SoS RARE Namibia field work, 2015
Between the 25th September and the 14th October, 2015, myself, Ed Loye, Delia Cangelosi and Frances Wall undertook fieldwork sampling carbonatites and alkaline rocks in northern Namibia. We were specifically looking at the Lofdal alkaline and carbonatite complex, supported by our industry partners Namibia Rare Earths. However, we also took the opportunity, while in-country, to visit and sample other carbonatites and alkaline rocks in the region. These included the monazite-rich Eureka deposit, the Amis alkaline complex, the Okorusu fluorite mine and the Kalkfeld carbonatite.
Our objectives, as part of work package 1, were to understand more about the chemistry and temperature of REE-bearing fluids. Accordingly, the deposits visited on the trip were selected to sample occurrences where REE minerals crystallise from different rock types and, therefore, a range of potentially different fluids. The localities included light REE-enriched carbonatites, REE-bearing peralkaline granites and the unique heavy REE carbonatite deposit at Lofdal. At Lofdal, our work was to try and understand how REE-bearing fluids fractionated to form such a HREE-enriched deposit.
Ed flew out a day early and arranged the vehicle, which was kindly provided by Namibia Rare Earths, as well as visiting colleagues at the geological survey. Delia and I arrived the following day and the three of us proceeded towards the Eureka carbonatite. Here we were following up on some preliminary research where we had found fluid inclusions in large (up to 10 cm) monazite grains. We returned to investigate a wider area of the deposit, and to obtain a better understanding of the extent of the deposit at the surface. There is not much exposure here (Figure 1), but historic trenching from the 1980s means that the different carbonatites present intermittently crop out.
Following Eureka, we proceeded north towards the Amis complex. This complex occurs as a series of sills on the western flank of the magnificent Brandberg intrusion (Figure 2). Previous work at this locally had identified that the volatile and REE content increases towards the top of the sills, ultimately culminating in remobilisation of the REE in bastnasite and fluorite veins (Schmidt et al., 2002). Our objective was to sample these veins, giving us an example of REE mobility in a fluid derived from a peralkaline granite. After some scampering around on the first day, we found some candidate samples and further scampering on the second day found some more at different parts of the intrusion. These will be sectioned in the lab to check if our sampling was correct. Also sampled from Amis were late aplite dykes which contained pyrochlore, lamprophyllite (or astrophyllite), arfvedsonite and (probably) a plethora of REE minerals. Again, these will have to be sectioned to find out more!
Our next locality was the Okorusu fluorite mine. While the mine is currently on care and maintenance, a crew was available to meet us and show us around the pit – for which we are very grateful. Their help meant that we were able to spend a large amount of time to understand the field relations, and to sample the fluorite which formed through reaction with both carbonatite and marble. Delia extensively sampled here, to understand the redistribution of the REE in low temperature carbonatite-derived environments.
Frances joined us for the last part of the trip and, after a brief introduction to Lofdal, Ed returned to Europe. We enjoyed a week at Lofdal, looking at core from areas 4, 6 and 8, as well as being guided around the field by Boni, the resident NRE geologist. Fluid inclusions remain enigmatic at Lofdal, but we were able to obtain fluorite from the various core sections, as well as some well-spotted fluorite in outcrop. We also sampled some quartz from the notorious Thorium Hill which may be suitable for laser ablation, if we can find inclusions. Time was well spent looking at field relations, following up on previous fieldwork carried out by Frances. The breccia quarry and 'composite dyke' (Figure 3), were particularly interesting for establishing the geological succession.
We finished up the trip by visiting colleagues at the geological survey in Windhoek and sending off >50kg of rock back to the UK. For SoS RARE colleagues, these samples will shortly be on the SharePoint sample list.
Sam Broom-Fendley 29 October 2015
