Copper, gold, silver, mercury

1. Copper, gold, silver, mercury

2. 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

3. 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.

4. 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 hydrothermalprocesses as copper is concentrates in late magmatic stagesduring crystallization. The principal minerals of copper are sulfides such as chalcocite (Cu2S) and chalcopyrite (CuFeS2).

5. 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 neutralizingrocks, copper can be precipitated in the form of carbonates(e.g. malachite and azurite) from contact with limestone or silicatesand oxides (dioptase/chysocolla or tenorite-cuprite). A gossan of oxidizedferric iron oxides generally remains in place of the originalcopper-iron sulfide. Under ideal conditions, Cu2+ can reachthe water table and encounter reducing conditions where it isreduced to Cu1+. The reduced form of copper can then substitutefor Fe in iron sulfide to produce chalcocite, Cu2S, digenite, covellite etc.

6. 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 organicmatter is commonly associated with the sedimentary depositional environment.

7. 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 inacidic 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 complexeswith CO3 and OH and weak complexes with SO4 and Cl. In soils outside zones of mineralization, copper concentrationsapproximate the local country rocks; however, concomitantwith the economic recovery of copper ores is theanthropogenically promoted dispersal of copper in the terrestrial environment.

8. 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

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

10. 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.

11. Gold in magmatic processes The epithermal gold occurrences formed by hydrothermal activity within 1 km of the surface and atrelatively low temperatures (50-200C). These deposits arebelieved to underlie many modern hot springs and steam vents and are characterized by quartz and carbonate veining. The veins are formed by hot meteoricwaters 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 includegold-electrum-quartz-carbonate with silver, arsenic, antimony, and iron sulfides.

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

13. Gold in weathering and sediments Gold is only sparingly soluble in dilute, low temperature waters, the maximum gold concentration measured in natural freshwatersis in the order of0.15 ppb. However, in the presenceof ligands such as chloride, thiosulfate, cyanide,bisulfide and organic acids, and favorable conditions for complexformation, gold can be appreciably dissolved and transportedat 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, providesa favorable environment for gold migration.

14. 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 precipitatedby any chemical change which renders the thiosulfateligand unstable, including reduction to bisulfide, oxidation toother sulfur-oxyanions, or acidification. Complexes of goldwith humic acids or cyanide-bearing ligands are also proposedto be stable in organic-rich environments. The sedimentary gold deposits can be further divided into true residualplacer gold deposits and those in which gold has been chemicallytransported and reprecipitated during ore deposit weathering.The latter are termed supergene ore deposits.

15. 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

16. 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 measurablequantities of one or more of the following elements: Au, Hg,As, Sb, Bi, Te, Cu, Fe, Sn, Pb, Co, Ni, Pt and Ir. Silveramalgam can contain up to 20% Hg. Ag may also reach highconcentrations dissolved in native Au, Cu, Te, Sb. Itis also frequently concentrated in sulfides, e.g. argentiferous galena, tellurides, selenides etc.

17. Silver in magmatic processes Epithermal vein deposits account for a large proportion of thesilver mined in the world. They are formed by volcanic-relatedhydrothermal activity at shallow depths ( < 1.5 km) and low temperatures (50-300°C). Silver occurs as sulfide and sulfosaltminerals, and as the native metal. Associated metals ofteninclude 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 igneousintrusions. They often contain a higher concentration ofbase metals than epithermal deposits. Silver occurs as tetrahedrite, tennantite etc.

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

19. 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 nativemetal, it is frequently found in supergene enrichment zonesassociated with hydrothermal systems. The solubility of Ag+increases with increasing Eh; it is therefore dissolved fromprimary silver-bearing minerals by oxygenated near-surfacewaters. Subsequent transport to reduced zones below resultsin deposition of silver sulfide or native silver; where chlorideis available, chlorargyrite may deposit. This process of supergeneenrichment has increased the grade of many hydrothermal silver deposits.

20. Silver in weathering and sediments Silver is found in some sediment-hosted disseminated deposits,the most common of which is the Carlin-type ('invisiblegold') 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 primarilyimportant for their base metals; silver is sometimes animportant accessory metal. The majority of the deposits formin non-volcanic marine environments. Sediment-hosted (Sedex) deposits are principally strataboundPb-Zn sulfides hosted in shales, siltstones, carbonates and chemical sediments.

21. Silver in weathering and sediments In oxide- and hydroxide-containing sediments, Ag may be adsorbed onFe and Mn compounds. Deep-sea abyssal clays contain verylittle 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 dioxideswill adsorb up to 90%. In some geothermal areas, scale onthe inside of pipes contains up to 7 wt% silver.

22. Silver in environment Plants appear to concentrate silver in greater concentrationsthan the substrate upon which they grow. Coal and peat oftencontain appreciable silver, suggesting that the original plantsgrew on Ag-mineralized rocks. Silver is also preferentially concentratedin marine and terrestrial animals; this may explainthe abundances of silver in black shales.the abundances of silver in black shales. The highest concentrations of silver in soils are found overlyingAg-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 ligandsin the soil.

23. Silver in environment Some complexing anions, such as SO4, NO3,HC03 and organic acids, increase the solubility ofsilver. Others (e.g. PO4, Cl-, Br-, I-, H2S, S2-) causeprecipitation 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 adsorb20-30% of all silver in solution in streamsediments.

24. 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

25. 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 (cubicHgS) and livingstonite (monoclinic HgS · Sb2S3) etc. Mercury isa trace constituent of some sulfides (e.g. tetrahedrite -Cu3SbS3, sphalerite- ZnS). All mercury deposits are formedfrom 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 beenfractured, thus permitting ingress of the hydrothermal solutions.

26. 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 atmosphereand atmospheric transport of gaseous Hg is thepredominant 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.