Multi-trace element distribution in olivine from the Eastern Deeps deposit, Voisey’s Bay Intrusion, Labrador: olivine composition as a record of ore-forming processes in evolved magma systems

摘要

The distribution of Ni-Mg-Fe in olivine is used extensively to characterize mafic intrusions, and, in combination with other geochemical tools, assess their Ni-Cu-Co sulfide mineralization potential. Despite the availability of sensitive analytical techniques (e.g., Secondary Ion Mass Spectrometry – SIMS) and the proven importance of olivine multi-trace element studies to igneous petrology, a more comprehensive geochemical approach of determining multiple trace elements in olivine has never been adopted for economic geology applications. For this dissertation I implemented a refined SIMS analytical protocol for the determination of multiple major-trace elements in olivine from the Eastern Deeps Intrusion (EDI), a part of the Voisey’s Bay Intrusion (VBI), Labrador. The study demonstrated that systematic, lithology-dependent trace element variations in olivine, particularly in Ni, Co, Cr, Mn, and Zn, characterize critical ore-forming processes and indicate the proximity to zones of massive sulfide mineralization in the EDI.udSIMS is an analytical technique for the in-situ, micrometer-scale determination of elements in a solid sample with very low detection limits. Chapter 2 introduces the SIMS analytical parameters applied for the olivine analyses and further discusses the reference material development and the sequential empirical calibration.udThe subsequent chapters detail the SIMS studies of olivine from the EDI and Pants Lake Intrusion (PLI). Chapter 3 demonstrates that lithostratigraphic variations in olivine Ni-Co contents (~80–2,500 ppm Ni; ~170–370 ppm Co) in the EDI indicate magmatic episodicity and multiple sulfide saturation events. The basal, incongruent olivine Mn-Zn enrichment (up to 12,000 ppm Mn; up to 680 ppm Zn) reflects country rock contamination followed by a diffusive trace element exchange with surrounding sulfide liquid. Chapter 4 discusses the application of the olivine trace chemistry as a lateral vector towards sulfide mineralization in the EDI, and as a fertility indicator for mafic intrusions on a regional scale (ex. PLI). Progressively increasing contents of Mn-Zn in olivine towards the inner basal margin of the EDI indicate spatial (vertical and lateral) proximity (~150m) to massive sulfides. A strong compositional bimodality (e.g., Ni-enriched and Ni-depleted) of olivine also increases the potential for inherent economic mineralization (ex. VBI). This study considerably improved our understanding of systematic compositional variations in olivine as a response to essential ore-forming processes in mineralized mafic magmatic systems.