Alteration of Mn-Ilmenite in Horton Area of Lena WestSoapbox Wednesday, November 21, 2018 - 16:30 to 19:00 Multiplex Gym (DND)
Mn-ilmenite was recognized as a kimberlite indicator mineral (KIM) in the Lena West diamond region of the Northwest Territories by Darnley Bay, Talmora and Sanatana. It includes compositions that match those found as inclusions in type IIa diamonds from Brazil and Venezuela that formed in the lower mantle. The recent determination that large, high value type IIa diamonds like the Cullinan, Koh-I-Nor, etc. also formed in the lower mantle increases the importance of Mn-ilmenites not only as a KIM resistant to tropical weathering but as a possible indicator of large high value stones. The Mn-ilmenite alteration products, pseudorutile (Fe2Ti3O9) and ferropseudobrookite (FeTi2O5) may also be used as KIMs and provide useful additional information.
Mn-ilmenites found as inclusions in diamonds range from 51 wt% TiO2 (total wt% 100) to 56 wt% TiO2 (total wt% 95). The shortfall in wt% of the high TiO2 grains was ascribed by Kaminsky and Belasouva (2009) to some element not included in the analysis. The shortfall may also occur when some ferric iron is calculated as ferrous iron?
As most Lena West Mn-ilmenite analyses have high totals those with totals less than 96 wt% have been considered an alteration product (“pseudorutile”). “Pseudorutile” is produced by the oxidation of FeO in ilmenite to Fe2O3 which results in an apparent loss of total weight percent when Fe is calculated as ferrous iron. A range of values approximating “ferropseudobrookite” with totals close to 100 wt% is another alteration product of ilmenite with a loss of iron but without its oxidation to ferric iron.
The Horton area consists of a cluster of magnetic anomalies averaging ~ 200m diameter east of a very large magnetic anomaly beneath Seahorse Lake. The Seahorse anomaly is at the focus of a train of kimberlite pathfinder elements coincident with a NNW trending KIM train characterised by Mn-ilmenite, picro-ilmenite and chromite. A parallel train of similar KIMs is focused on the cluster of smaller anomalies to the east.
“Pseudorutile” is found over the cluster of anomalies that lie within the Horton River drainage and in the area north towards Darnley Bay. It was especially abundant with some unaltered Mn-ilmenite grains in the cuttings of a Packsack drill hole that penetrated a few feet of rusty coloured clay coincident with one of the anomalies. It does not appear to travel far. “Ferropseudobrookite” is found mostly west of the Horton River drainage, about 100 kilometers down-ice in the trains coming off the Seahorse anomaly and the cluster of smaller anomalies respectively. It represents the weathering of Mn-ilmenite in the upper part of anomalies that was carried furthest by glaciation indicating a resistance to mechanical wear.
Mn-ilmenite is a useful KIM in areas of tropical weathering and is also an indicator of rare large high value diamonds. The distribution of pseudorutile and ferropseudobrookite suggests that the initial alteration of Mn-ilmenite is to pseudorutile and then ferropseudobrookite. The presence of one or the other is therefore a measure of the distance to the source.