The Way in Which Ore Is Formed

One practical way of classifying mineral deposits is to be able to distinguish deposits that were formed at the same time as the host rocks from the ones that were formed afterward. Epigenetic mineral deposits form in rocks that already are in exsistance. For example, solid rocks may fracture and the veins may be deposited in the fractures. Syngenetic mineral deposits the ones which are formed from igneous bodies or by way of sedimentary processes.

Ores have the possibility of forming by the processes that produce rocks – there are mineral deposits that seem 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 are also formed by another process which is 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 are solids that have come out of solutions.

In a hydrothermal process, hot water which is 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 some action makes them precipitate. A variety of things can occur to do this action. At times the temperature falls or the confining pressure of the rock unexpectedly decreases. At other times, the water finds another rock type that has a chemical reaction with the metal which is dissolved, this forms new minerals. In occasions, one fluid encounters another fluid which has different chemical species in solution, and there is a fluid reaction from each dissolved species.

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 do not contribute anything to the value of the deposit. For instance, gold veins are often made up of large amounts of carbonate gangue and quartz, with a little gold and some pyrite. The only amount and form there that is worth extracitng is the gold.

Whichever place that the hot water goes, it has a chemical reaction with the rock, and causes alteration. Alteration is the chemical destruction of some, if not all of the existing minerals in a rock, and the creation of new rocks. Mafic minerals like pyroxene can be converted to chlorite; feldspars are converted to clays and micas; sulphide and carbonate 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 the clearest messages of nature that there is a possibility of finding a mineral deposit closeby.