What is Economic Geology?
Economic geology refers to the specialized branch of geology concerned with the scientific analysis and systematic exploration of Earth’s valuable natural resources, such as minerals,ores, rocks, and groundwater. This discipline facilitates the identification and assessment of new mineral deposits and conducts their thorough investigations (Pohl, 2011). The field of economic geology encompasses all dimensions related to the characterization, interpretation, and utilization of mineral resources. Furthermore, it constitutes the foundational training for professional Earth scientists employed in the global mineral extraction and associated industrial sectors (Robb, 2015). This scientific discipline integrates methodologies from:
- Geochemistry
- Mineralogy
- Geophysics
- Petrology
- Structural Geology
—or a combination of these approaches. Hence, economic geology offers essential insights into the mechanisms, rationale, and spatial distribution of mineral concentrations within particular rock types, structural configurations, and tectonic environments of the Earth’s crust.
What is Ore Geology?
Ore geology constitutes a subdivision of economic geology, specifically focusing on the systematic study of metallic minerals. In contrast, economic geology encompasses both metallic and non-metallic mineral resources. Thus, ore geology can be viewed as a subset within the broader framework of economic geology.
Why is a Mineral Classified as an Ore?
The distinction between a mineral and an ore hinges on economic viability. For instance, chalcopyrite, a naturally occurring copper iron sulfide mineral with the defined chemical composition CuFeS₂, qualifies as a copper ore when discovered in substantial and concentrated quantities. Its abundance in a localized setting makes it an economically significant resource, thereby classifying it as an ore deposit.
Ore and Gangue
An ore is described as a naturally occurring mineral or a combination of minerals from which metal(s) can be profitably extracted. Typically, ores are found in association with non-valuable or commercially unviable materials known as gangue. Gangue refers to the economically insignificant substances that are intermixed with the desired mineral components in an ore body. These include impurities such as sand, soil, or other unwanted elements, which are removed during the mineral processing and refinement stages. 
Beneficiation
Beneficiation refers to the crucial process through which gangue minerals are eliminated from ore, thereby yielding a product of higher grade. This process is essential for enhancing the economic value of raw mineral material.
Ore Deposits
An ore deposit represents a natural accumulation consisting of both ore minerals and gangue minerals. Such deposits are typically found enclosed within the surrounding country rock.
Host Rock
The term host rock denotes the geological medium that contains a mineral concentration or an ore body. It acts as the geological environment in which valuable minerals are hosted.
Country Rock
Country rock is defined as the geological formation that encases or surrounds an ore deposit. It is distinct from the mineralized zone and plays a key role in structural and tectonic analyses of mineralization.
Mineral Deposit
A mineral deposit may be characterized as a rock formation containing one or more elements or minerals in concentrations significantly higher than their average crustal abundance, rendering them of economic importance. Mineral deposits are broadly classified into two principal categories:
a) Metallic Mineral Deposits
These include deposits of copper, lead, zinc, iron, gold, and similar substances from which one or more metals can be commercially extracted.
b) Non-Metallic Mineral Deposits
These involve deposits of clay, mica, fluorite, asbestos, garnet, and others, which are valued for their distinct physical or chemical properties, rather than metallic content.
Common metals and their ore minerals
Ore Minerals of Important Metals
| SN | Metal | Ore Mineral | Category | Composition | Tenor |
|---|---|---|---|---|---|
| 1 | Iron | Magnetite | Oxide | Fe₃O₄ | 74% Fe |
| Hematite | Oxide | Fe₂O₃ | 70% Fe | ||
| Goethite | Hydrated Oxide | FeO(OH) | 68.5% Fe | ||
| Siderite | Carbonate | FeCO₃ | 48.3% Fe | ||
| Pyrite | Sulphide | FeS₂ | 46.8% Fe | ||
| 2 | Manganese | Pyrolusite | Oxide | Mn₂O₃ | 63% Mn |
| Braunite | Oxide | MnO₂·H₂O | 64.3% Mn | ||
| Psilomelane | Hydrated Oxide | MnO₂ | 40–60% Mn | ||
| Rhodochrosite | Carbonate | MnCO₃ | 47.8% Mn | ||
| 3 | Chromium | Chromite | Oxide | Fe₂Cr₂O₄ | 46.4% Cr |
| 4 | Aluminium | Bauxite | Al Hydroxide (Mixture) | Variable | — |
| 5 | Tin | Cassiterite | Oxide | SnO₂ | 78.8% Sn |
| 6 | Copper | Native Copper | Native | Pure Cu | Up to 100% Cu |
| Chalcopyrite | Sulphide | CuFeS₂ | 34.5% Cu | ||
| Chalcocite | Sulphide | Cu₂S | 79.9% Cu | ||
| Bornite | Sulphide | Cu₅FeS₄ | 63.3% Cu | ||
| Covellite | Sulphide | CuS | 66.4% Cu | ||
| Malachite | Hydrated Carbonate | Cu₂CO₃(OH)₂ | 57.4% Cu | ||
| Azurite | Hydrated Carbonate | Cu₃(CO₃)₂(OH)₂ | 55.3% Cu | ||
| 7 | Lead | Galena | Sulphide | PbS | 86.6% Pb |
| 8 | Zinc | Sphalerite | Sulphide | ZnS | 67.1% Zn |
| Smithsonite | Carbonate | ZnCO₃ | 52% Zn | ||
| 9 | Nickel | Pentlandite | Sulphide | (Fe,Ni)S | — |
| Nicolite | Arsenide | NiAs | — | ||
| 10 | Sulphur | Arsenopyrite | Sulphide | FeAsS | 46.0% As |
| Orpiment | Sulphide | As₂S₃ | 61% As | ||
| Realgar | Sulphide | As₂S₂ | 70.1% As | ||
| 11 | Gold | Native Gold | Native | Au, Ag | 80–98% Au |
Common non-metallic or industrial minerals
| Agate | Gypsum |
| Andalusite | Jasper |
| Baryte | Kaolin |
| Asbestos | Laterite |
| Limestone | Mica |
| Halite | Ochre |
| Garnet | Pyrophyllite |
| Corundum | Quartz |
| Diaspore | Quartzite |
| Dolomite | Glass Sands |
| Graphite | Shale |
| Wollastonite | Magnesite |
| Feldspar | Slate |
| Fireclay | Steatite / Talc / Soapstone |
| Industrial Diamond | Phosphate |
Reserve and Resource
Reserves refer to those ore bodies that are economically viable for extraction and do not face any legal, engineering, or technical constraints to mining. In contrast, resources are defined as mineralized materials that possess the potential for future extraction, though not currently viable due to economic or technical limitations.
- Reserves
- Natural resources that have been discovered & can be exploited profitably with existing technology
- Resources
- The term “resource” refers to the total amounts of a commodity of particular economic use that is present in an area. These estimates include both extractable and non-extractable amounts of this commodity.
- Deposits that we know or believe to exist, but that are not exploitable today because of technological, economical, or political reasons.
The classification of an ore body as a reserve is determined by the degree of geological certainty regarding its existence, generally categorized as measured, indicated, or inferred. Within a mineral deposit, materials that are identified yet currently sub-economic are regarded as potential resources. These may be further differentiated into paramarginal and submarginal classes, based on their economic feasibility for future exploitation. 
Tenor
- The term tenor defines the metal content present within an ore body.
- Tenor represents the minimum acceptable concentration of a metallic component in an ore.
- It signifies the lowest metal quantity permissible for a material to be considered an ore.
- Typically, tenor is denoted as a percentage of the specific metal content.
Grade (Subject to Industrial Classification)
- Grade refers to the average concentration of a targeted material within a mineral deposit.
- It is often utilized interchangeably with the term tenor.
- The grade is generally quantified as weight percent, parts per million (ppm), or tons per ounce (ton/oz) in the case of metallic contents.
- Ore grades are commonly classified into:
- High grade
- Medium grade
- Low grade
Cut-Off Grade
- Cut-off grade denotes the minimum concentration required for a mineral or metal to reach the break-even threshold for economic extraction.
Important Terminologies
- Ore magma: Refers to an anomalously enriched magma that undergoes crystallization to form ore bodies, predominantly in cases involving sulfide or oxide minerals.
- Ore guides: These are structural, textural, or geological features serving as indicators of an ore body’s location. The most effective guides are those that can be clearly illustrated on geological maps, cross-sections, or 3D models.
- Ore genesis: This term defines the geological processes responsible for the formation of ore deposits.
- Hypogene: Describes mineralization resulting from ascending hydrothermal fluids within the Earth’s crust.
- Supergene: Represents mineral enrichment caused by descending solutions, typically associated with the oxidation and weathering of sulfide and oxide ores near the surface.
- Metallogeny: The scientific study of ore deposit formation, with a focus on their spatial and temporal relationships to the geological architecture of the Earth’s crust.
- Metallotect: Refers to any geological, tectonic, lithological, or geochemical feature that facilitates the localization and concentration of one or more elements.
- Metallogenic epoch: Defines a geological time interval that is favorable for ore formation, characterized by the presence of a distinct assemblage of mineral deposits.
- Metallogenic province: A geographic region distinguished by the presence of specific mineral deposit types occurring under similar geological conditions.
- Industrial minerals: These include any naturally occurring rocks or mineral substances of economic value, excluding metallic ores, fossil fuels, and gemstones, but utilized in industries for their physical or chemical characteristics. Common examples include limestone, clay, sand, gravel, diatomite, kaolin, bentonite, silica, barite, gypsum, and talc.
- Syngenetic deposit: An ore deposit that forms simultaneously and through the same geological process as the surrounding host rock, often seen in stratified sequences such as iron-rich sedimentary layers.
- Epigenetic deposit: An ore deposit that forms subsequent to the formation of the host rock, typically introduced into pre-existing country rock, as observed in vein-type mineralization.
Stratiform Ore
- As implied by the name, a stratiform deposit exhibits layering or bedding consistent with stratification.
- Example: Stratiform copper deposits, such as the Kupferschiefer-type.
- Most concordant mineralized bodies are categorized as stratiform.
- These deposits can comprise sulfides, oxides, and sulfates. Occurrence is possible in both sedimentary and volcanic geological environments.
Stratabound Ore
- The term stratabound implies that mineralization is restricted to a pecific lithological unit.
- Stratabound deposits may be concordant or discordant in nature. Example: Mississippi Valley-Type (MVT) deposits, Zawar Belt, Rajpura-Dariba, Rampura Agucha.
- Note: A stratiform deposit may qualify as a stratabound deposit, but not all stratabound deposits are stratiform.
- Strategic minerals are those for which a nation must rely on external sources, due to minimal or nonexistent domestic availability. Notable examples include molybdenum, chromium, graphite, boron, rare earth elements, gypsum, tungsten, gold, antimony, and platinum group elements.
- Critical minerals are those in which a nation faces supply deficiencies, yet known occurrences exist that could be mined regardless of cost during emergency scenarios such as war. These include germanium, beryllium, light and heavy rare earths, rhenium, tantalum, and are essential for sectors such as automotive, aerospace, defense, electronics, nuclear energy, and medical imaging.
- Essential minerals encompass those for which there are ample global resources and sufficient production levels. These do not fall under the classifications of strategic or critical, and their availability is considered stable and secure for industrial purposes.
Styles of Mineralization and Morphology of Mineral Deposits
The style of mineralization pertains to the spatial distribution pattern of ore minerals within the host rock, and can range from being extremely subtle—sometimes imperceptible to the unaided eye as observed in certain precious metal deposits—to highly prominent, such as in massive sulfide accumulations. The morphological forms of mineral deposits are notably diverse, encompassing configurations such as concordant tabular bodies, stratiform layers, and discordant geological features including veins and breccia zones.
| Mode of occurrence | Typical examples |
|---|---|
| Disseminated Ore minerals dispersed through the host rock | Diamond in kimberlite pipes |
| Stockwork An interlacing network of small and narrow (commonly measured in centimeters), close-spaced ore-bearing veinlets traversing the host rock | Footwall alteration zone of volcanic-hosted massive sulfide lenses |
| Massive Mineralization comprising >50% of the host rock | Volcanic-hosted massive sulfide lenses |
| Tabular An ore zone that is extensive in two dimensions, but has a restricted development in its third dimension | Sandstone-type uranium deposits |
| Vein-type Mineralization in veins, commonly discordant to the host rock layering (depositional) | Base- and precious metal veins |
| Stratiform Mineralization confined to a specific bed and, thus, broadly conformable to the host rock layering (depositional) | Kupferschiefer-type stratiform copper deposits |
| Strata-bound Mineralization discordant to host rock layering (depositional), but restricted to a particular stratigraphic interval | Mineralized breccia bodies in Mississippi Valley-type deposits |
Classification of Mineral Deposits
- The fundamental objective of any classification system is to organize comparable entities into categories or sets, primarily to enhance systematic understanding and practical utility.
- To date, a multitude of classification frameworks for ore deposits have been proposed.
- The majority of these taxonomies are founded upon the genetic models that were either contemporaneously accepted or favored by their respective proponents.
- Each classification methodology offers distinct benefits and limitations, depending on its applicability and underlying assumptions.
Lindgren’s Classification
- Waldemar Lindgren (1911) was the earliest scholar in the 20th century to propose a comprehensive genetic classification of mineral deposits, which he subsequently revised (Lindgren 1922) based on his extensive empirical observations.
- This system has evolved into one of the most universally recognized and applied classification frameworks in the field of economic geology.
- Lindgren’s original classification (1911) distinguished mineral deposits into two principal categories:
- i. Those originating through mechanical concentration processes
- ii. Those resulting from chemical interactions within aqueous solutions
- The primary criterion for differentiation among the groups in Lindgren’s model is the temperature and pressure conditions under which the mineralization processes occur.
| Temperature °C | Pressure | |
|---|---|---|
| I. Deposits by Mechanical Processes. | ||
| II. Deposits by Chemical Processes. | ||
| A. In surface waters. | ||
| 1. By reactions. | 0-70 | Medium to high |
| 2. Evaporation. | ||
| B. In bodies of rocks. | ||
| 1. Concentrations of substances | ||
| contained within rocks: | ||
| a. By weathering. | 0-100 | Medium |
| b. By ground water. | 0-100 | Medium |
| c. By metamorphism. | 0-400 | High |
| 2. By introduced substances. | ||
| a. Without igneous activ- | ||
| ity. | 0-100 | Medium |
| b. Related to igneous | ||
| activity. | ||
| (a) By ascending waters. | ||
| (1) Epithermal de- | ||
| posits. | 50-200 | Medium |
| (2) Mesothermal | ||
| deposits. | 200-300 | High |
| (3) Hypothermal | ||
| deposits. | 300-500 | High + |
| (b) By direct igneous | ||
| emanations. | ||
| (1) Pyrometasomatic | ||
| deposits. | 500-800 | High + |
| (2) Sublimates. | 100-600 | Low to medium |
| C. In magmas by differentiation. | ||
| 1. Magmatic deposits. | 700-1500 | High + |
| 2. Pegmatites. | 575 ± | High + |
- The classifications, divisions, and subdivisions outlined in the system are not inherently suitable as specific designations for the individual deposits included within them; for instance, a deposit falling under class IIA1—“Deposits produced by chemical processes of concentration in bodies of surface waters by interaction of solutions”—would be more appropriately and concisely referred to as a sedimentary deposit. With the exception of bog iron ores, this term more accurately encompasses the nature of all occurrences described under this category.
- The terminology used in class IIB2b (a), described as “deposits produced by hot ascending waters of uncertain origin, but charged with igneous emanations,” corresponds more broadly with what are commonly referred to as hydrothermal deposits.
- The Lindgren classification framework lacks applicability to a number of zoned deposits. A notable example includes the Butte, Montana deposits, which are classified as mesothermal. However, the Gagnon vein notably extends from the central high-temperature zone, through an intermediate zone, and into the outer low-temperature zone, thereby transcending a single thermal classification.
Recognizing the limitations imposed by overly concise terminology in his earlier scheme, Lindgren (1922) introduced additional nomenclature to address such complexities more effectively.
DEPOSITS OF ORIGIN DEPENDENT UPON THE ERUPTION OF IGNEOUS ROCKS
Hydrothermal deposits.
- a) Epithermal.
- b) Mesothermal.
- c) Hypothermal.
B. Emanation deposits.
- Sublimates.
- Exudation veins, surface type.
- Pyrometasomatic deposits.
- Exudation veins, deep-seated type.
C. Magmatic deposits.
- Orthotectic. 1. Differentiation in situ. 2. Injected.
- Pneumotectic. 1. Differentiation in situ. 2. Injected.
This scheme was met with criticism, primarily because it differentiated between emanation-type deposits—those presumed to form from magmatic vapors—and those derived from liquid solutions. However, the absence of definitive criteria for reliably identifying deposits formed exclusively from gaseous emanations rendered this distinction problematic. Consequently, this proposed modification was not incorporated by Lindgren in subsequent editions of his seminal work Mineral Deposits.
Niggli’s Classification
In 1925, Niggli introduced a novel classification system based on a primary dichotomy between “plutonic” and “volcanic” origins, analogous to the established categorization of igneous rocks. His classification is as follows: I. Plutonic:
- (A) Hydrothermal
- (B) Pegmatitic-pneumatolytic
- (C) Ortho-magmatic
II. Volcanic:
- (A) Exhalative to hydrothermal
- (B) Pneumatolytic
- (C) Ortho-magmatic
Schneiderhöhn’s Classification
In 1932, Schneiderhöhn introduced a more comprehensive genetic classification of mineral deposits, expanding upon previous schemes with increased complexity and detail.
| Category | Subcategory | Type/Process | Specific Examples/Details |
|---|---|---|---|
| A. Magmatic rocks and ore deposits. | (a) Intrusive magmatic. | I. Intrusive rocks and liquid magmatic deposits. | |
| I-II. Liquid magmatic-pneumatolytic. | |||
| II. Pneumatolytic. | 1. Pegmatite veins. | ||
| 2. Pneumatolytic veins and impregnations. | |||
| 3. Contact pneumatolytic. | |||
| II-III. Pneumatolytic-hydrothermal. | |||
| III. Hydrothermal. | |||
| (b) Extrusive magmatic. | I. Extrusive-hydrothermal. | ||
| II. Exhalation. | |||
| B. Sedimentary deposits. | 1. Weathered zone (oxidation and enrichment) | ||
| 2. placers | |||
| 3. residual | |||
| 4. biochemical-inorganic | |||
| 5. salts | |||
| 6. fuels | |||
| 7. descending ground water deposits. | |||
| C. Metamorphic deposits. | 1. Thermal contact metamorphism | ||
| 2. metamorphic rocks | |||
| 3. metamorphosed ore deposits | |||
| 4. rarely formed metamorphic deposits. |
| Process | Deposits |
|---|---|
| I Magmatic concentration | 1 Early magmatic |
| High T and P | A. Disseminated crystallization |
| B. Seggregation | |
| C. Injection | |
| 2 Late magmatic | |
| A. Residual liquid seggregation | |
| B. Residual liquid injection | |
| C. Immiscible liquid seggregation | |
| D. Immiscible liquid injection | |
| II Sublimation (low T & P) | Sublimates (unimportant) |
| III Contact metamorphism | Contact metasomatic Fe, Cu, Au etc. |
| Int. low high T & P | |
| IV Hydrothermal processes (T & P conditions low to high) | Different kinds of cavity filling and replacement deposits |
| 1. Telethermal | |
| 2. Epithermal | |
| 3. Leptothermal | |
| 4. Mesothermal | |
| 5. Hypothermal | |
| V Sedimentation (exclusive of evaporation) | Sedimentary products of Fe, Mn, Phosphate etc. Sulfides (sedimentary exhalative) |
| VI Bacteriogenic | Bacterial products of reduction |
| VII Submarine exhalative | Submarine volcanic (sulfides of Cu, Zn, Pb) |
| volcanism, low to high T & P | |
| Evaporation, low T & P | Sulfate (barite, gypsum), chloride (rock salt) |
| IX Residual and mechanical concentration, low T & P | |
| 1. Residual concentration | |
| Residual deposits of Fe, Mn, Al etc. | |
| 2. Mechanical concentration | |
| Placer deposits of Au, PGE, ilmenite, monazite | |
| X Surficial oxidation and | Oxidised supergene sulfides |
| supergene enrichment, | |
| low T & P | |
| XI Metamorphism, Int. to high | 1. Metamorphosed deposits |
| T & P | 2. Metamorphic deposits |
Jensen and Bateman’s (1981) Classification of Ore Deposits
- Bateman (1942) initially presented a classification of mineral deposits, which was subsequently expanded and refined several decades later by Jensen and Bateman (1981).
- This revised scheme represents a simplified adaptation of Lindgren’s classification, wherein the subdivisions are determined based on the processes of deposition and the morphological characteristics of the deposit.
Conclusion
- The formation of mineral deposits constitutes a highly complex phenomenon, where numerous geological processes transition into one another due to variations in temperature,pressure, host lithology, and fluid composition within the system.
- Hence, it is essential that classification frameworks for mineral deposits maintain a degree of flexibility, allowing for the inclusion of intermediate or gradational categories.
- The classifications proposed by Niggli, Schneiderhöhn, Lindgren, and Bateman share fundamental similarities, with the genesis or nature of the ore-bearing fluid serving as the primary criterion for their initial categorization.
- The systems introduced by Niggli and Schneiderhöhn incorporate subdivisions based on mineral assemblages or metal content, rendering them practically useful for field-based geological investigations.
- Lindgren endeavored to categorize deposits by linking physicochemical processes to specific depth-temperature zones within the Earth’s crust.
- Among the various models, the Bateman classification stands out as the most straightforward, placing emphasis on both the mechanism of formation and the structural configuration of the ore deposit.