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. This broad high chargeability response was attributed to the widely disseminated pyrite (Figs. 13 & 14)..
Additional IP/R lines run with different arrays and modified instrumentation. From these tests it was concluded that the very strong response was due to the pyritised Calp and likely to mask weaker signals from zinc or lead mineralization. Attention was turned instead to the investigation of the northern zinc-high directly by drilling.
Drilling - on Northern Zinc-High, Line 36N (Figs. 14 & 16)
DH 6: 36N inclined 50NW Wavy nodular bioclastic, crinoidal limestone carrying pyrite and a few blebs of sphalerite to 55ft and brecciated pyritzed reef limestone to 450 ft. The reef was strongly mineralised from 55ft to 105ft, averaging about 5% Zn, 1% Pb over 50ft, weakening downwards to under 1% Zn.
DH 7: 36N inclined 50NW This hole was sited 200ft southeast of DDH 6 to test the extension of the higher grade mineralisation. It intersected muddy, crinoidal limestone (Calp) from the surface to 39ft, nodular limestone to 45ft, and pale grey reef limestone to the base of the hole at 310ft. The reef was strongly mineralised from 45 to 110ft averaging about 6.5 Zn, 1.5% Pb over 65ft; weakening downwards to about 1.5% Zn, with several higher grade sections, throughout the rest of the reef.
DH 8: 38N, Vertical, 200ft N of DH7 This hole located intersected muddy limestone (Calp) with disseminated pyrite down to 160ft, wavy nodular limestone with negligible sulphides to 230ft, pale grey reef limestone with weak sulphides to 295ft and mixed shales, limestones and sandstones carrying scattered pyrite, sphalerite and galena to 390ft.
DH 9: 36N, V, 300ft W of DH 6. Muddy limestone to 120ft, wavy nodular limestone to 220ft, reef to 405ft , mixed shales and limestone to 475ft. Negligible sulphides throughout.
DH 10: 34N, inclined 50NW, 200ft SE of DH 9. Entirely in reef limestone with negligible sulphides down to 565ft.
DH 11: 32N, V, 200ft S of DH 10. Reef limestone with negligible sulphides to 340ft, bioclastic limestones to 500ft, a fault at 500ft, reef limestone to 700ft, limy sandstone and shale to 700ft with green steeply dipping shales to 875ft.
DH 12: 36N, inclined 50NW, 200ft E of DH 7. Calp to 55ft, nodular limestone to 65ft, reef to 225ft. Reef strongly mineralised, 75 to 105ft, 7.3% Zn, 0.9% Pb (with higher grade5ft sections, up to 13% Zn), weakening downwards.
DH 13: 37N, inclined 50NW, 100ft north of DH 12. Calp to 190ft, nodular limestone to 230ft, reef to 385ft. Reef strongly mineralised, 230 to 235ft, 13% Zn, 7.8% Pb 245 to 250ft, 6% 0.5% 270 to 310ft, 8% 0.6% .
These 8 drill holes, totalling some 3700ft (1100m) suggested that higher grade sulphide mineralization might be preferentially located at the top of the reef similar to spatial relationships between zinc-lead mineralisation and reef geometry observed in the Eastern U.S.A zinc-lead deposits.
To elucidate the reef geometry and lithology Dr. Alan Lees, who had published a paper on Irish Waulsortian ‘reefs’ (1964), was invited to examine the Ballinalack drill cores. Lees confirmed that the Ballinalack ‘reefs‘were typical Waulsortian mud banks and suggested they might be present as two roughly circular masses or knolls draped against a rise in lower sandy beds.
To trace the sub-outcrop of the reefs close-spaced soil samples were taken and tested for zinc and lead. This work confirmed the outlines of both zinc-highs and in particular the very abrupt zinc termination of the northern zinc-high along a NW-SE line coinciding (as drilling subsequently revealed) with the margin of the muddy Calp limestone overlying the reef; but gave no indications of new drilling targets.
Fill-in Drilling on Northern Reef Knoll A further 11 holes (DH 14 to DH 24)* totalling some 4000ft (1200m) were put down on the northern reef knoll (fig 14). These holes simply confirmed the rather patchy and overall sub-economic grade mineralization found in holes DH 6 to 13 and with the lack of further IP/R targets substantially reduced expectations of finding ore-grade mineralization on the property. Drilling was stopped but resumed several weeks later to investigate the deeper reef intersected in drill hole D14. Summary logs for these holes given in the appendix.
Drilling on Lower ‘Blind’ Reef This set of 13 holes (DH 25, 26, 30 & 33 to 42)*, totalling some 10,000ft (3000m), progressively outlined a zone of higher grade mineralisation in the blind lower reef plunging to the northeast as shown in the longtitudinal section; and effectively concluded the discovery phase of work at Ballinalack (fig 17). Summary logs given in the appendix.
At this stage (in late 1970) Edwards, Patterson and the writer resigned from Syngenore Explorations. Additional drilling at Ballinalack has indicated a total of some 7.8 million tons of 7.4% Zn/Pb ore (Mining Journal, Special Report 2004).
The discovery at Ballinalack of virtually blind base metal mineralisation in ground apparently devoid of favourable geology, at an early stage in the exploration programme, confirmed the effectiveness of the regional geochemical approach. Coverage by stream sediment sampling was progressively extended over an area of some 10,000 ml 2 (26,000 km2) ; and by roadside soil sampling over 6900 ml2 (16,000 km2). These extended surveys shown that many of the other base metal occurrences found in Ireland, including Abbeytown, Galmoy, Gortdrum, Keel, Navan, Silvermines and Tynagh, could have been discovered by a regional geochemical survey.
R.H.C. Holman April 2000 .
APPENDIX :
Summary drill logs:
Holes Drilled on Northern Reef Knoll
DH 14: 38N, inclined 50W Calp to 110ft, wavy nodular limestone to 365ft, reef to 475ft, shaley limestones with reef Scattered sphalerite and down to 535ft. galena in reef. This hole gave the first indication of a second development of reef at depth
DH 15: 37N, inclined 50W Calp to 90ft, wavy nodular limestone to 165ft, reef limestone to 310ft. Reef strongly mineralised, 164 to 200ft, 11.5% Zn, 0.6% Pb 239 to 249ft , 7.4 0.8
DH 16: 37N, inclined 50W Calp to 180ft, wavy nodular limestone to 245ft , reef limestone to 380ft. Negligible sulphides.
DH 17: 37N, inclined 40S Calp to 75ft, (fault), reef to 160ft shaley reef limestone to 215ft. Negligible sulphides in reef.
DH 18: 32N, Vertical Reef limestone to 210ft, rusty patches, negligible sulphides.
DDH 19: 37N, inclined 50W Calp to 130ft , nodular bioclasstic limestone to 425ft. Weakly pyritised throughout.
DH 20: 37N, inclined 50W Calp to 45ft, nodular limestone to 185ft, reef limestone to 315ft. Sulphides between 185 and 245ft averaging about 1.8% Zn over 60ft, first 5ft section 6.7% Zn.
DH 21: 36N, Vertical Nodular limestone to 30ft, reef limestone to 165ft; sulphides between 30and 140ft averaging about 2.3% Zn with a single 5ft section of 5% Zn.
DH 22: 37N, inclined 50E (at DH 19) Calp to 205ft, nodular bioclastic limestone to 475ft, reef to 645ft with negligible sulphides, shaley limestone to 670ft.
DH 23: 35N, Vertical Reef limestone to 175ft, negligible sulphides.
DH 24: 34N, Vertical Nodular limestone to 25ft, reef limestone to 530ft, shaley limestones to 675ft and calcareous sandstones and shale to 875ft. Negligible sulphides.
Holes Drilled on Lower Blind Reef
DH 25: 39N, Vertical Calp to 165ft, nodular limestone to 35ft, reef limestone to 325ft, muddy limestone to 335ft, mixed limestone and calcareous sandstone to 785ft, quartzitic sandstones to 875ft , greenish shales (pre- Carboniferous) to 885ft. Weak sphalerite and galena in reef, 1.3% Zn between 350 and 440ft in calcareous sandstone, with 5ft sections up to 4% Zn.
DH 26: 40N, Vertical Calp to 150ft, nodular limestone to 180ft, reef limestone to 205ft, shaley limestone to 435ft, mixed calcareous sandstone and limestone to 755ft, shelly limestone to 785ft. Weak sphalerite (< 1%Zn) between 415 and 470ft.
DH 30: 40N, Vertical Calp to 110ft, WN limestone to 255ft, reef limestone with shaley sequences to 535ft, shaley limestone to 625ft, calcareous sandstones with mudstones to 758ft. Mineralised between 395 and 535ft averaging 2.3% Zn over 140ft with higher 5ft sections up to 9.5%. Disseminated low grade mineralisation in the sandstones below 600ft.
DH 33: 42N, Vertical Calp to 100ft, WN limestone to 360ft, reef limestone with shales to 590ft, bioclastic limestones to 620ft. Reef mineralised throughout averaging 2% Zn between 370 to 460ft; and 1.9% between 500 to 570ft with 5ft sections up to 9 % Zn.
DH 34: 44N, Vertical Calp to 45ft, WN limestone to 330ft, reef limestone to 700ft and dark shaley limestone to 875ft. The reef limestone was mineralised throughout averaging 8% between 355 and 380ft, 6.6% between 425 and 475ft and 3.8% between 540 and 610ft with several higher 5ft sections running up to 12%.
DH 35: 38.5N, Vertical Calp to 115ft, WN limestone to 295ft, reef limestone with shaley sequences to 460ft, bioclastic limestone to 550ft, mixed sandstones, shales and quartzitic sandstones to 730ft. Between 335 and 445ft the reef averaged 2% Zn with 5ft sections up to 7.5%.
DH 36: 40N Vertical Calp to 75ft, WN limestone to 335ft, reef limestone with shaley sequences to 445ft, bioclastic limestone to 650ft. Between 340 and 435ft the reef averaged 2.1% Zn with a few 5ft sections up to 9% Zn.
DH 37: 42N, Vertical Calp to 45ft, WN limestone to 230ft, reef limestone to 520ft, shaley limestone to 555ft, reef limestone to 620 and shaley bioclastic limestone to 690ft. Reef mineralised from 240 to 520ft averaging 1.7% Zn with a few higher grade 5ft sections up to 15%; and from 555 to 620ft averaging 1.8% with one 5ft section of 4.9% Zn.
DH 38: 48N Vertical Calp to 49ft, WN limestone to 380, reef limestone to 506, shaley Bioclastic limestone to 528, reef limestone with shaley sequences to 792 and shaley bioclastic limestone to 873ft. Mineralised from 380 to 506 ft averaging 2.3% Zn with 5ft sections up to 14%, from 532 to 602ft averaging 0.5% and from 659 to 791ft averaging 6.9% with several sections up to 12.5% Zn.
DH 39: 44N Vertical Calp to 63ft, WN limestone to 422ft, reef to 493, shaley bioclastic limestone to 532, reef limestone with shaley sequences to 720 and Bioclastic limestone 770ft. Mineralised from 427 to 460ft averaging 6.7% Zn with two 5ft sections of 7 and 12%; and from 552 to 652ft averaging 0.5% Zn.
DH 40: 44N Vertical Calp to 88ft, WN limestone to 365, reef limestone to 609 and shaley Bioclastic limestone to 904ft. Mineralised from 400ft downwards averaging 2% Zn overall with 5ft sections up to 7% Zn.
DH 41: 52N Vertical Calp to 103ft, WN limestone to 392, reef limestone to 1028. Weakly mineralised down to 900ft averaging less than 1% Zn with two 10ft sections up to 2.5% Zn.
DH 42: 48N Vertical WN limestone to 275ft, reef limestone to 877 and Bioclastic limestone to 905ft. Mineralised throughout, averaging less than 1% Zn down to 615ft with higher grade sections of 4.3% between 615 and 630, 2.4% between 750 and 760 and 3.9% between 770 and 835ft. Figure 1 Republic of Ireland, Prospecting Licence Map 1966
Figure 2 Geological Map of Ireland
Figure 3 Location of Dromod, Strokestown, Longford, Edgeworthstown & Mullingar
Figure 4 Regional Linear Zinc-High between Strokestown & Mullingar
Figure 5 Zinc in Stream Sediment, Rathowen-Ballinalack-Bunbrosna Area 5 5 8 8 5 5 8 5 8 8 8 55 5 5 5 10 8 8 10 8 8 5 8 5 3 8 8 10 3 8 3 3 53 5 5 Zn ppm/10 5 8 5 13 5 8 8 5 5 8 5 5 8 5 8 36 5 8 8 5 5 5 4 5 5 8 5 8 5 3 3 10 5 34 3 3 85 8 5 3 5 5 10 32 10 5 5 8 54 5 5 5 13 5 5 30 3 8 5 5 4 28 15 3 5 5 8 1010 8 5 4 26 5 3 3 13 5 8 10 25 5 108 3 8 53 24 38 3 5 5 4 3 15 8 3 Edgeworthstown8 8 10 5 5 22 8 5 5 5 10 13 10 5 20 5 8 5 5 8 5 3 8 5 18 13 5 8 10 10 16 8 8 5 5 5 8 3 3 14 20 10 5 3 3 5 8 8 10 8 10 8 5 8 8 12 13 13 3 15 10 13 8 5 10 3 3 5 8 8 5 5 3 5 8 5 3 8 4 3 5 5 5 6 8 5 8 Rathowen 108 3 3 55 4 10 5 8 3 5 13 5 3 2 13 5 3 10 13 108 3 20 5 (5) zn ppm/10 8 8 8 3 13 5 3 10 13 8 8 25 10 3 13 8 3 8 8 10 5 8 1010 3 13Ballinalack5 13 5 10 5 5 Fig 6 8 10 3 25 10 5 15 158 13 5 50 20 10Bunbrosna13 10 8 30 5 10 5 10 3 8 0ml 1ml 2ml 0km 1km 2km
Figure 6 Zinc in Roadside Soils, Ballinalack Area
Figure 7 Zinc in Soil, Ballinalack
Figure 8 Prospecting & Geological Mapping, Ballinalack
Figure 9 Ballinalack, Location of of Pits
Figure 10 Ballinalack – Zinc in Overburden, Line 36N Section
Figure 11. Ballinalack – Zinc in Overburden, Line 16N Section
Figure 12 Ballinalack - Resistivity
Figure 13 Ballinalack - Chargeability
Figure 14 Location of Drill Holes Nos 1 to 42
Figure 15 Ballinalack – Section on Line 20N
Figure 16 Ballinalack – Section on Line 36N
Figure 17 Ballinalack – Section on Line 15E from 32N to 52N (approx NE- SW)
Figure18.. Nova Scotia - Zinc in Stream Sediment