Mineralium Deposita

, Volume 43, Issue 5, pp 519–532 | Cite as

High resolution X-ray computed tomography studies of Grasberg porphyry Cu-Au ores, Papua, Indonesia

  • J. Richard Kyle
  • Alison S. Mote
  • Richard A. Ketcham
Article

Abstract

High-resolution X-ray computed tomography (HRXCT) provides unique information of the geological and metallurgical significance for gold and related ore minerals in the supergiant Grasberg porphyry Cu–Au deposit. Digital radiographs have proved to be an effective means of screening samples for the presence of gold for HRXCT studies. Digital radiograph effectiveness is limited by the thickness of samples (typically to ≤2 cm), as well as the associated minerals. Thus, preselecting samples for gold studies using HRXCT is most effective using digital radiographs combined with assay information. Differentiating between metallic mineral grains with relatively small differences in density, e.g., bornite (5.1 g/cm3) from chalcopyrite (4.2 g/cm3), is relatively straightforward for isolated monominerallic grains or composites in a similar lower-density matrix, but difficulties are encountered with the interpretation of typical intergrown ore minerals. X-ray beam-hardening artifacts lead to inconsistency in attenuation determination, both within and among slice images, complicating quantitative processing. However, differentiation of chalcopyrite and bornite has been successful in smaller-diameter (≤22-mm) cores of Grasberg ores. Small-diameter (≤10 mm) cores of the Grasberg stockwork Cu–Au ore were analyzed using HRXCT methods scanned at the minimum spacing currently available (7.5 μm), and data reduction protocols using the Blob3D program were modified to improve the quantification of grain sizes and shapes. Grains as small as 6.5 μm have been identified. All of these grains are in direct contact with chalcopyrite, providing support for gold distribution in porphyry copper systems being a result of exsolution from copper sulfides. HRXCT scanning (±digital radiography) precisely defines the in situ location of mineral grains of interest within a sample, which then can be studied in conventional petrographic sections, and other types of analytical studies conducted, e.g., gold trace element geochemistry.

Keywords

X-ray computed tomography Porphyry Cu–Au Mineralogy Exsolution Ore processing Grasberg 

References

  1. Baline LM (2007) Hydrothermal fluids and Cu–Au mineralization of the Deep Grasberg porphyry deposit, Papua, Indonesia. M.S. thesis, Univ Texas Austin (226 p)Google Scholar
  2. Cooke DR, Hollings P, Walshe JL (2005) Giant porphyry deposits: characteristics, distribution, and tectonic controls. Econ Geol 100:801–818CrossRefGoogle Scholar
  3. Kesler SE (2004) Gold in sulphide minerals and ore deposits. The Gangue: mineral deposits division, issue 38. Geological Association of Canada, St. John’s, NL, pp 1, 4–8Google Scholar
  4. Ketcham RA (2005) Computational methods for quantitative analysis of three-dimensional features in geological specimens. Geosphere 1:32–41CrossRefGoogle Scholar
  5. Ketcham RA (2006) Accurate three-dimensional measurements of features in geological materials from X-ray computed tomography data. In: Desrues J et al (ed) Advances in X-ray tomography for geosciences, 2nd International Workshop on X-ray CT for Geomaterials. ISTE, London, pp 143–148Google Scholar
  6. Ketcham RA, Carlson WD (2001) Acquisition, optimization and interpretation of X-ray computed tomographic imagery: applications to the geosciences. Comput Geosci 27:381–400CrossRefGoogle Scholar
  7. Ketcham RA, Mote AS (2004) Accurate in situ three-dimensional measurement of economic trace phases in geological materials using high-resolution X-ray computed tomography. Geol Soc Am, Abstracts with Programs 36:3Google Scholar
  8. Kyle JR, Ketcham RA (2003) In-situ distribution of gold in ores using high resolution X-ray computed tomography. Econ Geol 98:1697–1701CrossRefGoogle Scholar
  9. Kyle JR, Ketcham RA, Mote AS (2004) Contributions of high resolution X-ray computed tomography to ore studies. In: Muhling J et al (ed) Extended abstracts, predictive mineral discovery under cover. University of Western Australia, Perth, pp 387–390Google Scholar
  10. MacDonald GD, Arnold LC (1994) Geological and geochemical zoning of the Grasberg igneous complex, Irian Jaya, Indonesia. J Geochem Explor 50:143–178CrossRefGoogle Scholar
  11. McDowell FW, McMahon TP, Warren PQ, Cloos M (1996) Pliocene Cu–Au-bearing igneous intrusions of the Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia: K–Ar geochronology. J Geol 104:327–340CrossRefGoogle Scholar
  12. Mealey GA (1996) Grasberg: mining the richest and most remote deposit of copper and gold in the world, in the mountains of Irian Jaya, Indonesia. Freeport-McMoRan Copper & Gold, New Orleans, LA (384 p)Google Scholar
  13. Meinert LD, Hefton KK, Mayes D, Tasiran I (1997) Geology, zonation, and fluid evolution of the Big Gossan Cu–Au skarn deposit, Ertsberg District: Irian Jaya. Econ Geol 92:509–534Google Scholar
  14. Mote AS, Kyle JR, Ketcham RA, Melker MD, Jahraus MJ, Brown TR, Wawrzyniec TF (2005) High resolution X-ray computed tomography investigations of high grade gold ore zones in the Cripple Creek District, Colorado. In: Rhoden HN, et al. (eds) Proceedings of the Geological Society of Nevada Symposium: Window to the World, Reno, NV, pp 1169–1175Google Scholar
  15. Pollard PJ, Taylor RG, Peters L (2005) Ages of intrusion, alteration, and mineralization at the Grasberg Cu–Au deposit, Papua, Indonesia. Econ Geol 100:1005–1020CrossRefGoogle Scholar
  16. Rubin JN, Kyle JR (1997) Precious metal mineralogy in porphyry-, skarn-, and replacement-type ore deposits of the Ertsberg (Gunung Bijih) District, Irian Jaya, Indonesia. Econ Geol 92:535–550CrossRefGoogle Scholar
  17. Sapiie B, Cloos M (2004) Strike–slip faulting in the core of the Central Range of West New Guinea, Ertsberg mining district, Indonesia. Geol Soc Amer Bull 116:277–293CrossRefGoogle Scholar
  18. Sillitoe RH (2000) Gold-rich porphyry deposits: descriptive and genetic models and their role in exploration and discovery. In: Hagemann SG, Brown PE (eds) Society of Economic Geologists, Reviews in Economic Geology, vol 13, pp 315–345Google Scholar
  19. Simon G, Kesler SE, Essene EJ, Chryssoulis SL (2000) Gold in porphyry copper deposits: experimental determination of the distribution of gold in the Cu–Fe–S system at 400° to 700°C. Econ Geol 95:259–270CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  • J. Richard Kyle
    • 1
  • Alison S. Mote
    • 1
  • Richard A. Ketcham
    • 1
  1. 1.Department of Geological Sciences, Jackson School of GeosciencesUniversity of Texas at AustinAustinUSA

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