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  •  gold veins like the quartz fields of CA (10+ / 0-)

    are always produced by geothermal activity.  

    The phrase that all gold comes from asteroids is only correct in the sense that all metals were formed in the fusion of an exploding star.

    If the earth formed suddenly and at once, heavy metals would sink to the core, but in the slow and violent accretion process of earth's formation, the core was disrupted throwing heavy metals up and allowing them to solidify at the crust.  

    Since earth was our familiar size, gold in veins have been present on earth, and were not the result of secondary impact events.

    •  I'm sorry, that's just not correct. (0+ / 0-)

      Earth didn't have a permanent crust until nearly a billion years after its formation.  Unless you're a geologist, I have to disagree with your characterization.

      Pour yourself into the future.

      by Troubadour on Fri Feb 15, 2013 at 12:23:58 PM PST

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      •  What say we cite some references? (1+ / 0-)
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        Troubadour

        Others have simply gotten old. I prefer to think I've been tempered by time.

        by Just Bob on Fri Feb 15, 2013 at 02:04:57 PM PST

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        •  If I understand the article correctly (0+ / 0-)

          the Nat Geo source supports what I'm saying - that the gold deposits moved over time with ancient crust into mountains and were only then exposed and washed away by streams.  In other words, not gushed up in lava or hydrothermal processes.  Also, they're talking about South Africa, which has some of the oldest crust on Earth, which is why it has so many precious metals in it - they haven't been subducted.

          Pour yourself into the future.

          by Troubadour on Fri Feb 15, 2013 at 05:55:47 PM PST

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          •  One of those articles cited a modified (0+ / 0-)

            placer theory...a combination of both.

            Of course in one sense you are absolutely right. We are all star dust. That's all there is for both us and the planet.

            Others have simply gotten old. I prefer to think I've been tempered by time.

            by Just Bob on Fri Feb 15, 2013 at 06:00:04 PM PST

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            •  Right, but I don't just mean "in a sense." (0+ / 0-)

              I'm saying that there is no physical geological process I'm aware of - as a scientifically literate semi-layman - that would prevent any significant quantity of primordial gold, platinum, etc. from sinking into the core over billions of years of tectonic convection.  South Africa is rife with these metals because it's among the oldest crust on the planet and has been absorbing impacts for 3.6 billion years.  

              Humbler deposits - e.g., the California Gold Rush - were caused when older crust migrated into younger, more geologically active areas and got warped into hills and mountains where erosion could expose the metals.  But you go to the youngest crust on Earth, you simply will not find gold and platinum.  That's why Polynesian culture didn't have all the precious metal jewelry of cultures like the Inca, Hittites, or Upper Egyptians.

              Pour yourself into the future.

              by Troubadour on Fri Feb 15, 2013 at 06:08:45 PM PST

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              •  We have to properly define the question (1+ / 0-)
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                Troubadour

                We know the earth doesn't create gold (to any significant degree[1]) and platinum, so the question is how is it that some areas have more than others. There is a hydrothermal explanation and even the placer theory relies on water transport from areas of concentration.

                I guess it might be interesting to consider how gold would be concentrated in asteroids to a degree that impacts would create enriched concentrations on earth. A greater age for the gold than the surrounding material doesn't address the question of transport and deposit.

                The question has been argued for over a hundred years, and while we have more knowledge, there still is no clear answer.

                [1] http://news.discovery.com/...

                Right. The bacteria doesn't create gold. It only concentrates it. I still thought it was interesting and provides another possible means of concentration and transport.

                 

                Others have simply gotten old. I prefer to think I've been tempered by time.

                by Just Bob on Fri Feb 15, 2013 at 06:28:44 PM PST

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                •  The answer lies in asteroid formation itself. (1+ / 0-)
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                  Just Bob

                  Asteroids (except for the handful of largest, like Ceres and Vesta) don't have enough gravity to have differentiated interiors - i.e., the different elements don't separate out, and there's no erosive process that would evenly distribute them either, so they occur in heterogeneous blobs throughout the interior.  Not blobs of one pure substance, obviously, but highly enriched ones.  When they impact, lighter material is mostly blasted away while the heavier material is mostly deposited.  

                  Over time the crust where the impact occurred moves and fragments, and once parts of it get uplifted into mountains, the continental crust is eroded and exposes the veins of precious metal.  This also means pieces of it can wash downstream, but as the history of any gold rush will tell you, finding gold in a river means the main strike is somewhere up in the mountains closer to the river's source.  It's not that there's more gold to be found in mountains than flat land - it's just where the material is more likely to have been exposed by erosive processes.

                  So that's why these metals occur in heterogeneous blobs on Earth: Because they occur in heterogeneous blobs in the asteroids that deliver them.

                  Pour yourself into the future.

                  by Troubadour on Fri Feb 15, 2013 at 06:41:10 PM PST

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                  •  More meteorites strike the oceans than on land (1+ / 0-)
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                    Troubadour

                    Of course any fragments are buried under miles of ocean sediment and isolated from chemical reactions and transport other than upthrusts.

                    There are sea shells found on Sandia Crest at an elevation of 10,700 feet. That implies exposure to weathering, transport, and concentration. Of course that area has also had huge lava flows from time to time.

                    http://vulcan.wr.usgs.gov/...

                    It seems that each of the Southwestern states has an apparent geologic specialty. If so, and Arizona is the big Canyon state, Utah is the Mesozoic fauna state, and Colorado is the big snow-capped Rocky Mountains state, then what is New Mexico? New Mexicans need only look out their windows for the answer: New Mexico is the Volcano state. New Mexico has one of the greatest concentrations of young, well-exposed, and uneroded volcanoes on the continent. And as a bonus, it is also the Rift Valley state; it has one of only four or five big continental rifts in the world, East Africa being one of the other ones. The fact is, New Mexico is one of the best places to study the natural history of volcanoes. Twenty percent of the U.S. National Parks and Monuments based on volcanic themes are in New Mexico. There are more here than Arizona, Idaho, Oregon, and Washington combined.

                    Volcanism in New Mexico is not "extinct," but is dormant. The record of volcanism in New Mexico is continuous over tens of millions of years, and there is no reason to think it stopped magically 3000 years ago with the eruption of several cubic kilometers of basalt (McCartys lava flow, El Malpais). New Mexico has one of only three large mid-crustal active magma bodies (Socorro) in the continent. (The others are Long Valley, California and Yellowstone, Wyoming.) The Socorro area is one of the few areas where there is a dearth of young volcanoes, so perhaps the Rift is working on filling out its volcano landscaping.

                    Every major type of volcanic landform (composite volcano, shield volcano, volcanic caldera, major ash-flows, pahoehoe and aa lava, maar crater, fissure eruptions, cinder cones) occurs in New Mexico.

                    "There's gold in them thar hills!"

                    http://geoinfo.nmt.edu/...

                    Our staff have prepared a free packet of information about gold panning in New Mexico (in PDF format 5.14 Mb). It includes a brief history of gold mining in NM, where to look, what equipment is required, what regulations apply and who to contact, and an extensive reference list.

                    Others have simply gotten old. I prefer to think I've been tempered by time.

                    by Just Bob on Fri Feb 15, 2013 at 06:59:26 PM PST

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    •  Better explanation of "permanent crust." (0+ / 0-)

      I meant that no crust from before that point survives today - it's all been recycled.  And most of the continental crust today is much younger than the oldest parts.

      Pour yourself into the future.

      by Troubadour on Fri Feb 15, 2013 at 12:27:39 PM PST

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