Download - Gold mineralogy
-
7/29/2019 Gold mineralogy
1/21
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
-
7/29/2019 Gold mineralogy
2/21
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
-
7/29/2019 Gold mineralogy
3/21
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
-
7/29/2019 Gold mineralogy
4/21
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
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545528
-
7/29/2019 Gold mineralogy
5/21
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
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 529
-
7/29/2019 Gold mineralogy
6/21
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545530
-
7/29/2019 Gold mineralogy
7/21
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).
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 531
-
7/29/2019 Gold mineralogy
8/21
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545532
-
7/29/2019 Gold mineralogy
9/21
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
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 533
-
7/29/2019 Gold mineralogy
10/21
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545534
-
7/29/2019 Gold mineralogy
11/21
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 535
-
7/29/2019 Gold mineralogy
12/21
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545536
-
7/29/2019 Gold mineralogy
13/21
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 537
-
7/29/2019 Gold mineralogy
14/21
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545538
-
7/29/2019 Gold mineralogy
15/21
+ 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
B4-4 Anhedral lump marcasite 12.1
B4-8 Anhedral lump marcasite 27.4
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 539
-
7/29/2019 Gold mineralogy
16/21
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
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545540
-
7/29/2019 Gold mineralogy
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.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 541
-
7/29/2019 Gold mineralogy
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,
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545542
-
7/29/2019 Gold mineralogy
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.
References
Ashley, P.M., Craw, D., 1995. Carrick Range Au and Sb miner-
alisation in Caples Terrane, Otago Schist, Central Otago, New
Zealand. New Zealand Journal of Geology and Geophysics 38,
137149.
Barnicoat, A.C., Henderson, I.H.C., Knipe, R.J., Yardley, B.W.D.,Napier, R.W., Fox, N.P.C., Kenyon, A.K., Muntingh, D.J., Stry-
dom, D., Winkler, K.S., Lawrence, S.R., Cornford, C., 1997.
Hydrothermal gold mineralization in the Witwatersrand basin.
Nature 386, 820 824.
Barnicoat, A.C., Phillips, G.M., Law, J.D.M., Walshe, J.L., Phillips,
G.N., Fox, N.P.C., 2001. Refuting the irrefutable: a new look at a
well-known sample of Witwatersrand gold mineralisation. In:
Williams, P.J. (Ed.), A Hydrothermal Odyssey, Extended Confer-
ence Abstracts, Contribution, vol. 59. James Cook University
Economic Geology Research Unit, Townsville, pp. 1617.
Barton, E.S., Hallbauer, D.K., 1996. Trace-element and UPb isotope
compositions of pyrite types in the Proterozoic Black Reef,
Transvaal Sequence, South Africa: implications on genesis and
age. Chemical Geology 133, 173199.
Brown, C.J., Coates, J.D., Schoonen, M.A.A., 1999. Localized sul-
fate-reducing zones in a coastal plain aquifer. Groundwater 37,
505516.
Brown, C.J., Rakovan, J., Schoonen, M.A.A., 2000. Heavy minerals
and sedimentary organic matter in Pleistocene and Cretaceous
sediments on Long Island, New York, with emphasis on pyrite
and marcasite in the Magothy aquifer. Water-Resources Investi-gations Report. United States Geological Survey, pp. 994216.
22 pp.
Canfield, D.E., Teske, A., 1996. Late Proterozoic rise in atmospheric
oxygen concentration inferred from phylogenetic and sulphur-
isotope studies. Nature 382, 127132.
Clough, D.M., Craw, D., 1989. Authigenic gold-marcasite associa-
tionevidence for nugget growth by chemical accretion in
fluvial gravels, Southland, New Zealand. Economic Geology
84, 953958.
Craw, D., 1992. Growth of alluvial gold particles by chemical accre-
tion and reprecipitation, Waimumu, New Zealand. New Zealand
Journal of Geology and Geophysics 35, 157164.
Craw, D., 1994. Contrasting alteration mineralogy at an unconformity
beneath auriferous terrestrial sediments, central Otago, New Zeal-and. Sedimentary Geology 92, 1730.
Craw, D., Chappell, D.A., 1999. Evolution and sulphide mineral
occurrences of an incipient nonmarine sedimentary basin, New
Zealand. Sedimentary Geology 129, 37 50.
Craw, D., Norris, R.J., 1991. Metamorphogenic AuW veins and
regional tectonics: mineralisation throughout the uplift history of
the Haast Schist, New Zealand. New Zealand Journal of Geology
and Geophysics 34, 373383.
Craw, D., Youngson, J.H., 1993. Eluvial gold placer formation on
actively rising mountain ranges, Central Otago, New Zealand.
Sedimentary Geology 85, 623635.
Craw, D., Hall, A.J., Fallick, A.E., Boyce, A.J., 1995. Sulphur iso-
topes in a metamorphogenic gold deposit, Macraes mine, Otago
Schist, New Zealand. New Zealand Journal of Geology and
Geophysics 38, 131136.
Craw, D., Falconer, D.M., Youngson, J.H., 2003. Environmental
arsenopyrite stability and dissolution: theory, experiment, and
field observations. Chemical Geology 199, 71 82.
Dimroth, E., 1979. Significance of diagenesis for the origin
of Witwatersrand-type uraniferous conglomerates. Philoso-
phical Transactions of the Royal Society of London 291,
227287.
England, G.L., Rasmussen, B., Krapez, B., Groves, D.I., 2002.
Palaeoenvironmental significance of rounded pyrite in siliciclastic
sequences of the Late Archaean Witwatersrand Basin: oxygen-
deficient atmosphere or hydrothermal alteration? Sedimentology
49, 11331156.Falconer, D.M., 1987. Detrital and authigenic gold in quartz gravels,
Belle-Brook, Southland. Unpublished Dip. Sci Thesis. University
of Otago. 112 pp.
Falconer, D.M., 2003. Sediment-hosted gold and sulphide mineralisa-
tion, Belle-Brook, Southland, New Zealand. Unpublished M.Sc.
Thesis. University of Otago. 373 pp.
Falconer, D.M., Craw, D., submitted for publication. Authigenic gold
and surface textures indicative of authigenic gold remobilization
on placer gold, Waimumu, New Zealand. Economic Geology.
Feather, C.E., Koen, G.M., 1975. The mineralogy of the Witwaters-
rand reefs. Minerals Science and Engineering 7, 189 224.
Fleet, M.E., 1998. Detrital pyrite in Witwatersrand gold reefs: X-ray
diffraction evidence and implications for atmospheric evolution.
Terra Nova 10, 302306.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 543
-
7/29/2019 Gold mineralogy
20/21
Fox, N., 2002. Exploration for Witwatersrand gold deposits, and
analogs. In: Cooke, D.R., Pongratz, J. (Eds.), Giant Ore Deposits:
Characteristics, genesis and exploration, CODES Special Publi-
cation vol. 4. University of Tasmania, Hobart, pp. 243 269.
Frimmel, H.E., Gartz, V.H., 1997. Witwatersrand gold particle chem-
istry matches model of metamorphosed, hydrothermally altered
placer deposits. Mineralium Deposita 32, 523 530.Frimmel, H.E., Minter, W.E.L., 2002. Recent developments
concerning the geological history and genesis of the Witwaters-
rand gold deposits, South Africa. Society of Economic Geologists
Special Publication, vol. 9, pp. 1745.
Frimmel, H.E., Le Roex, A.P., Knight, J., Minter, W.E.L., 1993. A
case study of the postdepositional alteration of the Witwatersrand
Basal Reef gold placer. Economic Geology 88, 249265.
Giusti, L., Smith, D.G.W., 1984. An electron microprobe study of
some Alberta placer gold. Tschermaks Mineralogische und Petro-
graphische Mitteilungen 33, 187 202.
Grosovsky, B.D., 1983. Microbial role in Witwatersrand gold depo-
sition. In: Westbroek, P., de Jong, E.W. (Eds.), Biomineralization
and Biological Metal Accumulation. D. Reidel Publishing, Dor-
drecht, Netherlands, pp. 495498.Hallbauer, D.K., 1975. The plant origins of the Witwatersrand
bcarbonQ. Minerals Science and Engineering 7, 111 131.
Hallbauer, D.K., 1981. Geochemistry and morphology of mineral
components from the fossil gold and uranium placers of the
Witwatersrand. United States Geological Survey Professional
Paper 1161M, 122.
Hallbauer, D.K., Barton, J.M., 1987. The fossil gold placers of the
Witwatersrand. Gold Bulletin 20 (3), 6879.
Hallbauer, D.K., von Gehlen, K., 1983. The Witwatersrand pyrites
and metamorphism. Mineralogical Magazine 47, 473479.
Hallbauer, D.K., van Warmelo, K.T., 1974. Fossilized plants in thu-
cholite from Precambrian rocks of the Witwatersrand, South
Africa. Precambrian Research 1, 199212.
Hallbauer, D.K., Utter, T., 1977. Geochemical and morphological
characteristics of gold particles from recent river deposits and
the fossil placers of the Witwatersrand. Mineralium Deposita 12,
293306.
Harrison, A.G., Thode, H.G., 1958. Mechanism of bacterial reduction
of sulphate from isotope fractionation studies. Transactions of the
Faraday Society 54, 8492.
Isaac, M.J., Lindqvist, J.K., 1990. Geology and lignite resources of
the East Southland Group, New Zealand. New Zealand Geological
Survey Bulletin 101 (202 pp.).
Kaplan, I.R., Rittenberg, S.C., 1964. Microbiological fractionation of
sulphur isotopes. Journal of Genetic Microbiology 34, 121195.
Kiba, T., Takagi, T., Yoshimura, T., Kishi, T., 1955. Tin (III)-strong
phosphoric acid. A new reagent for the determination of sulfate byreduction of hydrogen sulphide. Bulletin Chemical Society Japan
28, 641644.
Knight, J., Mortensen, J.K., Morrison, S.R., 1994. The shape and
composition of lode and placer gold from the Klondike district,
Yukon, Canada. Department of Indian and northern affairs, ex-
ploration and geological services division, Yukon region. Bulletin
3, 1141.
Krouse, H.R., Mayer, B., Schoenau, J.J., 1996. Applications of stable
isotope techniques to soil sulphur cycling. In: Boutton, T.W.,
Yamasaki, S. (Eds.), Mass Spectrometry of Soils. Marcel Dekker,
New York, pp. 247 284.
Krupp, R.E., Oberthur, T., Hirdes, W., 1994. The early Precambrian
atmosphere and hydrosphere: thermodynamic constraints from
mineral deposits. Economic Geology 89, 1581 1598.
Kusakabe, M., Rafter, T.A., Stout, J.D., Collie, T.W., 1976. Sulphur
isotopic variations in nature: 12. Isotopic ratios of sulphur
extracted from some plants, soils and related materials. New
Zealand Journal of Science 19, 433440.
Li, R., Sieradzki, K., 1992. Ductilebrittle transition in random
porous Au. Physical Review Letters 68, 1168 1171.
Loftus-Hills, G., Solomon, M., 1967. Cobalt, nickel and selenium insulphides as indicators of ore genesis. Mineralium Deposita 2,
228242.
MacKenzie, D.J., Craw, D., 2005. The mercury and silver con-
tents of gold in quartz vein deposits, Otago Schist, New Zea-
land. New Zealand Journal of Geology and Geophysics 48,
265278.
MacLean, P.J., Fleet, M.E., 1989. Detrital pyrite in the Witwatersrand
gold fields of South Africa: evidence from truncated growth
banding. Economic Geology 84, 2008 2011.
MacPherson, E.O., 1937. The geology of the Waimumu alluvial gold-
field and notes on quartz conglomerates in Southland. New Zeal-
and Journal of Science and Technology 18, 772778.
Meyer, F.M., Robb, L.J., Oberthur, T., Saager, R., Stupp, H.D.,
1990. Cobalt, nickel, and gold in pyrite from primary golddeposits and Witwatersrand reefs. South African Journal of Ge-
ology 93, 7082.
Minter, W.E.L., 1999. Irrefutable detrital origin of Witwatersrand
gold and evidence of eolian signatures. Economic Geology 94,
665670.
Minter, W.E.L., Goedhart, M., Knight, J., Frimmel, H.E., 1993.
Morphology of Witwatersrand gold grains from the Basal
Reef: evidence for their detrital origin. Economic Geology 88,
237248.
Mossman, D.J., Dexter-Dyer, B., 1985. The geochemistry of Witwa-
tersrand-type gold deposits and the possible influence of ancient
prokaryotic communities on gold dissolution and precipitation.
Precambrian Research 30, 303319.
Mossman, D.J., Reimer, T., Durstling, H., 1999. Microbial processes
in gold migration and deposition: modern analogues to ancient
deposits. Geoscience Canada 26, 131140.
Myers, R.E., Zhou, T., Phillips, G.N., 1993. Sulphidation in the
Witwatersrand Goldfields: evidence from the Middelvlei Reef.
Mineralogical Magazine 57, 395405.
Oberthur, T., Saager, R., 1986. Silver and mercury in gold particles
from the Proterozoic Witwatersrand placer deposits of South
Africa: metallogenic and geochemical implications. Economic
Geology 81, 20 31.
Paktunc, A.D., Dave, N.K., 2002. Formation of secondary pyrite
and carbonate minerals in the Lower Williams Lake tailings
basin, Elliot Lake, Ontario, Canada. American Mineralogist 87,
593602.Park, J., 1994. The preservation of pre-metamorphic colloform
banding in pyrite from the Broken Hill-type Pinnacles deposit,
New South Wales, Australia. Mineralogical Magazine 58,
461471.
Phillips, G.N., Law, J.D.M., 1997. Hydrothermal origin for Wit-
watersrand gold. Society of Economic Geologists Newsletter,
vol. 31, pp. 2632.
Phillips, G.N., Law, J.D.M., 2000. Witwatersrand gold fields: geolo-
gy, genesis and exploration. Society of Economic Geologists
Reviews, vol. 13, pp. 439500.
Phillips, G.N., Myers, R.E., 1989. The Witwatersrand Goldfields:
II. An origin for Witwatersrand gold during metamorphism
and associated alteration. Economic Geology Monographs 6,
598608.
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545544
-
7/29/2019 Gold mineralogy
21/21
Phillips, G.N., Law, J.D.M., Myers, R.E., 2001. Is the redox state of
the Archean atmosphere constrained? Society of Economic Geol-
ogists Newsletter, vol. 47, pp. 118.
Raiswell, R., Plant, J., 1980. The incorporation of trace elements into
pyrite during diagenesis of black shales, Yorkshire, England.
Economic Geology 75, 684699.
Reid, A.M., le Roex, A.P., Minter, W.E.L., 1988. Composition of goldgrains in the Vaal Placer, Klerksdorp, South Africa. Mineralium
Deposita 23, 211217.
Reimer, T.O., Mossman, D.J., 1990. Sulfidization of Witwatersrand
black sands: from enigma to myth. Geology 18, 426 429.
Robb, L.J., Meyer, F.M., 1990. The nature of the Witwatersrand
hinterland: conjectures on the source area problem. Economic
Geology 85, 511536.
Robinson, B.W., Kusakabe, M., 1975. Quantitative preparation of
sulfur dioxide, for 34S/32S analyses, from sulfides by combustion
with cuprous oxide. Analytical Chemistry 47, 11791181.
Turnbull, I.M., Allibone, A.H., 2003. Geology of the Murihiku Area.
Institute of Geological and Nuclear Sciences. 1:250 000 Geolog-
ical Map 20 (97 pp.).
Utter, T., 1978. Morphology and geochemistry of different pyritetypes from the Upper Witwatersrand System of the Klerksdorp
Goldfield, South Africa. Geologische Rundschau 67, 774804.
Utter, T., 1979. The morphology and silver content of gold from the
Upper Witwatersrand and Ventersdorp systems of the Klerksdorp
gold field, South Africa. Economic Geology 74, 2744.
von Gehlen, K., 1983. Silver and mercury in single gold grains from
the Witwatersrand and Barberton, South Africa. Mineralium
Deposita 18, 529534.
Wood, B.L., 1956. The geology of the gore subdivision. New Zealand
Geological Survey Bulletin 53 (128 pp.).
Youngson, J.H., 1995. Sulphur mobility and sulphur-mineral precip-
itation during early MioceneRecent uplift and sedimentation inCentral Otago, New Zealand. New Zealand Journal of Geology
and Geophysics 38, 407417.
Youngson, J.H., submitted for publication. Occurrence, Composition
and Morphology of Hydrothermal, Secondary and Artifact Gold
SilverMercury Alloys. Economic Geology.
Youngson, J.H., Craw, D., 1993. Gold nugget growth during tecton-
ically induced sedimentary recycling, Otago, New Zealand. Sed-
imentary Geology 84, 7188.
Youngson, J.H., Wopereis, P., Kerr, L.C., Craw, D., 2002. AuAgHg
and AuAg alloys in Nokomai and Nevis valley placers, northern
Southland and Central Otago, New Zealand, and their implica-
tions for placersource relationships. New Zealand Journal of
Geology and Geophysics 45, 5369.
Youngson, J.H., Craw, D., Falconer, D.M., 2006. Evolution of Cre-taceousCenozoic quartz-pebble conglomerate gold placers dur-
ing basin formation and inversion, southern New Zealand. Ore
Geology Reviews. 28, 451-474. doi:10.1016/j.oregeorev.
2005.02.004 (this volume).
D.M. Falconer et al. / Ore Geology Reviews 28 (2006) 525545 545
http://dx.doi.org/10.1016/j.oregeorev.2005.02.004http://dx.doi.org/10.1016/j.oregeorev.2005.02.004http://dx.doi.org/10.1016/j.oregeorev.2005.02.004