FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

Continuing on the theme of practical mineralogy, I would like to toss out the idea...not new, but perhaps worthy of discussion...that replacement pseudomorphs are excellent indicators of the "strength" and "length" of genesis of the ore deposits they come from.

Hydrothermal ore deposits are the result of ore fluids that evolve chemically as the system passes from initial prograde heating, through thermal leveling off, and finally through retrograde cooling. The products of the early stages are thus out of equilibrium with those of the later stages and in many cases are destroyed and/or replaced. Where this results in clays replacing early formed silicates we pay little attention to the generally ugly results (although in the case of pegmatites we bemoan the loss of huge gem crystals that were converted late to grungy clays)...but where the late replacements are one attractive species following another, some of us avidly accumulate them as pseudomorphs.

We are learning that most big magmatic ore systems do not go simply through one prograde-retrograde cycle, but go through a series of them as new batches of magma are injected into the roots of the system, triggering new thermal pulses which send repeated waves of ore fluids with overlapping thermal characteristics through the products of the earlier stages. But it gets worse/better...the magmas themselves are evolving in composition through differentiation, contamination, assimilation and other crustal/magmatic processes so each fluid pulse is compostionally different from another.

The final results of this repeated, multi-stage process is an ore deposit which imperfectly and incompletely preserves the products of the component stages. This suggests that deposits sharing this general genesis will show a wide range of replacement pseudomorphs with some very complex pseudomorphing relationships.

As an example, the Santa Eulalia system in Chihuahua,Mexico shows a huge range of these relationships. Early anhydrous skarn silicates are replaced by later hydrous silicates, in turn replaced by sulfides and even carbonates. We have already talked about some of the mutual sulfide pseudomorphing relatioships, especially those involving pyrrhotite. There are also silicate/carbonate replacements and fluorite-calcite pseudos. More puzzling are replacements of one species by the same species...quartz after quartz or calcite afer calcite...in some cases without evidence for a n intermediate "placeholder".

Skarns and carbonate replacement deposits are superb sources of pseudos, as are some porphyry copper systems. I am hoping that folks can chime in with comments and observations about other or additional pseudo-rich systems and whether/how these fit this general thesis.

Clearly a comparable argument can be made for repeated overprinting in the oxidation environment as wet/dry cycles and uplift history repeatedly modify the oxiding environment and the resultant mineralogy. Places like Santa Eulalia even show evidence of multi-stage oxidation pseudomorphs replacing multi-stage primary pseudomorphs. I'll see if I can take some photos to illustrate that.

Be happy to see this thread evolve in a similar fashion....

[alfredo]

Peter, you are unfortunately inspiring me to visit Santa Eulalia someday, as if I didn't have enough to do already...

Interesting similarties to the subvolcanic Llallagua/Siglo XX, which also had multiple phases of mineral deposition in the same veins. Apart from the pyrite after pyrrhotite there, there are wavellite after apatite, very abundant tourmaline after feldspar, more rarely pyrite after feldspar and cassiterite after feldspar. Oruro has siderite after feldspar. Cerro Rico has hematite after feldspar, rare lazulite after feldspar. Animas has wurtzite after pyrrhotite. In all of these subvolcanic deposits, very little feldspar is left - almost all now converted into other minerals.

[Pete Richards]

Peter and Alfredo are better ore geologists/mineralogists/petrologists than I am by far, but I can offer some thoughts on Peter's most recent comment, in the context of Mont Saint-Hilaire. In addition to being an igneous stock that penetrated many sedimentary layers including (apparently) evaporites, there were several stages of intrusion, each of which cooled and left room for the next intrusion. Each following intrusion brought fresh magma with a different composition, which reworked the earlier deposits (over and over again). It's an incredibly complex geology encapsulated in a rather small mountain (I think of it as a "dumpling"), much of the most interesting parts (at least that we know about) fortunately exposed in the quarries. It is truly amazing to walk around and see the veins, xenoliths, and rock masses exposed in the quarry walls, and to ponder what has been quarried away.

Yet this is not a major ore deposit - it is quarried for road stone and gravel for asphalt roof shingles. The mineralogical complexity and the kinds of parageneses represented here suggest that while these replacement phenomena may be common in major ore deposits, they are not unique to them, and may not even be good indicators of them. Perhaps the critical factors are the historical sequence of geological events, the richness of elements from whatever source, and the length of reaction time (which in this case I would link with an intrusive as opposed to extrusive system).

[Peter Megaw]

Pete's points are well taken...but I would argue that MSH is an excellent analogue in terms of having multiple alteration/mineralization events associated with a long-lived complexly evolved intrusuve system. Just because we choose to crush it up for road metal does not make it a major concentration of certain metallic elements, they just happen to be ones more readily and cheaply derived from sedimentary sources.

Just a thought

[Pete Richards]

Yes, I would agree with Peter's response. Certainly Mount Saint-Hilaire (MSH) has "been through the mill". My only point was that I thought that you, Peter, were proposing this kind of replacement history as a marker for discovering large metal ore bodies, and MSH does not appear to be one of them. As a largely academic mineralogist, I tend to expect markers to be 100% reliable (which I'm sure exploration geologists would laugh at). So perhaps MSH is one important mineral locality that keeps that marker from being 100% reliable.

And actually, MSH is relatively speaking a major concentration of unusual metallic elements, such as titanium and zirconium and rare earth elements, but as Peter says, not enough to justify economic exploitation.

[Peter Megaw]

Pete, you're absolutely right...explorationists would be DELIGHTED to be right 10% of the time...never mind anything approaching 100%! We are in a high risk, high reward, high failure rate business and most never make a significant discovery in their entire careers. Only about .001% of exploration projects make mines! These unfortunate statistics have led some in the business to recognize that if you always say "no" you will be right 99.99% of the time...stultifying careers of considerable longevity have been built on this philosophy. Of course they never find anything, which is part of why I went independent 25 years ago!

Mayhaps I shoud be clearer about this line of reasoning. We are not looking for 100% markers, but rather a series of features that tell us whether more work is justified or we should head down the road. First priority in seeking a metals rich system would be evidence that the metals of interest are present in desirable quantities....MSH will not pass this first test despite its igneous complexity, so we keep moving (after filling up a few flats with cool minerals). Down the road we find several metals bearing systems but only have the funds to to focus on one...I will start with the ones that look the longest lived and most complex...and multiple stages of pseudomorphing will definitely fit into that assessment.

My purpose in starting this thread was to get folks thinking a little more about what their specimens are telling them and see if perhaps observations made from different perspectives will bolster some of these concepts...or not. You have already reminded me of the importance of first principles and getting that message across first. Thanks

[alfredo]

Re Peter's comment: "Clearly a comparable argument can be made for repeated overprinting in the oxidation environment as wet/dry cycles and uplift history repeatedly modify the oxidizing environment and the resultant mineralogy".....

An excellent example of this is the famous native copper after aragonite pseudomorphs from Corocoro, Bolivia. The copper after aragonites look nice, so those are the only ones that show up on the collector market, but the area also produces chalcocite after copper after aragonite, and cuprite after copper after aragonite, and even malachite after cuprite after copper after aragonite. Native silver is also (rarely) present.

On a humorous side note: I once received a small lot of cuprites after copper after aragonite, rather ugly, swollen and cracked because of the volume expansion involved. I separated them into two groups, those that were completely pseudomorphed all the way through, and those that still had cores of native copper. A well-known German mineralogist approached my table at the Munich show and accused me of irresponsible labelling because there was no way I could know what the center of the pseudo looked like without cutting them open. (The gentleman in question had recently published a study of these pseudos in which he had in fact sawn several of them open to show the interior structure.) I responded that I knew exactly which ones still had a core of native copper because I'd checked them with an electronic metal detector. I wish I had a photo of the fellow with his mouth hanging open ;-))

[Peter Megaw]

A similar example of this is the less famous native copper after chalcocite after azurite pseudomorphs from the cleverly named "Rose Mine" near Silver City, New Mexico. You see the shiny copper jobs on the collector market, but the copper layer is typically thin and if you grind/cut through you see chalcocite and sometimes malachite or cuprite after the original azurite. This indicates you had a primary copper source nearby (the Santa Rita Porphyry Copper Mine is practically next door) that was leached of copper to make secondary oxides that then got sulfidized to chalcocite and then reduced to native copper. All that geologic history from a single specimen.

Amazing how such obvious approaches fail to occur to those who get too close to their subjects! (part of why I put ideas out here in case I overlook the obvious myself) Since early days metal detectors have been used to find these pseudos at the Rose Mine...I suspect the wash may be full of examples not sufficiently converted to native copper to register.

[alfredo]

Another strange pseudomorph story: Some miners at the Viboras silver mine in the Machacamarca mning district, Potosi, had a sackfull of very obvious cogwheel bournonite crystals, so obviously bournonite that it would never have occured to me to have them analyzed. But the miners claimed it was rich silver ore! I told them it was bournonite and unlikely to contain any significant amount of silver, but they insisted they had already sold some and were getting good money for its silver content. This raised some doubt in my mind, along with the fact that the pieces (mostly broken crystals, as they weren't trying to save them as specimens) had a somewhat grainy matte luster on broken surfaces, not the brilliant and smooth conchoidal fracture you usually see on fresher-looking bournonites from this district. End result was that it was silver-rich tetrahedrite, which would seem to imply that lots of Pb was removed, along with a small amount of the Sb and S, and a little silver was introduced. Yellow nonmetallic secondary films of crud (bindheimite?) are present (which one doesn't see on the fresh unpseudomorphed bournonites), so the replaceent might have happened under slightly oxidizing conditions.

[Peter Megaw]

Aren't there spectacular tetrahedrite pseudos after bournonite from Julcani..or is it after enargite...or both?

[alfredo]

I've seen big Julcani enargites that I'd thought were totally encrusted with younger drusy tetrahedrite, but perhaps they really were pseudos; I'm not sure. Jaroslav Hyrsl and Rock Currier know Julcani minerals better than I do.

[Jordi Fabre]

The large crystals with white Baryte from Julcani, mined 10 years ago (approximately) were Freibergite after Tetrahedrite. We analyzed them several times, so not doubts about it.

I add a photo of an other interesting pseudo from Yaogangxian Mine, a sharp Tetrahedrite pseudomorphosed by Bournonite (analyzed too).

Jordi

[Tony L. Potucek]

Good thread, Peter. Here's an interesting pseudomorph for you--calcite after vanadinite from the Apex mine, San Carlos, Chihuahua, Mexico. I picked it up a while back. It was formerly in Gene Tribbie's collection and he acquired it back in the 1970s-1980s era. The crystals on the specimen have the typical orange-brown color and maintain a reasonably faithful morphological shape (somewhat skeletal hexagonal xls) of the former vanadinites. It was sold to me as a pseudomorph specimen, and I don't have a reason to doubt it. Among other reasons, it has lost its heft as a vanadinite. I am not willing to conduct destructive testing on it, but I am reasonably satisfied as to its claim. I did not see reference to these pseudomorphs in the MR article, but I would not be surprised by the occurrence given the minerals and wholesale changes I have observed in other lead-zinc and magnetite replacement orebodies. Did you see any critters like this when you visited the San Carlos mantos?

This is probably a specimen you need to pry out of my cold dead hand ,after I expire under the weight of a large slab of limestone and oxides riddled with wulfenite pockets. Be careful of the broken cabernet bottle fragments as you come up on the carcass.

tlp

[Peter Megaw]

[Peter Megaw]

Roger...be careful with Santa Eulalia pyrrhotite... at least 3 different polymorphs are known

[Roger Warin]

Hello Carles, Peter.
Thank you for these good points. I appreciate the Peter’s discussion. I think it corresponds to oxidative degradation of the initial pyrrhotite deposit.
I appreciate even more my specimen. It has a very low sulfurous odor. It has not changed in appearance in 40 years in my drawer.
You were right to insist on the presence of isomorphic phases. The basic phase diagram of Fe - S system is very complex, but a little less in space since pyrite does not appear.
Pyrrhotite also exists in meteorites (even in chondrites), but its proportions are low. It would be responsible for a remanence of magnetization in the SNC meteorites (from Mars).
Thank you.
Roger.

[Antonio Alcaide]

[Jordi Fabre]