The petrology and geochemistry of the Bigspruce Lake syenite complex N.W.T. Canada

The aeromagnetic anomaly high at Bigspruce lake N.W.T. is located over an alkaline and carbonatite complex. Approximately two-thirds of this complex is submerged, but sufficient rock is exposed on the shores and islets to reconstruct the original intrusive form and sequence.
The rock-types are distributed in five geographic and petrographic centres showing a successive southerly migration. These are -
Centre 1
saturated diorite, gabbro, olivine-magnetite-pyroxenite.
Centre 2
saturated and oversaturated syenites.
Centre 3
undersaturated foyaites and ijolites.
Centre 4
saturated (high potassic) shonkinites, minettes, biotite-pyroxenites.
Centre 5
silico-carbonatite and carbonatite, sovite and dolomite carbonatite.
Centre 1 is located to the northeast of the main lake. It is composed of an outer diorite, and a later inner gabbro, layered intrusion, surrounded by arcs of olivine-magnetite-pyroxenite, and syenite intruded along a marginal ring fracture.
Both the diorite and the gabbro possess broad wall-rock zones in rapid gradational contact with central laminated differentiates. The diorite lacks significant cryptic variation, but the gabbro is divided into anorthositic and gabbroic series showing the crystal accumulation of olivine, apatite and calcic plagioclase towards the base of the intrusion. The operation of continuous convection currents is evidenced by the radial dip of the lamination and the winnowing of the mafic minerals. Stronger intermittant currents in the gabbro are evidenced by radial trough banding, and rhythmic banking.
The olivine-magnetite-pyroxenites are composed of olivine Fo66–68, clinopyroxene Ca44.6Mg55.8Fe9.6, magnetite and ilmenite in the approximate modal proportions 19 : 59 : 17 : 5. Minor barkevikite, biotite and apatite occur as an alkaline matrix. Quenching to magnetite-pyroxenite at the margins, remobilization of the contact granite or gabbro, and comparison of the modal composition with the field boundaries in the synthetic system ol-cpx-iron oxide demonstrate that these rocks crystallized from an ultramafic magma containing a small percentage of olivine and pyroxene phenocrysts. A temperature of intrusion of 1100–1200° is deduced. Although no descriptions of comparable rocks have been found these are regarded as the alkaline, ilmenite-rich, equivalents of the Alaskan-type zoned ultramafic intrusions.
Centre 2 spans the northern half of the lake. It contains two concentric ring dykes of quartz syenite to the south, and two quartz free syenites around the margin of centre 1. The intrusion of these syenites followed an episode of transcurrent faulting which bisected centre 1.
Mineralogically and in geochemistry the syenite series forms a continuation of the gabbro-diorite series, and spans the transitions pyroxene-alkali amphibole, and plagioclase → alkali feldspar (An52 → An28 → Or38). A few late syenite dykes are peralkaline, and include aegirine bearing comendites.
Centre 3 occupies the southern half of the lake. It is dominated by a 3.4 × 2.1km body of pegmatitic foyaite rimmed by chilled vertically laminated foyaite, and 10–100m of zoned fenite, A finer grained more mafic foyaite covers Bigspruce and Littlespruce islands. Both foyaites are composed of cryptoperthite Or66, nepheline Ne69, and aegirine and show a differentiation into feldspathic, nephelinic, and ijolitic horizons with low angle contacts, and a concentric distribution. This form is interpreted in terms of the preferential crystal settling of feldspar plates in the absence of convection currents. Both the foyaites show a chill composition lying within the low temperature "foyaite sink" in the system Qtz-Ne-Ks. Breccia pipes of sodic ijolite transitional into cancrinite foyaite are marginal to the carbonatite pegmatite focus of Bigspruce island. A possible origin through breccia-fluid exchange reactions involving the loss of K2O and SiO2 in the presence of abundant H2O and Na2CO3 is suggested.
High potassic rocks occur predominantly within a subsiduary complex 5km to the southeast of the lake, but identical types occur in close association with the carbonatites. They are composed of pyroxene, olivine, biotite, oxide, apatite and cryptoperthite Or50, with a maximum feldspar content of circa 50%. The shonkinite forms dykes and breccia pipes of similar form and composition to those of the Navajo area, Arizona. The biotite-pyroxenites also form pipes, are transitional into shonkinite through the loss of feldspar, and are surrounded by zoned fenites. They are closely comparable to the stock of Libby, Montana, and to the "O.B.P." nodules found in the East African potassic provinces. Comparisons of the volume and composition of the fenite, pyroxenite and shonkinite are used to show that the biotite-pyroxenite may have originated from the shonkinite through the loss of normative feldspar to the wall rock in a fluoride-rich aqueous fluid phase.
Both the foyaite and pyroxenite fenites resemble those of Fen, Alno and Chishanya, with concentric zones of aegirine-perthite-syenite, arfvedsonite-albite-quartz-syenite and biotite microgranite. The principal geochemical changes are the transfer of silica into the microgranite, and the transfer of feldspar with similar K : Na, ratio to the foyaite or "shonkinite" magma into the syenites.
The carbonatites of centre 5 outcrop within two cone sheet and radial dyke foci of dolomitic and calcitic composition respectively. The dolomite focus lies on the east side of the lake within a fault bound structure. It consists of a central breccia plug of biotite-ankerite carbonatite surrounded by inwardly dipping sheets of dolomite defining a focus ≱ 300m deep. The silicate content of the rock and iron content of the dolomite decrease away from the centre. There is a complete transition within these rocks from calcite bearing biotite-peridotite to carbonatite. On the basis of these replacement reactions a temperature of intrusion of 500–560°C is deduced.
The sovite focus lies on the west side of Bigspruce island. It shows a well-defined radial dyke and cone-sheet distribution of silicate-calcite pegmatites associated with Fe, Ti, Ca, Nb, R.E., CO2, H2O, Cl metasomatism. Multiple fracturing and intrusion within these pegmatites shows a sequence of:- Aegirine + magnetite + calcite; Biotite + magnetite + calcite + pyrochlore; Biotite + calcite + bastnasite. Biotite carbonatite, fluorite carbonatite, and bastnasite carbonatite occur as peripheral satellite bodies.
Mineral textures in the carbonatites give evidence of chilling, quenching and the crystal settling of apatite within the magmas. The close similarity of the trace element content and volatile content support the derivation of the carbonatites from the high potassic parent magma. The Sr87/86 ratio of 0.7020 from the calcite and dolomite supports a mantle derivation for this parent.
Within the exposed rock types three main differentiation trends may be distinguished. These are -
Alkali gabbro – alkali diorite – syenite – quartz syenite-comendite.
Alkali gabbro – rhomb syenite – sanidine syenite – foyaite.
Biotite peridotite – shonkinite – biotite pyroxenite.
The geochemical series 1 and 2, as illustrated by the Ca : Na : K and Mg : Fe : Alk diagrams, more closely resemble those of the Gardar province than those of the Oslo or Monteregian provinces but show a lesser degree of iron enrichment, and a greater enrichment in soda relative to Ca and K. These differences are thought to be due to the early disappearance of olivine as a fractionating phase, and to the dominating influence of feldspar fractionation in both series.
Although both the saturated and undersaturated trend may ultimately be derived from the same parent and the composition of the magnetite-pyroxenite shows that it is related, the potassic series is believed to be derived from a distinct high potassic peridotitic parent of mantle origin.
The differentiation of these magmas is believed to have taken place simultaneously in a series of independent magma chambers located within the prism of shattered rock at the intersection of a system of N-S and E-W transcurrent faults. The intrusion of the derived magmas was closely related to movement on the N-S fault system.
On a regional scale the complex is located at the centre of a dome of granitic rocks close to the border between the Slave and Bear provinces. The age of the complex at 1820 m.y. is similar to the age of uplift of the neighbouring Snare orogenic belt. At the present time these rocks form the oldest known alkali-carbonatite centre.