How ore forms: uranium deposits

–The following is an excerpt from Mining Explained, published by The Northern Miner.

One useful way to classify mineral deposits is to distinguish between those that were formed at the same time as the host rocks from those that were formed afterward. Syngenetic mineral deposits are those that form from igneous bodies or by way of sedimentary processes. Epigenetic mineral deposits form in rocks that already exist: for example, solid rock may fracture and veins may be deposited in the fractures, forming an epigenetic deposit.

Ores can be formed by the processes that produce rocks — there are mineral deposits that appear to have been created by the crystallization of a magma or from the erosion and redeposition of material that comprises sedimentary rocks. But mineral deposits also form by another process, called hydrothermal activity, which is the action of heated fluids in the earth. Many mineral deposits are chemical precipitates from hydrothermal solutions — that is, they have come out of solution as solids.

In a hydrothermal process, hot water, circulating through rocks by way of fractures and pore spaces, can leach minerals out of the rocks through which it passes and transport the minerals in solution. The minerals remain dissolved in the water until something makes them precipitate. A number of things can make this happen. Sometimes, the temperature falls or the confining pressure of the rock suddenly decreases. Other times, the water encounters another rock type that reacts chemically with the dissolved metal, forming new minerals. Sometimes one fluid meets another with different chemical species in solution, and the dissolved species from each fluid react.

A mineral deposit is made up of ore minerals, which carry the metal, and gangue minerals, which are formed along with the ore minerals but contribute nothing to the value of the deposit. For example, gold veins often are made up of large amounts of quartz and carbonate gangue, with some pyrite and a little gold. Only the gold is there in a form and amount that is worth extracting.

Wherever the hot water goes, it reacts chemically with the rock, causing alteration. Alteration is the chemical destruction of some or all of the existing minerals in a rock and the creation of new ones. Mafic minerals like pyroxene can be converted to chlorite; feldspars are converted to micas and clays; carbonate and sulphide minerals and quartz are left behind in the rock. Hydrothermal alteration is a sign that fluids have passed through a rock, and is one of nature’s clearest messages that there may be a mineral deposit nearby.

Sedimentary Uranium Deposits

Sandstone-hosted uranium deposits and shale-hosted uranium deposits are exactly what their name implies, that is, uranium minerals in a sandstone or shale host rock. The uranium minerals are usually disseminated through the sedimentary rock.

The sandstone-hosted types appear to have been formed in a similar manner to the stratiform copper deposits, with circulating fluids carrying uranium through the rock and depositing it when they reach a chemical trap. The uranium deposits of Niger and the southwestern U. S. are sandstone-hosted.

Uranium in these sorts of settings may have occurred through erosion and chemical removal of traces of uranium from other rocks and concentration in the basin where the shales were forming.

Sediment-hosted deposits such as carbonate lead-zinc, red-bed copper and uranium can be deformed by later folding and faulting. However, because they form in stable sedimentary basins where there is usually less tectonic activity, they remain undeformed more often than the exhalative types.

Unconformity Uranium Deposits

The unconformity-type deposits are the world’s main source of uranium. These deposits form at or near the contact between an overlying sandstone and underlying metamorphic rocks, often metamorphosed shales. The orebodies have lens or pod-like shapes, and most often occur along fractures in sandstone or in basement rocks. The host rocks often have disseminated uranium minerals and show hydrothermal alteration, which may indicate that the deposits formed after the rocks. The mineralized bodies may carry minor amounts of sulphide minerals like pyrite, arsenopyrite, galena and sphalerite, as well as nickel-cobalt arsenides.

Because this type of deposit is a relatively recent discovery — the first huge uranium deposits in Saskatchewan and northern Australia were found in the late 1960s and early 1970s — geologists are still trading theories about their origin. A model that has gained favour in recent years suggests that fluids with dissolved uranium and other metals, moving through the sandstone, encountered the basement rocks, where chemical conditions were ideal to cause the metals to precipitate from solution.