Ian R. Dasti's MSc Thesis Abstract
The Shakespeare intrusion, which hosts the Shakespeare Ni-Cu-PGE sulfide deposit, is part of the 2.2 Ga Nipissing Gabbro magmatic event, a large mafic-ultramafic suite located in the southern Superior and Southern Province. The Shakespeare intrusion intrudes the 2.2-2.45 Ga Huronian Supergroup and is located in the northern part of the Southern Province, only a few km from the Archean- Proterozoic unconformity which separates the Superior Province from the Southern Province and is adjacent to the ~ 2450 Ma East Bull Lake and Agnew Lake intrusions.
The Shakespeare deposit is the largest Ni-Cu-PGE deposit in the Nipissing Large Igneous Province (LIP). Field mapping and drilling have provided a detailed stratigraphic understanding of the deposit and provide the framework for geochemical models of sulfide segregation related to assimilation. The U-enriched rocks that interacted with the intrusion during ascent through the crust provide an excellent opportunity to gauge the role of assimilation during the formation of the Shakespeare deposit while providing further evidence that crustal sulfur is necessary in order to generate economic concentrations of magmatic sulfides.
A detailed study of three drill holes representing poor, average, and above average intercepts of Ni-Cu-PGE sulfides was performed in order to establish the mechanism by which an immiscible sulfide liquid was produced and also to establish the contamination history of the Shakespeare deposit. Various geochemical techniques were employed, including SEM-EDS, ICP-MS, ICP-AES, total sulfur, sulfur isotope, and XRF analysis. During the course of the SEM-EDS study primary magmatic textures within hollingworthite-irarsite-platarsite were observed, suggesting limited remobilization during metamorphism. Rheniite, cerium-rich allanite, and various tellurides and selenides were also observed.
δ34S analysis yielded values between 0.01‰ and 2.38‰, averaging 1.14‰. These values are indicative of an unfractionated sulfur source, suggesting that the source of sulfur in the Shakespeare deposit was either derived from the mantle or an unfractionated crustal source, such as the quartz pebble conglomerates of the Matinenda Formation. Sulfur: selenium ratios yielded values between 1245 to 3271, averaging 1810. The average value is significantly lower than accepted mantle values (2850-4000) and may point to an assimilant rich in selenium.
Spider diagrams of mantle normalized data from the Shakespeare intrusion indicate that all of the rocks of the Shakespeare intrusion display a geochemical profile which is characteristic of magmas that have undergone significant degrees of crustal contamination. Negative Nb and Ti anomalies, enrichment in HFSE relative to MREE, and enrichment in LREE relative to HREE indicate that ascending Shakespeare magmas assimilated significant quantities of upper crustal material.
The results of the study show that the Shakespeare intrusion is highly enriched in U, Th, and LREE, relative to other Nipissing intrusions. The most likely explanation for the enrichment is that the Shakespeare intrusion interacted with the pyritic quartz pebble conglomerates of the Matinenda Formation, possibly near the past producing Agnew Lake uranium mine. Geochemical and textural evidence suggests that magma mixing was the dominant mechanism responsible for the precipitation of an immiscible sulfide liquid. R factor calculations and sulfide metal concentrations suggest a second immiscible sulfide liquid precipitated from later mixing with the PGE- and base metal-poor biotite quartz diorite magmas which ponded in the roof zone of the intrusion.
The results of this study have major implications for Ni-Cu-PGE exploration in the Nipissing LIP and indicate that any economically significant concentrations of magmatic sulfides will be found in gabbroic rocks which have assimilated significant amounts of U- and Th-rich crustal material from the Matinenda Formation, resulting in discernable differences in the U-contents of potential host intrusions versus intrusions that are not likely to host a Ni-Cu PGE deposit.
A copy of the thesis can be downloaded here