12 February, 2008

The solution: Zebra ore

I think it is about time to solve my weekend fun picture. A few people made some guesses regarding carbonates and dolomite and indeed these do play a role here. However, no one got close enough to solve it. So what are you seeing? Everyone can notice the distinct light/dark colored layers that may remind some people of how a Zebra looks like. Indeed this is called a Zebra ore! It usually can be found in lead-zinc Mississippi Valley Type deposit that can usually be found in dolostone and less frequently in limestone. What you see here is rhytmic inter-layering of dark replacement and sparry dolomite typically called zebra structure. The brown to yellow layers are Sphalerite (with some minor Galena) occuring between dark and white sparry dolomite.

The fundamental mechanism in the formation of these structures is mainly selective replacement of primary rock by hydrothermal fluids but also of previous generations of hydrothermal dolomite along fractures, layers of higher permeability, stylolites and others.

The sample shown is from the MVT deposit of San Vicente in Peru.

  • V. Badoux, R. Moritz & L. Fontboté, The Mississippi Valley-type Zn-Pb deposit of San Vicente, Central Peru: an Andean syntectonic deposit, In: A. Piestrzyñski et al. (eds. 2001), Mineral deposits at the beginning of the 21st century, Balkema, Amsterdam, p. 191-195.

09 February, 2008

Weekend fun - picture time!

Everyone appears to be busy posting Geopuzzles, Friday Field Fotos 1 and 2 or Airliner Chronicles. Today I felt like posting something interesting to bother you with, too. Blogging is quite a long way down my lists of priorities, meaning I really got a lot of useful stuff done in the last days. Here is a foto of a sample outcropping right on my bookshelf. I am curious who can tell me what it is and how it forms. No need to write an essay though - we are not working here, right?!

So what do you see?