Dear Mr. Wang,

In principle, you are faced with two minerals which are representative of two different redox regimes. Barite used to show up during the younger stages of vein mineralization with more oxiding conditions and close to the surface in shallow mineralizations. Chalcopyrite reflects the opposite regime. What is your evidence for an IS epithermal deposit ? The current mineral association of chalcopyrite and barite is very widespread and may be found from stratiform to vein-type deposits, avoiding any premature genetic conclusions drawn from this association. Have you already done the geological and mineralogical field and laboratory work in this deposit. You can, of course, jump immediately into the field of S, Ba, Cu isotope studies or carry out F.I. studies using other gangue minerals, but a careful analysis of the trace elements in CuFeS2 (Cd, In, Se..) and BaSO4 (Sr) will perhaps also shed some light on this part of the deposit. There exists plenty of literature on these minerals in relation to the various methods I have referred to here.

I wish you much success.

H.G.Dill

"But what is your aim?" might be the first thing to know before answering (why are you doing it)? And is this one of many similar examples that you have seen? Overall I would agree with Harald's explanation as the most probable, (although I think that chalcopyrite and barite can sometimes form under similar redox conditions, eg at higher pH,  this is usually in a small field of conditions and not a very likely one in your case of a gold deposit). However keep in mind that you are looking at a single, 2-Dimensional  section, you really need lots of examples to study paragenesis. For example, a younger mineral can be surrounded by an older mineral in a 2D section because of 3D geometry (although this looks unlikely in your photo), and progressive change of fluid composition can result in replacement of earlier minerals by later minerals (eg  we see this with anhydrite being replaced by ore sulphides in some submarine white smoker vents). Also, does mineralogy vary in different parts of your vein such as depth or with generation of infill? Do you have other sulphide minerals diagnostic of redox conditions etc (eg enargite versus tennantite)? I would do all these things before even considering other things like isotope studies - you need a full picture of spatial variation, and most deductions from a single section are a bit meaningless.

I've found the sphalerite-chalcopyrite-galena and minor tetrahedrite-tennantite as the major ore minerals, and there are also abundant Mn-calcite, both of which suggested a IS characteristic. The phenomenon I presented here is very restricted since till now I only just found one. I just feel a little confused about why the IS weakly acid, reduced fluids could produce the barite implying the oxidized condition.

Yes, that sounds probable as to type.

Do you have magnetite and what Fe sulphide phases are present?

Fluid evolution with time is the most likely answer - that fluids were more oxidising at one time and evolved to more reducing with time. There are many ways this can occur in the relatively shallow environment of epithermal systems, which can have varying input of unevolved meteoric and more evolved water (ie water reacted with rock at high T +/- mixed with a magmatic component) during their history.

However as I mentioned, chalcopyrite and barite can also be stable under the same intermediate pH / slightly oxidizing conditions, as I mentioned. One method to resolve this might be sulphur isotopes - if the chalcopyrite and barite precipitated under similar redox conditions. I would expect the del S of chalcopyrite that precipitated under oxidising conditions similar to the barite to be very different to chalcopyrite that precipitated under much more reducing conditions than those under which the barite precipitated (and the del S of the barite would also assist). At the minimum. I would analyse a couple of grains of every mineral in your barite-bearing sample (often results from galena, sphalerite and barite are easier to interpret than chalcopyrite and Fe sulphides I have found). That was one reason I asked for the aim and purpose of your study (potgrad, postdoc, mining industry) to understand what you are trying to achieve (eg a qualification, or this might be incidental to mineral processing studies). I mention sulphur isotopes because (i) they have a reasonable chance of shedding light on the problem, (ii) analyses are usually failrly inexpensive and easily and rapidly available, (iii) they can be taken rapidly in a pure form from a polished section using a diamond point screwed into an objective holder on a multi-objective stage (i.e. use an objective to centre it over the grain, rotate to the diamond point, then lower it onto the polished section and rotate the stage to get sufficient powdered mineral for analysis).Make sure you get enough, and rotate back to the objective lens and check the cavity to see if it all came from one grain (as best you can tell in 2 dimensions).

If you get interesting results, you might then see if you find a lot of variation of sulphur isotopes ratios with the same minerals in different samples. If your aims are largely genetic and academic, you may find that looking at variation with depth in veins, or within various depositional layers across the width of a vein, give an interesting story.  If interesting patterns result, this might give the incentive to look at fluid inclusions, initially just Th (homogenisation) and freezing stage to look for evidence of different T and salinity fluids, and that can lead to oxygen and hydrogen isotope studies....such is the nature of experiment and finding the most effective tools for a particular problem (of course if you are doing an undergraduate project it is very different to a major postdoctoral study :-) However the highest priority beyond very preliminary sulphur isotope studies would always start with knowing the spatial variation of minerals in your vein ( I suspect Harold might agree)?

Thanks for your warm-hearted guidance, Prof. Harald and Martin. In fact, this is my PhD research project. I have some ideas about this phenomenon and I may know what I should do next. Thanks a lot, Martin.

Le

Dear colleagues:

The question and answer process reminds me of an event when I was a freshman in geosciences. Prof. Walliser, from Göttingen University told the audience at the end of his oral presentation about the Variscan Orogeny in Central Europe: " Do not take one  sample (showing it to the audience) and create a new orogeny". I am not glad about any classification, too hastily done, into HS, LS, IS epithermal, VMS, MVT .......These are abbreviatons to make  a deposit attractive either in scientific or in economic terms; but at the very end they often blur the true nature of them and form a stumbling block for making real progress in geosciences.

Sorry for this excursion into science philosophy

H.G.Dill

Aha Harald - a splitter not a lumper. Both have their place, and one should not need to apologise for discussing science philosophy as it is fundamental to all we do.. I don't think the classifications of deposit types are too hastily done, in terms of end-member type examples at least, but the classification of an individual deposit as one of them often is. I have more trouble with things like orogenic gold or mesothermal gold, which can be applied so as to cover a multitude of sins. I am not even fond of epithermal (alone( since I think temperature/pressure are inadequate descriptors and they are no longer applied in the Lindgren sense anyway, and many deposits given this name extend up into the "mesothermal" temperature range. However the terms are so entrenched, that I make do with qualifications such as "epithermal related to volcanic centres", "so-called mesothermal". "which are commonly termed orogenic". If you don't, and don't include them in your key words, your papers don't come up on searches and are less likely to be discussed or cited.

You are very welcome Le Wang. One sentence had its meaning altered a bit by an accidental full-stop and poor structure, and this punctuation may make the meaning confusing, especially if not a native-English speaker.

It read:

"One method to resolve this might be sulphur isotopes - if the chalcopyrite and barite precipitated under similar redox conditions. I would expect the del S of chalcopyrite that precipitated under oxidising conditions similar to the barite to be very different to chalcopyrite that precipitated under much more reducing conditions than those under which the barite precipitated (and the del S of the barite would also assist)".

It should read (reformulated):

"One method to resolve this might be sulphur isotopes. I would expect the del S of (i) chalcopyrite that precipitated under oxidising conditions similar to the barite to be very different to (ii) chalcopyrite that precipitated under much more reducing conditions. The del S of the barite would also assist this interpretation, since it MIGHT approximate to the del S of the ore fluid".

I hope that is clearer.

  Dear Harald, to a large extent, I agree with your idea  that mineral deposists should not be classified too arbitrarily. Since when these deposits formed, our nature won't give them any name and any boundaries deliberately between each other. They all are the sons and daughters of the geological processes. However, the classification of so many deposits is our human beings' own understanding. The real truth can only be approached, but never be reached, especially for geology. The classification of deposits facilitate the communications between thousands of economic geologists in different countries. But at the same time, the boundaries we set will slower our progress to the real truth. It was just like a paradox. Thanks for your fundamental remind.

Dear Martin, I am so happy to see your detailed, kind, and thoughtful reply. The reformulated answer above is easier to understand for me. Thanks again.