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Akelikongo is Sipa's flagship discovery in Uganda. During 2015 and 2016, geochemistry, drilling and geophysics defined a sizeable body of nickel-copper sulphide mineralisation which has strong similarities to other globally significant, intrusive-related magmatic nickel copper sulphide systems such as Nova-Bollinger (14Mt @ 2.3% Ni and 0.9% Cu), Voisey's Bay (141Mt @ 1.6% Ni and 0.8% Cu) and Raglan (30Mt @ 3.4% Ni and 0.9% Cu).
The key elements of these systems are a plunging magma channel or conduit with a high magma fluid flux which then interacts with the country rock during emplacement to form a mixing zone, which triggers sulphur saturation and the formation of nickel-copper sulphide mineralisation.
At Akelikongo, the conduit essentially sub-crops with an intense nickel and copper anomaly in residual soil. The infill soil samples have now confirmed the circular pipe like geometry of the shallowly plunging intrusive complex. This anomaly has a surface footprint of about 300m by 300m which has been traced by drilling for up to 1km and remains open in all directions (Figure 1).
Figure 1: Akelikongo Schematic Long Section.
Drilling shows it then continues down plunge for at least 500m and is interpreted to extend more than a kilometre where holes to the north (AKD003 and 12) have detected alteration due to a probable nickel system in form of anomalous nickel copper and platinum group element (PGE) geochemistry in the felsic paragneisses.
The discovery at Akelikongo West of a second mineralised intrusive body with a different geological and geochemical character, demonstrates the Akelikongo field contains multiple mineralised intrusive systems indicative of a mineral field with a complex history enhancing its economic potential.
Data integration indicates the magmatic disseminated sulphides at Akelikongo show a linear nickel tenor trend up to 12% with massive sulphides averaging 4-6%. These nickel tenors are encouraging and an early stage indicator that any larger accumulations of sulphide mineralisation are likely to be economic.
Results of the recent drilling during 2016 have shown that the previously sparsely drilled western footwall to the system has a prospective basal position and has formed an embayment where sulphides have accumulated within a much larger up to 110m wide disseminated zone.
Drilling in late 2016 resulted in the widest disseminated and thickest semi-massive sulphide zones intersected to date with results such as:Matrix to semi-massive zones:
Continuous disseminated and semi-massive intercepts:
Recent down hole EM surveys conducted earlier in 2017 show a number of untested off hole targets and in particular show a second trend of massive to semi-massive sulphide mineralisation about 200m to the east of the western trend which outcrops and has been drilled over a strike length of 500m. Figure 2 is a section across the deposit and shows the eastern mineralised zone as intersected in hole AKD014 and detected off hole in EM.
Figure 2: Cross Section Akelikongo
Figure 3 shows the processed gravity and an interpretation of the more massive mineralised zones showing up as linear trends. Note trend AB is untested down plunge and D is completely untested.
Figure 3: Processed gravity with linear gravity ridges A-D which correlate to mineralisation.
History and Exploration Program to date
In June 2014 the discovery of disseminated nickel copper sulphide mineralisation was made during Sipa’s first RAB program. The initial RAB program intersected a number of mineralized intercepts from surface like:
Figure 4: Akelikongo drilling plan at June 2014 with RAB results. Striped area highlights NiCuS mineralised zone.
An attempt to apply geophysics at an early stage was made to attempt to understand the geology and mineralisation in 3 dimensions.
To this end a number of fixed loop electro-magnetic (EM) surveys were undertaken over the Akelikongo area. The surveys identified some strong late time EM conductors which were drilled in early 2015. Whilst these surveys did not directly detect mineralisation they highlighted some zones of abundant pyrrhotite in upper amphibolite to granulite facies para-gneisses which are spatially related to the known mineralisation in ultramafic intrusive rocks.
Figure 5: Plan of EM channel 25. Showing EM conductor west of Akelikongo and 1km to the north-west.
The disseminated mineralisation is occurring immediately adjacent to the mineralisation and as such cannot be resolved in the fixed loop data (Figure 5).
Down Hole Electro Magnetics (DHEM) is routinely conducted on drillholes. Modelling of the data shows the calculated conductive sources occur at a similar depth to known mineralisation but in some cases the modelled anomalies are due to off hole positions within the ultramafic unit. The conductance of the modelled plates may indicate the presence of conductive sulphides with values that are not dissimilar to DHEM results at other known nickel sulphide deposits.
Following the EM in December 2014 an initial diamond program was planned and conducted in February March 2015. The program was designed to test the EM conductors and under the newly discovered mineralisation identified in the reconnaissance RAB. Diamond holes AKD001 and AKD003 intersected strongly disseminated pyrrhotite in felsic gneiss. The diamond core drill holes under the RAB discoveryand soil anomaly returned positive results from AKD002 and AKD004 confirming the presence of a nickel copper sulphide bearing ultramafic sequence. A high MgO ultramafic sequence was intersected around 50 to 80m wide with the lower 30m containing disseminated nickel-copper-sulphides generally in the order of >0.3% Ni.
A small area was ground-gravity surveyed around Akelikongo. The gravity surveying (Figure 5) indicates there are distinctive gravity highs (red colour) associated with Akelikongo ultramafic intrusion. Based on the gravity, the intrusion is interpreted to extend to the north a further 400-500m beyond the known extent as identified by drilling to date. There are also a significant number of other gravity highs within the wider survey area that may also be due to ultramafic intrusive bodies.
Figure 6: Drill location plan on gravity.
Drill intersections of massive nickel sulphide occur within a zone of ultramafic near the contact with the footwall granite gneiss. This position corresponds to a sharp gravity gradient in the data that can be mapped out to the north of the presently known nickel sulphide intersections. Drilling campaigns were conducted in March and September 2015, and April and October 2016.
Drilling shows the disseminated magmatic nickel and copper sulphides now extend across the width of the intrusive complex as defined by gravity modelling and confirm the pipelike shape. They also show that the mineralisation extends continuously over more than five hundred metres and is plunging at around 20-25 degrees to the north-west.
Figure 6 also shows the drillplan of all holes to date, with Table 1 (Click here for Table 1 - opens in new window) showing a summary of all drill intercepts >0.2% Ni obtained since inception in 2015 to date (Dec 2016).
The October 2016 phase of drilling follows the highly successful RC program completed by Sipa in May 2016. The results from the May program included the highest grade and widest matrix to semi-massive intercepts drilled at Akelikongo to date, including a significant semi-massive sulphide intercept of 10m grading 1% Ni, 0.22% Cu and 0.05% Co from 63m down-hole in the footwall of the disseminated mineralisation in hole AKC004.
This program also returned some of the widest disseminated intercepts obtained to date from the project, including 119m @ 0.4% Ni, 0.12% Cu and 0.02% Co from surface in disseminated mineralisation in hole AKC005. The drilling program, which was designed to further delineate zones of massive and disseminated sulphides intersected earlier in 2016, consisted of 9 RC holes, 6 RC holes with diamond tails, and 1 diamond hole drilled from surface for a total of ~1,800m of drilling. 12 holes were drilled to test the Akelikongo Ultramafic Complex with the remaining four holes testing additional targets in the immediate Akelikongo area.
The footwall matrix to semi-massive zones, which lie at the footwall of the wide and shallow zones of disseminated sulphides, are interpreted to represent the high-grade basal position. This sequence has been intersected in hole AKD004, in the south, continues up to AKC001, approximately 250m to the north, and remains open down-plunge to the north-west.
The basal position in other, better-understood nickel deposits is where massive sulphides (which have higher grades of nickel and copper) originally pooled during the initial formation of the deposit. AKD017 and AKCD006 intersected massive and disseminated sulphides with some of the best nickel and copper assays of up to 2.5% Ni and 2.4% Cu returned to date.
The results support the Company's geological model for the Akelikongo discovery with the recent drilling providing a much better indication of the orientation and controls on the better mineralised basal position – which appears to be strengthening and potentially thickening down-plunge – and the thick overlying zone of disseminated mineralization.
This thicker and better mineralised basal zone can now be demonstrated to plunge shallowly to the north west and is continuous where drilled from AKD004 in the south for well over 300m to the north, where it is thickening and demonstrates the presence of higher copper values in association with the strong matrix textured zones, as seen in hole AKD017 from 213.1m to 221.9m down-hole.
Figure 7: Close-up of matrix textured sulphides in NQ core 218m AKD017 showing chalcopyrite (Cu), pyrrhotite (Fe) and pentlandite (Ni).
Comment from Managing Director Lynda Burnett … “The drilling has extended the high-grade matrix to semi-massive sulphide zone at least 100m down-plunge and, more importantly, has given us a much clearer understanding of the orientation and controls on this zone in relation to the overall Ultramafic Complex and the thick disseminated zone which sits on top of it."
“The presence of strong matrix sulphide textures as seen in the drill core photos suggests that we are close to a bigger magmatic sulphide pool within this dynamic sulphide conduit system. This is an exciting development which validates our exploration approach this year which has been to follow the better mineralised basal position down-plunge to the north.”
“As indicated recently, the presence of economic grades of nickel and copper within a system of this scale and fertility is an important development which elevates and strengthens the potential of this system. We are very much looking forward to the next stage of evaluation, with down-hole EM surveys commencing early in 2017 to help refine planning for the next stage of exploration.”
During June 2015, reconnaissance RAB drilling intersected nickel and copper sulphides 800m south west of Akelikongo. Following pXRF indications that Nickel and Copper was significantly enriched, two diamond holes were drilled to test the depth extend of the mineralisation. The Regional Exploration tab describes the discovery history of this prospect.
Diamond holes AKD008 and AKD009 drilled in July, were targeted to test shallow RAB intersections of coarse disseminated sulphides at Akelikongo West from LMR 137 and 138 (ASX 24 August 2015). The diamond holes were drilled 60m apart and oriented -60 degrees to the north to test apparently shallowly south dipping nickel copper and sulphide mineralisation.
AKD008 intersected tonalite from 0 to 46m followed by coarse grained pyroxenite from 46m to 156m. Nickel and copper sulphide mineralisation occurs as coarse grained disseminated zones up to 15% total sulphide with sulphides identified as pyrrhotite, chalcopyrite, and pentlandite. Where the tonalite is close to the footwall and hanging wall pyroxenitic intrusion, it exhibits partial melting textures. An 8m zone from 47m to 55m assayed 0.2% Ni.
AKD009 collared in tonalite and intersected mineralised ultramafic pyroxenite at 38.3m. A 47.3m zone with strong 15% to 50% very coarse grained disseminated to matrix nickel and copper sulphide mineralisation occurs from 38.3m to 85.6m, with some minor zones of massive to semi massive sulphide. At the upper contact at 38.3m there is a 1.6m zone of semi-massive sulphides (pyrrhotite, chalcopyrite and pentlandite).The best mineralised zone assayed 41m at 0.5% Ni and 0.1% Cu from 38m.
AKD010 and AKD011, were drilled in September at Akelikongo West for a total of 201.7m. These holes did not intersect the target mineralised intrusion. The recent data integration exercise provides a probable explanation as to why these holes missed the target. The current interpretation is that the body dips to the south east.
Figure 8 shows a geological section of the Akelikongo West drilling and nickel and copper assay results.
Figure 8: Section through Akelikongo West Nickel and Copper Sulphide system.
AKD012 drilled in September 2015, one kilometre to the northwest of Akelikongo under AKD003 (ASX Release 25 March 2015).
The results from AKD012 and further new assay intervals collected from AKD003, now confirm nickel copper and PGE anomalism in migmatitic gneisses. These rocks appear similar to those drilled in the footwall of the Akelikongo Ultramafic disseminated mineralisation. In AKD012, from 151m to 173m, assays average 174ppm Ni and 95ppm Cu. The AKD012 core assay dataset has a 92% correlation between Ni and Cu and an 83% correlation between Cu, Ni and Pd. The most anomalous >100ppm Cu and >200ppm Ni assays correlate with 3PGE (Au+Pt+Pd) assays of between 20ppb and 48ppb. The strong correlation of nickel copper and PGEs is typical of nickel sulphide systems.
It was the nickel copper and PGE anomalism from selected samples of variably migmatised paragneiss assayed in AKD003, plus some untested late time EM anomalies which led Sipa to drill hole AKD012 in this area in September 2015.
The geochemistry results now point to the strong possibility that these holes have intersected migmatitic gneisses which are proximal to the Akelikongo chonolith conduit. These holes AKD003 and 012 are over 500 meters northwest of AKD007 (the most northern mineralised drill hole).
Moving loop time domain EM surveys were conducted over Akelikongo and Akelikongo West during October and early November 2015. The surveys, have highlighted a number of conductors detectable from mid to late time and detected conductors within the known modelled ultramafic chonolith both at Akelikongo and Akelikongo West and also outside.
Down hole EM detected a number of off hole conductors in addition to the known mineralisation in holes AKD008, 009, 006 and 14. AKD005 and 007 were blocked and therefore not surveyed.
Integrated modelling of these data with known geology assays and petro physical measurements of the core such as specific gravity, magnetic susceptibility and conductivity is continuing. The targeting of drill holes AKD015 and 16 was guided by the moving loop and down hole EM surveys in conjunction with the ground gravity and down hole gravity modelling as shown in Figure 9.
Figure 9: Drillhole locations on late channel EM Image – Akelikongo area.
Conclusion from detailed tenor analyses of various mineralisation styles and variations with depth show the magmatic disseminated sulphides exhibit a linear nickel tenor trend up to 8% with massive sulphides averaging 6.5%. These nickel tenors are encouraging and an early stage indicator that possible ore concentrates could be economic. Further the variability of Ni Tenors in massive sulphides indicates evidence of multiple mineralised magma pulses, an important feature in most economic Ni deposits.