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The Geology of Metallic Mineral Deposits the Metals Used by Early

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The Geology of Metallic Mineral Deposits the Metals Used by Early Metal compounds Increased usage of metals necessitated the development of smelting methods to separate metals from impure forms. Many metals mined today are derived from metal-bearing mineral compounds (most combined with sulphur or oxygen) Metal Madness: The geology of metallic mineral deposits Galena Sphalerite Magnetite PbS ZnS Fe3O4 (lead sulphide) (zinc sulphide) (iron oxide) Native metals Classification of Metals The metals used by early humans were undoubtedly in pure elemental or native form. Metals are all united by having high electrical conductivity, luster, and malleability/ductility, and the ability of their The great value of metals in ancient times reflected, in part, atoms to lose electrons to form positive ions (cations). the rarity of native metal deposits. In our culture, the term “metal” generally refers to “fusible metals”- metals of moderate hardness and relatively low melting point that can be fused with other metals to form alloys (metal mixtures). In turn, we commonly classify these fusible metals in two main categories: Native copper Native gold Native silver • Precious metals •Base metals 1 2 Abundance of Elements in Earth’s Crust Precious metals Most metals (excluding iron and aluminum) that are useful Metals of high economic value (often used to make coins to society occur in very low abundances in Earth’s crust, or jewelry due to their high metallic lustre and distinctive relative to other elements. colour). Approximately 98.5% of Earth’s crust (by weight) is accounted for by Examples: Silver, gold, platinum (and other PGEs). only 8 of the 92 naturally occurring elements: 46.6% Oxygen (O) 27.7% Silicon (Si) These are typically less reactive than other metals and 8.1% Aluminum (Al) melt at higher temperatures. 5.0% Iron (Fe) 3.6% Calcium (Ca) Many of these metals (with the exception of silver) are 2.8% Sodium (Na) 2.6% Potassium (K) desirable for the manufacture of electrical components 2.1% Magnesium (Mg) due to their resistance to corrosion and oxidation. All other elements: 1.5 % Note that the majority of the familiar metals used in society (aside from aluminum and iron) don’t even appear on this list Base metals Abundance of Culturally Significant Metals Metals of low inherent value (as compared to precious Base metals (other than iron, aluminum and titanium) metals). in weight percent of continental crust. Most base metals oxidize readily (rust/tarnish) in air. As Copper 0.0055 % a result of the high reactivity of these metals, most are Nickel 0.0075 % found naturally primarily in the form of ore minerals and Zinc 0.0070 % other compounds. Lead 0.00125 % Tin 0.00020 % The most widely used base metals include iron, lead, copper, zinc, and tin. Precious metals in weight percent of continental crust. Cultural Significance of Term: Name derived from the Silver 0.000007 % practice of alchemists who attempted to make Gold 0.0000004 % (transmute) gold (and other noble metals) from less Platinum 0.0000005 % valuable (base) metals. 3 4 Importance of Enrichment Igneous Processes: Magmatic Cooling Some metals, such as iron and Metals do not occur in uniform abundance throughout the chromium can be concentrated by crust. fractional crystallization and simple gravitational settling of There are certain geological circumstances in which heavy minerals such as culturally valuable metals occur in higher concentrations. magnetite- Fe3O4 and chromite- [(Fe, Mg)Cr2O4] from cooling Knowledge of the geological processes that lead to magma within an intrusive enrichment of metals, and of the geological environments body. in which these processes operate, are essential to the discovery of metal-rich deposits. These heavy minerals are formed during the earliest stages Geologists impact your life at the most fundamental level ! of cooling and sink to the bottom of the magma chamber. Dark bands: magnetite, chromite, platinum Light bands: silicates The Economics of Metal Extraction Igneous Processes: Immiscibility The concentrated nickel deposits An important cultural aspect in determining the feasibility of of the Sudbury Igneous Complex extracting any given metal is the cost of extraction. are thought to be the product of immiscibility – the separation of a A mineral body that can be mined at a profit is called an ore. metal-rich magma from silicate- rich magma (in a similar manner as oil separates from water). Economic factors that qualify a mineral body to be called an ore are numerous. These factors include the following: The Sudbury Basin is believed to be a meteorite impact crater • The concentration of the metal(s) of interest (due to (circular, later compressed into enrichment) an oval shape by tectonic forces). The impact of a ~10 km bolide • Size of the ore body. penetrated through, or at least heated, the entire crust causing • Cost associated with infrastructure (e.g. cost of mining, shock melting and mixing of transportation and smelting). layers of various compositions. Most of the nickel probably came • Market value of the metal(s) of interest. from the lower crust. 5 6 Igneous Processes: Immiscibility Hydrothermal Processes Among the best-known and most important ore deposits are those formed from hydrothermal (hot-water) solutions. As the molten material rose Such solutions may be left-over fluids of cooled magma, or can be to the surface, metal-rich generated by the warming of groundwater or seawater that seeps liquid separated from the into rock below the surface (the heat source is always assumed to silicate liquid into density- be magma). stratified layers. A critical factor in the ability of hot water to dissolve metals appears The molten metal material to be its “saltiness” – brine solutions containing dissolved salts are was concentrated in cracks able to dissolve metals more readily than pure water. surrounding the impact crater (forming a nickel-rich Also, the the higher the water temperature, the greater the amount of sublayer). ions and other substances the water can hold in solution (most compounds are more soluble at higher temperatures). The crater basin was later infilled with sediment. A warm brine containing lots of dissolved metals drops (precipitates) its metals (usually combined with other elements, e.g. sulphur, as ore minerals) when it is cooled, forming a metallic mineral deposit. Igneous Processes: Immiscibility Hydrothermal Processes: Convergent Boundaries Recall that magma generated at This is a section of a sample of convergent boundaries (by hydration nickel ore from Sudbury. melting) tends to be intermediate to felsic in composition. The nickel occurs in the rounded bodies of bronze- Only a little bit of fluid can exists in a coloured pentlandite- (Fe, magma chamber when most Ni)9S8 that crystallized from the of the silica has crystallized into metal-rich melt. minerals – this fluid is very salty and is also highly concentrated in heavier Dark coloured mafic silicate metal ions that can’t easily be minerals surround the incorporated into the common rock- pentlandite bodies. forming minerals. The ore also contains lots of This fluid can penetrate cracks around copper (in chalcopyrite), iron igneous intrusions– once cooled, metals (pyrrhotite) and significant are precipitated out of solution in the concentrations of platinum form of mineral compounds or native group elements (PGEs). metals. 7 8 Disseminated Deposits Vein (Lode) Deposits Pressure from the intrusion of an igneous body can generated many microscopic cracks into which residual fluid (thermally buoyant) can penetrate and cool. Minerals containing copper, molybdenum, gold and silver are distributed throughout the fractured rock in low concentrations (disseminated). Although disseminated ore deposits are of a very low grade (low concentration of metals), they are immense in scale and can be mined at a profit assuming that large volumes of Gold-bearing quartz veins (Timmins, Ontario) rock and the metal can be extracted at relatively low costs. Vein (Lode) Deposits Vein (Lode) Deposits Fluids superconcentrated in metals can penetrate larger fractures, forming mineral-filled veins. Native gold and silver can be found in the spaces between quartz crystals. Vein deposits are sometimes called “Lode Deposits” . The common term “Mother Lode” refers to a belt of quartz veins in California that was the source of the gold in the famous California gold rush of the mid 1800s. “Leaf silver” from vein deposit (Cobalt, Ontario) 9 10 Hydrothermal Processes: Divergent Boundaries Volcanogenic Massive Sulphide (VMS) deposits Magma generated at convergent boundaries (by decompression melting) The heated water, now tends to be mafic in composition. concentrated in metal ions, Accordingly, metallic mineral deposits rises to the surface along in divergent boundary settings tends to fractures and escape through be associated with mafic igneous volcanic vents called “black rocks. smokers”. As in convergent settings, metal- When the solution hits cold bearing brines left over from the cooling water, the metals precipitate of magma can penetrate cracks in the as sulphide minerals (or in crust and form metal deposits. native form in the case of gold). However, a more important contributor to the concentration of metals is the Fine crystals of the metallic interaction of seawater with oceanic sulphides that emanate from crust. black smokers accumulate in pods at the surface below). Volcanogenic Massive Sulphide (VMS) deposits Volcanogenic Massive Sulphide (VMS) deposits Note: At a midocean ridge, Black smoker chimneys salty seawater flows the “black smoke” down into faults and is actually fine- other cracks of the grained metal ocean floor. sulphide crystals that are As it is warmed, it precipitated from dissolves sulphide hot vent water. ions, and metal ions (including iron, copper, lead and gold). VMS pod surrounding black smokers 11 12 Aqueous Placer Deposits Many rock fragments eroded from rocks are ultimately affected by running Volcanogenic Massive Sulphide (VMS) deposits water. Particles of gold and other heavy minerals can be concentrated in areas affected by stream currents (remember light particles are preferentially washed The resulting pod-like away, leaving heavy particles behind).
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