(Intense heat and pressure are instrumental in rock "morphology.")
Essentially, metamorphic rocks are rocks that have undergone a tremendous amount of change or alteration over time. Most metamorphics were formed during the Archean and Tertiary Epochs or Periods. Many of you are already familiar with the Tertiary Period (65 million-1.8 million years ago) and its significant relationship to gold, including those "ancient rivers of gold" known as Tertiary Channels that famed geologist Waldemar Lindgren spent much of his professional life studying and writing about. I'm no expert by any stretch, but I have a solid foundation of knowledge when it comes to metaphorphic rocks, the Tertiary Period, and their combined relationship to gold mineralization and genesis. On the other hand, I know very little about the Archean Epoch which goes so far back in time (2,500 million years ago) that it makes the Tertiary Period look like a VERY "new kid on the block."
What you need to know about both periods or epochs when it comes to gold though, is not their relative age, but their dynamics in terms of intrusive chemical solutions, heat, pressure, and finally...the three main rock types they ultimately transformed or "morphed:"
Sedimentary rocks are typically formed when sand, mud, or even silt are deposited as layers (usually by water or sometimes by the wind) on the earth's surface. We refer to sedimentaries more commonly as sandstone or mudstone and they're found in many different locations throughout the known world. One of the real identifiers of sedimentary rocks is their capacity for capturing or entrapping fossils, which is why sedimentaries are frequently associated with ancient rivers, swamps, lakes, or oceans.
(Sedimentaries in Arizona's Grand Canyon.)
From a strictly gold genesis or mineralization standpoint I wouldn't give a plug nickel for a ton of sedimentary rock. No way, no how. Although I've read of isolated instances where sedimentaries contained a bit of gold, I wouldn't waste one iota of time trying to prospect them, nor should you (unless your true intent is recovering fossils and not gold). Burn this tip deep into your gold-fevered brain: "Forget sedimentaries!" That is, unless those very same sedimentaries were transformed into metamorphic rocks during the Archean or Tertiary Epochs.
Igneous rocks are invariably formed underground from volcanic activity and emerge on or near the earth's surface as lava or magma. Igneous rocks typically have a high silica content. Once again from a gold-production standpoint, I wouldn't cash in the house, cars, or your 401k to rush out and buy a stockpile of igneous rock thinking you're gonna "hit the big one." Moreover, I can't tell you how many times I've had people (usually the greenhorn element) ask me about volcanoes and lava and their relationship to gold. When I reply that there's very little in the way of a DIRECT relationship between volcanoes, lava, and gold, some of these folks get miffed or think I'm blowing smoke up their butts. I'm not...honestly. You see, without some sort of metamorphic process taking place in and with those very same igneous rocks, you're wasting valuable mining time trying to pan for gold in Hawaii. Get the picture?
(Igneous or "fire" rock is typified by magma or lava.)
OK, I admit this can get a bit confusing here. I've already established that sedimentaries and igneous rocks can be transformed into one form of metamorphic rock or another, but now I'm going to tell you that some existing metamorphics can be "morphed" again and changed into something else given enough time and with the necessary physical or geological forces in play. Metamorphics into metamorphics...quite a concept isn't it?
("Blue" schist is a common form of metamorphic rock.)
You see brothers and sisters, it's all about change...sometimes significant change. Nothing stays the same forever, not the skies above us nor the earth beneath our weary feet.
There's more to come, so stay tuned.
(c) Jim Rocha (J.R.) 2014
Questions? E-mail me at email@example.com