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TitleAnomalous zircon Ce4+/Ce3+ ratios from the Jurassic of Yukon, Canada: implications for exploration applications of a zircon magma redox proxy
AuthorChapman, J B; Jackson, S E; Ryan, J J
SourceApplied Earth Science (Trans. Inst. Min. Metall. B) vol. 121, no. 4, 2012 p. 173
Alt SeriesEarth Sciences Sector, Contribution Series 20130010
PublisherManey Publishing
Meeting36th Annual Winter Meeting of the Geological Society's Mineral Deposits Studies Group; Leicester; GB; January 2-4, 2013
Mediapaper; on-line; digital
File formatpdf
Subjectsgeochronology; economic geology; zircon dates; zircon; cesium; magmas; mineral occurrences; porphyry deposits; copper; gold; molybdenum; mineralization; Mesozoic; Jurassic
ProgramGEM: Geo-mapping for Energy and Minerals, Multiple Metals - NW Canadian Cordillera (Yukon, B.C.)
AbstractNumerous studies suggest that a genetic link exists between oxidized magmas and porphyry-style Cu (¡Au¡Mo) mineralisation. However, traditional indicators of magmatic redox state can be reset during both hydrothermal alteration and weathering. Zircon (ZrSiO4) is an abundant accessory mineral in granitoid igneous intrusive rocks and is both refractory and resistant to alteration. Trivalent heavy rare earth elements readily substitute for Zr within the zircon crystal lattice, but the degree of substitution decreases markedly with increasing ionic radius. However, Ce shows significant deviations from this model behaviour due to its additional stable z4 oxidation state. Zircon commonly displays a significant positive Ce anomaly, as Ce4z is preferentially incorporated into the zircon lattice over Ce3z due to its identical charge to Zr4z. The magnitude of this anomaly should correlate to the Ce4z/ Ce3z ratio in the parent magma, and as such can be used as a qualitative proxy for oxidation state. In this study, we used laser ablation inductively-coupled plasma mass-spectrometry (LA-ICP-MS) microanalysis to determine zircon Ce4z/Ce3z ratios for a suite of Triassic to Jurassic granitoid intrusions from southern and western Yukon, and for various intrusive phases associated with the Cretaceous Casino Cu-Mo-Au deposit. Results for the Casino samples show that intrusive phases associated with mineralisation conform to established models for Ce systematics within porphyry-style Cu ore-forming settings. The major mineralising intrusive breccia phases have average zircon Ce4z/Ce3z values of 200-350, and peak values approaching 500, in contrast to mid-Cretaceous granitoid host rocks that have average values of 50-100 and peak values ,250. Hence, it may be possible to use zircon Ce ratios as a powerful exploration fingerprint and vectoring tool. Although variability within an individual sample appears to be quite high, basing prospectivity thresholds on population statistical parameters may allow the number of individual analyses required to be reduced. More work is needed to establish possible provenance determination protocols, but in time it may be possible that Ce4z/Ce3z ratio determination could be applied to stream and soil sediment samples. Data from the Triassic/Jurassic suite are more equivocal, however, and seem to indicate that the controls for these rocks are substantially different to those of the Cretaceous. Both the Minto and Carmacks Copper deposits are hosted within Triassic-Jurassic plutonic rocks of the Granite Mountain batholith, part of a regionally extensive suite of plutonic rocks of similar age and composition. Within the Carmacks Copper Belt of south-central Yukon these have been extensively explored for Cu (¡Au) mineralisation, and numerous showings have been identified. However, outside the belt few other mineralisation targets have been identified. The Triassic-Jurassic suite appears to fall into two distinct groupings: those with modal Ce4z/ Ce3z values .700, and those with values ,600. However, all are above the values identified by Ballard et al. (2002) and Liang et al. (2006) as the threshold for prospective bodies in the Chuquicamata-El Abra district, Chile, and Yulong district, China, respectively. Petrogenetic indicators for this suite suggest that the depth of emplacement was significantly deeper than was the case for the mineralisation associated rocks of the Cretaceous suite. Hence, it may be that the zircon Ce4z/ Ce3z ratio in these samples is not dominantly controlled by magma redox. In this case, the technique may be of little use.
Summary(Plain Language Summary, not published)
Models for formation of the type of copper ore bodies known as "porphyry deposits" suggest that there is a direct link between these and oxygen-rich magma. During their growth in magma, crystals of the mineral zircon can incorporate significant quantities of a suite of metals known as the rare-earth elements (REE). By studying the relative concentrations of REE in zircon crystals and their parent igneous rocks, we can determine whether the magma that formed both was relatively oxygen-rich or not. By applying this theory to different igneous rocks in Yukon we were able to test whether this technique might increase the effectiveness of copper exploration in the region. While the zircon REE method does seem to be highly effective at identifying Cretaceous (i.e. 145 to 66 million years old) rocks with high ore-forming potential, rocks of Early Jurassic age (201 to 174 million years old) have anomalous REE patterns so the technique is not applicable to these.