Friday, April 18, 2008

Weekend Field Foto - A Peruvian VMS Prospect

This weekend's field foto is actually 3 field fotos. A single one just won't do with what I am trying to illustrate. In the end of 2005 I spent 3 months doing my pre-professionals practise in a major Peruvian Mining Company. The majority of the time I spent accompanying Geologists to a VMS prospect in Northern Peru only 25km away from the coast. The climate can be described as arid and almost desert-like. Agriculture was only possible with intensive irrigation. The water melons were really good though! Anyways. Here is the first of 3 fotos:

What you see above is a good overview of the exploration prospect. We needed one day to climb that mountain the fotos is shot from and had to walk half a day through the desert at 40 degrees C without any water. Not very pleasent! Anways. Two gossaneous zones we discovered are marked. The left hand one is the bigger and most significant one. The rocks are mainly andesitic Basalts, Pillow Lavas and calcareous Black Shale metamorphosed underlain by the Peruvian Coastal Batholith that is also exposed at the tip of the right hand ridge. Except for the Batholith that is of Cretacious-Teriary age all rocks belong more or less to the Casma Group/Formation that is of Upper Cretacious age. The mountain we stand on is likely a Dacite belonging to the Calipuy Group that surpringsingly has been mapped as a Granite in the Geological Map we had from the 1960ties. The rocks of the Casma Formation are all metamorphosed and Greenschist Facies affiliated. The next foto...

...illustrates the contact between the actuall Gossan and the overlieing bleached and silified Andesites. You can see a very sharp contact between Gossan and Andesites. As you can see informal mining was going on by "mineros informales" who were mainly after the tiny gold content of the Gossan. They dug quite deep into the Gossan that we did not dare to enter. Also they used the most primitive kind of mercury extraction you can imagine. We had to call the police and have them removed to avoid contamination of the ground and drinking water. Local farmers told us it was 3 guys armed with guns. Gladly, they were not at the scene the day we visited this spot that is actually a 2nd prospect 10km south of the one I present here but likely belongs to the same cluster of VMS deposits and shares many characteristics. It is concealed by the mountain in the top foto. The last foto... back in the left gossaneous zone of the first fotograph. Here also was a pit about 30 to 40m wide and 5m deep dug by "mineros informales" who were after the little gold it contained. Mining here had been given-up some years ago. You can see the beautiful color-play of the gossaneous zone. White is Kaolonite (Dickite), orange is Limonite, red Hematite and black/grey mostly Manganese weathering traces. We took around 390 - 400kg of samples in this pit for further analysis. We managed to break one shock absorber of the car and the rear axis was severly bending through. I guess the 7 farmers (+ the 4 of us) we gave a ride together with the samples and equipment was a bit too much for our pick-up. The road was...well...Silver Fox has a real Super Highway gravel road in Nevada compared to his. The info I have is all back from early 2006. I don't know if the project was advancing beyond the geophysics and explorational drill phase. When I left we were still a little uncertain but I think it is safe to say that this would most likely be classified as a Kuroko Type VMS deposit with Lead, Zinc and Copper. I could go into much more detail but I think that would expand the scope of this post beyond the entertaining level. Hope you enjoyed!


highway8a said...

Hello, that looks like an interesting place to work, although it's too bad that the roads weren't very good. (Of course, the roads here in the deserts of Nevada and California aren't usually as good as the one I photoed in the "lost" gold mine!)

Lost Geologist said...

Well, in parts there were no roads at all so we drove over the weathering debris that was everywhere. After 2 hours of driving over that I felt like throwing-up any minute because the shaking was so bad.