Postby Davidizer13 » Mon Jun 17, 2013 9:35 am
Let's start this story around 1.8 billion years ago. Back then, the only sign of life were stromatolites, these huge cabbage head shaped mats of photosynthetic algae, built over thousands of cycles of getting covered over by sediment and growing through that sediment, over and over again. At that point, all the landmasses in the world were mashed together into a supercontinent known as Columbia. It was made up of what eventually became the ancient cores of the continents we know and love today. Antarctica and Siberia next to North America, South America against western Africa. But this arrangement was slowly pulling apart, creating a huge rift valley along the margins of what became North America. This started filling up with sediment, bits and pieces of the continent behind it. First, it was muds, deposited under water along with some lenses of carbonate, representing huge masses of stromatolite growth, or lenses of coarse sands. In the next stage, the sediments became dominated by coarser grains, deposited as beach sands or as alluvial fans, huge half-cones of messy sand deposited by flash flood deposits in a desert climate. Oh yeah, and there were little traces of sulfur in there too. Maybe there was a bit of pyrite, or a pocket of hydrogen sulfide-rich methane, but there's sulfur available.
Now would be a good time to discuss oxidation/reduction reactions. Oxidation is a pretty familiar topic - iron rusts, metal corrodes, etc. What is happening at the chemical level is that oxygen, in the air or as water, is reacting with these metals, and wrenching electrons off of them, converting them from elemental or other forms into ions or oxides. The reverse of this is reduction, where atoms gain electrons, reversing the process. This is relevant in our case, because the presence of sulfur and the absence of oxygen often creates an environment where reduction occurs.
Anyway. So you've got this sandstone with a bunch of sulfur in it, whether as hydrogen sulfide or iron sulfide. Along comes a brine rich in metal ions, especially copper, pouring off the continent in the form of groundwater, and passing through this sandstone, creating a reducing environment. The metals react with the sulfur and create different metal sulfides, forming tiny crystals or masses throughout the thing. There's a zonation to this mineralization, too: at its center, where the groundwater flow lasted the longest, you have chalcopyrite, a copper-iron sulfide, along with feldspar after altered quartz grains, chlorite, and traces of hematite that color the rock pink. Next, there's a band of chalcocite and digenite, copper sulfides, along with microscopic traces of native silver; then a band of bornite (another copper sulfide), chalcopyrite and the occasional calcite nodule. Then chalcopyrite, galena (a lead sulfide) and calcite, then just galena and calcite, then pyrite-calcite, and then no alteration at all. These bands of alteration follow along the beds of sand/sandstone, bounded by beds of finer material like clay or silt, which block the flow of the solution and limit mineralization across them.
So while all that's going on, there's still more sediment getting dumped on top of all this. There's a good layer of thinly bedded mud, then a bunch of carbonate mixed with clays and silts. The last thing to get dumped on is a bunch of quartz-rich sand, probably beach or dune environments. All these got baked into good, solid sedimentary rocks. And then due to the pressure and increasing heat as they went deeper and deeper below the surface, some got cooked a lot more. The quartz grains reformed, losing their structure and becoming interlocking masses of quartzite; the muds became shales and then argillites, thinly bedded soft rocks that come in a variety of colors like green, red, purple... A few got metamorphosed even more, forming schists with scaly layers of mica between garnet-rich feldspars and quartz crystals.
You remember how all this was happening in a continental rift, where the continent's splitting apart? Yeah. That happened. Part of it gets shipped off to what becomes Siberia, the rest stays attached to the western US. All these rocks remain relatively unchanged, apart from some folding and faulting. But that's expected - the earth doesn't stand still. Over time, they becomes more and more distant from the shoreline, as chunks from other continents glom onto the western coast of the continental core, building up everything west of Boise. There's a bunch of volcanoes that form along the continental margin caused by a downgoing plate, similar to what we see in the Cascades today, that push a bunch of granite blobs through this arrangement, forming a solid mass of plutonic rock miles across. All that pushing and shoving of magma up to the surface breaks our first set of rocks a bunch, creating more faulting and folding, fractures and joints in them, shoving chunks from an ancient time over those from a slightly less ancient time.
A rather clever species of warm-bloods with funny-shaped hands starts poking holes in the rocks, and - remember that bornite I was talking about earlier? Those copper sulfides? The silver? This species loves that stuff. Can't get enough. So they dig a bunch of bigger holes, blasts apart the rock, scrapes it out, grinds it to a powder, pours it through a bunch of chemicals, and gets the copper and silver out. Some of the rock starts moving again, everyone gets out, and these clever animals try their best for a very short time to figure out how best to get back at the shiny rocks. But not all of them.
And that's how I got laid off from the best job I've ever had.