Metallic Deposits

Metals occur in all kinds of rocks but usually in concentrations that are too low to be mined. Metallic ore deposits, however, are relatively rare concentrations of metal‐bearing minerals (usually sulfides) that contain enough metal to be profitably mined. Again, the profit line is dependent on a number of economic factors. Our most important metals are iron, copper, aluminum, lead, zinc, silver, gold, chromium, nickel, cobalt, manganese, molybdenum, tungsten, vanadium, tin, mercury, magnesium, platinum, and titanium.

Mineral exploration is the practice of exploring for and discovering new ore deposits. Exploration is becoming progressively more challenging as the ore deposits exposed at the surface are discovered and mined. Future exploration will focus on developing techniques that will help find ore deposits that are hundreds or thousands of feet below the surface and impossible to detect at the surface.

Metallic ores occur in every kind of rock and some varieties of soil. The metallic minerals are concentrated into rich masses by igneous, hydrothermal, or erosional/weathering processes. Metals such as chromium, platinum, nickel, copper, and iron can precipitate as sulfide minerals in a cooling body of magma. Magmatic deposits result when the minerals settle to the bottom of the intrusive body and form thin, high‐grade layers. Hydrothermal deposits rich in copper, lead, zinc, gold, silver, molybdenum, tin, mercury, and cobalt form from hot solutions that circulate through fractured country rock. The solutions come from nearby intrusions or heated meteoric water. Much of the dissolved metal in the solutions is leached from the surrounding rocks through which the solutions migrate. Changing pressures and temperatures precipitate the metals as sulfides or pure metal, such as gold, silver, and copper. This process is usually repeated many times until the heat source has cooled or the fracture systems have become filled with mineral deposits.

Common types of hydrothermal deposits are contact metamorphic, hydrothermal, disseminated, and hot springs deposits. Contact metamorphic deposits result from hot solutions that migrate from a cooling intrusion and deposit minerals in cracks in the surrounding country rock. Hydrothermal veins are also mineral deposits in faults and cracks but are not necessarily related to an intrusive body. The fluid can be meteoric water that has moved downward toward a heat source, been heated, and ascended, leaching metals along its path. The sulfides are later deposited a considerable distance from the heat source. Some of the richest gold and silver deposits in the world are hydrothermal veins. Disseminated deposits are those in which the metal is evenly distributed in generally low concentrations throughout large masses of rock. An important type of disseminated deposit is the porphyry copper deposit, in which copper and molybdenum are found in porphyritic intrusive rocks. Huge, low‐grade, multimillionounce disseminated gold deposits have been found in sedimentary rocks in Nevada. Hot springs deposits are minerals that formed in response to hot spring activity at the surface of the earth. These can be rich in gold, silver, antimony, arsenic, and mercury.

Ore deposits can form also by other processes at the earth's surface. Mississippi Valley‐type deposits are concentrations of lead and zinc that are thought to be deposited in porous limestones and sandstones by low‐temperature water that was driven out of deeper sediments by compaction. These deposits are common in the central United States over relatively stable crust and may be one of the few deposit types not related to some kind of igneous heat source. The ore minerals in most of the world's iron and manganese reserves were chemically precipitated in the ocean and accumulated on the sea floor. Placer deposits are heavy metallic minerals, such as iron or titanium minerals, or native gold or diamonds, that have been concentrated by wave or water action in a river or beach environment. The source of the minerals may be far upstream and contain very low amounts of these minerals. The weathering, erosion, downstream transport, and deposition result in concentrations of the minerals that can be profitably mined. Lateritic weathering results in residual deposits that became enriched through the chemical breakdown and removal of most of the elements of the rock. For example, in tropical climates, nickel and aluminum are left behind as their host rocks are chemically weathered, forming enriched, high‐grade supergene deposits that can be mined. High‐grade supergene gold and copper deposits can form also when a low‐grade deposit is weathered down ward, and the metal accumulates in place. A rusty, iron‐bearing cap called a gossan is often the only remnant of a weathered metallic ore deposit at the surface. Finding a gossan may indicate additional minerals exist below the zone of weathering.

Most metallic ore deposits are a result of plate tectonic activity. High heat flows and convection currents at divergent plate boundaries, such as midoceanic ridges, create submarine hot springs called black smokers that deposit solid masses of metallic minerals. These are important environments for the deposition of iron, copper, zinc, lead, gold, and silver. Metal‐laden solutions that are denser than water can collect in basins on the ocean floor, forming rich deposits. Island arc systems that develop along converging boundaries also create massive sulfide deposits rich in base metals, as well as hot‐spring gold‐silver deposits on the flanks of andesitic volcanoes. The development of metallic ores in intrusive rocks, such as porphyry copper deposits, is related to the partial melting and rising of crustal material along subduction zones. Metal in these systems may also be contributed from sea‐floor deposits that were subducted and became part of the new magma. Chromite may occur also in ultramafic intrusions in the new oceanic crust that forms at divergent boundaries.