Download ES 3210 ECONOMIC MINERAL DEPOSITS ORE MICROSCOPY II

ES 3210 ECONOMIC MINERAL DEPOSITS
ORE MICROSCOPY II
Stephen J. Piercey
ORE MICROSCOPY
•
Continued ……………………..
INTERNAL REFLECTIONS
•
A second property (best) observed with crossed polars is
INTERNAL REFLECTION
INTERNAL REFLECTIONS
•
Internal reflections are generated by light reflected back
to the surface from subsurface cracks or flaws in grains
of a mineral that is not totally opaque.
•
•
Many transparent minerals, like quartz, will often
appear as masses of internal reflections under
reflected light.
If we were dealing with gems these features would be
termed “flaws”
Quartz
Photo: JLM Visuals
INTERNAL REFLECTIONS
•
Internal reflections are best observed under crossed
polarizers and with the illumination brightness increased.
•
Not all grains of a mineral will necessarily show them, but
they can be useful diagnostics.
INTERNAL REFLECTIONS
•
Sphalerite commonly shows yellow, brown, green or red
internal reflections.
•
This property can be very useful in distinguishing
sphalerite from another cubic mineral, magnetite, which
does not.
•
Internal reflections may also be useful in identifying
cassiterite, hematite, wolframite and chromite
Sphalerite
Photo: JLM Visuals
Sphalerite
HARDNESS
•
Hardness has several useful manifestations in ore
microscopy
•
Not necessary to use a sophisticated microindentation hardness tester for all applications
POLISHING HARDNESS/RELIEF
•
Good preparation techniques always attempt to minimize
polishing relief
•
•
Relief makes grain boundary features and textural
relationships more difficult to observe.
Nevertheless, harder minerals inevitably stand out
topographically to some degree from adjacent softer
minerals.
Polishing Relief
from Craig&Vaughan, 1994
POLISHING HARDNESS/RELIEF
•
If the relief is sufficient, it is possible to utilize the Kalb
Light Line test
•
Sometimes called the “Pseudo-Becke” line test,
although it is caused by a completely different
physical phenomenon)
POLISHING HARDNESS/RELIEF
•
Kalb Light Line Test:
•
•
•
•
1.Focus on a clear boundary line between two mineral
grains.
2. Lower the stage to defocus the sample.
3. Observe a “Becke-like” line of light, which will
move into the softer mineral.
In practice, this is similarly difficult to execute to the
Becke Line Test. But is still sometimes very useful for
difficult identifications.
SCRATCH HARDNESS
•
Good preparation techniques also attempt to completely
eliminate the scratches accumulated during progressive
stages of grinding and polishing the section.
•
Nevertheless, the sample surface always displays some
scratches, which can be used to advantage
……………………..
SCRATCH HARDNESS
•
The relative depth of a
scratch as it traverses
different minerals
allows one to judge
their relative hardness.
tn
gn
Which mineral is harder?
Photo: Craig&Vaughan, 1994
SCRATCH HARDNESS
•
One has to remain aware, however, that deep scratches
produced early in preparation may be preferentially
retained by very hard minerals.
•
In fact, this is a diagnostic characteristic of pyrite
Pyrite is, in fact, harder than
sphalerite, and so displays higher
relief – but also displays deep
retained scratches from initial
preparation steps
py
sph
Photo: Craig&Vaughan, 1994
SCRATCH HARDNESS
•
Very soft minerals such as gold, silver, molybdenite and
graphite almost always retain a scratched or dimpled
appearance even after final polishing.
Py
Au
Scratch Hardness
PPL
Differential Interference Contrast
(Nomarski)
STRUCTURAL AND
MORPHOLOGICAL PROPERTIES
•
These are also invaluable aids to mineral identification in
Ore Microscopy
•
So don’t forget all the mineralogy we made you learn!!
CRYSTAL FORM AND HABIT
Covellite - Bladed
xp
pp
Millerite - Acicular
Photo: Marshall et al, 2004
Millerite
Photo: JLM Visuals
Pyrite – Framboidal/Spheroidal
Common in shalehosted pyrite,
this texture is a
product of
bacterial activity.
CLEAVAGE AND PARTING
Arsenopyrite rhombs
(with chalcopyrite and sphalerite)
Galena
Photo: JLM Visuals
Galena – Triangular Pitting
This a consequence of the perfect cubic cleavage of galena, which
causes characteristic plucking features during polishing.
mrc
mrc
Marcasite – Irregular Longitudinal Fractures
Photo: Marshall et al, 2004
Molybdenite
Photo: JLM Visuals
Molybdenite – Basal Cleavage/Lath Shape
Photo: Marshall et al, 2004
EXSOLUTION TEXTURES
Ilmenite with exsolved magnetite lamellae
©RA Ixer & PR Duller
Pentlandite Flame
Lamellae in Pyrrhotite
Marshall et al, 2004
Chalcopyrite Disease
Sphalerite from VMS and other types of
deposit often contain small inclusions of
chalcopyrite. These often appear to be
oriented along crystal planes, and were
originally thought to be an exsolution texture.
Photo: Marshall et al, 2004
Chalcopyrite Disease
Photo: Augustithus, 1995
In fact, these chalcopyrite inclusions are
a later hydrothermal replacement
feature, commonly termed “chalcopyrite
disease”
from Craig&Vaughan, 1994
Paragenesis
•
Paragenesis refers to the time sequence of formation of
mineral phases.
•
The textural relationships of minerals in thin or polished
sections provide essential information for establishing
paragenetic sequences.
Simultaneous growth texture – Galena and Sphalerite
from Craig&Vaughan, 1994
Myrmekitic chalcopyrite and bornite
chc
cpy
chc
Chalcocite crosscutting earlier Chalcopyrite
Photo: Augustithus, 1995
Typical textures of vein-filling minerals. Note the bilaterally symmetrical
sequence of deposition from edge(s) to centre in (b).
from Craig&Vaughan, 1994
Most ore deposits form in dynamic
environments. Brecciation of both ores and
host rocks is a common feature due to
faulting and collapse events.
Brecciated Sphalerite Ore
Photo: Marshall et al, 2004
Subsequent to brecciation events, successive generations of mineralization
often nucleate on individual fragments.
from Craig&Vaughan, 1994
PARAGENESIS:
TWO EXAMPLES FROM AN MVT PBZN DEPOSIT
dolo
gn
sph
cc
Photo: Ixer, 1990
Photo: Ixer, 1990
TIPS AND TRICKS
GOOD HABIT I
•
Look at the section surface and any existing offcut hand
sample without the microscope before you start.
•
Knowing one or more of the more abundant constituent
minerals to begin with is a great help.
GOOD HABIT II
•
Always do a low magnification reconnaissance of the
entire sample first.
•
Try to look at more than one sample from the same suite if
possible.
GOOD HABIT III
•
•
•
Don’t stop being a mineralogist.
Learn to recognize X-sectional shapes.
Be cognizant of crystal system.
•
e.g., iIsometric/cubic minerals are generally isotropic
Spry and Gedlinske (1987)