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Unit One - Geological Field
Methods Objectives - The student will be able to:
Explanation Ore & Gangue Mineral Identification This unit will cover ore minerals, which are the ones which contain valuable commodities. Where these minerals are sufficiently concentrated in a rock, the rock is described as “ore”. The need to search for mineral resources should be obvious to everyone. Every product that makes our society “tick” is derived in some way, directly or indirectly, from minerals. Think about all the metals which are present in the materials which comprise everything from a house, to an automobile, to a computer. Minerals are naturally occurring substances in the earth’s crust. These substances consist of chemical elements or compounds consisting of two or more elements. The elements or compounds are repeated as patterns, which gives the mineral its crystalline structure. Rocks are simply an aggregate of one or more minerals, or (less often) an aggregate of organic matter (such as coal), a glassy substance (such as obsidian) which has no crystalline structure. There are approximately 3,000 minerals, often referred to as mineral species, which are known, and approximately 60 new minerals are discovered each year. Naturally occurring minerals are typically grouped according to their chemical formula and crystal structure, and tend to fall into one of the following categories (after Dana, 1971):
Identification Procedure Determining the name of a mineral involves testing of the physical properties of an unknown mineral for identification purposes. The tests for ore minerals are the same as for any other mineral. The observer needs to systematically note as many properties as possible for the unknown mineral before consulting the mineral tables. Below are the key properties to note: Color: Color is not a good characteristic to identify minerals by because many silicate and other minerals have multiple colors for the same mineral. For example “fluorite” comes in at least eight colors. There are rare times when color is a useful indicator, for example the color green or blue often indicates the presence of copper in an oxide setting. Luster: Three main types are metallic, submetallic and nonmetallic:
Other Optical Properties:
Magnetism: indication of the minerals magnetic susceptibility, or strength of its own magnetic field. The mineral “magnetite” is strongly magnetic; the mineral “pyrrhotite” is weakly magnetic. Specific gravity: this is the ratio of the weight of a substance to the weight of an equal volume of water. For most minerals the specific gravity ranges from 2 to 5. Some minerals, like gold (specific gravity 15 to 19), are extremely dense. Cleavage: tendency to break in preferred directions along bright, reflective plane surfaces. Examples:
Fracture: this is similar to cleavage, but is the character of breakage along non-planar surfaces. Examples
Crystal Habit: this is the crystal system the mineral follows by nature. Examples:
Hardness: this is the toughness on a scale of 1 to 10, having key minerals as indices, named after Moh:
TABLE T2: Moh’s Hardness Scale Effervescence: this is the bubbling behavior which happens when a carbonate mineral is exposed to hydrochloric acid. This test needs to be done very carefully, by using only a drop or two of HCl. Some carbonate minerals effervesce readily, while others effervesce very slowly.Host Rock Identification Prospectors and geologists need to be able to recognize rock types so they can know where to search for mineralization or petroleum. Once mineralization is discovered, the “host rock” which contains the mineralization must be described in detail, leading to a field call of the rock type, called “lithology”. After defining a lithology in the field, the object is often to trace or map out the lithology of interest in order to attempt to find valuable minerals.Rocks are grouped into three main types known as igneous, sedimentary, and metamorphic:
Igneous rocks  Igneous
rocks are those which crystallize from a silicate or other type of “magma”
(molten rock) either deep underground or near the surface. Igneous
rocks which form deep underground are called “plutonic” and
igneous rocks which occur near or at the surface are called “volcanic”.
Igneous rocks are composed mostly of crystals. It is the size of the crystals, called “grain size”, which provides an important clue as to the origin of the igneous rock in a plutonic or volcanic setting. The insulating effects rock material surrounding a magma chamber below the surface and allows it to cool slowly enough to grow larger crystals and thus create a coarse-grained texture. Volcanic rocks are generally composed of very small crystals or even glass because the magma is “quenched” rapidly when exposed to air or water at the surface of the earth. There are many different classification schemes for igneous rocks. Most of the main magma types can occur either at depth or at the surface, so their origin as a plutonic or volcanic rock must be based on texture unless direct evidence is known. The general color of igneous rocks can also be used to identify the rocks. This is because rocks rich in minerals like quartz and feldspar tend to be light colored, and rocks rich in mafic minerals (those containing abundant iron and magnesium) tend to be dark colored. Below is an igneous rock identification chart based on color, composition and texture:
Sedimentary Rocks  Sedimentary
rocks are those which form by the consolidation
of sediments and are generally layered. They form by a wide variety
of processes, including both water movements and wind action.
Like igneous rocks, they are classified according to their composition and
grain size. Sedimentary rocks can be important host rocks for either
mineral or petroleum deposits. The most common types of sedimentary
rocks which are found in the field include the following:
Metamorphic Rocks
Like igneous and sedimentary rocks, the classification of metamorphic rocks is based largely on composition and texture. The rock name is likely to include the minerals which are characteristic of the rock, followed by the rock name. The most common types of metamorphic rocks include:
Rock type images courtesy USGS
Field Methods | Geochemical Methods | Geophyscial Methods | Drilling Methods | Petroleum Exploration
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Metamorphic
rocks are those derived from pre-existing rocks of any type (including
igneous, sedimentary, or pre-existing metamorphic rocks) by subjecting
them to heat and/or pressure. They are generally older rocks which
have been exposed to multiple deformation events over time. The changing
conditions also cause different minerals to form over time, with later
minerals sometimes completely replacing earlier ones. As
heat and pressure are applied to a rock, the original mineralogy changes
to form new minerals which are stable at the new temperature/pressure
conditions.