#metasomatic

The “K2 Granite”

I’ve been writing here for years and honestly, this might be the first time I’ve known so little about a rock I’m showing you. I don’t know how old this rock is or exactly where it comes from, but this is an amazing rock. I just wrote a thousand words on it. Just look at it – the rock has freaking blue dots on it!

This rock, especially in polished cabochon forms, has been appearing at mineral and rock shows for sale for several years. It gets sold under the names “K2 Granite” or “K2 Jasper”, but it’s definitely not Jasper (a type of quartz/chalcedony). Fascinatingly, no one has ever found the actual outcrop that it comes from. The best descriptions suggest that it has been hand quarried, literally picked up and carried out of the area of northern Pakistan near the mountain known as K2 or Mount Godwin-Austen, the 2nd highest peak above sea level on the planet Earth. Since no one has reported an official outcrop where these stones come from, it is possible that the stones are found in a moraine along the edge of one of the glaciers in this area. These glaciers carry ice and rock tens of kilometers through the range; the actual outcrop could be anywhere along that stretch and might even be buried beneath the ice.

When I saw a picture of this stone, I started googling for published papers on it and found…none. I searched for other things and found articles that repeated the basic description of it – found near K2, gemstone, the blue color looks like the mineral azurite and the greenish specks around it look like the mineral malachite, polishes readily, acts like a rock, and it seems like the blue color is inherent to the stone rather than a dye, but what the heck is it?

Finally, I tracked down a discussion forum at Mindat.org, cited below, where some geoscientists had (in their spare time and at their own expense) pulled apart a few of these stones and analyzed the mineralogy and texture. Here’s what they were able to get.

The rocks appear to be metamorphic, with a weak foliation. The bulk mineralogy looks a lot like an igneous rock, something between a granite and a tonalite, dominated by the mineral plagioclase but with a fair amount of potassic feldspar in the rock, in addition to mica minerals such as biotite. The blue material is a real mineral, not a chemical dye added later, and it really is the copper carbonate azurite, which produces brilliant blue crystals when it is present – the presence of that mineral was confirmed by X-ray analyses, the best way to confirm mineral ID. However, the blue color isn’t centimeter-sized grains of the azurite, the azurite is found as tiny inclusions in the plagioclase feldspar. The azurite creates centimeter-sized blue spots by existing as tiny grains in that volume of feldspar. Furthermore, the azurite is associated with tiny flakes of metallic copper, suggesting how this rock formed.

Plagioclase sometimes has copper inside it; the copper dissolves at high temperatures then separates as the plagioclase cools down, occasionally forming a gemstone called a sunstone (https://tmblr.co/Zyv2Js1r-65Bl). Since plagioclase doesn’t have a strong color, when the copper carbonate mineral azurite is present, it dominates the color of the grains and turns everything that brilliant blue color. The azurite is also associated with a bit of sulfur, which suggests that the sequence forming the blue color in this rock was: 1. Plagioclase formed with copper dissolved inside it at high temperatures 2. As the plagioclase slowly cooled, the copper exsolved into a separate, metal phase 3. Carbonate rich fluids worked their way through the rock along grain boundaries and fractures. These fluids carried other elements, probably including water and sulfur. 4. The carbonate chemically reacted with some of the copper, producing azurite within the plagioclase or on grain boundaries. This reaction left behind a trace of the sulfur.

One scientist who looked at this rock under thin section described it as being weakly foliated; a weak foliation but a metamorphic rock. The area around K2 contains a number of rocks this could possibly be related to, both igneous and metamorphic.

120 million years ago, some of the rocks near K2 were forming as part of a volcanic arc. The oceanic plate India was riding on was moving north, subducting beneath Asia and forming volcanoes. This subduction formed batholiths; huge bodies of igneous rock, and it lasted tens of millions of years. Some of these batholiths would have produced things resembling pegmatites – rich in volatile elements and sometimes metal elements. Some of the copper could have been residing here.

Sometime between 40 and 60 million years ago, India ran into Asia. When these continents collided, many of the igneous plutons that are today near K2 were thrust deep into the crust. At this point, they were heated again and metamorphosed, altering and foliating the rocks. This could be one of the places where these rocks formed; start off with one of those pegmatites that formed earlier, send those deep into the crust, metamorphose them, and we might have a place where copper-bearing plagioclase grains could form.

However, it’s not the only place this could have formed. When these rocks were pushed downwards into the crust, some of them started melting and formed new igneous rocks. Some of these igneous rocks are as young as 20 million years old, and some of them never rose up all the way to the surface, which means they formed at high enough pressures to develop weak foliations. Igneous rocks that form in continental collisions can be really, really weird – they have chemistries rich in elements like sodium, potassium, and rare earth elements. Copper might also get enriched there, along with a few other metallic elements.

However these rocks formed, finally, they moved up to the surface, during which time the copper exsolved and was altered into azurite.

Without an age date on a rock like this, we can’t know if it formed before or after the Himalayan orogeny started. In either case, it probably was foliated while it was tens of kilometers deep in the Himalayan crust. It was then thrust up to the surface along the huge thrust faults that helped form K2.

Somewhere near K2, there is a rock out there with little blue measles of azurite. I would really like to know where this rock came from. Honestly, with enough money to pay for it, someone could probably put a chunk of this stuff into an isotope analyzer, get an age for it, and that might actually help us figure out where it came from. I’m wondering if anyone has put together a funding proposal to do an age date on this stuff.

-JBB

Image credit: James St. John. https://flic.kr/p/LWtEyh

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