Copper, gold, silver, mercury

Copper (Cu) Universe: 0.06 ppm (by weight) Sun: 0.7 ppm (by weight) Carbonaceous meteorite: 110 ppm  Earth's Crust: 50 ppm  Seawater:  Atlantic surface: 8 x 10-5 ppm     Atlantic deep: 1.2 x 10-4 ppm

Copper in magmatic processes It occurs in the Earth's crust as elemental Cu, or in minerals as Cu1+ or Cu2+. It has strongly sulphophil character in the Earth crust. It shows enrichment in early differenciates with chalcopyrite-pyrrhotite-pentlandite association. From basic to acidic magmas the amounts of Cu shows strong deacresing. There are some Fe2+ subsitute by Cu2+ in rock-forming minerals, e.g. tourmaline (it results blue colors). Its silicates compounds are rare: dioptaz, chrysocolla etc. and form in sedimentary environs.

Copper in magmatic processes It concentrates in the post-magmatic processes, from the high to low temperatures. Its inclusions can show the origin, the Co-Ni-Bi inclusions verify the high temperature, while As-Sb-inclusions verify low temperature process. Primary copper mineralization is associated with hydrothermal processes as copper is concentrates in late magmatic stages during crystallization. The principal minerals of copper are sulfides such as chalcocite (Cu2S) and chalcopyrite (CuFeS2).

Copper in weathering and sediments The aqueous solutions associated with such weathering, commonly copper-bearing acidic iron sulfate solutions, percolate downward toward the water table. If the solutions contact acid neutralizing rocks, copper can be precipitated in the form of carbonates (e.g. malachite and azurite) from contact with limestone or silicates and oxides (dioptase/chysocolla or tenorite-cuprite). A gossan of oxidized ferric iron oxides generally remains in place of the original copper-iron sulfide. Under ideal conditions, Cu2+ can reach the water table and encounter reducing conditions where it is reduced to Cu1+. The reduced form of copper can then substitute for Fe in iron sulfide to produce chalcocite, Cu2S, digenite, covellite etc.

Copper in weathering and sediments Copper has been observed in modern swamps, where it appears to be reduced through the oxidation of organic matter common in these environments. Similarly, copper enrichment is noted in shales and sandstones where organic matter is commonly associated with the sedimentary depositional environment.

Copper in weathering and sediments In natural waters copper is commonly a trace constituent (10 mg/l), but can range up to a few hundreds of mg/l in acidic dramage from metal mines or naturally weathering ore deposits. Copper readily goes into solution. It can exist in solution as either Cu1+ or Cu2+. Cu2 + readily forms strong aqueous complexes with CO3 and OH and weak complexes with SO4 and Cl. In soils outside zones of mineralization, copper concentrations approximate the local country rocks; however, concomitant with the economic recovery of copper ores is the anthropogenically promoted dispersal of copper in the terrestrial environment.

Gold (Au) Universe: 0.0006 ppm (by weight) Sun: 0.001 ppm (by weight) Carbonaceous meteorite: 0.17 ppm  Earth's Crust: 0.011 ppm  Seawater: 5 x 10-5 ppm

Gold in magmatic processes The average concentration in the Earth's crust is in the order of 5 ppb and gold occurs mainly in discrete ore deposits. Gold-bearing ore deposits fall into two main categories: quartz or quartz-carbonate veins or vein systems, related to igneous activity or other heating events, and placer (sedimentary) deposits. Auriferous vein-type deposits can be further subdivided, on the basis of structure, geochemistry and mode of emplacement, into replacement or space-filling veins and shallow low temperature epithermal deposits.

Gold in magmatic processes It appears mainly as native gold in the Earths crust. It forms rare tellurides, selenides very rare sulphides with silver, or sometimes with other metals. sylvanite AgAuTe4 monoclinic calaverite AuTe2 krennerite orthorhombic

Gold in magmatic processes The epithermal gold occurrences formed by hydrothermal activity within 1 km of the surface and at relatively low temperatures (50-200C). These deposits are believed to underlie many modern hot springs and steam vents and are characterized by quartz and carbonate veining. The veins are formed by hot meteoric waters circulating near a magma body or other heat source, which leach precious metals such as gold and silver from the host rock or magmatic fluids carrying these metals.Typical mineral associations include gold-electrum-quartz-carbonate with silver, arsenic, antimony, and iron sulfides.

Gold in magmatic processes Gold may be reprecipitated in response to fluid boiling, due to temperature/pressure changes and loss of the sulfide ligand as gaseous H2S. Precipitation in the sinters of hot springs and other hydrothermal surface features may also be caused by gold adsorption onto amorphous mineral surfaces. Gold sulfide complexing has also been proposed to account for gold transport in fluids forming auriferous quartz veins at higher temperatures and pressures in the Earth's crust. Gold occurs both in the quartz vein and in the altered wallrock adjacent to the vein and typical mineral associations are gold-quartz with iron and copper sulfides.

Gold in weathering and sediments Gold is only sparingly soluble in dilute, low temperature waters, the maximum gold concentration measured in natural freshwaters is in the order of 0.15 ppb. However, in the presence of ligands such as chloride, thiosulfate, cyanide, bisulfide and organic acids, and favorable conditions for complex formation, gold can be appreciably dissolved and transported at low temperatures. In an oxidizing, acid environment, for example, gold can be dissolved and transported as a gold chloride complex, Au(Cl)4. Acid conditions can arise in weathering fluids as a result of iron sulfide oxidation and, coupled with the high salinity of some groundwaters, provides a favorable environment for gold migration.

Gold in weathering and sediments In low temperature fluids of more neutral or alkaline pH, thiosulfate ions form during ore sulfide oxidation, and gold may be transported as a thiosulfate complex. Gold will be precipitated by any chemical change which renders the thiosulfate ligand unstable, including reduction to bisulfide, oxidation to other sulfur-oxyanions, or acidification. Complexes of gold with humic acids or cyanide-bearing ligands are also proposed to be stable in organic-rich environments. The sedimentary gold deposits can be further divided into true residual placer gold deposits and those in which gold has been chemically transported and reprecipitated during ore deposit weathering. The latter are termed supergene ore deposits.

Silver (Ag) Universe: 0.0006 ppm (by weight) Sun: 0.001 ppm (by weight)  Carbonaceous meteorite: 0.14 ppm  Earth's Crust: 0.07 ppm  Seawater: Pacific surface: 1 x 10-7 ppm     Pacific deep: 2.4 x 10-6 ppm

Silver in magmatic processes Silver is found in the native state, and in combination with other elements, primarily S, Sb, Se, Pb, As, Bi, Cu, and Au, chiefly in sulfides and sulfosalts. It is strongly chalcophile. Native silver is rarely pure; it usually is alloyed with measurable quantities of one or more of the following elements: Au, Hg, As, Sb, Bi, Te, Cu, Fe, Sn, Pb, Co, Ni, Pt and Ir. Silver amalgam can contain up to 20% Hg. Ag may also reach high concentrations dissolved in native Au, Cu, Te, Sb. It is also frequently concentrated in sulfides, e.g. argentiferous galena, tellurides, selenides etc.

Silver in magmatic processes Epithermal vein deposits account for a large proportion of the silver mined in the world. They are formed by volcanic-related hydrothermal activity at shallow depths ( < 1.5 km) and low temperatures (50-300°C). Silver occurs as sulfide and sulfosalt minerals, and as the native metal. Associated metals often include Au, Pb, Cu, Zn, Fe, Sb and Hg. Mesothermal vein deposits (Cordilleran-type) are formed at depths of 1-4.5 km, and are associated with calc-alkaline igneous intrusions. They often contain a higher concentration of base metals than epithermal deposits. Silver occurs as tetrahedrite, tennantite etc.

Silver in magmatic processes Silver's transport in and deposition from hydrothermal solutions is greatly dependent on the presence of complexing ligands in the solutions. Within the range of temperatures ( < 350°C), pH (acidic) and fluid salinities of most hydrothermal systems, chlorosilver complexes appear to be the most important transporters of silver. In near neutral solutions, the bisulfide complex Ag(HS)2 may be important. Deposition of silver-bearing minerals from hydrothermal solution (i.e. destabilization of the soluble complexes) occurs in response to decreasing temperature, decreasing oxygen fugacity, increasing pH, fluid dilution and/or increasing activity of sulfide.

Silver in weathering and sediments Because silver is relatively soluble when combined with common anions existing in the oxidized zone of an ore deposit, but is very insoluble in the reduced sulfide form or as a native metal, it is frequently found in supergene enrichment zones associated with hydrothermal systems. The solubility of Ag+ increases with increasing Eh; it is therefore dissolved from primary silver-bearing minerals by oxygenated near-surface waters. Subsequent transport to reduced zones below results in deposition of silver sulfide or native silver; where chloride is available, chlorargyrite may deposit. This process of supergene enrichment has increased the grade of many hydrothermal silver deposits.

Silver in weathering and sediments Silver is found in some sediment-hosted disseminated deposits, the most common of which is the Carlin-type ('invisible gold') deposit. The host rocks are generally sandstones, dolomites, and limestones. Silver occurs as pyrargyrite, chlorargyrite, acanthite, in all cases finely disseminated throughout the host rocks. Stratiform sulfide deposits of sedimentary affiliation are primarily important for their base metals; silver is sometimes an important accessory metal. The majority of the deposits form in non-volcanic marine environments. Sediment-hosted (Sedex) deposits are principally stratabound Pb-Zn sulfides hosted in shales, siltstones, carbonates and chemical sediments.

Silver in weathering and sediments In oxide- and hydroxide-containing sediments, Ag may be adsorbed on Fe and Mn compounds. Deep-sea abyssal clays contain very little Ag, suggesting that most Ag in the ocean is removed by near-shore processes. Iron hydroxydes will adsorb about 60% of the available silver. Manganese dioxides will adsorb up to 90%. In some geothermal areas, scale on the inside of pipes contains up to 7 wt% silver.

Silver in environment Plants appear to concentrate silver in greater concentrations than the substrate upon which they grow. Coal and peat often contain appreciable silver, suggesting that the original plants grew on Ag-mineralized rocks. Silver is also preferentially concentrated in marine and terrestrial animals; this may explain the abundances of silver in black shales. the abundances of silver in black shales. The highest concentrations of silver in soils are found overlying Ag-bearing bedrock. Soil pH appears to control the mobility of silver; Ag is more soluble in acidic conditions, and fairly immobile in more alkaline conditions (pH> 4). Silver mobility is also controlled by the availability of ligands in the soil.

Silver in environment Some complexing anions, such as SO4, NO3, HC03 and organic acids, increase the solubility of silver. Others (e.g. PO4, Cl-, Br-, I-, H2S, S2-) cause precipitation of silver as insoluble complexes and compounds. Silver exists in fresh water in a variety of soluble complexes. Clays such as montmorillonite and illite will adsorb 20-30% of all silver in solution in stream sediments.

Mercury (Hg) Universe: 0.001 ppm (by weight) Sun: 0.02 ppm (by weight) Carbonaceous meteorite: 0.25 ppm  Earth's Crust: 0.06 ppm  Seawater: Atlantic surface: 4.9 x 10-7 ppm     Atlantic deep: 4.9 x 10-7 ppm

Mercury in magmatic processes Mercury is chalcophile and so when the Earth's crust solidified, it separated out in the sulfide phase. The most important Hg minerals are sulfides: cinnabar (trigonal HgS), metacinnabar (cubic HgS) and livingstonite (monoclinic HgS · Sb2S3) etc. Mercury is a trace constituent of some sulfides (e.g. tetrahedrite - Cu3SbS3, sphalerite- ZnS). All mercury deposits are formed from hydrothermal solutions at relatively low temperatures. The Hg-contant minimal in the early and main magmatic processes. Mercury deposits may occur in any kind of rock that has been fractured, thus permitting ingress of the hydrothermal solutions.

Mercury in weathering and sedimentary processes High levels of mercury have been reported from shales and soils enriched with organic matter. Normal soils typically contain 20-150 ppb Hg. Anthropogenic and natural sources emit Hg to the atmosphere and atmospheric transport of gaseous Hg is the predominant mechanism for mercury dispersion at the surface of the Earth. Natural inputs to the atmosphere are emissions from volcanoes, erosion, soil degasification and evasion from the ocean. Man-made release includes coal and petroleum combustion, chloralkali production and wood-paper industry. Compared to pre-industrial fluxes, about 70-80% of the current emissions are of anthropogenic origin.