I may be about to anger a bunch of geologists, but I’m going to try to boil all of economic geology down to a few critical points. The goal is to help non-geologists sort through the geo-jargon so they can have a clearer idea of what’s being said.
Even a basic grasp of how deposits are formed can help investors spot… shall we say, unlikely claims. It can also help them see when a story makes sense and the results are genuinely great.
I hope my geologist friends will forgive me for simplifying.
There’s one central idea that helps understand everything else: plumbing.
Mineral deposits are formed by fluids moving through the earth’s crust, driven by a heat source below. These weakly-acidic fluids often dissolve metals and other minerals as they travel through large volumes of rock, adding to the metals they carry as they go. When they meet physical or chemical conditions that cause those minerals to drop out of solution, they form deposits. For that to happen, you need weaknesses in the rock that allow the fluids to move: plumbing.
That can take the form of:
- Faults. These are cracks in the crust formed when immense forces break it, and the rock on each side moves in different directions. These can be huge, extending many kilometers and going deep into the earth’s crust.
- Joints. These are cracks in rock, but with no movement. Joints tend to be smaller, cracks in local rock that broke, but stayed put.
- Contacts. This is where one type of rock meets another type. For example, when a lava flow covers a sedimentary rock, the boundary between them would be the contact. An “unconformity” is a contact with a gap in time between the rocks in contact.
- Porous rocks. These are permeable rocks like sandstone, through which fluids can flow.
- Magmatic intrusions. Picture lava forcing its way up through the crust. Sometimes this is the heat source that drives fluids through other types of plumbing, as mentioned above. Sometimes the fluids travel along the margins of the magma itself. Whether the magma breaks the surface and forms a volcano (leaving extrusive rock), or stops before reaching the surface (leaving intrusive rock) doesn’t change its possible roles in the geological plumbing system.
Conditions that cause minerals to drop out of solution can be:
- Pressure changes. Picture hot fluids under great pressure moving up a fault and encountering an open void. They would boil in a flash, depositing the minerals they carry. The same thing can happen simply as the fluids come closer to surface. This is the genesis of “epithermal” deposits, so called because they form closer to surface. The boiling zone tends to be limited to 200–300 meters. Sometimes there’s more than one such mineralizing event over millions of years, and the boiling zones occur higher or lower along the fault, resulting in overlapping layers of deposition. Otherwise, you know an epithermal deposit is likely to be limited to roughly this vertical extent.
- Temperature changes. Imagine superhot fluids moving up within or along the margins of a magma pipe and then traveling away along a fault or through a porous layer of rock. Their temperature drops as they get farther from the heat source, and minerals that require high temperatures to stay in solution start dropping out.
- Chemical changes. Now picture mineralized fluids moving up through whatever plumbing is carrying them until they hit a different rock type. Its different chemical composition reacts with the fluids, causing them to deposit the minerals they carry.
Understanding which combination of these things we’re dealing with helps us understand and predict where to look for (more) pay dirt.
“Pay dirt,” by the way, often takes the form of metal sulfides. Subsequent events often oxidize (rust) these sulfides, resulting in mineralization from which it’s easier and less expensive to extract metals.
No two mineral deposits are completely alike, but they do tend to come in a few broad categories:
- Veins. Common minerals that drop out of solution when fluids flow through open spaces along faults or similar planar weaknesses in the crust include quartz and calcite. Sometimes these bear valuable metal sulfides, sometimes not. That depends not only on whether the fluids carry metals in solution, but whether conditions (pH, temperature, pressure, etc.) are just right. So, just finding a vein doesn’t mean anything valuable has been discovered. Even when the vein does have metals, they aren’t evenly distributed. It usually takes a lot of drilling to figure out where the bits worth mining are. Depending on how rich the mineralizing fluid that forms the vein is, a vein can be barren, low grade, high grade, or extremely high (bonanza) grade. Under certain conditions, veinlets of pure metal can form.
- VMS lenses. These are formed on the bottom of oceans when hot reduced sulfur and metal-bearing fluids rising along a magmatic pipe of some sort escape the rock and mix with cold, non-acidic seawater, causing minerals to drop out of solution. Such plumes deposit sulfur and metal sulfides, often forming quite large (and heavy) aggregates of sulphide minerals of lead, zinc, iron and copper, called massive sulfides. Hence the name “volcanogenic massive sulfide” or VMS. Since the ocean floor isn’t flat, the deposits tend to form in troughs and depressions around the plume. This is why, when we find them after being covered with ocean sediments for millions of years and then brought closer to surface by tectonic action, VMS lenses tend to occur in clusters. Because VMS lenses form from massive sulfides dropped directly onto the ocean floor near their source, they are often quite high grade.
- Porphyries. A Porphyry is a large, low grade deposit, usually containing copper. Sometimes there’s byproduct molybdenum, gold, tungsten or other metals. A “gold porphyry” is one in which the value of the gold exceeds the value of other minerals in the deposit. Porphyries are associated with magmatic intrusions of particular chemical compositions, which solidify before reaching the surface. There is some confusion about the term. “Porphyry” can refer to the intrusion, which has a “porphyritic” texture (big crystals in a fine groundmass) or to the deposit itself. Some porphyry intrusions are mineralized, some are not. In mineralized porphyries, most of the valuable minerals tend to occur in tiny veinlets and individual sulfide crystals. Sometimes that’s throughout the porphyry intrusion. Often it’s in the enclosing rocks on its margins. The magma that generates a mineralized porphyry is the primary source of fluids and metals that travel up the plumbing system. Even a well-mineralized porphyry is rarely very high grade—but it’s big. It’s a volume of intrusive rock, rather than a narrow vein formed in a crack somewhere. Many of the biggest copper and gold deposits in the world are porphyries.
- Shear zones. These are areas where prolonged movement along a fault results in not just a crack in the earth’s crust, but a wide volume of ground-up rock. Deposits in shear zones can be quite large, since the zones are usually much wider than veins are. But because the fluids have so much more volume to flood, the grades of the deposits tend not to be as high as in veins.
- Stockworks and breccias. These occur when mineralizing fluids flood a volume of busted-up rock, forming not just a vein or two, but a mesh of crisscrossing veins and veinlets. A stockwork can be much wider than a simple vein, like a shear zone, but it’s not a result of grinding. A breccia looks more like peanut brittle, with the deposit forming around the busted up rock (pieces of which are called “clasts”). Stockworks and breccias can also make for larger deposits, but rarely as high grade as simpler vein deposits. The veins in porphyries can be seen as giant stockworks.
- Carbonate replacement. This type of deposit is formed when mineralizing fluids meet a specific type of rock, and the chemical interactions result in the replacement of much of the original rock with mineralized rock, usually with high base metal content. These areas can be quite high grade, and they tend to come in bunches, but their shapes and distribution tend to be irregular. That can make them hard to find and define. One type of carbonate replacement deposit is a “Carlin type” gold deposit. In this case, the mineralizing fluids contain gold but little base metals.
- Other Disseminated. Mineralizing fluids rising up through different layers of rock can come to a permeable sandstone or porous tuff (rock made of volcanic ash) and spread through this layer as through a sponge, depositing minerals as they go. Sometimes a previous flow of acidic fluids can partially dissolve what would normally be impermeable rock, leaving little holes (called vugs) in which minerals can be deposited. There are other scenarios. The point is that under certain conditions, an entire layer of a certain rock gets flooded with mineralizing fluid and becomes potential ore. Such deposits tend to be lower grade, but very large.
Other factors to keep in mind:
- Alteration. Hot mineralizing fluids moving up through the earth’s crust are often acidic and they react with the rocks they encounter, altering them. There are characteristic patterns of alteration. These can tell us when we’re getting closer to the plumbing that brought the fluids up from the heat source—and where they may have deposited more minerals.
- Zonation. Some minerals tend to drop out of solution before others—those that require higher pressures or temperatures, for example. In many deposits, base metals tend to drop out first, and then precious metals. Thus, in some porphyry-related and epithermal deposits, we often see a zonation of copper, lead, and/or zinc lower down, changing to include more gold and silver higher up, and ending with mostly silver-gold toward the top.
- Dilation. The earth’s crust is not a homogenous crystal that cracks in nice straight lines or planes. If the faults formed as perfect planes, the rock on either side would suffer little damage as it moved, and would leave no open spaces. But reality is messy. Faults often kink and change directions. So as the rocks move, the kinks often result in open spaces, called dilation zones. These can run very deep and can be associated with areas of lower pressure and temperature that drop valuable minerals out of solution. In fact, we rarely find deposits in major faults, but in splays and subsidiary faults, where more dilation zones tend to form. These dilation zones give rise to veins that form as minerals are deposited in the open spaces.
- Textures. An epithermal boiling zone leaves typical textures in the crystals formed. A porphyry also has typical textures in the rock crystals as they form when the magma cools. These and other rock textures can tell us what kind of mineralized zone we are in, where we are in a mineralized system, and hence where the pay dirt is likely to be.
- Plate tectonics. Mineral-bearing fluids generally move up through plumbing in the earth’s crust, migrating from higher pressure zones deeper down to lower pressure areas near surface. But the tectonic plates of Earth’s crust are constantly moving. They rip some rocks apart, crush others together, thrust some up into mountain ranges, push others down into the mantle, accordion others into sawtooth patterns, and even turn whole layers of rock completely upside down. All of this activity is important in that it generates the magma that so often drives the plumbing systems we’re interested in. It’s also important because it impacts the geometry of deposits after they form. I’ve been to places where you can walk along rock exposed on the surface—flat as an ironing board—and see the zonation in the minerals under your feet as you go. There are places where the minerals you would expect at the top of a boiling zone are found beneath those formed lower down. So it’s important to understand not only how minerals were deposited, but what happened to them between deposition and the present.
I don’t have any own cartoon showing all these plumbing systems, but if you’re curious, you can see one on this geology page.
All of this may seem confusing. Truth is, it’s rarely easy to figure out what’s going on in any rocks in the ground. This is true even for the best geologists in the world. That’s why exploration takes years—and often ends in failure. Solving the earth’s jigsaw puzzles requires careful examination of all the clues. Some of the pieces are hundreds of miles long, and others take a microscope to see. Usually, we don’t get all the pieces, and we have to fill in the gaps with guesses.
But if it were easy, there would be no risk. And without risk, there would be nothing to speculate on.
It’s the very difficulty of the task that makes it so profitable when it’s done right.
Add it all up, and that’s why it’s so exciting when the pieces and guesses fall together and a valuable discovery is made.
Well… that and the impact on the share price of the company making then discovery, of course.
Wednesday, August 29, 8:32pm, EDT, 2018