Dear Christian,

Your question can only be answered if the major and trace element behavior of rocks involved was measured and was shown graphically in a detailed geological profile.

Presumably you have a black wall zoning with the chromite-containing ultramafic source rock and talc-carbonate schist as a product of a metasomatic exchange in an open system.

For comparison, I recommend three publications that deal with the development of metasomatic zones in metamorphic rocks.

1) Curtis, C. D. and Brown, P.E. (1969): The development of metasomatic zoned ultrabasic bodies in Unst, Shetland. Contrib. Mineral. Petrology, v. 24, p. 275-292.

2) Curtis, C. D. and Brown, P.E. (1971): Trace element behaviour in the zoned metasomatic bodies of Unst, Shetland. Contrib. Mineral. Petrology, v. 31, P. 87-93.

3) Brady, J.B. (1977) Metasomatic zones in metamorphic rocks. Geochim. et Cosmochim. Acta, v.41, p.113-125.

Best regards,

G. Grundmann

Dear Mr. Guenther,

a simple question: " What type of alteration do you speak about, is it hypogene or supergene ?"  Pentlandite and pyrrhotite are sulfides  most sensitive to hydration and as such need to be viewed in context with the minerals associated with them.

Best regards

Harald G. Dill

Hi Christian,

You state that your pentlandite-bearing lithologies are genetically related, so given the ultramafic context, I’m guessing you are dealing with rocks from a co-genetic chromitite-bearing tectonized, serpentinized, ultramafic intrusive sequence which has been subject to low-to-moderate p-t (greenschist to lower amphibolite) metamorphism.

Firstly, have you already established that your pentlandite Ni content variations are not a primary magmatic feature that pre-dates alteration? If your observed variation is within the range seen in magmatic pentlandites you could check if both rock types share the same overall sulphide/opaque mineral paragenesis, because large variations in the Ni:Fe of  magmatic pentlandite are closely correlated to differences in the primary sulphide  mineral assemblage in which it occurs (see the paper by Harris & Nickel 1972). Also, in the chromitite, check the Ni:Fe of any pentlandite included in the chromite grains (see, for example, Lorand, 1987) and compare with your overall pentlandite compositional spectrum in both rock types.

With regards to metamorphism, I am unaware of any study whose primary focus is relating Ni:Fe:Co variation in pentlandite to metamorphic grade or degree of alteration; however several studies suggest that Ni and Co in sulphides increase at the expense of Fe as ultramafic rocks are progressively serpentinized (e.g. Shiga 1987). Eckstrand (1975) also discusses nickel sulphide paragenesis, including pentlandite, in the context of serpentinization of ultramafics and the subsequent calc-carbonatization of the serpentinites.

Finally, if you are able to measure trace concentrations of the PGE’s in your samples you may be able to discriminate between magmatic and secondary pentlandite; hydrothermally-mediated re-equilibration can result in very low-PGE concentrations in secondary pentlandite; you might then be able to correlate this with Ni:Fe compositional variations.

Hope this helps,

Regards

Graham

Shiga, Yoshihide. "Behavior of iron, nickel, cobalt and sulfur during serpentinization, with reference to the Hayachine ultramafic rocks of the Kamaishi mining district, northeastern Japan." The Canadian Mineralogist 25.4 (1987): 611-624.

Eckstrand, O. R. "The Dumont serpentinite; a model for control of nickeliferous opaque mineral assemblages by alteration reactions in ultramafic rocks." Economic Geology 70.1 (1975): 183-201.

Harris D C and Nickel E H “Pentlandite Compositions and Associations in Some Mineral Deposits” Canadian Mineralogist 11 (1972): 861-878

Lorand, J. P. "Cu-Fe-Ni-S mineral assemblages in upper-mantle peridotites from the Table Mountain and Blow-Me-Down Mountain ophiolite massifs (Bay of Islands area, Newfoundland): Their relationships with fluids and silicate melts." Lithos 20.1 (1987): 59-76.

Christian,

Start with the basics. First of all, verify that the pentlandite analyses are good quality, and that apparent Ni-depletion is not due to electron beam overlap on a nearby or underlying pyrrhotite or similar grain (apologies if you already checked this).

Secondly, determine the high-temp mineral paragenesis that the pentlandite is/was in equilibrium with. The Ni and Fe content will vary systematically depending on whether it is in contact or in equilibrium with troilite, hexagonal pyrrhotite or monoclinic pyrrhotite, or pyrite as the main Fe-sulphide mineral (see ref. Harris and Nickel in previous answer). These are the main high-temperature minerals that pentlandite would be in equilibrium with as it cools and exsolves from the monosulphide solid solution. These equilibria are controlled rather by fO2 and FS2 variations than P and T.

Thirdly, look to see whether this high-temperature assemblage has been modified or obliterated by low-temperature alteration (as seems likely in your case) - evidence of alteration of pyrrhotite to pyrite or Fe-oxides, presence of millerite or heazlewoodite, bravoite, awaruite etc. If the Ni depletion of pentlandite relates to particular assemblages, then you can start to develop a hypothesis of your own.

Another reference dealing with progressive serpentinization and talc-carbonate alteration and its effect on sulfides is that by Donaldson (1981) Economic Geology Vol. 76, p 1698. Redistribution of ore elements during serpentinization and talc-carbonate alteration of some Archean dunites, Western Australia.

Good luck.

Dave

I propose that you are looking at primary magmatic pentlandite which has been re-equilibrated in during metamorphism/metasomatism.  Talc has a strong affinity for Ni, therefore I would expect the Ni to move out of the pentlandite and into the surrounding talc as talc-forming reactions proceeded.