Indicator mineralogy of kimberlite boulders and sand samples from the Lac Baby and Sharp Lake eskers, Lake Timiskaming kimberlite field, western Quebec and northeastern Ontario

Six kimberlite boulders and 20 sand samples were collected from the Sharp Lake and Lac Baby eskers in the Lake Timiskaming kimberlite field of northeastern Ontario and northwestern Quebec. The boulders were processed to recover heavy mineral concentrates fromwhich grains of Mg-ilmenite, chromite, garnet, clinopyroxene, olivine, and perovskite were picked, counted, and analyzed by electron microprobe. Of the three boulders from the Sharp Lake esker, two boulders (SD-029, SD-030) were heavily altered and did not contain many kimberlite indicator mineralgrains. A third boulder from this locality (SD-040) yielded abundant indicator minerals dominated by Mg-ilmenite, Cr-diopside, and garnet. The SD-040 boulder contains diverse garnet and diopside populations, e.g. from lherzolite, sheared lherzolitic, wehrlite, websterite/pyroxenite, eclogite, andmegacryst parageneses. Of particular interest are 22 grains of orange garnet recovered from this small boulder (0.6 kg sample weight), which are of potentially diamondiferous Group I eclogite paragenesis. Based on Mg-ilmenite compositions of this boulder, its source is not among the knownkimberlites in the Lake Timiskaming field.Three boulders from the Lac Baby esker contain abundant fresh olivine. Two of the three boulders (SD-041 and SD-043) have very similar indicator mineral abundance and composition, which closely resemble garnet and Mg-ilmenite compositions from the Guigues kimberlite 10 km upstream. They contain highproportions of Mg-ilmenite and garnet, but very little Cr-diopside and chromite. Eclogite garnet grains occur in these two boulders, several of which show the compositional characteristics of potentially diamondiferous Group I eclogite garnets. A third boulder (SD-042) contains only peridotite(wehrlite, lherzolite, and harzburgite) garnets, as well as chromite, Cr-diopside, and comparatively little Mg-ilmenite. Although the garnet compositions show similarities to those of the Notre-Dame-du Nord kimberlites, the Mg-ilmenite compositions are not similar. Its indicator mineral compositionsare also different from those of the other two boulders and those of the recently discovered SC118 kimberlite northeast of Lake Timiskaming, suggesting this boulder is derived from an unknown kimberlite.The sand samples were processed to recover indicator minerals for visual counting only. Samples from the Sharp Lake pit contain <300 indicator mineral grains/10 kg, while samples from five sites along the Lac Baby esker contain >10,000 grains/10 kg. The high indicator mineralabundance combined with the presence of numerous kimberlite fragments in the Lac Baby esker suggests that subglacial meltwater that deposited the Lac Baby esker directly eroded a kimberlite. The much lower indicator mineral grain contents at Sharp Lake suggest that the kimberlite boulders found inthis pit are eroded from till, and thus have undergone at least two phases of transport, glacial transport to the southeast or south followed by glaciofluvial transport southward.