Hydrothermal veins

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Hydrothermal veins

This is a transcript of a QuickTime movie (14.3 MB) recorded in the Pilbara in 2005. Martin Van Kranendonk, of the Geolological Survey of Western Australia, describes hydrothermal veins at the Dresser formation.


In this area there are two sets of hydrothermal veins. One is the type that comes up through here and breaks across the bedded chert. The second set are these big massive barite veins. You can see that they are very coarsely crystalline. There is no wrinkly laminate texture in most of them. They are very regular/ shapes, some are sort of splitting up there, and it goes up into the fine sets of barite crystals. That big outcrop up at the top is a sandstone and that sandstone totally overlies this barite vein. It contains clasts of barite in the sandstone. There’s a lot of different types of fluid flushing through this system, in most cases after this part of the sedimentary pile has accumulated. We are lower down in the sequence than where we were at the stromatolites, which was quieter, more normal sort of sedimentation and there’s only a little bit of barite left. So that will be a little bit higher up in the sequence. In this case the sandstone has eroded down to a slightly deeper level and cut under that upper part. But it’s interesting to look around and see the complexity of events here and one thing that helped us to convince ourselves that these were hydrothermal fluids is the fact that we found in these brecciated chert horizons, hydrothermal kaolinite. These are intergrown with the silica and the fragments that you see in them are just the bedded chert that are incorporated in. But it’s broken apart by fluids and if you map them out I can show you this map here. This is the horse shoe barite thing in this fleck pattern, right by my feet, a block of bedded chert, bedded chert all the way down that hillside. There are two types of sorts of breccias and they form these vein arrays then it’s cut by this last barite vein that goes all the way to the top and the sandstone overlies all of that. So it shows that these fluids passing through the crust, which are a mix of silica and barite, and some of the veins have both silica and barite, that they are overlain by this sandstone.




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