Gold mineralogy

7/29/2019 Gold mineralogy

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Gold and sulphide minerals in Tertiary quartz pebble conglomerate

gold placers, Southland, New Zealand

D.M. Falconera,*, D. Craw a, J.H. Youngson b, K. Faure c

a Department of Geology, University of Otago, P. O. Box 56, Dunedin, New Zealandb

Placer Solutions (2004) Ltd, P. O. Box 5284, Dunedin, New Zealandc Institute of Geological and Nuclear Sciences Ltd, P. O. Box 31312, Lower Hutt, New Zealand

Received 15 February 2004; received in revised form 2 May 2004; accepted 14 March 2005

Available online 9 January 2006

Abstract

Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district,

Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-

forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths

on detrital AuAg and AuAgHg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained AuAg and AuAgHg

alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au

Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous

mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and

arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite ismarcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has

also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and

anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from 45x to +18x

(relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from 9x to 1x and are most likely

derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (b20x) reflecting

microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations

of Ni, Co, As and Cr, commonly with compositional banding of these metals.

Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from

Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the

Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers.

There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and

post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young depositsthus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of

these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Quartz pebble conglomerate; Authigenic gold; Sulphidation; Marcasite; Framboidal pyrite; Colloform pyrite

0169-1368/$ - see front matterD

2005 Elsevier B.V. All rights reserved.doi:10.1016/j.oregeorev.2005.03.009

* Corresponding author. Fax: +64 3 4821175.

E-mail address: [email protected] (D.M. Falconer).

Ore Geology Reviews 28 (2006) 525545

www.elsevier.com/locate/oregeorev


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1. Introduction

Gold deposits hosted by quartz pebble conglomer-

ates are a significant source of gold worldwide. The

most important are the Archaean QPC gold deposits of

the Witwatersrand Basin, South Africa, which haveproduced about 48,670 t of gold between 1886 and

2000, equating to nearly 40% of all gold produced

worldwide (Frimmel and Minter, 2002). The well-docu-

mented Witwatersrand orebodies are characterised by

gold and sulphide minerals that feature both detrital and

authigenic textures (see reviews by Phillips and Law,

2000; Frimmel and Minter, 2002). However, the gene-sis of QPC-hosted gold and sulphide mineralisation

Fig. 1. (A) Relief map of eastern Otago and Southland showing low relief areas dominated by Tertiary non-marine sequences that contain QPC.Provincial boundary between Otago and Southland indicated. (B) Schematic geological map of Waimumu area showing location of selected sites.

D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545526


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from the Witwatersrand orebodies are the focus of

considerable debate because of the distinction, or lack

thereof, between detrital and authigenic textures

(MacLean and Fleet, 1989; Phillips and Myers, 1989;

Robb and Meyer, 1990; Minter et al., 1993; Myers et

al., 1993; Barnicoat et al., 1997, 2001; Phillips andLaw, 1997; Fleet, 1998; Minter, 1999; England et al.,

2002). Specifically, one of the central issues in these

debates is the lack of criteria to distinguish between

diagenetic sulphide or gold textures and those resulting

from low-grade regional metamorphism or hydrother-

mal mineralisation. In the absence of well-constrained

unmetamorphosed diagenetic assemblages, it is inevi-

table that a variety of diagenetic textures and geochem-

ical signatures could be erroneously attributed to

metamorphic processes. In particular, gold-grain tex-

tures in the Witwatersrand deposits are ambiguousbecause of potential effects of metamorphic recrystalli-

sation (Minter et al., 1993; Barnicoat et al., 1997,

2001).

One approach to resolving such controversies is to

examine gold and sulphide textures in younger unme-

tamorphosed QPC that feature gold and sulphide miner-

alisation, as potential analogues for Witwatersrand-style

gold deposits. Such QPC occur in southern New Zeal-

and where both sedimentary and diagenetic processes

are able to be better constrained, and therefore more

easily understood than those of Archaean QPC (Young-

son et al., 2006-this volume). Although some workersclaim that the Archaean environment was unique in the

Earths history with respect to the occurrence of sul-

phide minerals in sediments (Phillips and Myers, 1989;

Phillips et al., 2001), there is abundant evidence for

detrital and diagenetic sulphide mineral occurrences in

young non-marine sediments (Clough and Craw, 1989;

Youngson, 1995; Craw and Chappell, 1999; Brown et

al., 1999, 2000; Paktunc and Dave, 2002; Falconer,

2003). Despite such occurrences, the search for Witwa-

tersrand analogues remains focussed on similar Archae-

an terranes (Fox, 2002) rather than expanding this

approach to include much younger QPC.This paper documents some aspects of the gold and

sulphide mineralogy in QPC gold placers from South-

land, New Zealand, especially aspects that might be

relevant to interpretation of mineralised Archaean

deposits. Thus, despite their economic insignificance,

young QPC gold placers such as those in southern New

Zealand (Fig. 1A) are a potential analogue for Witwa-

tersrand-style gold mineralisation.

2. Regional geology

The QPC described in this study occur in non-ma-

rine, fluvial and colluvial sedimentary units of late

Oligocene to Pliocene age (Fig. 2). The non-marine

strata are commonly underlain by Oligocene marine

sedimentary rocks, and the Tertiary sedimentary se-

quence rests on Permian to Jurassic greywacke base-

ment (Wood, 1956; Isaac and Lindqvist, 1990; Turnbull

and Allibone, 2003). The Tertiary non-marine strata are

dominated by the Gore Lignite Measures of late Oligo-

ceneMiocene age, deposited by a large meandering

fluvial system (Isaac and Lindqvist, 1990). Basal del-

taic beds are overlain by extensive, generally fine-grained lower-delta plain beds and lignites, which are

in turn overlain by extensive upper-delta plain con-

glomerates and mudstones (Isaac and Lindqvist, 1990;

Fig. 2). These sediments are at least 500 m thick,

although late Tertiary uplift has caused significant ero-

sion of parts of the section (Isaac and Lindqvist, 1990).

Fig. 2. Generalised stratigraphic sequence for the Waimumu area (modified after Isaac and Lindqvist, 1990).

D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 527


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The upper part of this non-marine sequence is domi-

nated by conglomerates with abundant basement grey-

wacke clasts, and ca. 30 vol.% quartz pebbles derived

from the Otago Schist belt to the north (Fig. 1A). This

schist belt contains numerous mesothermal vein sys-

tems (Craw and Norris, 1991) that have shed gold toform placer accumulations throughout Otago and

Southland (Youngson et al., 2006-this volume).

Recycling of Tertiary non-marine conglomerates

during late Cenozoic uplift resulted in local redeposi-

tion of variably quartz-rich QPC (N80 vol.% quartz

pebbles) during the late Miocene and Pliocene (Wai-

mumu and Waikaka Quartz Gravels; MacPherson,

1937; Wood, 1956; Falconer, 2003). Pliocene to Pleis-

tocene immature fluvial conglomerates (Gore Piedmont

Gravels; Wood, 1956) were derived from uplifting

basement ridges and the pre-existing Tertiary sedimen-tary sequence, and locally overlie the QPC described

above (Fig. 2). Pleistocene fluvio-glacial gravels are

restricted to the present major drainage systems. In

both Otago and Southland, many of these middlelate

Tertiary QPC host economically significant gold pla-

cers (Youngson et al., 2006-this volume).

2.1. Mining history

In the Waimumu area (Fig. 1B), alluvial gold was

historically dredged from several of the principal

streams, with 30,000 oz produced before 1904. Hydrau-lic sluicing was practiced intermittently from the 1930s,

with hydraulic excavators and gravity recovery systems

utilised until the late 1990s. Numerous small under-

ground workings throughout the district followed rela-

tively rich gold leads in the WaimumuWaikaka Quartz

Gravels. The Belle-Brook placer, 5 km south of the

historic gold workings, was not discovered until the late

1970s, and has been sporadically mined on a small

scale since 1980. The Parker Road QPC have been

sporadically mined as a source of road-building aggre-

gate, with gold recovery from this deposit about tocommence. The Waimumu area also contains signifi-

cant economic lignite deposits in the Tertiary non-ma-

rine sequence (Isaac and Lindqvist, 1990; Fig. 2).

2.2. Geology of the study area

This study focuses on sediments in the Waimumu

area (Fig. 1B). Structurally, this area is dominated by

the Dunsdale Fault System, a series of north- to north-

east-striking reverse faults that dip steeply to the west,

and trend for at least 50 km. Some of these faults have

uplifts of 400 to 500 m, exposing Murihiku basement

that comprises the nearby Hokonui Hills (Fig. 1B).

Belle-Brook, one of the principal localities for this

study, is in a structural basin within this fault system,

on the downthrown side of the Hedgehope Fault. A

second site at Parker Road (Fig. 1B) is similarly located

adjacent the Hedgehope Fault. Marcasite for this studywas also collected from Hedgehope Stream (Fig. 1B),

where the stream is eroding through a section of faulted

Gore Lignite Measures (Isaac and Lindqvist, 1990).

The upper part of the Tertiary non-marine sequence

(Gore Lignite Measures) exposed at Belle-Brook is

typically a coarse (up to 15 cm clasts), clast-supported

fluvial conglomerate, with poorly defined stratification.

Minor stratified sandstone of similar mineralogical

composition is interbedded with these conglomerates.

Unaltered Gore Lignite Measures conglomerates are

bluish in colour and composed of approximately70 vol.% greywacke cobbles and 30 vol.% quartz

clasts. However, most of the Gore Lignite Measures

conglomerates at Belle-Brook exhibit moderate to

strong kaolinisation of the greywacke component,

resulting from the alteration of labile minerals. The

heavy-mineral suite is dominated by marcasite, zircon,

and garnet, with minor pyrite, arsenopyrite, chromite,

rutile, magnetite, and rare gold. Gore Lignite Measures

conglomerates at Belle-Brook are unconformably over-

lain by thin (ca. 1 m) QPC of inferred Pliocene age that

have been formed by the erosion and recycling of

underlying conglomerates (Clough and Craw, 1989;Falconer, 2003; Youngson et al., 2006-this volume).

The quartz component is higher (ca. 95 vol.%) in

these recycled sediments, as the altered greywacke

clasts disaggregated during erosion and transport. The

heavy-mineral suite in these recycled QPC includes

zircon, garnet, gold, framboidal and coarse anhedral

marcasite, and, less commonly, framboidal pyrite, euhe-

dral pyrite and arsenopyrite. Magnetite is notably rare.

Localised deposits of mature Pliocene QPC consist-

ing of 99 vol.% quartz cobbles occur in the non-marine

sequence. One such QPC, at Parker Road (Fig. 1B), isat least 5 m thick and overlain by a thin (b2 m)

carbonaceous mudstone. Palynology on the Parker

Road carbonaceous mudstone indicates that the envi-

ronment was an acid swamp, and the climate was

temperate to sub-tropical. Pollens and spores from the

carbonaceous mudstone indicate an age of 5 to 3.1 Ma

which is consistent with deposition following QPC

formation. The heavy-mineral suite in the Parker

Road QPC is dominated by unaltered chlorite, zircon

and garnet, with rare gold and magnetite. However, in

rare well-defined channels, comparatively less mature

QPC have abundant gold and magnetite. The QPC at



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Parker Road and QPC elsewhere in the Waimumu

district do not contain framboidal or anhedral sulphides

in the heavy mineral fraction (Craw, 1992; Falconer,

2003).

3. Methods

Heavy mineral concentrates from the QPC were

collected by panning disaggregated material from out-

crops, and from a gold concentration plant at Belle-

Brook. Mineral concentration did not involve the use of

mercury. Sulphide and gold grains for all analytical

procedures were handpicked under a stereomicroscope

to ensure contamination-free phases were selected.

Bulk samples of sulphide material for trace element

analyses were selected by hand and analysed by X-

ray fluorescence in the Geology Department, Universityof Otago. Analysis followed standard methods and used

international rock standards. Follow-up trace element

analysis using ICP-MS was carried out by Australian

Laboratory Services, Brisbane, Australia. Particular

care was taken during bulk sample preparation to

avoid cross-contamination between samples. Quartz

blanks were routinely prepared in the same way as

the sulphide samples, and analysed by the same tech-

niques. Sulphide phases were confirmed optically

(using reflected light, oil immersion), and by X-ray

diffraction and Gandolfi camera powder patterns.

Electron-probe microanalysis (EPMA) of gold andsome sulphide minerals were carried out on carbon

coated samples with a JEOL JXA 8600 instrument

(operated at 25 kV, 20 nA (2108A) beam current,

with a 2 Am beam diameter). A procedure documented

by Youngson et al. (2002) was used to correct for peak

interference between Au and Hg. EPMA detection

limits were approximately: As, 0.4%; Cu, 0.1%; Zn,

0.1%; Ni, 0.1%; Co, 0.1%; Fe, 0.1%; S, 0.1%; Au,

0.4%; Ag, 0.2%; Te, 0.5% and Hg, 0.4%. Pure metal

standards were used for As, Bi, Sb, Te, Cu, Ni, Co, Fe,

Au and Ag, crystalline cinnabar was used for the Hgstandard, and sphalerite for Zn and S standards. Core

and rim compositions of gold grains were obtained

separately. dCoreT analyses refer to the central part of

the grain in polished section. Element maps were

obtained over a 450500 Am grid with a semi-quan-

titative analysis every 1 Am. Scanning electron micro-

scope (SEM) examination of grains was carried out

using a Cambridge S-360 instrument, in which operat-

ing conditions varied from 15 to 35 kV, and a JEOL

6700F Field Emission SEM operated at 2.5 to 5 kV.

Samples were not etched or cleaned in any way prior to

mounting on SEM stubs using a fine paintbrush under a

Zeiss stereomicroscope. Although the Cambridge SEM

did not have an EDS detector, micron-sized dgoldT

forms are inferred based on back scatter response, the

use of uncoated samples (i.e., less stable minerals read-

ily alter, and/or, vaporise under high kilovolts), along

with similarities to macroscopic gold forms or surfacetextures that are known to be gold.

Sulphur isotope analysis of sulphide minerals was

analysed by conventional methods at the Institute of

Geological and Nuclear Sciences Stable Isotope Labo-

ratory, Lower Hutt, New Zealand. Sulphur for isotope

measurement was liberated from the sulphide minerals

(single crystals) using the Robinson and Kusakabe

(1975) and Kiba et al. (1955) methods, respectively.

Results are expressed in the familiar d34S notation as

per mil (x) relative to Canon Diablo troilite (VCDT)

standard with a variability ofF

0.2 per mil (x

).

4. Gold

4.1. Morphology

Four gold sub-types are identified based on their

gross morphology: 1. fine-grained gold; 2. coarse nug-

get gold with intergrown quartz; 3. jagged-edged gold;

and 4. porous, sheet-like gold. At least 95% of the gold

occurs as fine-grained gold, with minor coarse gold (ca.

4%) and rare jagged-edged and porous gold (b1%).

Fine-grained gold occurs predominantly as flattened,rounded platy particles that range from 300 to 800 Am

in length (Fig. 3A). Most grains are multiply refolded

and flattened to a thickness of ca. 15 Am. Grain edges

are variably thickened, but not to the extent of the

well-developed toroids reported from elsewhere (e.g.,

Giusti and Smith, 1984; Minter, 1999). Other gold

forms that are included in the fine-grained category

include variously ball-like, stubby, and elongated

cigar-shaped particles. Stubby particles are short (typ-

ically 100 Am wide by 300 Am long) and although

multiply refolded, they are not flattened (Fig. 3B).Stubby particles are a common component of the

fine-grained gold that occurs at Parker Road. These

stubby particles typically have deep cavities between

refolded and rolled particle limbs, in which a variety of

spheroidal, bud-like protrusions and spongy gold forms

are well preserved within parts of such cavities (Fig.

3C). Surface textures of rounded plate-like gold parti-

cles are characterised by a variety of gold forms that

occur either in protected cavities or on open surfaces:

spheroidal forms and bud-like protrusions (Fig. 3D),

chain and ring structures (Fig. 3E, F); and, rarely,

euhedral gold forms (Fig. 3G). All of these inferred



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gold forms are well preserved with no physical defor-

mation. Iron oxide coatings are rare on the fine-grained

gold particles.

Coarse nugget gold (2 to 6 mm in length), generally

with intergrown quartz, is less common than fine-

grained gold, and occurs only at Belle-Brook (Fig.

3H). This variety is chunky and irregular in shape

and has not been flattened. Most particles have com-

plex, and or, branched forms (see Knight et al., 1994)

and are variably crystalline in appearance (Clough and

Craw, 1989; Falconer, 2003; Falconer and Craw, sub-

mitted for publication). Many particles exhibit cavities

Fig. 3. SEM photomicrographs illustrating morphological features of alluvial gold from Parker Road and Belle-Brook. (A) Typical example of fine-

grained refolded platy gold. (B) Fine-grained gold from Parker Road featuring deep cavities with authigenic gold as shown in C. (C) Authigenic

gold forms occurring as chain structures within cavities (scale pertains to left hand image). (D) Spheroidal and bud-like gold forms occurring within

shallow cavity on surface of fine-grained gold from Parker Road. (E) Stepped chain structure illustrating polygonal development of individual gold

forms (steps) protruding from general gold surface. (F) Variously developed polygonisation of bud-like gold forms and chain structure occurring on

general gold surface. (G) Triangular plate gold forms occurring on gold surface. (H) Coarsely crystalline gold from Belle-Brook showing intergrown

quartz in bottom right of grain. (I) Jagged-edged gold from Belle-Brook showing thin jagged edges. (J) Close-up of I, showing smooth, clean,striated gold surface that resembles a slickensided surface.



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or large smooth faces reminiscent of sites from whicheuhedral quartz crystals have been plucked. Irregular

grain edges and protrusions are typically rounded and

variably infolded. The margins of such infolded rims

have not been flattened, except in cases where the

infolded protrusion is thin (b100 Am). Euhedral pyrite

containing micron-sized gold blebs has been found

within some euhedral quartz crystals and occurring

within host gold grains (Falconer, 1987).

Very small (b150 Am), thin (1 to 20 Am), jagged-

edged particles (Fig. 3I) have been observed from one

specific sandy-gravel lithofacies within the QPC atBelle-Brook. A notable feature of these particles is

the gouged and striated nature of the particularly

clean (contamination-free) surface that resembles slick-

ensides (Fig. 3J).

Porous, irregular sheet-like particles of gold occur

within a carbonaceous horizon at Parker Road (Fig.

4A, B). Such gold particles are extremely thin (b10 Am),

up to 4 mm in length, very delicate, and easily

damaged even by careful handling. Despite the irreg-

ular and pseudocrystalline appearance of these grains,

SEM examination shows that well-formed crystals are

absent (Fig. 4C). The porous sheet-like gold has a

lacy appearance and abundant delicate protrusions thatare generally ca. 5 Am across, many of which are

either interconnected or joined to form dchainsT (Fig.

4C, D, E). Most of the delicate features and protru-

sions that make up the surface of the grains are

smooth and lack planar faces. The protrusions show

no obvious crystallographic alignment or control, but

there is a subtle orthogonal association similar to that

exhibited by authigenic gold forms on fine-grained

gold (Falconer, 2003). Grain edges are very irregular

and thin, with a typical thickness ca. 5 Am. There is

no deformation of any of the delicate features on thegrain edges, and grain surfaces show no indications of

abrasion and little evidence of chemical modification

(dissolution along grain boundaries) (Fig. 4E). Crys-

talline stepped-chain structures are common on the

gold surface (Fig. 4F).

4.2. Gold composition

Fine-grained gold is predominantly AuAg alloy

(72%), with lesser AuAgHg alloy (25%) and minor

pure gold (3%) (Table 1). These different gold types are

optically indistinguishable and do not appear to be

Fig. 3 (continued).



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characterised by specific morphological forms or sur-

face textures. Less than 1% of fine-grained gold parti-

cles are compositionally homogeneous (Table 1). An

element map of one coarse gold grain showed that

silver-depleted rims are not developed on either the

grain surface or the interior quartzgold grain bound-

aries. Compositional analysis has not been undertaken

for either jagged-edged or porous sheet-like gold, how-

ever, the latter has been examined by EDS to confirm

suspected gold forms.

The silver content of AuAg alloy ranges from 0.2

(detection limit) to 10.2 wt.% in grain cores, with an

average of 3.5 wt.% Ag. However, a single electrum

grain contained between 27 and 32 wt.% Ag. AuAg

Fig. 4. SEM photomicrographs illustrating morphology and surface texture of authigenic porous sheet-like gold from carbonaceous material at

Parker Road. (A) Porous, sheet-like gold grain. (B) Portion of grain showing irregular and delicate nature of grain exterior margins. (C and D)

Interior of grain illustrating porous nature of interconnected gold forms and bud-like protrusions. (E) Close-up of smooth bud-like gold protrusions

that shows no physical deformation. (F) Close-up of crystalline stepped gold forms on porous sheet-like gold surface.



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alloy grains were free from quartz and primary sulphide

inclusions. The single sample of coarse gold with inter-

grown quartz that was analysed is an AuAg alloy grain

containing 2.3 wt.% Ag. Trace amounts of Cu were

detected in only two samples (ca. 0.1 wt.%). No other

trace elements were detected (i.e., As, Bi, Zn, Sb, Fe, S,

Te; detection limits listed above). Silver content in Au

AgHg alloys is typically higher than in AuAg alloys,

and ranges between 1.3 and 13.9 wt.% in grain cores,

with an average of 6.6 wt.% Ag. Mercury ranges from0.4 to 9.1 wt.% in grain cores, with an average of

2.2 wt.% Hg. The AuAgHg alloys do not contain

inclusions of quartz or primary sulphide. Mineralogi-

cally, AuAgHg alloys at Belle-Brook are a-phase

alloys of hydrothermal origin, rather than secondary

AuAgHg alloys that contain N17 wt.% Hg (Young-

son, submitted for publication).

Many AuAg alloys and AuAgHg alloys exhibit

well-defined Ag-depleted rims that, in polished sec-

tions, are either conformable with the grain margin or

irregular in appearance. These two contrasting styles ofAg-depletion are clearly defined by electron micro-

probe element mapping. Both styles of Ag-depleted

rims show a sharply defined and steep gradient sepa-

rating the bulk of the grain from the depleted rim, and

typically contain less than 0.5 wt.% Ag. Element map-

ping also shows that, in both styles of Ag-depletion, the

rim is commonly made up of an extensive inner zone

that is Ag-depleted, and a minor outer zone of pure gold

on grain margins.

Silver enrichment in the rims of gold alloys is

less common, with 28% of the AuAg alloys and

6% of the AuAgHg alloys showing Ag-enriched

rims. Gold alloys that were observed to have Ag-

enriched rims were indistinguishable from grains

with Ag-depleted rims in terms or morphology. Gen-

erally, enrichment of 0.3 to 2 wt.% Ag occurs,

although up to 5% Ag-enrichment relative to core

composition is exhibited by a single electrum grain.In contrast to the steep compositional gradients typ-

ically associated with Ag-depletion, the Ag-enriched

rim from this electrum grain shows a more gradual

increase in Ag content. With the exception of the

electrum grain data were not collected to establish

the nature of the core-rim contact for Ag-enriched

gold alloys.

The distribution of Hg in AuAgHg alloy gener-

ally mimics that of Ag, whereby Hg is depleted in

rims relative to the core. However, there are subtle

differences in the distribution of Ag and Hg withindepleted rims, with Hg forming more extensive and

wider zones of depletion than Ag. Also, Hg was not

detected in 75% of grain rims, whereas Ag is com-

pletely absent in only 22% of AuAgHg alloy rims

(Table 1).

Approximately 20% of all fine-grained gold ana-

lysed exhibits a pure gold rim (i.e., N99.9% Au).

These pure gold rims occur in conjunction with more

extensive Ag-depleted rims (FHg), as well as on grains

that do not have Ag-depleted rims (FHg) (Table 1).

Unlike Ag-depleted rims, the pure Au rims are com-

monly discontinuous around a grain margin, and are ofvarying width.

5. Sulphide minerals

The sulphide suite at Belle-Brook is predominantly

marcasite (ca. 97 vol.%) that occurs both as fine-

grained (b1 mm) framboidal marcasite (Fig. 5A) or

anhedral dlumpT marcasite (Fig. 5B) that ranges from

1 mm to 25 cm. Both fine-grained framboidal marca-

site and dlumpT marcasite are ubiquitous throughout

the matrix of QPC about the Belle-Brook site. Fram-boidal pyrite (Fig. 5C) is rare in comparison to fram-

boidal marcasite and comprises ca. 1 vol.% of the

sulphide suite. In contrast to framboidal marcasite

which is widely dispersed throughout the QPC, fram-

boidal pyrite is observed rarely, and generally con-

fined to specific settings within the QPC, such as in

narrow (10 cm) horizons above the water table. In

outcrop, the occurrence of framboidal marcasite and

framboidal pyrite is almost mutually exclusive, with

only rare framboidal marcasite found in a framboidal

pyrite-bearing locality. Lump marcasite and framboidal

pyrite are mutually exclusive. Euhedral pyrite and

Table 1

Summary of compositional data for fine-grained gold from Belle-

Brook and Parker Road, Waimumu

AuAg alloy AuAgHg alloy

Fine-grained gold (n = 200) 72% 25%

Range Ag (wt.%) 0.2410.2 1.3213.85Av. Ag (wt.%) 3.48 6.55

Rim silver content cf. core

Enrichment 28% 6%

Depletion 52% 72%

Absent (i.e. gold rim) 19% 22%

Range Hg (wt.%) 0.439.06

Av. Hg (wt.%) 2.17

Rim mercury content cf. core

Enrichment 9%

Depletion 16%

Absent (i.e. gold rim) 75%

Coarse nugget gold with

intergrown quartz (n = 1)

Av. Ag (wt.%) 2.3



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arsenopyrite are common in heavy mineral concen-

trates (Fig. 5D) from the mining operation at Belle-

Brook. Rare euhedral pyrite is found intergrown withanhedral lump marcasite (Fig. 5E). Variably modified

euhedral pyrite (Fig. 5F, G) comprises ca. 1 vol.%, of

the total sulphide fraction, whilst arsenopyrite (Fig.

5H) makes up less than 1 vol.% of the sulphide

fraction. There is no evidence for Fe-oxide replace-

ment of sulphides, although minor Fe-oxide over-

growths do occur on some pyrite grains (Fig. 5G).

Lump marcasite (2 to 25 cm) was also collected from

a raised gravel bar at a second site at Hedgehope Stream.

At this locality, goethite pseudomorphing marcasite is a

notable feature. Marcasite has not been observed from

other QPC in the Waimumu district, despite other large

QPC exposures (commercial aggregate quarries) exist-

ing within 3 km of Belle-Brook at Parker Road.

5.1. Framboidal and anhedral iron sulphides:

morphology and textures

Framboidal marcasite is composed of aggregates of

framboids that commonly have irregular (Fig. 5A)

and more rarely branched forms. The mineralogy of

the framboidal masses was confirmed optically, and

that of individual framboids by XRD analyses (Gan-

dolfi camera). Individual spheroidal framboids range

from 50 to 150 Am in diameter and are composed of

interpenetrating marcasite crystals ranging in size

from 10 to 40 Am. Crystal faces are generally well

Fig. 5. Morphological features of sulphides from Belle-Brook. (A) Irregular cluster of framboidal marcasite composed of individual framboids made

up of microcrystalline marcasite. (B) Variation of anhedral lump marcasite masses (centimeter scale bar). (C) Framboidal pyrite mass, made up ofpyrite framboids. (D) Predominantly euhedral sulphide concentrate, illustrating euhedral and rounded pyrite morphologies (py). Dark sooty rounded

grains are small anhedral marcasite masses (mc). Well-preserved euhedral arsenopyrite crystals are relatively rare (aspy) (characteristic grains

notated for reference). (E) Cubic pyrite in altered marcasite groundmass on anhedral marcasite. (F) Intergrown pyrite cubes illustrating lack of

modification to some euhedral pyrite. (G) Abraded and etched pyrite cube with minor patches of Fe-oxide overgrowths. (H) Euhedral arsenopyrite

illustrating lack of physical abrasion and well-preserved cleavages. (I) Close-up of microcrystalline marcasite faces (that make up a marcasite

framboid) that is undergoing dissolution resulting in a dwool-ballT texture. (J) Close-up of I illustrating aligned octahedral microcrysts. (K) Marcasite

cemented QPC (centimeter scale bar). (L) SEM photomicrograph of polished-section surface showing recrystallised marcasite mass with

incorporated quartz grains (black) displaying marcasite veining and angular grain boundaries indicating dissolution. Quartz detritus also occurs

along former framboidal marcasite grain boundaries.



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developed and well preserved, and are usually free of

alteration products such as gypsum. Although marca-

site microcrysts are generally well preserved, they

commonly show various degrees of surface dissolu-

tion that affects crystal edges and corners. This dis-

solution of marcasite microcryst faces has theappearance of a wool-ball texture (Fig. 5I) that

reveals individual marcasite microcrysts are composed

of yet smaller octahedral microcrysts 0.3 to 1 Am

across (Fig. 5J). In polished section, individual mar-

casite framboids consist of finely divided fibrous to

radial marcasite crystals. Marcasite framboids rarely

have a discrete core, although pyrite microcrysts

making up a single pyrite framboid are found as a

core in rare cases.

Aggregates of framboidal marcasite form anhedral

masses, ranging from 0.5 mm up to 20 cm in length,in samples from Belle-Brook (Fig. 5B), and up to

25 cm in length from Hedgehope Stream. Regardless

of size, anhedral masses are generally sub-rounded to

rounded in appearance. In hand specimen, anhedral

dlumpT marcasite has a variety of surface textures

Fig. 6. Anhedral dlumpT marcasite textures as viewed under reflected light (oil immersion). (A) Coarsely bladed marcasite, outer zoning reflects

elevated Ni and As. (B) Radiating finely divided marcasite showing fan-shaped plumose texture. (C) Finely divided radial marcasite with multiple

concentric bands truncated by adjacent quartz grain(s). Note development of polygonisation of banding. (D) Cluster of recrystallising radial

marcasite spheroids that are colloform banded. Note framboidal pyrite core in more coarsely bladed marcasite spheroid. (E) Concentric,

crystallographically zoned marcasite with swallow-tail marcasite twins. (F) General texture of lump marcasite undergoing recrystallisation showingvariably deformed individual marcasite framboids. Replacement of cellular structures occurs in the centre of field of view.



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ranging from smooth to finely or coarsely crystalline,

to knobbly. Variably recrystallised marcasite over-

growths are common on quartz clasts. These marca-

site overgrowths reach 2 to 3 cm in length, and 1 to

2 mm in thickness. Rarely, some horizons in QPC

outcrops are cemented with marcasite on the cm scale(Fig. 5K). Quartz grains (up to 1 mm across) consis-

tent with those from the QPC matrix are commonly

observed occurring on anhedral lump marcasite sur-

faces. Although quartz grains on the surface are

rounded, in section quartz grains incorporated are

typically angular (Fig. 5L). Veinlets of recrystallised

framboidal marcasite (micron-to-millimeter scale) ex-

tend through adjacent quartz grains along fractures,

and some such quartz grains have become disaggre-

gated and brecciated by veinlet emplacement. These

quartz grains are commonly characterised by angularor irregular grain boundaries that feature concave

embayments (Fig. 5L).

In polished section, lump marcasite typically con-

sists of a wide range of variably recrystallised tex-

tures as the sulphide develops a foam texture and

becomes increasingly massive during recrystallisation.

Variably recrystallised lump marcasite include the

following textures: finely divided (2 to 20 Am) acic-

ular needles; medium-grained lath-like marcasite;

coarse-grained bladed marcasite (Fig. 6A); coarse-

grained prismatic marcasite; plumose fans of fine-

grained acicular marcasite (Fig. 6B); coarse-grainedpolygranular marcasite; marcasite that has replaced

cellular structures in plant material; as well as foam

textured and massive marcasite. Concentric banding

defined by differences in reflectance and incorpora-

tion of impurities also occurs (Fig. 6C, D). Concen-

tric banding may consist of either widely spaced

single bands, or, closely spaced multiple bands.

Well-defined crystallographically zoned marcasite fea-

tures prominent swallow-tail contact twins, common

on {101} faces (Fig. 6E). More rarely, clusters of

framboids that are variably modified by polygonisa-tion are preserved (Fig. 6F).

Framboidal pyrite (Fig. 5C) is composed of sub-

spherical to irregular aggregates of framboids typically

200 to 600 Am in length. Individual framboids are

compact and form well-constrained spherical to sub-

spherical masses, ranging from 10 to 40 Am in diameter.

Framboids are characteristically composed of unor-

dered uniformly sized octahedral pyrite microcrysts

up to 1 Am across. Microcrysts, framboids and fram-

boidal masses are well preserved, with little develop-

ment of alteration products, evidence of dissolution, or

overall physical degradation.

5.2. Euhedral sulphides: morphology

Euhedral pyrite cubes range in size from 0.5 to 4

mm, with an average size of ca. 2 mm (Fig. 5F, G).

Pyrite occurs predominantly as single cubes, less com-

monly as intergrown cubes, and rarely, as pyritohedraor pyritohedral clusters. Octahedral forms were not

observed. Crystal faces and edges generally show

very little, if any, physical degradation such as abrad-

ed edges or corners, and striations are well preserved.

Pyrite faces are untarnished and typically free of

visible alteration products such as sulphates or Fe-

oxides, although rare marcasite overgrowths do occur

on the faces of some crystals. Despite a lack of

significant physical modification and abrasion of crys-

tals, heavily etched and pitted crystals are common

(Fig. 5G). Euhedral pyrite associated with anhedralmarcasite is observed with relatively significant etch-

ing along corners and edges despite a physically

protected setting (Fig. 5E).

Arsenopyrite is rare in the sediments studied and

occurs as well-preserved prismatic crystals up to 4 mm

in length with minor abrasion (Fig. 5H). Striations and

cleavage surfaces are well preserved. Crystals are typ-

ically untarnished and free from visible alteration pro-

ducts. Some euhedral arsenopyrite has overgrowths of

anhedral pyrite.

5.3. Sulphide composition

All of the recrystallised anhedral lump marcasite

analysed during this study contained elevated Ni, Co,

As, Cr and, to a lesser extent, Cu, Zn, Pb and W (Table

2). Element mapping shows that NiFCo occur in

concentric bands (Fig. 7) that correspond with relatively

Table 2

Summary of selected trace element concentrations (ppm) for anhedral

lump marcasite from Belle-Brook and Hedgehope Stream

Sample Cr Ni Cu Zn As Pb Coa Wa

Belle-Brook marcasite

Minimum 34 180 12 17 219 6 195 16.4

Maximum 1449 20 157 129 260 6861 27 7010 139

Average 412 5753 70 78 1624 15 3527 60.6

Number 29 29 29 29 29 29 9 9

Hedgehope marcasite

Minimum 49 449 15 16 1494 6 295 214

Maximum 402 5195 108 40 2666 12 3360 442

Average 185 2291 26 30 1844 8 1628 312

Number 11 11 11 11 11 11 3 3

Samples analysed by XRF, with the exception of ICP-MS for W and

Cr.a ICP-MS.



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rare, but well defined, concentric bands observed under

reflected light. Simple concentric compositional bands

occur in individual framboidal marcasite grains (Fig.

7A), whereas complex truncated bands are characteris-

tic of the variably recrystallised lump marcasite (Fig.7BD). Nickel-rich bands are 1 to 3 Am wide and

contain up to 12 wt.% Ni (EPMA) as Ni-bearing mar-

casite, with subordinate Co enrichment. Bands contain-

ing both Ni and Co have an average Co:Ni ratio of 0.6

(EPMA analysis, n =80). Elevated Ni occurs rarely as a

compositionally zoned, crystallographically controlled

overgrowth within some anhedral lump marcasite. Ar-

senic distribution in both fine-grained and anhedral

lump marcasite is less clearly defined, as As is gener-

ally present at, or below, the detection limit of the

microprobe. However, compositionally zoned As-rich

marcasite is observed in rare samples.

Microprobe analysis of euhedral pyrite indicates the

occurrence of rare arsenian pyrite containing up to

1.3 wt.% As. Compositionally distinct overgrowths

were not apparent in the arsenian pyrite, but a Ni-

bearing (6 wt.% Ni) alteration rim was observed forone euhedral detrital pyrite crystal.

5.4. Sulphur isotope data

Sulphur isotope analysis was carried out on fram-

boidal marcasite (b400 Am in diameter), rounded

framboidal masses (2 to 3 mm in diameter), anhedral

lump marcasite, euhedral pyrite, and euhedral arseno-

pyrite from Belle-Brook. One sample of anhedral

lump marcasite from Hedgehope Stream was also

analysed. Analyses of sulphide minerals show a

wide range in d34S values, between 45x and

Fig. 7. Electron microprobe element maps of marcasite from Belle-Brook. (A) Nickel map illustrating elevated Ni in marcasite framboids. (B) Lump

marcasite that is variably recrystallised showing elevated Ni bands that correspond with concentric bands observed under reflected light. (C) Lump

marcasite showing large-scale depletion and enrichment of elevated nickel-rich bands. (D) Massive marcasite featuring colloform style banding of

elevated Ni-rich marcasite. All element maps are 500 Am across, darker colours indicate more elevated Ni concentration.



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+ 18x (Fig. 8, Table 3). Marcasite samples generally

have the lowest values (typically less than about

20x), pyrite samples the heaviest (N0%) and arse-

nopyrite has values in-between.

6. Discussion

6.1. Gold morphology and composition

The morphology of the fine-grained gold at Belle-Brook and Parker Road clearly shows that it is detrital

(Fig. 3A, B). The gold composition is consistent with

derivation from the Otago Schist, where low-silver Au

Ag and AuAgHg alloys occur in primary and placer

deposits (Youngson and Craw, 1993; Youngson et al.,

2002; MacKenzie and Craw, 2005). Relatively high-

silver (N10 wt.% Ag) AuAgHg alloy occurs sporad-

ically in many placers throughout the region, but the

electrum observed from Parker Road contains the most

Ag-rich electrum yet documented in the South Island.

The textures of the coarse nuggetty gold from Belle-

Brook are ambiguous. Many grains show little or no

transport-induced modification. However, there are no

known auriferous primary sources proximal to Belle-Brook.

Authigenic gold mobilisation and deposition has

been inferred for gold particles from many Southland

and Otago QPC placers (Falconer, 1987; Clough and

Craw, 1989; Craw, 1992; Craw and Youngson, 1993;

Youngson and Craw, 1993; Falconer, 2003; Falconer

and Craw, submitted for publication). Authigenic gold

in these deposits typically has a low Ag content, and is

commonly confined to irregular overgrowths on detrital

grains. In this study the authigenic addition of gold is

inferred for silver-free gold rims on the exterior ofapproximately 20% of both gold alloy types (Table

1). The porous, sheet-like gold grains from Parker

Road (Fig. 4) are inferred to be entirely authigenic

based on their extremely delicate textures which

could not survive sedimentary transport. Such discrete

grains of authigenic gold have not previously been

documented from Southland, although they have been

reported from Quaternary eluvial sediments in Central

Otago (Craw and Youngson, 1993; their Fig. 4). As

with the porous, sheet-like authigenic gold, many of the

smaller inferred authigenic gold forms of this study are

of a more spheroidal rather than crystalline nature.

Table 3

Sulphur isotope compositions (x) for detrital and diagenetic sul-

phides from Belle-Brook and Hedgehope Stream

Sample number Sulphide description d34S

Belle-Brook

BS-1 Framboidal marcasite 28.6B4-17 Anhedral lump marcasite 28.6

B4-1 Anhedral lump marcasite 21.5



BFG-1 Fine-grained (b3 mm) lump marcasite 45.4

B4-20 Recrystallised diagenetic pyrite 2.0

BP-1 Pyrite (detrital) + 13.3

BP-2 Pyrite (detrital) 0.9

BP-3 Pyrite (detrital) + 16

BP-4 Pyrite (detrital) 8.8

BA-1 Arsenopyrite (detrital) 9.9

BA-2 Arsenopyrite (detrital) 1.3

Hedgehope

H2-1 Lump marcasite 18.2

Fig. 8. Distribution of sulphur isotope values for marcasite, pyrite and arsenopyrite from Belle-Brook.



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However, they commonly have a poorly developed

polygonal appearance and subtle crystallographic con-

trol (Fig. 3E, F).

Jagged-edged particles (Fig. 3I) have been found at

only one locality. Despite their small size, they are

conspicuous in samples because of their particularlybright and shiny appearance. Rare coarse gold with

intergrown quartz and abundant framboidal pyrite (no

marcasite) are also features of this specific locality.

Although there is no surface expression, faulting is

suspected at this locality based on ground conditions

and groundwater activity (spring) in the sandygravely

sediments.

6.2. Origin of sulphide minerals

Approximately 97% of the sulphide minerals exam-ined in this study are marcasite. This marcasite occurs

in a variety of forms that are considered to be diagenetic

in origin, and formed as an authigenic phase in pore

spaces in the Tertiary non-marine sediments and their

recycled derivatives (QPC). This diagenetic interpreta-

tion is based on morphology (framboidal), together

with the lack of primary or basement source, and is

supported by sulphur isotope data. Both lump and fine-

grained marcasite from Belle-Brook and Hedgehope

that contain Ni and Co enriched bands, had an average

Co:Ni ratio of 0.6, which is consistent with the as-

sumption that Co: Ni values less than one are oftenassociated with a sedimentary (diagenetic) origin (Lof-

tus-Hills and Solomon, 1967). Use of Co:Ni ratios to

discriminate between different formation environments

is questioned by some workers (Utter, 1978; Raiswell

and Plant, 1980; Meyer et al., 1990). The broad spread

of Co:Ni ratios between 0.1 and 2.9 in this study shows

that the assertions of Loftus-Hills and Solomon (1967)

may be an oversimplification.

There are three main probable inputs of sulphur into

the sediments: (1) atmospheric deposition, (2) weather-

ing of parent material and; (3) plant residue. Atmo-spheric sulphur reaches soil via sulphate aerosols from

volcanoes and sea spray. Volcanoes are not considered

to be a likely source of sulphur in Southland. High d34S

values (+ 15x to +21x) of soluble, adsorbed sulphate

in New Zealand soils (including those in Southland) are

attributed to addition of modern sea spray or precipita-

tion high in marine sulphate (Kusakabe et al., 1976).

This appears to be the case even for sites distant from

the sea with low rainfall. Plant residues have similar

values (about 1x to 3x less) as the soils.

The most significant source of sulphur in most soils

is from minerals, usually locally derived (Krouse et al.,

1996). Some of these minerals will be detrital sulphides

from local formations or they may be authigenic sul-

phides precipitated from dissolved sulphate. Euhedral

pyrite that occurs at Belle-Brook may be from the

Otago Schist, however, Tertiary marine sediments of

Otago and Southland commonly contain pyrite as cubesand granular masses, and these may also constitute a

source of detrital sulphide in the Southland QPC. In the

Waimumu district, Chatton Marine Formation sedi-

ments are exposed along the northern extension of the

Hedgehope Fault, thus providing a local source of

euhedral diagenetic pyrite. It is also possible that dia-

genetic euhedral sulphides were derived from deltaic

and lower delta plain sediment of the Gore Lignite

Measures overlying the marine strata. Euhedral pyrite

grains from these different sources (Otago Schist, Gore

Lignite Measures and Chatton Marine Formation) areapparently indistinguishable in terms of morphology.

The wide range of d34S values (45x to +18x)

for sulphides in the Southland QPC suggests varied

sources for sulphur and fractionation effects during

sulphide mineral formation. Possible sources of detrital

sulphides (euhedral pyrite and euhedral arsenopyrite) or

sulphur from weathering of these minerals are: (1) the

Otago Schists (d34S values 5x to +1x; Ashley and

Craw, 1995; Craw et al., 1995); (2) pyrite in the Gore

Lignite Measures (d34S values about 6x; Table 3)

and; (3) the Chatton Marine Formation which is a

potential source of sulphur with values similar tothose in the adsorbed soil sulphate (+15x to +21x;

Kusakabe et al., 1976).

Inorganic and microbial sulphate reduction (MSR)

are two important controls on isotope fractionation of

sulphur during diagenesis. Both these processes yield34S-depleted sulphide. The sulphur isotope fraction-

ation between sulphate and sulphide is approximately

+ 22x during inorganic sulphate reduction (Harrison

and Thode, 1958), but laboratory studies have shown

that fractionations are variable and larger during MSR

(up to ca. 45x; Kaplan and Rittenberg, 1964). Still,larger fractionations (N+ 70x; Canfield and Teske,

1996) have been documented in nature. An enrichment

of the lighter 32S in the hydrogen sulphide as a conse-

quence of MSR during diagenesis results in a

corresponding accumulation of the heavy isotope

(34S) in the residual water. MSR can take place as

long as; organic matter is present to be metabolised

by bacteria; reactive iron is present to neutralise H2S

and; sulphate is available as a reactant.

The low d34S values of the marcasite in this study

are consistent with a diagenetic or authigenic origin.

The d34S value of dissolved sulphate from which the



17/21

marcasite would have formed is not known, but the

presence of marine formations (Chatton Marine For-

mation) and the likelihood that sea spray may have

been a source for sulphur, suggest that values would

have been relatively elevated (between +10x and

+ 20x). This indicates sulphatesulphide fractiona-tions between + 20x and +60x and that microbial

sulphate reduction was probably the dominant control.

Euhedral pyrite and arsenopyrite, which occur only in

minor quantities (ca. 2 vol.%) in the QPC, have

relatively elevated values (10x to +13x) that

may reflect a detrital or possibly diagenetic/authigenic

from either microbial or inorganic sulphate reduction

of 34S-enriched residual sulphate.

In Otago and Southland, sulphide deposition is part

of the widespread diagenetic processes that occur in

non-marine sediments (Craw, 1994; Youngson, 1995;Youngson et al., 2006-this volume). These processes

are also responsible for kaolinisation of greywacke

clasts and mobilisation of gold (Clough and Craw,

1989; Craw, 1994). It is likely that the most spectacular

marcasite from Belle-Brook formed by post-Pliocene

authigenic deposition from groundwater in the Pliocene

QPC (Clough and Craw, 1989; Falconer, 2003). A

similar occurrence of authigenic pyrite and marcasite

formation during shallow diagenesis is documented

from Pleistocene sediments within localised sulphate

reducing zones associated with lignite in the Mogothy

aquifer, Long Island, New York (Brown et al., 1999,2000). Similarly, authigenic framboidal pyrite occurs at

Elliot Lake, Canada, in 30-year old partially saturated

tailings dumps that previously did not contain framboi-

dal pyrite (Paktunc and Dave, 2002).

6.3. Gold textures resembling Witwatersrand gold

textures

Fibrous gold (Utter, 1979) and filamentous gold

(Hallbauer and van Warmelo, 1974; Hallbauer, 1975,

1981; Hallbauer and Barton, 1987) extracted from car-bonaceous matter (kerogen) is similar to the porous,

sheet-like gold from carbonaceous mudstone at Parker

Road (Fig. 4). The Witwatersrand samples were derived

from ashing carbonaceous material at 500 8C. In con-

trast, the Parker Road gold was hand-panned from

moderately lithified near-surface carbonaceous mud-

stone. However, the gold forms themselves are strik-

ingly similar. A biomineralisation origin involving

prokaryotic communities (i.e., algal and fungal mats)

has been suggested for the Witwatersrand (Grosovsky,

1983; Mossman and Dexter-Dyer, 1985; Mossman et

al., 1999). Li and Sieradzki (1992) document similar,

but not sheet-like, porous gold derived as a result of

silver dissolution from AuAg alloy.

Some authigenic gold textures described from Belle-

Brook resemble Witwatersrand secondary gold textures

ascribed to metamorphic processes. Linked crystalline

and stepped structures from the Witwatersrand (Minteret al., 1993) are similar to chain structures and crystal-

line gold forms from this study (Fig. 3E, F and 4F).

Irregular grains with jagged edges are commonly

reported from the Witwatersrand (Hallbauer and Utter,

1977; Utter, 1979; Minter et al., 1993). Although sim-

ilar grains are generally absent from the QPC in this

study, rare very small jagged-edged grains do occur at

Belle-Brook (Fig. 3I).

Compositionally, the gold of this study is similar to

that from the Witwatersrand (Feather and Koen, 1975;

Hallbauer and Utter, 1977; Utter, 1979; Von Gehlen,1983; Oberthur and Saager, 1986; Reid et al., 1988;

Frimmel et al., 1993; Frimmel and Gartz, 1997). Fine-

grained gold from this study is heterogeneous, in con-

trast to the generally homogeneous Witwatersrand gold.

Limited data from the Belle-Brook coarse-grained gold

indicate that it may be homogeneous.

6.4. Marcasite textures resembling Witwatersrand

pyrite textures

The resemblance of concretionary Witwatersrand

pyrite textures to those commonly exhibited by marca-site has been noted by a number of workers (Dimroth,

1979; Hallbauer and von Gehlen, 1983; Barton and

Hallbauer, 1996; England et al., 2002). Marcasite tex-

tures resembling oolitic-colloform pyrite documented

by England et al. (2002) are ubiquitous at Belle-

Brook (Fig. 6CF). England et al. (2002) inferred that

such grains may result from the pyritization of carbon-

ate or evaporite ooids. Similarly, England et al. (2002)

inferred chevron and swallow-tail pyrites to be pseudo-

morphs after gypsum. The occurrence of these styles of

replacement in some places is not disputed. However, atBelle-Brook, there is no evidence for pseudomorphic

replacement of carbonate, evaporite, or Fe-oxides for

these natural marcasite textures. Many of these

bpseudomorphic replacementQ forms are similar to a

variety of bladed marcasite textures that occur at a

range of scales in recrystallised lump marcasite from

Belle-Brook (Fig. 6A, E). Porous pyrite surrounded by

a pyritic cement, and concretionary pyrite with euhe-

dral-to-subhedral microcryst cores documented by Eng-

land et al. (2002) are similar to the common pyrite

marcasite association in which pyrite framboids are

surrounded by variously massive to radial marcasite.



18/21

Witwatersrand pyrite with concentric banding is com-

monly ordered into hexagonal arrays similar to those

observed in Belle-Brook framboidal and anhedral mar-

casite (Figs. 6C, D and 7A). Hence, it is suggested that

at least some of the sulphide textures seen in Witwa-

tersrand sulphides were initially the result of diageneticmarcasite formation. Although sulphur isotope signa-

tures are probably preserved, it is not known whether

marcasite textures as observed at Belle-Brook could

survive solid state transformation to pyrite at tempera-

tures N160 8C during metamorphism. However, diage-

netic pyrite textures that have survived greenschist

facies metamorphism are commonly reported, and col-

loform banding in diagenetic pyrite has reputedly even

survived granulite facies metamorphism (Park, 1994).

Belle-Brook marcasite is compositionally similar to

porous round pyrite grains from the Witwatersrand interms of its elevated Ni and Co levels and Co : Ni ratios

(Meyer et al., 1990). Oscillatory-zoned pyrite from

Witwatersrand orebodies (MacLean and Fleet, 1989)

is similar to the less-common compositional style of

As zoning at Belle-Brook.

6.5. Sulphide stability in a fluvial environment

The detrital nature of rounded Witwatersrand sul-

phides has received much attention, particularly re-

garding their stability during transportation in an

oxygen-poor atmosphere (Krupp et al., 1994; Fleet,1998; Phillips et al., 2001; England et al., 2002). The

Southland placer environment shows that both marca-

site and pyrite can form by diagenetic processes with-

in millimeter to meter of the surface under an

oxygenated atmosphere (Youngson, 1995; Craw and

Chappell, 1999; Falconer, 2003). Furthermore, these

sulphides can survive erosion and transport in surface

streams, to be reburied in younger sediments. Marca-

site can be transported for at least short distances

under oxic, but acid, conditions and remain stable

without alteration to Fe-oxide. Pyrite and arsenopyritecan be transported for tens of kilometers in fluvial

systems (Craw et al., 2003).

6.6. Sulphidation textures

The role of sulphidation remains a central issue in

the Witwatersrand debate (Phillips and Myers, 1989;

Reimer and Mossman, 1990; Myers et al., 1993; Phil-

lips and Law, 2000; Phillips et al., 2001). Magnetite,

ilmenite and hematite are generally absent from the

Witwatersrand orebodies (Feather and Koen, 1975).

These Fe and Ti oxides are also rare at Belle-Brook,

but are present elsewhere in Otago and Southland QPC.

Notably, Fe and Ti oxides are abundant in some QPC

material recycled from Gore Lignite Measures at Parker

Road, some 3 km north of Belle-Brook. There is no

textural evidence to support replacement of these miss-

ing Fe and Ti oxides by sulphides at Belle-Brook, orelsewhere in Otago and Southland placers where they

are scarce (Falconer, 2003; Youngson et al., 2006-this

volume). Consequently, it is considered unnecessary to

invoke a sulphidation process to account for their scar-

city or absence. Although the replacement of Fe and Ti

oxides by leucoxene is suggested by some in the Wit-

watersrand (Feather and Koen, 1975; Reimer and

Mossman, 1990), the lack of leucoxene precludes this

interpretation at Belle-Brook. The resemblance of

bmud-ballQ pyrite to pisolites both in terms of morphol-

ogy and trace element composition has been noted(Phillips and Myers, 1989; Phillips et al., 2001) but

as suggested in this study, such textures and composi-

tions are common in diagenetic marcasite.

Belle-Brook anhedral marcasite with truncated con-

centric bands occurs as authigenic grains adjacent to

detrital quartz grains (Fig. 6C). Thus, truncated bands

in diagenetic sulphides are not necessarily associated

with transportation, but rather, may be the result of

dissolution processes occurring amongst the precipi-

tating sulphides, detrital quartz and the pore fluid.

However, minor transportation and abrasion of con-

centrically banded sulphides at Belle-Brook wouldalso result in truncated banding similar to that docu-

mented from the Witwatersrand (MacLean and Fleet,

1989; Fleet, 1998; Phillips et al., 2001; England et al.,

2002).

7. Conclusions

The Belle-Brook and Parker Road gold placers are

characterised by detrital gold and diagenetic sulphides

exposed by small-scale alluvial gold mining in Tertiary

to recent non-marine QPC. At Waimumu the QPC areunmetamorphosed, poorly lithified, relatively unde-

formed, and are unlikely to have been buried more

than 100 m since deposition. Well-preserved, porous,

sheet-like gold and microscopic gold overgrowths at

Belle-Brook and Parker Road localities suggest authi-

genic gold mobility at the micron and individual-grain

scales. Sulphur isotope data support an authigenic or

early diagenetic origin for marcasite (45x to

20x), as it most likely formed from hydrogen sul-

phide produced by microbial sulphate reduction. Pyrite

and arsenopyrite d34S values (1x to +16x) are

consistent with a detrital origin from the Otago Schist,



19/21

the Gore Lignite Measures, or formation by inorganic

sulphate reduction from waters with elevated d34S

values. The sulphide suite at Belle-Brook is dominated

by variably rounded and transported diagenetic marca-

site that features a range of concentric banded textures

locally enriched in NiFCo. This diagenetic marcasiteis compositionally and texturally similar to the rounded,

concentrically banded pyrite commonly reported from

Witwatersrand QPC. Therefore, the QPC placer envi-

ronment at Belle-Brook, in particular, can be used to

constrain the continuum of sulphide mineral deposition

and transformations that probably also occurred during

diagenesis in ancient fluvial environments.

Acknowledgements

The findings documented are from the first authors

MSc thesis. Financial support was provided by Univer-

sity of Otago Geology Department research funds.

Expert assistance with SEM analysis was provided by

the South Campus Electron Microscopy Unit, in par-

ticular, M. Gould, S. Johnstone and L. Girvan. Acqui-

sition of microprobe data was facilitated by D. Chappell

and L. Patterson. D. Walls expertly assisted with XRF,

XRD and Gandolfi camera work. B. Pooley, M. Trinder

and S. Read provided technical assistance. We appreci-

ate the assistance of P. Warnes (GNS, Lower Hutt) who

provided analyses of sulphur isotopes. J. Smith is

thanked for access to mine sites and general assistancein the field. Useful discussions with J. Knight are

gratefully acknowledged. Prompt and constructive

reviews by J. Mauk and G. Els significantly clarified

the manuscript.

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