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Diamond Drilling in the Lisle Valley

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Diamond Drilling in the Lisle Valley Tasmanian Geological Survey TASMANIA DEVELOPMENT Record 1996/04 AND RESOURCES Diamond drilling in the Lisle Valley by R. S. Bottrill Ab stract The sandstone, quartzite, siltstone and pelite (phyllite and slate) strike 340–360 and are locally An attempt was made, by diamond drilling, to ° hornfelsed close to granitoid bodies. The folding and intersect the contact between the Lisle granodiorite syntectonic metamorphism (greenschist facies) in and the hornfelsed Mathinna Beds near Lisle. The the Mathinna Beds are considered to predate the two diamond-drill holes failed to intersect the intrusion of the granitoids. The metamorphic hornfels, or discover significant mineralisation, but aureoles are commonly sharply defined, varying they did provide good intersections of fresh from 800 m to about 5 km in width, depending upon granodiorite and other igneous rocks in the Lisle the dip of the contact (McClenaghan et al., 1982). pluton, with some minor alteration and very weak Within these aureoles the pelite is commonly spotty gold mineralisation in the granitoid. and/or hornfelsed, and may contain biotite, andalusite and/or cordierite. A very hard INTRODUCTION Fe-cordierite (sekaninaite)-rich hornfels (with The Lisle–Denison goldfield lies in the southwestern quartz, feldspars and biotite) makes up much of the part of the Mathinna Beds in northeastern rim around the Lisle Valley, and many of the other Tasmania. The locality and geology of the area are high ridges in the area. shown in Figure 1. Geophysical interpretation (Roach, 1994) suggests The Lisle goldfield was discovered in 1878, and that a range of granitoid types are present at Lisle, produced about 8–9 t (3000 oz) of gold by 1902, but these are mostly deeply weathered and poorly mostly from Cainozoic placers in the Lisle valley exposed. Hornblende-biotite-magnetite–bearing (Twelvetrees, 1909). The valley is an ovoid basin granodiorites are present, but more alkali-feldspar with a drainage area of only about 20 km2. The rich phases also occur. Small quartz and greisen source of the gold is uncertain, despite several small veins and granitic dykes occur in the metamorphic occurrences of primary gold in the area. The gold aureole. appears to be closely related to the underlying granodiorite, and may be related to the Tertiary sediments and basalt, and Quaternary hornfels-granodiorite contact (Bottrill, 1994). ironstone and quartz gravel beds, are preserved as remnants of old plateau surfaces in many areas. The Two diamond-drill holes were sited on the eastern river valleys are partly filled with Cainozoic flank of the Lisle valley (at 527 890 mE, 5 434 704 alluvium, which is possibly partly Tertiary in age. mN) above some of the main alluvial workings, in an attempt to test the granitoid/hornfels contact. MINERALISATION The Lisle area is locally highly mineralised, with at REGIONAL GEOLOGY least 66 significant known gold deposits within or in The oldest rocks exposed in the area are the close proximity to the area (Bottrill, 1994). About Mathinna Beds, quartzwacke to pelitic turbidite half of these deposits are in lodes, but more than 99% sequences of (?)Ordovician to Early Devonian age, of the gold produced was from the alluvial and generally correlated with the Melbourne Trough in residual deposits. The various styles are discussed the Lachlan Fold Belt (Powell and Baillie, 1992). The below. Mathinna Beds are intruded and locally contact metamorphosed by small granitic to dioritic Gold deposits are widespread in northeastern intrusive bodies, perhaps related to the Scottsdale Tasmania, mostly as gold-quartz veins in the Batholith, of probable Upper Devonian to Lower Mathinna Beds. The greatest concentration of Carboniferous age. deposits occurs in a belt running through the Tasmanian Geological Survey Record 1996/04 1 Tb Qh Qh Qh Qh Qh Qh N Golconda m Tb D/Oq 0 0 0 2 Qh 4 4 Quaternary sediments Qp 5 Dgr Tb Dgr Ts Tertiary sediments Tb Tertiary basalt D/Oq Qh N m 0 0 0 Jd Jurassic dolerite 0 4 4 5 D/Oq Pl Qh D/Oq Plc Permo-Triassic sediments Pu Qp Ts Dgr Devonian granodiorite N m 0 0 0 8 Mathinna Beds 3 Qp 4 D/Oq (Devonian–Ordovician?) 5 pelite dominant Lisle goldfield Qp Qp D/Oq N m 0 0 0 6 Drill site 3 Ts 4 5 Qp Dgr Dgr N D/Oq m Dgr 0 0 LOCALITY MAP 0 4 3 4 D/Oq 5 Qp LAUNCESTON Dgr D/Om D/Oq N m HOBART 0 Plc Tb 0 Pl 0 2 3 4 5 Qp Jd Tb 523 000 mE 525 000 mE 527 000 mE 529 000 mE Figure 1 Location and geological map, Lisle area Mathinna Beds from around Waterhouse, near the to represent an old Tertiary lake bed (Reid, 1926; north coast, south for about 80 km almost to Fingal Marshall, 1969) overlying a weathered granitic (Noldart and Threader, 1965; Bottrill et al., 1992; stock. There were numerous patchy gold-rich Bottrill, 1992; Taheri, 1992; Taheri and Bottrill, horizons in the possibly lacustrine sediments, and in 1994). Other deposits, however, occur with spatial carbonaceous horizons underlying talus, which and possible genetic relationships to granitoid produced relatively pure, free, angular (crystalline?) intrusions, and include deposits in the Upper gold (Noldart in Marshall, 1969). The mineralogy of Scamander (Hogans Road or Brilliant Creek) and the gold suggests that some gold is detrital and some Lisle–Denison areas (Bottrill et al., 1992; Bottrill, secondary (i.e. in situ reprecipitation of dissolved 1994; McNeil, 1995). gold from groundwater; Reid, 1926; Bottrill, 1986, 1991; Roach, 1991). Workings at Lisle were in alluvium and eluvium in the basin-shaped depression, which was considered Tasmanian Geological Survey Record 1996/04 2 Auriferous quartz was relatively rare at Lisle, both granodiorite and other igneous rocks in the Lisle in alluvium and bedrock. Twelvetrees (1909) found pluton, with minor alteration and mineralisation. evidence for gold originating in the contact metamorphosed sandstone of the Mathinna Beds Thirty-one samples were taken for geochemistry surrounding the basin, near the contact with and petrology studies, mostly for signs of alteration Devonian granitoid intrusive rocks. Inclusions of and mineralisation (Table 1). mica, rutile and magnetite in the gold grains also suggest that the gold was more likely to have been Min er ali sa tion disseminated in the hornfels or granitoids than in quartz veins, while rare gold-limonite composites in The only indication of mineralisation encountered in placers suggest gold-bearing pyrite may have the two drill holes was a small zone of brecciation originally been present (Bottrill, 1986). Some gold with minor pyrite from 94.4 to 94.95 m in BH1. This was, however, found in small quartz veins in the zone contained 0.05 g/t Au, and is associated with granitic rock underlying the alluvial sediments quartz-sericite-carbonate alteration; base metals (Thureau, 1882; Montgomery, 1894). and arsenic are low (Table 2). A zone of similar appearance from 70.15–70.4 m in BH2 contains Gold veins in granitoids are poorly represented in considerable carbonate, pyrite and chert, but is not Tasmania, and those described at Lisle by anomalous in gold or other elements. Montgomery (1894) consisted of a belt, 1–1.6 m wide with veinlets 6–40 mm wide, striking ‘076° Geo chem is try (magnetic?), dipping 48° NW’, in weathered The results of full, major and trace element analyses ‘granites’ in the Titmus and Dodgson Sections. ‘A by XRF are shown in Tables 2 and 3. Some little gold’ was apparently recovered from both the representative CIPW norms are listed in Appendix veins and granite (Montgomery, 1894). 3. Thureau (1882) described other veins in ‘syenite’ as The pink to reddish granodiorite zones are similar in reaching widths of up to 600 mm, and being composition to the normal grey granodiorite, except disjointed by the host rock. He noted ‘the whole of for slightly elevated CO2 and H2O contents. The the granitoid formation to be traversed by dykes are lower in Fe, Mg and Ti, and higher in Si attenuated quartz veins charged with very fine gold’. and K. The xenoliths are the opposite. The rocks, in Thureau (1882) also described weakly gold-bearing, general, are all very low in Rb/Sr compared with ferromanganese-stained quartziferous formations other granitoids from northeastern Tasmania at the granitoid/hornfels contacts, and the presence (fig. 3), reflecting their low degree of fractionation, of relatively coarse gold with quartz detritus in the as noted by Roach (1994). vicinity of these contacts. In general, however, quartz veins were rare in the Lisle valley. The base metals (except Pb) are mostly slightly Disseminated gold in turbiditic greywacke has been anomalous, but closely reflect the contents of Mg, Fe described in very similar settings, with quartz-gold and the other transition metals, and the mafic veins, in the Meguma area of Nova Scotia, but is very content of the rocks. They may largely reside in solid low in grade (Crocket et al., 1986). solution in biotite, hornblende, magnetite and ilmenite, although some occur as rare sulphides PREVIOUS MINERAL EXPLORATION (Cu). AND DRILLING There is no anomalous enrichment in any other Previous mineral exploration and Department of element, with the exception of gold in the weakly Mines studies have been summarised by Bottrill mineralised zone described above. (1994). The only recorded drilling was undertaken by BP Pe trol ogy Minerals/Seltrust (Storer, 1985), who drilled 29 The petrography is summarised in Table 4. The percussion holes (to 60 m) in the area to test for gold petrographic analyses (Table 4 and Figure 2) mineralisation. Only three holes encountered the indicate that the major rock types vary from granitoid/hornfels contact and the gold results were granodiorite to tonalite.
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