Saturday, May 24, 2008

How I understand Nonsulfide Zn-Pb deposits

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Some time ago I wrote about the Special Issue on Nonsulfide Zn-Pb deposits published in the Ore Geology Reviews magazine. I finally got around to spending a bit of time reading those articles and making a few, simplified notes that I would like to share. Why are Nonsulfide deposits so interesting or why is the focus moving back to them after a long period of dis-interested in many parts of the world?
Most commonly nonsulfide deposits host a series of different minerals that all share a common characteristic: They lack sulfur and are usually oxides, hydroxides or carbonates. The benefication process for ores compromised of these minerals is somewhat more complicated than for sulfides, however, with the increase in environmental protection the benefit of sulfur absence in the ores is returning nonsulfides back to the agenda.
Mineralogically the most common minerals are smithonite, hydrozincite, hemimorphite, cerussite, and willemite along with gangue (calcite, gypsum, barite, etc.) and other ore minerals.
The principle idea in the entire Special Issue is that the formation of Nonsulfide Zn-Pb deposits requires the hypogene or supergene weathering of primary sulfide ores by oxidised fluids and subsequent reprecipation of the metals.
It needs a mechanism to allow for weathering and formation of oxidised fluids. Palaeoweathering, carstification and reprecipation in carstic cavities do play an important role in this process. Also these fluids need a pathway in the form of faults, joints, etc.
The influence of meteoric fluids and the penetration downward along faults and carst into the primary sulfides is one central mechanism. In Eastern Belgium the fluids likely were a mix of hydrothermal fluids and meteoric waters. Additionally the presence of willemite in nonsulfide deposits in Eastern Belgium appears to be connected to low salinity and high silica, oxidised fluids. Although the presence of willemite is often associated with high temperature hydrothermal fluids easily above 100°C, in the Belgian deposits their formation must have taken place well below 70°C under arid weathering conditions with a high silica activity or by low temperature hydrothermal fluids.
The emplacement of nonsulfides can be divided into two phases based on examples from Iran: An "oxidation stage" and a following "post-oxidation stage". Low amounts of Fe-minerals in the primary sulfide ore will controll the oxidation process. When oxidised they will lower the pH, carbonate host rocks will neutralise the acids and release CO2 which controls the paragenetic sequence (I don't quite understand how though). Last but not least the presence of oxidising bacteria, that i.e. oxidise pyrit, can increase the oxidation rate of sulfides to more easily facilitate the formation of nonsulfides. Last but not least a buffer is needed for the acid stabilising process and subsequent neutralisation.

The relationship of sulfide and nonsulfide ores in deposits from Belgium. The influence of palaeoweathering on the deposits should be obvious.

To make a quick summary the essentials for the formation of Nonsulfide Zn-Pb deposits are: Palaeoclimate and weathering, host-rock lithologies (to supply a buffer for acid neutralisation), the mineralogy of the primary sulfide ores and ore body geometry/tectonics to develop pathways of fluid migration and reprecipation in carstic cavities or replacement of the primary sulfides.

I must admit that I am still having some problems understanding especially the geochemical details. Dispite this I think that the articles are a wonderful contribution especially for people like me who had no prior contact with this kind of deposits. I can only recommend!
References:
A Special Issue devoted to Nonsulfide Zn–Pb Deposits
H.A. Gilg et al. / Ore Geology Reviews 33 (2008) 117–133
J. Reichert, G. Borg / Ore Geology Reviews 33 (2008) 134–151
J. Schneider et al. / Ore Geology Reviews 33 (2008) 152–167
G. Balassone et al. / Ore Geology Reviews 33 (2008) 168–186
V. Coppola et al. / Ore Geology Reviews 33 (2008) 187–210

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