The geology and nickel-copper sulphide ...

New Zealand Journal of Geology and Geophysics

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The geology and nickel-copper sulphide mineralisation in the Graham Valley, North-West Nelson K.R. Gill & M.R. Johnston To cite this article: K.R. Gill & M.R. Johnston (1970) The geology and nickel-copper sulphide mineralisation in the Graham Valley, North-West Nelson, New Zealand Journal of Geology and Geophysics, 13:2, 477-494, DOI: 10.1080/00288306.1970.10423979 To link to this article: http://dx.doi.org/10.1080/00288306.1970.10423979

Published online: 20 Jan 2012.

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Date: 30 January 2016, At: 11:58

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THE GEOLOGY AND NICKEL-COPPER SULPHIDE MINERALISATION IN THE GRAHAM VALLEY, NORTH-WEST NELSON K. R. GILL * and M. R. JOHNSToNt

New Zealand Geological Survey, Department of Scientific and Industrial Research, Lower Hutt (Received for publication 29 January 1969)

Downloaded by [82.85.8.99] at 11:58 30 January 2016

ABSTRACT

In the Graham Valley, North-West Nelson. mafic and ultramafic rocks intruding lower Paleozoic sediments, are referred to the Riwaka Complex. At this locality the complex consists of two major rock types; the most extensive is pyroxenite with minor gabbro (Pokororo Pyroxenite) which encloses small discrete masses of peridotite (Graham Peridotite). Sulphides associated with the pyroxenite include pentlandite, pyrrhotite, and chalcopyrite; and chrome-spinel is associated with the peridotites.

LOCALITY The mapped area consists of about 5 square miles of the steep sided eastern foothills of the Mount Arthur Range, North-West Nelson. It is situated between and around the North and South Branches of the Graham River (Fig. 1), a small western tributary of the Motueka River, and covers a The area is accessible by public small portion of the Cobb District (S 13 road from the Motueka Valley and a private metalled road continues from it along the south bank of the South Graham River to join the Flora Track (Fig. 3) at Thorns Creek. Another foot track runs north from the road near the junction of the rivers and follows the western bank of the North Graham River. The lowest point in the area, near the junction of the two branches of the Graham River is 350 ft in elevation, while westward, the ridge between the two branches rises to 2,651 ft at Trig. VIII. Except in the extreme west, most of the original beech vegetation has been removed, only small, often isolated pockets remaining in steep gullies. Much of this former farmland has reverted to scrub.

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PREVIOUS GEOLOGICAL WORK The area forms a small portion of a belt of lower Paleozoic sedimentary rocks, intruded by both basic and acidic rocks, all of which have a northnorth-east regional strike, and extended along the eastern edge of the Mount Arthur Range (Fig. 2). *Present address, Grant Institute of Geology, University of Edinburgh, Scotland. tPresent address, c/o DSIR, Private Bag, Nelson. :j:Sheet number of the 1 : 63,360 topographical map series (NZMS 1).

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N .Z.G .S. photo FIG.

I-Aerial photograph of the Graham Valley, looking north-west. B, C, and E are locations of mineral assemblages described In text.

The first study I)f the area was by McKay, whose mapping included the area from south of the Baton River, northwards to the Graham River (McKay, 1879). McKay recognised that the basic rocks, west of the granite, were of igneous origin. Since they intruded rocks at Baton River, then considered to be of Silurian age (and now known to be of lower Devonian age), he assigned them a Devonian age. Further north in the Riwaka Valley - Takaka Hill area, both Cox (1881) and Park (1890) concluded that the basic rocks were of metamorphic origin. Both authors also explained the greater apparent thickness of these rocks in the north as due to repetition in the core of a syncline. Park considered that they were the youngest of all the lower Paleozoic rocks in the Riwaka - Takaka Hill district. In their detailed mapping of the Motueka Subdivision in the 1920s, Henderson et at. (1959) showed the basic rocks as a discontinuous, linear, intrusive belt. In a major revision of the stratigraphy of North-West Nelson, Grindley (1961) referred the basic rocks to his metamorphic Riwaka Metavolcanics of Devonian age. These rocks were the youngest in the Riwaka Syncline extending south from the Riwaka Valley to the Baton River. Willis (1965), mapping in the Baton Valley, confirmed the earlier

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work of McKay (1879) and Henderson et al. (1959) and considered the rocks as post-Devonian intrusives. At Takaka Hill, Cooper (1965) placed the rocks in his Riwaka Group, consisting of a lower gneissic metasedimentary unit and an upper metavolcanic unit, of Devonian age.


RECENT WORK

The basic rocks have been known to exist as a continuous belt from the Riwaka Valley to the Graham River, and recent mapping by one of the authors (M.R.J.) has shown that they continue without interruption to the Baton River. All observed contacts, other than fault contacts, are intrusive and cut across the regional strike of the metasediments at an acute angle. During this mapping, an area of peridotite was found on the ridge separating the North and South Graham Valleys, and massive sulphide mineralisation was found in pyroxenite to the south of the ridge. The petrology, geochemistry, and mineragraphy of the basic rocks, formerly described as Riwaka Metavolcanics and Rameka Intrusives (Grindley, 1961), form the subject of a more detailed study by one of us (K.R.G.). This work has shown that the majority of the basic rocks are of igneous origin, although many have undergone extensive amphibolitisation and serpentinisation, and some have been partly recrystallised. Lack of recognition of these secondary processes may account for the variety of opinions put forward by earlier workers. The metavolcanics, confined to the Riwaka Valley, are now mapped as Onekaka Schist (G. W. Grindley, pers. comm.). Principal rock types in the complex range from peridotites, pyroxenites, and gabbros to pyroxene-mica diorites and hornblende-quartz diorites. A preliminary account of the sulphide mineralisation has been published by the present authors (1967).

STRATIGRAPHY

Grindley (1961) divided the lower Paleozoic sediments of North-West Nelson into several groups, of which rocks belonging to the two younger groups occur at the Graham Valley. The younger of these two groups, the Baton River Group, consisted of two formations, Ellis and Baton. However, Willis (1965) included the older of these, the Ellis, in the underlying Mount Arthur Group so that the upper part of the Group consisted of four formations; Flora, Mount Arthur Marble, Wangapeka, and Ellis. The name Mount Arthur Marble has since been shortened to Arthur Marble (G. W. Grindley, pers. comm.). Only two of these formations, Flora and Arthur, are present in the Graham area, but the Ellis is partly represented by thick quartzites in the Onekaka Schist.

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Flora Formation Sediments belonging to the Flora Formation crop out to the west of the Skeet Fault and are the oldest rocks known in the Graham Valley. The formation consists of dark shale, grey to silvery phyllite with no obvious sedimentary features, and alternating beds, ! in. to 2 in. thick, of sandstone and siltstone with occasional grading and cross bedding. Dark grey to black, finely crystalline limestone occurs in beds up to 30 ft thick throughout the formation. In the finer beds, cleavage is well developed, and is generally steeper than the bedding. Only the upper part of the formation is exposed in the Graham area. Because there is an incomplete sequence and structural complexity no estimate of the thickness of the formation can be given. Although no fossils are known from the formation within the area mapped, graptolitic shale and crinoidal limestone occur further west. The graptolites indicate an upper Middle to lower Upper Ordovician age (Cooper, 1968)

Arthur Marble The formation consists of a white, to grey, to black, often laminated, finely to coarsely crystalline marble. The largest area of marble is at Sugar Loaf in the South Graham Valley. The marble has been intruded in the east by granite which, north-east of Sugar Loaf has invaded further west than shown by previous authors, leaving only a thin strip of marble. Small lenses of marble occur within the granite and small areas of marble crop out in the west, in the upper South Graham Valley and in Goat Creek. The marble exposed on Sugar Loaf is laminated and consists of alternating light and slightly darker grey layers ! in. to !- in. thick, which show better on weathered surfaces. Subspherical to elongate quartz nodules !- in. to over 8 in. in length are common in planes parallel to the laminations and from a few inches to several feet apart. Sandy layers ranging in thickness from less than !- in. to 3 in. in thickness, and from an inch to a foot or more apart, are common within parts of the marble. Where the marble has undergone deformation and flowage individual sandy layers have become disrupted so that pieces of sandy limestone appear to float within the marble. A common mineral in the marble is phlogopite which occurs as small crystals clearly visible on weathered surfaces. On the west side of the main mass of marble in the south Graham Valley, thin impure marble layers contain tremolite crystals up to -! in. long. Tremolite also occurs in the marble on the north side of the South Graham Gorge and in the lens of marble that extends north-east from Trig. FF. To the west of the Skeet Fault the Arthur Marble conformably overlies the Flora Formation. A complexly folded contact is exposed on the south side of the South Graham Valley and in Goat Creek. The thickness of the marble is difficult to estimate due to the absence of a continuous sequence and its lensoidal nature. A minimum thickness of 3,000 ft is exposed on Sugar Loaf.

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No fossils were found in the formation, but further west towards the Mount Arthur Range, corals, crinoid and sponge remains have been recorded from the marble (McKay, 1879). These fossils are not sufficiently diagnostic to give a precise age and an Upper Ordovician age is inferred from the formation's stratigraphic position, and from better preserved fossils in the Takaka Valley to the north (Cooper 1965, 1968).


Onekaka Schist Quartzite and interbedded biotite schist cropping out discontinuously on either side of the north-north-east-trending Riwaka Complex, were mapped as Onekaka Schist, the metamorphic equivalent of Ellis Formation and the underlying Wangapeka Formation (Grindley, 1961). In the south Graham River, and in a small tributary of the north Graham River, draining north-east from Trig. FF, a gradation from the Arthur Marble into the Onekaka Schist is exposed. The Arthur Marble becomes well-bedded, progressively more quartzitic and grades up conformably into well-bedded, light grey to white metaquartzites with interbedded biotite schist. On the Flora Track, the quartzite beds range from a few inches to 5 ft thick and contain interbedded biotite schist a few inches thick; the thicker quartzite beds have a well-developed lineation. Similar beds continue northeast to the North Graham River, where thinner quartzite beds are interbedded with foliated biotite schist bands, up to 10 ft thick. In the lower part of Goat Creek, a complexly folded sequence of alternating limestone and quartzite, in 1-8 in. beds, is considered to belong to the lower part of the formation. In the eastern strip of Onekaka Schist, between the Separation Point Granite and Riwaka Complex, several high-grade metamorphic assemblages have been found. The significance of these assemblages and the relative effects of the two igneous complexes have not yet been fully evaluated. Sillimanite-bearing pelitic schist, containing abundant secondary white mica, are similar to those reported by Ghent (1968) from the Canaan Road area of the Separation Point Granite contact, and were interpreted by him as thermal hornfelses formed during emplacement of the granite. Near Takaka Hill, Cooper (1965) has mapped a quartzite, the Hailes Knob Quartzite, of probable uppermost Ordovician to Silurian age, overlying the Arthur Marble. This quartzite may be equivalent to the Ellis and Onekaka Formations. The Wangapeka Formation, although known to conformably underlie the Ellis Formation in the south, is considered to thin northwards, so that the Onekaka Schist conformably overlies the Arthur Marble. No top was seen to the formation and its thickness is at least 800 ft in the Graham area. No fossils were found within the Onekaka Schist in the Graham area and its age is assumed to be uppermost Ordovician to Silurian.

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QUATERNARY

In the south Graham River upstream of the marble gorge, the river occupies an open valley with terraces and has a gentle gradient. The river has formed these terraces by cutting down into laminated, flat-lying, dark carbonaceous, unindurated muds and silts, probably representing lake or swamp deposits. Towards the head, and at the sides of the valley, coarse gravels were deposited, probably during the latter part of the Pleistocene. At the mouths of most of the smaller streams, alluvial fans have been cut into by both the side and main streams. On the steep slopes below marble outcrops, marble talus has been cemented together by calcium carbonate to form coarse angular breccias. These breccias are commonly situated on non-marble terrain. Recent alluvium occurs at Graham Forks and remnants of high terraces are mapped in the north Graham River. IGNEOUS GEOLOGY

Separation Point Granite The Separation Point Granite is the most easterly of the three meridional granite belts of West Nelson and is composed of many rock types. In the Graham River, the dominant rock type is a medium-grained foliated, biotite granite, with both orthoclase and oligoclase feldspar. Attitudes of foliation planes are variable but the majority strike parallel to the granitesediment contact. Dips range from vertical to 50° towards the contact. Near the contact the granite is commonly gneissic, and at the contact itself is cataclastic. Further from the contact, foliation planes are less readily apparent. In North Graham River a small area of dark, fine-grained, hornblende granite or diorite crops cut. Thin aplite and pegmatite veins are numerous throughout the granite, and garnetiferous aplite veins are concentrated within the marble adjacent to the granite. Some pegmatites are simply zoned and may contain accessory muscovite and garnet. No preferred orientation of any of these veins was noted. The granite is younger than the Riwaka Complex; a pegmatite vein from the Motueka River was dated (Rb-Sr isochron method) at 105 m.y. (late Lower Cretaceous) by Aronson (1965).

Riwaka Complex Introduction The amended name, Riwaika Complex is proposed for the complex suite of mafic and ultramafic rocks, extending from the Baton River north to the Riwaka Valley. Also included are similar rocks at Thomson Hill (Baton Valley) to the south-east and Rameka Creek (Takaka Valley) to the north-west. The complex includes the majority of the rocks mapped by Grindley (1961) as Riwaka Metavolcanics and Rameka Intrusives. Geology-21

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The igneous rocks most commonly intmde quartzite and biotite schist of the Onekaka Schist, but in the north and south of the area, the Arthur Marble forms the country rock. The contacts make an acute angle with the regional strike of the sediments, and high-grade contact metamorphic effects have been observed at several places. A diorite from the complex at Rocky River to the north of the Graham River, was dated by the Rb-Sr isochron method by Aronson (1968) at 285 m.y. (late Carboniferous). Description In this study, some 60 thin and polished sections have been examined from the Graham area. Preliminary petrological examination indicates the presence of two main rock types within the Riwaka Complex at Graham Valley. 1. Graham Peridotite-chrome-spinel bearing peridotite 2. Pokororo Pyroxenite-(l) olivine-bearing clinopyroxenite (2) feldspathic biotite-hornblende pyroxenite containing disseminated massive sulphides. 1. Graham Peridotite The Graham Peridotite consists of a chrome-spinel bearing peridotite and has been found in two localities (Fig. 3). The main exposures on the east-west ridge between the North and South Graham Valleys suggests that the peridotite is a pod-like body completely surrounded by pyroxenite. No gross stmctures in the peridotite can be seen in the field but thin section studies show that the proportion of interstitial clinopyroxene changes from outcrop to outcrop. Olivine occurring as rounded subhedral crystals, is always partially serpentinised, and forms from 50% to 90% of the rock. Where the rock is olivine rich, interstitial clinopyroxene occurs as cuspate areas between the olivine grains and euhedral chrome-spinels are usually completely surrounded by pyroxene; only very rarely do they occur within the olivine. As the augite content of the rock increases, all spinel-type minerals tend to decrease in size and abundance. The spinels usually contain minute exsolution lamellae of ilmenite and magnetite, and are often surrounded by reaction rims of a "magnetite type" of spinel. The significance of the various corrosion features is not known. Clinopyroxenite veins and xenoliths are found in the peridotite. 2. Pokororo Pyroxenite The Pokororo Pyroxenite contains clinopyroxene and other minerals and is petrologically divided into two informal units.

(1) Olivine clinopyroxenite Olivine clinopyroxenite, usually containing some brown hornblende, is the most common rock type present in the Riwaka Complex in the Graham Valley. A pale green clinopyroxene is always the principal constituent and large, anhedral, partially serpentinised olivine forms 5% to 10% of the rock. Very large crystals of poikilitic brown hornblende

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commonly show reaction relationships with the host pyroxene and are very similar to those found in the small irregular hornblendite veins cutting the pyroxenite. Titaniferous magnetite and homogeneous ilmenite are. the most common opaque minerals in the pyroxenite, but accessory sulphides described below, have been found at several localities. Most outcrops of the pyoxenite are amphibolitised and localised shearu:g and crushing of the rock is widespread. The actual contact of the pyroxemte and the peridotite is not exposed, but since only a chain separates the more northerly peridotite plug from severely crushed and slickensided pyroxenite on the eastern side, the contact may be faulted. (2) Feldspathic biotite hornblende pyroxenite Feldspathic biotite hornblende pyroxenite crops out in the north-west tributaries of the south Graham River close to the eastern contact of the complex. In the creek bed at locality C (Fig. 3), the rock contains massive nickeliferous sulphides. Much of the rock is completely amphibolitised so that hand specimens have the appearance of an altered gabbro. Clinopyroxene, commonly twinned, and brown-green primary hornblende are the most abundant minerals in the pyroxenite; pleochroic hypersthene, partially serpentinised olivine, rutilated biotite, and granular basic plagioclase are also present. Most of the minerals are anhedral and show some reaction or replacement by others. Small plagioclase crystals are often enclosed within larger grains of pyroxene and hornblende, and both biotite and hornblende are derived in part from pyroxene. Exactly the same silicate minerals are found in the sulphide-bearing pyroxenite, and unaltered assemblages are more common than those showing secondary amphibolitisation. Two important facts emerge from the thin section study of the mineralised rocks: (a) many silicate minerals, especially clinopyroxene, plagioclase and brown hornblende show some evidence of replacement by sulphide minerals; (b) in the majority of samples from the mineralised zone there is very little secondary alteration of the silicates (uralitisation, etc.) that is commonly associated with the hydrothermal introduction of sulphides. Because of the lack of exposures in the creek it has not been possible so far to judge just how widespread the feldspathic pyroxenite is, or what its relationships are with the more common clinopyroxenite and peridotite. Available information apparently demonstrates a close association between the feldspathic pyroxenite and the massive nickeliferous sulphides, although similar sulphide assemblages are found as accessory minerals in the other ultrabasic rocks.

Mineralisation in the Riwaka Complex The sulphide assemblages and their field relationships at five localities in the Graham area, marked A to E on the geological map (Fig. 3) are now described. Identifications of some of the sulphide minerals were made by X-ray diffraction, and X-ray fluorescence analyses for nickel and copper were made on representative samples from the more massive sulphides. Geology-22

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Locality A


Mineralised pyroxenite occurs in the bluffs to the west of a small flood plain at the junction Q1f the North Graham River and Goat Creek. Much of the rock is amphibolitised and stained brown by the oxidation of sulphides. The pyroxenite consists of granular pyroxene, marginally replaced by green brown hornblende, olivine, and sporadically distributed interstitial sulphides (P36100)*. The sulphide assemblage usually consists of disseminated aggregates of pyrrhotite, pyrite and chalcopyrite. Pyrrhotite is the most common and locally shows secondary alteration to a lamellar marcasite phase. The assemblage is shown to be nickeliferous by the presence of characteristic pentlandite flames at both grain boundaries and crystal margins. With the exception of marcasite, all the sulphides appear to be primary minerals. There are no primary oxide minerals.

In Goat Creek, a few chains from the junction with the North Graham River, a thin sliver of completely altered calcite-bearing feldspathic pyroxenite containing rare sulphide blebs crops out (P36101). It contains pale-green uralitic amphibole, apatite prisms, and saussuritised plagioclase. The sulphide blebs consist mainly of homogeneous pyrrhotite, with small amounts of pyrite, very rare chalcopyrite, and pentlandite lamellae. The pyrite grains are sometimes engulfed by the pyrrhotite and are of early formation. No well exposed contacts between these ultramafic rocks and the enclosing quartzite and marble have been seen, but it may be significant that the apatite-bearing feldspathic pyroxenite in Goat Creek is adjacent to marble, whereas the more sulphide-rich hornblende clinopyroxenite is adjacent to quartzite.

Locality B This locality is on the ridge be~ween the North and South Graham Valleys and consists of the northern peridotite plug and the pyroxenite to east and west. To the east of the peridotite both fresh (P36108, P36207) and amphibolitised pyroxenite crops out. Titaniferous magnetite and homogeneous ilmenite, in variQlus stages of alteration, and sphene are present. Pyrite blebs do occur rarely but these are associated with hydrothermal alteration and secondary amphiboles. In the peridotite the opaque minerals are titaniferous chrome-spinel and complex titanmagnetite-ilmenite-spinel intergrowths. Extremely rare minute pyrrhotite blebs, are included within some of the chrome-spinel. A mineragraphic study of these rocks is in progress and a detailed examination of the various recation and exsolution phenomena may give some evidence of the crystallisation and post-consolidation history of the peridotite. *Number refers to New Zealand Geological Survey Petrological Catalogue.


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In the pyroxenite to the west of the peridotite sulphide minerals are. a common accessory and serve to distinguish this rock type from the pyroxemte to the east. A granular pale-green clinopyroxene forms the matrix of the rock and contains interstitial sulphide blebs, partially serpent:nised olivines, and some green-bro~n hornblende (P3611S). Chalcopyrite is the principal mineral in the sulphide assemblage, and the only primary oxide mineral present is homogeneous magnetite. Pyrrhotite, showing partial replacement by marcasite, is associated with pentlandite grains that are sometimes euhedral and completely enclosed in homogeneous magnetite. A common feature seen in many of these sqlphide assemblages, is that although they ocmr as disseminated blebs, they usually comprise two or three of the minerals present. Fractures in the sulphide aggregates are often filled by later oxides suggesting a definite age relationship. Both the chalcopyrite and pentlandite contain small lamellae of the birefiecting, highly anisotropic, tetragonal Ni, Fe sulphide, mackinawite. It ocmrs either as regular intergrowths within the other sulphides, or as irregular, randomly distributed replacement lamellae. The homogeneous magnetite and the lack of ilmenite and spinel suggests some fundamental difference in rock type between the pyroxenites found on either side of the peridotite. Locality C In a north-west tributary of the South Graham River feldspathic pyroxenite, containing the richest sulphide bodies so far discovered in the Riwaka Complex, crops out. The feldspathic pyroxenite intrudes calcareous quartzite, amphibolite and semi-pelite of the Onekaka Schist. The principal mineralised outcrop, approximately 12 ft high and 30 ft wide, ocmrs close to the contact and forms the eastern wall in the steepsided creek. All rocks in this massive exposure are covered by a bright orange-brown oxide film about i-! in. thick. Detailed examination of the walls shows that only about 6-8 ft of the exposure, measured across the apparent strike, contain abundant sulphides, the rest being the host pyroxenite and metasediments. Immediately adjacent to the mineralised zone the country rock is a clinopyroxene-bearing hornblende granulite, but in the creek bed opposite the lode calcareous quartzite appears to have been intruded by quartzofeldspathic material. To the west of the principal mineralised outcrop clinopyroxenite similar to those at locality A and containing some accessory sulphides crop out. The sulphides ocmr as blebs and usually comprise aggregates of chalcopyrite, pentlandite flame-bearing pyrrhotite, and rare pyrite. Because of the bush cover in the steepsided creek and the lack of exposures, the sulphide body cannot be traced laterally for any distance and the extent of the massive miner!llisation is unknown. Figs. 4 and 5 show typical sulphide development in samples from the centre and margins of the ore body; it is clearly apparent from the photographs that as the percentage of silicate host increases the sulphides tend to assume a more mspate, interstitial habit, and the silicates themselves lose the subhedral shapes evident in Fig. 4. Polished mounts of these rocks contain two principal primary sulphide assemblages: 1. Pyrrhotite-pentlandite-pyrite with accessory chalcopyrite. 2. Pyrrhotite-pentlandite-chalcopyrite with accessory mackinawite.


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K. R. Gill, photos

FIG. 4 (l efl )-Massive sulphides (white) enclosing subhedral silicate minerals (grey). The sulphide ass emblage is pyrrhotite-pyrite-pentlandite and the silicate gangue is augite-hypersthene- olivine-hornblende-plagioclase. FIG. 5 (right)-Interstitial sulphides partially replacing silicate minerals in feldspathic pyroxenite. The sulphide assemblage is pyrrhotite-pentlandite-chalcopyrite. The sulphide minerals, especially chalcop}'rite, often occur as small veinlets and filaments in fractures and cleavages within the silicates which are usually quite unaltered even though they appear to be partially corroded by the sulphides. Samples are fr om locality C.

Both of these assemblages have been modified by secondary replacement and alteration. The alteration is irregular and not obviously associated with uralitisation and serpentinisation of the co-existing silicate minerals. Pyrrhotite is often extensively altered to lamellar marcasite (Fig. 7) and subsequent weathering gives rise to secondary hydrous iron oxides. Pentlandite often shows incipient marginal and overall alteration to the nickeliron sulphide, violarite. It commonly has a pinkish colour, instead of straw yellow, and a slightly porous surface suggesting incipient alteration. (Fig. 6). Sometimes this alteration is restricted to grain margins and the distinction between fresh and slightly altered pentlandite is readily apparent (Fig. 8) . 1. In the first assemblage the sulphides are massive and form a composite network of granular crystals enclosing both mafic and fe1dspathic silicate minerals. Pyrite occurs as rounded subhedral grains always engulfed by homogeneous pyrrhotite showing deformation twins and complex grain boundaries.


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FIG. 6 (left)-Pentlandite (Pe) with characteristic cleavages, associated with pyrite blebs (Py) enclosed in optically homogeneous pyrrhotite (Ph). The pentlandite has lost its characteristic straw yellow colour and the surface is slightly pitted, suggesting incipient alteration . FIG. 7 (right)-Veinlets of pentlandite with characteristic cleavages in a matrix of lammellar marcasite (?) that has almost completely replaced the pyrrhotite. A few small patches of residual pyrrhotite can be seen in the centres of the altered grains. Samples are from locality C.

Cracks and angular voids in the pyrite are often filled with pyrrhotite indicating the early formation and possible replacement of pyrite. The amount of pyrite is variable up to a maximum of approximately 15 % of sulphides present. Pentlandite usually occurs as small veinlike bodies within the pyrrhotite (Fig. 7) and only rarely forms composite aggregates with the other principal sulphides (Figs. 6 and 8). Some flames of pentlandite are seen at grain boundaries of pyrrhotite, but are far more common in assemblages where pentlandite is less abundant. Chalcopyrite distribution is sporadic and it usually occurs alone within the silicates but it is sometimes present as discrete grains in composite sulphide blebs. It often penetrates cleavages and fractures suggesting that it is probably formed late'r than the majority of phases present (Fig. 9).

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Pyrrhotite is always the dominant sulphide and X-ray diffractometer runs of concentrates from this locality suggest that both monoclinic and hexagonal modifications are present; these phases cannot be distinguished optically, and so far none of the stoichiometric hexagonal "troilite" phase, with its characteristic lamellar intergrowths, has been observed. 2. In the second assemblage towards the margin of the sulphide body pyrite is not usually present and chalcopyrite becomes more prominent. As the silicates increase in proportion, the pentlandite tends to become less vein-like within the pyrrhotite and forms more compact, granular crystals, pentlandite flames are commonly found on grain boundaries and crystal edges. The more abundant chalcopyrite now occurs as discrete masses, and is often associated with green-brown hornblende filling minute cracks within adjacent silicates (Fig. 9). Vermicular intergrowths are another characteristic development, and occasionally chalcopyrite appears to be replacing massive pyrrhotite.

FIG. 8 (le/t)-Homogeneous straw yellow pentlandite (Pe) with rare cleavages, showing marginal alteration tQ pale violet violarite (?) (dark grey), associated with chalcopyrite (C) enclosed in homogeneous pyrrhotite (Ph). FIG. 9 (right )-Homogeneous chalcopyrite (C) occurring in fractures and cleavages in the silicate minerals of the host pyroxenite. Samples are from locality C.


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Preliminary analyses of the samples from the principal mineralised outcrop indicate a variation in nickel content from 1·2% to 1·7%, and an analysis ~f a composite chip sample from a dozen different points across the sulphide body gave an average Ni content of 1'5 %. Copper appears to be much more variable from 0'05% to 1'8% with an average on the chip sample of 0'25%. Cobalt was also detected by the X-ray fluorescence spectrometer but no determinations have been made for this element. . The textures observed are very similar to others that have previously been described from basic igneous complexes (Sudbury, Insizwa, Muskox, etc.), the form of the pentlandite suggesting unmixing from a high temperature pyrrhotite-pentlandite solid solution. Corrosion textures of the silicates by the sulphides briefly mentioned in the thin section descriptions above, certainly suggest post-silicate crystallisation of the sulphides, but whether this is a primary magmatic feature as described from elsewhere (e.g., Sudbury) or whether it results from local remobilisation of the sulphides by the later granitic intrusions has yet to be determined. Present evidence suggests that the massive sulphides in the primary pyroxenite are comagmatic with the silicates although there has been some later hydrothermal movement, probably along shear zones.

Locality D Boulders of ultrabasic rocks found in the small alluvial fan where the Flora Track crosses Thorn's Creek are of hornblende-clinopyroxenite sometimes containing poikilitic accessory sulphides. Nickeliferous pyrrhotite and assessory chalcopyrite are the only minerals found, and iron oxides appear to be lacking. The ultra-basic rocks found in situ on the Flora Track are highly fractured and altered olivine-bearing clinopyroxenite containing no accessory sulphide minerals. Locality E To the west of locality C, another small tributary of the South Graham River flows across the eastern contact of the Riwaka Complex. Although an intrusive contact it has been complicated by later fault movement along it. Three samples (P36274, P36271, P36275) were collected close to the contact. One sample (P36274) is a feldspathic, biotite-bearing clinopyroxenite containing appreciable accessory sulphides. Nickeliferous pyrrhotite, granular subhedral pentlandite and irregular grains of chalcopyrite form composite sulphide blebs; irregular pentlandite flames commonly occur on the grain boundaries between chalcopyrite and the enclosing pyrrhotite. Alteration of the primary silicate minerals gives rise to secondary amphiboles containing much disseminated magnetite, and rutile-sphene symplectites are presumably derived from the original oxide minerals during the amphibolitisation. The second sample (P36271) is a mineralised pyroxenite and contains a prominent opaque mineral assemblage. Large interstitial pyrrhotite blebs with many coarse pentlandite flames are often enclosed by a homogeneous magnetite, which also occurs as wide veinlets in the amphibolitised host.


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Textural relationships, on the whole, point to the later formation of the oxide minerals, but there is still some ambiguity in a few cases. Rare tablets of homogeneous pink ilmenite also occur in the silicate matrix. The third sample (P36275) is a mineralised mi~aceous granuli~e containing the same sulphide assemblages as the adjacent ultrabaslC rock suggestting that there has been some migration of sulphides away fr?m the igenous body. A biotite-sericite-quartz schist contains rounded sulphIde masses approximately 5 mm in diameter; nickeliferous pyrrhotite,. with some granular pentlandite, is often partially replaced along its margms by a low reflectivity, highly anisotropic and bireflecting mineral that has yet to be determined. Mackinawite lamellae are found in the associated massive chalcopyrite, but the optical characteristics of this replacement mineral are not identical with published properties of the tetragonal sulphide.

Other Areas At two localities in the Graham area (Grid Refs S13/191409 and 178393) poorly exposed, crushed igneous rocks on the eastern contact of the Complex are covered in an iron oxide film. A similar film covers rocks of the Riwaka Complex on the west bank of Field Creek (Grid Ref. S13/141348), a tributary of the Pearse River, to the south of the Graham. The nature of the sulphides forming these films has not yet been investigated. STRUCTURE

Within the Graham area faulting is widespread, with a dominant northnorth-east trend. Grindley (1961) showed the major Skeet Fault separating the Riwaka Complex from the sediments to the west. Detailed mapping has shown that this contact, except locally, is unfaulted from the Graham Valley south to the Baton River. A major fault, ! to ! mile west of the basic rocks, is probably the continuation of the Skeet Fault north from the Baton River. Although this fault can be recognised in the area south of the Graham area, it is poorly exposed in the South Graham Valley. In Goat Creek there is a zone of crushing, with an abrupt change in the attitude and nature of the rocks on either side. Several large springs emerge and large boulders of quartz, up to 10 ft in diameter are present. This fault is present within the Arthur Marble in the head of the North Graham Valley (G. W. Grindley, pers. comm.). West of the Skeet Fault, sediments of the Flora Formation and Arthur Marble are complexly folded along north to north-west trending axes. In the upper part of the South Graham Valley, a number of tight, commonly overturned folds, form a complex north to north-west trending anticlinal structure. Near the Skeet Fault the marble has been folded into recumbent mesoscopic folds, about south-west dipping axial planes. East of the Skeet Fault, the Arthur Marble and Onekaka Schist have been folded into the north-north-east-trending Riwaka Syncline, and are intruded by basic and acidic igneous rocks. In the north-west, the west limb of the syncline consists of complexly folded alternating quartzitic and

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calcareous beds. Elsewhere the strike of the sediment is regular. The proximity of the Riwaka Complex to the Riwaka Syncline suggests that it may have been intruded along the synclinal axis. Two faults, parallel to the Skeet Fault, are well exposed in the South Graham Gorge. The most eastern one can be traced northwards into the Riwaka Complex where it dies out north of Goat Creek. Almost at right angles to these faults are two other faults, one in the South Graham Valley and the other to the south of Goat Creek, which vertically offset the Riwaka Complex.


CONCLUSIONS

Rocks forming the Riwaka Complex intrude sediments of the Mount Arthur Group, and consist of gabbro, diorite, pyroxenite and peridotite. Nickel and copper-bearing sulphide minerals are found as both accessory and major minerals in ultramafic members of the Complex in the Graham Valley. Many of the contacts of the Riwaka Complex with the adjacent metasediments of the Onekaka Schist are obscured, but the richest sulphides discovered lie close to the eastern side of the complex. Thin section and mineragraphic studies of many samples suggest that the sulphides are of primary magmatic origin and that some, at least, crystallised after the silicate minerals. The host rock for the massive sulphides is a feldspathic biotite pyroxenite that is found at several localities along the eastern margin. Some migration of metals and sulphur into the country rocks has occurred but the degree of mineralisation, and its extent, have not been determined. In order to prove the extent of both the massive and disseminated sulphides and the nature of the contacts more detailed sampling and geophysical and field geochemical studies are necessary, and these might need to be followed by shallow drilling from favourable sites. ACKNOWLEDGMENTS The writers gratefully acknowledge constructive criticism received from Mr G. W. Grindley and Dr A. Wodzicki. REFERENCES ARONSON, ]. 1. 1965: Reconnaissance rubidium-strontium geochronology of New Zealand plutonic and metamorphic rocks. N.Z. 11 Geol. Geophys. 8 (3): 401-23. - - - - 1968: Regional geochronology of New Zealand. Geochim. cosmochim. Acta 32 (7): 669-98. CoOPER, R. A. 1965: Lower Paleozoic rocks between Upper Takaka and Riwaka, North-west Nelson. N.Z. 11 Geol. Geophys. 8 (1): 49-61. - - - 1968: Lower and Middle Paleozoic fossil localities of North-west Nelson. Trans. R. Soc. N.z. (Geol.) 5 (7): 75-89. Cox, S. H. 1881: On certain mines in the Nelson and Collingwood districts, and the geology of the Riwaka Range. N.Z. geol. SUrt). Rep. geol. Explor. 1879-80 (13): 1-11.


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GHENT, E. D. 1968: Petrology of metamorphosed pelitic rocks and quartzites, Pikikiruna Range, North-west Nelson, New Zealand. Trans. R. Soc. N.Z. (Geol.) 5 (8): 193-213. GILL, K. R.; JOHNSTON, M. R. 1967: Nickel and copper mineralisation in the Riwaka Basic Complex, Graham V:llley, North-west Nelson. Rep. geol. Surv. N.Z. 26. GRINDLEY, G. W. 1961: Sheet 13 Golden Bay "Geological Map of New Zealand, 1:250,000". New Zealand Department of Scientific and Industrial Research, Wellington. HENDERSON, ].; MACPHERSON, E. 0.; GRANGE, L. I. 1959: The geology of the Motueka subdivision. Bull. N.z. geol. Surv. n.s. 35. McKAY, A. 1879: The Baton River and Wangapeka districts and Mount Arthur Range. N.Z. geol. Surv. Rep. geol. Explor. 1878-79 (12): 121-31. PARK, J. 1890: On the geology of Collingwood County, Nelson. N.Z. geol. Surv. Rep. geol. Explor. 1888-89,20: 186-243. WILLIS, I. 1965: Stratigraphy and structure of the Devonian strata at Baton River, New Zealand. N.z. 11 Geol. Geophys. 8 (1): 1-48.