Geology & GeoMetallurgy

Geology & GeoMetallurgy 2017-03-23T09:44:23+00:00
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Sulphide Formation (13 replies and 1 comment)

Bob Mathias
1 year ago
Bob Mathias 1 year ago

I'm working on sulfides from a VMS deposits that were deformed at amphibolite grade, so pyrite seemed to have deformed by plastic deformation, but what is unusual it seems to form pressure shadows around euhedral sphalerite. I was expecting sphalerite or chalcopyrite to be in pyrite pressure shadows since it is more refractory.

John Koenig
1 year ago
John Koenig 1 year ago

There are a number questions that you need to ask first. Why are the sphalerite crystals euhedral? What lead to them forming first? Lastly was it formed during retrograde or pro-grade metamorphism? The last question will be vital as it will direct you to why there is actually pyrite pressure shadows instead of sphalerite. Send us a picture if possible.

Bob Mathias
1 year ago

According to the literature these deposits are the Besshi-type VMS deposit. They are located within the Damara Orogen. To be more specific they are within the Matchless Amphibolite Belt that traverse Namibia.So there is no doubt they are VMS deposits.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

I think that Your exposure is total insufficiency.  Only a microscopic image , you will not be able to realize the tectonic-metallogenetic events that occurred in that deposit. It is even VMS? I think that it is necessary to make a structuralist-analysis especially when the deposits is hosted in metasediments or other phace types.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

The literature indicates that euhedral grains form when there is no obstruction of other minerals, as such they formed earlier. i also know that increasing metamorphic grade sphalerite and chalcopyrite they deform by plastic deformation while it is opposite for pyrite, hence i suggest that sphalerite was formed during retrograde and pyrite was then remobilized during deformation by pressure solution processes, which is suggested by McClays and Ellis, (1983) as a mechanism which pyrite deform by plastic deformation at low grade metamorphism. other indicators of retrograde conditions that i have observed is in gangue minerals where chlorite is invariably replacing biotite and plus there are no primitive pyrite textures like framboidal or colloidal forms. just to give more info; the literature has indicated that this deposits are associated with a late granite intrusion in addition to regional metamorphism.

JohnnyD
1 year ago
JohnnyD 1 year ago

I would recommend you use BSD to map the crystal lattice orientations in the various sulphides to see if they are really plastically deformed. Search for papers by PhD student Craig Barrie to see what the technique can do with both pyrite and sphalerite.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

Yes i am still going to use SEM to prove this, although there is macroscopic indicators like pyrite grains elongation. i will check the paper.

JohnnyD
1 year ago
JohnnyD 1 year ago

Pyrite can deform plastically down to low grade metamorphic conditions of 200-250C. See this paper:
Barrie, C. D., Boyle, A. P. & Salter, M., 2009. How low can you go? - Extending downwards the limits of plastic deformation in pyrite. Mineralogical Magazine, 73(6), 895-913.
Sphalerite, chalcopyrite, galena are still softer, but it is now well understood that pyrite is not as brittle as once thought.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Lacking a fuller context, I hesitate to comment. However, sheared massive sulphides will sometimes show a grain-size reduction of relatively more brittle pyrite, while other sulphide grains resist fracturing and are rotated but maintain more of their original size. Look for crushed/ partly comminuted pyrite as a clue.

David Kano
1 year ago
David Kano 1 year ago

You may want to make sure about the timing of sulphide re-crystallization during the complex metamorphic cycle of the Matchless. I seem to remember seeing indications of a short retrograde metamorphic event at high pressure. Also the Matchless is well known for the proliferation of huge perfectly formed twinned euhedral staurolite crystals suggesting a large fluid source and high pressure at an extended metamorphic peak which occurred late during the tectonic history. To me this suggest that very little or no deformation occurred during the main pro-grade metamorphic event and that you have to place the formation of your pressure shadows earlier in the metamorphic history than you might want to. I suggest that you study the metamorphic and tectonic history of the alteration zone below the sulphide body to try and find an answer to your question.

Helena Russell
1 year ago
Helena Russell 1 year ago

One alternative line of enquiry that you should investigate is that sulphides can easily melt, when the correct assemblage is present in the rock, even at temperatures as low as 300C. There are some distinctive criteria by which you may either recognize this or rule it out in your study area. Try as starting points:

Tomkins AG, Pattison DR, Zaleski E (2004) The Hemlo gold deposit, Ontario: An example of melting and mobilization of a precious metal-sulfosalt assemblage during amphibolite facies metamorphism and deformation. Economic Geology 99 (6):1063-1084
Cockerton AB, Tomkins AG (2012) Insights into the Liquid Bismuth Collector Model Through Analysis of the Bi-Au Stormont Skarn Prospect, Northwest Tasmania. Economic Geology 107 (4):667-682

Victor Bergman
1 year ago
Victor Bergman 1 year ago

In my first intervention I haste and asked you if it is really a VMS deposit ? "So there is no doubt they are VMS deposits" - you have told my.

I do not know that work - perhaps you can send it by e-mail. I worked in Africa for a period of time, I know that the Great Lufilian Arch was formed by closing the Lofdal rift basin (Mezi-Neoproterozoic) during Pan-African orogenesis (550 Ma).

The sequences of continental and marine sedimentary rocks of the Damara district,(Namibia) and Kantonga district,(Zambia) were submitted during the opening of bazin / rift; these rocks hosting Cu-Co deposits of the Zambian Copperbelt.

If it exist Besshi type deposits, then : which subtype are ?

  • are there ophiolites and basalts in the area ?
  • are there green schist ?
  • are they of siliciclastic subtype ?

It exists there the alkaline magmatism - described by Lobo-Guerrero - which took place in different stages of formation of the rift, and continued after the closing of the rift, probably during continent-continent collision.

The proofs are the magmatic massifs neat to Lusaka , Kamanjab, Otjiwarongo Batholith, etc

So in which moment it occurs the Besshi type deposits: at the cossing or opening "pool" ?

In Namibia the hematite/magnetite massive ore bodies occur in many places, accompanied by sodo-calcic alteration characteristic feature of IOCG type deposits. But in the marginal parts of alteration zones it contains sulfides: pyrite, calcopyrite, bornite, sphalerite, galena etc

Won`t you think it`s possible that "this Besshi" to be a marginal area of IOCG alterations metamorfosed ?

The problem that I have raised it is more generally : are they some parts of Lufilian Arc, VMS type deposits ??

Alan Carter
1 year ago
Alan Carter 1 year ago

Google "Matchless belt". The sulphide deposits that is under discussion occur in close proximity to a 400 km linear occurrence of basalts in the Southern Damara mobile belt. It is interpreted to be ocean floor basalts within a failed rift system. So not part of the Lufilian arc and not part of an IOCG system.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

I have seen this commonly at the Ming mine in Baie Verte Newfoundland, however it usually occurs along contacts if mafic dikes. This usually associated with the remobilization of amorphous chalcopyrite into the selvedges of the mafic dikes.
This feature Is certainly a result of the contrasting melting and crystallization temperatures of the various sulphide minerals. The melting temperature of pyrite is significantly higher than that of chalcopyrite. You mentioned that the deposit had undergone amphibolite grade metamorphism it is possible that the temperature pressure conditions could have exclusively melted the chalcopyrite, or that the pyrite crystals had significantly more time to mature. The pressure shadows you are seeing are likely the result of any syn or post kinematic deformation around the pyrite porphyroblasts.

Alan Carter
1 year ago
Alan Carter 1 year ago

One of the remarkable features of these deposits is the presence of huge pristine twinned (90 degree) Staurolite crystals. That speaks of high pressure at the peak of metamorphism and no significant deformation at or after the peak of metamorphism. This is probably consistent with enough time for pyrite crystals to mature. The twinned Staurolite crystals found a place in history when they were used as part of the German upper leadership insignia prior to WW1. There are still heaps of twinned Staurolite crystals which did not fit the required quality.

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