Oh, don’t forget to find the full “Topology and Geology” video in the members area of the Gold Prospectors Bonanza Club.
Right now, up on the board I have a couple of things I wanted to bring up. One is the concept of topology. This has to do with the elevation changes of the geography around you. What the land shape looks like vertically. Typically, that’s shown in a topologic map as a series of what you’d call … They’re really not circles, but they’re closed loops around the same level. Okay? Those are called isolines or contours. The contour line represents the same elevation or altitude at all points along the line. The objective is that what you’re going to be looking for isn’t so much the same elevation as it is the line that crosses that elevation line at a 90-degree angle. The reason is that’s the steepest slope downhill.
Whenever gold breaks away from lode, it’s going to move downhill, down that steepest slope, or steepest descent slope. That’s something to keep in mind. Every time that you look at a map, you want to keep in mind that if there’s a gold lode source found nearby or a placer source that’s up on a mountainside, such as a bench placer, the gold will move out of that region and it will immediately start to descend downhill because of the way gravity drives it and the way water and flooding drives it during the flood seasons. It’s going to drive it straight downhill, so you want to find out where that steepest slope is and map out a line that goes down that slope. That’s shown also, if you look at it down in the valley and you’re looking at something in real life, it’s going to look like this. The profile from the side will look like a mountain. In this case it’s cut off a little bit and has a flat top, so at the top we have this X marking a lode spot or a gold lode source. The gold will flow downhill. Straight down. Notice there’s another lode source on this hill on the other side. Same thing. It’s shown on this X on this side of the map.
Both of these are telling you kind of where the gold goes. Doesn’t tell you anything about the source of the gold. It doesn’t tell you anything about the geology of the area, the kind of rocks you’re going to find, or what you’re really looking for when you’re looking down in the streambed or digging things up because there’s other clues that you want to map together. Remember I said about integrating or adding things up? You want to add those things up so that they give you a bigger picture. Now, that bigger picture would say I’m going to take my topology now. I now know where the gold historically may have been because I looked that up on lode maps or I found some gold maps online or something like that.
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So, now we look at the side profile down in our canyon where we’re working, and we see this cross section of the canyon. There’s a river working the bottom. There was a river back in ancient history up on the sides in the bench area. That’s shown on here because of the little glitch in the contour lines. You would see in these maps, you’d see it where’s there’s an area where it’s nice and level or it might be sloping gently and wrapping around the side. That is an important thing to look for, but it still doesn’t tell you enough because what you’re really looking for is in that side contour are there gravels, are there rounded, are there things buried there that would indicate that there once was river up on this side? If there was, this could easily have been an old tertiary gravel pit full of gold placer material that you want to go investigate.
Now, we also had our lode on the top. Now, let me explain a little bit. The lode source is one source of gold and if you can find one of these, they can be worth a fortune, but they’re difficult to find. Without knowing what you’re doing, they’re really difficult to find in today’s age partly because of the obscurity and partly because all the easy ones have been found. Now, that doesn’t mean you won’t find them. They’re still around, but you’re more likely to find eluvial or alluvial gold. Now eluvial gold is gold that’s left behind or a residue of gold that’s left behind from the decay of the rock and minerals around it.
The key thing about an eluvial is it’s nearby where the gold lode source was originally, so it may be a bench placer nearby or it may just simply be that there once was a gold lode that decayed down into this and formed a pocket placer. That could be very well worth investigating and then only sign of it might be a dimple in the side of the mountain. A dimple that would show up on your topo map possibly. It might not. It might be too small for the contour lines on this topo map. That would be where your eyes come into play. You’re looking for a dent or some big depression that doesn’t have any explanation. There’s no in, there’s no out, there’s no way the water could have flowed into that thing in the past. What is this thing about? So, you need to go investigate it and perhaps take a metal detector over it.
Next thing, alluvial gold. This is where most of our gold is going to be found at first and that’s the placer deposits where water moved this material that decayed, down into the streambeds to settle into traps and gold hiding places. The are the S curves and the bends in the river where the gold will naturally be driven by the hydrologic processes. That’s the water flow. In our world, we have a lot of things called hydrology. Really, all it amounts to is the same concept that you have when wind blows. You’re driving this thing by putting some kind of moving fluid next to it. That force of that fluid, like we saw in the hurricanes recently, drives the material sideways through whatever it has to go through, but what it’s going to do because of its mass is it’s going to go to the bottom of that fluid bed. That fluid bed consists of rocks, cobbles, mud, gravel, anything, and as you’ve seen in some of the flooding, they can be like a cement slurry going downhill at full bore. Some of those boulders are as big as locomotives.
So, what’s happening is the gold is going to go find its way underneath those boulders. It’s going to find its way into cracks in the bedrock, stuff that doesn’t move in the flood. That’s where we’re headed when we go into alluvial gold or placer gold. Now, we’ve gone through these concepts. In addition, those things have to do with geology. Geology has kind of two parts. One is how do these things get to be formed? What are the processes that make mountains and what are the processes that tear them down? The fluids tear them down. That’s water. Okay? Sometimes air, but 99.99% of it’s water. So, what’s going to happen is as that water and the slurry and gravel cut through the material. As we saw recently, water in the form of ice ripped out a big sheet of granite the size of several apartment buildings in Yosemite Valley here just last week. That sheet moved downhill, literally fell a thousand feet down into the valley. Several people were wounded and there was a person killed, but what happens is those processes exist and create the situation where gold will move from uphill to downhill. As it does, we take advantage of that knowledge and know where to go start looking as we’re prospecting for gold. That’s the thing we want to keep in mind as we’re thinking about topology and how it integrates with geology.
The other thing about geology is the mineralogy and chemistry of the materials plays a role in how they erode and what kinds of minerals we’ll see. So, for example, if this were lode and it were chocked full of all kinds of iron pyrite, which is FES, that sulfur combines with water and air to form sulfuric acid and can erode the quartz. Very strong acid can erode the quartz and the gold will simply decay in situ and you’ll get some other minerals in here. Some oxides of that iron and sulfur and whole pile of gold. It will be very concentrated. Now, one of the unique things about eluvial gold over alluvial gold is that the eluvial gold will be very rough and course and sometimes even crystal. If you find crystalline gold, it can be extremely valuable because most of it rolls downhill and becomes crushed in the placer deposits and become alluvial, which is rounded. That’s one of the distinguishing characteristics between these two types of gold finds.
As you go through this, you’ll want to be aware of what that means. If I am looking at a gold placer and I see rounded material, I know that it has traveled a ways down the hill. If I see eluvial kinds of placer gold, even if it’s found in the streambed, I know there’s a potential that there was a lode nearby, probably within a half a mile of where I am. Whereas, here, this could have traveled for tens of miles, you just don’t know. Big difference because if I’m looking to find this lode and I start seeing a lot of this crystalline stuff, let’s see if we can’t trace where that’s coming from and do a little more panning and sampling. See the picture? Let’s get back to our discussion, questions and answers. We had several come in and I want to get to them, but that’s today’s summary of topology and geology in the short.
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