 01-10-2010, 11:06 AM

Can you get access to any geological maps of the area you live in?? If you have one, and a topo map on which you can mark where you found the rock it's a fairly easy matter of transposing one to the other and identifying where the rocks lie w.r.t. one another. It'll give you a better idea also of where the faults occur and how many faults there are in the area. If there is a major one 3km away, there will be others around.

sjastro · #7 01-10-2010, 11:09 AM

Quote:

Originally Posted by renormalised

Can you get access to any geological maps of the area you live in?? If you have one, and a topo map on which you can mark where you found the rock it's a fairly easy matter of transposing one to the other and identifying where the rocks lie w.r.t. one another. It'll give you a better idea also of where the faults occur and how many faults there are in the area. If there is a major one 3km away, there will be others around.

I'll see what I can do.

Thanks

Steven

renormalised (Carl) · #8 01-10-2010, 11:26 AM

With those Fe rich basalts (basalt does contain a lot of Fe anyway), the reason why you don't pick it up on a metal detector is because the Fe is disseminated throughout the rock in the minerals making it up, e.g. Biotite, Olivine, Hornblende etc. To actually pick it up, you need to have it congeal into a lump within the rock....you need lumps of metallic Fe to appear. One way you could possibly get that is if you metamorphosed the basalt, but even then the chances of that occurring are slim. The other way would be to weather it out in a lateritic profile...that would produce an Fe rich gossanous cap or layer within the weathered rock. Which would be the most likely route to getting Fe concretions.

sjastro · #9 01-10-2010, 02:05 PM

Quote:

Originally Posted by renormalised

With those Fe rich basalts (basalt does contain a lot of Fe anyway), the reason why you don't pick it up on a metal detector is because the Fe is disseminated throughout the rock in the minerals making it up, e.g. Biotite, Olivine, Hornblende etc. To actually pick it up, you need to have it congeal into a lump within the rock....you need lumps of metallic Fe to appear. One way you could possibly get that is if you metamorphosed the basalt, but even then the chances of that occurring are slim. The other way would be to weather it out in a lateritic profile...that would produce an Fe rich gossanous cap or layer within the weathered rock. Which would be the most likely route to getting Fe concretions.

Metal detectors perform worst on volcanic soils because they do in fact pick up a signal. The signal however is more of a background noise. The high end detectors can isolate and subtract the background noise generated from any given soil much like the use of dark frames in astroimaging.

I think the sample is looking more like a breccia. I was able to pull out rectangular section of quartz out of the rock and left a depression in the rock as shown.

Regards

Steven

renormalised (Carl) · #10 01-10-2010, 02:16 PM

On closer inspection there, it's looking more like a cataclasite...a really busted up piece of granite that's been altered via faulting and brecciation. That's one messy bit of rock!!!. Seen plenty of them, but it's still a good example of that type of geology.

I can see plenty of alteration of minerals in this shot....embayed quartz, micaceous alteration of feldspars, engulfed crystals etc.

That could possibly be part of an ore body....maybe gold. Or at least be associated with the formation of one. The fault would act as a conduit for ore bearing fluids percolating through the rock.

sjastro · #11 01-10-2010, 02:32 PM

Now the question is the matrix. I wonder if the matrix is the same igneous material that swept over my property a few million years ago and has turned me into a amateur rock farmer.

Regards

Steven

renormalised (Carl) · #12 01-10-2010, 03:37 PM

The rock was most likely a granite...judging by the mineral contents of the rock and the coarseness of the grain. There are some relict textures present in that rock, but it's pretty much been busted up and re-crystalised for the most part. A bulk composition analysis would tell you what the source rocks were. A basaltic cataclasite looks different to this because it's bulk composition doesn't match granite...it would be a gabbro or dolerite instead.