All About Schist
I recently received an e-mail from Doug B., a Bedrock Dreams reader who had read an earlier post on the blog that contained a segment on schist. After reading that post, Doug was still unsure about what schist is and why it can have importance in gold prospecting and mining. Any time I fail to fully explain myself in Bedrock Dreams I think it only right that I try to clarify things. So this post is all about schist.
Change and More Change
Many of you already know the significance I place on metamorphic rocks as gold producers, especially in the Western and Southwestern United States. The key to understanding metamorphic rocks or structures is that they have always undergone moderate or alternately, massive changes or alterations due to geological forces such as heat, pressure, and the presence of certain important chemical solutions. The latter add another important ingredient to the geologic "stew" at work during metamorphic alteration. Schist falls into this category because it too is a metamorphic that can be a gold producer in contact zones (i.e., where two different rock structures come into contact) or it can become country rock (i.e., the prevailing bedrock underlying a given gold area). We'll talk more about the importance of schist as bedrock later in this post but right now I want to give you the low down on schist itself from a geological standpoint. Again, the whole idea behind metamorphic rock is that it has changed from one rock type to another or different rock type. So if schist itself undergoes metamorphic geologic change it becomes gneiss!
(An Arizona metamorphic desert range.)
Schist and the Geological "Stew"
Schist is a foliated type of metamorphic rock that is typically composed of large, plate-shaped grains or scales. What does "foliated" mean? It simply means that schists tend to form in layers that alternate or repeat themselves. Some of these layers can be quite thin while others can be very thick. But the upshot is that most schists will display this "layering," although this isn't true in every instance. Schists can also show a variety of dull colors...typically greys, black, and white or even iron staining (an indicator of iron sulfide oxidation). They usually form on boundaries where shales or sedimentary rocks have been subjected to those geological "stew" ingredients I've already mentioned, but only to a moderate degree. Once these geologic forces have run their course, schist is formed because of the alteration of the clays, mud, and sands contained in most sedimentary rocks. On the other hand, schist can be formed when shales metamorph into slate or phyllite (a mixture of quartz, mica, and chlorite). The most common metamorphic minerals found in schist are chlorite, biotite, and muscovite. Most schists have a dull luster or look to them and appear pretty much nondescript in most instances, although they can have a "sparkly" appearance due to those scaly minerals they're composed of. In other words, they don't visually scream "Gold!" or gold mineralization to an experienced prospector but they can attract or even fool those with minimal gold prospecting field experience because of the shiny minerals they contain (remember, mica is one form of "fool's gold!"). It's good to note here that schists containing muscovite or mica can be associated with crystallization or semi-precious gems like garnet (as well as others).
(Note the presence of large flakes or scales of mica and/or muscovite in this chunk of schist.)
The Old Timers Were Wrong and Right
So why is it that schists are important to gold production in certain areas here in the West and Southwest (as well as other parts of the world)? There's probably no better example to use here than the massive California gold belt or "Motherlode" found in the Sierra Nevada Mountains of Northern California. Although a good part of the Motherlode was formed from granitic batholiths (i.e., large volcanic intrusions that go deep into the earth), schists abound there as well. Geologically speaking, numerous schists and schist contact zones were formed in the northern part of the Motherlode after sedimentary and igneous (again, volcanic) rocks underwent metamorphic alteration millenia ago. Granite or granitic geologic structures then "intruded" into these schist zones creating gold-bearing schists or what the old timers called "gold slates." By now you know the old timers were wrong calling these auriferous schists "slates," but they were damn close to getting things right despite their lack of formal geological knowledge. And one reason they used the term slate in describing schists was due to its common layered structure. The other reason the old timers compared schist to slate (a common building product of the day) was the fact that many schists tend to break up easily into plate-like pieces. All this said, there actually are slates in a number of Motherlode locations so this probably added to the general confusion. But remember, slates and sedimentary rocks like sandstone form the bases for the metamorphic "birth" of schist. Confusing? Maybe, but schists were an important geologic "ingredient" in forming the massive gold-bearing region known as the Motherlode. Here are a few areas of the Motherlode that contain gold-bearing schists or "slates:"
Mariposa Formation: east of the modern-day town of Mariposa, California and composed of bands of schist formed from ancient clays and slates.
Monte de Oro ("Mount of Gold" or "Mountain of Gold") Formation: just north of Oroville, California and a Jurassic Period area of metamorphed slates (schist) often containing fossils.
Sailor Canyon Formation: found near the Sierra Nevada summits and composed of altered clays, slates, and sandstones.
Milton Formation: very close in geographic area and composition to the Sailor Canyon Formation.
(Mariposa Formation outcropping.)
So you can see that gold-bearing schist belts or areas can be quite extensive in their geographic coverage. This, in turn, underlies their significance as gold generators in certain instances.
Oh I Know...
I know that some of you reading this are wondering why it's necessary to understand the geology of schists or just plain geology in general. The wiser and more experienced small-scale gold miners and prospectors among you already know the answer to this question. Essentially, "knowledge is power." The more you know about gold geology the better off you are...not only in terms of educating yourself, but the more you learn about how gold is formed and where its likely sources are, the better the chances that you'll find yourself on top of that gold at some point. Oh I know, the ass clowns on the TV gold shows don't care about geology or schist or whatever and they get huge amounts of gold. Isn't that right? Yes they do, but so could you if I dropped your butt in the middle of pre-proven gold ground with thousands of ounces available and the high-priced equipment and gear to "scoop" it all up with. And here's one more thing...guess who delineated that proven gold ground for those TV reality "stars" in the first place? Geologists.
(The most famous gold geologist of all time and my hero, Waldemar Lindgren.)
Schist as Gold-Trapping Bedrock
Since the majority of us are placer gold miners, our main interest lies in the realm of how schist could help us recover gold. And the answer to that questions is...as bedrock either in its pure form or its altered form, gneiss. No rocket science course is needed to know that the coarser and more angular a bedrock, the greater its gold-trapping abilities as that yellow is carried over or across it. While its true that most schist country rock or bedrock tends to be horizontally laid out, there are numerous instances where various geologic upheavals have pushed it upward or forced it downward. Remember that schist bedrock is, at least in certain situations, an altered form of slate. Any of you who have worked with slate (Don F. perhaps) know how brittle it is and how easily it can be fractured. If that slate (or schist) bedrock was fractured this way with many upturned and angular pieces or stretches, just imagine its gold trapping ability in a wash or streambed. But even if schist bedrock isn't highly fractured its very coarseness or surface roughness tends to stop or trap placer gold in motion. Just run your hand across a piece of coarse sandpaper and you'll get my drift here.
(One form of schist bedrock.)
So that's what I have to say about schist in more "concrete" terms. I hope this post helped explain things a bit better for Doug and the rest of you, but if not let me know and I 'll try my best to take things further for you.
Be good to one another.
(c) Jim Rocha 2019
Questions? E-mail me at email@example.com