FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

I would like to enlist Jordi's mineralogical "Baker Street Irregulars" in an on-going research project/crusade of mine. In my "day-job" as an exploration geologist I am always looking for ways to apply specimen mineralogy to my work...in other words "practical mineralogy" Admittedly, this could be regarded as cheap justification for spending hours of "work" time poring over dumps and old stopes looking for minerals, but I argue that if you can recognize the species and know something about their compositions you can justify the effort as "visual geochemistry" and defuse the issue promptly and return to having fun at work!

It is actually amazing how few geologists can play this card...and a large part of why I titled this Practical Mineralogy #1, because I am sure there are other examples out there and perhaps this is a way to lure (semi) educated professionals into a better appreciation of mineralogy and thereby into the hobby. Or, it might give us a tie to lure these guys and gals into our shows for reasons very different from why most people go to mineral shows...

I also try to use practical mineralogy in pursuit of indications whether whatever property I am evaluating has potential to contain a large ore deposit...or not. In my work on skarn, carbonate replacement, porphyry copper and vein deposits I (and others) have noted that pyrite pseudomorphs after pyrrhotite only seem to occur in major systems: Dalnegorsk, Santa Eulalia, Leadville, Ertzburg, Freiberg etc, and think this could be a useful fact if properly examined.

There may be a very good theoretical reason for this,: the fact that amany hydrothermal systems change from depositing pyrrhotite to depositing pyrite over time indicates increased sulfidation (and perhaps somewhat more oxidized conditions) with time but not necessarily in space. At a deeper level, and within the volume of a crystal, the pseudomorphing of pyrrhotite by pyrite requires addition of sulfur to the exisiting crystal structure, a change within both time and space. This reaction has been experimentally shown to have extremely slow kinetics, meaning it takes a LONG time to happen. (See Murowchick, 1992...citation below).

This indicates that this reaction will only happen in long-lived systems, and long lived systems tend to be large and juicy, so if you can find these pseudos somewhere that is not yet known to be large, you have immediate justification to explore the system aggressively.

This gets us to the crux of my request for help. I have found these psuedomorphs as specimens from a number of large well-known systems and they have been reported from massive ores from major districts as oblong pyrite blades or bars in sulfide ores (most famously from Gilman and Leadville, Colorado). I have also found them in projects I am exploring...that are showing signs of getting big. However, scientifically we need a lot more examples to be comfortable with this concept...which is where you all come in. Please check the list below and see if you can add to it, I promise to compile and update periodically and let you know if the idea holds up.

(Almost all big systems of these types produce good mineral specimens [for very good reasons that are the possible subject of another post] and they are active mines for many years, so their specimen potential should be all the reason you need to help out here!)

Note that many assume these are pyrite pseudomorphs after marcasite because of the tabular hexagonal habit of the pseudo. This would obviously be a much simpler reaction and it is known to be rapid or even inevitable over time. However the temperature and conditions of these systems, especially during their early stages, make it much more likely that pyrrhotite was the initial phase. The experimental and petrographic data support this, and in many cases a simple swing magnet can confirm that unreacted magnetic pyrrhotite remains in the core of the specimen. It is also likely that many geologists and petrologists who have encountered these are largely unfamiliar with well crystallized pyrrhotite, but have seen good crystals of marcasite in freshman mineralogy classes. (The general woeful level of mineralogical ignorance in the geological community might make another entertaining posting sometime).

So here's my list, please add to it if you can. This is not primarily a fishing expedition for places to explore, but I make no promises that I won't follow up on occurrences in apparently minor systems! On the other hand, if it turns out there are numerous occurrences in small systems it would disprove this hypothesis and we go back to the drawing board. If a major find comes from this...ore or specimens, I promise to give you a good piece!


Py Ps Po: Locality, specimen owner

Free-standing crystal specimens

Freiberg, Germany: British Museum
Madan, Rumania: Carnegie
Baie Marie, Rumania: Megaw
Trepca, Yugoslavia: Carnegie
Dalnegorsk, Russia: Carnegie and many others
Grasburg, Irian Jaya: Joe Knue
Santa Eulalia, Mexico: Megaw
Naica Mexico: Megaw
Concepcion del Oro, Mexico: Megaw
San Martin, Zacatecas Mexico: Megaw
Fresnillo, Mexico: Megaw
Platosa, Durango, Mexico: Megaw
Cinco de Mayo, Chihuahua, Mexico: Megaw
Taxco, Mexico: Megaw

Ore Petrographic or Core Occurrences
Leadville, Colorado, USA
Gilman, Colorado, USA
Silvertip, BC, Canada
Lustdust, BC, Canada

Here's the best readily available literature on the issue, from the generally available journal "Economic Geology". If you don't have access to it, I can probably figure out how to get you a copy.

Murowchick, J.B., 1992, Marcasite inversion and the petrographic determination of pyrite ancestry: ECONOMIC GEOLOGY V. 87, p. 1141 - 1152.

[Peter Megaw]

Yes, I know the title needs spelling assistance!!!

[Jordi Fabre]

Done Peter, I already corrected it.

Jordi

[Peter Megaw]

Examples....

[Peter Megaw]

Both are from the Potosi Mine 10th Level, Santa Eulalia, Chihuahua, Mexico. First piece is about 12 cm across...second is 3 cm across. Second piece is somewhat unstable and has cracked over the last 20 years

[alfredo]

Peter, Very interesting topic!

In big bolivian Sn/Ag mines, pseudomorphs of pyrite after pyrrhotite are much more common than pyrrhotite itself. I've seen nice sharp pseudos of this type from Cerro Rico, and there are huge ones in Siglo XX (I don't know for sure, but I'd bet there would be some examples in the Bandy collection at the L.A. County museum - Ask Tony Kampf).
(NB: I should confess that I haven't x-rayed these pseudomorphs, I just judge them not to be pyrrhotite anymore by their lack of magnetism. Technically, if they were a low-magnetism "troilite" type of pyrrhotite, I wouldn't be able to distinguish them from pyrite.)

Question (perhaps a stupid question): Why is it assumed that the change involves addition of sulfur, rather than loss of Fe?

Cheers,
Alfredo

[mmauthner]

Excellent topic and idea, Peter!

1) Alfredo's question also came to my mind as I was reading your thought on increased sulfidation...why not decreased Fe?

2) I have a small rosette of pyrite after pyrrhotite from the Husky Mine in the Yukon. At least, that is what it might be. The only pyrrhotite in the camp occurs at depth along with lower Ag values, but the material is massive; no well developed crystals are known. Another possibility is that this is a pseudomorph after polybasite, which is relatively more common as rosettes. Not sure what would give rise to that.

It adds at best another possibility to your list and if it IS Py ps Po, it fits with your idea. The Keno-Galena Hill system is no slouch.

Cheers,
Mark

[mmauthner]

Oh, I almost forgot.
You can add Nanisivik to your list. I had 3 specimens of nice, sharp "blocky" rosettes of pyrite after pyrrhotite from there. Alas, pyrite rot claimed them all in rather short order. I have pictures somewhere...snapshots unfotunately.

I don't know how Nanisivik ranks in size but there you go.

Mark

[Matt_Zukowski]

I know this doesn't expand the locality list but I have what appears to be pyrite and sphalerite replacing pyrrhotite from Trepca. Perhaps this speaks to the changes in environment there.

[alfredo]

I wonder how Peter's theory is affected, for better or worse, by mineral species other than pyrite, replacing pyrrhotite? Jaroslav Hyrsl has an almost fist-size pyrrhotite crystal replaced by galena (if memory serves), from the Porco Ag-Zn mine in Bolivia. I have a strange arsenopyrite pseudo from Cerro Rico (photo attached) that I think is after pyrrhotite (just because typical habit of bolivian pyrrhotites), but no real proof that it's after pyrrhotite; I suppose it could have been after apatite or some silver mineral.

The Tomitaka gold mine in Japan had massive pyrrhotite replaced by pyrite; no good pseudomorphed crystal specimens, just a deduction from textural study of the massive ore.

[Matt_Zukowski]

Attached are picts of the sphal/pyrite after pyrr from trepca.

[Montanpark]

Peter,
you may add Cartagena, Spain, to the list - not a small system of ore deposit :-)
During a field trip with Aachen University in 1989 we found a lot of samples with sphalerite crystals accompanied by pyrite(or marcasite) after pyrrhotite in an open pit which was active at that time (Brunita?, Jordi may help out, it was not Emilia but more in the east). I did not x-ray the crystals but they weren´t magnetic anymore, the habit was undoubtedly that of well developed pyrrhotite crystals - similar to the Trepca example posted here.

There should be some samples left at the university - unfortunately i am quite sure that i swapped all my samples in the past. If i find a remaining specimen i will post a picture. BTW a very interesting idea on practical mineralogy.

Cheers
Roger

[Montanpark]

.. just checked MTI homepage .. it should have been Mina Sultana if i compare the pictures there with the situation i remember ..
cheers
Roger

[Jordi Fabre]

Correct Roger, it is Sultana Quarry, Llano del Beal, Cartagena, Murcia, Spain. I have some fine specimens of them in my own collection. As I haven't photos of them where I am currently, I add to this post one photo from the great MTI web page ( https://www.mtiblog.com ) where most of the main Spanish localities are listed with lot of images of them and their minerals.

For more info about Sultana Quarry, please use this MTI web page: https://www.mtiblog.com/search?q=sultana

Jordi

[Peter Megaw]

This is GREAT...go to bed and wake up with a whole raft of new occurrences...and ideas!

Digging a bit fartehr baqck into my files I find references to these at Mont St. Hillaire (what doesn't happen there) and Kutnahore, Czech republic.

Let me try to address some of the queries and bounce some of my own back. I'll take them more or less in the order of the postings.

Alfredo: Specifically which are the "Big" Bolivian Sn/Ag systems...beyond what you mentioned. I am further intrigued by the geochemical similarity between the Bolivian Sn/Ag Belt and the common coincidence of Sn and Ag in Mexico...another major py ps po region. Anyone know of an Sn component at Dalnegorsk or Trepca... I would expect that there might be some py ps po in the Chinese Sn systems? (From a scientific standpoint being able to use this concept predictively would be very cool).

As to Fe loss versus S addition...that's actually considered to be a significant mechanism for this replacement...but under oxidizing conditions at low temperatures (<250 degrees C). Under those conditions the mechanism is believed to be dissolution of the pyrrhotite followed by reprecipitation of pyrite...which has a 30% volume loss leaving huge pore spaces that get filled with siderite or magnetite. Most documentation of this has been in petrographic work rather than specimens. Most of the systems that produce decent specimens formed under reducing conditions at higher temperature, which is where the sulfidation process is a more likely process.

Mark: What is the question about Keno Hill...that there really is pyrite after pyrrhotite? This is based on massive ores or specimens?

I have nice rosettes of py after po (presumably) from Taxco...where the possibility of polybasite is real. However, I have logged tens of meters of core characterized by spectacular "snowflake" rosettes of pyrite after pyrrhotite from the Silvertip/Midway property in exteme northern BC. It is very unlikely that this was polybasite to begin with.

Nanisivik is a bit more problematic...most researchers think its a Mississippi Valley type deposit formed at relatively low temperature. I have marveled at those beautiful pyrites after platy hexagonal precusors (when new and in the few that survive) and seen them labled both as replacing pyrrhotite and marcasite. Murowchick indicates that pyrite after marcasite tends to result in orientation of the pyrite crystals...which I reemember as being the case in the Nanisivik pieces?

Many of the Mexican deposits...especially Santa Eulalia...show pyrrhotite replaced by arsenopyrite, sphalerite and galena...closely matching examples mentioned by several of you. In some cases these sulfides coat an earlier pyrite ps po stage, in others it goes to the core. I suspect this tells us more about the multi-stage nature of these systems than whether or not sulfidation reactions are going on...This is actually another thread I am thinking about posting.

In fact I think I'll do that now...

[Pete Richards]

You're right, Peter - pyrrhotite is found replaced by either pyrite or marcasite at Mont Saint-Hilaire - I have found both on the same specimen. Both are commonly oriented replacements (topotaxy), which adds to the interest and the esthetics of the samples. The one drawback is that they're almost always micros. I don't seem to have any photos of the pyrite after pyrrhotite, but I think you'll like this one of marcasite after pyrrhotite.

[Jordi Fabre]

More Spanish localities: Brunita Quarry and Los Blancos Quarry both in the Cartagena-La Unión mining district as well as the Lomo de Bas tunnel in the works of the highway between Cartagena-Vera, Murcia, Spain.

As Peter can read Spanish language, for the details please visit this post: https://www.foro-minerales.com/forum/viewtopic.php?p=18852#18852

Jordi

[mmauthner]

Peter,
The question is not so much about Keno Hill (Galena Hill, actually, in this case) as it is about this one specimen. I have never seen others, nor read about the occurrence of pseudomorphs in these mines. To my knowledge...there is just so little literature on what has to have been one of the 20th century's major silver camps...no one has written about it (pseudomorphism/replacement) or maybe even taken note of these features.
I'll get a picture up as soon as I get my new iMac back from the shop (the irony is disgusting).

I shall also have to get an image scanned of one of the Nanisivik specimens. There is no possible confusion that this is a pyrite (or marcasite) pseudomorph after pyrrhotite. There are Py ps after cyclically twinned marcasite, but these are quite distinctive and the angles are all wrong to be after pyrrhotite.

Cheers,
Mark

[Jesse Fisher]

Hello Peter,
The Cambokeels Mine in Weardale was famous during the 1980s for producing some of the best pyrrhotites found in Britain. While I don't have a "killer," I do have several small specimens of what morphologically appears to be pyrrhotite rather than pyrite. None show the slightest magnetism so I am assuming that they are now pyrite pseudomorphs. While a typical MVT-type deposit like others in the region, the Cambokeels Mine was the only one I am familiar with that produced pyrrhotite in the orefield. It was also one of the deepest mines in the region and closed when the lowest levels were literally meters from penetrating the underlying granite.

Cheers,
Jesse

[Peter Megaw]

Jesse...would like to see it next time I'm out there,...or a picture sooner. Would note that there are non-magnetic polymorphs of pyrrhotite, so magnetism alone is not definitive...