How do I decide what resources are where?

[satyesu]

Gold, marble...there are probably things I don't think of, so if you'd list some too I'd appreciate it.

[Adversary]

Gold is usually found with quartz (but unfortunately not the other way around) and granite which is are igneous materials so there would have to have been some magma near the surface of the earth. I have also heard that some gold was found in meteor impacts but I can't believe that would be a major source.

Marble on the other hand is metamorphosed sedimentary rock high in carbonates (think limestone). The original limestone was formed on a seabed with lots of shell forming creatures. The limestone must then be buried deep enough for the pressure and heat to bake (or pressure cook) the limestone into marble. You may find both types or rock in the same area, but unless there was a lot of deformation of the landform, you will not find marble under granite. Granite (or more likely Gneiss) would lay below the marble.

Above the marble, or by itself, you may find limestone. Limestone makes a great building material and is valuable in its own right. It is useful for gravel, walls, sculptures or for making cement.

Iron veins formed from the oceans. Iron is highly soluble in water and likes to be there. But iron likes oxygen even more and will leave water in a heartbeat if there is enough oxygen around. Before the earth had a high concentration of O2 most of the iron was in the oceans. Once the O2 level reached a critical point it joined with the iron to form iron-oxide which precipitated out of the sea water to form layers on the seabed. When the sea level dropped, or the land was uplifted it left these bands of iron available for relatively easy mining. These bands are again often found with limestone.

Many useful materials or minerals are found in mountains. Not because they are only found in mountains, but because it is easier to find them exposed. It would be possible to find gold, silver, copper, iron, marble and limestone in one mountain, but not likely.

That is probably more than you wanted, but I hope it helps.

[Midgardsormr]

More, more! That's incredibly useful information, and it would be great if this thread turned into a general guideline for placing resources similar to the river guidelines threads.
So how about tin? That was another very important mineral during the bronze age that influenced settlement and trade patterns. If I recall correctly, it was a major export from Britain in the classical era.

[Gidde]

What Midgardsormr said. For those of us who obsess over realism (yep, I've spent the odd week working out tectonics for a map) this is a treasure trove. Thanks and rep to you, sir. *bonk*

[Adversary]

I had to do some research for this one to make sure but... Tin is another metal found in Granite deposits. As I said before Granite is an intrusive rock formed by magma cooling underground. It is rich in many metals, rich being a relative term. Tin is normally found, like iron, in its oxidized state in the ore Cassiterite (SnO2). It was most likely found on the surface or in streams as it weathered out of granite exposures (like gold). It is also apparently often found with copper. I read that the earliest bronze items only has trace (2-3%) of tin in the mix. It is possible that this was how the alloy was discovered. For our purposes (in a fantasy or ancient world) Tin, Copper, lead and gold would be found on the surface downhill from granite mountains. The mining would most likely be surface mining or mountain side mining where a vein of the desired ore was initially exposed by erosion. Deep mines or strip mines would be rare but possible. It is interesting to me that tin is much more rare than iron in the earth's crust. Tin makes up 2 parts per million of earth's crustal material were iron makes up 50,000 ppm.

Copper mining would be similar to the other metal mining. The surface deposits would be used first, then shallow mining and mountain side mining would follow. As a side note I went to the Bingham Canyon mine in Utah with a geology trip. I don't have the photos on this computer but do a search if you are interested, it is an amazing strip mine!

Another easily mined substance that is often overlooked is kaolin. Kaolin is a high quality clay. It was deposited in slow moving or still water. Clay particles are so fine that it requires very little water movement to move them. They can collect in tidal flats, swamps or river deltas. Clay can be found near or in still water or in some places on the surface. A source of clay would necessary for just about any town or city. It will be found almost every where there are sedimentary rocks. I have a couple of photos of a local kaolin mine (central Georgia, USA):

KaolinMine.JPG
The kaloin is the white stuff on the bottom and is soft like... well.... clay. If you ask a mining engineer what the stuff on top is they will tell you it is overburden.

Just as a side note (another one). The sea level in this area rose after the clay was deposited (the over burden) and you can see the wave action preserved in the cross bedding:
CrossBedding.JPG

By the way, The kaolin from this quarry is used to make bricks for kilns... like the kilns they use to process this kaolin to make bricks for kilns...

[Gidde]

That's really cool, the clay thing never would have occurred to me.

[Adversary]

Gems, Jewels and other shiny stones. There are too many to list all of them so I will select a sample of useful or popular stones. I will start with the obvious jewel the diamond. Most people know that diamond are pure carbon, not so many people know that diamond can burn and dissolve. Forming carbon into diamonds requires heat and pressure, but too much heat and the carbon will burn. For these reasons diamond are mostly found in mines, but of course do appear on the surface in the pipe where they formed... usually. There was a diamond rush in Wisconsin until it was realized that the diamond pipe was actually in Canada and the diamonds were relocated by the glaciers during the ice age. Speaking of glaciers, ice is a mineral and can be quite valuable especially in warmer climates. Ice can be mined from glaciers or frozen lakes. The ice blocks are then packed in sawdust and put in ice houses for the summer. Ice from natural sources was a valuable commodity until well the 1920's.

Amber is another exception to the valuable stone rule. Amber is not really a stone but is petrified tree resin. Amber is a lot more common than you might think. Amber is often found with coal but the grains are usually sand sized and not worth anything (more on coal in another post). Amber is not always.. well... amber in color. It can come in shades of blue, red or black as well. While mostly used for jewelry, amber has also been used as a medicine and in perfume.

Quartz, is the second most common mineral on earth. It is found with... you guessed it, granite. Chemically quartz is SiO4 which forms a clear crystal, but impurities can give quartz different colors and properties. Varieties of quartz include: rose quartz, tiger eye, amethyst, citrine, agate and many more. Quartz crystals of unusual size and clarity are thought by some to have magical properties and my be used in some types of healing.

Jade is actually two different rocks. Nephrite and jadite, they are both metamorphic silica minerals. Jade comes in a wide variety of colors, but green is the most well known. It is usually weathered out of... not granite. Jade is a metamorphic rock and granite is not. But, granite or sedimentary rocks can be metamorphosed into gneiss (nice) and that is where jade is found (as well as garnets). Jade was/is used for its mystical properties as well as in art and jewelry.

Rubies are cool because we don't know exactly how they form in nature. Rubies are a form of corundum and are made from aluminum oxygen a touch of chromium (for color) They form where there is little silica (the most abundant mineral) and iron (the most abundant metal). They are most often found in marble deposits. Rubies are slightly fluorescent. Under UV light (like in sunlight) they give off more visible light than they absorb. Burmese warriors would imbed rubies under their skin so they would be immune to injuries in battle. Rubies were also used in medicine for flatulence, stomach pains and indigestion.

Sapphires are similar to rubies, take out the chromium and add a bit of titanium, magnesium, copper or iron for color. They come in colors from red to yellow and blue. They are found in alluvial deposits. Sapphires do not erode easily so they fall out of the rock matrix where they formed and drift down stream until they are deposited in a loose matrix (alluvial deposit). They may be found with gold, platinum or other gem stones (a valuable alluvial deposit is also known as a placer deposit).

I think those are the major ones.

Thanks Waldronate, I typed a bit to slow and we got some overlap but that is cool. I forgot about hydrothermal vents as a mineral source.

[Adversary]

The results of more research. Bitumen/Pitch/Tar/Asphalt these are all the same thing. Ancient algae and other organic creatures on sea and ocean beds get buried deep and then cooked in the absence of oxygen. The land containing the deposit then either gets uplifted of erodes down to the deposit and it leaks out. The La Brea Tar Pits are an excellent example, but there are surface deposits on most continents. The deposits are found infused in sandstone or around hydrothermal vents and even in tar lakes (Pitch Lake in Trinidad and Tobago and Lake Bermudez in Venezuela). The Canadian oil sands is a huge deposit of tar. Bitumen has the consistency of thick molasses or corn syrup (not as tasty though).

Bitumen has been used for thousand of years for everything from waterproofing ships to assembling statues or as a pigment for ink and paint. It is used as adhesive for road stones or brick walls.

If your city has ship building, walls, artists or streets a supply of bitumen will be a great asset.

[waldronate]

A lot of economically important minerals (lead, copper, silver, gold, tin, zinc, etc.) are associated with hydrothermal systems, as are many types of gems (beryls like emeralds, oxide gems like sapphires, and silica gems such opals are commonly formed in hydrothermal systems; diamonds are not).

Hydrothermal systems come in a number of forms, but the basic idea is that hot water dissolves out soluble things like the common metals and sulfur in the hot areas and deposits them in cooler areas.

What often happens is that an intrusion of melted material enters the bottom of a stack of rock and slowly begins to cool. As the melt cools, it fractionates into different components. The first things to freeze out are minerals such as quartz and feldspar (the primary component of granite). Water lowers the melting point of rock, so the water-associated things will tend to crystalize out last. The water-holding material percolates and infiltrates into surrounding rocks, metamorphosing them into other minerals and depositing a lot of the things that are dissolved (there is a lot of silica in solution, so often things like quartz veins with minerals in them are formed). This sort of intrusive process tends to form veins of minerals around the edges of granitic systems rather than bulk deposits. As the sulfides oxidize at the surface, they become oxides. Most sulfide ores need to be converted to oxides in order to extract the metal content, so the oxidation saves a step.

Another common type of hydrothermal system forms bulk sulfide ores on the ocean floor. We've probably all seen pictures of black smokers; these are examples of water percolating down, hitting hot rocks, dissolving things, and moving back up to the ocean floor, where the cool temperatures let things precipitate back out. Lots of economically important metal sulfides such as copper get set up this way. A deposit can be refined to very high purity through a few iterations of this process and the deposits can eventually be raised above the surface by tectonic forces.

In addition to the primary deposition, there are secondary deposits such as placer deposits. These deposits form as rivers erode the mineral-bearing rocks and the action of the flowing water separates the dense materials such as gold, gems, or native copper from the lighter minerals. These dense materials tend to deposit where the water flow slows down such as the inside bends of rivers or where a canyon exits a mountain range. The gold deposits of South Africa, for example, are actually an ancient and buried placer deposit. Many of the surface diamond and sapphire deposits are also secondary deposits, where the dense gems were worn out the places were they formed and moved downriver.

Diamonds are formed at great pressure and temperature, and brought to the surface as part of magma bodies. The diamonds may erode out or the primary deposit and collect in placer deposits for mining, but miners may also choose to mine the primary kimberlite (a kind of frozen magma) deposit.

An amusing side note on bitumen deposits is that the economically important bitumen is often found associated with a far more fluid fraction that is hazardous to have around and reduces the value of the bitumen. These days we refer to that stuff as petroleum (lit. "rock oil") and it's more important than the bitumen.

[waldronate]

Ah, alluvial deposits. Important both in bulk and as density-separated things like gems and metals. Waterborne materials eventually come out of the water to form various sorts of deposits. Bulk materials are useful for construction and crafts. Cobbles (6" or so) are important for a number of things such as building materials and paving. Gravel and sand are important for concrete if your culture has developed such a thing. Silt and clay are useful for making brick and various ceramic items. After the finest materials (clays) settle out, there are still dissolved materials, which can form evaporate deposits in endorheic basins. Minerals such as halite (table salt), gypsum (useful for plaster of paris), and various borate minerals (useful for glassmaking) can all form from such deposits. Most of the major evaporate deposits tend to come from ocean basins which have been closed off and buried (the major salt mines in Europe fall into this category).

I have a nasty habit of stomping on other people's threads, Adversary, sorry about that. I've had a bit too much geology training and I get a little enthusiastic on the subject. I live on the desert side of the Sierra Nevada in California, where there are huge numbers of mines for all manner of things in the area. Most of them derive in one way or another from hydrothermal action. One of my favorite places nearby is a quartz-core pegmatite. From the road about a mile away, it looks like a hill with a largish white shed on top. The shed is actually the tip of a huge quartz crystal (it's microcrystalline quartz, so it's a block of snow-white material rather than clear) that forms the center of the hill. Around the quartz core is a whole sequence of minerals left over from the original event. It's a fun place to visit. Not as much fun as the area between Darwin and Talc City (a huge hydrothermal system with minerals changing according to temperature from the central lead/silver sulfide deposit at Darwin down to the talc deposits at Talc City).