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11 New Zealand Geothermal Workshop 1989 STRUCTURAL BACKGROUNDTO GOLDFIELD MINERALISATION: A REVIEW

M.R. Gadsby and K.B. Sporli

Epithermal Mineralisation Research Unit Department of Geology, University of , Private Bag, Auckland ABSTRACT Hauraki goldfield epithermal mineralisation formed in Late Miocene-Early Pliocene time in a calcalkaline arc setting associated with some intra-arc rifting. The country rocks record widespread post-Miocene eastward tilting of the . Some faults show both dip slip and strike slip phases of movement. Mineralisation is restricted to the Mio-Pliocene volcanics and the top of the greywacke basement . Dominant quartz vein trends are northeasterly and dips are sleep, although there are regional and local deviations from these trends. Vein patterns range from simple, planar to complex, stockwork Style of veining and mineralisation are in part controlled by lithology. Some veins appear be tension gashes in dilational jogs, others are localised along faults. Vein fills can be separated into a banded Golden Cross type and a Tokatea type, dominated by buck quartz. Ore shoots in the veins often plunge steeply, possibly indicating control by strike slip faulting.

INTRODUCTION objective information on the structural controls of veins, alterations and mineralisations in the Hauraki epithermal deposits is present in theses and publications, mainly from the Department of Geology , University of Auckland. None of this information has been previously compiled to provide an of the structural characteristics of the Hauraki goldfield. The Hauraki goldfield is part of Coromandel Peninsula, which is underlain by a basement of Mesozoic greywackes, dominantly of Jurassic and older age Manaia Hill Group, Schofield, but Tokatea Hill Formation (Skinner, may be Cretaceous (Smale, They are overlain by thin early Tertiary shelf carbonates, followed by Miocene Coromandel Group andesitic volcanics and volcaniclastic sediments, and plutonics The Pliocene Group showsa change to rhyolitic volcanism. Omahia andesites andesites of and a thin series of ignimbrites are the youngest volcanic units. occurred in the Late Miocene and Early Pliocene Brathwaite et Structural control can be considered at four scale levels: : involving plate tectonic and large scale regional processes, such as subduction and associated rifting, regional uplift, block faulting and tilting. scale:involving one or several mineralised fields and relating to overall patterns of veining, alteration and Fig. Schematic geological map of Coromandel peninsula, with mineralisation. scale: considering the major geographic features in capitals) and location of Auckland shape and structural development of one vein or vein system,its University theses (by author, in ordinary script). The main relation to fractures and faulting. Vein theses used for this paper are underlined. Inset shows location of scale: considering the internal structure of vein map and additional theses on Great Barrier Island. and mineral deposits to determine propagation directions, pulses of opening ,fluid flow and mineral deposition. FRAMEWORK The Hauraki gold deposits are dominantly epilhermal Brathwaite et al., Some base metal deposits may range into Basement rocks in the Coromandel peninsula are the the lower temperature region of the mesothermal product of Mesozoic accretion at the margin of structural framework of epithermal deposits is typically upper crustal and is associated with the seismogenic regime of 1978).The New Zealand micro - continentwas formed during the dominantly faulting (Sibson. opening of the Tasman Sea,80 to SO m. y. ago. Associated rifting thinned the New Zealand crust and and created a pattern of Because of dense forest cover, deep weathering and fault blocks which still dominate the structural widespread alteration, the general geology of this region is still pattern Reactivation of such faults poorly Much diligent, objective field work needs to be must be an important feature of Tertiary tectonics in Coromandel. done (Skinner, until the stratigraphic,volcanological and The influence on the Coromandel Peninsula of a further rifting structural framework for these mineral deposits is adequately episode, during opening of the Late Eocene to Oligocene u Challenger Rift is not known at present. 150

Gadsby and Sporli At the end of the Oligocene, the Alpine fault transform as transfer faults between segments of the rift and should plate boundary between the Pacific and the therefore have some strike slip became established in New Zealand Sparli, Cartwrtight, However, on the east side of the Initial subduction under Northland and Coromandel led to peninsula (e. g.at Kuaotunu. Parkinson, samedirection formation of andesitic arcsalong the west coast and of faults can be expected to be normal, because it is influenced by the east Coromandel and east coast of Northland, Ballance, the Havre trough Ridge extension (Sparli. The vector of convergence for that time is almost at right angle to the tectonic trends of Coromandel peninsula The two volcanic arcs differ from each other, in that the one along the east coast records volcanic activity over a longer period Because of the abundance of volcaniclastic sediments in the and has plutonic intrusions, and significant economic mineral Coromandel Group, where depositional dips up to could be deposits, which are absent in the Waitakere arc. The cause for possible, it is difficult in some areas to determine with any these differences is not yet known, but differences in certainty whether tectonic tilting has taken place. lithosphere structure, probably due to the Tasman Sea (and Challenger Rift?) opening could be involved. Post CoromandelGroup pre-Whitianga Group eastward and northeastwards tilting is recorded from southern Great Barrier During the Pleistocene, the arc volcanicity migrated to its Island (Hayter. 1984;Ramsay. 1972;Henrys. At Coromandel, present site in the Taupo Volcanic Zone and the presently active the eastward tilting may postdate Whitiangs Group Hauraki Rift (Hochstein et was formed. At Kuaotunu (Parkinson, eastward tilting of to is recorded by the unconformity at the top of the greywacke basement and must postdate the Miocene REGIONAL SCALE CONTROLS sediments onlapping it. At Table tilting is again eastwards and is post - Whitianga Group and pre - Oniahia Mineralisation is most common in the Miocene sediments Andesite (Hayward. 1971). De Ronde recorded past and andesitic volcanics but also exists in the Pliocene W Whitianga Group eastward tilting at Golden Cross. Coromandel Group. The Mesozoic basement appears to be mineralised at Group units at Te Aroha mostly appear to dip towards the east its very top. This means that in general the highstanding (Cochrane, ,but attitudesoccur in some areas basement blocks are devoidof workings and the mines are located (Cartwright. 1982) in grabens g the belt trending north from township, Very few folds have been recorded in the Tertiary sequence of the peninsula. At Neavesville, a very open syncline trends NW (Torckler, and the Tertiary sediments at Kuatounu seem to Faults patterns in the Coromandel peninsula are poorly form a N-S trending syncline (Parkinson, The relation of known, because of the lack of good marker horizons, the these folds to mineralisation is unknown, as is their widespread alteration and weathering in the Tertiary rocks. A significance. Drag on nearby faults is the most likely cause for large number of the faults described in the literature and in the folds. theses are very conjectural and are often based on subjective interpretations of photo or magnetic lineaments. When factual of veins information only is used, the number of faults that can be plotted is reduced drastically. However, in the northern part of Karangahake and Golden Cross seem to lie on a NNE the peninsula a rectangular pattern of NNW trending and trending lineament which could be a major fault (DeRonde and trending fault appears to be well established (Skinner, Blather, If the model of for the Martha Lode Some of these patterns are also visible offshore to the NE and at Waihi is correct, the quartz veins there represent true tension north of the peninsula (Thrasher, gashes associated with a N-S trending strike slip fault system.In such systems there should be a more or less simultaneous Slip vectors have been determined on a f e r fault systems development of faults and veins. Some of the only (Stevens, 1980; Parkinson; 1980. Price, Most workers hanging wall and footwall gouges described Couper. 1975, have not made any distinction between apparent and true may be such structures. although much more displacement ("separation" and "slip" respectively of Hill, 1981 careful study is necessary to determine whether the faults are Determination of slip vectors based on assumed tectonic models not significantly older than the vein or,alternatively, are much g Skinner, have to be considered suspect younger reactivationsof the vein walls.

At Manaia ,Stevens recorded offsets of bedding and a A persistent question is that of the role of the faults mineralised dextral horizontal separation fault on a later, E-W themselves as aquifers. Clay gouges should be aquicludes. and trending fault. resulting in a slipvector plunging the east therefore could act as traps along which hydraulic fracturing under a sinistral. dominantly strike slip regime of faulting. At can take place. Breccia faults, on the other hand, acting as Kuaotunu (Parkinson, dominant fault movements are aquifers. would allow direct precipitation of gangue and ore normal dip slip, both on earlier WNW- trending faults and later minerals within the fault zone. Mechanisms of this kind may ENE to NE- trending NNW - trending faults have a have operated atTui Mine ,Te Aroha (Cochrane where the dominant downthrow to the west. Some of the NE-trending faults main mineralised veins appear to coincide with a pair of have experienced reverse movement This may correspond to a conjugate faults. N- trending faults also seem to have localized similar regime of NW-SE directed compression recorded from the mineralisation at Kuaotunu (Parkinson. Miocene Waitemata Group in the'vicinity uf Auckland (Sporli, 1982). Vertical, Coromandel Group tuffs in the Other'mechanisms of vein formation also need to be Waitekauri Valley, near Waihi are offset by vertical, considered, such precipitation on joints due gentle NNE-trending dextral separation faults (Rabone, 1971, p 20 bending, and in dilatant cracks due to rotation of rigid blocks Because of the vertical dips of both the faults and the offset against other. During the heyday of the porphyry copper marker, a component of dextral strike slip must be involved in model. ring structuresdue to updoming or caldera collapse were the diplacement. In the same area, columns in a post- Omahia also postulated as structural controls Merchant. Group ignimbrite have been rotated to low plunges near an E-W Care needs to be taken to distinguish true ring structures due to trending fault, to suggest an upthrow (separation to the north, igneous processes from pseudo - rings resulting from which is confirmed by the higher position of the ignimbrite in intersection of three or more straight fracture sets. that area. Dextral strike slip on N-S trending faults is implied in the fault jog model for Martha Lode at Waihi (Sibson , At Te Kaka Ridge, near Tapu, both strike slip and dip slip striations occur on E to NE - trending and NW - trending faults,which seem to be mostly younger than the vein systems (Price Lithology should exercise some control on the formation of Complex fault reactivation is indicated the fractures in which veins are precipitated and along which alteration takes place. The differential response of various rock The Pleistocene to Recent Hauraki Rift influences types deformation may also be influenced by any pre- fracture post-mineralisation structures, especially in the west of the alteration. peninsula (Merchant. and is represented by N-S to NNW trending normal faults, which in part control some of the At Manaia, Coromandel(Stevens, very different styles upfaulted basement blocks. trending faults can be regarded of mineralisation occur in the greywacke basement (restricted to 151

Gadsby and Sporli

Apparent control by alteration has , been observed at Coromandel Edwards, where highly altered greywacke chalcopyrite-rich base metal sulphideveins. whilst those in weakly altered greywacke are sphalerite - rich. At Monowai 1973) and Waitekauri Valley (Rabone, more quartz veinsare present in highly alteredzones.

Veins also occur in (Ramsay, 1972;Torcltler, On Great Barrier Island (Ramsay. there is a change from fracture fill reefing to replacement reefing with increasing distance from the sinter.

VEIN SYSTEM SCAIE Vein Simple vein patterns are straight and planar (Tokatea Big Reef, Coromandel; but can include low angle en echelon arrays (Golden Cross, De Ronde. De Ronde and Blattner, Segments can be linked, as at Karangahake leading to systematic changes in vein orientation and constant axes of vein deflection steeply north plunging at Karangahake, see fig. 2 A). These structures record plane (biaxial) strain In the extensional jugs at Martha Mine, Waihi (Sibson. vein deflections and intersections occur both on en echelon segments horizontal and vertical axes (Fig. 2 and record strain. Vein patterns at Hauraki Mine, Coromandel Triaxial strain veins (Skinner, are even more complex and may indicate with steep tensional opening (hydrofracturing) of a pre-existing fault system. and low angle or S Stockwork has been described by Merchant, 1978; Stevens, 1980 and Erceg. 1981. Although intersection sometimes are zones of high grade mineralisation, S their internal geometry and structural development are poorly S understood. Vein On Great Barrier Island and the northern part of the CoromandelPeninsula, the major trend of quartz veins is NNW to N. A minor NE trend is often present and is usually the younger of the (Smale. 1962; Skinner. Ramsay, 1972; Robson. 1979; Parkinson, 1980; Erceg. 1981; Henrys, 1982; Engleback, 1984; Fisher. Moving southward, down the peninsula, the NNW direction becomes progressively less important. whilst a NE to N trend is increasingly important. From Coromandel township, the latter is the dominant trend, though the fomer usually persists as a minor trend . The NNW veins here are younger than those of NE strike Cornwell, Ferguson. Fig. 2. Examples of vein geometries. A. Karongahake, 1970; Ovens, 1976, Stevens, At Tararu (Couper. a interpreted after Anstiss Workings (thin lines) represent significant number of - striking veins appear. though veins contours along the lodes.Deflection linesare boundaries between with a strike dominate. Immediately to the south, at Thames. en echelon deomains of different strike. B.Martha Mine, Waihi, the two directions are equally important, but NNW - trending after Sibson Arrows with numbers indicate dip direction veins still represent a minor group 1973; Merchant. and dip of veins. 1978 East of Thames. at Broken Hills and Neavesville, NNW trending veins are more common than ENE and N striking veins (Moore. 1976;Torckler. At Broken Hills. the trend is joints and fractures and the older than the ENE trend Moore, argillites) and the Tertiary rocks disseminated mineralisation with localised veins only). In the area (Main, Moving southward again, N to NE (usually is once there is mineralisation only in Tertiary and not in the the again the dominant and older vein trend, with secondary NW basement. In the Tararu area (Thames, Couper, there are trends still occasionally present Main, 1971; Rabone. 1971; more, and better mineralised veins in the Miocene rocks than in Cartwright, 1982; Anstiss, 1984; De Ronde, At Te Aroha the Mesozoic, elsewhere there are mines which work minor E-W trending veinsoccur beside the normal set (Cochrane. both in the basement and the Tertiary (Cornwell, For the Tertiary, Ramsay, 1972 (Great Barrier Island). Engleback, 1984 (Waikawau Bay), and Cartwright , 1982 VEIN MICROSCOPIC)FEATURES Aroha). all observe that more veins are present volcanic rocks than in pyroclastics. due to the more brittle behaviour of veins the flow rocks. In some this appears to be compensated by the veins in pyroclastics tending to be larger than those in the Veins are mostly parallel- sided, but a downward-thinning flow rocks (Engleback, Atother localities the lithological veins have been described from the Waiorongomai area control seems to be reversed in that veins are more common (but (Cartwright, Terminations of veins can be wedge - shaped more irregular) in the pyroclastics Hayward, 1971, Table or involve 'horsetails' (Price, Little curvature due to Mountain ; Moore Whitianga Group) than in flow rocks. interaction of en echelon segments Pollard and Aydin, Group flow banded spherulitic rhyolites at Paku has been seen. veins have been described from Island, (Rutherford, 1970) contain soft opal, whilst hard. Waitekauri Valley (Rabone, Tui Mine, Te Aroha dense, opaline veins are present in glassy rhyolites the Coromandel area (Edwards, and Phenocrystic lavas have no veins. 152

Gadsby and Sporli

There are two major types of quartz vein filling: Tokatea There are a large variety of breccias. 'Pebble dikes' have consists of coarse buck or comb ,often amethystine, quartz been described by Cornwell Merchant Edwards. and is common in the gold field and the Thames Stevens Fisher Two phases of brecciation Bonanza deposits. of quartz deposition appear to have have occur within veins at Karangahake Breccias have been slow and concentrations low. 2. Golden type consists also been reported from veins within Whitianga Group Golden of crustiform, intricately layered veins, including 1976 and from basement hosted veins cryptocrystalline, flinty, sulphide rich phases, indicating rapid near Coromandeltownship (Edwards, Couper records jigsaw breccias from Tararu Valley near Thames. Clasts of greywacke within quartz veins Waikawau Bay east coast of Coromandel peninsula) indicate considerable vertical or horizontal transport, since the nearest greywacke outcrop is 12 km away (Engleback. The origin of most of these breccias not definitely determined It is not certain whether the pebble dikes are due to volcanic processes or are more closely associated with hydrothermal activity during vein formation, although the clastic dikes described by Fisher may well be due to rupturing during or shortly after the deposition of the Whitianga Group ignimbrites in which they occur. In the future it will also be necessary to distinguish ordinary fault breccias from implosion breccias suggested by the fault jog model of Sibson

Vertical zonations of ore minerals has been noted by Main Moore and Merchant Some of these Fig. Example of cryptic structures in cryptocrystalline, black, zonations are cone shaped with a downward pointing apex (Main. macroscopically featureless K-feldspar sulphide vein fill at 1971 Karangahake (after Anstiss, Non - oriented specimen. Numbers indicate sequence of res. Note alternation Within certain veins, mineralisation is concentrated in the between brittle and ductile states. form of 'shoots'. Mushroom shaped elongate bodies of high base metal concentration plunge 38south Tui Mine (Cochrane, Zones of high gold concentration at Karangahake (Anstiss, pitch steeply north within the veinsand are parallel to the deflection axes of vein orientation shown in A. At Kuatounu Parkinson, the reefs are lens-shaped and display lens - shaped distribution of ore. Structures crosscutting reefs are precipitation from solutions. Examples include often important in controlling mineralisation. In the Golden Cross Ronde, Broken Hills (Moore, Great Thames area, they include faults. cross reefs and "flinties" Barrier Island and parts of the Thames district Merchant, faults at Maratoto Main, and the broad (Merchant, The old miners used the name for corridor of silicification at Broken Hills (Moore, such veins. Black, K-feldspar rich, cryptocrystalline phases Karangahake (Anstiss, show microscopic structures Other controls of optimum conditions for ore deposition indicating of brittle and ductile deformation include . Locations where veins change direction , and even a limited amount of pressure solution, although this Main, an eastward swing is important); veins of a type of vein fill is macrosopically massive. This could indicate particular orientation Cochrane, Parkinson, reefs alternations between gel and sol state in the newly vein material. which display a minor amount of strike slip movement (Main, On Great Barrier Island , mineralisation is to be Repeated pulses of carbonate deposition can be recognized found in breccias, whilst As mineralisation is restricted to quartz veins Erceg, At (Robson. in several vein systems Ronde, 1985; De Ronde and Blather, sulphides are most commonly found as coatings on joint planes, 1988). Bladed calcite replaced by quartz indicates boiling A few systems have distinct late vein phases (e. De Ronde particularly in highly zones and whilst pyrite is and Blather, Manganese - rich phases such as ubiquitous. Au and Hg are restricted to quartz veins. and rhodonite are occasionally present as gangue minerals Main, 1971; Ramsay, 1972; Couper, It will important in the future to establish how many of these changes in composition are due to tectonic deformation of the fracture In many instances, propylitic alteration predates formation sustems hosting the veins. of quartz veins (Edwards, 1979;Stevens, 1980;Anstiss, This must have a strong influence on the strength of the rocks in Replacement reefing, with -indistinct margins, and much which veins are emplaced. Depending on type alteration, included country rock material can be distinguished from simple it may lead to formation of highly irregular and discontinous fracture fill veins with sharp wall rock contacts and little veins. included material 1971;Ramsay. 1972; Anstiss, At several localities mineralised late oblique veins occur within the .Structural influences on alteration include main Cochrane, Merchant, 1978) "telescoping" of a concentric pattern of alteration zones (Robson, Faults may also provide windows into more highly altered In higher temperature systems Mine: Cochrane parts of the system (Robson, Earlier faults and reefs may early precipitated galena shows ductile deformation and complex provide the locus for hydrothermal fluids. At Neavesville, for pulses of deposition of quartz, chalcedony and different sulphide instance. K-silicate alteration followssteep faults assemblages has occurred Massive ore is present in or and at Monowai, silicification occurs along the hanging wall of multiple bands within the quartzveins, sometimesdipping in the the reef (Merchant, At Broken an important same, sometimes in the opposite direction as the quartz vein. trending "corridor" of silicification cuts across quartz reefs Within these bands , the ore minerals are themselves zoned. so (Moore. that galena is dominant in the hanging wall and chalcopyrite in the footwall Deformed galena has also been reported from the A late iron oxide and phase of staining associated Coromandel district (Edwards. In the same area, with veining present at several localities, for instance chalcopyrite-rich base metal mineralisation is again at Maratoto (Main, Broken Hills Moore, Hot Water concentrated in the footwall of the veins. At Maratoto (Main. Beach (Ovens. Te Kaka (Price, and is due to 1971). fractured galena crystals have later been cemented by downward percolation of acid - sulfate waters and to hessite and ,whilst fractures in pyrite are healed by While this late phase appears be mainly chalcopyrite and sphalerite following topography, locally it is strongly influenced by pre - existing fractures. Gadsby and Sporli

DISCUSSION Black, Petrology of the Cuvier and Plutons and their The theses and reviewed provide interesting 334 p. insights into the structure of the Hauraki goldfield.Some overall structural trends are already becoming evident, but more field work is necessary to confirm these trends and to gather L., Christie, A. B., and Skinner, D. N. B. 1989: The information on possible mechanisms. Hauraki Goldfield - regional setting, mineralisation and recent exploration. (Ed.) Mineral deposits of New A problem in understanding the deposits is the Zealand. dearth of detailed geological information on the host rocks, due to 45-56. various difficulties in mapping. Well exposed sections need to be studied again to provide models for understanding the more Cartwright, A.J. 1982: Geology of the Waiorongomai Valley, Te and bush - covered areas. Modern Aroha. 163 p. volcanological principles need to be applied to the mapping of the Cenozoic volcanic sequence. It is important to obtain more Cochrane, R. Geology of the Tui Mine, Mount Te Aroha. objective information on the location and slip history of faults I 70 , and on the timing of their movement in relation to mineralisation. The presence or absence of calderas needs to be Couper. P. G. 1975: Geochemical investigations of Tararu Valley, more critically assessed and the structure of those calderas that Thames. 121 p. have been positively identified studied in detail. De Ronde, E. C. 1985: Studies on a fossil hydrothermal system at Regional vein patterns are dominated by northeasterly Golden Cross, Waihi, New Zealand. strikes, but in detail the patterns are more complicated. It is 166 probably to assign all the veins to one unified stress system, especially since in a number of areas at least two De Ronde. E.J. C. and Blather, P. 1988: Hydrothermal alteration. different phases of veining are present. These phases need to be stable isotopes and fluid inclusions of the Golden Cross better studied, and the boundaries of homogenous domains of epithermal gold- silver deposit. Waihi, New Zealand. 83: mineralisation history e. of contiguous geothermal systems) . established. The mechanisms of formation of the two vein fill types recognized will have to be further investigated. What are Edwards, P W. 1979: Alteration and mineralisation of the relative roles of incremental crack - seal opening versus and associated rocks near Coromandel, New Zealand. filling of fissures held open for considerable time spans? 148 p. Breccias associated with the mineralisation need to be more closely studied to their mechanisms of formation, Engleback, 1984: Geochemistry and hydrothermal of which may include hydrothermal eruption, tectonic brecciation, greywackes and associated rocks near Coromandel, New hydrofracturing and implosion (Sibson. as well as Zealand. Auckfmd 134 p. rocksfalls from the walls of open fissures. Erceg. M.M.1981: The Te Ahuamta fossil geothermal system. aspects of its geology, geochemistry and mineralogy. The types of structural controls on veinsand mineralisation 34 p, need to be more thoroughly studied and classified . Direct structural controls include alignment along faults and joints, , G. K. 1970: Propylitisation and ore mineralisation, opening of gashesand cavities.Faultswith clay Stoke’sCreek. Waiomu. gouges may also act a aquicludes. More indirect controls are 66 pressure drops and resultant boiling) due to uplift or dilatant deformation (e. Sibson, Tectonic overpressuring of fluids may cause hydrofracturing. Some of these processes will be Fisher. G. D. 1986:Geological aspects of an area near Whitianga, recorded in the variations in texture and composition of the vein Coromandel Peninsula. University fills. Uplift or burial also cause changes in fluid 131p. temperatureand tectonic opening of pathways may lead to mixing of different fluids. Harvey, C. C. Ruck alteration in the SE Whitianga area. Auckfmd I 36 p . this framework, the structural causes of formation of the steeply plunging ore shoots in the Hauraki Gold field will Hayter. I. 1954:The geology of the southern part of the central have to be investigated in more detail. It will also be important to portion of Great Barrier Island. study structural controls of alteration patterns. p.

Structural studies of this kind should improve the Hayward, B. 1971: The geology and eruptive history of The Table predicitive of geological assessment of goldfield Mountain region, Coromandel Peninsula. mineralisation In addition they will provide a detailed record of of 173 p. the tectonic history of the calcalkaline arc associated with the establishmentof the present plate boundary in New Zealand. Hill, M.L. 1981:San Fault: History of concepts. America I : 112- 3

ACKNOWLEDGMENTS Hochstein, M. P., Tearny, K., S., Davey, F. J., Davidge, S., Henrys. S., and D., We wish to thank D. Clarke and S. Simmons,who provided Geophysical structure of the Hauraki Rift (North constructive criticism and L. who draughted the Island, 24: figures. Gadsby is supported by a grant from Heritage Mining N. L. Henrys, S. A. 1982:A geophysical reconnaissance of Great Barrier Island. 139

Kamp, P.J. 1986: The mid-Cenozoic Challenger Rift system of western New Zealand and implication for the age of Alpine Fault inception. Society America

Anstiss, R. G. 1984: Maria Lode and associated hydrothermal Lawton. D. C. 1973:The delineation of the southern of alteration of andesites, Karangahake fossil geothermal the Monowai Reef. system. Coromandel Peninsula. 56 p. 53 p. Main, 1971: Geology of the Marototo - Waipaheke area, Waiomu Ballance. P. F., 1976. Evolution of the upper Cenozoic magmatic Valley, Coromandel Peninsula. arc and plate boundary in northern New Zealand. 158 p. 28: 154

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Merchant, R. J. 1973: Aspects of the geology, and Stevens, M.R. 1980: The geology of a porphyry - copper type mineralisation of the Monowai Comstock area, Waiomu deposit at Manaia, Coromandel Peninsula, New Zealand. Valley, Thames. 66 p. Thrasher, G.P. 1988:Subsurface geology of the continental shelf, Merchant, R.J. 1978: Metallogenesis in the Thames - Tapu area, Bay of Plenty to the Three Kings Islands.New Zealand. Coromandel Peninsula, New Zealand. G 363 p. Torckler, L. 1978:The geology of the Neavesville area. Moore, C. R. 1976:Gold - silver mineralisation of the Broken Hills 109 p area, . 0 G.P. 1978;A geophysicalstudy of the Whitianga Graben. p. Ovens, S.A. 1976:Geology of the Tairua - Hot Water Beach area. 175 p . Parkinson, P. 1980:Geology of the Mesozoic, Tertiary and Au - mineralised rocks Coromandel Peninsula.

Pollard, D., and Aydin. 1988: Progress in understanding jointing over the past century. America 100 1181-1204.

Price, L. 1988:Quartzveining in Miocene volcanics at Te Kaka Ridge, Tapu Valley, Coromandel Peninsula , New Zealand. 60 p.

Rabone. S. D. C. 1971: Igneous geology of the western Valley. Ohinemuri thesis, 125

Ramsay R. H. 1972: Geology of south-central Great Barrier Island. 147 Robson, R. N. Geology of a mineralised porphyry system. W hangapouo. Coromandel Peninsula. 143p . Rutherford, N. F. 1970:Geology of Paku Island, wilh comments on the geology of Whiritoa, and 49 p .

Schofield.J C. 1974 Stratigraphy, facies, structure, and setting of the and Manaia Hill Groups, East Auckland. I 4 - 838 Sibson. R. H., 1987 Earthquake rupturing as a hydrothermal mineralising agent. Skinner, D. N. B. The geology of the Muehau, district, Coromandel Peninsula. of 158 p . Skinner, D. N. Geology of the Coromandel Region, wilh emphasis on some economic aspects. 3vols.,414 Skinner, D.N.B. 1976: Sheet N40 and part sheet N N36, N39 Northern Coromandel, (1st edition). Map sheet) and notes New Zealand Department of Scientific and Industrial Research, Wellington. Skinner, D. N.B. 1986: Neogene Volcanism of the Hauraki volcanic region. Smith, I. E. M., late Cenozoic Volcanism in New Zealand. Society New 23: 47. Smale,D The geology of the Coromandel - area. 33 Sparli, K B. 1978: Mesozoic tectonics, North Island, New Zealand. 89: 1982 Review of directions in Northland, New Zealand, and of the structure of the Northland Allochthon. 36 Sparli, K.B. 1987 Development of the New Zealand microcontinent. Monger, J.W.H.,Francheteau. J. Geodynamics Series Vol American Geophysical Union. Washington: