The first step to identify a rock is to try to categorize the rock into
one of the three main types or groups of rocks.
These include igneous, sedimentary or metamorphic types.
The only rocks which do not fall into one of these categories are meteorites.
Igneous, sedimentary and metamorphic rock types are distinguished by the
processes which form them.
form by crystallization of a melt (molten rock material).
formed at significant depth below the surface.
formed at or near the surface.
form by the compaction small or large grains or fragments of pre-existing
rocks, or by the precipitation of mineral matter from a body of water,
such as an ocean, lake or stream.
formed from pre-existing igneous, sedimentary or metamorphic rocks by
subjecting them to heat and/or pressure and/or migrating fluids, causing
the original mineral assemblage of the rock to change to a new assemblage
of minerals. The origin is not always obvious, but sufficient training
will enable recognition of certain features which point to the most likely
origin. Examples include the common presence of bedding or layering in
sedimentary rocks, and the presence of mineral foliations or lineations in
metamorphic rocks. One must also consider the geologic environment where
the rock is found.
For example, in a young volcanic terrane one is less likely to find
sedimentary or metamorphic rocks.
When the origin is completely unobvious, the composition and texture must
be relied upon to make the best guess.
The rock composition is found by determining which minerals make up the
By definition, a rock is a solid mass or compound consisting of at least
two minerals (although there are some exceptions when a rock may consist
entirely of one mineral). The minerals comprising the rock can be
identified using common field testing methods for individual minerals,
particularly where the texture is sufficiently coarse-grained enough to
distinguish the individual minerals with the naked eye or a hand lens.
Where the grain size of the minerals comprising the rock are too
fine-grained to recognize discrete minerals, “petrographic” methods (those
using a microscope) can be used for reliable identification in many cases.
Petrographic methods involve the use of a microscope to examine the
optical properties of discrete minerals magnified through the microscope
Properties include the behavior of refracted, reflected and transmitted
light either through a thin wafer slice of the rock (called a thin
section), or of a sample plug (for reflected light).
The light source is adjusted to provide light which polarized in one or
Different minerals have characteristic optical properties, which can be
used with tables of optical mineral properties to identify the mineral.
Other instruments which can be used to make mineral identification include
the electron microscope.
These methods are reliable but expensive, and require somewhat tedious
The image is obtained by exposing the sample to electron bombardment and
imaging the results.
X-Ray Diffraction Techniques
Another method to identify small mineral grains is using X-ray powder
A small amount of material is ground into a powder and bombarded with
The results are recorded on a film strip in a camera, or in the form of
The reflections of the X-rays are measured to determine the ‘d-spacings’
of the unknown mineral.
Each mineral has a unique set of peaks corresponding to d-spacings, which
are related to the crystal structure.
In X-ray spectrometry, another method to identify minerals, the X-rays
cause the emission of photons from the surface of the mineral.
The sample is prepared by obtaining a very high polish on its surface.
The photons emitted from the surface atoms have characteristic energies
for specific elements.
By measuring the energy levels of the photons, the mineral composition can
The texture of a rock is defined by observing two criteria:1) grain
sizes,2) grain shapes.
the average size of the mineral grains.
The size scale used for sedimentary, igneous and metamorphic rocks are
the general shape of the mineral grains (crystal faces evident, or
crystals are rounded).
Examples of the size classifications for each of the three major rock
types include: FINE-GRAINED > > > > > > > > > > >
> > > > > COARSE-GRAINED Sedimentary: Shale Siltstone Sandstone Wacke Conglomerate Metamorphic: Slate Phyllite Schist Gneiss Igneous: Rhyolite Granite
Very Fine Grained
Very Coarse Grained
.06 - .125 mm
.125 - .25 mm
.25 - .5 mm
.5 1 mm
1 2 mm
< .25 mm
.25 1 mm
1 2 mm
> 2 mm
< 1 mm
1 5 mm
5 20 mm
> 20 mm
Sizes are median diameter of grains in millimeters.
One of the main goals of mineral exploration is to predict the geometry
and relationships of different rock types under the surface where they
can’t be seen either below the surface or beyond the immediate exposures.
This is essential to know in order to plan a mine.
Much effort and a variety of techniques are used to analyze the timing or
“geologic history” of the area
There are three main principles, or “laws”, which are used in field
geological studies to guide in determining the relative timing of events.
Law of Cross-cutting Relationship
The “Law of Cross-cutting Relations” is a principle which is useful to
employ in igneous provinces.
It states that invading rocks are younger than those invaded.
an igneous dike invading a sedimentary or metamorphic rock.
Another example is a situation where there are multiple intrusions are
found; the sequence of igneous events can be sorted out by observing which
intrusions cut which other intrusions.
The sequence might give an indication of a particular differentiation
pattern of the magma.
The same law applies to veining relationships:younger veins cut across
older vein sets
Often times where there are gold-bearing quartz veins there are also other
veins which are barren, and may have a different orientation due to
different structural conditions during formation.
Vein crosscutting relations.
Vein A is cut by Vein B.
Vein C cuts both A and B, so it is youngest.
Law of Superposition
The “Law of Superposition” is a law which applies to sedimentary rocks.
It states that where undisturbed, layered, sedimentary rocks occur,
younger rocks will be situated on top (above) older rocks.
The same law can apply to layered volcanic flows, where the ages of the
succeeding layers going up section will be relatively younger than the
lower part of the section.
This law is also one which is employed to determine age relationships of
different rock units.
In mineral exploration, a situation where this principle could be employed
would be to project the underground geometry of a mineralized or petroleum
Principle of Uniformitarianism
The “Principle of Uniformitarianism” states that the earth is a result of
natural forces which are presently active and have persisted over the
course of geologic time.
Rocks form most often as a result of slow, gradual developments resulting
from various geologic processes.
Catastrophic events do occur and contribute to the overall development and
history of rocks, but these events are less frequent and contribute to
only a small percentage of the net effect of natural forces in general.
This principle has been used to study the history of ancient volcanic
rocks by observing present day volcanic activity.
For example, a certain type of massive sulfide deposit has been documented
along an active sea floor rift.
This knowledge can be used to better understand a certain type of
Copper-Lead-Zinc ore deposits, called “volcanogenic massive sulfide
depsits”, or “VMS”.
see also Some rocks exposed at the surface are very young, but most are
very old, in fact are much older than the historical records of humanity.
These “old” rocks are generally many millions of years in age.
The vastness of the concept of ‘millions” of years can be difficult to
comprehend since human life times are so much shorter (generally less than
Units of geologic time which have established include the “era” (longest),
“period”, and “epoch” (shortest).
All of geologic time has been divided into 4 main eras, called (from
oldest to youngest) the Precambrian, the Paleozoic, the Mesozoic and the
Sites 1 6 provide illustrations and summaries of the geologic time
scale. The earth has slowly changed throughout its history, and continues
to do so as a result of a very slow cooling and differentiation process.
As a result, certain time periods during the earth’s history had
conditions more conducive to formation of specific types of mineral
deposits (Site 7).
For this reason, knowing the approximate age of rocks can be a rough guide
to the types of mineral deposits most likely to be found. When evaluating
the ages of rocks we speak of two types of terms of ages called “absolute
age” and “relative age”.
“Absolute age” is measured in years, and depends on having some type of
time scale to measure against, typically by using a highly technical
chemical dating method.
“Relative age” simply means placing one geologic event or feature in
context with another in a timing sequence. Absolute Age: During the early 1900’s, shortly after the discovery of radioactivity,
it was discovered that radioactive decay involves the transformation of
radioactive atoms into completely different elements.
Each radioactive substance disintegrates at its own rate and forms a
unique set of daughter products (elements).
The rate of decay is generally very slow.
For example, uranium changes into lead at a rate such that half of the
original amount will be converted to lead after a period of 4,500 million
Half of the remaining uranium will convert to lead in another 4,500
million years, and so on.
Therefore the “half life” of uranium is 4,500 million years.
By measuring the ratio of unchanged uranium to lead in a sample, and
knowing the rate of decay, we can calculate the length of time the sample
has been disintegrating, or in other words, the age of the rock.
Besides the Uranium-Lead method, several other radiometric techniques are
available, including Carbon 14 and Rubidium-Strontium. Relative Age Where different rocks are in physical contact and
observable, the relative ages of the rocks can often be determined
evaluating superposition and cross-cutting relationships.
Rocks comprising the upper strata are younger than rocks comprising the
Rocks formed from an intruding magma are younger than the rocks they
Inclusions within an igneous rock are older than the magma which formed
the matrix. When different rocks are in close proximity but their actual
contacts are not visible, a geologic map and cross-section can be made
which illustrate the geometric relationships of the rocks, and allows the
determination of relative age. Difficulty is encountered when attempting
to correlate rocks which are not in direct contact or even close
Fortunately geologists have worked out the evolutionary succession of
It was found that sedimentary rocks containing fossils could easily be
placed in a successive sequence with respect to time by identifying the
fossil assemblages present.
The natural outgrowth of this effort was to begin comparing rocks from all
parts of the globe.
Fossils could now be used to attach relative ages to a wide variety of
different sedimentary rock types.
They have been used to construct what is referred to as the “Geologic Time
Scale”, which is a chronology of the earth’s history largely based on the
fossil record. Since the oldest rocks and the oldest fossils are the ones
most likely to become obliterated due to age, we have much more fossil
data available for younger rocks, and hence these contain the smallest
subdivisions of time.
The Paleozoic Era was when invertebrates and simple vertebrates (fish,
amphibians and primitive reptiles) were the dominant life forms.
The Mesozoic Era was when reptiles, including the dinosaurs, ruled.
The Cenezoic Era is best characterized as the time when mammals became
The following terms are useful to know: Ore:
the rock material or minerals which are mined for a profit. Ore Minerals:
the specific minerals within the ore which contain the metals to be
recovered. Gangue Minerals:
the minerals having no commercial value, they just happen to be mixed up
with the ore minerals. Prospect:
potential ore deposit, based on preliminary exploration. Mine: Excavation for the extraction of mineral deposits, either at the
surface (open pit mine) or below (underground mine). Orebody or Ore Deposit:
naturally occurring materials from which a mineral or minerals of economic
value can be recovered at a reasonable profit. Mineral Deposit:
similar to an ore deposit, but is implied to be subeconomic or
incompletely evaluated at present. Mineral Occurrence:
anomalous concentration of minerals, but is uneconomic at present. Grade:
this means the concentration of the substance of interest, usually stated
in terms of weight per unit volume. Cut-off Grade:
the lower limit of concentration acceptable for making a profit when
mining. Host Rock:
the rock lithology (type) which contains the ore.
May or may not comprise ore. Country Rocks:
the rocks of no commercial value surrounding the host rocks and/or the
ore. Anomalous: above or below the range of values considered to be normal.