Fathom Geophysics Newsletter 23

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Geochemistry Research News: Beware of TiO2 masquerading as rutile

RUTILE is by far the most common form of titanium dioxide found at and near the earth's surface. And yet mineral explorers preferring to avoid a merry goose chase should be prepared to successfully parry occasional encounters with rutile's impostors during ore-search efforts.

In recent years, mineral explorers have become alert to the fact that, when present in association with gold and base-metal mineralization, rutile contains certain telltale trace-element signatures. [1]

But unsuspecting explorers who use rutile as part of geochemical techniques employed to explore for ore bodies could find themselves led astray by minerals whose broad composition is identical to rutile but that are devoid of rutile's explorationally useful ore-pathfinder elements.

"[Correctly] characterizing the type of titanium dioxide polymorphs is critical if rutile is to become a new pathfinder mineral to mineralization," researchers from Western Australia, California, and South Australia said in their recent journal paper. [2]

The polymorphs

Titanium dioxide has four main polymorphs:

  • anatase and brookite, both of which form at relatively low temperatures and pressures, such as those occurring during the formation of authigenic minerals and during low-temperature metamorphism (below 500 degrees Celsius);
  • TiO2(II), which forms at very high pressures in conjunction with lower temperatures, such as conditions occurring at subduction zones and meteorite-impact sites; and
  • rutile, which forms at medium to ultra-high temperatures and pressures, such as those occurring during relatively intense metamorphism.

"[D]ifferences in trace-element compatibility amongst different titanium dioxide polymorphs have the potential to complicate the use of rutile as a pathfinder mineral if anatase and brookite are mistaken for rutile," the researchers said.

Pathfinder-element profiles are different

Recognizing the presence of anatase and brookite during the reconnaissance stage of mineral exploration was therefore critical, the researchers said.

This was because these two minerals had a propensity to contain systematically smaller abundances of the pathfinder elements iron, chromium, vanadium, tin, and tungsten.

If anatase or brookite were present in geological samples but were mistaken as rutile, their lower pathfinder-element concentrations could potentially produce false negatives during exploration-related geochemical analyses, they said.

To help avoid such false negatives, the researchers recommended that explorers plot on a rutile-anatase-brookite ternary discrimination diagram the geochemistry of the full suite of titanium dioxide grains hosted in their exploration samples — before going ahead with any procedures designed to identify potential mineralization-associated rutile grains.

If a large number of the plotted grain geochemistries cluster in the anatase and brookite regions of the ternary diagram, detailed inspection of the grains is advisable, the researchers said.

Polymorph identification

In theory, distinguishing rutile from its lower-temperature polymorphs should be relatively straightforward using an optical microscope, given the polymorphs' different inherent crystallographic properties and their propensity to be associated with differing rock types, differing suites of co-accompanying minerals, and differing microtextures. [3]

However, in practice, even when examined in polished section under ideal circumstances, anatase and brookite appear almost identical to rutile.

Distinguishing signs that one is looking at anatase crystals include their lack of twinning (rutile commonly shows twinning), their very weak anisotropy (rutile's grays are strongly anisotropic, but often these grays are masked by internal reflections), and the fact that they're uniaxial negative a.k.a. length fast (rutile is uniaxial positive a.k.a. length slow). Meantime, brookite crystals are orthorhombic (rutile is tetragonal), and their biaxial indicatrix has a small positive optic-axial-angle a.k.a. 2V (rutile, being tetragonal, belongs to the group of uniaxial crystals and therefore has no biaxial indicatrix).

These diagnostic features become exceedingly difficult to discern when the mineral grains in question take the form of very fine-grained aggregates or multiple-polymorph intergrowths. In the worst cases, even the most basic observations under the microscope — such a mineral color, birefringence, extinction angle, and the presence of twinning — can become equivocal.

In such cases, even closer inspection of mineral grains was called for, the researchers said. They recommended the use of electron backscatter diffraction analysis or Raman spectroscopy.

In their paper, the researchers concluded by saying that a combined approach involving uranium-lead geochronology and rapid, high-resolution electron backscatter diffraction-based microtextural mapping of in-situ grains within thin sections or mounts "shows great potential as a new and relatively inexpensive exploration method" for better understanding the nature and origin of the rutile in ore-bearing versus barren mineral systems.


[1] See, for example: K.M. Scott, N.W. Radford, R.M. Hough and S.M. Reddy (2011) "Rutile compositions in the Kalgoorlie Goldfields and their implications for exploration", Australian Journal of Earth Sciences, 58, 7, 803-812.

[2] D. Plavsa, S.M. Reddy, A. Agangi, C. Clark, A. Kylander-Clark and C.J. Tiddy (In Press) "Microstructural, trace element and geochronological characterization of TiO2 polymorphs and implications for mineral exploration", Chemical Geology.

[3] For a discussion of the general principles of optical microscopy, see, for example: C.D. Gribble and A.J. Hall (1994) "A practical introduction to optical mineralogy", Chapman & Hall, 249 pages.

About Fathom Geophysics

In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

We offer automated data processing, automated exploration targeting, and the ability to tailor-make data processing applications. Our automated processing is augmented by expert geoscience knowledge drawn from in-house staff and from details relayed to us by the project client. We also offer standard geophysical data filtering, manual geological interpretations, and a range of other exploration campaign-related services, such as arranging surveys and looking after survey-data quality control.