High resolution X-ray computed tomography studies of Grasberg porphyry Cu-Au ores, Papua, Indonesia
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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.
KeywordsX-ray computed tomography Porphyry Cu–Au Mineralogy Exsolution Ore processing Grasberg
The University of Texas at Austin HRXCT facility was created with support of the W.M. Keck Foundation, the National Science Foundation (EAR-9406258), and the Geology Foundation of the University of Texas at Austin. Continuing support for laboratory operations as a multiuser facility and for software development is also provided by NSF (EAR-0345710, EAR-0113480). We are grateful for support provided by PT Freeport Indonesia (JK-04042-001) for field work and other research expenditures and to the many PTFI geologists and other employees that have contributed to our understanding of Ertsberg District geology; John Mosher provided valuable counsel on the complexity of the PTFI milling issues as they relate to our study. Reviews by Peter Pollard and Cameron Huddlestone-Holmes provided valuable suggestions to improve the presentation of this material.
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