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.

加拿大西北地区斯纳普湖的寒武纪（5.23亿年）浅成金伯利岩侵入体，是一处复杂的分段式含金刚石矿体。详细地质调查表明，该金伯利岩属于多期侵入体，至少包含四种岩浆岩相。其中已识别出两类浅成金伯利岩变种：富橄榄石型（ORK）与贫橄榄石型（OPK）。关键观测结果显示，富橄榄石岩相多集中分布于侵入体厚度最大的区域，且侵入体厚度、橄榄石晶体粒径与晶体含量三者间存在良好相关性。岩相的非均质性可归因于侵入体厚度变化与构造复杂性。岩相的几何形态与分布特征指示二者为岩浆同步侵入，并受两相之间的基础流变学差异驱动发生流动分异。我们推测，低黏度的贫橄榄石岩浆可作为高黏度富橄榄石岩浆的润滑剂。这类低黏度、贫晶体的岩浆的存在，或许可以解释为何富含晶体（体积占比＞60%）的金伯利岩岩浆能够在金伯利岩喷发过程中抵达地表。此外，本次研究还记录到缺乏晶体沉积分异现象，并观察到一种罕见的拉长状橄榄石晶体近垂直组构，该现象可通过侵入过程中及侵入后由快速脱气诱导的岩浆淬火结晶作用予以解释。