ES 3210 ECONOMIC MINERAL DEPOSITS ORE MICROSCOPY

ES 3210 ECONOMIC MINERAL DEPOSITS
ORE MICROSCOPY
(aka REFLECTED LIGHT MICROSCOPY)
Stephen J. Piercey
Modified from original notes of Graham Layne
ORE MICROSCOPY
•
The microscopic study of ore minerals largely involves
the use of reflected light
•
Many textbooks introduce reflected light microscopy as a
qualitative technique
•
•
Don’t take this to heart!!
Reflected light microscopy still involves careful
evaluation of mineral properties to enable identification
MD Barton, ASU
`
Highly coloured copper sulfide minerals in plane polarized
reflected light.
Bornite
(Bn; Cu5FeS4; tetragonal)
- peach to tan to bronze
Digenite (Dg; Cu9S5; cubic)
- blue
Covellite (BbCv; CuS; hexagonal)
- dk blue to red to gray (Covellite is spectacularly
bireflectant, colour depends heavily on orientation)
You can see some of the variation in covellite colors, due to variable orientation, in the central part
of the central veinlet.
ORE MICROSCOPY
•
Compared to the transmitted light microscopy you have
been using for thin section petrology, it can be somewhat
more equivocal in certain cases
•
•
There are also some differences in the skill set involved
As the term progresses, we will begin using both
transmitted and reflected light microscopy to study ore
deposits
ORE MICROSCOPY – RELEVANCE
AND APPLICATION
•
Useful and accurate way of studying the reactions and
processes that form ore deposits ⇨ ES 3210
•
Invaluable in the development of efficient means of
processing ores through the milling, separation and
refining required to produce a final raw metal product
(GEOMETALLURGY).
ORE MICROSCOPY – RELEVANCE
AND APPLICATION
•
Essential pre-preparation for more sophisticated
instrumental assessment techniques:
•
•
•
•
Scanning Electron Microscopy (SEM/MLA)
Electron Probe Microanalysis (EPMA)
Laser Ablation Mass Spectrometry (LA-ICP-MS)
Secondary Ion Mass Spectrometry (SIMS)
ORE MICROSCOPY – COURSE
OBJECTIVE
•
Central goal of the laboratory component of this course
is that you gain skill and experience in:
•
•
Identifying both common and (important) trace ore
minerals with the microscope
Recognizing and interpreting the textures and interrelationships of these minerals
Native Gold
Photo: JLM Visuals
Native Gold
Photo: Marshall et al, 2004
“Paragenesis”
Py1
Py2
Ccp
In what sequence did these mineral phases form?
ORE MICROSCOPY – COURSE
OBJECTIVE
•
These skills will allow you to develop and understand
paragenetic (time) sequences for the formation of ores in
a given deposit, and insight into the processes that
caused ore deposition.
•
The only viable way to learn these skills is to spend time
looking at the wide variety of polished sections that will
be presented during the labs.
•
Gladwell’s 10,000 hour concept - you won’t get good at
something without putting in the time.
Paragenetic sequence for the Tri-State MVT deposits
from Hagni and Grawe, 1964
OPTICAL PROPERTIES IN
REFLECTED LIGHT
•
Like a standard petrographic microscope, the reflected
light microscope contains a pair of polarizing filters.
•
[In fact, our teaching scopes are equipped for both
transmitted/reflected light]
Typical Dual Illumination Petrographic Microscope
Nikon.ca – Microscopy U
OPTICAL PROPERTIES IN
REFLECTED LIGHT
•
These filters are referred to as the polarizer (incident light
path) and the analyzer (reflected light path) and are
(generally) set at exactly 90º to each other.
•
[Some older texts (and old instructors) refer to the
polarizing filters as “nicols”]
OPTICAL PROPERTIES IN
REFLECTED LIGHT
•
Observations are made either with only the polarizer
inserted:
•
•
“Plane Polarized Light”
Or with both polarizer and analyzer inserted:
•
“Crossed Polars” or “Cross Polarized Light”
OPTICAL PROPERTIES IN
REFLECTED LIGHT
•
Properties that are observed under plane polarized light:
•
•
•
•
colour
reflectance
bireflectance
reflection pleochroism
OPTICAL PROPERTIES IN
REFLECTED LIGHT
•
Properties that are observed under cross polarized light:
•
•
anisotropism
internal reflections
COLOUR
•
A small number of minerals are strongly and distinctively
coloured
•
The following minerals are usually readily identifiable on
this basis:
MINERALS WITH OBVIOUS COLOUR
COLOUR
MINERAL
OTHER PROPERTIES
Blue
Covellite
Intensely Pleochroic
Chalcocite, Digenite
Weakly Anisotropic
Gold
Chalcopyrite
V. High Reflectance, V.
V. WeakSoft
Anisotropy
Millerite, Cubanite
Strongly Anisotropic
Bornite
Weakly Anisotropic
Copper
High Reflectance, V. Soft
Yellow
(Red-)Brown
after Craig & Vaughan,
1994
Native Copper
Photo: JLM Visuals
Native Copper
Photo: Marshall et al, 2004
Native Silver
Photo: JLM Visuals
Bornite
Photo: Marshall et al, 2004
pp
xp
Covellite
Photo: Marshall et al, 2004
COLOUR
•
As a rule, however, most minerals are very weakly
coloured !!
•
PRACTICE will enable you to recognize the many subtle
colour (aka colour tint) differences that help identify other
minerals.
pp
xp
Pyrrhotite
Photo: Marshall et al, 2004
FACTORS AFFECTING OBSERVED
COLOUR
•
Observer perception (books, charts and instructors are
therefore a guide only)
•
•
The specific microscope being used
•
It is important to “get your eye in” when first using the ore
microscope, or when using a new microscope
The settings of the microscope (lamp type, illumination
brightness, filters etc.)
FACTORS AFFECTING OBSERVED
COLOUR
•
Apparent colour depends on surrounding minerals
(mutual colour interference)
•
The good news is that the eye can detect relatively subtle
colour differences between different adjacent minerals.
•
For example, gold and chalcopyrite…..
Native Gold
Photo: Marshall et al, 2004
FACTORS AFFECTING OBSERVED
COLOUR
•
Tarnishing can affect colour, e.g:
•
Bright blue “peacock bloom” on chalcopyrite or
bornite can cause confusion with an actual coexisting
mineral like covellite
cpy
cc
cv
Bornite
Photo: Marshall et al, 2004
Effect of Tarnish
REFLECTANCE
•
The percentage of light incident on the polished surface
of a mineral that is reflected back through the microscope
objective, to the observer.
•
Without special metering attachments to the microscope,
we will deal with reflectance as it manifests as the relative
“brightness” of mineral phases.
REFLECTANCE
•
It is fairly easy to determine the RELATIVE reflectance of
the different minerals in a section
•
These can be compared to the known reflectance of easily
identified minerals in the same section……………………..
REFLECTANCE
•
For example;
•
•
•
•
Magnetite ~20%
Galena
~43%
Pyrite
~55%
Mounting plastics (epoxies) and many (though not all)
gangue minerals have very low (dull) reflectance (~5%).
FACTORS THAT MODIFY
REFLECTANCE
•
For a given mineral, the absolute reflectance in a polished
section may be modified by:
•
•
•
Colour (and the wavelength of incident light and/or
filters used)
Polishing quality (poor quality reduces reflectance)
Tarnish
BIREFLECTANCE AND REFLECTION
PLEOCHROISM
•
Most non-cubic minerals show some change in
reflectance and/or colour when sections are rotated under
plane-polarized light
•
These are termed BIREFLECTANCE and REFLECTION
PLEOCHROISM, respectively
•
Cubic (isometric) minerals generally do not show these
properties
BIREFLECTANCE AND REFLECTION
PLEOCHROISM
•
BIREFLECTANCE and/or REFLECTION PLEOCHROISM
may occur as very weak, weak, moderate, strong or very
strong properties in a given mineral.
pp
xp
Covellite
Photo: Marshall et al, 2004
MINERALS THAT EXHIBIT
REFLECTION PLEOCHROISM
Mineral
Colour Range (Darker : Lighter)
Bireflectance Range
Graphite*
Brownish Grey : Greyish Black
6-27
Covellite*
Deep Blue : Bluish-White
6-24
Molybdenite*
Whitish Grey : White
19-39
Stibnite*
White : Greyish-White
31-48
Bismuthinite
Whitish-Grey : Yellowish White
37-49
Pyrrhotite
Pinkish Brown : Brownish Yellow
34-40
Niccolite
Pinkish Brown : Bluish White
46-52
Cubanite
Pinkish Brown : Clear Yellow
35-40
Valeriite
Brownish Grey : Cream Yellow
10-21
Millerite
Yellow : Light Yellow
50-57
after Craig & Vaughan, 1994
pp
xp
Graphite
Photo: Marshall et al, 2004
Pleochroism in Pyrrhotite w Asp
PPL, 0o
PPL, 180o
PPL, 90o
PPL, 270o
STRONGLY BIREFLECTANT
MINERALS
•
•
•
•
Graphite
C
Hexagonal
Molybdenite
MoS2
Hexagonal
Covellite
CuS
Hexagonal
Stibnite
Sb2S3 Orthorhombic
MODERATELY BIREFLECTANT
MINERALS
•
•
•
•
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Marcasite FeS2
Orthorhombic
Hematite Fe2O3
Hexagonal
Pyrrhotite Fe1-xS
Hex/Mono
Cubanite CuFe2S3
Orthorhombic
Niccolite NiAs
Hexagonal
pp
xp
Niccolite (NiAs)
Photo: Marshall et al, 2004
Pleochroism in Pyrrhotite w
Arsenopyrite
WEAKLY BIREFLECTANT MINERALS
•
•
•
Arsenopyrite FeAsS
Monoclinic
Ilmenite
FeTiO3
Hexagonal
Enargite
Cu3AsS4
Orthorhombic
EFFECT OF CRYSTALLOGRAPHIC
ORIENTATION
•
Like PLEOCHROISM and BIREFRINGENCE in transmitted
light microscopy –
•
BIREFLECTANCE and REFLECTION PLEOCHROISM are a
function of the crystallographic orientation of the grain
relative to the incident polarized light……………..
EFFECT OF CRYSTALLOGRAPHIC
ORIENTATION
•
•
Cubic minerals do not display these properties
•
Non-cubic minerals may display anywhere from their
maximum effect to no effect, depending on the grain
orientation
Neither do basal sections of tetragonal and hexagonal
(i.e., uniaxial) minerals
DETECTING BIREFLECTANCE AND
REFLECTION PLEOCHROISM
•
Look at closely adjacent grains or grain aggregates of the
same mineral:
•
•
•
These will have varying relative orientations
In this manner you can detect very small differences
in behaviour as the stage is rotated
A classic example of this is the identification of
pyrrhotite…………
ANISOTROPISM
•
A property evident under crossed polars
•
Cubic minerals remain uniform in appearance when the
stage is rotated, although not necessarily completely dark
•
An exception to this rule is the fairly common
observation of weak anomalous anisotropy in pyrite
Pleochroism in Pyrrhotite w Asp
ANISOTROPISM
•
Most orientations of non-cubic minerals will show some
variation in brightness and/or colour as the stage is
rotated
•
As with BIREFLECTANCE/REFLECTION PLEOCHROISM
this effect can range from maximum to none, depending
on relative orientation.
Millerite (NiS)
Photo: JLM Visuals
pp
xp
Millerite
Photo: Marshall et al, 2004
XPL, 0o
PPL
XPL, 45o
XPL, 90o
Stibnite changing anisotropism
with stage rotation
XPL, 135o
xp
pp
Pyrrhotite
Photo: Marshall et al, 2004
xp
pp
Covellite
Photo: Marshall et al, 2004
ANISOTROPISM
•
Similarly it is often best detected at the junctions of grains
or grain aggregates of the same mineral.
•
Restricting the field of view with a field aperture
diaphragm may also help.
ANISOTROPISM
•
The maxima and minima of these anisotropic effects will
each occur four times in a 360º rotation, offset 45º from
each other.
•
The degree of anisotropism is also described as very
weak, weak, moderate, strong or very strong.
XPL, 0o
PPL
XPL, 45o
XPL, 90o
Stibnite changing anisotropism
with stage rotation
XPL, 135o
ANISOTROPISM
•
The anisotropic colours themselves are sometimes
distinctive, e.g.:
•
•
Deep blue/green/yellow displayed by marcasite.
False anisotropy can be induced by fine parallel
scratches,
•
These are especially common in very soft minerals
(e.g., Au and Ag)
pp
xp
Native Silver
Photo: Marshall et al, 2004