Mineral Resources Tasmania TASMANIA DEVELOPMENT REPORT 1993/10 AND RESOURCES Notes on the geology of the Lyell Quadrangle between the East Queen River and Linda Creek Scm by E. Williams This report outlines the geology of the Queens­ town area in the far northwestern corner of the Lyell Quadrangle. This area is bounded by: (a) the East Queen River/ Reservoir Road in the west; (b) Linda Creek/Idaho Creek in the east (grid line 385000 mE); (c) grid line 5339500 mN to the south; and (d) to the north by grid line 5343500 mN. STRATIGRAPHY margin of Little Owen Spur [823395J and has a maximum thickness of about 120 metres. It is within the dominantly felsic feldspar-phyric pyroclastic rocks and agglomerate of KING RIVER ASSOCIATION unit £kvr and faces west. Units of the King River association are usually laterally The unit is well-layered and shows no obvious impersistent, steeply dipping, and trend NW to NNW. hydrothermal aheration. It consists of tuff, sometimes South of the Lyell Highway and north of grid 39500 mN graded; siliceous and calcareous siltstone, which is very sparse facing data indicate that the units are folded commonly current bedded and displays convolute folding; about a horizontal NNW-directed synclinal axis located on laminated slaty mudstone; and agglomerate beds of up to the western flank of Little Owen Spur [823395J. The fabric 12 m in thickness. The unit is generally dark grey to black of the rocks is dominated by a steep WNW -trending in colour with finer layers weathering dark green or white. Devonian cleavage. -Ckx Typically, larger fragments within the pyroclastic deposits are vaguely defined and are commonly of altered fe ldspar This laterally impersistent, steeply-dipping unit occurs as porphyry with phenocrysts of altered feldspar and quartz a strike section in Conglomerate Creek along the eastern grain s with flecks of chlorite and carbonate in a fine matrix REPORT 1993/10 of chlorite, feldspar, carbonate and micaceous material rocks, quartz rods and pyritised lenses, which are up to (Corbett, 1979). 800 mm in width and similar length, are common. To the west the £kx sequence passes upwards, with an For most part the £kx unit of Conglomerate Creek is increasing proportion of agglomerate, into the bounded on the east by northerly-trending felsic feldspar hydrothermally altered £kvr (of £kvu) of Little Owen porphyry intrusive and extrusive rocks (£kvp of £kvu), Spur. which are hydrothermally altered along with other rock types of the King River association [830401] present £kvu (hydrothermally altered £kvp and £kvr) between the £kx unit and the Owen Conglomerate to the east. At the boundary of the £kx unit is a steeply-dipping This unit is of dominantly laterally impersistent felsic layer of massive, usually grey-green feldspar porphyry, feldspar porphyry, felsic feldspar-phyric pyroclastic rocks which at Waterfall Creek [83340 I] includes a flow-banded and agglomerate. rare mudstone and sandstone, and some horizon. This porphyry is separated from extensive chlorite-rich intermediate/mafic layers. The feldspar northerly-trending pale green to grey felsic feldspar porphyries are usually massive intrusions and extrusions, porphyry extrusive rocks to the east by steeply-dipping which are occasionally flow-banded or coarsely Lyell Schist consisting of pale green schistose felsic fragmental. East of Queenstown the rocks designated £kru agglomerate with chlorite-rich zones. The felsic extrusives are hydrothermally altered and include the Lyell Schists commonly display 5-25 mm thick vertical bands which (Wade and Solomon, 1958), the structures of which have have been flow folded, and the folds have axial surfaces of resulted from Devonian deformation. and lesser altered very variable trend. The extrusives, which show rock types. Hydrothermal alteration is readily recognised autobrecciation at some localities, contain phenocrysts of by the replacement of feldspar with phengitic-mica and altered feldspar and uncommon embayed quartz grains in quartz, and Na-depletion in comparison with the low-grade a matrix with textures that have resulted from Devonian metamorphic assemblages of originally similar rock types deformation. Lyell Schist types similar to those (Solomon, 1964; Cox, 1981). downstream occur adjacent to the Owen Conglomerate at the base of the waterfall [835400] and include a steeply The northerly-trending £kru unit exposure is more than dipping intermediate layer which has been described six kilometres long and up to 1.75 km wide, but it rapidly (Corbett, 1979) as containing altered feldspar and thins to the north and south. To the east, the belt of chloritised pyroxene phenocrysts in a ground mass of hydrothermally-altered rocks is faulted against members of intergrown quartz and feldspar with chlorite actinolite, the Owen Conglomerate, whereas to the west it is for the epidote and sericite. most part abruptly transitional with other units of the King River association. The feldspar porphyry extrusive rocks of Waterfall Creek extend north to a northerly-trending gully [830405] which Unit £kx of Conglomerate Creek [824400] grades is immediately east of the Lyell Highway. North of the upwards to the west into dominantly felsic feldspar-phyric Lyell Highway, at Philosophers Ridge [830412], grey and agglomerate and tuff (£kvr of £kvu) of Little Owen. The light green siliceous flow-banded impersistent extrusive textures of these grey-green to pale green laterally rocks, sometimes displaying autobrecciation, occur in a impersistent beds have been masked to varying degrees by generally NNW-trending, steeply-dipping sequence with hydrothermal alteration and the development of Devonian schistose siliceous agglomerate and tuff. The extrusive cleavage, and in a number of localities, particularly at the rocks display folded bands with a very wide range of fold northern end of the Little Owen Spur [821406], the rocks hinge and axial surface trends due to the superimposition grade into schist - the Lyell Schist (Wade and Solomon, of tight, short wavelength (> 1m), NW-trending Devonian 1958). The hydrothermal alteration is readily recognised folds upon flow folds developed during extrusion. The by the replacement of feldspar with muscovite and quartz extrusives. as noted in thin section, commonly contain (Cox, 1981). Hydrothermal alteration extends west to a resorbed quartz phenocrysts and devitrified spherulites, northerly-trending boundary along the western slopes of and, in addition to hydrothermal alteration products, Little Owen Spur, and further west rock textures are well-defined Devonian cleavage seams of micaceous modified by Devonian deformation alone. material. Devonian deformation has also resulted in the development of ubiquitous quartz rods, quart-rich boudins Immediately west of £kvu on the Lyell Highway [821408] and pencil structures. occur west-younging, felsic layers (up to 100 mm thick) of light green and grey graded tuff interbedded with felsic This unit contains all the known economic base metal laminated tuff and usually grey feldspar-phyric deposits of the region (Hills, 1990) and has been termed agglomerate (£kvr) of a northerly-striking, steeply the Mine Sequence (Cox, 1981). In this region the east-dipping sequence. This succession apparently generally lenticular metal deposits are of similar trend to stratigraphically overlies the £kvu unit to the west (Cox, the units of the enclosing volcanic rocks. Much of the 1981 ), which consists of hydrothermally altered, variably mineralisation is considered to have developed in a pyritised, layered felsic feldspar-phyric agglomerate and hydrothermal phase related to the volcanism, which tuff. Further east, very thick and poorly-sorted dark grey resulted in disseminated pyrite-chalcopyrite, agglomerate layers are common and contain usually bornite-chalcopyrite, massive pyrite-chalcopyrite, and angular clasts of up to some 350 mm diameter in a mass ive stratiform pyri te-galena- sp haleri te-chalcopyrite tuffaceous matrix. These massive agglomerates are mineralisation (\Valshe and Solomon, 1981). Discussions evidently deposited from dense turbid currents. In areas continue as to the influence of later Devonian events, where Devonian deformation has imparted a schistose particularly in the evolution of enriched ore (Hills, 1990). structure to hydrothermally-altered siliceous volcanic The barite-chalcopyrite deposits are associated with lenses REPORT 1993/10 2 [432832] of brecciated hematitic chert, chert (ch) and intrusion are a number of uniformly thick chlorite-rich hematitelbarite (hb) (Walshe and Solomon, 1981; Reid, intrusions truncated to the south by an ENE-trending fault. 1975). The irregular lenses generally parallel the bounding Owen Conglomerate, and the hematitic deposits have been Near the mine mill a WNW-trending elongate occurrence suggested to be metamorphosed pre-Owen Conglomerate [420814], some 100 m wide and more than 400 m long, gossan, both in situ and transported (Solomon, 1969). consists of a chlorite-rich red and brown weathered However, the Owen Conglomerate shows hematite and succession of green to dark grey interlayered agglomerate, barite alteration at the margin, which suggests the outcome tuff and fine-grained porphyries (£ke). This intermediate of a Devonian hydrothermal event (Hart, 1992). The sequence contains common phenocrysts of albite and relict dull-yellow chert is of very fine quartz, patchily clinopyroxene in some tuffs (Cox, 1981). recrystallised, and the hematitic chert contains minor amounts of muscovite,