Geology of the Snap Lake kimberlite intrusion, Northwest Territories, Canada: field observations and their interpretation

The Cambrian (523 Ma) Snap Lake hypabyssal kimberlite intrusion, Northwest Territories, Canada, is a complex segmented diamond-bearing
ore-body. Detailed geological investigations suggest that the kimberlite is a multi-phase intrusion with at least four magmatic
lithofacies. In particular, olivine-rich (ORK) and olivine-poor (OPK) varieties of hypabyssal kimberlite have been identified.
Key observations are that the olivine-rich lithofacies has a strong tendency to be located where the intrusion is thickest
and that there is a good correlation between intrusion thickness, olivine crystal size and crystal content. Heterogeneities
in the lithofacies are attributed to variations in intrusion thickness and structural complexities. The geometry and distribution
of lithofacies points to magmatic co-intrusion, and flow segregation driven by fundamental rheological differences between
the two phases. We envisage that the low-viscosity OPK magma acted as a lubricant for the highly viscous ORK magma. The presence
of such low-viscosity, crystal-poor magmas may explain how crystal-laden kimberlite magmas (>60 vol.%) are able to reach the
surface during kimberlite eruptions. We also document the absence of crystal settling and the development of an unusual subvertical
fabric of elongate olivine crystals, which are explained by rapid degassing-induced quench crystallization of the magmas during
and after intrusion.

加拿大西北地区的寒武纪（5.23亿年前）Snap Lake深成金伯利岩侵入体，是一个复杂的分段含金刚石矿石体。详尽的地质调查表明，该金伯利岩是一种多阶段侵入体，至少包含四个岩相。特别是，富含橄榄石的（ORK）和贫橄榄石的（OPK）深成金伯利岩品种已被识别。关键观察结果为富含橄榄石的岩相倾向于位于侵入体最厚处，且侵入体厚度、橄榄石晶体大小与晶体含量之间存在良好的相关性。岩相的非均质性归因于侵入体厚度的变化和结构复杂性。岩相的几何形态与分布指向了岩浆共侵入，以及由两相间基本流变学差异驱动的流动分离。我们设想低粘度的OPK岩浆作为高粘度的ORK岩浆的润滑剂。这种低粘度、晶体贫乏的岩浆的存在可能解释了为何富含晶体的金伯利岩浆（>60体积%）能够在金伯利岩浆喷发过程中到达地表。我们还记录了晶体沉降的缺失和一种不寻常的亚垂直橄榄石晶体织构的发展，这被解释为岩浆在侵入期间和侵入后因快速排气诱导的淬火结晶。