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Eluvial gold placer formation on actively rising mountain ranges, Central Otago, New Zealand
Sedimentao' Geology, 85 (1993) 623-635
623
Elsevier Science Publishers B.V., Amsterdam
Eluvial gold placer formation on actively rising mountain ranges, Central Otago, New Zealand D. Craw and J.H. Youngson Geology Department, UniL'ersity of Otago, P.O. Box 56, Dunedin, New Zealand Received June 12, 1992; revised version accepted November 17, 1992
ABSTRACT Craw, D. and Youngson, J.H., 1993. Eluvial gold placer formation on actively rising mountain ranges, Central Otago, New Zealand. In: C.R. Fielding (Editor), Current Research in Fluvial Sedimentology. Sediment. Geol., 85: 623-635. Eluvial gold deposits in Central Otago, New Zealand, have formed and are still forming on the flanks of actively rising antiformal mountain ranges. These gold deposits are derived mainly by erosion and concentration of fine-grained ( < 500 /xm) gold in mature Miocene fluvial quartz gravels. Chemical processes during Pleistocene-Recent uplift and eluvial sedimentation have resulted in crystalline and amorphous authigenic gold precipitation and up to 2 orders of magnitude gold grain-size increase. The eluvial gold deposits are hosted by thin lithic soil and sequences (up to 60 m thick) of poorly sorted immature schist gravels. The gravel sequences consist mainly of matrix-supported mass flow deposits and channellised proximal fan deposits, intercalated on a 1-10 m scale. Gold is concentrated in coarse lag gravels (up to 40 cm clasts) at channel bases. Topographic slopes on the rising ranges show an evolutionary trend in space and time, from gentle weakly dissected surfaces, through slightly degraded but convex slopes, to deeply incised convex streams. Eluvial gold occurs sporadically on the gentle slopes, but the most efficient concentration processes occurred where steeper convex slopes yielded an apron of fan sediments. Gold concentration at these sites resulted from selective and localized removal ("winnowing") of most schist debris, leaving coarse lag gravels and gold. The combination of authigenic grain size increase and residual concentration ensures that the eluvial deposits retain coarse-grained gold, and that only fine-grained gold is released to the alluvial systems downstream.
Introduction Detrital gold deposits, or placers, have always been important sources of gold. So-called "giant" placer gold deposits are found in numerous geological terranes around the world, and these have formed by repeated recycling of gold-bearing sediments (Henley and Adams, 1979). Economic placer deposits are generally presumed to form by these recycling processes during rejuvenation of topography in areas where low-grade gold deposits already occur (Adams et al., 1978; Boyle, 1979). The necessary topographic rejuvenation results from changes in the longitudinal profiles
Correspondence to: D. Craw, Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand.
of rivers due to, for example, uplift in the headwater regions. Alluvial placers form in the rivers as they equilibrate to the new base levels (Adams et al., 1978). Placer deposits fall into two main types: those with gold distributed through large volumes of fluvial sediments, and those which accumulate more irregularly near to a gold source (Boyle, 1979). The former type of placer generally contains fine-grained gold, and these typically occur in braided river deposits and distal alluvial fans (Smith and Minter, 1980; Day and Fletcher, 1991). The latter type of placer generally contains a wide range of gold grain sizes including coarse irregular nuggety grains and these occur in proximal tectonic sediments and glacial deposits which are commonly associated with the tectonic sediments (Henley and Adams, 1979: Eyles and Koc-
0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
624
sis, 1989). Studies of alluvial placer formation have mainly concentrated on the first of these types, and studies of both placer types have examined fluvial processes in well-defined drainage systems (Smith and Beukes, 1983; Nami and James, 1987: Eyles and Kocsis, 1989; Day and Fletcher, 1991). The processes which occur in the headwaters of alluvial systems for both types of placers, the "eluvial" processes (Boyle, 1979), have been largely ignored. This is partly because the deposits are generally small and partly because eluvial deposits are notoriously irregular and unpredictable, as slope processes which are responsible for the deposits are poorly understood. Nevertheless, eluvial deposits are an important link in the chain of processes which result in alluvial gold concentrations, particularly when topographic rejuvenation is due to hinterland uplift.
D. CRAW AND J.}l ~ (.~UNGSON
Eluvial processes in rising mountains are spatially transient, as further uplift causes erosion of early-formed eluvial deposits, while new eluvial deposits form nearby on the same mountain range. Ultimately, all record of the eluvial environment is removed if erosion rates excced ratc~ of eluvial processes. All alluvial gold deposits have passed through an eluvial environment at some stage, but very little cvidence of this is preserved in the geological record. It is only by studying currently active, or very you,,g, eluvial deposits that some insight can be gaincd into thc processes which govern the nature of gold which enters the more distal alluvial systems. This paper is a preliminary account t)f thc development of small but locally rich eluvial deposits in Central Otago, New Zealand, one of the "giant" placer regions of Henley and Adams (t979). The processes which give rise to these
Fig. 1. Location and s u m m a r y geological m a p of Central Otago in the South Island, New Zealand, showing the prommen! northeast-trending Otago Schist ranges, and basins filled with cover sediments. Major gold-bearing quartz veins (squares) and placer deposits (triangles) are indicated (after Williams, 1974).
E L U V I A L G O L D PLACER F O R M A T I O N ON A C T I V E L Y RISING M O U N T A I N R A N G E S
crease in strain and metamorphic grade to the north and south and grade into Palaeozoic/Mesozoic greywackes. The central greenschist facies portion of the Otago Schist has a well-defined flat-lying schistosity. Uplift and erosion of the schist belt from late Mesozoic to middle Cenozoic resulted in a regional low-relief surface which lies sub-parallel to the schistosity (Stirling, 1990)• This low-relief surface was unconformably overlain by Miocene fluvio-lacustrine sediments of the Manuherikia G r o u p (Douglas, 1986). Coarser fluvial sediments of the Manuherikia G r o u p consist largely of mature quartz gravels which are auriferous, particularly near the base of the section on the unconformity (Williams, 1974). Extensive kaolinitisation of schist has occurred up to 10 m below the unconformity beneath Miocene sediments. This kaolinitised zone, and the overlying unconformity, is traceable throughout Central Otago (Benson, 1935; Stirling, 1990).
eluvial placers are of some significance for the economics of the immediate deposits, but are also important for alluvial deposits which form farther downstream. The Central Otago region is tectonically active, and the eluvial placers are very young or actively forming; hence, some of the parameters involved in placer formation can be constrained. This p a p e r examines development of eluvial placers in space and time in the regional tectonic context. We also attempt to identify processes and mechanisms which are responsible for maximising gold concentrations in the tectonic sediments, and show that the differences in grain size between the two main types of placer deposits (above) are due to eluvial processes.
Regional geology The Otago Schist which underlies Central Otago (Fig. 1) is a belt of highly deformed predominantly greenschist facies schists which de-
sediment •" ~ recent alluvium " " • ~fan
gravels
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r
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MaoriBotto'~ Formation H Manuherikia Group
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degraded i unconformity
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N
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/ :
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Fig. 2. Geological map of the middle portion (see Fig. 1) of Central Otago (modified after Mutch, 1962, and Wood, 1962), showing topographic subdivisions of the antiformal ranges, and generalised stratigraphy of basin sediments. Eluvial gold occurs on the preserved Miocene unconformity and in proximal portions of fans (close stipple) derived from degraded unconformity during uplift of the ranges. The gold was derived primarily from erosion of the auriferous Manuherikia Group (preserved outcrops in black).
626
D, ( ' R A W A N D .I.tL "t ( )t INGS( )N
Southern New Zealand now lies astride the Australian/Pacific plate boundary, the Alpine Fault (Fig. 1; e.g. Norris et al., 1990). The Alpine Fault is currently an oblique-slip fault with approximately 10 m m / y r vertical (east side up) and dextral horizontal movement, Near the southern end of the Alpine Fault, the Otago Schist structural block abuts the fault and is being deformed by plate boundary processes up to 200 km east of the Alpine Fault (Norris, 1979). Late Cenozoic strain in the Otago Schist block has resulted in a set of northeast and northwest trending folds and reverse faults (Norris, 1979). Regional scale folding has resulted in asymmetrical warps of the flat-lying schistosity to form a regular pattern of mountain ranges (antiforms) and intervening basins (synforms) with a 20 km wavelength and an amplitude of approximately 2 km (Figs. 1, 2). Antiformal ranges have gently dipping northwestern limbs and broad crests (Fig. 3). Southeastern limbs are typically steeper dipping (Fig. 3) and are locally overturned, with minor reverse faulting along the range margin or in the immediately adjacent basin (Beanland et al., 1986). The Miocene unconformity is partially preserved on the gently dipping parts of the ranges and fully preserved in the basins. Uplift in the region began at least 2 million years ago, initially in the greywacke zone to the north of the schist belt. Sediment shed from the
rising mountains spread over many parts of the schist belt as the widespread greywacke conglomerate, the Pliocene Maori Bottom Formation (Bishop, 1974; Fig. 2). These ranges are still actively rising along a prominent northwest striking fault zone, and rivers from these ranges flow as axial streams through the synformal Central Otago basins, with a greywacke bedload. Northeast trending schist ranges began to rise later than the greywacke ranges, and the Maori Bottom Formation has been locally folded and faulted during schist range uplift. Most schist range uplift has occurred within the past million years, and a considerable amount of uplift has occurred within the last 400,000 years (McSaveney et al., 1992). Alluvial fans are poorly dated, but some fans can be traced to merge with better-dated glacial outwash terraces, some as young as 65,000 years (Officers, 1983). The Central Otago region has been extensively mined for gold (Fig. 1), with over 240 tonnes extracted, more than 90% of which was from placer deposits. The following paper concentrates on the middle portion of the region (Figs. l, 2), to avoid the complications of glacial deposits to the west and more complex pre-Miocene geology to the east. In this middle portion, eluvial deposits are very common on margins of mountain ranges, particularly in the proximal portions of associated fans (Fig. 2).
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Fig. 3. Topographic profiles (no vertical exaggeration) of the Dunstan Range and the Raggedy Range (section lines in Fig. 2L showing the scale and overall concave shapes of the ranges. Dunstan Range has been uplifted more than Raggedy Range, and consequently the Dunstan Range unconformity is more degraded, and eluvial gold is concentrated in proximal fan deposits. Miocene sediments are stippled,
ELUVIAL (IOLD PLACER FORMATION ON ACTIVELY RISING MOUNTAIN RANGES
Authigenic gold There are two main sources of gold for the eluvial placers described below. Rare primary gold-bearing veins crop out in the schist basement as steeply dipping quartz-filled fault zones traceable for up to 1 km along strike (Craw and Norris, 1991). Fine-grained gold (1-100 /~m) occurs associated with sulphides in these veins. In the near-surface region ( < 20 m), supergene processes have caused gold mobility in solution followed by reprecipitation to increase grain size of some particles (Craw and MacKenzie, 1992). Resultant gold grains are highly irregular in shape, and up to 2 cm across (Fig. 4). Once released into the soil there is further chemical mobility of gold, combined with physical modification during down-slope creep, which results in formation of irregular nuggets with rounded extremities.
627
The other source of gold is detrital gold in the Manuherikia Group fluvial sediments which overlie the basement. This gold is generally very fine grained (100-500 /xm; Fig. 5), and concentrated principally in coarse gravel facies in channels near the base of the sequence. Chemical gold mobility has occurred within the Manuherikia sediments during uplift (Craw, 1992), and in tectonic sediments after uplift and recycling, resulting in grain size increase (Clough and Craw. 1989; Youngson and Craw, 1993; Figs. 5, 6). Authigenic gold grains are generally >500 /xm across, and numerous grains > 1 mm occur. Nuggets up to 1 cm across and larger are rare but widespread. Evidence for authigenic gold precipitation can be seen in the morphology of these coarse gold grains. Many of the grains are highly irregular in shape, with numerous delicate protrusions which could not have survived fluvial
Fig. 4. Photograph of eluvial gold nugget from Central Otago; complex aggregate of wire gold which has partially or wholly overgrown lithic soil material (mainly angular schist fragments, e.g. top and bottom left, and centre), central Raggedy Range. Specimen is 2 cm across.
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P. ( ' R A W AND ,LH. YOUNt},'-,ON
Fig. 5. Photograph showing relative size and morphology of detrital gold distributed through the Miocene Manuherikia Group sediments (fine flaky particles in background), and coarse irregular nuggets which occur in proximal alluvial fan deposits immediately overlying the Manuherikia Group at Matakanui (Fig. 2). The nuggets consist principally of authigenic gold (cf. Youngson and Craw, 1993), but delicate protrusions have been subjected to minor rounding by transport, especially on the centre and right nugget. Scale is in millimetres.
transport. Crystalline overgrowths, in the form of skeletal octahedra, and colloidal encrustations are also found. Nuggets partially or wholly enclose detrital mineral grains and authigenic clay minerals (Youngson and Craw, 1993). Some coarser authigenic grains have one or more recognizable relict detrital cores (Craw, 1992; Youngson and Craw, 1993). The grain size increase due to authigenic growth has occurred to a similar extent in soil in areas with little erosion, and in repeatedly recycled proximal fan sediments, during Pleistocene-Recent uplift.
Miocene fluvial qua~z gravels 20
"(~ "6 E 10 +~luggets 1 to2cm
Pleistocene schist
Sediments "
The sediments deposited on the slopes of the rising ranges consist of sequences (from t to 60 m thick) of poorly sorted gravel and sand units 1- 10 m thick. Units are typically poorly bedded or unbedded internally. Many units are chaotic and
2000
' 4060
''~06o""
'
8obo
'
' '
gold maximum dimension (microns) Fig. 6. Comparison of gold grain size in the Manuherikia Group, which provided much of the placer gold. and in the youngest part of a complex fan at Matakanui where authigenic precipitation of gold has occurred. Grain size was determined on gold pan concentrates using binocular microscope and scanning electron imagc~.
E L U V I A L G O L D P L A C E R F O R M A T I O N ON A C T I V E L Y RISING M O U N T A I N R A N G E S
(clasts up to 30 cm) at their bases. Lag deposits occur locally at the bedrock interface at the base of channels and in scours cut into mass flow deposits. The lag gravels have a matrix of sand, silt and mud, including concentrations of magnetite (coarse sand to silt size). Gold occurs principally as local concentrations in channel lag deposits, many of which have been worked by early miners. Minor, but locally rich concentrations of gold occur irregularly in the soil and in unchannellised hollows and basins. The
matrix-supported, with angular schist clasts up to 20 cm floating in matrix of immature sand and silt, and kaolinite clay. Reverse grading of clast sizes is visible in the lower 30 cm of some horizons. These sediments are typical of mass flow deposits (Heward, 1978; Nemec and Steel, 1984) which have accumulated as a series of sheet-like layers 1-3 m thick with near-planar bases (Fig. 7). Some units are channellised accumulations of weakly stratified, clast-supported, imbricated gravels, with coarse clast-supported lag gravels
max. clast size
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629
silt and mud weakly stratified ) 3 matrix supported ~.~2clast supported
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1 weakly stratified, matrix supported horizontally stratified, clast supported horizontally stratified, clast supported, clasts fining up
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massive/imbricated, clast supported muddy sand, scattered pebbles
poorly stratified clast supported, minor imbrication unstratified muddy sand, scattered pebbles & cobbles
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clast supported, some matrix supported zones matrix supported increasing matrix up, clasts lining up 0 ctast supported
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unconformable on dipping Miocene strata Fig. 7. Sketch section through proximal alluvial fan sediments in a complex fan sequence at Matakanui (Fig. 2).
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recoverable gold is commonly highly irregular in shape, coarse (up to 2 cm), and is presumed to have a substantial authigenic component (see above). Topographic evolution of the ranges in space and time
The Central Otago range topography can be broadly subdivided into areas which have undergone different degrees of uplift, and therefore different degrees of dissection of the unconformity and the underlying schist. These subdivisions, which are outlined below in order of increasing uplift, grade from one to another spatially along and across the ranges. The more uplifted portions of the ranges have also evolved with time through different topographic subdivisions. Amount of uplift of the ranges increases from east to west, so little remains of the unconformity on the Dunstan Range (Fig. 2).
Preserved unconformity The Miocene unconformity is largely preserved in those portions of ranges which have undergone little uplift. The topographic slope of these areas is generally around 7 ° or less. These areas are predominantly on the western sides of the ranges, but are locally on the crests and eastern sides of the ranges as well (Fig. 2). The exposed unconformity consists of highly kaolinised schist on very gentle slopes and oxidised and weakly kaolinised schist on steeper slopes. A characteristic feature of many parts of the preserved unconformity is widespread occurrence of dismembered silicified Manuherikia Group conglomerate, common, for example, along the western slopes of Rough Ridge (Fig. 2). This silicified conglomerate breaks up on erosion into approximately equant boulders typically from 20 cm to 2 m across. These limonite-stained boulders, or "sarsen stones", are scattered widely over low-slope hillsides, and locally accumulate in the small stream channels on these hillsides. The gold found on the preserved unconformity areas consists of two populations: finer grains with the morphological characteristics of detrital
P . C R A W A N D J.l [. Y O U N ( I S Q N
Manuherikia gold; and a rough and irregular component which exhibits most of the characteristics of authigenic growth (Fig. 4). Coarse gold is typically found in the upper 10 cm or so of the thin ( < 50 cm) lithic soil and, less frequently, on the unconformity surface. Gold nuggets, up to 1 ounce (troy), are found amongst the sarsen stones in the associated soil. Significant gold accumulations are rare but are found at or near the base of former cracks opened during erosion of silicified conglomerate sheets and in schist joints or fractures which traverse the slope. At one locality where the unconformity has intersected a mineralised vein in the schist, some spectacular wire gold can be found in cavities in the oxidised vein, and also in the adjacent soil where gold has overgrown angular schist detritus (Fig. 4),
Degraded unconformity Preserved unconformity areas grade laterally into slopes underlain by degraded unconformity. Large areas of the range surfaces consist of this degraded unconformity (Fig. 2), from which the altered schist and overlying sediments have been stripped by erosion during uplift. These surfaces are dotted with tors of more resistant schist up to 10 m high. Degraded unconformities occur over a wide range of slope angles, including the more or less flat range summits, and have an overall convex profile (Fig. 3). Soil is typically thin (5-30 cm) and highly lithic. Hollows and unchannellised basins are common, and these are partially filled with soil creep and mass flow deposits. Small channels have formed on the steeper parts of the degraded unconformity, and portions of these act as sites of accumulation for mass flow and channellised gravel deposits up to 1 m or more thick. Gold is haphazardly distributed through soils in the hollows and unchannellised basins and on the degraded unconformity surface in general. Small concentrations occur locally in the channellised gravel deposits and several such deposits were worked by early miners. The morphological characteristics of the gold are similar to those for gold on the preserved unconformity, with coarse and irregular particles relatively common. The degraded unconformity deposits and their con-
E L U V I A L G O L D PLACER F O R M A T I O N ON A C T I V E L Y RISING M O U N T A I N R A N G E S
tained gold represent a transition between the preserved unconformity areas and the proximal margins of the single fans described below. In some cases, however, degradation has not advanced sufficiently to form a down-slope fan.
Single fans Degraded unconformity surfaces grade laterally and down-slope into steeper range fronts from which some of the fresh schist basement has been removed. The slope profile of these areas is still convex, and thin (typically < 5 m) fan deposits extend from the slopes of the range into the basin. These thin fan gravels are well developed on the eastern margin of the Raggedy Range and on less uplifted portions of the Dunstan Range (Fig. 2). Discrete channels up to 3 m deep and 10 m wide with steep sides have been cut into schist and Manuherikia Group across the range margins. Channels are traceable for hundreds of metres from unchannellised basins at their heads, downslope to low-angle fans which merge with valley fill. Sediment in the channels is massive to poorly stratified schist gravel (see above) with minor quartz gravel components especially near the base. Mass flow material continues across interfluves, and some channels are hidden by overlying matrix-supported slope wash. Most of the gold in the fans is coarse, irregular, and presumably dominantly authigenic, although a significant fine (recycled Manuherikia Group) component is present also. The gold is concentrated almost exclusively in the channel lag deposits, at internal unconformities and particularly at the base of the channels. Gold has been extracted from many of the fans by early miners, who generally ceased operations when the Manuherikia Group was reached.
Complex fans The steepest parts of the ranges have an apron of fan sediments extending from the lower schist slopes out into the adjacent valley. These fans are most common on the slopes of the Dunstan Range (Fig. 2). The fans are complex sequences of conglomerates which have formed progressively as
631
the ranges rose. Early-formed parts of the fans have been deformed and are unconformably overlain by younger fan conglomerates which are commonly channellised into the underlying strata. The fan sediments become younger up-section, with locally recognizable stratigraphic order. Gold in the complex fans is concentrated in lag gravels at unconformities within the fans, as in single fans (above). Most of the gold is highly irregular in shape, commonly with delicate surface textures indicative of authigenic precipitation. Some coarse grains have been rounded by recycling, and further addition of authigenic gold has occurred. Nuggets up to 2 cm across occur in the youngest gravel sequence.
Major streams A small number of major streams emanate from, or cross, the ranges. Those which emanate from the ranges have large concave drainage basins with steep headwalls and deeply incised channels. Stream gradient decreases downstream, and streams emerge on to broad low-angle alluvial fans. Fans consist of well-stratified, poorly sorted, fresh schist debris with clasts up to 30 cm. The fans merge downstream with terraces associated with the axial rivers. Fans do not extend sufficiently far, however, to contribute significantly to either the terraces or the axial river bed load. Rivers which cut through the ranges (e.g., Pool Burn, Manuherikia River, Fig. 2) are antecedent, and form deeply incised gorges in which the streams maintain low gradients. Bed load in and below the resultant gorges has a low schist content (< 10%) and is dominated by greywacke cobbles derived from the stream headwaters. Significant gold concentrations are relatively rare in the deposits of the major streams, but minor concentrations occur at unconformities within the fans, and some gold is dispersed through the fan gravels. Discussion and conclusions: eluvial gold concentration
The above descriptions allow some generalisations of the processes of eluvial gold mobility and
632
concentration for the Central Otago region. These are: (1) Chemical gold mobility and authigenic growth of grains is a common phenomenon which is important in grain size increase in the eluvial deposits. (2) Eluvial gold placers form where topographic slopes steepen due to progressive uplift. The placers form on degraded unconformity areas and steeper convex slopes on which single and complex fans have formed. These fans contain a high proportion of matrix-supported mass flow material. (3) Physical concentration of gold occurs in surficial deposits eroded from the degraded unconformity, and gold is found primarily in coarse clast-supported lag gravels. (4) Eluvial placer formation ceases when further uplift and erosion has caused development of substantial concave streams which erode below the unconformity into fresh schist. Influx of this schist debris, without gold, dilutes potential placer deposits in the early stages of stream evolution. Investigation of the first of these identified processes revolves around groundwater chemistry, and is the subject of further study. The last process represents the transition from the eluvial environment to the truly alluvial environment where placer formation, farther downstream, is better understood (Adams et al., 1978; Nami and James, 1987; Dyson, 1990). However, the processes of physical gold concentration in an eluvial environment in which mass flow is clearly an important transport mechanism is more problematical, and is discussed further in the following section.
Heavy mineral concentrations in steep mountain streams The Central Otago eluvial placers are concentrated in lag deposits in which the predominant clast size is greater than 10 cm. The cobbles are angular to subrounded, and clearly have not been subjected to significant water transport. The clasts have interstitial sand-silt sized silicates and magnetite, and variable gold grain sizes up to 2 cm. Similar associations of placer gold concentrations
D, ( ' R A W A N D ,I.H ~c,)[JNI.IS()N
with coarse lag gravels have been noted commonly elsewhere (e.g. Cheney and Patton, 1967: Boyle, 1979). Most experimental studies of placers have used magnetite as the heavy mineral to be concentrated (e.g. Adams et al., 1978). as magnetite is readily transported in streams with silicates. However, there is a very large difference in specific gravity between magnetite (S.G. = 5.2) and gold (S.G. = 19), and it is unlikely that gold will respond in the same manner as magnetite in an alluvial system. The following simple model is intended to compare likely responses of silicates, magnetite and quartz to alluvial processes in a steep mountain stream. The model assumes thal a stream develops in mass flow deposits on a mountain slope, and the material available for transport has a very wide range of grain sizes, from boulders to mud. The finer-grained silicate material is assumed to be moved by the stream while the coarse cobbles are not moved. The model differs from models of alluvial systems in which the whole bed becomes mobile in occasional floods (Day and Fletcher, 1991 ). The coarse cobbles accumulate to form a lag with portions of cobbles protruding from the bed which protect small particles from entrainment (Slingerland and Smith, 1986; Komar, 1987). The shear stress, related to the stream discharge, required to mobilise clasts of various sizes can be determined theoretically from the Shields equation: L = Tc* ( d s - d w ) g D where Tc is the critical shear stress for initial bed material movement, d S and d w are the densities of bed material and water, g is the acceleration due to gravity, and D is the particle diameter (e.g. Bathurst, 1987). To* is the Shields parameter, or dimensionless critical shear stress, whose magnitude depends on the ratio of particle dianaeter to the median particle diameter in poorly sorted sediments. Values of T~* can be calculated from experimental and natural studies, although there is disagreement over exact relationships (Komar, 1987). Large density differences between gold and other minerals dominate the Shields equation calculations, so we use the 1~.* relationship of Andrews (1983) for simplicity.
ELUV1AL
GOLD
PLACER
FORMATION
ON ACT1VELY
RISING
MOUNTAIN
A median particle diameter=lO mm
.80
(D L.
-40
co
I
0.01
0.1 I
_4o
B
median particle diameter=l mm
I
I
1
10
I
100
/ma 7 I
-20
particle diameter/median particle diameter Fig. 8. Critical shear stress (vertical axis) requirecJ to move particles of differing grain sizes (horizontal axis) in poorly sorted sediment with (A) median grain size = 10 mm; and (B) median grain size = 1 ram. Curves for quartz, magnetite and gold are presented in each diagram.
Model curves (Fig. 8) depict theoretical shear stresses required to mobilise different grain sizes in poorly sorted gravels. Curves are presented (Fig. 8) for two different median grain sizes: 10 mm and 1 ram, which encompass the typical Central Otago eluvial deposits. The significance of the curves is that for sediments such as those present in Central Otago surficial deposits, most grain sizes are mobilised at about the same shear stress, leaving only the coarse cobbles (Andrews, 1983; Komar, 1987). Similar curves drawn for gold shows that shear stresses which are capable of moving most schist debris are not capable of moving typical eluvial gold grains, especially for sediments with median particle diameter of 10 mm (Fig. 8). The curves for magnetite lie close to those of quartz. The model provides some theoretical basis for the field-based inference that gold apparently
RANGES
633
accumulates residually by removal of all but the coarse silicate debris ("winnowing", Slingerland and Smith, 1986). Magnetite apparently behaves in a similar manner to silicate material with similar grain size, and can be readily mobilized (Fig. 8). Hence, magnetite and the fine-grained silicate material are probably deposited after entrainment and transport, in voids between lag clasts (Reid and Frostick, 1985; Day and Fletcher, 1991). Thus, in the eluvial environment, magnetite and gold concentrations occur by different mechanisms. Gold concentration by this "winnowing" mechanism requires a constant supply of goldbearing sediment which is presumably the matrix-supported mass flow deposits so commonly found in unchannellised and locally channellised basins on the uplifting unconformity (cf. Dietrich et al., 1986; Alger and Ellen, 1987). The mass flow deposits are continually degraded during flood discharges to concentrate gold until the resultant lag is buried by stream activity or new mass flows. A new lag then develops higher in the section until that too is buried. In this way a fan is built up as the range rises• Channelling of pre-existing fans can occur after continued uplift, and this results in recycling of gold by stream and mass flow action into the new channel as a complex fan is built up. An important corollary of the conclusions outlined above is that authigenic grain-size increase causes entrapment of gold in the eluvial environment. Only the finer-grained gold is transported downstream into the truly alluvial environment to form placers.
Implications for other placer deposits All detrital gold must pass through an eluvial stage at some time. Grain size increase by secondary processes is an important step in ensuring entrapment of gold in the source region. Grain size increase may have been hindered if the primary veins were uplifted and eroded very rapidly, such as the high-rainfall sides of orogenic belts (Eyles, 1990; Koons and Craw, 1991), or if the groundwater chemistry was not favourable for gold mobility (perhaps in a low-oxygen atmosphere, e.g. Witwatersrand, Krupp and Weisser, 1992). However, in most tectonic environments
634
some or all of the eluvial processes discussed in this paper will have occurred. This is especially true for slow to moderate uplift rates in semi-arid regions analogous to Central Otago, such as the numerous circum-Pacific young giant placers including the Mother Lode district of California and the Klondike district of northern Canada (Henley and Adams, 1979). The eluvial processes discussed above ensure that a large proportion of available detrital gold is trapped in the source region, and only the finest-grained fraction is released to the alluvial system downstream. With continued uplift, eluvial placers will ultimately be eroded and recycled into younger alluvial systems to form placer deposits with coarse-grained irregular nuggety gold (Boyle, 1979; Henley and Adams, 1979). The gold will move only short distances during this recycling because of its coarse grain size. Glaciation may cause transport of the coarse gold farther downstream, but fluvial outwash processes can result in reconcentration of a coarse gold residue by winnowing, with little or no transport (e.g. Eyles and Kocsis, 1989). Hence, we suggest that the distinction between the two main types of placer deposits (discussed above) is due primarily to eluvial processes (or lack of them) in the source region.
Acknowledgements This work was funded by the University of Otago Research Committee and a University of Otago Division of Sciences student bursary. Discussions with B.J. Douglas, C.A. Landis and J.K. Lindqvist helped to clarify some of the ideas in the paper, although they do not necessarily agree with these ideas. Constructive reviews of the manuscript by N.D. Smith and an anonymous referee considerably improved the presentation.
References Adams, J., Zimpfer, G.L. and McLane, C.F., 1978. Basin dynamics, channel processes, and placer formation: a model study. Econ. Geol,, 73: 416-426. Alger, C.S. and Ellen, S.D., 1987. Zero-order basins shaped by debris flows, Sunol, California, USA. In: Erosion and Sedimentation in the Pacific Rim. Int. Assoc. Hydrol. Sci. Publ., 165: 111-119.
I). ('RAW AND J.H. YOtINGSON Andrews, E.D., 1983. Entrainment of gravel from naturally sorted riverbed material. Geol. Soc. Am. Bull, 94: 12251231. Bathurst, J.C., 1987. Critical conditions for movement in steep boulder-bed streams. In: Erosion and Sedimentation in the Pacific Rim. Int. Assoc. HydroL Sci. t'ubl.. 165: 309318. Beanland, S., Berryman, K.R., Hull, A.G. and Wood. P.R., 1986. Late Quaternary deformation at the Dunstan Fault. Central Otago, New Zealand. R. Soc. N.Z. Bull., 2,1: 293-306. Benson, W.N., 1935. Some landforms in southern New Zealand. Austr. Geogr., 2: 3-23. Bishop, D.G., 1974. Tertiary and Early Quaternary geology of the Naseby and Kyeburn areas, Central Otago. N.Z.J. Geol. Geophys., 17: 301-335. Boyle, R.W., 1979. The geochemistry of gold and its deposits. Geol. Surv. Can. Bull., 280, 584 pp. Cheney, E.S. and Patton, T.C., 1967. Origin of the bedrock values of placer deposits. Econ. Geol., 62: 852- 853. Clough, D.M. and Craw, D., 1989. Authigenic gold-marcasite association: evidence for nugget growth by chemical accretion in fluvial gravels, Southland, New Zealand. Econ. Geol., 84: 953-958. ('raw, D., 1992. Growth of alluvial gold particles by chemical accretion and reprecipitation, Waimumu, New Zealand. N.Z.J. Geol. Geophys., 35: 157-164. Craw, D. and MacKenzie, D.J., 1992. Near-surface secondary gold mobility and grain-size enhancement, Barewood mine, east Otago, New Zealand. N.Z.J. Geol. Geophys., 35: 151-156. Craw, D. and Norris, R.J., 1991. Metamorphogenic Au-W veins and regional tectonics: mineralisation throughout the uplift history of the Haast Schist. New Zealand. N.Z.J~ Geol. Geophys., 34: 373-383. Day, S.J. and Fletcher, W.K., 1991. Concentration ot magnetite and gold at bar and reach scales in a gravel-bed stream, British Columbia. J. Sediment. Petrol.. 6t: 871 882. Dietrich, W.E., Wilson, C.J. and Reneau, S.L., 1986. Hollows. coUuvium, and landslides in soil-mantled landscapes. In: A.D. Abrahams (Editor), Hillslope Processes. Allen and Unwin, Boston, pp. 361-388. Douglas, B.J., 1986. Lignite resources of Central Otago. Publ. P104, N.Z. Energy Research and Development Committee, University of Auckland, Auckland, 368 pp. Dyson, I.A., 1990. Fluvial models in placer gold exploration and their evaluation. Placer Deposits. Australasian Inst. Min. Metall. Symp., pp. 11-43. Eytes, N., 1990. Glacially derived, shallow-marine gold placer~ of the Cape Yakataga district, Gulf of Alaska. Sediment. Geol., 68: 171-185. Eyles, N. and Kocsis, S.P., 1989. Sedimentological controls on gold in a late Pleistocene glacial placer deposit, Cariboo Mining District, British Columbia, Canada. Sediment, Geol., 65: 45-68.
ELUVIAL GOLD PLACER FORMATION ON ACTIVELY RISING MOUNTAIN RANGES Henley, R.W. and Adams, J., 1979. On the evolution of giant gold placers. Trans. Inst. Min. Metall., 88: B41-B50. Heward, A.P., 1978. Alluvial fan sequence and megasequence models, with examples from Westphalian D-Stephanian B coalfields, N. Spain. Can. Soc. Pet. Geol. Mem., 5: 669-702. Komar, P.D., 1987. Selective grain entrainment by a current from a bed of mixed sizes: a re-analysis. J. Sediment. Petrol., 57: 203-211. Koons, P.O. and Craw, D., 1991. Gold mineralization as a consequence of continental collision: an example from the Southern Alps, New Zealand. Earth Planet. Sci. Lett., 103: 1-9. Krupp, R.E. and Weisser, T., 1992. On the stability of goldsilver alloys in the weathering environment. Miner. Deposita, 27: 268-275. McSaveney, M.J., Thomson, R. and Turnbull, I.M., 1992. Timing of relief and landslides in Central Otago. In: D.H. Bell (Editor), Landslides. Sixth International Symposium on Landslides Proceedings. Balkema, Rotterdam, pp. 1451-1456. Miall, A.D., 1978. Lithofacies types and vertical profile models in braided river deposits: a summary. Can Soc. Pet. Geol. Mere., 5: 597-604. Mosley, M.P. and Schumm, S.A,, 1977. Stream junctions--a probable location for bedrock placers. Econ. Geol., 72: 691-694. Mutch, A.R., 1962. Geological map of New Zealand 1 : 250000. Sheet 23--Oamaru. Wellington, Department of Scientific and Industrial Research. Nami, M. and James, C.S., 1987. Numerical simulation of gold distribution in the Witwatersrand placers. Soc. Econ. Paleontol. Mineral. Spec. Publ., 39: 353-357. Nemec, W. and Steel, R.J., 1984. Alluvial and coastal conglomerates: their significant features andsome comments on gravelly mass-flow deposits. Can. Soc. Pet. Geol. Mere., 10: 1-31.
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Norris, R.J., 1979. A geometrical study of finite strain and bending in the South Island. Bull. R. Soc. N.Z., 18: 21-28. Norris, R.J., Koons, P.O. and Cooper, A.F., 1990. The structure of the present obliquely-convergent plate boundary in the South Island of New Zealand: implications for the interpretation of ancient collision zones. J. Struct. Geol., 12: 715-725. Officers of the Geological Survey, 1983. Seismotectonic hazard evaluation of the Clyde dam site. N.Z. Geol. Surv. Rep. EG375. Reid, I. and Frostick, L.E., 1985. Role of settling, entrainment and dispersive equivalence and of interstice trapping in placer formation. J. Geol Soc. London, 142: 739-746. Slingerland, R. and Smith, N.D., 1986. Occurrence and formation of water-laid placers. Annu. Rev. Earth Planet. Sci., 14:113-147. Smith, N.D. and Beukes, N.J., 1983. Bar to bank convergence zones: a contribution to the origin of alluvial placers. Econ. Geol., 78: 1342-1349. Smith, N.D. and Minter, W.E.L., 1980. Sedimentological controls of gold and uranium in two Witwatersrand paleoplacers. Econ. Geol., 75: 1-14. Stirling, M.W., 1990. The Old Man Range and Garvie Mountains: tectonic geomorphology of the Central Otago peneplain, New Zealand. N.Z.J. Geol. Geophys., 33: 233-244. Williams, G.J., 1974. Economic geology of New Zealand. Australasian Inst. Min. Metall. Monog. 4, 4~0 pp. Wood, B.L., 1962. Geological map of New Zealand 1:250000. Sheet 22--Wakatipu. Wellington, Department of Scientific and Industrial Research. Youngson, J.H. and Craw, D., 1993. Gold nugget growth during tectonically-induced sedimentary recycling, Otago, New Zealand. Sediment. Geol., in press.
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Elsevier Science Publishers B.V., Amsterdam
Eluvial gold placer formation on actively rising mountain ranges, Central Otago, New Zealand D. Craw and J.H. Youngson Geology Department, UniL'ersity of Otago, P.O. Box 56, Dunedin, New Zealand Received June 12, 1992; revised version accepted November 17, 1992
ABSTRACT Craw, D. and Youngson, J.H., 1993. Eluvial gold placer formation on actively rising mountain ranges, Central Otago, New Zealand. In: C.R. Fielding (Editor), Current Research in Fluvial Sedimentology. Sediment. Geol., 85: 623-635. Eluvial gold deposits in Central Otago, New Zealand, have formed and are still forming on the flanks of actively rising antiformal mountain ranges. These gold deposits are derived mainly by erosion and concentration of fine-grained ( < 500 /xm) gold in mature Miocene fluvial quartz gravels. Chemical processes during Pleistocene-Recent uplift and eluvial sedimentation have resulted in crystalline and amorphous authigenic gold precipitation and up to 2 orders of magnitude gold grain-size increase. The eluvial gold deposits are hosted by thin lithic soil and sequences (up to 60 m thick) of poorly sorted immature schist gravels. The gravel sequences consist mainly of matrix-supported mass flow deposits and channellised proximal fan deposits, intercalated on a 1-10 m scale. Gold is concentrated in coarse lag gravels (up to 40 cm clasts) at channel bases. Topographic slopes on the rising ranges show an evolutionary trend in space and time, from gentle weakly dissected surfaces, through slightly degraded but convex slopes, to deeply incised convex streams. Eluvial gold occurs sporadically on the gentle slopes, but the most efficient concentration processes occurred where steeper convex slopes yielded an apron of fan sediments. Gold concentration at these sites resulted from selective and localized removal ("winnowing") of most schist debris, leaving coarse lag gravels and gold. The combination of authigenic grain size increase and residual concentration ensures that the eluvial deposits retain coarse-grained gold, and that only fine-grained gold is released to the alluvial systems downstream.
Introduction Detrital gold deposits, or placers, have always been important sources of gold. So-called "giant" placer gold deposits are found in numerous geological terranes around the world, and these have formed by repeated recycling of gold-bearing sediments (Henley and Adams, 1979). Economic placer deposits are generally presumed to form by these recycling processes during rejuvenation of topography in areas where low-grade gold deposits already occur (Adams et al., 1978; Boyle, 1979). The necessary topographic rejuvenation results from changes in the longitudinal profiles
Correspondence to: D. Craw, Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand.
of rivers due to, for example, uplift in the headwater regions. Alluvial placers form in the rivers as they equilibrate to the new base levels (Adams et al., 1978). Placer deposits fall into two main types: those with gold distributed through large volumes of fluvial sediments, and those which accumulate more irregularly near to a gold source (Boyle, 1979). The former type of placer generally contains fine-grained gold, and these typically occur in braided river deposits and distal alluvial fans (Smith and Minter, 1980; Day and Fletcher, 1991). The latter type of placer generally contains a wide range of gold grain sizes including coarse irregular nuggety grains and these occur in proximal tectonic sediments and glacial deposits which are commonly associated with the tectonic sediments (Henley and Adams, 1979: Eyles and Koc-
0037-0738/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
624
sis, 1989). Studies of alluvial placer formation have mainly concentrated on the first of these types, and studies of both placer types have examined fluvial processes in well-defined drainage systems (Smith and Beukes, 1983; Nami and James, 1987: Eyles and Kocsis, 1989; Day and Fletcher, 1991). The processes which occur in the headwaters of alluvial systems for both types of placers, the "eluvial" processes (Boyle, 1979), have been largely ignored. This is partly because the deposits are generally small and partly because eluvial deposits are notoriously irregular and unpredictable, as slope processes which are responsible for the deposits are poorly understood. Nevertheless, eluvial deposits are an important link in the chain of processes which result in alluvial gold concentrations, particularly when topographic rejuvenation is due to hinterland uplift.
D. CRAW AND J.}l ~ (.~UNGSON
Eluvial processes in rising mountains are spatially transient, as further uplift causes erosion of early-formed eluvial deposits, while new eluvial deposits form nearby on the same mountain range. Ultimately, all record of the eluvial environment is removed if erosion rates excced ratc~ of eluvial processes. All alluvial gold deposits have passed through an eluvial environment at some stage, but very little cvidence of this is preserved in the geological record. It is only by studying currently active, or very you,,g, eluvial deposits that some insight can be gaincd into thc processes which govern the nature of gold which enters the more distal alluvial systems. This paper is a preliminary account t)f thc development of small but locally rich eluvial deposits in Central Otago, New Zealand, one of the "giant" placer regions of Henley and Adams (t979). The processes which give rise to these
Fig. 1. Location and s u m m a r y geological m a p of Central Otago in the South Island, New Zealand, showing the prommen! northeast-trending Otago Schist ranges, and basins filled with cover sediments. Major gold-bearing quartz veins (squares) and placer deposits (triangles) are indicated (after Williams, 1974).
E L U V I A L G O L D PLACER F O R M A T I O N ON A C T I V E L Y RISING M O U N T A I N R A N G E S
crease in strain and metamorphic grade to the north and south and grade into Palaeozoic/Mesozoic greywackes. The central greenschist facies portion of the Otago Schist has a well-defined flat-lying schistosity. Uplift and erosion of the schist belt from late Mesozoic to middle Cenozoic resulted in a regional low-relief surface which lies sub-parallel to the schistosity (Stirling, 1990)• This low-relief surface was unconformably overlain by Miocene fluvio-lacustrine sediments of the Manuherikia G r o u p (Douglas, 1986). Coarser fluvial sediments of the Manuherikia G r o u p consist largely of mature quartz gravels which are auriferous, particularly near the base of the section on the unconformity (Williams, 1974). Extensive kaolinitisation of schist has occurred up to 10 m below the unconformity beneath Miocene sediments. This kaolinitised zone, and the overlying unconformity, is traceable throughout Central Otago (Benson, 1935; Stirling, 1990).
eluvial placers are of some significance for the economics of the immediate deposits, but are also important for alluvial deposits which form farther downstream. The Central Otago region is tectonically active, and the eluvial placers are very young or actively forming; hence, some of the parameters involved in placer formation can be constrained. This p a p e r examines development of eluvial placers in space and time in the regional tectonic context. We also attempt to identify processes and mechanisms which are responsible for maximising gold concentrations in the tectonic sediments, and show that the differences in grain size between the two main types of placer deposits (above) are due to eluvial processes.
Regional geology The Otago Schist which underlies Central Otago (Fig. 1) is a belt of highly deformed predominantly greenschist facies schists which de-
sediment •" ~ recent alluvium " " • ~fan
gravels
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625
r
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MaoriBotto'~ Formation H Manuherikia Group
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degraded i unconformity
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Fig. 2. Geological map of the middle portion (see Fig. 1) of Central Otago (modified after Mutch, 1962, and Wood, 1962), showing topographic subdivisions of the antiformal ranges, and generalised stratigraphy of basin sediments. Eluvial gold occurs on the preserved Miocene unconformity and in proximal portions of fans (close stipple) derived from degraded unconformity during uplift of the ranges. The gold was derived primarily from erosion of the auriferous Manuherikia Group (preserved outcrops in black).
626
D, ( ' R A W A N D .I.tL "t ( )t INGS( )N
Southern New Zealand now lies astride the Australian/Pacific plate boundary, the Alpine Fault (Fig. 1; e.g. Norris et al., 1990). The Alpine Fault is currently an oblique-slip fault with approximately 10 m m / y r vertical (east side up) and dextral horizontal movement, Near the southern end of the Alpine Fault, the Otago Schist structural block abuts the fault and is being deformed by plate boundary processes up to 200 km east of the Alpine Fault (Norris, 1979). Late Cenozoic strain in the Otago Schist block has resulted in a set of northeast and northwest trending folds and reverse faults (Norris, 1979). Regional scale folding has resulted in asymmetrical warps of the flat-lying schistosity to form a regular pattern of mountain ranges (antiforms) and intervening basins (synforms) with a 20 km wavelength and an amplitude of approximately 2 km (Figs. 1, 2). Antiformal ranges have gently dipping northwestern limbs and broad crests (Fig. 3). Southeastern limbs are typically steeper dipping (Fig. 3) and are locally overturned, with minor reverse faulting along the range margin or in the immediately adjacent basin (Beanland et al., 1986). The Miocene unconformity is partially preserved on the gently dipping parts of the ranges and fully preserved in the basins. Uplift in the region began at least 2 million years ago, initially in the greywacke zone to the north of the schist belt. Sediment shed from the
rising mountains spread over many parts of the schist belt as the widespread greywacke conglomerate, the Pliocene Maori Bottom Formation (Bishop, 1974; Fig. 2). These ranges are still actively rising along a prominent northwest striking fault zone, and rivers from these ranges flow as axial streams through the synformal Central Otago basins, with a greywacke bedload. Northeast trending schist ranges began to rise later than the greywacke ranges, and the Maori Bottom Formation has been locally folded and faulted during schist range uplift. Most schist range uplift has occurred within the past million years, and a considerable amount of uplift has occurred within the last 400,000 years (McSaveney et al., 1992). Alluvial fans are poorly dated, but some fans can be traced to merge with better-dated glacial outwash terraces, some as young as 65,000 years (Officers, 1983). The Central Otago region has been extensively mined for gold (Fig. 1), with over 240 tonnes extracted, more than 90% of which was from placer deposits. The following paper concentrates on the middle portion of the region (Figs. l, 2), to avoid the complications of glacial deposits to the west and more complex pre-Miocene geology to the east. In this middle portion, eluvial deposits are very common on margins of mountain ranges, particularly in the proximal portions of associated fans (Fig. 2).
NW .2000
moderately degraded unconformity (minor eluvial gold concentration)
A
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........
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Fig. 3. Topographic profiles (no vertical exaggeration) of the Dunstan Range and the Raggedy Range (section lines in Fig. 2L showing the scale and overall concave shapes of the ranges. Dunstan Range has been uplifted more than Raggedy Range, and consequently the Dunstan Range unconformity is more degraded, and eluvial gold is concentrated in proximal fan deposits. Miocene sediments are stippled,
ELUVIAL (IOLD PLACER FORMATION ON ACTIVELY RISING MOUNTAIN RANGES
Authigenic gold There are two main sources of gold for the eluvial placers described below. Rare primary gold-bearing veins crop out in the schist basement as steeply dipping quartz-filled fault zones traceable for up to 1 km along strike (Craw and Norris, 1991). Fine-grained gold (1-100 /~m) occurs associated with sulphides in these veins. In the near-surface region ( < 20 m), supergene processes have caused gold mobility in solution followed by reprecipitation to increase grain size of some particles (Craw and MacKenzie, 1992). Resultant gold grains are highly irregular in shape, and up to 2 cm across (Fig. 4). Once released into the soil there is further chemical mobility of gold, combined with physical modification during down-slope creep, which results in formation of irregular nuggets with rounded extremities.
627
The other source of gold is detrital gold in the Manuherikia Group fluvial sediments which overlie the basement. This gold is generally very fine grained (100-500 /xm; Fig. 5), and concentrated principally in coarse gravel facies in channels near the base of the sequence. Chemical gold mobility has occurred within the Manuherikia sediments during uplift (Craw, 1992), and in tectonic sediments after uplift and recycling, resulting in grain size increase (Clough and Craw. 1989; Youngson and Craw, 1993; Figs. 5, 6). Authigenic gold grains are generally >500 /xm across, and numerous grains > 1 mm occur. Nuggets up to 1 cm across and larger are rare but widespread. Evidence for authigenic gold precipitation can be seen in the morphology of these coarse gold grains. Many of the grains are highly irregular in shape, with numerous delicate protrusions which could not have survived fluvial
Fig. 4. Photograph of eluvial gold nugget from Central Otago; complex aggregate of wire gold which has partially or wholly overgrown lithic soil material (mainly angular schist fragments, e.g. top and bottom left, and centre), central Raggedy Range. Specimen is 2 cm across.
628
P. ( ' R A W AND ,LH. YOUNt},'-,ON
Fig. 5. Photograph showing relative size and morphology of detrital gold distributed through the Miocene Manuherikia Group sediments (fine flaky particles in background), and coarse irregular nuggets which occur in proximal alluvial fan deposits immediately overlying the Manuherikia Group at Matakanui (Fig. 2). The nuggets consist principally of authigenic gold (cf. Youngson and Craw, 1993), but delicate protrusions have been subjected to minor rounding by transport, especially on the centre and right nugget. Scale is in millimetres.
transport. Crystalline overgrowths, in the form of skeletal octahedra, and colloidal encrustations are also found. Nuggets partially or wholly enclose detrital mineral grains and authigenic clay minerals (Youngson and Craw, 1993). Some coarser authigenic grains have one or more recognizable relict detrital cores (Craw, 1992; Youngson and Craw, 1993). The grain size increase due to authigenic growth has occurred to a similar extent in soil in areas with little erosion, and in repeatedly recycled proximal fan sediments, during Pleistocene-Recent uplift.
Miocene fluvial qua~z gravels 20
"(~ "6 E 10 +~luggets 1 to2cm
Pleistocene schist
Sediments "
The sediments deposited on the slopes of the rising ranges consist of sequences (from t to 60 m thick) of poorly sorted gravel and sand units 1- 10 m thick. Units are typically poorly bedded or unbedded internally. Many units are chaotic and
2000
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'
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gold maximum dimension (microns) Fig. 6. Comparison of gold grain size in the Manuherikia Group, which provided much of the placer gold. and in the youngest part of a complex fan at Matakanui where authigenic precipitation of gold has occurred. Grain size was determined on gold pan concentrates using binocular microscope and scanning electron imagc~.
E L U V I A L G O L D P L A C E R F O R M A T I O N ON A C T I V E L Y RISING M O U N T A I N R A N G E S
(clasts up to 30 cm) at their bases. Lag deposits occur locally at the bedrock interface at the base of channels and in scours cut into mass flow deposits. The lag gravels have a matrix of sand, silt and mud, including concentrations of magnetite (coarse sand to silt size). Gold occurs principally as local concentrations in channel lag deposits, many of which have been worked by early miners. Minor, but locally rich concentrations of gold occur irregularly in the soil and in unchannellised hollows and basins. The
matrix-supported, with angular schist clasts up to 20 cm floating in matrix of immature sand and silt, and kaolinite clay. Reverse grading of clast sizes is visible in the lower 30 cm of some horizons. These sediments are typical of mass flow deposits (Heward, 1978; Nemec and Steel, 1984) which have accumulated as a series of sheet-like layers 1-3 m thick with near-planar bases (Fig. 7). Some units are channellised accumulations of weakly stratified, clast-supported, imbricated gravels, with coarse clast-supported lag gravels
max. clast size
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unconformable on dipping Miocene strata Fig. 7. Sketch section through proximal alluvial fan sediments in a complex fan sequence at Matakanui (Fig. 2).
630
recoverable gold is commonly highly irregular in shape, coarse (up to 2 cm), and is presumed to have a substantial authigenic component (see above). Topographic evolution of the ranges in space and time
The Central Otago range topography can be broadly subdivided into areas which have undergone different degrees of uplift, and therefore different degrees of dissection of the unconformity and the underlying schist. These subdivisions, which are outlined below in order of increasing uplift, grade from one to another spatially along and across the ranges. The more uplifted portions of the ranges have also evolved with time through different topographic subdivisions. Amount of uplift of the ranges increases from east to west, so little remains of the unconformity on the Dunstan Range (Fig. 2).
Preserved unconformity The Miocene unconformity is largely preserved in those portions of ranges which have undergone little uplift. The topographic slope of these areas is generally around 7 ° or less. These areas are predominantly on the western sides of the ranges, but are locally on the crests and eastern sides of the ranges as well (Fig. 2). The exposed unconformity consists of highly kaolinised schist on very gentle slopes and oxidised and weakly kaolinised schist on steeper slopes. A characteristic feature of many parts of the preserved unconformity is widespread occurrence of dismembered silicified Manuherikia Group conglomerate, common, for example, along the western slopes of Rough Ridge (Fig. 2). This silicified conglomerate breaks up on erosion into approximately equant boulders typically from 20 cm to 2 m across. These limonite-stained boulders, or "sarsen stones", are scattered widely over low-slope hillsides, and locally accumulate in the small stream channels on these hillsides. The gold found on the preserved unconformity areas consists of two populations: finer grains with the morphological characteristics of detrital
P . C R A W A N D J.l [. Y O U N ( I S Q N
Manuherikia gold; and a rough and irregular component which exhibits most of the characteristics of authigenic growth (Fig. 4). Coarse gold is typically found in the upper 10 cm or so of the thin ( < 50 cm) lithic soil and, less frequently, on the unconformity surface. Gold nuggets, up to 1 ounce (troy), are found amongst the sarsen stones in the associated soil. Significant gold accumulations are rare but are found at or near the base of former cracks opened during erosion of silicified conglomerate sheets and in schist joints or fractures which traverse the slope. At one locality where the unconformity has intersected a mineralised vein in the schist, some spectacular wire gold can be found in cavities in the oxidised vein, and also in the adjacent soil where gold has overgrown angular schist detritus (Fig. 4),
Degraded unconformity Preserved unconformity areas grade laterally into slopes underlain by degraded unconformity. Large areas of the range surfaces consist of this degraded unconformity (Fig. 2), from which the altered schist and overlying sediments have been stripped by erosion during uplift. These surfaces are dotted with tors of more resistant schist up to 10 m high. Degraded unconformities occur over a wide range of slope angles, including the more or less flat range summits, and have an overall convex profile (Fig. 3). Soil is typically thin (5-30 cm) and highly lithic. Hollows and unchannellised basins are common, and these are partially filled with soil creep and mass flow deposits. Small channels have formed on the steeper parts of the degraded unconformity, and portions of these act as sites of accumulation for mass flow and channellised gravel deposits up to 1 m or more thick. Gold is haphazardly distributed through soils in the hollows and unchannellised basins and on the degraded unconformity surface in general. Small concentrations occur locally in the channellised gravel deposits and several such deposits were worked by early miners. The morphological characteristics of the gold are similar to those for gold on the preserved unconformity, with coarse and irregular particles relatively common. The degraded unconformity deposits and their con-
E L U V I A L G O L D PLACER F O R M A T I O N ON A C T I V E L Y RISING M O U N T A I N R A N G E S
tained gold represent a transition between the preserved unconformity areas and the proximal margins of the single fans described below. In some cases, however, degradation has not advanced sufficiently to form a down-slope fan.
Single fans Degraded unconformity surfaces grade laterally and down-slope into steeper range fronts from which some of the fresh schist basement has been removed. The slope profile of these areas is still convex, and thin (typically < 5 m) fan deposits extend from the slopes of the range into the basin. These thin fan gravels are well developed on the eastern margin of the Raggedy Range and on less uplifted portions of the Dunstan Range (Fig. 2). Discrete channels up to 3 m deep and 10 m wide with steep sides have been cut into schist and Manuherikia Group across the range margins. Channels are traceable for hundreds of metres from unchannellised basins at their heads, downslope to low-angle fans which merge with valley fill. Sediment in the channels is massive to poorly stratified schist gravel (see above) with minor quartz gravel components especially near the base. Mass flow material continues across interfluves, and some channels are hidden by overlying matrix-supported slope wash. Most of the gold in the fans is coarse, irregular, and presumably dominantly authigenic, although a significant fine (recycled Manuherikia Group) component is present also. The gold is concentrated almost exclusively in the channel lag deposits, at internal unconformities and particularly at the base of the channels. Gold has been extracted from many of the fans by early miners, who generally ceased operations when the Manuherikia Group was reached.
Complex fans The steepest parts of the ranges have an apron of fan sediments extending from the lower schist slopes out into the adjacent valley. These fans are most common on the slopes of the Dunstan Range (Fig. 2). The fans are complex sequences of conglomerates which have formed progressively as
631
the ranges rose. Early-formed parts of the fans have been deformed and are unconformably overlain by younger fan conglomerates which are commonly channellised into the underlying strata. The fan sediments become younger up-section, with locally recognizable stratigraphic order. Gold in the complex fans is concentrated in lag gravels at unconformities within the fans, as in single fans (above). Most of the gold is highly irregular in shape, commonly with delicate surface textures indicative of authigenic precipitation. Some coarse grains have been rounded by recycling, and further addition of authigenic gold has occurred. Nuggets up to 2 cm across occur in the youngest gravel sequence.
Major streams A small number of major streams emanate from, or cross, the ranges. Those which emanate from the ranges have large concave drainage basins with steep headwalls and deeply incised channels. Stream gradient decreases downstream, and streams emerge on to broad low-angle alluvial fans. Fans consist of well-stratified, poorly sorted, fresh schist debris with clasts up to 30 cm. The fans merge downstream with terraces associated with the axial rivers. Fans do not extend sufficiently far, however, to contribute significantly to either the terraces or the axial river bed load. Rivers which cut through the ranges (e.g., Pool Burn, Manuherikia River, Fig. 2) are antecedent, and form deeply incised gorges in which the streams maintain low gradients. Bed load in and below the resultant gorges has a low schist content (< 10%) and is dominated by greywacke cobbles derived from the stream headwaters. Significant gold concentrations are relatively rare in the deposits of the major streams, but minor concentrations occur at unconformities within the fans, and some gold is dispersed through the fan gravels. Discussion and conclusions: eluvial gold concentration
The above descriptions allow some generalisations of the processes of eluvial gold mobility and
632
concentration for the Central Otago region. These are: (1) Chemical gold mobility and authigenic growth of grains is a common phenomenon which is important in grain size increase in the eluvial deposits. (2) Eluvial gold placers form where topographic slopes steepen due to progressive uplift. The placers form on degraded unconformity areas and steeper convex slopes on which single and complex fans have formed. These fans contain a high proportion of matrix-supported mass flow material. (3) Physical concentration of gold occurs in surficial deposits eroded from the degraded unconformity, and gold is found primarily in coarse clast-supported lag gravels. (4) Eluvial placer formation ceases when further uplift and erosion has caused development of substantial concave streams which erode below the unconformity into fresh schist. Influx of this schist debris, without gold, dilutes potential placer deposits in the early stages of stream evolution. Investigation of the first of these identified processes revolves around groundwater chemistry, and is the subject of further study. The last process represents the transition from the eluvial environment to the truly alluvial environment where placer formation, farther downstream, is better understood (Adams et al., 1978; Nami and James, 1987; Dyson, 1990). However, the processes of physical gold concentration in an eluvial environment in which mass flow is clearly an important transport mechanism is more problematical, and is discussed further in the following section.
Heavy mineral concentrations in steep mountain streams The Central Otago eluvial placers are concentrated in lag deposits in which the predominant clast size is greater than 10 cm. The cobbles are angular to subrounded, and clearly have not been subjected to significant water transport. The clasts have interstitial sand-silt sized silicates and magnetite, and variable gold grain sizes up to 2 cm. Similar associations of placer gold concentrations
D, ( ' R A W A N D ,I.H ~c,)[JNI.IS()N
with coarse lag gravels have been noted commonly elsewhere (e.g. Cheney and Patton, 1967: Boyle, 1979). Most experimental studies of placers have used magnetite as the heavy mineral to be concentrated (e.g. Adams et al., 1978). as magnetite is readily transported in streams with silicates. However, there is a very large difference in specific gravity between magnetite (S.G. = 5.2) and gold (S.G. = 19), and it is unlikely that gold will respond in the same manner as magnetite in an alluvial system. The following simple model is intended to compare likely responses of silicates, magnetite and quartz to alluvial processes in a steep mountain stream. The model assumes thal a stream develops in mass flow deposits on a mountain slope, and the material available for transport has a very wide range of grain sizes, from boulders to mud. The finer-grained silicate material is assumed to be moved by the stream while the coarse cobbles are not moved. The model differs from models of alluvial systems in which the whole bed becomes mobile in occasional floods (Day and Fletcher, 1991 ). The coarse cobbles accumulate to form a lag with portions of cobbles protruding from the bed which protect small particles from entrainment (Slingerland and Smith, 1986; Komar, 1987). The shear stress, related to the stream discharge, required to mobilise clasts of various sizes can be determined theoretically from the Shields equation: L = Tc* ( d s - d w ) g D where Tc is the critical shear stress for initial bed material movement, d S and d w are the densities of bed material and water, g is the acceleration due to gravity, and D is the particle diameter (e.g. Bathurst, 1987). To* is the Shields parameter, or dimensionless critical shear stress, whose magnitude depends on the ratio of particle dianaeter to the median particle diameter in poorly sorted sediments. Values of T~* can be calculated from experimental and natural studies, although there is disagreement over exact relationships (Komar, 1987). Large density differences between gold and other minerals dominate the Shields equation calculations, so we use the 1~.* relationship of Andrews (1983) for simplicity.
ELUV1AL
GOLD
PLACER
FORMATION
ON ACT1VELY
RISING
MOUNTAIN
A median particle diameter=lO mm
.80
(D L.
-40
co
I
0.01
0.1 I
_4o
B
median particle diameter=l mm
I
I
1
10
I
100
/ma 7 I
-20
particle diameter/median particle diameter Fig. 8. Critical shear stress (vertical axis) requirecJ to move particles of differing grain sizes (horizontal axis) in poorly sorted sediment with (A) median grain size = 10 mm; and (B) median grain size = 1 ram. Curves for quartz, magnetite and gold are presented in each diagram.
Model curves (Fig. 8) depict theoretical shear stresses required to mobilise different grain sizes in poorly sorted gravels. Curves are presented (Fig. 8) for two different median grain sizes: 10 mm and 1 ram, which encompass the typical Central Otago eluvial deposits. The significance of the curves is that for sediments such as those present in Central Otago surficial deposits, most grain sizes are mobilised at about the same shear stress, leaving only the coarse cobbles (Andrews, 1983; Komar, 1987). Similar curves drawn for gold shows that shear stresses which are capable of moving most schist debris are not capable of moving typical eluvial gold grains, especially for sediments with median particle diameter of 10 mm (Fig. 8). The curves for magnetite lie close to those of quartz. The model provides some theoretical basis for the field-based inference that gold apparently
RANGES
633
accumulates residually by removal of all but the coarse silicate debris ("winnowing", Slingerland and Smith, 1986). Magnetite apparently behaves in a similar manner to silicate material with similar grain size, and can be readily mobilized (Fig. 8). Hence, magnetite and the fine-grained silicate material are probably deposited after entrainment and transport, in voids between lag clasts (Reid and Frostick, 1985; Day and Fletcher, 1991). Thus, in the eluvial environment, magnetite and gold concentrations occur by different mechanisms. Gold concentration by this "winnowing" mechanism requires a constant supply of goldbearing sediment which is presumably the matrix-supported mass flow deposits so commonly found in unchannellised and locally channellised basins on the uplifting unconformity (cf. Dietrich et al., 1986; Alger and Ellen, 1987). The mass flow deposits are continually degraded during flood discharges to concentrate gold until the resultant lag is buried by stream activity or new mass flows. A new lag then develops higher in the section until that too is buried. In this way a fan is built up as the range rises• Channelling of pre-existing fans can occur after continued uplift, and this results in recycling of gold by stream and mass flow action into the new channel as a complex fan is built up. An important corollary of the conclusions outlined above is that authigenic grain-size increase causes entrapment of gold in the eluvial environment. Only the finer-grained gold is transported downstream into the truly alluvial environment to form placers.
Implications for other placer deposits All detrital gold must pass through an eluvial stage at some time. Grain size increase by secondary processes is an important step in ensuring entrapment of gold in the source region. Grain size increase may have been hindered if the primary veins were uplifted and eroded very rapidly, such as the high-rainfall sides of orogenic belts (Eyles, 1990; Koons and Craw, 1991), or if the groundwater chemistry was not favourable for gold mobility (perhaps in a low-oxygen atmosphere, e.g. Witwatersrand, Krupp and Weisser, 1992). However, in most tectonic environments
634
some or all of the eluvial processes discussed in this paper will have occurred. This is especially true for slow to moderate uplift rates in semi-arid regions analogous to Central Otago, such as the numerous circum-Pacific young giant placers including the Mother Lode district of California and the Klondike district of northern Canada (Henley and Adams, 1979). The eluvial processes discussed above ensure that a large proportion of available detrital gold is trapped in the source region, and only the finest-grained fraction is released to the alluvial system downstream. With continued uplift, eluvial placers will ultimately be eroded and recycled into younger alluvial systems to form placer deposits with coarse-grained irregular nuggety gold (Boyle, 1979; Henley and Adams, 1979). The gold will move only short distances during this recycling because of its coarse grain size. Glaciation may cause transport of the coarse gold farther downstream, but fluvial outwash processes can result in reconcentration of a coarse gold residue by winnowing, with little or no transport (e.g. Eyles and Kocsis, 1989). Hence, we suggest that the distinction between the two main types of placer deposits (discussed above) is due primarily to eluvial processes (or lack of them) in the source region.
Acknowledgements This work was funded by the University of Otago Research Committee and a University of Otago Division of Sciences student bursary. Discussions with B.J. Douglas, C.A. Landis and J.K. Lindqvist helped to clarify some of the ideas in the paper, although they do not necessarily agree with these ideas. Constructive reviews of the manuscript by N.D. Smith and an anonymous referee considerably improved the presentation.
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