Bornite oxidation with formation of hematite and digenite/chalcocite (and perhaps some more Cu-S phases ?)

Thank you for your answer Gavril!

Although the hematite seems to be formed after bornite, but it does not seem to be formed due to oxidation of bornite. Some chalcosite, digenite and covellite are present, but I did not recognize any other Cu-S phases.

Formula of a bornite oxidation is: 4Cu5FeS4 +9O2 = 10Cu2S +2Fe2O3 + 6SO2.

According to your photo it is impossible that such large volumes of hematite were obtained from so small volumes of bornite and chalcocite. In other words, presence of hematite in the association isn't connected with bornite->chalcocite alteration. Otherwise bornite wolume should predominate over hematite one in about 5 times. But we don't observe this/

I am think, that hematite here is primary (very probably prebornite) mineral phase.

What is the main (matrix) phase on the photo?

By the way here presents very minor, bright-blue laths of covellite in the association (it should be easy recognizable under crossed nickols by it strong pleochroism - http://www.mindat.org/photo-471850.html).

Many thanks Pavel! 

Yes, I as well think that the hematite is primary, although to me it seems that it is post-bornite (please see the SE corner of the photo, where a hematite lath has crosscut a fracture of bornite).

The matrix is andesite. In fact the picture shows part of a veinlet in andesite.

Pavel is right, the modal composition of the field does not account for the stoichiometric oxidation reaction. Yet, dissolution-reprecipitation may result in odd reactant-product ratios. Bornite apparently never coexists with hematite, but coexists with Fe-free copper sulfides, phase stabilities displaying at least three degrees of freedom (redox potential, iron activity, and temperature). In the picture bornite is clearly corroded by a bluish phase (digenite? - is it isotropic?) while hematite is hosted by the dominant phase (is it chalcocite? - Pavel also asked about it, but you did not get the question right) in apparent textural equilibrium, never touching bornite relics, as well as its bluish corona.

Thank you so much Gavril!

The matrix of the photo is chalcocite. The bluish phase is covellite.

Nima, I have found very similar textures with the same mineralogy in Cretaceous Chilean Manto Type Cu deposits. I also think hematite is primarily. Quite often, bornite and “chalcocite” also appears in symplectitic intergrowths. Have you found such feature?

I use “” for chalcocite because I found XRD evidences and even EPMA evidences for other Cu-sulfides phases such djurleite. I hypothesize, in the deposits I studied, a very low-grade waning stage led to slow cooling and reactions in Cu-phases.

In addition, have you found tiny Ag-bearing phases, or also Co-bearing phases?

Best regards

Dear Javier, many thanks for your interesting and helpful answer. 

I have already found some symplectitic intergrowth between chalcocite and bornite. I did not have access to EPMA, I just utilized SEM-EDS, therefore I am not sure about djurlite.

I have quit often found Ag-bearing phases.

Best wishes,

Paul Ramdohr (1955) described in his book "The ore minerals and their intergrowth" the intergrowth of hematite (with 0001) parallel to the (111) of bornite. The formation of this paragenesis may be due to ascending oxidation conditions. Schneiderhöhn (1923) describes this occurrence in his paper: 'Vorläufige Mitteilung über pyrometamorphe Paragenesen in den Siegerlaender Spateisensteingängen.--' In: Zeitschrift für Kristallographie, Vol. 58, pages 309-329. I have this original material in my own collection, the locality is the siderite mine 'Neue Haardt' north of Siegen in Germany. Please find enclosed an ancient picture postcard view of the Neue Haardt Mine, also out of my own collection.

Dear Gunter, Nice photo and reference to the classics. I like both.

Nevertheless I think there is no cristalographic link between hematite and "chalcocite". I think in addition, hematite formed previously to chalcocite.

Best regards

Javier

Dear Dr. Guenter Grundmann,

Many thanks for your useful comment and the nice postcard. Your collection should be very interesting.

Kind regards,

Nima

Dear Javier, Many thanks for your comment. I just would like to mention that in my opinion, it seems that hematite is post chalcocite-bornite. Please see the SE corner of the photo, where a hematite lath has crosscut a fracture of chalcocite. 

Please have a look at this black and white photomicrograph No. 0060, which I found in the famous 'Card index of ore photomicrographs', published in 1961 (Editors: A. Maucher and G. Rehwald). It shows a so-called "Druse" of unhedral chalkopyrite and bornite in the centre surrounded by subhedral radially grown 'iron-hydroxides' from the 'Neue Haardt Mine' I mentioned above. I am sure that the radially grown aggregates actually are hematite. What do you think?

Dear Dr. Grundmann,

Wow! An outstanding comment! Many thanks (herzlichen Dank!)! 

Yes, very similar to what I have as hematite laths grown in copper sulfides. Is there any explanation on the "card index" or its supplement over the genesis and formation conditions of such a texture?

Dear Nima,

Thank you, too. Unfortunately there are no further explanations on the cards. But, anyway, we can interpret the views by ourselves.

Let me add one more photomicrograph (No. 0010) of the 'Card index', which tells me the following story: 1. Crystallization of euhedral pyrite crystals in bornite matrix. The grain size of the bornite mosaic texture is up to 0,5 mm. 2. Subvertical deformation produces tensional fractures inside the pyrite crystals and the bornite matrix (brittle deformation). Other parts of the surrounding bornite matrix  react by ductile deformation. The grain boundaries of the mosaic texture are slightly opened and give way to replacement by chalcocite. 3. The smaller network of fractures is healed by mobilized chalcocite, the larger fractures (between the large pyrite clasts) are healed by siderite. This sequence of crystallization and deformation events shows that in this case we are dealing with an old generation of Cu-Fe-sulfides.

I am going to finish the preparation of 3 polished section from the Neue Haardt Mine, which I mentioned above, and which contain exactly the siderite/bornite/hematite assemblage we are talking about.

You will hear me soon.

P.S.: Please leave the Dr. title alone, I am Guenter.

Many thanks Guenter for your kindness and your interesting comment.

I just should mention that pyrite has only scarcely been observed in my deposit. Copper-(Fe)-sulfides and hematite are the main ore minerals.

I am looking forward to hearing from you soon.

In the case of the bornite/hematite paragenesis from the Neue Haardt Mine the pyrite plays an important role in the reconstruction of the crystallization and deformation events. Please find enclosed a scan of the first polished section, I prepared for you, containing the pyrite-bornite-chalcocite-hematite assemblage, that is shown on the card No. 0010 (card index).

By comparison you will see exactly the same pattern representing subvertical tensional fractures filled by a second generation of siderite, bornite, chalcocite and hematite! The secondary hematite crystals not only occur in the bornite/chalcocite but also inside the tensional fractures of the pyrite. This result underlines the late stage circulation of oxidizing fluids with the formation of  late hematite.

The two remaining polished sections containing siderite-bornite-chalcocite-hematite are still in progress.

Dear Guenter, very interesting. Thank you!

Dear Javier, dear Nima,

may I confirm your statements related to the photomicrograph of Nima's question and Javier's answer, that there is no crystallographically oriented intergrowth between hematite and the copper sulphides. I statistically measured the angles of the around 140 single tabular hematite crystals exposed to the cutting surface of the polished section (see my evaluation: one red point = one single hematite crystal).The diagram definitely shows no preferential orientation!

Best regards

Guenter

Dear Guenter, really outstanding! Thank you so much!

Some remarks for Gavril, Pavel, Javier and Nimar

The first view is a scan (dark field dispersed light) of the second polished section from the Neue Haardt Mine. It clearly shows siderite (light brown and red internal reflections) surrounded by newly grown, fine-grained hematite (pinkish grey) in a matrix of bornite/chalcocite (black). It is obvious that hematite predominantly replaces the precursor siderite. This reaction produces a significant amount of excess iron, ready to be consumed by new growing hematite rims.

The enlargement (B-5-ok) of a portion of partly altered siderite (dark gray) containing bornite (orange brown) and subhedral hematite inclusions (light gray) clarifies the development of corrosion. The bluish grains at the rim of the bornite are secondary chalcocite.

Photomicrograph (B-2-ok) clarifies that even the bornite (pinkish brown) is partly overgrown and replaced by hematite crystals forming angular network (light gray). Please note the rounded bornite inclusions in hematite.

Now, it can be concluded that the ore type of Neue Haardt Mine:

1) represents an assemblage of primary siderite (euhedral to subhedral) and bornite (including exsolution lamellae of chalcopyrite).

2) Oxidizing fluids corroded i) the siderite to be replaced by hematite, and ii) the bornite to be replaced by chalcocite. Armored relics of bornite remained stable and uneffected by corrosion.

3) larger growing hematite crystals cut into adjoining copper sulfides (B-8-ok). That explains, why hematite grain boundaries are sometimes directly in touch with bornite.

P.S.: Sorry for the low quality photos, just keeping my IPad by hands on to the eye-piece.

I think if we have a Fe-Cu-S-O system with high fogacity of oxygen and high activity of cu, will be formed  bornite and hemetite as paragenetic  mineralas.

Dear Nima,

the lath-shaped hematite is primary and the bornite infilling the interstices. Therefore we have to take into account a reaction during which the Eh went down. Can you disclose some more data on the mineral assemblage and its position within the deposit? Is it a Cu or Fe deposit? I would avoid the term boxwork-texture even if the term may be justified by the outward appearance. There is too much connotation to gossan-style mineralization. A significant amount of Cu sulfide only signals an influx of a Cu-bearing solution. For a full-blown answer which I am reluctant to give here the mineralogy alone does not get us out of the field of speculation and comparing it with this or another mineralization will not get to a solution. I would only speak of a hematite mineralization prior to the Cu sulfide mineralization which took place as the redox regime became more reducing.

Best regards

Harald Dill

Dear Professor Dill,

Many thanks for your helpful comment. The deposit is a vein type copper deposit. The major ore minerals are chalcocite and bornite which are accompanied with some hematite.

As you have mentioned, part of the hematite has probably occurred prior to sulfide minerals. Nevertheless, some features make me think that parts of hematite could be post-sulfide (please see the attached picture).

With kind regards and best wishes,

Nima

Dear Nima,

May I ask you for one or two additional photos from different areas of your polished section. It is difficult to derive the relationship between crystallization, deformation and alteration just from this single area.

Thank You very much.

Best wishes

Guenter

Dear Dr. Nezafati,

Dear Colleagues,

I agree with the request of some more photographs on this matter but I am also realistic. Even if we have more images on a microscopic scale, a full-blown picture of the succession of minerals can only be provided in a reliable way as we integrate the field observations, which are with yours. If you claim another stage of mineralization under oxidizing conditions conducive to another generation of hematite, I can live with. There are e.g., vein-type deposits that start off with oxidic minerals go through reducing stages and end up eventually in another oxidizing regime. This is not anything out of the ordinary and has again and again been written in textbooks on ore mineralogy. Uplift, hydraulic conditions etc. are fluctuating very much in ore deposits.

I think you got a lot of good information, but too much of a remote-controlled diagnosis might at the very end only put you on the wrong scent. There is only one who knows all about this deposit. The person who works from the pit to the publication on that site. Sorry for my very personal opinion on this matter.

Good luck !

Harald G. Dill