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PhotosMagnetite - Marmoraton Mine, Marmora Township, Hastings Co., Ontario, Canada

20th Jul 2017 21:58 UTCRichard Gunter Expert

Magnetite, Andradite-Grossular Series, Quartz

Marmoraton Mine, Marmora Township, Hastings County, Ontario, Canada

Hi John:

Have you any data on whether these Magnetite plates are ps. after Hematite? All of the Hematite I have seen from here has been very late-stage, after the Calcite.

20th Jul 2017 22:07 UTCReiner Mielke Expert

What kind of data? They occur as tabular aggregates of small magnetite crystals so I would say they are pseudomorphs but of what I don't know.

20th Jul 2017 22:21 UTCRichard Gunter Expert

That kind of data would do. I don't think there is too much doubt as to their original mineral phase. There is a similar occurrence to the Marmoraton skarn at the Phoenix Mine at Greenwood, B.C. The massive oxide has similar platy magnetite of small crystals but the surrounding Andradite skarn has non-pseudomorphic aggregates of specular hematite (non-magnetic). Since both the magnetite and hematite have very similar "Iron Rose" aggregates it is likely that the magnetite is pseudomorphed after hematite. I have not seen any equivalent early hematite from Marmoraton but is might exist.

20th Jul 2017 23:08 UTCJohn R. Montgomery 🌟 Expert

Hello Richard and Reiner

I have no knowledge about the topic under discussion, but I hope others might weigh in.

John

21st Jul 2017 00:05 UTCAndrew Debnam 🌟

Hello Richard, the "platy" Magnetite has been interpreted by many as ps after Hematite. I have never seen any info or documents to substantiate this theory. I have seen Hematite found on the site in micro rosettes just this past June but as you said they appear to be post calcite

21st Jul 2017 00:09 UTCRichard Gunter Expert

Hi John:

I mentioned the magnetite as I am trying to fit Marmoraton into other, better-described, andradite skarns. Platy magnetite after early-stage hematite seems to be common in the British Columbia Cu-Au skarns. As far as I know Marmoraton did not have major sulphides; at least none I saw.

21st Jul 2017 00:14 UTCRichard Gunter Expert

Hi Andrew:

Have a look at the Phoenix Mine skarn in British Columbia. It is one of the few where non-pseudomorphic hematite and pseudomorphic magnetite after hematite co-exist. The B.C. Minfile system can be of help as it often has mineralogical data.

21st Jul 2017 00:33 UTCAndrew Debnam 🌟

Thanks Richard, I did find some interesting Pyrite bearing material in 2016-pic attached

21st Jul 2017 01:07 UTCReiner Mielke Expert

"Marmoraton did not have major sulphides;" Next to magnetite, pyrite is the most common iron mineral ( not counting garnet) found in the dumps. It occurs in the dumps as massive pyrite or heavily fractured and broken crystals in calcite or dolomite. I have seen pieces of massive pyrite 2 m in diameter. Even though there is so much pyrite, good complete crystals are rare.

21st Jul 2017 01:23 UTCMatt Neuzil Expert

I'm not sure how you could tell if they are pseudomorphed or not. Perhaps a piece could be polished in a resin and looked under a scope? Maybe some hematite would be left over or these are complete pseudomorphed ?

I am not knowledgeable on these crystals, just throwing an idea out there. Perhaps it wouldn't make any difference.

21st Jul 2017 01:33 UTCAlan Pribula

I don't know how similar the mineralogy might be to that at Marmoraton, but at the French Creek Mine in PA, magnetite was found as a pseudomorph of platy hematite. It was also found there in octahedral crystals.

21st Jul 2017 08:54 UTCMark Heintzelman 🌟 Expert

Even more similar would be Cornwall mine, where both original hematite and Magnetite pseudos exists, sometimes simultaneously. At French Creek, actual hematite is fairly scarce. A discussion of which I had with Ron Slotto some years ago, and after a search through his extensive stash of materials from the dumps at French Creek, only a few scattered plates in tiny pockets in white massive calcite was found. None of my material, neither more recent from the dumps nor vintage mine run material, had even a trace of original Hematite.

As a side note, Reiner's observation of pyrite is as I understand it as well, although Mamoraton did produce some fair crystals, even octahedral pyrite crystals of a very similar form and association as French Creek, found within radiating actinolite/tremolite amphibole (although I'm not aware of how commonly it was encountered there).

21st Jul 2017 10:40 UTCJohn R. Montgomery 🌟 Expert

All very interesting.

Mark, nice specimen. I recently found some radial sprays of actinolite so they are still around!

21st Jul 2017 12:59 UTCAlan Pribula

Just to clarify: In my previous post, when I said "It is also found...." the "it" referred to magnetite. I have not seen unaltered hematite from French Creek, or even heard of it before Mark's post.

21st Jul 2017 15:33 UTCRichard Gunter Expert

Thanks for the observations everyone. I did not know there was as much pyrite at Marmoraton as there is. I do not remember much rust-coloured rock on the dump, but that was a long time ago.

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