Saturday, March 29, 2008

Nonsulfide Zn-Pb deposits

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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!

Friday, March 28, 2008

Weekend fun No. 3

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Weekend fun still is a bit of an irregular feature on my blog however, for this weekend I managed to find something interesting on my bookshelf that should give you something to think about. This deskcrop is about 3 to 4 cm wide to give you an idea of scale. It also features fossil remains to stay within this weekends general theme of Friday Fossil Day.

What are you seeing? What kind of rock is it? What kind of fossil is it? Why is this kind of rock so special (aside of the fossil)?



The fossil is rather difficult to recognise but this is the best I could do. To give you a hint. This is not a complete fossil. A big part is missing but what is there is enough to recognise it for what it is. Also the rock is slightly more grey than can be seen on a foto. Thus are the limitations of digital cameras I guess.

Saturday, March 22, 2008

Happy Easter!

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I ike to wish you all a Happy Easter and that you may all have a few quiet days of relaxation and reflection. Here is a perfect easter egg for us geo-scientists:

Enjoy!

Wednesday, March 19, 2008

Green mineral finally found!

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I know I am horribly late but it took me forever to find something among my collection of rocks and minerals that even remotely has some sort of greenish color. Finally, I succeeded and here I present to you the only piece of green rock/mineral that I have.



There are a lot of other minerals in this sample as anyone can notice. But just tell me what the green one is and I'll be happy! Of course you are free to speculate ahead on the others just, as well. The pencil was included for scale; the background palaeo-geographic maps have no connection to the sample. It's just the stuff I am studying these days and I didn't want to clean up.

How to make Uranium from wood

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Yes! You read the title correctly! Today I found an article in Ore Geology Reviews from M.-Z. Min et al. that I find fascinating. Why? It dates from 2001 but it demonstrate how wood is being replaced by uranium minerals! Pitchblende and coffinite to be exact. To some of the old geos around this may be yesterday's news but I am truly amazed. I'm not an expert on this so here is the abstract:
A fossil wood cell texture with pitchblende and coffinite found at a sandstone-hosted roll-type uranium deposit, Xinjiang, NW China, is first reported here for the country. In the mineralized sandstone, detrital grains consisting of quartz, feldspar, rock fragments, carbonaceous trashes, mica and accessory minerals were deposited in early Turassic time and were cemented by clays and minor authigenic calcite and quartz. Pitchblende and minor coffinite are principal ore minerals at the deposit, and selectively replaced carbonized fossil wood remnants or filled fossil wood cells. An excellent fossil wood cell texture with primary uranium minerals formed. Replacement of organic debris by primary uranium minerals may be due to a local reducing environment resulting from the production of CH4, H2S or H2SO3 in its decomposition, and a biochemical reaction indicated by the common presence of framboidal pyrite. (c) 2001 Elsevier Science B.V. All rights reserved.
And also a nice picture taken from the article of M.-Z. Min et al.



I knew that the presence of organic matter was involved in the formation of sandstone hosted roll-type deposits by creating reducing environments but that you can actually get wood replaced by uranium minerals with such great preservation of single cells is totally new and amazing for me. If you are interested I recommend getting a copy of the article!

References:
  • M.-Z. Min, X.-Z. Luo, S.-L. Mao, Z.-Q. Wang, R.C. Wang, L.-F. Qin and X.-L. Tan (2001): An excellent fossil wood cell texture with primary uranium minerals at a sandstone-hosted roll-type uranium deposit, NW China, In: Ore Geology Reviews, Elsevier

Sunday, March 16, 2008

What Phyllites can give you...

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...when you treat them nice and with care is organic-walled microfossils! Yep, I managed to get a few (only 3) well preserved (hear the sarcasm!) chitinozoa out of highly metamorphosed rocks of the Erzgebirge! My professor read and loved my report (meaning I cannot be too wrong) which is wonderful news! This is the first time I was told "it was a joy reading it"! Ok, enough exclamation marks! As I wrote in an earlier post I made a handful of thin section to examine with a polarisation microscope in the hope to prove or disprove the presence of organic-walled microfossils in these so far undescribed rocks. My seminar paper grew to 18 pages and far beyond the initally set aim of 8 but fortunately only to add a lot of detail and quality as I think. I never worked on organic-walled microfossils before, and actually I never work with fossils because I'm deeply into mineral deposits, but this was really fun. Also I have to admit it is amazing what can be done with a budget of exactly 0 Euros. It did cost me a lot of time in the lab to prepare the thin sections myself and I surely was an annoyance to the poor lab technician who had to demonstrate me everything. So what did I find? These:



A slender chitinozoan example.



This one is getting a bit rounder...



...just like this one, as well. Perhaps they are related.

Above you see 3 neat microfotos of the suspects sought. They are all a bit tiny in comparison to the literature but still within the realm of the possible. Considering the results and occurence time of chitinozoa I dismissed my original ideas and decided these must be part of the Hermsdorf Phyllites. Precambrian or Upper Carboniferous chitinozoa would have been a bit of a far fetch and the rock samples themselfs shared more characteristics with Graphite-Phyllites than Precambrian or Carboniferous Schists. I still should mail this seminar paper to the mine geologist who originally gave is the task to map out the Hermsdorf Phyllites. If I remember correctly he once told me it would be a first to find microfossils in those rocks. So these are Ordovician chitinozoa. Cool. Also cool are some metamophic microtextures. Here you go:


A nice foto of crenulation cleavage under the microscope. Note the quartz in the centre and clay minerals and organics at the sides.



Parallel foliation and segregated quartz. There is no foto of it but it forms a very nice mosaic in another sample.



Quartz with I assume fluid inclusions and signs of dissolution.

Well, enough of micro-palaeontology and metamorph rock fabrics. Hope you liked it! :-)

Saturday, March 15, 2008

Blog, Blog, Blog...

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...knock, knock, knock! Who is on the door? Blogs, blogs, blogs...

Literature about Peru and Zambia?

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I am a bit busy these days with preparing my 2nd diploma exam (there are 4 in total) that is scheduled for the 28th of March. Posting will be rather infrequent because of that.

I do have a little question not related to my preperations though. Can anyone suggest me good introductory and advanced literature/articles about the geology of Peru and Zambia? Right now I have a rather random collection of very specialised nature mainly on individual deposits. Unfortunately good general geology info about these countries is hard to come by in Germany especially when you do not know what exactly to look for. I am not restricted to German; English, Dutch or Spanish sources would also be fine.

So...any suggestions welcome and many thanks in advance!

Thursday, March 6, 2008

The Hohentwiel

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In the last Weekend Fun I demonstrated a sample of volcanic ash tuff from the Singen-Hohentwiel former volcanoe complex in southern Germany. Today the actually volcanoe can no longer be observed, it has been eroded during the last periods of glaciation. Volcanic activity was seperated into two phases. First there was an extensive ash production phase that created the very extensive tuff cover of much of the modern Hegau-region. Second was a phase of magma production that, with one exception, never reached the surface and created a number of intrusions into the tuff layers. In the Hohentwiel this formed the central Phonolite body we see today. Volcanic activity lasted roughly from the Miocene to the Pliocene. In the post-volcanic phase from the Pliocene to Pleistocene the volcanoes were eroded by glaciers down to the current geologic level. The tuff used to surround the entire phonolite body, however, theory says that it could only be preserved in the "wind-shadow" of the hard phonolite where it was protected against glacial movements. Below you can see a schematic cross-section of the Hohentwiel.
At the end of the day I took a foto that - by chance - shows roughly the same section as the above schematics. It can easily be observed how the tuff in the SW has been shielded by the phonolite. Today it is home to a nature reserve and lovely wine gardens.

The tuff sample from Weekend Fun was taken from the tuff section. Actuall location is on the backside of the hill though. Below is a nice example of the wonderful volcanic bombs that can be encountered in the tuff cover when taking the long way down the castle through the wine gardens. I forgot a scale on these fotos...how careless... :-(

And finally a wonderful and beautiful flower from the nature reserve. Unfortunately I don't have a clue what it is called but it looks very nice!

Except for the schematics fotos were all taken by myself.

References:

  • Henningsen, D. & Katzung, G. (2006): Einf├╝hrung in die Geologie Deutschlands, 7. Auflage, Elsevier

Wednesday, March 5, 2008

Looking for Detachment and Geology Happens

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Those are two new Geoblogs I discovered in the last days and am glad to welcome into the Geoblogosphere. As it seems Silverfox is re-starting her blogging activity with Looking for Detachment. Geology Happens is a completely new Geoblog that has been inspired by the activity of the Geoblogosphere. I am looking forward for both to become interesting and active Geoblogs! Silverfox states to be an exploration geologist in her profile and I looking forward to read more especially about that aspect of geology.

Tuesday, March 4, 2008

Aquatic Geologist facing the tides

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It became quite an event in the geoblogosphere to talk about your own "geologist facing death" experience. Geotripper started it and some others like Ron, Chris and Mel joined in to name a few. Being a student I didn't have that many occasions to do something really stupid, however, I quickly could think of two occasions that were quite...exciting.

Both took place during my previous mapping course in Brittany, France, 2 and a half years ago. During those the two weeks on the peninsula south of Brest we had quite some fun. The weather was awesome and the mapping area directly on the seaside. We could take a swim in our breaks and enjoy the sun. There are quite some steep cliffs. Actually the entire coast consists of bays seperated by steep, up to 1o to 20 m high, cliffs only accessible when the tide is low. Some were pretty densely vegetated but we had to get that one rock to see what the only unaccessible bay was composed of. So how do you get a rock from a vertical cliff you cannot climb down? Easy! You lay down on your belly and hang head-down with two people holding on to your feet while you work with your hammer to get some samples.

But that wasn't so scary. I had to good people sitting/holding onto my legs and I was only hanging over from by chest onwards.

Remember the tides? Yep, those almost got us drowned a few days earlier. The tides are very strong in Brittany, much stronger than in Germany were I come from. Every group of 3 had tables that noted exactly the onset of the rising and falling of the tides. Take a look at this:

Do you see those houses? There is a road in front of them. That's where we wanted to go crossing the sand you see. Lovely landscape. Like vacation. Originally we wanted to go back how we came, however, that would have meant walking into the rising tide. Then I had the bright idea to just keep going to the houses. You don't see any obstacles - neither did I. We made good progress until about 40 m in front of the road, the water was already tipping our shoes slightly, when we did encounter a tiny problem - a channel 10 m wide with a very strong current!

There was no real choice but taking off our shoes quickly, storing anything sensitive to water (GPS, compass, mobile) into them and put them around our necks to stay dry. My two friends and I decided to cross the channel holding onto each others hands tightly. That current was strong! The water was reaching well up to my chest, my not so tall friend was in to her neck! 10 m sure can seem like a long way to go! We got drifted off by the current by about 30 to 40 m down-current. You likely can imagine our excitement to make it across the channel safely. Totally wet but safe. Phewww!!! Odd enough - we loved it!

P.S. That's me on the left of the foto.

Sunday, March 2, 2008

The Rhinefalls of Schaffhausen

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While you may keep working on my Weekend Fun I'll show you two very nice fotos of one of the places I visited in the last two weeks. This was just for fun and I usually didn't pick up any rocks either, except the one you see in the Weekend Fun, however, the Rhinefalls of Schaffhausen in Switzerland very close to Germany have some very interesting geology, too, which I will tell you a bit about in the future. Right now I want to share two wonderful fotos with you. If you want you may do some guessing of what you see, too, but these are just being posted here for sharing and enjoying the beauty of nature.

This is the view from the side towards the castle.

And here you see a front view of the full width of the Rhinefalls.

Weekend Fun again!

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I just spent two nice weeks in the south of Germany. That's the explaination for the total lack of activity here. Some things are just more important...
Anyways. On one of our sight-seeing tours I couldn't help but pick up some rocks. Here I show you one for this weekend's fun! I included a pencil for scale. I hope you will have some fun guessing, however, I think this one is pretty easy to get.