High-grade silver-lead-zinc mineralisation formed within the Neoproterozoic Tapley Hill Formation (THF) was historically mined 100 years ago at Yerelina in the northern Flinders Ranges. Despite the existence of numerous poorly documented small-scale historical surface and underground workings, and outcropping sulphide mineralisation that can be mapped over 100s to 1000s of metres extent in repetitious gossanous vein sets exposed over a very broad area, no systematic modern exploration has been undertaken and this area has never been drill tested. The Adelaide Geosyncline has long been considered prospective for sedimentary basin - hosted base metal mineralisation styles (e.g. MVT or sedex), but no definitive examples of these deposits have yet been identified therein. Core Exploration (CXO) believes that the mineralisation at Yerelina, which occurs in thick basinal sediments located just to the west of the Mount Painter Inlier, may represent a surface exposure of possibly part of one of these large regionally active mineralisation styles. Recent petrological investigations of samples of the outcropping Yerelina mineralisation infer that there are lithological, textural, mineralogical and paragenetic similarities to the Coeur d’Alene (Idaho) high-silver network vein mineralisation style which, if substantiated, would introduce a potential new economic mineralisation model for many of the metasedimentary rocks occurring in South Australia. CXO acquired EL 5015 Yerelina in September 2012, and has progressively advanced this project through its conduct of reviews of historical data, geochemical analyses and petrological studies of surface samples, and the acquisition of high-resolution airborne geophysical surveys. The company's field observations have revealed that the mineralised gossans and historical workings are associated with evidence of the former passage of multiple low temperature hydrothermal fluid pulses, rich in lead, zinc and silver, which took place within brittle fault breccia zones that are mm to 20 m wide and up to 2 km long. CXO believed that the logical next step for managing the project would be via drill testing, ideally with orientated diamond core, to explore underneath the known outcropping mineralisation to obtain subsurface information that could give a better understanding of the area's mineralisation potential and geological controls. Reactive calcareous shales of the THF that are tightly folded on a regional scale dominate the geology in the tenement area. The base metal prospects identified to date could be described as occurring on an east-west striking limb of a large-scale fold within the THF. The identified north – south trend of the local scale gossans is perpendicular to the regional geological trend controlled by the axial planar orientation of the Gladstone Anticline. On 13/4/2015, as part of South Australia’s PACE Initiative Year 8 Discovery Drilling programme of collaborative drilling projects agreed to between the government (DSD) and industry, CXO was granted subsidy funding up to $75,000 to defray part of the cost of performing a maiden core drilling campaign at Yerelina. The company's successful work bid application was designated as project DPY8-21. It was implemented in September- October 2015, when 6 inclined HQ2 and NQ2 wholly diamond cored holes were drilled for a total penetration of 1043.2 m. Three of the holes were designed to test the Great Gladstone mine prospect, where high-grade mineralisation outcrops for over 2 km strike length. The remaining three holes tested other occurrences which lie on parallel, north-south orientated structures, with one hole put in at the East Great Northern mine located 5 km east of Great Gladstone, and two drilled into the Big Hill gossan, with one hole being completed to target depth and the other being terminated prematurely. The 2015 PACE drilling at Yerelina was undertaken with the following objectives: • to evaluate of the depth-potential and grade-continuity of the known outcropping Ag-Pb-Zn mineralisation; • to characterise and map structural controls and alteration systems; • to enhance present understanding of the geology of the THF and its capacity to host an economic ore system; • to acquire quality core samples for undergoing drilling data value-additive studies including hyperspectral scans, petrological examinations, petrophysical property determinations, and isotopic and geochemical analyses. Hole YRDH001 was designed to test under the East Great Northern workings, where inspection of abundant spoils of mined rock indicate complex carbonate and base metal sulphide veins are developed in strongly brecciated THF slate having a pronounced north-south strike. This hole did not intersect a well-developed breccia in the predicted target interval (100-130 m hole depth), but the slate cored here does contain common thin carbonate-quartz veining with some visible 1-2 mm thick stringers of sphalerite and minor galena. No alteration of the slate is evident, aside from brittle fractures infilled by polyphase carbonate cements, and the amount of (inferred later stage) base metal mineralisation is generally scant. The best drill core assay results obtained were 6 m @ 0.2% Pb+Zn from 92-98 m hole depth, and 2 m @ 0.3% Pb+Zn from 112-114 m. Although the hole could have been extended beyond its eventual total depth of 150.6 m to test for any eastwards dip to the mineralisation, the last 20 m drilled were not encouraging, and the proposition was not supported by structural logging of the drill core, which indicated a sub-vertical dip to the mine lode. Hole YRDH002 was the first of three PACE 2015 drillholes to test along the strike of the Great Gladstone mine line of lode (LOL), that had been extensively worked prior to 1914. This test was the most southerly located of the three, and was directed towards the south-east to intersect any depth extensions to the vertically dipping LOL exposed on the southern hill, close to the old mine adit. Initially designed as a 200 m deep test, the hole was later extended to a total depth of 348.3 m as a result of emergent difficulties of rig access that placed the collar too far west to effectively test the target, without such extension. Throughout its entire length this hole drilled monolithic THF slate containing rare pyrrhotite-pyrite diagenetic ellipsoids and very little carbonate veining. No mineralised fault breccia was encountered. A minor 60 cm wide interval of carbonate infilled fractured ground was noted at 214 m hole depth. Bedding-parallel trace sulphide layers are present throughout the drill core, becoming more frequent between 257.5 and 283 m. The sulphide is dominated by pyrite, with lesser intergrown pyrrhotite plus rare sphalerite-galena-chalcopyrite flecks. The only significant mineralised intercept occurs as a solitary banded carbonate-quartz-siderite vein, which returned an assay value of 2 m @ 0.9% Pb+Zn and 24 g/t Ag from the cored interval 307.7-308.8 m. This steeply eastwards dipping vein (86° towards 070°) lies subparallel to the axis of the drill core, and only has a true thickness of 30 mm. It displays ribbons of galena up to 1 mm thick and blebby coarse pyrite situated close to the vein margins, but no sphalerite. This intercept is located directly under the outcropping LOL at a vertical depth ~280 m below surface. Because YRDH002 hole did not intersect the anticipated mineralised breccias, which subcrop and have been worked by the mine, it is likely that they are situated to the east of the actual hole trajectory, which by 350 m was dropping and swinging away from such potential targets. An easterly dip or jog-offset of the target mineralised breccia is possible. Hole YRDH003 was drilled at a shallower dip angle than YRDH002 from the same rig pad, aiming to test as close as possible under the bottom of the main shaft of the Great Gladstone mine, to investigate the lode features at ~85 m vertical depth, or 110-120 m depth in-hole. No notable geology was seen in the unaltered THF slate down to 140 m hole depth, after which carbonate-quartz veining and fine fracture brecciation gradually intensifies. A porous silica-carbonate matrix, iron-oxide stained fault zone was cut at 146.6 m depth (orientated at 70° towards 069°), which suggests recent reactivation and groundwater movement. The zone has a 2 m wide selvedge of siderite-quartz alteration in the slate. The target lode rocks were entered deeper than expected at 148.5 m hole depth, and continued to 153.7 m. They are intensely brecciated, with rotated slate clasts in a siderite-carbonate-quartz matrix. Polyphase banded vein overprinting and highly varied vein fill mineralogy occur across the lode, including disrupted silica bands and coarse pyrite clots, disrupted parallel bands of 1-3 mm thick red sphalerite, and disseminated trace fine grained galena. Drusy silica is evident in voids. Beyond the lode edge, very fractured slate continues to ~172 m hole depth, with common quartz-carbonate-siderite veining occasionally bearing pyrite and sphalerite. The following are significant core assay intercepts: 17 m @ 1.4% Pb+Zn and 19 g/t Ag [145-162 m], including 1 m @ 3.0% Zn, 3.0% Pb and 152 g/t Ag [151-152 m], 1 m @ 6.0% Zn, 0.6% Pb and 34 g/t Ag [153-154 m], and 1 m @ 1.3% Zn and 0.2% Pb [160-161 m]. The last 40 m of the hole only exhibit unaltered and sparsely veined slate. YRDH003 was terminated at 210.3 m as there was no evidence to suggest any repeats of brecciation or mineralisation. Hole YRDH004 was directed to pass underneath a historic costean at Great Gladstone that had returned significant channel sampling results to CXO (2 m @ 109 g/t Ag, 4.9% Pb and 7.4% Zn). It was collared as close as possible to the workings and was declined -50 degrees to the north-east to intersect the mineralisation as shallowly as possible, to verify the surface sampling results; however, because close access for the drill rig was restricted by heritage clearance and steep topography factors, the collar position finally used was still 100 m from the surface anomaly. For its entire 249.6 m length, this hole cored only monotonous unaltered and sparsely veined slate having sporadic thin carbonate / dolomitic interbeds with rare trace pyrite (+/- pyrrhotite). As in the previous holes, ellipsoidal concretions and random small blebs of pyrite are present. The amount of veining present, compared to that seen in the other two Great Gladstone holes, is notably less, as is the extent of brittle deformation and brecciation. Overall, this hole was very disappointing, and did not reveal any mineralisation or contribute towards understanding the prospect geology. At the Big Hill gossan, where CXO's channel sampling had returned 10 m @ 0.25% Pb and 0.4% Zn, PACE hole YRDH005 was collared ~30 m west of the anomalous sample points and was declined shallowly (-50°) eastwards to test the oxidised zone beneath the gossan outcrop. The Big Hill gossan was thought to be considerably thicker but of an order of magnitude lower grade for base metals, compared to the Great Gladstone LOL located ~5 km to the west. The subsurface geology encountered in this hole is notably different to what had been found in the previous PACE DPY8-21 holes. The cored THF slate sequence at Big Hill has far more clastic psammitic interbeds, and the apparent intensities of weathering, faulting and structural complexity in features that it embodies are significantly higher. Numerous more recently formed fault zones were identified. The visible proportions of veining, alteration selvedging and brecciation are greater, but strong weathering has leached away most of the sulphides, leaving goethite-stained vughs and recrystallised haematite (-marcasite) in the veins, presumably after sulphide. Beneath the target gossanous breccia that was intersected from 45.7 to 46.9 m hole depth, a well developed competent breccia layer extends for 1.2 m, with the upper contact lost in a grey clayey fault zone 20 cm wide. Veining and weakly developed breccias associated with leached sulphides (goethitic voids) that lie above this breccia extend the probable target zone width to ~16 m in-hole, but the effects of evident recent faulting may have affected this estimated original width. The orientations of dominant carbonate-quartz-goethite veining and breccia contacts imply a 70°-85° dip to the west, which aligns the top-of-target intercept made in-hole at 33 m vertical depth with the outcropping gossan. In general, the target breccia, which has a pale green matrix, is now unmineralised aside from some coarse pyrite blebs, and it contains vughs (after ?other sulphides) distributed over 10 cm near the base of the breccia. Below the target interval the degree of brecciation of the THF slate wanes, but it remains frequently veined. The rock appears to be harder (more siliceous) than in the previous holes, and regular thick clastic interbeds persist. A second moderately brecciated interval, with rotated slate clasts in a white carbonate-dominant matrix, can be seen in the drill core at 60.5-61 m hole depth, with coarse pyrite clots occurring near the basal contact. This hole was terminated at 78.1 m depth in relatively unaltered and sparsely veined slate. It returned the following significant intercepts from later core sample assaying: 11 m @ 0.4% Pb+Zn [14-25 m] 11 m @ 0.1% Pb+Zn [31-42 m] 4 m @ 0.2% Pb+Zn [43-47 m]. The last hole of the PACE 2015 drilling campaign on EL 5015, YRDH006, was designed as a second test of the Big Hill gossan at a collar location ~100 m north of YRDH005. Again it was intended to pass through the subsurface oxidized zone to assess the gossan's along-strike heterogeneity. However, this hole had to be abandoned after reaching only 6.3 m depth due to logistical problems with drilling rig water supply, when the Mount Freeling homestead groundwater bore being used for this purpose became overstressed, leading to depleted yield. As no alternative water source could be located within trucking distance on Mount Freeling Station or on neighbouring pastoral leases, the drilling project had to be curtailed, in view of it having already exceeded the planned aggregate 1000 drilling metres. The near-surface rocks intersected by YRDH006 consist of heavily veined and partially brecciated slate with common psammitic interbeds. CXO has concluded that the subject drilling project has been successful in contributing much needed information regarding the occurrences of base metal mineralisation within the Tapley Hill Formation at Yerelina, which may lead to the discovery of new base metal deposits with the northern Flinders Ranges. Newly revealed features of the geological setting at Yerelina, combined with the metal assemblage recorded from cored occurrences, suggest ore genetic affinities with structurally influenced, sediment-hosted, carbonate rich low temperature silver-lead-zinc mineralisation (MVT to Isa-type sedex). Because rig access to planned drill sites became restricted during field implementation of the 2015 campaign, the resulting holes were not drilled to target at optimal angles for maximising the subsurface information sought. CXO now regards the requirement for drilling scissor holes at Great Gladstone prospect as very desirable, while conducting deeper exploratory diamond drilling into the transitional and reduced sulphide zones at Big Hill is also regarded as a high priority, to gain better knowledge of the basal THF metasediment sequences, which appear more prospective due to the presence of carbonate psammitic beds. Gaining further insights into the THF regional stratigraphic setting, and determining the spatial disposition of more carbonate-rich facies within it, is obviously warranted as it is likely these rocks would host better grades and widths of mineralisation. Due to the local abundance of THF outcrop, there is scope for hyperspectral mapping to be a useful exploration tool for performing this task.