Holman RHC Ballinalack Discover
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BALLINALACK ZINC-LEAD DEPOSIT A DISCOVERY BY REGIONAL GEOCHEMISTRY List of Figures: Figure 1 Irish Prospecting Licence Map 1965 2 Geological Map, Ireland 3 Location of Dromod, Strokestown, Ballinalack 4 Linear Zinc-high between Strokestown & Mullingar 5 Zinc in Stream Sediment, Rathowen, Ballinalack, Bunbrosna Area 6 Zinc in Roadside Soils, Ballinalack Area 7 Zinc in Off-Road Soils, Ballinalack 8 Prospecting & Geological Mapping, Ballinalack 9 Location of Pits, Ballinalack 10 Zinc in Overburden, Line 36N Section 11 Zinc in Overburden, Line 16N Section 12 Resistivity 13 Chargeability 14 Location of Drill Holes DH 1 to 42 15 Section on Line 20N 16 Section on Line 36N 17 Section on Line 15E 18 Zinc in Stream Sediments, Nova Scotia Ballinalack Zinc-Lead Deposit A Discovery by Regional Geochemistry Summary This paper describes the exploration work by Syngenore Explorations Ltd in Ireland that started in 1966 and led to the discovery of ore grade zinc-lead mineralization at Ballinalack, County Westmeath, in 1969. The deposit was located by a regional geochemical survey based on wide-spaced stream sediment and soil sampling along roads; in an area where published records gave no indication of favourable geology or history of mineralization. Syngenore Explorations (a subsidiary of Noranda Mines Ltd) was set-up in 1965 by R.C.J. Edwards and M.E.Penstone with a world-wide exploration brief. Reviewing global possibilities, Edwards recognised the potential for zinc-lead deposits in Ireland and immediately initiated an exploration programme. The writer, a long-standing friend of Dick Edwards and Mike Penstone (since graduating together at the Royal School of Mines in 1952) was invited to join Syngenore and manage the Irish Project. A regional geochemical approach to exploration in Ireland was highly appropriate at that time. Economic base metal deposits had recently been found in Carboniferous limestones at Tynagh, Silvermines and Gortdrum, all in the south of the country where roughly 100 of the total of some 120 prospecting licences then issued were clustered. Whereas, roughly north of a line between Dublin and Galway (except for a prospect being drilled at Keel in Co. Longford) exploration activity was evident on 18 prospecting licences only, scattered singly or in small groups (Table 1, Fig. 1). Table 1 Prospecting Licences held north of Dublin-Galway line 1966 No of Licences County Location 2 Dublin Swords Inlier 1 Meath Dunboyne 6 Westmeath Sion Hill & Moate 4 Longford Keel 1 Roscommon Mt Mary Inlier 2 Leitrim Mohill 2 Mayo Westport Carboniferous rocks in Ireland cover about 14,000 ml2. Ground under licence at that time totalled some 2000 ml2, leaving roughly 12,000 ml2 (30,000 km2) open and virtually unexplored (Fig. 2). The location of the known deposits adjacent to inliers suggested that other unmapped inliers, with minimal surface expression, as potential exploration targets. The feasibility of applying soil or stream sediment sampling to bedrock mapping on a regional scale was considered as the result of geochemical work on kimberlites in Sierra Leone (Holman, 1956) which showed that basic rocks could be mapped by testing soils for nickel or zinc. This idea was later extended to a regional geochemical survey in Nova Scotia during 1957 & 1958 for the Geological Survey of Canada. In the Nova Scotia project sediment samples were collected from all streams accessible by road over an area of about 10,000 sq ml at an average density of roughly 1 sample per 2.3ml2. The main aim of this experimental low density sampling survey was to test the feasibility of mapping regional-scale base metal variations, as opposed to the detection of strongly anomalous concentrations local to mineralization. This survey revealed broad patterns for zinc, lead and copper related to bedrock geology and the results were published as prototype regional geochemical maps of Nova Scotia (Holman 1959 & 1963) Fig. 18). Stream Sediment Sampling The same technique of collecting sediment from all streams directly accessible by road was adopted for the regional geochemical survey in Ireland. Sampling was started near the centre of the northern part of the Carboniferous Basin, at Strokestown, where a field office and laboratory was set up, and progressively extended along the surrounding network of roads (Fig. 3). All samples were analysed for zinc, lead and copper in the field laboratory, initially by a colorimetric dithizone method and subsequently by A.A. Early in the programme an increase in zinc, above 6 ppm/10 (60 ppm), was detected in sediments collected east of Strokestown. This rise was subsequently found to extend as a broken and roughly linear zone from Dromod through Longford and Edgeworthstown to Mullingar for some 30 miles (50km) (fig 4). Along this zone, between Rathowen and Bunbrosna , a group of 5 sediment samples gave higher zinc values of 20 to 50 ppm/10 (Fig. 5). Prospecting & Geological Mapping: The G.S.I geological map (Sheet 89) showed this area to be largely drift-covered area with just a single dip arrow (10 SE) at Ballinalack Bridge. Prospecting and float mapping by Roy Beavon . Dick Edwards and Jim Patterson indicated that the rising ground at Ballinalack between extensive bog to the west and the Inny River to the east was underlain by pale-grey reef limestone; surrounded to the west, north and east by dark nodular bedded limestone (Calp). Several scattered reef boulders carried pyrite and minute specks and threads of yellow sphalerite (Fig. 8). Pitting: The zinc-high zones at Ballinalack were examined by pits dug to bedrock with a JCB on lines L16N & L36N (fig 9). On the northern zinc-high, on line L 36N, 12 pits were put down over 700ft. 7 pits bottomed in dark grey limestone (Calp) containing fine pyrite grains and 5 pits at the southeast end of the line contained fragments of rusty reef limestone or bioclastic limestone. Soil/overburden samples gave very high zinc values (43 samples >0.1%Zn) decreasing with depth. Rock fragments from the pit bottoms gave low zinc figures of 30 to 350 ppm (Fig. 10). On the southern zinc-high 13 pits put down along line 16N, over 800ft, mostly bottomed in rusty pyritic pale grey reef limestone carrying specks of yellow sphalerite. Analyses again showed higher zinc in near-surface overburden and a general decrease with depth; but several reef fragments gave high zinc and lead values of several percent (Fig. 11). Both lines of pits showed localised zinc-highs in the overburden at 3 to 4 ft depth, with zinc decreasing downwards. In the 16N pits overburden zinc was comparatively low although rock fragments indicated underlying mineralised reef limestone. By comparison zinc in overburden in the 36N pits was markedly higher but rock fragments were virtual barren; and the presence of Calp indicating a reef /Calp transition. Permission to dig additional pits using a JCB to elucidate these results and locate a drilling site was refused by the landowner. At this stage it was decided to run an IP/Resistivity survey on the property. Geophysics: (Figs. 12 & 13) An IP/Resistivity survey (7.5kva high-powered pulse-transient system, current-on time 1 ½ sec, reading period 400 m/sec, 3-electrode array at 200ft ‘a’ spacing) was run over a rectangular area 3000 by 4000ft covering the zinc-highs along 120 lines 400ft apart. The resistivity plot showed no strong features apart from an increase from 500 up to 1000 ohm/metre over the central area where mapping indicated underlying reef limestone. The chargeability plot showed a linear high (+20 millisecond) in the west aligned roughly NE-SW surrounded by a broad +10 millisecond zone to the southeast narrowing to the south as a well-defined tongue. The narrow tongue was interpreted as an indication of sulphide mineralization and drilling recommended. Drilling on Southern Zinc-High: (Figs. 2, 14 & 15) Following the geophysical recommendation the following 4 inclined holes were drilled between February and May 69 to investigate the narrow chargeability- high crossing line 20N. Summary logs of these drill holes using down-hole depths are given below. DH 1: 20N, inclined 50NW Rusty, cavernous, grey argillaceous limestone to 355 ft. Weak pyrite and sphalerite <1% (vis. est.) down to 200ft. DH 2: 20N, inclined 50NW, 400ft S of DH 1 Grey argillaceous limestone with negligible sulphides to 530ft and limy sandstone with scattered sulphides to 835 ft. DH 3: 20N, inclined 50NW, 300ft NW of DH 1 Pale grey shaley reef limestone with weak sphalerite and pyrite to 475ft, and dark nodular argillaceous limestone devoid of sulphides to 700ft. DH 4: 20N, inclined 50W, 300ft SE of DH 1 Grey argillaceous limestone to 560ft, and limy sandstone to 700ft. Weak scattered sphalerite and pyrite throughout. These 4 holes (totalling 2600ft; 790m) confirmed low grade sphalerite with pyrite in bedrock as the local source of the mineralised reef fragments and high zinc in overburden found in the pits on line 16N. The presence of sandy basal beds at relatively shallow depth (between 350 &430ft vertically) in DH 2 & 4 gave the first indication of a ‘blind inlier. The consistently low grade of the sphalerite found in these 4 drill holes substantially downgraded the southern zinc-high/chargeability target and drilling was stopped. Five additional holes (DH 27, 28, 29, 31& 32) totalling some 2000ft (600m) subsequently put down in this area did not improve the grade (Fig. 14.) Drilling on Chargeability-High under bog DH 5: 31N, inclined 50SE This hole was put down to investigate the chargeability-high running NE-SW under bog. It was drilled to 500ft entirely in dark grey to black near-horizontally bedded limestone (Calp) containing abundant fine-grained pyrite; and a single sphalerite bleb at 340 ft.