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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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<blockquote data-quote="user 4386" data-source="post: 419502" data-attributes="member: 4386"><p>Yes, the reverse also occurs. It depends a bit on where you are. For example you will not find both together very often on the basalt lava plains of Victoria, although you do find little clusters (eg trachytes around Mount Macedon area, a bit around Omeo). It is far more common to find both together in some areas of eastern and central-western NSW and eastern Queensland, and a classic would be the area around Pambula NSW (the Boyd Volcanics). Often called bimodal volcanics (two modes - basalt and say rhyolite, thought to occur at the bottom and top of single magma chambers, respectively). So most of the time you get basalt flows but then a bit of rhyolite will be tapped and come to surface. Important in a lot of epithermal gold areas such as Nevada, Phillipines, Indonesia and Pacific Islands. I would suspect the area around Coonabarabran and the Glasshouse Mountains might also be of this type. Also important for some gems as we have discussed. The "greenstone belts" of the Eastern goldfields, WA are similar ancient examples - most of the volcanics are metamorphosed basalts etc (greenstone - metamorphism converts them to green minerals like chlorite and amphiboles) but there are minor rhyolites and porphyry intrusions within them.</p></blockquote><p></p>
[QUOTE="user 4386, post: 419502, member: 4386"] Yes, the reverse also occurs. It depends a bit on where you are. For example you will not find both together very often on the basalt lava plains of Victoria, although you do find little clusters (eg trachytes around Mount Macedon area, a bit around Omeo). It is far more common to find both together in some areas of eastern and central-western NSW and eastern Queensland, and a classic would be the area around Pambula NSW (the Boyd Volcanics). Often called bimodal volcanics (two modes - basalt and say rhyolite, thought to occur at the bottom and top of single magma chambers, respectively). So most of the time you get basalt flows but then a bit of rhyolite will be tapped and come to surface. Important in a lot of epithermal gold areas such as Nevada, Phillipines, Indonesia and Pacific Islands. I would suspect the area around Coonabarabran and the Glasshouse Mountains might also be of this type. Also important for some gems as we have discussed. The "greenstone belts" of the Eastern goldfields, WA are similar ancient examples - most of the volcanics are metamorphosed basalts etc (greenstone - metamorphism converts them to green minerals like chlorite and amphiboles) but there are minor rhyolites and porphyry intrusions within them. [/QUOTE]
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Gemstones, Minerals & Fossils
Gemstones and Minerals
❓Your Mineral Identification Questions answered here
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