PAPF.:RS AND PROCEF...DlNGS OF THE RoYAL SOCIETY 01<' TASMANIA. VOLUME 100 THE GEOLOGY OF THE TRIAL HARBOUR DISTRICT
By
T. H. GREEN* Geophys-ics Department, A.ustralian National University.
(With 2 plates and 4 text figures.)
ABSTRACT supports an open, stunted scrub flora, but the (?) A Devonian (?) grantte complex intrudes Cambrian volcanics are covered by a luxuriant Cambrian (?) volcanics, serpentinised dunite rain forest. Consequently inland outcrop of the \olivine Fo,), and siltstones and quartzites of the volcanics is poor except for ridges exposed by bush Precambrian (?) Oonah Quartzite and Slate and fire and rocks exposed in Pykes' Creek. Pebble the Silurian Crotty Quartzite and Amber Slate outcrop of WJeathered Precambrian ('!) rocks. Formations. The general trends of these country dunite, Silurian rocks and rounded granite boulders rocks are parallel to the granite margins. in situ provide suitable means of mapping boundaries of these rock types inland. Vegetation The complex is composed of grey, pink and and soil colour changes often can be used as white granites all with the same mineralogical guides to the rock type (e.g., dark red soil on composition (quartz, orthoclase dominant, plagio a unite, red son on Cambrian (?) volcanics, grey clase subordi.natel. The white granite commonly soil on Precambrian (?) rocks and granite). contains tourrnaline nodular Emplace- ment began with the pink grey granites, Outcrop along the coast line strip is about 20- followed closely by the white granite, and finally 100 wide and nearly continuous, apart from the intrusion of minor bodies (porphyritic aplite, IA' R.S.·-2. 1 2 GEOLOGY OF THg TRIAL HARBOUR DISTRICT KEY TO ROCK TYPES TRIAL HARBOUR GEOLOGY OF THE TRIAL HARBOUR AREA ------~------~------·------FIG, 1.--Geological Map of the Trial Harbour Area. The altitude of the coastal plain ranges from Drainage of the area is dominated by the Little about 300' at the top of the coastline cliffs to Henty River, as its tributaries extend as far north 650' at the base of the Heemskirk Range. This corresponds to the lower coastal surface of Davies as the Cumberland Dam. The north-western part 0959). A range of hills between the plain and the of the area has a characteristic trellis drainage main part of the Heemskirk Range has concor pattern, governed by jointing in the granite. These dant summits 1300'-1600' high and may be an streams are short, flowing directly from the Heem expression of the higher coastal surface, while the Heemskirk Range (2200'-2500') possibly reflects the skirk Range into the sea. Faulting in the southern St. Clair surface, with Mt. Agnew (2700') standing part of the area governs the direction of some slightly above this level. small streams. T. H. GREEN 3 TABLE 1 Summary of the Stratigraphy in the Trial Harbour Area. Thickness System Forma.tion or Rock Type (approxJ 4. Quaternary Dunes, alluvium, gravels 3. Silurian Amber Slate 1200' + Crotty Quartzite 1100' + 2. Cambrian('?) Siltstones, tuffs or tuffaceous sub-greywackes and 2500' + tuffaceous siltstones, volcanic breccia lavas 1. Precambrian (? l Volcanics (? l 2400' + Oonah Quartzite and Slate At least two and possibly three old shorelines are thinly bedded U"-2''). Sandstones grade into are evident along the coastline: sub-greywackes with increasing matrix A comparison can be made between these rocks (b) Tuffs and the Montana Melaphyre Volcanics aA; Zeehan. The best outcrops of these rocks in an unmeta Blissett 0962) considers ·the Montana Volcanics morphosed state occur to the south of the as part of the Oonah Quartzite and Slate. Both Cambrian(?) slates and are in faulted contact these volcanic· suites occur near the boundary with the Crotty Quartzite (fig.1). Tuft's are also between Precambrian(?) Oonah Quartzite and Slate exposed in Pyke's Creek about half a mile from and overlying Cambrian(?) formations, but .the the Little Henty River (specimens 31927, 31928, original definition of the Oonah Formation does 31929, 31930, 31931, 31932, 31933). not include volcanic rocks (Spry 1958). The rocks vary from dense, dark grey types with 2. CAMBRIAN(?) occasional white or grey fragments representing Volcanic sediments, interbedded lavas and silt chert, quartz or feldspar to pale grey or red-brown stones occur in contact with granite on the coast, (iron stained) types. The fragments range up to are present inland south of the Precambrian(?) 2 •em. in size and are usually sub-angular, but rocks and compLetely surround the serpentinised rounded particles are also present (specimen dunite. ~n the south-western part of the area 31931). Cambrian(?) formations are in faulted contact with An examination of the thin sec•tions shows that Silurian strata. these rocks are composed of 60-85% irresolvable These formations are termed Cambrian(?) only matrix and 10-40% clastic quartz, feldspar and because of probable similarity with other Tasmanian rock fragments. Specimen 31928 is slightly meta basic volcanic suites forming part of the Cambrian morphosed and biotite, opaques and actinolite can System (e.g., Waratah, Ulverstone, Smithton and be identified in the matrix. The matrix of all King Island) (Banks 1962). Blissett (1962) records these rocks is probably volcanic in origin (although the occurrence of a trilobite on a ridge ~ mile without analyses this is uncertain) so that the south-east of the Little Henty River and 2 miLes rocks are termed tuft's, with a subordina.te clastic south-east of Trial Harbour and correlates the non-volcanic component (non-volcanic quartz and host rock with the Dundas Group. However it is composite quartz fragments). Since specimen not known whether this trilobite bearing forma 31933 is composed of about 25% lithic ·fragments tion is part of the same Cambrian(?) succession it is more accurately termed a lithic tuft'. Thin as the Trial Harbour volcanic suite and Blissett bands of opaques or .thin bands of the irresolvable correlates the Cambrian(?) rocks at Trial Harbour material are frequently visible in the groundmass with the Crimson Creek Group. The writer prefers of the slides (e.g., 31933, 31929). not to attempt such a correlation until more is Quartz varies from 10-30% of the rock and has known about the relative stratigraphic posi.tions of a grainsize from minute to 1 mm. It occurs as the Oonah Quartzite and Slate, the Crimson Creek discrete crystals or composite quartz fragments of Group and the Dundas Group in Western Tas chert or quartzite, which show microsuturing and mania and until the nature of 1the contac.t between undulose extinction of their component grains. the Oonah Quartzite and Slate and the The rock fragments have moderate sphericity and Cambrian(?) volcanics is resolved Accessory minerals present in the tuffs include The pebbles in the grey pebbly sandstone are opaques (magnetite, pyrite and ilmenite), rutile, usually sub-rounded with a moderate sphericity and apatite and tourmaline. may reach 1.5 em. in size. They are enclosed in a Some suggestions regarding the possible ongm sand grade matrix and have an open framework. of these rocks can be made from the foregoing Rounded depressions common in the rock possibly observations. The volcanic components of the represent brachiopod moulds. Coarse cross bedding sediments were transported rapidly from the is also evident in this formation. volcanic source area to a nearby depositional This formation passes gradually into a series environment where they were deposited after little of thinly bedded siltstones which contain Tenta or no reworking. Fine volcanic dust, larger volcanic culites. The presence of Tentaculites suggests lithic fragments, feldspar, volcanic quartz and that the siltstones belong to the Amber Slate opaque minerals were intermixed with no apparent (Blissett 1.962) so ,that ,the conformably underlying sorting. Quartz grains and composite quartz sandstone is the Crotty Quartzite (from the Eldon fragments derived from a reworked sedimentary Group succession defined by Gill and Banks 1950l. or a metamorphic source area were also carried into the depositional environment and mixed with Amber Slate the volcanic components. The flow-type structures The rocks of this formation strike consistently of opaques and sometimes the irresolvable matrix at 100 °-125 o and dip steeply both to the north and may have developed as the result of movement of to the south, although minor folding occurs, the fine grained material during consolidation. especially in the north-western part of the coast line outcrop. (eJ Breccia A breccia bed 60' wide crops out in thick scrub This formation consists of thinly bedded purple H miles east of Remine (see fig. 1). It cannot be brown or grey siltstones, cherty siltstones, cal traced for more than about a quarter of a mile. careous siltstones and more coarsely bedded quartzite. The siltstone beds are usually ~"-3" It is poorly sorted, consisting of angular-rounded thick while the quartzite beds reach 12 feet. The fragments from 1 mm.-5 em. in size set in a green quartzites are pale green or yellow and frequently (actinolitic) matrix (specimen 31934). The matrix show well developed cross bedding. Lateral varia comprises 50-60% of the rock. The rock has an tion between quartzite and siltstone may be rapid open framework and the fragments have a low so that a quartzite bed may change abruptly in sphericity. Most of these fragments are quartz or thickness or be split into thinner quartzite beds chert but actinolite, and one case granitic, frag separated by siltstone horizons. In spite of this ments have been observed. The rock has been variation, the quartzites, especially those with metamorphosed and the development of the cross bedding, provide useful marker horizons along actinolite matrix suggests that the original sedi the coast. Some of the coarser calc-silicate bands ment had a considerabl-e basic volcanic component, can also be used as marker beds. so 'that it could be termed a volcanic breccia, although portions of the bed occur which have a Metamorphism by the granite has resulted in sand or silt-rich matrix. Graded bedding is baking of the siltstones and development of calc evident, suggesting that the top of the near vertical silicate hornfels from the calcareous hands. The breccia bed is to the south. The different pro preferential weathering of the calcareous bands portions of actinolitic and silt matrix probably and lenses has given rise to well developed honey reflect variations in the amount of volcanic comb weathering. activity in the depositional area. Fossils are present, but metamorphism has (d) Lavas obscured much of the fossil evidence, destroying the finer detail, but strongly ribbed brachiopods, Bodies, possibly interbedded lava :flows, occur gastropods and crinoid stems are evident. Tenta in the Cambrian(?) volcanics on the shoreline near culites is plentiful on some bedding planes. the mouth of Montagu Creek (see p. 6). 3. SILURIAN Sedimentary Structures Silurian rocks are exposed in the south-western Sedimentary structures are common in inter corner of the area, where a hill of Crotty Quartzite bedded coarse and fine siltstones on the coastline occurs and Amber Slate crops out on the shoreline outcrop 1 mile south-east of Remine. These from ~-H miles south of Remine. structures include scours of coarse siltstone in fine siltstone (plate I, fig. ll, intrastratal flow in coarse Crotty Quartzite Specimen 31936 siltstone, roll structure of coarse siltstone in fine This formation is composed mainly of massively siltstone, and a "pull-apart" structure. bedded pale grey pebbly sandstone but purple sand stones also occur (specimen 31935). This sand Two hundred yards north-west of these structure~ stone is very well sorted (quartz grains range from the best developed cross bedding is exposed (plate 0.1-0.8 mm. in size). The larger quartz grains tend I, fig. 2) and nearby parallel ,grooves are evident to he well rounded with a moderate to high on the bedding planes of the thinly bedded silt sphericity, but the small grains are usually sub stones. These extend over distances of 50'; the angular to sub-rounded with a low to moderate origin of this feature is unknown, but it appears sphericity. A few grains are composite with to be a relict lineament accentuated by weathering. undulose extinction suggesting a metamorphic Boudin-like structures consisting of quartzite origin. Minor muscovite and accessory apatite, boudins enclosed in siltstone also occur. It is not zircon, tourmaline and opaque minerals are also ce1i;ain whether they have a sedimentary or evident. structural origin. The presence of pos.'lible ten- 6 GEOLOGY OF' THE TRIAL HAltBOUR DISTRICT sional cross fractures in the " neck " zone suggests in size. The presence of lath-like feldspar crystals, a structural origin. in contrast to the usual development of grana· The truncated current bedding and the scour blastic feldspar in the metamorphic aureole, sug· structures indicate that 'the beds face to the south gests an igneous origin for the rock. west. Hence the Amber Slate is slightly over Another possible basic mass of igneous origin turned for the north-western part (about a mile) within the Cambrian sediments occurs on the shore of the coastal outcrop but swings over to the right line near the granite contact north of Remine; it way-up for the south-eastern part (about a third is about 6' wide and tracefl!ble for 50'. However, it of a mile) of the outcrop. The cross bedding also is entirely recrystallised by thermal metamorphism suggests that the depositional currents giving rise (except perhaps for a small olivine lens) to basic to the cross bedded quartzite came from the south and ultrabasic hornfelses and no relict igneous eastern quadrant. textures have been observed, either macro- or micro scopically. It appears to cut across banding which 4. QUATERNARY is possibly relict bedding in the surrounding horn The Quaternary System is represented in the fels. area by river alluvium, dunes and gravels. A small basic c~yke 4" wide is exposed on the Alluvium ocurs along the margins of the Little shore platform only about 40' from the granite Henty River and on marshy fiats where small contact. It Is not visibly connected with the granite streams fiow onto low lying areas behind sand mass ard is cut by pyroxene veins. The dyke rock dunes between Remine and the Ltttle Henty River. weathers to a red-brown colour but is grey on Sand-dunes up to 30' high occur extensively fresh surfaces. \Vhite phenocrysts of plagioclase between Hemine and the Little River, attain-- (Ab.",) laths up to 3 mm. in size are present in a ing their maximum development the northern metamorphic groundmass mosaic of biotite, hypers end of Ocean Beach where they extend inland for thene and opaques coast, and reaches up to three-quarters of a mile any evidence of relict pyroxene, so that the term inland. The eastern contact extends as far south peridotite cannot be used in the sense that infers as the Little Henty River, following a general the presence of pyroxene Fo., Fa, was obtained for specimen 31871 from from Zeehan. It varies from pale to dark green refractive index determinations (a = 1.650 ± .004, and from a fine grained interlocking texture of fJ = 1.671 ± .006, 'Y = 1.693 ± .004). In this case crystals of hornbLende and feldspar less than Deer, Howie and Zussman's (1962) determinative 1 mm. in size, to a coarse grained gabbroic texture curves were used. Finally, as a further check, with hornblende and feldspar crystals up to 20 mm. analyses of 5 relict olivine cores in specimen 13254 in size. Magnetite is present in irregular aggre (obtained from a locality close to specimen 31875) gates up to 10 mm. across. were carried out using an ARL electron microprobe. The composition of these grains ranged from Plagioclase feldspar laths observed in thin section Fo, Fa. to Fo•• Fa,, with an average composition of reach 2 mm. and are subhedral. The maximum Fa., Fa.. Hence the most probable composition of symmetrical extinction of 37° measured indicates the olivine in the serpentinised dunite is Fo,, Fa.. a composition of Ab,. (i.e., basic labradorite). Hornblende occurs as anhedral crystals 1 mm. in The Origin of the Magnetite size. It is pleochroic from paLe green-bluish green Assuming the primary olivine of .the dunite was olive green. Small crystals of hornblende enclosed not zoned with iron rich outer rims surrounding within larger crystals having a different optical magnesium rich cores, the generally low fayalite orientation, are common, No evidence of relict pyr content of the relict olivine bears out the proposed oxene or of hornblende pseudomorphs after pyr occurrence of primary magnetite, since release of oxene were observed. Minor amphibole veins cut iron on serpentinisation of the forsterite rich across the rocks (31886). dunite could not give rise to all the abundant magnetite now present within the ultrabasic mass. About 1 mile east of Remine a small outcrop of a This explanation does not agree with resuLts gabbro-like rock (specimen 31886A) occurs within obtained by Bowen and Schairer 0935) for ·the dunite mass, near its eastern margin. The crystallization within the system MgO-FeO-SiO,. chief component minerals are actinolitic hornblende If equilibrium had been maintained throughout, and irregular, cloudy sericite aggregates, but minor the iron present should have been taken up by amounts of relict augite and plagioclase are also crystallization of forsterite-fayalite, rather than evident. The plagioclase shows relict lath-like crystallization of nearly pure forsterite accompanied form and relict multiple and simple twinning. The by magnetite. If equilibrium was not maintained, pyroxene has simple twinning. Evidence of then primary crystallization could result in the ophitic relationship between the pyroxene and formation of zoned olivines with forsteritic cores feldspar is also preserved. Irregular aggregates of and fayalitic margins. Zoned olivines are frequent semi-opaque material, frequently with very straight in basic volcanic and hypabyssal rocks (Deer, edges probwbly represent the complete alteration of Howie and Zussman 1962, Vol. I p. 23) but the an iron-rich olivine. Other minor alter•ation pro writer has seen no reference to such olivines in ducts are talc and chlorite. Hence this rock is an dunites. altered gabbro, probably initially an olivine gabbro A second possible alternative if equilibrium was and is the first recorded occurrence of gabbro not maintained during the crystallization, is that actually present in contact with the serpentinised magmatic segregations of magnetite occurred dunite in the Trial Harbour area. within the dunitic magma. Later metamorphism by the granite could have then given rise to ,the Devonian Igneous Rocks present veins and large disseminations of magnetite The representative Devonian igneous rocks of the while the minute disseminated grains may have area are the various granite types comprising the resulted from the serpentinisa tion of the olivine. Heemskirk Granite Complex. This stock-like com This last suggestion provides the most satisfactory plex occupies an area of about 35 square miles explanation of the observed features of the magne extending up to 6 miles inland from Granville tite and olivine in the serpentinised dunite. Hughes Harbour in the north to Trial Harbour in the south. 0958, 1959) has indicated a somewhat similar The overall shape· is semi-circular with the western origin for magnetite deposits in the Tenth Legion margin cut off by the sea. The regional trends in and Sav·age River areas, since he reg·ards rthese the country rocks surrounding the granite are deposits as Cambrian magmatic segregations, but approximately parallel to the granite margins, but in the case of the Savage River deposits in ·basic the igneous rock cuts across the bedding of the dykes Solomon (1962) favours pre-intrusion con sediments directly in contact with it, so that on centration and Spry (19-62) suggests that the dykes a regional scale the granite is roughly concordant may be older Precambrian. but on a small scale it is markedly discordant. The contact itself is vertical (plate I, fig. 4) or else dips Origin of the Dunite steeply towards the country rocks. The high forsterite content of the olivine indicates that the dunite could be classed as an Only the south-western corner of the complex orogenic-type ultrabasic mass. The contact has been mapped, but in .this small area several relationship between the Cambrian(?) country different granite types are distinguishable and these rocks and the duntte are confused by the later have been mapped separately (figs. 1 and 2). The metamorphic effects of the granite, so that no granite complex has been divided into the follow statement concerning the origin of the dunite can ing types depending on the texture, composition, be made. colour and form (the two major types are the pink granite and white tourmaline granite) Hornblende Gabbro (a) grey granite, (b) pink granite, (c) white tour Hornblende gabbro crops out on the Zeehan maline granite, (d) porphyritic aplite, (e) grey Trial Harbour Road at Mcivor Hill about ·6 miles micro-granite, (f) tourmaline nodular aplite, (g) T. H. GREEN 9 aplite, (h) pegmatite, (i) tourmaline veins, (j) granite, indicating that the latter is discordant to quartz-tourmaline dykes, Ck> pyroxene dyke and relict lithological banding in the country rock veins. (plate I, fig. 5). (a) Grey Granite (Specs. 31396, 31897, 31398, The average size of the crystals in the granite 31899) is about L6 mm. Graphic and myrmekitic inter The best outcrops of this rock occur at the granite growth between quartz and feldspar is present. hornfels contact on the shoreline three-quarters of Both minerals are usually anhedral but some sub a mile north of Remine. Wherever an actual hedral crystals do occur. Minor small quartz ;igneous/sedimentary boundary has been observed, crystals are commonly poikilitically enclosed in the granite at the contact is this grey type. Pink larger orthoclase and plagioclase crystals, which granite has never been seen directly in contact with are slightly cloudy. The maximum symmetrical the country rock. The maximum width attained extinction of multiple twinning; in plagioclase is by the grey granite is about 100' on the coast. 16° and the refractive index is greater than balsam Isolated patches of this granite within the pink so that the composition is Abe, or acid andesine. granite have been mapped along the shoreline and Biotite in the granite has two forms·- a grey granite zone up to 30' wide occurs between (a) Red brown to pale brown pleochroic flakes the porphyritic aplite and the pink granite and (0.2 mm.l occurring poikilitically between the pyroxene dyke (see p. 12) and the .enclosed in quartz and feldspar crystals pink granite. Thus the grey granite is a marginal in the granite in the immediate vicinity phase of the pink granite, appearing in the contact of the contact. This biotite is unaltered zones of the pink granite with both the country and is similar to the biotite in the horn rocks and other granite types. In the road cutting fels. about a quarter of a mile from Remine grey (b) Dark brown to pale brown pleochroic sub granite containing disseminated molybdenite crops hedral flakes C0.8 mmJ occurring as out in contact with quartzites. crystals interlocking with quartz and The rock is coarse-grained and the chief com feldspar. It is commonly partly altered ponent minerals are quartz and orthoclase in to chlorite and contains minute zircon crystals up to 1 em. in size. Subordinate plagio inclusions with pleochroic haloes. clase and biotite are also present. Tourmaline is .t\cessory zircon crystals with rhombic section in very rare. Biotite sometimes occurs in concentra the granite reach 0.2 mm. in size. tions of up to 20o/o of the rock so that minor variations of the grey granite grade into biotite The hornfels is composed of a granoblasrtic granite, but in the immediate contact zone (spec. mosaic of xenoblastic qua.rtz, feldspar and red 31900) there is a biotite free area 1-2 em. wide. brown biotite crystals 0.1 mm. in size. Minor mag There is no distinct chilling of the granite at the netite and appa.tite are also evident. The refractive contact but a slight general decrease in grainsize index of the feldspar is greater than balsam and the from 10 down to 4 mm. occurs. maximum symmetrical extinction angle of 17° indicates a composition of Ab". · Microscopically the contact is very irregular, with many tongues of granitic quar,tz-rich material Other specimens of grey granite (31896, 31897, extending into the country rock (specimen 31899). 31898) characteristically contain sagentic quartz The most characteristic feature of the thin section The clusters of irregularly oriented needles respon of this specimen is the presence of parallel bands sible for this texture tend to be concentrated near of magnetite grains ending abruptly against the chloritised biotite and sometimes near the margins FIG. 2.-Geological Sketch Map .of Coastline and Cliff Outcrop 1 mile North-North-West of Trial Harbour, showing Interrelationships of the Granite Types. 10 (;E;QLOGY OF THFJ TRIAL HARBOUR DISTHICT of the host quartz crystals. The needles are yellow maline in the pink and grey granites) . This granite or dark brown and are probably rutile (or in some type is the host rock for most of the tourmaline and cases tourmaline). They sometimes occur in feld tin bearing bodies in the granite complex \tour spar, tending to be oriented parallel to cleavage maline nodular intrusions, tourmaline segregations, directions. The orthoclase is anhedral and usually tourmaline veins, and quartz--tourmaline dykes) strongly perthitic, while plagioclase is subhedral It has a granular texture and the grainsize varies euhed.ral. The plagioclase is oligoclase in these from coarse (spec. 31907) to medium (spec. 31906) specimens, cCJntrasting with the andesine in the but in general it is finer grained than the pink contact zone. Some of the crystals are zoned and granite since the normal grainsize of the pink show more strongly sericitized cores surrounded by granite is 5-15 mm. compared with 1-5 mm. for this unaltered rims. Biotite present is pleochroic ,from granite. dark brown to pale brown. Accessory minerals observed include zircon (often in biotite), apatite, The main component minerals are quartz and magnetite and sphene. orthoclase w~th minor plagioclase and tourmaline. The tourmaline occurs as disseminated crystals or (b) Pink Granite \Specs. 31901, 31902, 31903, 31904, nodules or as irregular segregations up to two feet 31905) in size, in which long tourmaline columns project This rock crops out over the southern part of the from the walls into the apparent cavity that the complex and according to Waterhouse walls surround. Black tourmaline is most common is the main rock type comprising the but green, blue and brown varieties have also been Heemskirk Range. It is pink and coarse-grained observed. Minor accessory minerals also present consisting chiefiy of quartz and orthoclase with include biotite, muscovite and fluorite. The two plagioclase and biotite. Black mieas occur disseminated throughout the slides is occasionally present. examined but the muscovite (and more rarely Biotite occurs in some places concentrated in fluorite) is typically associated with the margins of layers about 1-2 em. thick and extending up to .tourmaline veins and quartz tourmaline dykes 100' with a fairly uniform dip and strike, though since the host granite is usually greisenised by the sometimes an undulatory effect is supermiposed on late intrusions. the g.eneral trend (see plate I, fig. 6). These In thin section the tourmaline appears inter biotite bands have been observed only within a stitial to quartz and feldspar. Prequently the short distance (440 of the contact with the quartz exhibits straight margins against the tour porphyritic aplite whtte tourmaline granite. maline. The pleochroism of the tourmaline is from The best of the bands occurs within 0 = indigo blue to E = pale blue suggesting that it 100 yards of the granite margin. The banding is an iron-bearing tourmaline, schorlite. present in the porphyritic aplite (see p. 11) is (d) Porphyritic Aplite (Specs. 31910, 31911, 31912, distinct from this biotite banding. 3Hll:3, 31914J Textural variations range from the normal, granular varieties with a crystal size of 0.5-1.5 em. This aplite forms a minor rock type in the granite (specs. 31801, 31902, 31903) to porphyritic types mass. '!'he best exposure occurs on the shoreline (specs. 31904, 31905) in which phenocrysts of pink near a mutton bi.rd rookery one mile north-west of orthoclase up to 1.5 em. in size occur in an aplitic Remine. Small outcrops also occur in the Federa groundmass with a grainsize of about 0.5 mm. A tion Mine area and on the coastal plain, but the conspicuous feature of the pink feldspars both in .exposure is not good enough to map these bodies. the and. normal pink granites is a rim The texture and gra.insize within the mass is of commonly surrounding the main extremely variable ranging from coarse (5-10 mmJ part of the A thin section of specimen or normal phases of the grey micro-granite (type 31904 shows where the orthoclase phenocrysts CeJ to the normal porphyritic aplite phase in which come into contact with plagioclase a myrmekitic quartz phenocrysts up to 4 mm. in size occur studded intergrowth between the two develops. The white in a medium grain (0.5 mm.) aplitic groundmass. rims of the orthoclase crystals may thus represent a Macroscopic pegmatitic segregations in the aplite myrmekitic intergrowth, with white plagioclase are common and are composed of tourmaline, at the expense of the orthoclase. In quartz, feldspar (orthoclase) or biotite. Tour specimen a large lath-like crystal with the maline veining is pronounced. and greisen zones form of a subhedral orthoclase crystal, i,s are associated with these veins (see pla,te II, ftg. ll. seen in the section to be composed of Biotite nodules (specs. 31915, 17404) up to six crystals of with minor inter- inches across are present, and smaller nodules of stitial This suggests that the a red 18873J, dominantly almandine t"cldspar is being replaced by quart.z. Petrographi AL,., or Sp,,.), been identified. Disseminated cally the pink granite is identical to the grey grains of molybdenite and pyrite also occur in the granite. porphyritic aplite. Tourmaline nodules have (c) White Tourrnaline Granite (Specs. 31906, 31907, developed, but not to the extent observed in the 31908, 31909) tourmaline noC!ular aplite. This granite occurs to the north of the pink The porphyritic aplite .is light grey when fresh granite, beginning about one mile north-west of but weaLhers to a yellow colour. It consists chiefly Trial Harbour and extending up th.e coast to of quartz and orthoclase with minor amounts of Granville Harbour. It varies from yellow to white plagioclase, muscovite, tourmaline and biotite. and as the name tourmaline is an essential Thin sections of specimens 31910, 31911, 31912 accessory mineral contrast to the rare tour- show contacts between porphyritic aplite and grey 11 micro-granite, and for completeness the petro The quartz generally forms coarse myrm.ekitic graphic descriptions of both granite types will be intergrowth with orthoclase, but in the quartz given in this section. The contact between the two muscovite bands an intricate semi-reticulate inter types is sharp, but very irregular. The essential growth between the quartz and muscovite is present. components of both are quartz, orthoclase, plagio The muscovite is very ragged. clase and accessory chloritised biotite, muscovite Other variations of rock types in the porphyritic and tourmaline, but the quartz content seems to aplite include a greisen (spec. 31913), consisting of be higher at the expense of the feldspar, in the quartz (phenocrysts with vacuole streams and clear porphyritic aplite than in the grey micro--granite. groundma5's crystals) and muscovite. Porphyritic aplite Biotite nodules C3191f)) are also characteristic. This specimen consists of a central spherical core Quartz occurs in two forms: as anhedral-sub composed of plagioclase, biotite and quartz with hedral phenocrysts and as anhedral grains only subordinate muscovite, accessory apatite and sub about 0.4 mm. in size commonly enclosed in, or rounded zircon, surrounded by a zone 5 mm. wide forming myrmekitic intergTowth with, orthoclase. which is quartz-orthoclase rich and poor in biotite Separated crystals of quartz are frequently in and plagioelase. This zone is succeeded by a grey optical continuity. These anhedral quartz grains micro-granite phase extending irregularly into the do not contain minute inclusions, but streams of aplite host rock. The biotite of the these inclusions occur in the anhedral-subhedral core has the typical pleochroism of the quart?;. The " small crystal " type possibly repre biotite from the contact metamorphic zone 0 = red sents quartz replacing orthoclase. Orthoclase is brown, E ~c pale brown! which suggests that the markedly dusty and the crystals are usually nodule may be a xenolith. The plagioclase is anhedral, though some subhedral forms are albite. present. Plagioclase is strongly s.ericitized, giving a false impression of moderate relief. It occurs (el Grey Micro-granite as euhedral-subhedral laths. It is albite. Minor This granite is a minor but distinct textural chloritised biotite, shredded muscovite and rare variation occurring within the porphyritic aplite brown tourmaline crystals are also present. mass, most commonly developed at the margins of The porphyritic aplite in specimen 31912 appears this mass with either the grey granite or the white to have developed from the microgranite by super tourmaline granlte. Apart from its occurrence, position of the aplitic texture on the granitic it is distinguishable from the white tourmaline texture, due to the development of anhedral grains granite lac!' of tourmaline and higher per- and aggregates of quartz forming a myrmekitic centage biotite. It is distinct from the grey texture at the expense of the feldspar. granite because of its aplitic texture but some variations are coarse and very -similar to the grey Grey Micro-granite granite. The contact between the two is however The quartz is clear and colourless and sharp and I see plate I, fig. 6). The dctaiied contains streams of minute dark inclusions. descr~ption been given on this page with the crystals range from 0.5-1.5 mm. in size and are porphyritic aplite because the two occur in the anhedral-subhedraL Orthoclase forms interlocking same specimens (31910, 31911, 31912l. anhedral-subhedral with quartz; the (f) Tourmaline Nodular Aplite (plate II, fig. 3) crystal size varies from rnm. It is very dusty in contrast to the clear quartz and, to a lesser This body is mapped as a separate granite type extent, the plagioclase. Simple twinning and per because of the abundance of tourmaline nodules thitic texture are common. contained in it and because it occurs as discrete masses within the white tourmaline granite. The Plagioclase occurs in subhedral-euhedral laths, best development of the ncdular aplite is attained 0.5 mm.-1 mm. in strongly altered to musco- in the boundary zone between the tourmaline vite. It is albite relief appears to be less than balsam. granite and the porphyritic aplite. The tourmaline nodules range from 1 ern. to crystals of chlorltised biotite and clear, 15 em. in diameter. They usually form remarkable crystals of muscovite are common spherical bodies, although in some cases adjacent accessories. This muscovite occurs in interstitial nodules have joined to produce a " dumb-bell" aggregates in contrast to the muscovite developed shape. Black tourmaline, quartz and minor :feld by sericitization oi plagioclase. spar are the mcst common component minerals, but blue and tourmaline, Banded porphyritic aplite (Specimen 31914) fluorite and occur in nodules. Due A characteristic banded occurs at the to insufficient time. a detailed investigation of the contact zone between the tourmaline and the mineral associations in the nodules could not be porphyritic aplite (see P. 13 and plate II, fig. 4l carried out, but it is probable that definite associa Similar banded aplites, accentuated by weathering, tions (such as blue tourmaline, fluorite and cassite are exposed nearby, within the borders of the rite) occur. Another investigation which could porphyritic aplite mass (see plate II, fig. 2). The lead to profitable results relating to their origin is rock is pale yellow beneath a \Yhite weathered skin. a study of the textural relations between quartz It is with a medium grainsize (0.6 mm.). and tourmaline. In some nodules tourmaline The is formed by grey quartz-muscovite columns appear to project in sheaf-like aggregates bands from 0.5 to 3 mm. thick varying in from the centre to the walls of the nodule and the spacing, space between columns is fiiled with quartz, while 12 GEOLOGY OF THE TRIAL HARBOUR DISTRICT in other nodules It consists of large dark green euhedral-anhedral Stereographic Investigation of Aplite, Tourmaline pyroxene crystals enclosed in an aplitic ground and Quartz-Tourmaline Veins or Dykes and mass, which in turn is in contact with grey granite Biotite Bands in the Granite Complex merging into normal pink granite. On weathered Biotite Banding surfaces the aplite surrounding the pyroxene crystals in the dyke shows peculiar intricate Twenty poles to biotite banding in the pink "worm-like" patterns, the cause of which is granite are plotted in figure 3" A marked concen unknown. tration occurs, indicating that most bands strike at about 60' and dip to the south a,t 25°. This Single crystals of pyroxene may reach 10 em. in maximum is significantly close to the strike and size. In thin section it is pale green (31920, 31921). dip of the planar contact between the porphyritic The composition is 34% diopside, 66% hedenbergite aplite and the pink granite (the pole of this con ± 4% (refractive indices: a= 1.709 ± .003, {3 = tact is plotted on the same diagramJ . This sub 1.718 ± .005, 'Y = 1.735 ± .003). Other minerals stantiates the proposal that the biotite bands are present include axinite, sometimes showing comb margina1 flow phenomena in the pink granite, structure, sphene, apatite and calcite. possibly induced by the later intrusion of por phyritic aplite (see p. 15)" 'l'he aplitic groundmass to the pyroxene pheno crysts is white and medium grained (0.5-2 mm.). Aplite Veins or Dykes The chief component minerals, quartz, plagioclase, and orthoclase, occur in two distinct forms. One hundred poles to aplitic bodies have been plotted in figure 8. There are three marked con (a) Specimen 31918. In rthis specimen the quartz centrations, representing aplite veins with a strike is frequently embayed and contains many streams of 68" dip 60° N, and 98' dip 75o N, and 50° dip of minute inclusions" Crystals 2 mm. in size occur. 25° S. The last set has a similar orientation to the The feldspar is intensely altered and cloudy and biotite bands and represents aplitic veins or bands cannot be differentiated into feldspar types. The which commonly accompany the biotite banding" crystals reach 2 mm. in size, varying from anhedral The other two sets are unrelated to jointing in to subhedral. Accessory minerals in this sli.de are the granite, in contrast to the later forming tour characteristic:- maline veins and quartz-tourmaline dykes (see below) and so some factor other than jointing Sphene occurs as small anhedral aggregates must have governed the orientation of the aplitic or as discrete wedge-shaped crystals and intrusions. It would be of interest to determine is brown. whether these sets of aplite veins are constant in Apatite is evident as anhedral grains (0.05 mm) orientation throughout the complex, or whether with irregular fracturing" variations occur, and the pattern obtained for Chlorite is pale green with anomalous blue aplite in the south-western corner of the complex interference colours and moderate relief. is only a local expr.ession of an overall pattern for It is interstitial to the feldspar and has a the whole complex. shredded form. The interference figure is Tourmaline Veins and QuaTtz-Tmtrmaline Dykes negative with a low axial angle" F'igure 3 shows that the tourmaline veins and the F'ibrous mineral aggregates forming a fan quartz-tourmaline dykes have similar girdle con structure occur and are similar to chlorite entrations with a strike of 15° and a near ver,tical but have a stronger birefringence than is dip. The presence of a well developed girdle normal for chlorite. Interstitial carbonate maximum is probably slightly exaggerated because (probably calcite) is present, as well as in many cases the dips of the bodies could not be small brown weakly pleochroic crystals of determined accurately, but appeared to be very tourmaline. Finally a colourless, interstitial steep. Comparing the concentration of these poles mineral with moderate relief and strong with figure 4 showing a stereographic plot of the birefringence has been observed. Some poles of 500 joints in the granite, it is evident that slightly anomalous interference colours are the dykes and veins correspond with the weaker evident. It has a biaxial positive inter of the two girdle eonc.entrations of joint poles" It ference figure with a moderate axial is not known why these late intrusions should be angle. It typically occurs in acute-angled identical in orientation with one set of joints and sections surrounded by a low birefringent rim. It is probably prehnite pseudo not the other, more prominent set. morphing axinite, because axinite often Interrelationships of the Granite Types and the occurs as acute-angled crystals, and is Origin of the Complex present in the basic dyke (spec. 31921). At a point C1 mile north-west of Remine) (b) Specimen 31920. In this slide the ground on the shoreline near the southern boundary of mass quartz and plagioclase form a granular the white tourmaline granite, a characteristic mosaic (grainsize 0.2 mmJ . The plagioclase has a banded structure is exposed marking the contact composition of Abao (andesine>. It is clear and between the white tourmaline granite, the tourma unaltered in contrast to strongly sericitized ortho line nodular aplite and the porphyritic aplite (see clase and has a weak preferred orienta.tion. Acces plate II, fig. 4). The tourmaline granite passes sory sphene and prehnite are also present. The gradually into the tourmaline nodular aplite textur·e of this groundmass is quite distinct from due to increasing abundance of nodules and the texture of an aplite type (g) but similar to the decreasing grainsize, but the nodular aplite passes quartz-feldspar mosaic of the hornfels. abruptly into a microgranite phase marking the 14 GEOLOGY OF THE TRIAL HARBOUR DISTRIC'T 0 . 0. 01-5 7. 85-107. EEEBl0-20 7. a -20-407. DO-l '1. Bl-6 ·;. EEEB6-10 '/. c d -10-16 "/. FIG. 3.-Stereographic Plots (lower hemisphere) of- (a) Biotite Banding in the Granite (20 poles). (b) Aplite Veins and Dykes ( 100 poles). (c) Tourmaline Veins (100 poles). (d) Quartz-tourmaline Dykes (75 poles). T. H. GREEN 15 base of the porphyritic aplite. The microgranite the mass cooled. This would explain the apparent passes into a banded aplite and finally into the gradational contact between the tourmaline normal porphyritic aplite. nodular aplite and the white tourmaline granite described on page 11. Volatile rich segregations An actual contact between pink and white throughout the white tourmaline granite magma granites has not been observed because of poor gave rise to the isolated patches rich in tourmaline outcrop. Where rock exposures are good on the nodules and the tourmaline bodies observed. shoreline an intervening porphyritic aplite occurs between the two granites. As the aplite cannot The last major phase of igneous intrusion in the be traced away from the coastline, it is not known Heemskirk Complex involved the tourmaline veins whether it is merely a local intrusion or if it always and the quartz tourmaline dykes. These were occurs as an intermediate rock type between the accompanied by greisenization and mineralization. white and pink granites. The fact that the con The direction of intrusion of these bodies is related tact between the porphyritic aplite and the two to, and was probably governed by fissuring granites is sharp (e.g., plate II, fig. 1) and not developed in the granite (fig. 4). gradational suggests that the aplite is a late A possible alternative origin for the different intrusion. granite types in the complex could be argued from a metasomatic viewpoint by attributing the different There is no definite evidence for chilling at any types to changes taking place in the original pink of the granite/granite contacts. Any grainsize granite resulting from the activity of volatile rich variations that may ocur are also evident in areas fluids passing through the granite. The fluids were away from contact zones and so cannot be con active just prior to or simultaneously with the sidered as contact phenomena. However biotite intrusion of the tourmaline veins and quartz-tour banding in the pink granite (plate I, fig. 6) and maline dykes. However this theory would not muscovite-quartz bands in the porphyritic aplite explain the sharp contacts observed between the are more pronounced in rocks near the contact porphyritic aplite and the pink granite shown in between these two bodies (plate II, fig 2). The plate II, fig. 1, nor could it ,explain the rarity of writer considers that this banding is due to fiow tourmaline segregations in pink granite compared in the granitic magma during intrusion and is more with the common occurrence in white granite of pronounced near the margins of the intruding disseminated tourmaline segregations free of bodies where viscous drag in moving magma would surrounding alteration of the host rock. This be the greatest. lack of alteration points to actual magmatic The intrusion of the porphyritic aplite and the crystallization of tourmaline rather than a post white tourmaline granite probably occurred after crystallization metasomatic formation of the the intrusion of the pink granite magma, but mineral. before the latter mass had cooled, so that no Minor variations within the white tourmaline chilling or discordant brecciated structures granite mass are however, explainable by meta resulted. The porphyritic aplite and the white somatic activity aecompanying the intrusion of granite were separate magmas as the contact tourmaline veins and quartz-tourmaline dykes. between the two is sharp. The banding developed The chief result is the replacement of orthoclase by in the aplite is roughly parallel to the contact quartz. This is best developed in the porphyritic and suggests relative movement of the two bodies aplite and the white tourmaline granite where the producing fiow banding in the still fiuid aplite. tourmaline veins are most common. Thus the writer suggests that the porphyritic aplite magma was later than both the major granite The grey granite is only a marginal variation of types, and intruded between the white and pink the pink granite and is not a separate intrusion. magmas while they were still hot. Volatile-rich It is not known why the orthoclase changes colour segregations in the aplite magma formed tour but two possible explanations are:- maline and quartz patches when the magma crys (i) Temperature of formation is governing tallized. The quartz phenocrysts possibly represent factor in imparting a pink colour Gower original quartz crystals suspended in the magma. temperature of the margin results in The presence of almandine or almandine-spessar more rapid crystallization and the forma tite nodules in the porphyritic aplite and not in tion of grey orthoclase). other granite types suggests special condttions Oil Bleaching of the feldspars of the pink existing, at least locally, in the aplite magma granite caused by loss, to the rocks allowing the formation of the nodules on crystal adjoining the margins, of the component lization of the magma Other faults have also been mapped in the country rocks but they are only minor, with dis placements of only a few feet. Nature of the Contact between the Precambrian (?) and Cambrian (?) Rocks There is an unsettled controversy concerning the relaLion between the younger Precambrian sedi ments Bischoff", resulted in the erection of batteries, terite and pyrite. In one such prospect 200 yards driving of adits and sinking of shafts into outcrops south of the Black Face. cassiterite is associated which had not been sufficiently well prospected. with well developed cubic pyrite crystals (specimen This latter fact added to the confusing eft"ect of 12565). In spite of the extent of the Federation the abundant black tourmaline (frequently mis Workings, especially in the Black Face area, com·· taken for cassiterite) culminated in the collapse paratively little tin has been obtained (less than of most of the mining ventures, and gave the field 800 tons of cassiterite (Blissett 1962)). Large a bad name. seale mining operations finally ceased in 1938. Subsequent mining operations have centred In the Rederation area tourmaline nodular aplite, around the Federation area ,and at Mayne's Mine tourmaline veins, and quartz-tourmaline dykes (where a very r.ich pocket of tin was discovered) . occur in a white tourmaline granite host rock. Waterhouse 0916) thoroughly described all the Crustification texture in quartz may develop along mines and prospects of the area. Since many of the margins of the tourmaline veins (e.g., specimen these were still in operation or were still accessible 18650). Black tourmaline is the most common at the time of Waterhouse's investigation, his tourmaline variety, but green and blue tourmaline observa,tions were far more complete ,than any the are also evident (specimens 18649, 18650, 18651). writer could make. For this reason the descrip The cassiterite is usually associated with the latter tion of the economic geology of the area is limited. two varieties. Muscovite and fiuorite are also Since 1916 adits have been driven in the Black present. The cassiterite either occurs as dis Face area and at Goleman's workings, but these seminated grains in the granite host, in veins or have not resulted in any significant discoveries of in rich segregations and varies from a black to tin. dark-brown colour. Other minerals recorded include bismuthinite and wolframite (Waterhouse Mineralisation 1916). (a) Associated with the Serpintinised Dunite Coleman's Workings On a ridge a quarter of a mile east-south·-east These workings are the only ones at present of Remine, three adits and a shaft have been driven producing tin in the district. E. Coleman has mined into a nickel-bearing dunite. The writer has tin on this lease since 1943. It is situated on a identified heazlewoodite (see p. 7 J and zaratite 1.40'0' spur marking the source of Monta.gu Creek from the workings (specimens 10532 CD, 10532 OD J. and is about 500 yards south of the Central Work Waterhouse (1916) described the nickel workings ings of the Federation Mine. White tourmaline fully and Williams 11958) gives a deta,iled account granite containing some tourmaline nodules is cut of the mineralogy. Williams pmposed that the by tourmaline veins and quartz-tourmaline nickel accompanied the original intrusion and dykes the tin mined has been obtained from segregated before serpentinisation. He records the rich segregations, tourmaline nodules and dis presence of c'hromite and sphalerite as well as the seminated grains, similar to the occurrence in the nickel bearing minerals pentlandite (most Federation Workings. Brown and black cassiterite abundant), heazlewoodite, shandite CNLPb,S,) and are present, and minute crystals of native copper millerite (a breakdown product of heazlewoodite). or cuprite are occasionally evident (specimens Blissett (1962) records assays of 18.6% and 14.6'/a 12566-70, 10023). This depostt, is low grade (prob nickel from samples from the dumps. ably it would bulk about 1 o/o cassiterite) and its Local segregations and veins of magnetite also possible development is hampered by its location occur in the dunite but these are not extensive on top of a steep ridge. enough to he economically important. Another mine of interest is Sweeney's Mine, which Cb) Associated with the Granite is about half a mile south of Cumberland Lake. The lode occurs in quartz-tourmaline dykes in pink The mineralisation associated with the granite is granite and contains sphalerite, cassiterite and the only mineralisation of economic significance stibnite in a gangue of pyrite, siderite, quartz and in the area. The tin is invariably connected with fiuorite (specimens 10491, 13517). late tourmaline rich intrusions in the granite, and most of the old workings occur in the white tour The best cassiterite obtained by the writer came maline gmnite, where it is cut by quartz-tourma1ine from a dump at a prospect about a quarter of a dyk,es or tourmaline veins. A few mines are mile north of the Cliff Mine. This specimen present in the pink granite and are also associated (12572) contains cassiterite crystals up to 0.5 em with quartz-tourmaline intrusions, e.g., Empress, in size occurring in a gr.eisen groundmass consist Victoria, Sweeney's and Wakefield and in one case ing of muscovite, fluorite, quartz and green tour (Mayne's Mine) tin mineralisation occurs in the maline. The shaft from which the specimens Oonah Quartzite and Slate country rocks, accom originated is now flooded. panied by extensive tourmaline veining. The remaining mines in the district (Prince George, Cornwall, Cliff, Empress Victoria, Montagu, Federation Workings Wakefield and Mayne's) contained cassiterite Most of the mining operations in the area have associated with tourmaline and quartz, and been carried out in the Federation Mine Creek, occasionally pyrite and arsenopyrite. extending from the Western Workings (Geason's, A similar association of tin with tourmaline has Tributer's, Fowler and Dunn's) through the Central been recorded from granites in south-west England Workings (Black Face, Munro's, Long Tunnell to (Dewey 1948) and in Malaya (Hutchinson and the minor Eastern Workings. In between these Leow 1963). Tourmaline nodules and late cutting mines there are many prospect trenches and pits in quartz-tourmaline dykes have also been recorded in quartz-tourmaline bodies containing minor cassi- the Malayan granites. T. H. GREEN 19 GEOLOGICAL HISTORY CAREY, S. W., 1953: Geological Structure of Tasmania in Relation to Mineralisation. 5th Emp. Min. Metall. Congr., The geological record in the Trial Hal'bour area Vol. 1, pp. 1108-1128. began with the deposition of sands and silts in late and BANKS, M. R., 1954: Lower Palaeozoic Unconformities in Tasmania. Pap. roy. Soc. Tasm., VoL Precambrian(?) times, with possible volcanic 88, pp. 245-269. activity towards the end of sedimentation. Fold CHINNER, G. A., 1962: Almandine in Thermal Aureoles. J. ing, uplift and erosion may have occurred before Petrol., Vol. 3, pp. 316-340. DAVIES, J. L., 1959: High Level Erosion Surfa.ces and Land subsequent sedimentation and volcanic activity scape Development in Tasmania. Aust. Geogr., Vol. 7, pp. resulted in deposition of siltstones, tuffaceous silt 193-203. stones, tuffs and breccia, accompanied by inter DAVIS, G. A., 1963: Structure and Mode of 1E'mplacement of Cari1bou Mountain Pluton, Klamath Mountains, Cali ... mittent basic lava fiows on the ocean fioor. How fornia. Bull. geol. Soc. Amer., Vol. 74, pp. 331-348. ever conformable deposition of Cambrian (?) rocks DAVIS, G., 1962, p. 265 in SPRY, A., 1962: Igneous Activity. on Precambrian (?) cannot be rul.ed out, because of The Geology of Tasmania. J. geol. Soc. Aust., Vol. 9, pt. 2, pp. 255-2.84. uncertainty of the contact relationship. Emplace DEER, W. A., HOWIE, R. A., and ZuSSMAN, J., 19·62: Rock ment of serpentinised dunite also probably took Form'ing Minerals. Longmans, Green and Co. Ltd., Vols. place in the Cambrian Period. 1, 2, 3, & 5. DEWEY, H., 1948: British Regional Geology, South-West There is no record of the Ordovician Period in England. Dept. Sci. Ind. Research, Geol. Surv. & Mus. the area, and the next rock type to form was the EDWARDS, A. B., 1953: Heemskirk-Zeehan Minel'al Field. 5th Emp. Min. Metall. Congr., Vol. 1, pp. 1209-12.12. Silurian Crotty Quartzite, followed concordantly EvERNDEN, J. F. and RICHARDS, J. R., 1961: Potassium Argon by siltstones, calcareous siltstones and ·an occa Ages in Eastern Australia. J. geol. Soc. Aust., Vol. 8, pp. sional cross bedded quartzite, all comprising the 1-49. GILL, E. D., and BANKS, M. R., 1950: Silurian and Devonian Amber Slate. These two .formations were probably Stratigraphy of the Zeehan Area, Tasmania. Pap. roy. deposited in shallow water. Soc. Tasm. (1949), pp. 2519·-271. GREEN, D. H., 1959: Geology of the Beaconsfield District, Folding possibly took place before the next phase Including the Anderson's Creek Ultrabasic Complex. of activity, when in Devonian(?) times, a major Rec. Q. Viet. Mus. No. 10. plutonic igneous complex was intruded, beginning HEss, H. H., 1938: A Primary Peridotite Magma. Am. J. Sci., Vol. 35, pp. 321-344. with a pink granite, followed closely by a white -----,. 1949 : Chemical Compositions and Optical Pro tourmaline granite, and finally by minor intrusions perties of Common Clinopyroxenes, Part 1. Amer. Min., of porphyritic aplite, aplite, pegmatite, quartz Vol. 34, pp. 621...&66. HUGHES, T. D., 1958: Savage River Iron Ore Deposits. Tech. tourmaline dykes and tourmaline veins, accom Rep. Dep. Min. Tasm., No. 2•. panied by cassiterite mineralisation and local ------, 1959: Magnetite Deposits at Tenth Legion, greisenisation of the granite. Comstock District. Tech. Rep. Dep. Min. Tasm., No. 3. HUTCHISON ,C. S. and LEOW, J. H., 1963: Tourmaline No record is preserved of the Carboniferous Greisenisation in L·a.ngkawi, North-West Malaya. Econ. Geol., Vol. 58, pp. 587-592. Permian Periods. The final events recorded are REID, A. M., 1923: Osmiridium in Tasmania. Bull. geol. Surv. probably sub-aerial erosion giving rise ·to prominent Tasm., No. 32. topographic levels (e.g., lower ·coastal surface). SoLOMON, M., 1!962 : Mineral Deposits in Geology of Tasmania. Minor Quaternary changes in sea level are also J. geol. Soc. Aust., Vol. 9, pt. 2, pp. 285-310. SPRY, A. H., 1958: Some Observations of the Jurassic Dolerite evident. of the Eureka, Cone Sheet, near Zeehan, Tasmania. Dolerite Symposium. Univ. of Tasm., pp. 93-129. -----,, 1962a: The Precambrian Rocks in Geology of Tasmania. J. geol. Soc. Aust., Vol. 9, pt. 2, pp. 107-126. ACKNOWLEDGEMENTS -----,, 1926b: Igneous Activity in Geology of Tasmania. J. geol. Soc. Aust., Vol. 9, pt. Z, pp. 255-284. This work was c·arried out as part of an Honours ----- and FoRD, R. J., 1%7: Reconnaissance of the course and the writer in indebted to the staff and Corinna-Pieman Heads Area, Geology. Pap. roy. Soc. students of the Geology Department, University of Tasm., Vol. 91, pp. 1-7. TAYLOR, B. L., 1955: Asbestos in Tasmania. Miner. Resour. Tasmania for helpful discussion and suggestions Tasm., No. 9. during the year. Special thanks are due to Dr. THAYElR, T. P ., 1960: Some Critical Differences between Alpine A. H. Spry who supervised the work. The writer Type and Stratiform Peridotite-Gabbro Complexes. 21st Int. geol. Congr. Copenhagen. Pt. XIII, pp. 247-2519•. also wishes to gratefully acknowledge the help of TURNER, F. J., and VERHOOGEN, J., 1960: Igneous and Meta Mr. W. Brook, who carried out a plane truble and morphic Petrology. McGraw-Hill Book Co., 2nd edition, geological survey of the coastline outcrop in 1960 694 pp. TWELVETREES W. H., 1900: Report on the Mineral Deposits of and kindly allowed use of his map for a detailed Zeehar{ and Neighbourhood. Rep. Secy. Min. Tasm., coastline study. The outcrop outline of fig. 2 is (1900-1). pp. 5-108. taken from this map. Thanks are also due to ------,, 1913: Bald Hill Osmiridium Field. Bull. geol. Surv. Tasm., No. 17. Mr. and Mrs. T. McGee who generously allowed use WADE, M., and SOLOMON, M., 1958: Geology of the Mt. Lyell of their cottage at Trial Harbour during field work. Mines, Tasmania. Econ. Geol., Vol. 53, pp. 367-416. WALKER, K. R., 1957 : Geology of the St. Helens-Scamander Area. Pap. roy. Soc. Tasm., Vol. 91, pp. 109-114. REFERENCES WALLER, G. A., 1902 : Report on the Tin Ore Deposits of Mount Heemskirk. Rep. Dep. Min. Tasm. BANKS, M. R., 1962: Cambrian System in The Geology of and Hooo, E. G., 1903: The Tourmaline Tasmania, J. geol. Soc. Aust., Vol. 9, pt. 2. pp. 127-146. Bearing ' Rocks of the Heemskirk District. Pap. roy. BLISSETT, A. H., :1959: The Geology of the Rossarden-Storeys Soc. Ta81n., ( 1902), pp. 143-156. Creek District. Bull. geol. Surv. Tasm., No. 46. WARD, L. K., 1'9109: Contributions to the Geology of Tas ------, 1962: Explanatory Report. One Mile mania~Precambrian Rocks. Pap. roy. Soc. Tasm., P·P· Geological Map Series-eehan. Dept. Min. Tasm. 129-156. BoONE, G. M., 1962: Potassic Feldspar Enrichment in Magma; WATERHOUSE, L. L., 1916: The South Heemskirk Tinfield. Origin of Syenite in Deboullie District, Northern Maine. Bull. geol. Surv. Tasm., No. 21. Bull. geol. Soc. Amer.• Vol. 73, pp. 1451-147'6. BOWEN, N. L. and SCHAIRER, J. F., 1935: The System Mg0- WILLIAMS, K. L., 1958: Nickel Mineralisation in Western Fe0-Si0.. Am. J. Sci., Vol. 29, pp. 151-2!24. Tasmania. F. L. Stillwell Anniverstary Volume. Aust. BUDDINGTON, A. F., 1959: Granite Emplacement with Special Inst. Min. Metall., pp. 263-302. Reference to North America. Bull. geol. Soc. Amer., YODER, H. S. jr., and SAHAMA, Th. G., 1957: Olivine X-Ray Vol. 70, pp. 671-747. Determinative Curve. Amer. Min., Vol. 42, pp. 475-491. 20 GEOLOGY OF THE TRIAL HARBOUR DISTRIOT PLATE l, PIG. L~·~Scour of coarse siltstone in fine siltstone. FIG. 2.--Current bedding in quartzite of the Amber Slate. FIG. 8.--Minor basic intrusion ; plagioclase laths set in gra.no~ blastic mosaic of hypersthene and biotite ( Ord. light; s-cale mark == 0.5 mn1s). FIG. 4.~Granite (right)jcountry rock (left) contact on the shoreline outcrop. Outcrop about 20' high. FIG. ·5.-Photomierograph of granite/country rock contact. Banding in the country rock (right) ends abruptly against tbe granite (left). (Ord. light; s·cale mark == 0.5 mms). FIG. 6.-Biotite banding ,and quartz veining in pink granite. The contaet between the grey granite (marginal phase of the pink granite) and the grey microgJ~anite (marginal phase of the porphyritic aplite) occurs near the base o:f the photograph. PAPl1JRS A:ND PROCEEDINCS 0'/ THE RDYAL SOClElTY OF TASMAK:A, VOLl.JMf 2 4 6 F.P.20 PAPF_;RS AND PHOt'r::E;DING8 OF THE Rt)YAI, SOCIETY OP TAfiMAN!A, VOLUMb 100 PLATR II. FIG" 1.-Prominent tourmaline veining with grei-sen margins mid-right) and planar contact between biotite band€d pink granite and pm·phyritic aplite. FIG. 2.---Banded porphyritic aplite. FIG. 3.---Tourmalin€' nodular aplite and quartz-tourmali:r:e negre gation. Fw. 4. ---Contact betv.reen \Vhite tourmaJine granite (be1ow) gTading into tourmaline nodular aplite and porphydtic aplite (top). Note banding in porphyritic aplite paralle-l to the contact. FIG. 5.-~Pyroxene dyke intruding pink granite. FIG. 6.~Fo1ding in Silurian An1ber Slate. Scale 10' from base to top. PAPEta~ A~D PROCEEDii'.:"GS oP THE ROYAL SocmTY oF T:\S:VtA~TA, Vou_rME 100 2 3 6 4 5