Mineral Exploration Geology

1
Mineral Exploration Geology

• Geological Mapping in Exploration
• Reference Marjoribanks 1997. Geological Methods
  in Mineral Exploration and Mining. Chapman and
  Hall. Ch2.

2
General Considerations

• Why make a map?
• Maps and sections are essential tools in
  visualising spatial, 3-D geological
  relationships. Visit the Fractal Graphics website
  for a good example of this. They help with
  predictions on the location, size, shape and
  grade of potential ore bodies.
• Making or acquiring a geological map is
  invariably the first step in any mineral
  exploration program and is a fundamental skill
  for any exploration or mine geo

3
The Nature of a Geological Map

• Represents a selection of field observations and
  is useful in predicting those things that cannot
  be directly observed
• Different kinds of geological maps
• Large-scale maps with a small scale ratio
  11000 or 12500. The geo aims to visit and
  outline every significant rock outcrop in the
  area of the map. Often called outcrop maps.
• Small-scale 1100,000 or 1250,000. Only a
  selection of outcrops are examined in the field
  and interpolations made between the points. May
  involve a lot of remote sensing interpretation or
  projection. Therefore small scale maps are
  generally more interpretational.
• There is always a certain degree of subjectivity
  in map making, especially in highly weathered
  terrains where it is difficult to identify the
  primary rock type

4
Smart Mapping

• Producing a geological map is a process of
  problem solving. One of the best ways to approach
  problem solving is known as the system of
  multiple working hypotheses. This means that the
  geo doesnt start off with a completely blank
  mind but is armed with a number of ideas about
  the geology developed from looking at published
  maps, literature, air photos or aeromagnetics.
• Predictions are made, areas are selected and
  observations are made to test these predictions.
  It is extremely important to be prepared to
  discard or modify a particular model or
  prediction if evidence is found to the contrary.
  Never fall in love with a particular model or
  prospect. Good scientific theories are
  continually being modified or rejected as new
  evidence becomes available.

5
Smart Mapping

• Systematic mapping vs Smart mapping
• Smart mapper will start by looking at aerial
  photos to determine where the available outcrop
  is and then make 2-3 traverses across strike to
  groundtruth the photo interp
• Search patterns will focus in on key areas of
  interest eg shear zones and old workings
• Less time is spent in areas where the rocks are
  uniform and a lower density of observations will
  serve
• Most useful observations are the predictable
  geometrical relationships between bedding,
  cleavage, lineations and folds as well as
  movement indicators for brittle and ductile shear
  zones

6
Smart Mapping

• Features and relationships seen in thin-section,
  hand specimen or outcrop often exactly replicate
  features occurring at map scale. Informally
  called Pumpellys Rule after the 19th century
  USGS geologist who first described it. Today we
  recognise that geological processes are
  essentially chaotic (non-linear) and exhibit
  scale-invariance eg comparison in shape between
  a rock pool and a coastline, fault systems are
  self replicating
• Observations are not made randomly, nor are they
  collected on a regular grid according to a fixed
  search pattern. Rather they are selected to most
  effectively prove or disprove the current ideas.
  (Note An experiment either falsifies a
  hypothesis or expands the range of conditions
  under which it can be said to hold good it can
  never prove it Karl Popper, philosopher)
• Geological mapping is a scientific process and
  corresponds to the classic scientific method
  theorizing, making predictions from the theories
  and designing experiments (field observations or
  geochemistry) to test the predictions. (Note all
  theories in science must be formulated in such a
  way that they are capable of being falsified. Eg
  for field mapping purposes it is not very useful
  to postulate these outcrops represent a
  subduction complex because there is unlikely to
  be a single observation which can falsify that
  statement. Rather postulate this outcrop is
  basalt and chert, that outcrop is blueschist,
  that contact is a mylonite. Together these rocks
  are thought to have been accreted into a
  subduction complex

7
Scale Invariance in Strike-slip Fault Systems
8
Mapping techniques summarise

• Choosing the best technique depends upon the
  availability of suitable base maps on which to
  record field observations
• Scale depends on the purpose
• Satellite Navigation GPS
• Air Photo Mapping
• Geological Interpretation
• Scales
• Photo handling techniques
• Working with enlarged air photos
• Data transfer to base map

• Mapping with a plane table
• Mapping on a pegged grid
• Requirements of the grid
• Making the map
• Mapping with a tape and compass