Power of the microscope: Petrographic analysis and mineralogy in the exploration and mining industry

May 17, 2023

by Paul Ashley, Principal at Paul Ashley Petrographic and Geological Services

Most minerals industry geologists, after their initial university training, leave behind skills that they learned about the use of the petrographic microscope, much of which at university is aligned towards academic pursuits and not necessarily economic geology. In part, this situation arises due to the multiplicity of other industry work avenues that geologists must pursue, and due to the fact that most companies do not possess a suitable microscope. Consequently, the potential knowledge and deductions that can be obtained from petrographic examination of the wide range of geologic materials may be foregone. As an alternative, some companies use the services of consultant petrologists/mineralogists.

Petrographic examination generally follows careful examination of sample material with a hand lens and ideally uses both transmitted and reflected light optics on petrographic sections that can include standard thin sections, polished thin sections and polished blocks, prepared from the complete range of geological sample media. The latter can encompass outcrop samples, drill core and chips, mine samples, quarry products, regolithic materials, sand and heavy mineral concentrates, and processed material such as mill products and tailings (Fig. 1). Interpretations from petrographic observations can commonly be enhanced if there is access to accompanying geological and geochemical data. The information acquired can be the springboard for more detailed material investigations where necessary. Some examples may be techniques that include electron microprobe analysis, hyperspectral scanning, X-ray diffraction, QEMSCAN and fluid inclusion thermometric and compositional determinations.

Figure 1 - Typical materials used for petrographic and mineralogical examination: A) drill core, B) mine samples, C) regolith (gossan), D) RC drill chips, E) mineral concentrate.
Figure 1 – Typical materials used for petrographic and mineralogical examination: A) drill core, B) mine samples, C) regolith (gossan), D) RC drill chips, E) mineral concentrate.

In comparison to macroscopic and hand lens observations, e.g. during drill core or drill chip logging, petrographic examination can provide significant benefit in being able to give insights through the plethora of imposed processes that have commonly been imposed on rocks that are associated with mineral systems, e.g. one or more of hydrothermal alteration, metamorphism, deformation and supergene alteration.

Figure 2 – A) Typical texture in a porphyry Cu-related intrusive and with propylitic alteration characteristic of a peripheral position in
a mineralized system. B) Relict phenocrystal quartz grain in an intensely phyllic altered felsic volcanic rock associated with a massive sulphide system. C) Relict chromite grains (grey), enclosed by magnetite (pale brown-grey) and hosted in bornite-chalcocite-hematite rock, proving that the high-grade Cu mineralization replaced an ultramafic composition protolith.

Petrographic examination has the ability to deliver major benefits, including the following:

  • Assisting the development of geological models in greenfields exploration by being able to quickly verify rock types, alteration and mineralization potential.
  • Determination of primary rock types: relevant in exploration and assessment of ore systems such as porphyry Cu-Au (Fig. 2), layered intrusion and ultramafic-hosted Ni-Cu-PGE, granite-related Sn-W-Mo, carbonatite-related critical metals, etc.
  • Be able to ‘look through’ imposed processes to recognize features, such as relict textures and minerals, or assess metamorphic or hydrothermal mineral assemblages to assist the determination of primary rock type (Fig. 2).
  • Determine hydrothermal alteration mineral assemblages and use these as vectors towards mineralized systems, e.g. porphyry Cu-Au, volcanic-associated massive sulphides, orogenic Au, with related benefits of confirming hyperspectral results (Fig. 2).
  • Use the observed mineralogy to explain geochemical data: e.g. occurrence of particulate gold in gold-anomalous samples, the occurrence of enargite or tennantite to explain high As in a porphyry Cu system, presence of relict chromite in an altered rock to confidently state that the protolith was of ultramafic type (Fig. 2), presence of rutile in a high-Ti-altered rock to indicate that the protolith was of basalt type.
  • Use of observed mineralogy to explain geophysical data: e.g. graphite and Fe-sulphides in electrical conductivity anomalies, magnetite (and/or pyrrhotite) in magnetic anomalies, and K-feldspar and U- and Th-bearing minerals in radiometric anomalies.
  • Interpretation of vein and breccia textures, including timing relationships, formation processes and infill mineralogy.
Figure 3 – A) Where’s the deportment of high-grade Ag in the massive sulphides? In this case, it’s in tetrahedrite (pale olive-grey). B) Easy-to-liberate cassiterite grains (brown) in a quartz-tourmaline greisen. C) Different size gold particles in complex sulphide ore (sphalerite-galena-pyrrhotite-chalcopyrite) dictate optimal grind size.

Examples of mineralogical and textural characteristics that are directly relevant to the minerals industry include the following:

  • In sulphide ores, e.g. from porphyry Cu-Au-Mo, volcanic-associated and sediment-hosted massive sulphides, Ni-Cu-PGE deposits, and some epithermal precious metal, orogenic gold and skarn systems, the following aspects can be determined: ore mineralogy, mineralogical location of minor (including deleterious elements, e.g. As), metal deportment and textural relationships relevant to ore processing (e.g. particle size and intergrowths relevant to liberation characteristics) (Fig. 3).
  • In gold ores (e.g. epithermal systems, orogenic gold, intrusion-related, IOCG/ISCG and alluvial deposits), determination of gold mineralogy, location, and particle size relevant to processing recoveries, refractory (invisible gold) issues, gold fineness and the mineralogical presence of associated beneficial or deleterious elements, e.g. Cu, As, Sb, Te, Hg (Fig. 3).
  • In metal sulphide and gold ores, determination of the mineralogy in variably oxidized (supergene-affected) systems to enhance metal recoveries, e.g. liberated (particulate) versus sulphide-hosted gold, knowledge of oxidized/sulphide mineralogy in copper heap leach operations, and whether cyanide leaching is an optimal route in certain gold ores.
  • Critical metal deposits: mineralogy and metal deportment in deposits associated with pegmatites, carbonatites, fractionated granitic rocks and mafic-ultramafic igneous rocks, e.g. commodities such as Li, REE, Ta, W, Sn, Mo, V, Cr, Ni, Co and PGE (Fig. 2). Prediction of liberation characteristics of minerals during ore processing (Fig. 3). Mineralogical siting of trace critical metals such as Co, In, Ga, Ge.
  • Mine-focused environmental issues: due diligence before mining to determine potential problems and solutions with rock types and minerals, e.g. acid production and neutralization, and toxic element (base metals, As, Sb, Cd, Hg, U) release. Characterization of waste rock and mill tailings. Assessment of mineralogical issues with rocks that could produce hazardous dusts, e.g. fibrous and acicular minerals.
  • Engineering (including ground stability) issues at metalliferous and coal mines, and characterization of rock behavior in crushing and milling. Nature of quarry products such as the characterization of problematic rock types (e.g. sheared, layer-silicate dominated rocks, altered igneous dykes), assessment of suitability of quarry products for engineering and construction purposes (e.g. presence of swelling clays, other layer silicates, zeolites, sulphides, fractures/veins).

In conclusion, petrographic and mineralogical investigations on materials typically encountered in the minerals exploration and mining industries can add an additional layer of knowledge to standard techniques, particularly those involving geological observation and geochemistry. They can also provide augmentation to more detailed analytical methods, such as hyperspectral logging, and rapidly acquired field data from portable XRF and XRD analyzers. In mine development, mining (or former mining) and processing operations, this easily acquired information can assist in developing mineral processing routes without making ‘mineralogical mistakes’, prediction of waste rock and tailings behavior and their environmental consequences, and rock quality issues from an engineering perspective during mining, and in quarry products.

To better integrate knowledge gained from drill core/chip logging, geochemical and geophysical results, with petrographic and mineralogical information, some companies might be encouraged to utilize (or invest in) their own petrographic microscopes, and their geologists who still retain the observational and interpretive skills.

I thank Luke Milan and Damien Wilkinson for their exploration viewpoint comments.