I found that Fe-oxides were usually not used for vectoring Fe mineral deposits of skarn type in geochemical exploration. As an indicator element, the common practice told us Fe-oxides should be the most important indicator for Fe mineral exploration. Is there someone who can give an explanation? In addition, which elements can be used for prospecting skarn Fe mineralization?
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Skarn is metamorphosed sedimentary rock, it's iron deposits are in the form of sulfides. This means two things;
- There almost certainly will be iron oxide contamination throughout the unit, at least I have never seen a sedimentary rock that didn't have staining from iron oxides. In my Earth Science classes we were always told that copper, and particularly iron oxide(s) were next to useless as diagnostic tools because tiny volumes of metal cause large and widespread colour signatures and sample contamination.
- The commercial iron ore deposits in skarn units are unusual in that they are normally sulfides rather than the more usual oxides, thus tracking oxides is not as useful as it would usually be.
You're looking for sulfur based mineralisation so Gypsum may be diagnostic, you're also looking for areas of metal concentration so lower mobility metals like Molybdenum may also be useful.
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$\begingroup$ Thank you very much, Ash. I have two more questions. Firstly, whether the first thing you presented here only applies to local-scale geochemical exploration, say 1:1000? As far as I understand, this characteristic of sulfur-based mineralization is good for geochemical exploration in a regional scale, which take stream sediment samples. Secondly, it seems that Molybdenum is active under surficial oxidation environment? $\endgroup$– tunarCommented Sep 17, 2021 at 1:38
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$\begingroup$ @emberbillow Sorry I've read the first half of your request and have to ask for clarification of my own because I can't understand what you want me to clarify. On the Molybdenum my understanding was that it is only noticeably mobile under extremely acid conditions, down towards pH 3 or less, and will otherwise tend to stay in formation. $\endgroup$– AshCommented Sep 17, 2021 at 10:28
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$\begingroup$ Thank you, Ash. As far as I know, the common sulfide for Molybdenum is molybdenite, which formed under reduced conditions. Once the ore bodies was denudated to earth surface, it tend to be oxidized and became mobile in stream. Thus, I have no idea whether Molybdenum is immobile in surficial environment. $\endgroup$– tunarCommented Sep 17, 2021 at 12:42
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$\begingroup$ @emberbillow Ah I see what you're saying, I'm thinking about in situ rock sampling rather than looking at streambed erratics, in that case you'd need to look for the most chemically stable sulfides in the stream sediment as a clue to upstream sulfurous ore weathering. Molybdenum oxide is significantly less chemically mobile than iron oxide under neutral pH conditions (an assumed field standard condition that may or may not be valid for a given location) so it will stay concentrated close to the site of sulfide weathering. $\endgroup$– AshCommented Sep 17, 2021 at 12:53