Ore mineral 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

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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.

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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

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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

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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

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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

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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

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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

• • • • •

Marcasite FeS2

Orthorhombic

Hematite Fe2O3

Hexagonal

Pyrrhotite Fe1-xS

Hex/Mono

Cubanite CuFe2S3

Orthorhombic

Niccolite NiAs

Hexagonal

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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

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Millerite

Photo: Marshall et al, 2004

XPL, 0o

PPL

XPL, 45o

XPL, 90o

Stibnite changing anisotropism with stage rotation XPL, 135o

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pp Pyrrhotite

Photo: Marshall et al, 2004

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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)

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Native Silver

Photo: Marshall et al, 2004