Download Silver in magmatic processes

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
monoclinic
krennerite
AuTe2
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-quartzcarbonate 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 volcanicrelated 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 nonvolcanic 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.