Evolution of Cretaceous–Cenozoic quartz pebble conglomerate gold placers during basin formation and inversion, southern New Zealand

Abstract

Numerous auriferous fluvial quartz pebble conglomerates (QPCs) are present within the Late Cretaceous–Recent sedimentary sequence in southern New Zealand. The QPCs formed in low-relief settings before, during, and after regional marine transgression, in alluvial fan and a variety of fluvial and near-shore depositional settings: In particular, during slow thermal subsidence associated with Late Cretaceous–early Cenozoic rifting, and during the early stages of orogenic uplift following mid Cenozoic marine regression. QPC maturity characteristics are complex and vary with sediment transport and recycling history, stratigraphic proximity to the transgressive Waipounamu Erosion Surface, and the amount of first-cycle detritus incorporated during recycling. For pre-marine QPCs, the amount of first cycle detritus varies with tectonic intensity and proximity of the depositional setting to remnant Cretaceous topography. For post-marine QPCs, it varies with tectonic intensity and proximity to Late Cenozoic uplift of basement ranges.QPCs do not form during a single bedrock erosion–sediment deposition cycle: Non-oxidised and/or oxidized groundwater alteration (kaolinisation) of labile minerals in immature sediment and the upper part of underlying basement, and repeated sedimentary recycling, are fundamental processes of QPC formation regardless of the tectonic or sedimentary settings. Altered immature rock disaggregates easily upon erosion, and alteration clays are winnowed to leave quartz-rich residues containing resistant heavy minerals such as zircon and gold. Detrital sulfide survives recycling if deposition and burial in saturated sediments are rapid. QPCs result only if sediment recycling is not accompanied by excessive erosion of fresh basement rock. Uplift of many parts of the Otago Schist belt since late Miocene has raised rocks above the water table, increased erosion rates, and inhibited groundwater alteration and QPC formation. QPC formation is still occurring in Southland, where the water table is high, sediments are saturated and undergoing alteration, and uplift and erosion rates, topography, and fluvial gradients are all low. The QPCs accumulate as residual gravel on the valley floors of low-competence streams that are slowly incising pervasively altered dominantly late Miocene–Pliocene immature conglomerates.QPCs formation essentially represents physical and chemical lagging of precursor strata. Accumulation of detrital gold and other heavy minerals is an inevitable consequence, and most QPCs contain some gold. Three types of significant gold placer have developed in the QPCs. Type 1 placers are essentially eluvial and/or colluvial in origin and form without significant fluvial transport, by residual accumulation in low-competence valleys during low-rate uplift, fluvial incision and QPC formation. Type 2 placers have formed during significant fluvial transport and subsequent fluvial incision, mainly in higher energy proximal and medial reaches of larger pre-marine (Eocene) and post-marine fluvial systems. Type 3 placers formed by wave-base and marine current winnowing in the shallow shelf setting during low-rate regional marine transgression, especially in the Eocene.

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Evolution of Cretaceous–Cenozoic quartz pebble conglomerate gold
placers during basin formation and inversion, southern New Zealand
J.H. Youngson*, D. Craw, D.M. Falconer
Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand
Received 15 December 2003; received in revised form 5 July 2004; accepted 26 February 2005
Available online 23 January 2006
Abstract
Numerous auriferous fluvial quartz pebble conglomerates (QPCs) are present within the Late Cretaceous–Recent sedimentary
sequenceinsouthernNewZealand.TheQPCsformedinlow-reliefsettingsbefore,during,andafterregionalmarinetransgression,in
alluvial fan and a variety offluvial and near-shoredepositional settings: In particular,during slow thermal subsidence associated with
Late Cretaceous–early Cenozoic rifting, and during the early stages of orogenic uplift following mid Cenozoic marine regression.
QPC maturity characteristics are complex and vary with sediment transport and recycling history, stratigraphic proximity to the
transgressive Waipounamu Erosion Surface, and the amount of first-cycle detritus incorporated during recycling. For pre-marine
QPCs, the amount offirst cycle detritus varies with tectonic intensity and proximity of the depositional setting to remnant Cretaceous
topography. For post-marine QPCs, it varies with tectonic intensity and proximity to Late Cenozoic uplift of basement ranges.
QPCsdonotform during a single bedrock erosion–sediment deposition cycle: Non-oxidised and/or oxidized groundwater alteration
(kaolinisation) of labile minerals in immature sediment and the upper part of underlying basement, and repeated sedimentary recycling,
are fundamental processes of QPC formation regardless of the tectonic or sedimentary settings. Altered immature rock disaggregates
easily upon erosion, and alteration clays are winnowed to leave quartz-rich residues containing resistant heavy minerals such as zircon
and gold. Detrital sulfide survives recycling if deposition and burial in saturated sediments are rapid. QPCs result only if sediment
recycling is not accompanied by excessive erosion of fresh basement rock. Uplift of many parts of the Otago Schist belt since late
Miocene has raised rocks above the water table, increased erosion rates, and inhibited groundwater alteration and QPC formation. QPC
formation isstill occurring inSouthland, where the water tableishigh, sediments are saturated and undergoing alteration, and upliftand
erosion rates, topography, and fluvial gradients are all low. The QPCs accumulate as residual gravel on the valley floors of low-
competence streams that are slowly incising pervasively altered dominantly late Miocene–Pliocene immature conglomerates.
QPCs formation essentially represents physical and chemical lagging of precursor strata. Accumulation of detrital gold and
other heavy minerals is an inevitable consequence, and most QPCs contain some gold. Three types of significant gold placer have
developed in the QPCs. Type 1 placers are essentially eluvial and/or colluvial in origin and form without significant fluvial
transport, by residual accumulation in low-competence valleys during low-rate uplift, fluvial incision and QPC formation. Type 2
placers have formed during significant fluvial transport and subsequent fluvial incision, mainly in higher energy proximal and
medial reaches of larger pre-marine (Eocene) and post-marine fluvial systems. Type 3 placers formed by wave-base and marine
current winnowing in the shallow shelf setting during low-rate regional marine transgression, especially in the Eocene.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Quartz pebble conglomerate; alluvial gold placer; New Zealand; Formation mechanisms; Evolution; Cretaceous; Cenozoic
0169-1368/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.oregeorev.2005.02.004
* Corresponding author.
E-mail address: placer@xtra.co.nz (J.H. Youngson).
Ore Geology Reviews 28 (2006) 451–474
www.elsevier.com/locate/oregeorev

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1. Introduction
Quartz pebble conglomerates (QPCs) are an impor-
tant source of alluvial gold worldwide. The most impor-
tant deposits are those of the Archaean Witwatersrand
sediments of South Africa, which have yielded more
than 43,000 tonnes of gold between 1887 and 1994
(Robb and Robb, 1998). Despite their economic impor-
tance, the tectonic setting, compositional and textural
evolution, and gold concentration mechanisms for Wit-
watersrand and other Archaean QPCs are subjects of
ongoing debate. The Witwatersrand deposits are partic-
ularly controversial, because post-depositional over-
printing partly obscures primary sedimentary features
and imposes metamorphic and/or hydrothermal gold
and silicate mineral textures (Phillips et al., 1987; Phil-
lips and Myers, 1989; Robb and Meyer, 1991; Minter et
al., 1993; Barnicoat et al., 1997, 2001; Minter, 1999;
Kirk et al., 2001, Frimmel and Minter, 2002).
Features that make Archaean auriferous QPCs dis-
tinctive are textural and compositional maturity; con-
centration of gold at unconformities within a subsiding
sedimentary basin, complex inter-relationships between
marine and non-marine sedimentation and gold concen-
tration; and abundant detrital sulfide minerals, particu-
larly pyrite (some with Ni) and arsenopyrite (Minter,
1978; Pretorius, 1981; Saager, 1981; Els and Mayer,
1992; Els, 1998). Many of the sulfide minerals accu-
mulated in chemically mature sediments and have been
cited as evidence for a low-oxygen atmosphere in the
Archaean (e.g., Minter, 1978; Krupp et al., 1994; Eng-
land et al., 2002).
Fig. 1. Map of southern New Zealand showing basement terranes relevant to this study, the northwest-trending Otago Schist belt, and the main
onshore basins containing quartz pebble conglomerates of various ages. The boundary between the Torlesse and Caples terranes has been
metamorphosed, but lies north of the Tuapeka Fault Zone and trends generally northwest, sub-parallel to the axis of the Otago Schist belt.
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Auriferous QPCs are not confined to the Archaean;
Late Phanerozoic examples abound and include the
Eocene Ione Formation of California (Lindgren, 1911;
Peterson et al., 1968; Yeend, 1974), late Miocene–
Pliocene White Channel Gravels of the Yukon, Canada
(Lowey, 2001), and numerous Late Cretaceous and
Tertiary formations in southern New Zealand (McKay,
1894; Youngson and Craw, 1995, 1996, 2002; Young-
son et al., 1998). These younger deposits have not been
as economically productive as the Archaean QPCs, but
have nevertheless produced significant quantities of
gold over the past 150 years. Their importance lies
not in their gold content, but in their potential as
analogues for ancient auriferous QPCs such as those
in the Witwatersrand Basin. From examination of these
better exposed and better understood young deposits, it
is possible to gain insight into the processes of QPC
formation and associated gold concentration mechan-
isms. These insights have potential for unraveling the
geological evolution of the more economically impor-
tant and less-well understood Archaean deposits.
This study focuses on the tectonic and strati-
graphic evolution of QPCs in southern New Zealand
(Fig. 1), and examines these deposits in the context
of basin formation, QPC development, gold and
sulfidemineralconcentration,
QPC development by basin inversion. We document
the formation of auriferous QPCs from immature
sediments, progressive reworking of the QPCs and
the end of QPC development as excessive erosion of
immature detritus from rising mountains occurs. We
use the term QPC as a general descriptor for units
that are commonly dominated by quartz pebble con-
glomerate, but typically include quartz sandstone and
minor siltstone and mudstone (Flignite) beds. Sim-
ilarities in the appearance of QPCs in different
locations led to much stratigraphic confusion, and
use of local unit names by early workers (see
summary in Youngson et al., 1998) (Fig. 2). Con-
sequently, we avoid unit names in this paper and
relate the QPCs to tectonic and sedimentary settings
instead.
and disruptionof
Fig. 2. Occurrence of gold placers throughout the simplified Late Cretaceous and Cenozoic sedimentary sequence in southern New Zealand. Dotted
horizontal lines represent significant unconformities, and the widths of the columns of gold placer bars (left=Otago, right=Southland) are
approximately proportional to placer importance. Only selected stratigraphic names are shown. Legend: as for Fig. 3.
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2. Basin evolution
Basin inversion is a common component of the tec-
tonic evolution of many sedimentary basins of various
types around the world. Basin initiation is commonly
caused by crustal extension over a relatively short time
scale, involving progressive development of grabens or
half-grabens adjacent to hinterland blocks uplifted along
normal faults at the basin margins (e.g., McKenzie,
1978; Wernicke and Burchfiel, 1982). Large volumes
of syntectonic detritus derived from the rising hinterland
accumulate in the basins, typically as thick immature
sediments on the downthrown side of faults and thinner
coeval sediments on the upthrown side, yielding bsteer’s
headQ geometry in cross-section (White and McKenzie,
1988).Riftingissucceededbythermallyinducedpassive
subsidence over a relatively long time scale, and rifting-
related sediments are succeeded, commonly unconform-
ably, by near-shore and transgressive marine sediments
(McKenzie, 1978; Gawthorpe and Leeder, 2000). Fol-
lowing passive subsidence, a change to tectonic com-
pression initiates basin inversion and crustal shortening
by new folding and faulting, and reversal of movement
on some of the pre-existing normal faults (Hayward and
Graham, 1989). Differential uplift and folding of basin
sediments causes progressive exhumation, erosion, and
recycling of basin sediments, thus completing the basin
evolution cycle.
Southern New Zealand has undergone a cycle of
basin formation and subsequent inversion since the
mid-Cretaceous, with the inversion phase still active.
Rifting began as the SW Pacific portion of Gondwana
began to break up in the Cretaceous. This yielded
several sedimentary basins around New Zealand, the
largest of which are now offshore (Laird, 1993). Rifting
was followed by gradual thermal subsidence and ma-
rine transgression, which peaked in the Oligocene (Wil-
son, 1956). Reconfiguration of the tectonic plate
structure of the SW Pacific in the middle Tertiary
initiated compression and uplift (Walcott, 1978; Suther-
land, 1995). Currently, the Pacific–Australian plate
boundary passes through New Zealand, with opposite-
dipping subduction zones to the north and south linked
by the dextral, obliquely convergent, Alpine Fault
through South Island (Fig. 1). Mid-Tertiary–Recent
transpression has caused uplift of parts of the Creta-
ceous–early Tertiary rift basins along the 150–250 km
wide emergent New Zealand landmass straddling the
plate boundary. Uplift has occurred by reversal of many
pre-existing normal faults and development of new
reverse faults. Moderate compression and regional up-
lift during basin inversion have been sufficient to ex-
pose many of the geological features of the basin
formation–basin inversion cycle.
3. Geology of southern South Island, New Zealand
Southern South Island of New Zealand is underlain
by Paleozoic–Mesozoic greywacke and argillite, and
schist metamorphosed from those rocks. The greywacke
and argillite consists of two main suites: the volcano-
genic Caples Terrane and quartzofeldspathic Torlesse
Terrane. The boundary between these terranes generally
follows the northwest trending axis of the Otago Schist
belt (Mortimer, 1993) (Fig. 1). Metamorphic grade
ranges from zeolite facies and prehnite-pumpellyite fa-
cies (both unfoliated) to pumpellyite-actinolite and
greenschist facies (both variably foliated). The foliated
zones (Otago Schist, Fig. 1) are cut by metamorphic
quartzveins withsome albite, whichincrease infrequen-
cy and approach parallelism with the foliation in the
higher-rank schist. Veins can range from 1 mm to 20
cm thick in a single outcrop. Rocks in the central part of
the schist belt consist of up to 50% vein material, of
which15–30%isquartz.Metacherthorizons,commonly
hematitic, are rare but widely distributed throughout the
Otago Schist and its lower grade margins.
The Otago Schist is also cut by numerous fault-
hosted, gold-bearing, mesothermal vein systems includ-
ingtheMacraesdeposit(ca. 5MOz;Angus,1993)(Figs.
1 and 3). Historically, ca. 8 MOz of gold were produced
fromOtagoover140years,ataplacer/primarygoldratio
of ca. 27:1 (Williams, 1974; Henley and Adams, 1979).
The placer gold was recovered from alluvial deposits
formed during basin formation and inversion. The pla-
cers dominantly occur in Late Cretaceous–Recent QPCs
and immature sediments of the present streams (Young-
son and Craw, 1995, 1996). The QPCs are preserved
locally as remnants in sedimentary basins and crop out
sporadically on the flanks of rising basement blocks
(Fig. 1) (Youngson, 2005). QPCs are buried by younger
cover in most of the low-topography areas of east and
central Otago and Southland (Figs. 1 and 3).
Prominent NW trending ranges on the north and
south margins of the Otago Schist (Figs. 1 and 3)
were initiated during late middle-Miocene to Pliocene
compression (Youngson et al., 1998). These NW
trending ranges, and a system of NE trending ridges
in east Otago are being uplifted along faults (Tua-
peka, Waihemo, Titri) that originated as normal faults
in the Cretaceous (Mutch and Wilson, 1952; Bishop
and Laird, 1976; Els et al., 2003; Figs. 1 and 3). The
NE trending basin and range topography in central
Otago and northern Southland (Fig. 3) was initiated
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in the late Pliocene and Pleistocene and is still ac-
tively rising at rates of ca. 0.1–1 mm year?1(Jack-
son et al., 2002).
Southland and east Otago have a moist cool-temper-
ate climate (rainfall ca. 1200 mm year?1) and most hill
slopes and valleys are saturated with groundwater to
Fig. 3. Relief map of the area shown in Fig. 1 and representative vertical profiles of QPC-bearing sequences in Otago and Southland. Some of the
ranges have formed adjacent to reactivated Cretaceous normal faults since the late Miocene, in particular the Waihemo Fault system (WFS), Titri
Fault (TF), and Tuapeka Fault system (TFS). All other northeast-trending ranges in Otago and northern Southland have formed adjacent to
Quaternary faults and folds. CR=Crown Range, PR=Pisa Range, DR=Dunstan Range, RaR=Raggedy Range, RR=Rough Ridge, RPR=Rock
and Pillar Range, BM=Blue Mountains, UR=Umbrella Range. Low-relief areas between and beyond the ranges are dominantly Cenozoic
sedimentary basins containing QPCs, not all of which are labeled (cb=upper Clutha basin, mb=Manuherikia basin, ib=Ida Basin, mtb=Maniototo
Basin, sb=Strath Taieri Basin, tb=Taieri Basin). Sections A and B are shown in Fig. 4.
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