Help with rock identification, please?

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Hi Folks - Im hoping a rockhound or geo can help me identify the type/genesis of the rock in the pics.
It comes from an old, small mine that produced 768g/t at the turn of the century. Its not the actual ore, but is in association with it. The actual gold they were chasing is in quite clear, clean, pinkish quartz in conjunction with ironstone. The rock in the pics is from the same shaft. There is quite a lot of it.
The shaft is located at the intersection of two sets of shear zones. Banded sheared gossan is exposed along shear zones. Not sure if you can see it in the pics, but the inner sections are quite crystalline around the hollows. The whole rock is quite friable, including the ironstone components, and has voids inside. Any clues?
1445744366_img_0689.jpg

1445744860_img_0691.jpg

1445745060_img_0690.jpg
 
Atom - has similarities to both, but is not quite the same. It may be quite weathered though.
Martin - WA
5ft - the actual gold is precisely at ihfjoinijcwiofaoijmwjjw, bugger, my keyboard is playing up again
silver - yup, it's pretty strange, haven't seen anything like it in the fields before. Bit far from the coast to be an oyster ;)
 
Hi,

Just winging it off the pics and your post it looks like the protolith was initially made up of quartz and clay that was exposed to intense heat, almost certainly hydrothermal activity. It appears to have gone through neocrystallisation changing original mineral constitution and structure through chemical reaction due to high heat where the atoms from the original crystal structure dissolve and migrate/diffuse and reform in other atomic arrangements usually at grain boundries, often changing the molecular make up of the mineral and generally the mineral itself.

So original rock compostion me thinks: quartz and clay (could very possibly be wrong)
metamorphosed rock: Have no idea . . . Sorry

Cheers,
Burden
 
burden said:
Hi,

Just winging it off the pics and your post it looks like the protolith was initially made up of quartz and clay that was exposed to intense heat, almost certainly hydrothermal activity. It appears to have gone through neocrystallisation changing original mineral constitution and structure through chemical reaction due to high heat where the atoms from the original crystal structure dissolve and migrate/diffuse and reform in other atomic arrangements usually at grain boundries, often changing the molecular make up of the mineral and generally the mineral itself.

So original rock compostion me thinks: quartz and clay (could very possibly be wrong)
metamorphosed rock: Have no idea . . . Sorry

Cheers,
Burden

Ill give ya 6 out of 10 for havin a go and displaying 'big words' ;)

Atoms can't dissolve but I like the creativity
Basically your saying its an unknown silica form still :)
 
AtomRat said:
burden said:
Hi,

Just winging it off the pics and your post it looks like the protolith was initially made up of quartz and clay that was exposed to intense heat, almost certainly hydrothermal activity. It appears to have gone through neocrystallisation changing original mineral constitution and structure through chemical reaction due to high heat where the atoms from the original crystal structure dissolve and migrate/diffuse and reform in other atomic arrangements usually at grain boundries, often changing the molecular make up of the mineral and generally the mineral itself.

So original rock compostion me thinks: quartz and clay (could very possibly be wrong)
metamorphosed rock: Have no idea . . . Sorry

Cheers,
Burden

Ill give ya 6 out of 10 for havin a go and displaying 'big words' ;)

Atoms can't dissolve but I like the creativity
Basically your saying its an unknown silica form still :)

Thanks Rat,

I'll give you a 10/10 for trying to make me look like a fool, for having a guess at what the protolith or in other words original make up of the specimen was ;) and explaining from my perspective of what form of metamorphoses it underwent from the pictures.

I should have written the atomic structure dissolves, but meh, I didn't.

I'm not saying anything about what it is now. It would be pointless to do so without results of a hardness, streak, and cleavage :D

Like I posted before: I have no Idea.

As you know silica dominates the earths crust and as I just read on brittanica.com (gotta refresh the brain sometimes, can't remember everything I have studied) makes up the main constituent of 95% of rocks. So yeah, I guess it would be that. It sounds good, and that's the main thing ;)
 
Thanks, Burden - I appreciate your time in having a stab at this one. On this and another forum it's had over 300 views with only a few ideas - seems to have us all a bit stumped. I have a couple of experienced geo mates who I will show it to to see if they know. If anyone is interested I'll post their views.

Meanwhile, any other ideas will be gratefully accepted! A member of a different forum had this idea, which seems pretty close to the mark, I reckon, as it definitely looks as though it's borne from solution.

Hope he doesn't mind if I post it here:

"I will have a stab at it.... and I mean a guess only.
Based on your description and mineralised veins/gossans I have seen, it simply looks like a mineralised gossanous zone has been weathered and leached. Many minerals like pyrite form acids when they oxidize and dissolve away host rock and the mineralised zones leaving voids. The voids then fill with silica, evaporate and carbonate minerals. This can happen at room temperature or earlier on when the now exposed mineralised zone was at depth and when mild heat could of aided mineral deposition. The cracks in the secondary minerals in your images look a lot like silica minerals, (opalite or a range of other minerals) that have shrunk as they have dried and cracked (like opal/potch does). The white mould like coating looks like some type of evaporate type minerals or carbonate minerals (of which there are many). Many of these minerals in your rocks could also be zeolites which love forming in cavities surrounded by rocks with juicy chemistry. Minerals that look similar to the ones you have can be found forming on the walls of working and abandoned mines. Identifying many of these types of minerals requires Xray diffraction to be sure.
So I am having an each way bet on a range of about 80 minerals."

Once again, Burden, thanks for your input to solving the puzzle! And yes, it is a pretty interesting blob of rock, not one I've seen before in many, many years of prospecting.
 
Hi Since 1981,

I like it it! Sounds like the goods.

Yeah, I just jumped to the conclusion that original rocks minerals metamorphosed under the extreme heat without even thinking about other ways as mentioned in his excellent analysation of your pics.

I really enjoy being shown another persons perspective and learning from it. I certainly gets the brain going (even at 3am after a busy shift)

Cheers for sharing his post, I, and I am sure everyone else who is interested to find out what it is appreciates it.
 
I agree with Since1981, looks like a carbonate has moved in via groundwater infiltration and formed on something else later on in its life. The fact it crumbles probably suggests the coating isn't siliceous. Drop some HCl on it and see if it bubbles.
 

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