CHAPTER 9: ORE DEPOSIT GEOLOGY

The minerals we want have an economic value and can sometimes be in such concentration and in such quantity that they can be extracted with a profit and be used for many purposes. The rock mass with the mineral we want to extract is the orebody. The orebody has ore minerals, the ones we want, and gangue minerals, the minerals we do not want. The ore minerals become the product from the mine, metal or concentrate. The gangue minerals become tailings stored in a facility. Other rocks and soil on top of and around the orebody and that we need to move becomes waste rock and overburden stockpile.

The minerals we want from the mine have a value and their value pays for all the environmental protection that is needed at the mine. The minerals we do not want as they are stored in a facility alters the environment and can cause severe environmental impacts. The geology of the deposit is therefore extremely important for all aspects of environmental management of a mine. The characteristic and properties, chemistry, and mineralogy, of the deposit decides how waste rock and tailings will change the environment. The same characteristics also decide how resources are generated and made available for environmental protection, mine closure and mine rehabilitation.

In addition to the minerals generated at the mine, the mines main purpose, the mine can also contribute to taxes, jobs and economic development. The likelihood of these additional benefits is also much decided by the geology of the deposit.

We extract many raw materials from the Earth’s crust with which we construct and sustain our society. We extract minerals and rocks from mines, hydrocarbon liquids and gases and groundwater through pumping or where they rise to the surface under their own pressure, the heat of rocks as geothermal energy, etc. An ore deposit is ‘what is mined’. However, not all mineral resources are mined, some uranium is obtained by in situ leach extraction involving pumping solvent through uranium-bearing rocks, specifically sandstones, and some metals are extracted from saline brines pumped from sediments that underlie salars or salt lakes. Precise definitions of the term are based on economics rather than geology.

An ore deposit may include ores of metals (Cu, Zn, Pb, etc.); ores of gemstones; ores of minerals used as feedstock for production of industrial chemicals, e.g. Aluminium-bearing abrasives and refractory products; ores of minerals used in industrial products, e.g. diamond is both a gemstone and an industrial mineral; rock used as aggregate, for building stone; coal and oil shale.

Copper cathode with copper concentration of nearly 99,995% Cu metal. Photo: Jonathan Hamisi.

Ore deposit

An ore deposit consists of one or more ore bodies. An ore body is a mass of rock that contain ore and from which a commodity of value will be extracted. Not all ore within an ore body will be extracted. Ore bodies are divided into reserves and resources. Reserves are ore that is economically feasible to mine and for which there are no legal or engineering impediments to mining, while resources are ores that may potentially be extracted at some point in the future. Engineering constraints is one of the factors that will influence what ore is economic to mine (Fig. 1). The mining of an ore body may be from a river stream, an open pit, an underground mine, or a combination of the two.

Skouriotissa copper mine – open pit, Cyprus. Photo: Jonathan Hamisi.

Ore processing or extraction

Ore deposits contains ore minerals mixed with other ‘non-valuable’ minerals called gangue minerals from which ore will be separated separated through ore processing. Ore processing include milling, to separate the different constituent minerals. Thereafter a chemical separation called flotation allows to separate the ore minerals of economic interest from the gangue minerals. Ore minerals of metals can be ‘native’ however they are often chemical compounds in which the metals are bonded with other constituents from which they must be extracted. Similarly, several industrial minerals require to be processed before sale. Ore processing methods through flotation, refining and extraction are chosen based on ore mineralogy and on the chemical and physical characteristics of the ore and is the main purpose of extractive metallurgy (Fig. 2). Important ore minerals include native metals, sulfides, sulfosalts, oxides and hydroxides, and specific silicates, carbonates and minerals of other classes (Table 1).

Figure 2: Flowchart of steps and processes required to extract a metal from an ore deposit. Ridley, J. (2013) “What is an ore deposit?,” in Ore Deposit Geology. Cambridge: Cambridge University Press.

Processing of copper ore by flotation, Cyprus. Photo: Jonathan Hamisi.

Ore deposits – A geological perspective

A rock becomes ‘ore’ when it is economic to extract it from a given deposit. Costs of extraction vary according to the location of the deposit, its position (steeply dipping or relatively horizontal) and structure, the rock type and the mineralogy. The operating cost of mining are proportional to the volume of rocks to be moved and processed. The profit is made according to tonnage of commodity sold. Prices are often set either by tons, Kg, grams, ounces, etc. Considering all other factors (e.g. mineralogy) equal, the costs of extracting a commodity per mass are to a first approximation inversely proportional to the concentration (grade) of the element in the rock. In general, the most cost-effective extraction is from minerals that have a high concentration of the metal mined from rocks of high grade.

Table 1: Table of selected common ore minerals. Ridley, J. (2013) “What is an ore deposit?,” in Ore Deposit Geology. Cambridge: Cambridge University Press.

The significance of ore deposit size

The small and short-lived mining operations, like narrow veins stockwork, would typically be ore bodies of about 1 Mt. To illustrate it, it is the equivalent to a cube of rock about 75 m across, the volume will depend on rock density. Largest ore deposits are commonly a few gigatons, equivalent to an open pit of few kilometres long and several hundred metres deep. Regardless of the type of metal and ore deposit type, ore bodies vary in size, often by about two to three orders of magnitude.

Pyrite sample from a Copper mine in Cyprus. Photo: Jonathan Hamisi.

Geological factors affecting economics of ore extraction

Ore deposit grade and size are not the only deciding factor for an ore deposit to be mine or not. Other factors that vary such as the geological and societal, and the cost of extracting a metal are of importance and they will vary from deposit to deposit. An important factor in the economics of mining is the amount of overburden or waste rock that is required to be mined in order to access the ore bodies.

Additional geological factors that affect economics include:

Shape and depth of the deposit
Ore body that are flat and near the Earth’s surface are the cheapest to mine. Overall ore bodies with sub-spherical shape are easier and cheaper to mine than a thin stockwork vein. Similarly, open-pit mining is cheaper than underground mining. However, in case an operation would lead to a higher ratio of waste rock to ore (stripping ratio), the ore body will generally be mined from an open pit than from an underground operation. It is common though that a combined method is used, open pit for the upper part of the ore bodies and underground mining for deeply buried parts of the ore bodies.

Mineralogy and texture of the ore
Both have significant effect on the cost of mining, and ore processing. Sometimes the presence of deleterious elements in the ore will increase the processing cost, for instance, phosphorus in iron ores.

Polymetallic ore deposits
The presence of co-products and/or by products affect the economics of the mining operation. Co-products are often define as those additional metals which have a greater control on the economic feasibility of a mine, and by-products, stands for metals that are extracted from mined and milled ore or waste if the costs of metallurgical extraction are favourable, but which do not significantly affect the economics of the whole mining operation. By products often end up in the tailings. The distinction between the two categories is not well defined, however many mines produce multiple commodities, and some metals which are only extracted for specialist small-volume markets are extracted entirely as by-products (e.g. Sc, Te).

Ore deposits types and models

Geological research has enabled to define ore genetic types or ore genetic ‘models’, through empirical observations and interpretation of geological processes leading to ore deposit formation.

Genetic models aim to explain how deposits form (Fig. 5). As they are the result of rationalisation of knowledge, they are an effective way to organise data in a form that enhances understanding, prediction, and communication. Even though the ore genesis or geological processes leading to ore deposit formation rarely affect the exploitation of an ore body once it is well defined (position, grade, tonnage and mineralogy), these information are important in the exploration industry and the minerals-extraction industries during exploration and deposit evaluation. Ore deposit models are an important part of communication in the industry and provide a tool for exploration and deposit evaluation.

A genetic model will explain why an ore deposit occur in a specific geological and tectonic setting (fig. 5), and the geochemical and structural processes involved in its formation. Models sere as tool to guide where to search for a given deposit type within the Earth as a whole and, when combined with knowledge of local geology and geological history, on a much smaller scale within an exploration license.  A model provides a description and explanation to the shapes and forms of ore bodies. These are information that are needed to guide effective evaluation of a prospect, such as best positioning of drill holes.  A model takes into consideration the mineralogy of ore and provide a framework needed to evaluate, for instance, what co-products may be present, what ore grades can be expected, and what method of metal extraction may be best.

Ore deposits type and examples of environmental considerations

Table 2. Modified after Arndt et al. (2017) Future Global Mineral Resources. Geochemical Perspectives ; 6 (1): 1–2.

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