Understanding Mineral Deposits

A mineral deposit (or an ore deposit) may be defined as a rock body that contains one or more elements (or minerals) sufficiently above the average crustal abundance to have potential economic value. It has been a common practice to classify mineral deposits into two broad categories: (a) metallic mineral deposits (e.g., deposits of copper, lead, zinc, iron, gold, etc.), from which one or more metals can be extracted; and (b) nonmetallic (or industrial) mineral deposits (e.g., deposits of clay, mica, fluorite, asbestos, garnet, etc.), which contain minerals useful on account of their specific physical or chemical properties. The minerals of economic interest in a deposit are referred to as ore minerals and the waste material as gangue. Accessory sulfide-group and oxide-group minerals (e.g., pyrite, arsenopyrite, magnetite, ilmenite), especially in metallic mineral deposits, however, are sometimes described as ore minerals, although they actually constitute part of the gangue.

A comprehensive study of a mineral deposit comprises three essential steps.

The application of stable isotope ratios (¹⁸O/¹⁶O, D/H, ³⁴S/³²S and ¹³C/¹²C) to the characterization, interpretation, and exploration of hydrothermal mineral deposits is one of the major recent advances in the field of economic geology. As has been emphasized by Ohmoto (1986), isotopic data by themselves do not provide unique answers to any geological problem, because similar isotopic characteristics in a mineral deposit may be produced by different processes and the same general process may produce dissimilar isotopic characteristics under different conditions. However, isotopic data, complemented by relevant geologic, mineralogic, and geochemical studies, can provide information on several aspects of ore genesis:

Chromite deposits constitute the only primary source of chromium metal; in addition they record a remarkable phenomenon in the overall process of concentration by magmatic crystallization. Extensive literature exists on both aspects of chromite deposits. The readers are particularly referred to the review articles by Duke (1983) and Stowe (1994), and a book edited by Stowe (1987a).

Nickel is one of the most important ferro-alloy metals and its major use today, accounting for about half of the world’s total production, is for making nickel steels and nickel cast irons. It is also used for many other alloys, especially with copper (Monel metal) and chromium (stainless steel). Nickel imparts to its alloys toughness, strength, and anti-corrosion qualities; nickel steels and alloys are preferred for moving and wearing parts of many types of machinery. Other important uses of nickel include electroplating and coinage.

The platinum-group elements (PGE) comprise a geochemically coherent group of siderophile to chalcophile metals that includes osmium (Os), iridium (Ir), ruthenium (Ru), rhodium (Rh), platinum (Pt), and palladium (Pd). Based on association, the PGE may be divided into two subgroups: the Ir-subgroup (IPGE) consisting of Os, Ir, and Ru and the Pd-subgroup (PPGE) consisting of Rh, Pt, and Pd.

The most well known among the porphyry-type deposits are the porphyry copper deposits. Originally, the term porphyry copper was applied to stockwork copper mineralization in felsic, porphyritic igneous intrusions. With the development of large-scale open-pit mining methods and froth flotation techniques for efficient beneficiation of low-grade ores, the meaning of the term has been expanded to include economic considerations as well as engineering characteristics. It now refers to large (many with hundreds of million tonnes of ore), relatively low-grade (commonly <1 wt% Cu), epigenetic, intrusion-related, disseminated (stockwork), hypogene copper deposits that can be exploited by bulk-mining techniques. Similar large-tonnage and low-grade disseminated deposits related to igneous intrusions are also important for molybdenum (porphyry molybdenum) and tin (porphyry tin), and constitute part of the potential resources of uranium (porphyry uranium) and gold (porphyry gold). Association with porphyritic rocks is not considered an essential component of the definition of porphyry deposits, but the related intrusions almost always contain one or more porphyritic members, implying an epizonal environment (less than 5 km depth) of emplacement of partly crystallized magmas. Other common characteristics of the porphyry-type deposits include a large hydrothermal system dominated by magmatic ± meteoric fluids, pervasive hydrothermal alteration of host rocks, fracture-controlled ore mineralization, and association with breccias of diverse origins and variable degrees of mineralization. In this chapter we will focus on porphyry copper and porphyry molybdenum deposits.

Skarn deposits represent a very diverse class in terms of geologic setting and ore metals. They constitute the world’s premier sources of tungsten (more than 70% world’s tungsten production); major sources of copper; important sources of iron, molybdenum and zinc; and minor sources of cobalt, gold, silver, lead, bismuth, tin, beryllium, rare earth elements, fluorine, and boron (Einaudi et al. 1981, Meinert 1993). The deposits occur in a broad spectrum of geologic environments and range from Precambrian to late Tertiary in age. Most deposits of economic importance are relatively young, however, and are related to magmatic-hydrothermal activity associated with dioritic to granitic plutonism in orogenic belts. Owing to their economic importance and wide geographic distribution (Fig. 9.1), a wealth of literature is available on the descriptive and genetic aspects of skarn deposits, including some excellent review articles (e.g., Burt 1977, Shimazaki 1980, Einaudi et al. 1981, Einaudi 1982a and b, Meinert 1983, 1992, 1993, Kwak 1987, Ray & Webster 1991) and a special issue of Economic Geology (v. 77, no. 4, 1982). A continuum exists between the porphyry-type (Ch. 8) and skarn-type ore deposits, and at least some skarn deposits appear to be mineralization in carbonate wallrocks within porphyry systems. Nevertheless, skarn deposits do possess enough special characteristics to be treated as a distinct class.

Massive Cu-Zn-Pb sulfide deposits in predominantly volcanic terranes have been variously termed as ‘volcanic-associated’, ‘volcanic-hosted’, and ‘volcanogenic’. The term ‘volcanic-hosted’ is not entirely appropriate, because the deposits included in this class are not consistently hosted by volcanic rocks, nor do the deposits necessarily form as an integral part of the volcanic process as the term ‘volcanogenic’ implies. The term ‘volcanic-associated’ is considered more appropriate as it accommodates not only the deposits enclosed entirely within volcanic strata, but also those, such as the Besshi deposits (Japan), hosted by sedimentary rocks formed in a dominantly volcanic regime. The most important requirements of the volcanic-associated class of deposits are that penecontemporaneous volcanism must have accompanied the formation of the deposits, and that volcanic rocks must comprise an essential part of the immediate stratigraphic sequence (Franklin et al., 1981).

This class of Zn-Pb sulfide (±barite±Ag±Cu) deposits constitutes a major global resource of zinc (>50%) and lead (>60%), and contributes 31% and 25%, respectively, of world’s primary production of zinc and lead (Tikkanen 1986). This deposit class has been variously referred to as: Sullivan-type massive sulfide deposits (Sawkins 1976a); subclasses of stratiform sulfides of marine and marine-volcanic association (Stanton 1972); exhalative sedimentary group (Hutchinson 1980); sediment-hosted submarine exhalative deposits (SEDEX) (Large 1980, 1981, Carne and Cathro 1982); sediment-hosted Pb-Zn deposits (Badham 1981); sedimentary-type stratiform ore deposits in flysch basins (Morganti 1981); sediment-hosted stratiform lead-zinc deposits (Lydon 1983); syngenetic and diagenetic lead-zinc deposits in shales and carbonates (Edwards & Atkinson 1986); and shale-hosted deposits of Pb, Zn, and Ba (Maynard 1991b). By analogy with the volcanic-associated massive sulfide (VMS) deposits discussed earlier (Ch. 10), the descriptive term sediment-hosted massive sulfide (SMS) deposits is preferred, because it emphasizes the lithologic association of the deposits and excludes any genetic constraint. Most of the deposits included here are dominantly stratiform (i.e., the deposits are composed of sulfide layers parallel to the bedding of the host sedimentary rocks), but some are not, particularly those that have been highly deformed (McClay 1983). In addition, many deposits either contain, or are associated with, mineralization that is not stratiform. As has been pointed out by Large (1983), the term massive sulfide, which loosely describes mineralization containing more than 50% sulfides, separates this class from other classes of sediment-hosted sulfide deposits, such as the sediment-hosted (stratiform) copper deposits (see Ch. 12) and the Mississippi Valley-type Pb-Zn deposits (see Ch. 13); there are also significant differences in the lithologic association, nature of mineralization, and metal ratios among these three classes of sediment-hosted deposits. Also excluded from the present discussion are sediment-hosted barite deposits without significant base metal enrichment, such as those of the barite districts in Arkansas and Nevada (USA), although both deposit types are regarded as exhalative in origin. In contrast to SMS deposits, which are hosted by basinal elastics in dominantly intracratonic rift settings, barite deposits display geochemical signatures that indicate the influence of oceanic crust and appear to have formed in compressional continental margin settings (Maynard 1991b), perhaps from cooler and shallower hydrothermal systems.

The earliest mining of uranium ore, in 1727, was for recovery of radium from pitchblende (a poorly crystalline variety of uraninite) in a vein deposit at Joachinstal, Bohemia (Joachimsthal, former Czechoslovakia). The first important sources of radium outside Czechoslovakia were the sandstone-hosted uranium-vanadium deposits of Colorado and Utah (USA) from which about 275,000 tons of ore were produced during 1898–1923. This ore yielded about 200 g of radium, 2,000 tons of vanadium, and a small but undetermined amount of uranium most of which went into the mine tailings. The increased demand for uranium since the early 1960’s is almost entirely due to the development of nuclear reactors using uranium as the raw material. The Western world uranium production ranged between 55 to 61 million pounds of U₃O₈ during 1992–1995 (The Mining Record, March 1, 1995). Several intertwined factors — reactor safety, disposal of nuclear wastes, capital costs, political considerations — are responsible for the current stagnation and future uncertainty of the nuclear power industry and, along with it, of the uranium mining industry. Nevertheless, the uranium deposits offer a fascinating story because of their spatial, temporal, and genetic diversity.

Sedimentary rocks, including chemical precipitates formed by exhalative processes, often contain high enough iron to be considered ferruginous, or even iron deposits. Two major groups of iron-rich sedimentary rocks are recognized (James 1966): (a) ironstones, which are non-cherty, oölitic, poorly banded, and largely of Phanerozoic age; and (b) iron-formations, which are typically laminated with chert, generally non-oölitic, and largely (but not exclusively) of Precambrian age. Other groups of iron-rich rocks of lesser economic importance not considered here (but discussed, for example, by Borchiert 1960, Stanton 1972) include: (a) the blackband and clayband ores, most of which are diagenetic and post-diagenetic deposits of siderite found in coal measures and in some clays; (b) the bog iron ores found in many bogs and small lakes in higher latitudes; and (c) latentes derived by weathering of Fe-rich rocks (see Ch. 2). In this chapter we restrict our attention to the so called iron-formations, which constitute by far the most abundant and economically the most important iron-rich sediments.

Gold was among the first metals to be mined because it commonly occurs in the easily extractable native form, is beautiful and imperishable (a noble metal), and because exquisite objects can be crafted from this highly malleable and ductile metal. The earliest gold miners were the Sumerians, who were working deposits in the present-day Iran by 3800 B.C., and the Egyptians, who had organized gold mining on a significant scale by at least 3000 B.C. In ancient civilizations, gold was mostly used for lavish decoration of temples and kings’ tombs. Gold coinage appeared much later, around 700 B.C., and practically disappeared as a legal tender with the demise of the Roman Empire. Gold production slowed down in the Middle Ages but increased again with the great economic expansion of the 16th century. The ‘gold fever’ of the middle and late 19th century, which led to the discovery of many rich placer deposits in Siberia, Alaska, California, Australia, and South Africa, was probably triggered by the adoption of gold as the monetary standard by the British Empire in 1821.