Rafferty J.P. (ed.) Minerals

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7 Minerals 7

Hydrothermal deposits formed at shallow depths

below a boiling hot spring system are commonly referred

to as epithermal, a term retained from an old system of

classifying hydrothermal deposits based on the presumed

temperature and depth of deposition. Epithermal veins

tend not to have great vertical continuity, but many are

exceedingly rich and deserving of the term bonanza. Many

of the famous silver and gold deposits of the western

United States, such as Comstock in Nevada and Cripple

Creek in Colorado, are epithermal bonanzas.

Porphyry deposits

Among the

most distinctive hydrothermal deposits is

a class known as porphyry copper deposits, so called

because they are invariably associated with igneous intru-

sives that are porphyritic (meaning the rock is a mixture of

coarse and ﬁne mineral grains). Porphyry copper deposits

(and their close relatives, porphyry molybdenum deposits)

contain disseminated mineralization, meaning that a large

volume of shattered rock contains a ramifying network

of tiny quartz veins, spaced only a few centimetres apart,

in which grains of the copper ore minerals chalcopyrite

and bornite (or the molybdenum ore mineral molybde-

nite) occur with pyrite. The shattered rock serves as a

permeable medium for the circulation of a hydrothermal

solution, and the volume of rock that is altered and min-

eralized by the solution can be huge: porphyry coppers are

among the largest of all hydrothermal deposits, with some

giant deposits containing many billions of tons of ore.

Although in most deposits the ore averages only between

0.5 and 1.5 percent copper by weight, the tonnages of ore

mined are so large that more than 50 percent of all copper

produced comes from porphyry coppers.

Porphyry coppers are often associated with stratovol-

canoes. As a result of the volcanism that rings the Paciﬁc

idealized drawing of a porphyry copper deposit, showing the relationship

between the porphyry body, the altered and mineralized rock, and the overly-

ing volcano. Encyclopædia Britannica, Inc.

Ocean basin, porphyry coppers are conspicuous features

of mineralization along the western borders of North and

South America and in the Philippines. Among the major

deposits are El Teniente, El Salvador, and Chuquicamata

in Chile, Cananea in Mexico, and, in the United States,

Bingham Canyon in Utah, Ely and Yerington in Nevada,

and San Manuel in Arizona.

Skarns

When a limestone or marble is invaded by a high-

temperature hydrothermal solution, the carbonate

minerals calcite and dolomite react strongly with the

slightly acid solution to form a class of mineral deposit

called a skarn. Because solutions tend to have high temper-

atures close to a magma chamber, most skarns are found

immediately adjacent to intrusive igneous rocks. The

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solutions introduce silica and iron, which combine with

the calcium and magnesium in the parent rock to form sil-

icate minerals such as diopside, tremolite, and andradite.

The hydrothermal solutions may also deposit ore minerals

of iron, copper, zinc, tungsten, or molybdenum.

The mining of magnetite from a skarn deposit at

Cornwall, Penn., U.S., commenced in 1737 and continued

for two and a half centuries. Copper skarns are found at

many places, including Copper Canyon in Nevada and

Mines Gaspé in Quebec, Can. Tungsten skarns supply

much of the world’s tungsten from deposits such as those

at Sangdong, Korea; King Island, Tas., Australia; and Pine

Creek, Calif., U.S.

Volcanogenic Massive Sulfides

Wherever volcanism occurs beneath the sea, the poten-

tial exists for seawater to penetrate the volcanic rocks,

become heated by a magma chamber, and react with

the enclosing rocks—in the process concentrating geo-

chemically scarce metals and so forming a hydrothermal

solution. When such a solution forms a hot spring on

the seaﬂoor, it can suddenly cool and rapidly deposit its

dissolved load. Mineral deposits formed by this process,

which are called volcanogenic massive sulﬁde (VMS)

deposits, are known in ancient seaﬂoor rocks of all geo-

logic ages. In addition, deposits forming today as a result

of submarine hot-spring activity have been discovered

at a number of places along the oceanic ridge (the most

volcanically active zone on Earth), and in back-arc basins

associated with subduction zones.

VMS deposits constitute some of the richest deposits

of copper, lead, and zinc known. Some of the most famous,

found in Japan and called kuroko deposits, yield ores that

contain as much as 20 percent combined copper, lead,

and zinc by weight, plus important amounts of gold and

The relationship between Mississippi Valley-type deposits, the edges of

sedimentary basins, and the ﬂow of hydrothermal solutions. Copyright

Encyclopædia Britannica, Inc.; rendering for this edition by Rosen

Educational Services

silver. Other famous VMS deposits are the historic cop-

per deposits of Cyprus and, in Canada, the Kidd Creek

deposit in Ontario and the Noranda deposits of Quebec.

Mississippi Valley type

The central plains of North America, running from

the Appalachian Mountains on the east to the Rocky

Mountains on the west, are underlain by nearly ﬂat sedi-

mentary rocks that were laid down on a now-covered

basement of igneous and metamorphic rocks. The cover

of sedimentary rocks, which have been little changed

since they were deposited, contains numerous strata of

limestone, and within the limestones near the bottom

of the pile is found a distinctive class of mineral deposit.

Because the central plains coincide closely with the drain-

age basin of the Mississippi River, this class of deposit has

come to be called the Mississippi Valley type (MVT).

MVT deposits are always in limestones and are gen-

erally located near the edges of sedimentary basins or

around the edges of what were islands or high points in

the seaﬂoor when the limestone was deposited. The

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hydrothermal solutions that introduced the ore minerals

(principally the lead mineral galena and the zinc mineral

sphalerite) apparently ﬂowed through the sandstones and

conglomerates that commonly underlie the limestones.

Where they met a barrier to ﬂow, such as a basement high

or a basin edge, the solutions moved and reacted with the

limestone, depositing ore minerals.

Among the many famous MVT deposits are the

great zinc deposits of Pine Point in Canada’s Northwest

Territories; the Tri-State zinc district centred on Joplin,

Mo., U.S.; the Viburnum Trend of southeast Missouri;

deposits in Cumberland, Eng., and in Trepča, Serbia; and

the lead-zinc deposits of the central Irish plains.

Stratiform deposits

ﬁnal class of hydrothermal deposit is called stratiform

because the ore minerals are always conﬁned within spe-

ciﬁc strata and are distributed in a manner that resembles

particles in a sedimentary rock. Because stratiform

deposits so closely resemble sedimentary rocks, contro-

versy surrounds their origin. In certain cases, such as the

White Pine copper deposits of Michigan, the historic

Kupferschiefer deposits of Germany and Poland, and

the important copper deposits of Zambia, research has

demonstrated that the origin is similar to that of MVT

deposits—that is, a hydrothermal solution moves through

a porous aquifer at the base of a pile of sedimentary strata

and, at certain places, deposits ore minerals in the over-

lying shales. The major difference between stratiform

deposits and MVT deposits is that, in the case of stratiform

deposits, the host rocks are generally shales (ﬁne-grained,

clastic sedimentary rocks) containing signiﬁcant amounts

of organic matter and ﬁne-grained pyrite.

Several of the world’s largest and most famous lead-

zinc deposits are stratiform; they also are among the most

METASOMATiC

REPlACEMENT

Metasomatic replacement is the process of simultaneous solution and

deposition whereby one mineral replaces another. It is an important

process in the formation of epigenetic mineral deposits (those formed

after the formation of the host rock), in the formation of high- and

intermediate-temperature hydrothermal ore deposits, and in super-

gene sulﬁde enrichment (enriched by generally downward movement).

Metasomatic replacement is the method whereby wood petriﬁes (sil-

ica replaces the wood ﬁbres), one mineral forms a pseudomorph of

another, or an ore body takes the place of an equal volume of rock.

Replacement occurs when a mineralizing solution encounters

minerals unstable in its presence. The original mineral is dissolved and

almost simultaneously exchanged for another. The exchange does not

occur molecule for molecule, but volume for volume; hence, fewer

molecules of a less dense mineral will replace those of a more dense

mineral. Replacement takes place ﬁrst along major channels in a host

rock through which the hydrothermal solutions ﬂow. Smaller openings,

even those of capillary size, eventually are altered, the smallest by dif-

fusion at the very front of the exchange where solutions cannot ﬂow.

Early-formed replacement minerals are themselves replaced,

and deﬁnite mineral successions have been established. The usual

sequence among the commoner hypogene (deposited by generally

ascending solutions) metallic sulﬁde minerals is pyrite, enargite, tet-

rahedrite, sphalerite, chalcopyrite, bornite, galena, and pyrargyrite.

Although replacement can occur at any temperature or pres-

sure, it is most effective at elevated temperatures, at which chemical

activity is enhanced. Replacement by cold circulating waters mostly is

conﬁned to soluble rocks, such as limestone. These may be replaced

by iron oxides, manganese oxides, or calcium phosphates; vast sur-

face deposits of copper and zinc carbonates have also formed where

limestones were replaced, and valuable deposits have occurred

where supergene sulﬁde enrichment occurs. With higher tempera-

tures, replacement increases until, at high temperatures, hardly any

rock may resist. Solutions at intermediate temperatures form simple

sulﬁdes and sulfosalts for the most part, and those at higher tempera-

tures form sulﬁdes and oxides. Replacement deposits are the largest

and most valuable of all metallic ore deposits except those of iron.

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controversial in origin because there are no obvious aqui-

fers underlying the mineralized strata. Three examples

are in Australia: Broken Hill in New South Wales, Mount

Isa in Queensland, and McArthur River in the Northern

Territory. Another example is the famous Canadian

lead-zinc deposit at Sullivan, B.C. At Broken Hill, meta-

morphism has almost completely obscured the original

geologic environment, but in the other three cases evi-

dence suggests that hydrothermal ﬂuids moved upward

along a fault from deeper within a sedimentary basin,

then reacted with a shale while it was still a mud on the

seaﬂoor. Details of the actual processes involved remain

controversial.

Groundwater

Groundwater is that part of subsurface water that is below

the water table—that is, water in the zone of saturation.

For the purpose of the present discussion, the differ-

ence between groundwater and hydrothermal solutions

is that groundwater retains many of its original chemical

characteristics and remains within 1 km (0.6 mile) or less

of the surface. Such waters form two important classes of

deposit.

roll-Front deposits

Uranium

occurs in two valence states, U

and U

Weathering of rocks converts uranium into the +6 state,

in which state it forms the uranyl ion (UO

)

. Uranyl com-

pounds tend to be soluble in groundwater, whereas U

compounds are not. So long as the groundwater remains

oxidizing, uranyl ions are stable and uranium can be

transported by groundwater; however, when uranyl ions

encounter a reducing agent such as organic matter, U

uranium is precipitated as uraninite and cofﬁnite.

Because groundwater ﬂowing through an aquifer and

meeting a reducing zone will deposit a zone, or front, of

uraninite, and because the front tends to move slowly

forward through the aquifer, dissolving as the oxidiz-

ing groundwater moves in and precipitating at the front

of the zone, deposits formed in this fashion are known

as roll-front uranium deposits. Such deposits have been

extensively mined in the western United States, notably in

Colorado, Wyoming, Utah, and Texas.

caliche deposits

second class of uranium-bearing groundwater deposits

forms in dry land areas where evaporation of groundwa-

ter during summer months is an important process.

Evaporation causes precipitation of dissolved solids, and

the most abundant dissolved solid in dry land groundwater

is calcium carbonate. When deposited, this mineral forms

a hard, calcareous cement known as caliche. If uranium

is present in the groundwater, uranium minerals such as

carnotite will also be precipitated and thus form a ura-

niferous caliche deposit. Extensive deposits of this kind

have been identiﬁed in the Namib Desert in southwestern

Africa and in desert areas of Western Australia.

Seawater or Lake Water

When either sea or lake waters evaporate, salts are precip-

itated. These salts include sodium chloride, potassium and

magnesium chlorides, borax, and sodium carbonate. Such

salts are important economically, but they are not used

for the recovery of metals and thus do not warrant dis-

cussion here. One very important class of metallic mineral

deposit, though, is also formed by precipitation from lake

or seawater. This class of deposit comprises compounds of

iron or manganese and is known as a chemical sediment,

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100

because the mineral constituents are transported in solu-

tion and then precipitated to form a sediment as a result

of chemical reaction.

Iron deposits

By far the most important metal from an economic and

technical point of view is iron. Sedimentary iron depos-

its, from which almost all iron is obtained, can therefore

be viewed as one of the world’s great mineral treasures.

There are two major types of deposit. The ﬁrst, and by

far the most important, is banded iron formations (BIFs),

so called because they are ﬁnely layered alternations of

cherty silica and an iron mineral, generally hematite, mag-

netite, or siderite.

BIFs can be divided into two kinds. The ﬁrst, and

quantitatively most important, is found in sequences of

sedimentary rocks deposited in the shallow waters of

continental shelves or in ancient sedimentary basins.

These deposits are typiﬁed by the vast BIFs around

Lake Superior and are called Lake Superior-type depos-

its. Their individual sediment layers can be as thin as

0.5 mm (0.02 inch) or as thick as 2.5 cm (1 inch), but

the alternation of a siliceous band and an iron mineral

band is invariable. Several points about Lake Superior-

type deposits are remarkable. First, individual thin

bands have enormous continuity. During the 1980s, A.F.

Trendall, working for the Geological Survey of Western

Australia, studied deposits in the Hamersley Basin and

found that individual thin layers could be traced for more

than 100 km (about 60 miles). Such continuity suggests

that evaporation played a major role in precipitating

both the iron minerals and the silica. A second remark-

able feature of Lake Superior-type deposits is that they

only formed between 2.7 and 1.8 billion years ago. Such

a narrow time frame suggests that the chemistry of the

101

oceans and atmosphere at the time of formation dif-

fered greatly from that of the present (in today’s ocean,

iron is virtually insoluble because the oxidizing atmo-

sphere causes the precipitation of insoluble ferric iron

compounds).

Lake Superior-type BIFs are known and mined on

all continents. Among the most famous are the Lake

Superior deposits of Michigan and Minnesota, the

Labrador Trough deposits of Canada, Serra dos Carajas in

Brazil, the Transvaal Basin deposits of South Africa, and

the Hamersley Basin of Australia.

A second kind of BIF, known as an Algoma type,

formed over a much wider time range than the Lake

Superior type (from 3.8 billion to a few hundred mil-

lion years ago). Algoma-type BIFs are also ﬁnely layered

intercalations of silica and an iron mineral, generally

hematite or magnetite, but the individual layers lack the

lateral continuity of Lake Superior-type BIFs. Algoma-

type BIFs are found within rock sequences containing a

signiﬁcant proportion of submarine volcanic rocks, and

for this reason it is generally accepted that such deposits

formed as a result of submarine volcanism. Such a con-

clusion is supported by two simple observations: ﬁrst,

that many volcanogenic massive sulﬁde deposits, such

as those in New Brunswick, Can., are found in the same

stratigraphic horizons as Algoma-type iron deposits and,

second, that in the modern ocean iron-rich, chemically

precipitated siliceous layers can sometimes be observed

surrounding seaﬂoor hot springs. Important iron depos-

its of the Algoma type are also exploited in Western

Australia and Liberia.

Historically, a great deal of iron was mined from a sec-

ond major type of chemically precipitated marine iron

deposit. Containing pinhead-sized ooliths (small, rounded,

accretionary masses formed by repeated deposition of thin

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