How Ore Mineral Deposits Forms

    How Ore Mineral Deposits Forms

    One useful way to classify mineral deposits is to distinguish deposits that were formed at the time as the host rocks from those that were formed afterward. Syngenetic mineral deposits are those which 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.

    Ores can be formed by the processes that produce rocks – there are mineral deposits that appear to have been created by the crystallization of magma of from erosion and re-deposition of mineral that compromises 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 transports the minerals in solution. The minerals remain dissolved in the water until something makes them precipitate. A number of things can happed to do this. 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 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 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.