The gold mineralisation at Adumbi is associated with up to 130 m of interbedded banded ironstone, quartz carbonate and chlorite schist
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Gold miner Loncor Resources last December announced that the second core hole of a 7000 m drilling programme at its Adumbi deposit had intersected gold at its 84.6%-owned Imbo Project, in the eastern part of the Ngayu greenstone belt, in the Democratic Republic of Congo (DRC).
The second core hole has significant gold intersections of 33.30 m grading 3.25 g/t gold (including 5.7 m grading 7.00 g/t gold and 9.07 m grading 5.11 g/t gold) and 10.45 m grading 3.88 g/t.
Borehole LADD003 had an inclination of –57º and azimuth of 220º at the start of the hole and regular measurements of inclination and azimuth were taken at 30 m intervals.
All core was orientated, and it is estimated that the true widths of the mineralised sections are 80% of the intersected width. All intercepted grades are uncut. Borehole LADD003 was drilled in place of LADD002, which was stopped after deviating from its intended target depth.
At the time of the announcement, Loncor president Peter Cowley commented: “We are very encouraged by the results of the first two core holes of our 7 000 m drilling programme at Adumbi, where we are targeting a significant increase of resources, both within and below the openpit shell.”
“These two infill holes were drilled within the openpit shell where there is already an inferred resource of 2.19-million ounces (28.97-million tonnes grading 2.35 g/t gold). These results will further increase the inferred resource. Deeper drilling is now focused on the downdip mineralisation below the pit shell.”
In addition to core holes LADD001 and LADD003, the deeper hole LADD004, which was targeting mineralisation 140 m below previous borehole SDD53 (which intersected 23.5 m grading 6.08 g/t gold and was drilled in 2017), was completed and cores have been submitted for assay.
LADD007 and LADD006 are currently being drilled to intercept the downdip mineralised zone below the openpit shell, and at shallower depth towards the north-west base of the pit shell respectively.
The gold mineralisation at Adumbi i s associated with up to 130 m of interbedded banded ironstone, quartz carbonate and chlorite schist, with higher grade sections being found in a strongly altered siliceous unit termed ‘Replaced Rock’, where structural deformation and alteration has completely destroyed the primary host lithological fabric.
Disseminated sulphide assemblages include pyrite, pyrrhotite and arsenopyrite, which can attain up to 20% of the total rock in places.
The objective of the current drilling programme is to outline additional mineral resources to the current inferred mineral resource of 2.5-million ounces of gold on Loncor’s Imbo project, which contains the Adumbi, Kitenge and Manzako deposits (inferred mineral resource of 30.65-million tonnes grading 2.54 g/t gold).
Quality Control and Quality Assurance
Drill cores for assaying were taken at a maximum of one metre intervals and were cut with a diamond saw, with one-half of the core placed in sealed bags by company geologists and sent to the company’s on-site sample preparation facility.
The core samples were then crushed down to 80% passing –2 mm, with half of the sample (up to 1.5 kg) pulverised down to 90% passing 76 μm.
About 150 g of the pulverised sample was then sent out to global verification and testing company SGS’s laboratory in Mwanza, Tanzania. Gold analyses were carried out on 50 g aliquots by fire assay.
In addition, check assays were also carried out by the screen fire assay method to verify high-grade sample assays obtained, initially, by fire assay.
As part of Loncor’s quality assurance and quality control procedures, internationally recognised standards, blanks and duplicates were inserted into the sample batches prior to submitting material to the SGS laboratory.