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Title: Mineral indicators of porphyry Cu fertility
Author: Loader, Matthew
ISNI:       0000 0004 7963 7142
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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'Fertile' magmas which generate metal-rich porphyry Cu deposits are thought to form by similar processes to normal 'barren' arc magmas which are unrelated to mineralisation. However, fertile magmas contain distinctive trace element signatures, which highlight the processes by which they achieved high ore-forming potential. These 'indicators of fertility' make potentially useful tools to be used in the exploration of porphyry systems and may be amplified in minerals within these rocks. Intrusive rocks were collected from the Resolution and Oyu Tolgoi porphyry Cu deposits. A combination of whole rock geochemistry, and trace element analyses of apatite and zircon were carried out, and these data were compared to literature data for barren rocks. Rocks from both systems indicators of fertility, some of which (especially Sr/Y) were found to be susceptible to feldspar-destructive hydrothermal alteration. The presence of these anomalies (e.g. La/Yb, Eu/Eu*, V/Sc) is indicative of garnet control on deep crustal fractionation, which may occur due to loaclised increases in melt H2O. Zircons from Oyu Tolgoi show that Eu/Eu* and Ce4+/Ce3+ ratios in zircon were strongly controlled by the co-crystallisation of titanite, limiting their utility as measures of redox. Ce/Ce* may provide a better measure of melt redox in zircon, but is difficult to measure. Eu/Eu* may still be useful for distinguishing barren and fertile systems. Igneous apatite has high Sr and Eu/Eu* compared to barren systems. Hydrothermal apatite from a number of porphyry systems were found to contain uniquely high Fe, Mn, Pb, Sr, and Eu/Eu*, which possibly relate to their precipitation from metal rich brines. The magnitude of Mn enrichment may also be sensitive to bulk rock Cu content. These results highlight the potential of apatite and zircon as mineral indicators of porphyry fertility, and recommendations are made for how they may be used in exploration.
Supervisor: Wilkinson, Jamie Sponsor: Rio Tinto Exploration
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral