29 March, 2008

Nonsulfide Zn-Pb deposits

As you may have noticed in earlier posts of my blog I am a keen fan of carbonate hosted Zn-Pb deposits also known as Mississippi Valley Type deposits. Although named after the Lead Belt and Viburnum Trend in southeastern Missouri my personal interest into these has come from Peru. San Vicente is the only major producing mine these days but there are a number of prospects and projects. I hope that with good luck I will be able to conduct my Master thesis related to MVT deposits in northern Peru located in the Pucara Group carbonates that also hosts the San Vicente deposit in central Peru close to the city of San Ramon. As you may know MVTs usually are base metal sulfide deposits of mainly Sphalerite (Zn) and Galenite (Pb). I am not going to go into the further basics and details because there are excellent places to read about them already.

What got my attention today was the latest Special Issue devoted to Nonsulfide Zn-Pb deposits in Ore Geology Reviews going into press now and ready for April 2008. Perhaps to the veterans of exploration the topics discussed are yesterday's news however, I have found my favorite reading for the next couple of weeks! Without going into the details let me recapitulate the topics discussed (I skipped the Intro):
  • Stable isotope geochemistry of carbonate minerals in supergene oxidation zones of Zn–Pb deposits by Gilg et al. who present new equations for cerrusite and smithsonite oxygen isotopes and temperature fractionation by looking at deposits from Europe and Australia.
  • Numerical simulation and a geochemical model of supergene carbonate-hosted non-sulphide zinc deposits by Reichert & Borg who offer a fascinating metallogentic modell for nonsulfide and discuss it in great detail with some excellent illustrations (one good image can say more than a 1000 words).
  • Willemite (Zn2SiO4) as a possible Rb–Sr geochronometer for dating nonsulfide Zn–Pb mineralization: Examples from the Otavi Mountainland (Namibia) by Schneider et al. who demonstrate in this pilot study how the mineral willemite can be used as a geochronometer to directly date nonsulfide deposits and supplies good and reasonably accurate data.
  • Mineralogical and geochemical characterization of nonsulfide Zn–Pb mineralization at Silvermines and Galmoy (Irish Midlands) by Balassone et al. and The “calamine” nonsulfide Zn–Pb deposits of Belgium: Petrographical, mineralogical and geochemical characterization by Coppola discuss the historic districts of Ireland and Belgium. Especially the later also features a number of great sample fotos.
Perhaps if time and motivation meet I will talk a bit more about these papers but I think I can already recommend these to anyone interested in mineral deposits and exploration. I will certainly be busy ready. Enjoy!

UPDATE: Also read my more recent postings!


Silver Fox said...

These do sound interesting. Are they saying, then, that the non-sulfide Zn-Pb deposits are formed directly from sphalerite-galena Mississippi-Valley-type ores? Or do they form in other ways?

Thanks for the posting.

The Lost Geologist said...

As far as I understand from my first looks at the papers they do indeed argue that nonsulfide Zn-Pb deposits can form from sphalerite/galena deposits. though not limited to MVTs (in one of the articles Broken Hill and Sedex types are mentioned) they write carbonate hosted Zn-Pb sulfides are most favorable to this. the main influence here is oxidation and dissolution and re-precipation. there is a very nice illustration in reichert & borg how they imagine this to function. they also discuss the influence of (palaeo-)climate on the formation of these deposits.

Chuck said...

All the oxide mines I know about in Australia are simply the result of deep weathering of sulphides.

The Lost Geologist said...

Gilg et al. do discuss the Broken Hill district in NSW in their paper and talk about shallow and deep weathering down to 900m. Actually the consense seems that all nonsulfide deposits are somehow linked to weathering of supergene (one case hypogen) origen.

Classes at university never go into this much detail and hardly ever supply good examples of current investigations. It always felt more like a history class. That's already reason enough for me to like these papers so much.