Contrasting alteration mineralogy at an unconformity beneath auriferous terrestrial sediments, central Otago, New Zealand

SEDIMENTARY GEOLOGY ELSEVIER

Sedimentary Geology 92 (1994) 17-30

Contrasting alteration mineralogy at an unconformity beneath auriferous terrestrial sediments, central Otago, New Zealand D. Craw Geology Department, Universityof Otago, P.O. Box 56, Dunedin, New Zealand

Received July 26, 1993; revised version accepted January 19, 1994

Abstract

A regional unconformity cut into the Otago Schist belt in southern New Zealand is overlain by auriferous terrestrial sediments. Formation waters from the overlying sediments have reacted with the schist basement, resulting in a thick (up to 20 m) zone of intensely kaolinitized basement immediately below the unconformity. Primary rock textures are preserved throughout this alteration zone. Two distinct types of alteration zone have developed. Non-oxidizing alteration resulted in kaolinitization of muscovite, but some albite and chlorite was preserved even in the most altered rocks. Chlorite locally altered to ferrous-iron-bearing smectite-vermiculite during non-oxidizing alteration, and this unusual mineral occurs in basement and overlying sediments. Oxidizing alteration has resulted in degradation of almost all schist minerals, leaving kaolinite and goethite, with relict muscovite and quartz. A distinctive 12 A interlayered clay mineral occurs in altered schist and overlying sediment at one locality. Oxidizing alteration is responsible for localized gold mobility in basement gold deposits. Non-oxidizing fluids have mobilized detrital gold in auriferous terrestrial sediments. The formational waters were mobilized during late Cenozoic regional deformation of the unconformity and overlying sediments.

1. Introduction

Regional flow of formation waters in the terrestrial environment is now well established (e.g. summary by Garven, 1989). T h e r e is also widespread evidence for mobility of metals in formation waters in terrestrial sediments (Hoeve and Sibbald, 1978; Langmuir, 1978; Webster and Mann, 1984). Information on solution chemistry in some rocks can be obtained from mineral equilibria where there has been significant water rock interaction (Garrels and Christ, 1965; Merin and Segal, 1989). However, mature terrestrial sediments present a particular problem for deter-

mination of formation water chemistry, because these generally contain few minerals which can interact with formation waters. This p a p e r takes an alternative approach by examining mineralogical and chemical changes which have occurred in the underlying basement and clasts of basement in basal sediments. F r o m these diagenetic changes, some gross chemical characteristics of the terrestrial formation waters can be deduced. This study examines a very well preserved regional unconformity of Miocene age developed on the Otago Schist belt in the South Island, New Zealand (Fig. 1). The unconformity intersects numerous gold deposits in the schist belt, and alter-

0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0037-0738(94)00026-Q

18

D. Craw / Sedimentary Geology 92 (1994) 17-30

ation at the unconformity has b e e n instrumental in making some of these deposits economic in the past. T h e unconformity is overlain by auriferous terrestrial sediments in which gold mobility has been d o c u m e n t e d in several localities (Craw, 1992; Y o u n g s o n and Craw, 1993). As yet little is known about the chemistry of the gold-mobilizing fluids, partly because some of the sedimentary

hosts are m a d e up almost entirely of unreactive quartz and kaolinite (Craw, 1992). T h e unconformity alteration has b e e n ascribed to weathering prior to deposition of the v e n e e r of terrestrial sediments (Benson, 1935; Stirling, 1991), but arguments are presented below to show that the alteration was substratal and diagenetic (cf. Palmer et al., 1989; Nesbitt and Young, 1989).

•":. :. .-. ;.: ...'.-..-

i . - " "..

Alexandra 1~37 Roxburgh ~1)d2071 N v d2001 -,, ~ ~ 6

J OTAGO SCHIST

~Bamwood

\ \

t i %1o~roj,~,_~\,"

tnedin South Island New

Zealan~

sOtc~ig~t

~)

Fig. 1. Locality map showing the regional geological features of Otago. The unconformity is preserved beneath the cover rocks (stippled). Localities in which non-oxidized alteration mineralogy has been observed are shown with squares; oxidized localities examined in this study are indicated with small diamonds. Large triangles denote oxidized gold deposits in the basement schist. Numbered localities and drillholes are referred to in the text. Inset shows the location of the study area (square) in the South Island of New Zealand.

D. Craw / Sedimentary Geology 92 (1994) 17-30

Hence, this study has implications for studies of altered regional unconformities observed elsewhere in the world (Hoeve and Sibbald, 1978; Pinto and Holland, 1988), many of which are metamorphosed so that original mineralogy and textures are now obscured (Button and Tyler, 1981; Craw and Findlay, 1984; Nesbitt and Young, 1989).

2. Regional geological evolution The Otago Schist is a belt of Mesozoic prehnite-pumpellyite to greenschist facies schist which underlies about 10,000 km 2 of Otago, New Zealand. The schist is predominantly derived from quartzofeldspathic and volcanogenic greywackes which have been thoroughly recrystallized to the assemblage quartz, albite, muscovite, chlorite, epidote, titanite, and minor pyrite or pyrrhotite, with prehnite and pumpellyite in the lower grade portions, and rare biotite in the higher grade portions. The parent greywackes are preserved on low metamorphic grade margins to the north and south of the schist belt. Pervasive ductile deformation accompanied metamorphism of the schist throughout the belt, resulting in a penetrative flat-lying foliation. Greenschist facies schist is segregated into discontinuous micaceous and quartzofeldspathic lamellae from 1 to 10 mm wide. Native gold occurs in metamorphogenic quartz-pyrite-arsenopyrite vein deposits which formed periodically during uplift of the central portion of the schist belt (Craw and Norris, 1991). Most of these veins are steeply dipping, and cut the regional foliation at a high angle. The central portion of the schist belt, the subject of this study, is unconformably overlain by Miocene to Recent terrestrial deposits. The Miocene sediments (Manuherikia Group) which rest on the unconformity consist of basal fluvial sediments with locally substantial lignite deposits, overlain by lacustrine sediments of a Miocene lake complex (Douglas, 1986). Schist and sediments have been disrupted by Late Miocene folding and thrust faulting on a rectilinear pattern striking northeast and northwest (Beanland and Berryman, 1989; Stirling, 1990). These faults are

19

still active, and are resulting in broad antiformal ranges separated by synformal basins. Miocene sediments have been stripped off the ranges, but are still preserved in the basins between the ranges. As the ranges rise, aprons of immature schist debris spread o f f the eroding range fronts into the basins, and some of these fans have been recycled by further uplift and erosion (Youngson and Craw, 1993).

3. Manuherikia Group The Miocene auriferous fluvial sediments (Dunstan Formation, Douglas, 1986) which rest on the regional unconformity consist of gravels, sands, silts and muds which are poorly sorted and have matrix muscovite, kaolinite and minor smectite. Clasts are well rounded quartz, and rare more angular schist near the unconformity. Low sulphur lignite occurs within the fluvial sequence as seams UP to 55 m thick. Channels from 10 m to 10 km wide are cut into the underlying schist and are filled with quartz gravels (Douglas, 1986). The largest of'these channels, the St Bathans palaeovalley (Douglas, 1986), is traceable longitudinally for ;more than 20 km, but has been disrupted at both ends by late Cenozoic deformation. Channel sediments near the base of the sequence contain rare schist fragments which are rounded or sub-rounded, and are typically kaolinitized. Detrital gold is concentrated in these channel sediments. Lacustrine sediments (Bannockburn Formation, Douglas, 1986) locally interdigitate with, and overlie, the fluvial sediments. The Manuherikia Group, which includes the Dunstan and Bannockburn Formations, is up to 200 m thick over much of the area of this study (Fig. 1)

4. Regional unconformity The unconformity described in this paper is between the schist/greywacke basement and the overlying Miocene fluvial sediments. This unconformity is preserved beneath the sediments in the faulted and synclinal basins, where it can be observed in drillholes (Fig. 1), and crops out

20

D. Craw~Sedimentary Geology 92 (1994) 17-30

Fig. 2. Photomicrographs of altered schist immediately beneath the unconformity. Horizontal field of view is 4 mm; plane polarized light. (A) Non-oxidized alteration zone exposed in gold-mining excavations at Pennyweight Hill (Locality 4, Fig. 1), about 6 m structurally below the unconformity. Relict albite (a) occurs with quartz (q) in leucocratic segregations. Muscovite has been extensively kaolinitized (centre), but chlorite (c) is preserved. (B) Oxidized alteration zone near Alexandra (Fig. 1), about 10 m below the unconformity. Quartz (q) is preserved in original texture, but albite has been kaolinitized. Dark patches and veinlets are goethite. Muscovite (m) has been partially kaolinitized.

D. Craw/Sedimentary Geology 92 (1994) 17-30

along the margins of these basins. The unconformity surface has been stripped from the intervening ranges with the sediments, but the form of the unconformity is still preserved by the topography of the broad ranges of central Otago (Stirling, 1991; Youngson and Craw, 1993). The unconformity is parallel or sub-parallel to the underlying schist foliation throughout the schist belt (Stifling, 1990). There was little relief (< 50 m) on the unconformity over much of the area, but local relief as much as 600 m can be demonstrated (Douglas, 1986). The unconformity is marked in most places by a zone of highly altered schist up to 20 m thick. Schist structures such as foliation, folds and veins are preserved intact in the altered zone (Fig. 2). Schist alteration increases towards the unconformity surface, and at the surface it is commonly difficult to recognize primary schist features other than a faint foliation and relict segregations. The main alteration mineral is kaolinite, which replaces primary schist minerals. There are two mineralogically distinct alteration assemblages which occur in different outcrops due to nonoxidizing and oxidizing alteration, and oxidizing alteration on gold deposits forms a third mineralogical assemblage.

5. Non-oxidizing alteration Good exposures of these zones are relatively rare, as surface outcrops become oxidized due to modern weathering. However, a number of anthropogenic outcrops and drillholes (Fig. 1) provided material for this study. Grey friable altered schist extends up to 5 m below the unconformity at these localities, with the proportion of clays in the schist increasing towards the unconformity. Primary schist structures such as folds and foliation are well preserved in these alteration zones right up to the unconformity where the most altered schist consists of up to 30% kaolinite which is concentrated in micaceous laminae. Quartz veins and quartz-rich segregations retain their original structural integrity (Fig. 2A). Kaolinitization is centred on muscovite, which is altered by direct replacement along cleavages,

21

resulting in interlayered muscovite and kaolinite on the submicron scale (Fig. 3a; Craw et al., 1982). Kaolinite pools and veins up to 1 mm wide occur along and across quartz laminae and quartz veins. Smectite occurs in the clay fraction in X-ray diffraction analyses, but was observed in a few thin sections only (see below). The proportion of smectite increases towards the unconformity, and forms an important (but unquantifiable) component of the most altered rocks. Albite and chlorite (Fig. 3b; Table 2) remain unaltered in most outcrops, even when the surrounding muscovite is intensely altered, but albite content decreases in the most altered samples. Goethite is absent, and pyrite or pyrrhotite grains are commonly scattered along foliation planes. Some pyrite appears to cut the foliation and may be authigenic. Smectite-vermiculite. Altered rock taken from drillholes up to 2 m beneath the unconformity at Roxburgh (Fig. 1) has chlorite altered to coarsegrained iron-bearing interlayered smectite and vermiculite (Craw, 1984). Microprobe analyses of the interlayered material show that the composition is highly variable, with silica content ranging from 27%, which is chlorite-like, to 45%, which is kaolinite-like. Iron content varies from 10% to 30%, and bulk analyses of mineral separates suggest that about two-thirds of the iron is in the ferrous state (Craw, 1984). The interlayered structures are intermediate minerals formed during alteration of chlorite to kaolinite under nonoxidizing conditions (Fig. 3b; Craw, 1984). The same smectite-vermiculite mineral occurs in basal sediments immediately above the unconformity in the Roxburgh drillholes, as part of the clay fraction in the matrix, and as coatings on partings in the sediment. A similar interlayered smectite-vermiculite mineral occurs in deformed Dunstan Formation sediments immediately above the unconformity at Matakanui (Fig. 1). This locality lies astride an actively deforming range front, and the structure is complex (Walcott, 1988). Chemical analyses of this material are very similar to the Roxburgh material, and the response of the minerals to various X-ray diffraction tests is identical (Craw, 1984; Walcott, 1988). The host sediment is

D. Craw/Sedimentary Geology 92 (1994) 17-30

22

(a)~0•2

strongly sheared, and consists of deformed quartz clasts (coarse to fine sand) in a deformed matrix of kaolinite and muscovite. The smectitevermiculite material is coarse-grained (up to 0.2 mm) and crosscuts the deformed rock matrix and forms veins in quartz clasts. Kaolinitized schist below the unconformity at Matakanui has been highly sheared also, and no smectite-vermiculite was found. A similar smectite-vermiculite mineral also occurs authigenicaUy in auriferous Pleistocene immature schist fan gravels at Matakanui, structurally above the Dunstan Formation deposits (Youngson and Craw, 1993).

6. Oxidizing alteration ci

o

kaolinite

Fig. 3. Ternary plots of cationic proportions from microprobe analyses by energy dispersive system, 15 kV accelerating voltage and 0.02 /xA specimen current. (a) Muscovite Si, K, and AI from non-oxidized alteration zone at Locality 3 (Fig. 1). The analyses (small squares) fall on a trend between unaltered muscovite (large square) and ideal kaolinite, due to interlayering of kaolinite and muscovite on a scale finer than the microprobe beam (about 5 /~m). (b) Green phyllosilicate Fe, Mg, and AI from non-oxidized alteration zones at the numbered localities (Fig. 1). Fresh Otago Schist chlorite (Brown, 1967; Craw, 1981) is shown for comparison, and relict chlorites from localities 3 and 36 have essentially the same composition. Smectite-vermiculite from Roxburgh (Craw, 1984) defines a trend towards ideal kaolinite.

This most common type of alteration forms distinct white and orange zones at the unconformity in natural and anthropogenic outcrops. Areas up to several square kilometres of oxidizing alteration occur as natural outcrops where the unconformity is fiat-lying. There is a progression in degree of alteration towards the unconformity from below, over 1 to t0 m. Incipiently altered schist has chlorite oxidized from green to brown (cf. Brown, 1967; Craw, 1981), and albite is partially altered to kaolinite. There is a gradation towards the unconformity to more intensely altered schist in which chlorite has altered to swelling clays (beidellite and vermiculite; Churchman, 1978, 1980), and albite is fully kaolinitized. Muscovite is incipiently altered to kaolinite along cleavages. Nearer the unconformity, chlorite and swelling clays are replaced by kaolinite, goethite, and minor carbonate. The carbonate contains up to 10% iron, either as a siderite component to the calcite structure, or as a physical mixture of calcite and goethite. Muscovite is further degraded near the unconformity, but some muscovite remains in even the most altered zones. Quartz veins and quartz-rich segregations maintain their integrity approximately, through the most intense alteration, but some disaggregation of these has occurred due to local dissolution of quartz and replacement of albite by kaolinite• Immediately below the unconformity, the schist consists of kaolinite and goethite, with relict foliation de-

D. Craw/Sedimentary Geology 92 (1994) 17-30 Table 1 X-ray diffraction and electron optical properties of 12 ,A clay minerals, Bannockburn, central Otago Sample treatment

Diffractogram response

Dry Water 10% HCI, 1 h 10% HCI, overnight Saturated KCI Glycerol Ethylene glycol

12.1,A sharp; broad 6 (merges with kaolinite 7 ,~) no change no change no change no change no change no change

300°C, 1 h 600°C, 1 h 750°C, 1 h

12 ,~ collapses to 9.8 ,~ 9.8 .A unchanged 9.8 ,~ intensity decreased

Electron microprobe qualitative analysis of clay fraction: elements present in appreciable amounts: Si, AI, K, Mg, Fe.

Scanning electron microscopy: flakes only observed. Transmission electron microscopy: flakes only observed.

fined by quartz-rich lamellae and remnants of muscovite flakes. Localized goethite-rich zones form bright yellow a n d / o r red bands about 30 cm thick up to 1 m below the unconformity at some localities Bannockburn clay mineralogy. The oxidized unconformity at Bannockburn (Fig. 1) consists of about 10 m of variably kaolinitized schist. The unconformity is overlain by 10 m of lignite-bearing fluvial sediments, and then at least 30 m of lacustrine sediments. A distinctive but poorly characterized clay mineral (12 A) forms a significant proportion of the clay fraction of the kaolinitized zone immediately ( < 2 m) below the unconformity. The same mineral is also found, with kaolinite, in the clay fraction of fine sandy and silty sediments up to 5 m above the unconformity. Properties of this mineral and the clay fraction are listed in Table 1. Magnesium is the only element detected which is not found in the other minerals known to be present (oquartz, kaolinite, goethite, muscovite), so the 12 A mineral is presumed to be Mg-bearin~. The electron optical data suggest that the 12 A mineral is a phyllosilicate, so sepiolite can be discounted. The properties summarized in Table 1 are not consistent with any one common clay mineral (Brindley and Brown, 1980). This mineral is more likely to be a

23

randomly interlayered phyllosilicate, although the identities of the interstratified minerals are not clear. Interstratified muscovite or illite and nonswelling vermiculite is aoPlausible possibility. A similar unidentified 12 A mineral has been described from other altered schist localities (Fieldes et al., 1974).

7. Oxidizing alteration of gold-bearing vein systems

Gold-bearing deposits consist of quartz veins and hydrothermal breccias of host schist in fault zones 1-5 m wide which dip at an angle to, and are truncated by, the unconformity. Mineralized fault rocks are locally silicified and minor kaolinite a n d / o r chlorite occur as hydrothermal minerals up to 1 m from large veins. Pyrite and arsenopyrite, with minor sphalerite, chalcopyrite and galena, constitute up to 20% of veins and also occur as replacement minerals in schist breccia fragments and immediate host rock. Gold is almost invariably associated with these sulphides, as micron-scale inclusions, veinlets, and rare free grains up to 0.5 mm attached to coarse sulphide grains. Mineralized zones are oxidized and kaolinitized down to about 20 m below the unconformity. Host schist is extensively kaolinitized, as described in the previous section, so that no distinction can be made between hydrothermal kaolinite (minor) and unconformity-related alteration kaolinite. Chlorite is oxidized and variably altered to goethite, kaolinite a n d / o r smectites in incipiently altered rocks, and is completely replaced by kaolinite near the unconformity. Muscovite, which commonly survives as relict grains during hydrothermal silicification, is incipiently kaolinitized immediately beneath the unconformity. AIbite does not survive intense hydrothermal alteration, but is locally preserved in less hydrothermally altered rocks near the unconformity. Sulphides have been largely altered to goethite and scorodite, although relict sulphide cores to goethite knots occur right to the unconformity in some outcrops, particularly in silicified schist. Goethite and kaolinite veins and fracture coat-

D. Craw~Sedimentary Geology 92 (1994) 17-30

24

ings are common. These are locally accompanied by scorodite and rare pharmacosiderite (Craw and Mackenzie, 1992). Gold has been remobilized during oxidation and reprecipitated with kaolinite and goethite (and rarely arsenates) in veins and on fractures. Reprecipitated gold is typically lower in Ag content than primary gold (Craw and Mackenzie, 1992). Oxidation of gold-bearing vein systems has been important in economics of gold mining in Otago. Many of the mineralized zones (Fig. 1) were mined primarily in the surface alteration zones, and mining became uneconomic as unoxidized rock was reached (Park, 1907; Finlayson, 1908). This change in economics arose partly because of minor supergene enrichment due to gold mobility (Park, 1907), but mainly because of release of gold from sulphides and gold grain size increase which facilitated recovery (Craw and Mackenzie, 1992).

8. Whole rock geochemistry of altered zones

Four oxidized and four non-oxidized samples were selected to show differing degrees of alteration, and these were analysed by X-ray fluorescence (Table 2). Most major element concentra-

tions of altered rocks are similar to typical unaltered schist, but there are general increases in A120 3 and loss-on-ignition (LOI) for both types of alteration, and minor decreases in K20 and Na20 in some samples (Fig. 4). The increases in AlzO 3 and LOI presumably reflect the increasing proportion of kaolinite in the rocks, and are compensated for by minor but statistically insignificant decreases in the other major elements. The alkalis are more strongly depleted by the oxidizing alteration process, first by leaching of Na during destruction of albite, then by leaching of K during kaolinitization of muscovite. Sodium is ultimately leached from non-oxidized samples as albite is partially degraded, and some Na is derived from the degraded muscovite which contains up to 0.7 wt% Na20 (Brown, 1967). Potassium is not significantly depleted from nonoxidized samples in spite of muscovite degradation, and is presumably contained in smectites. Oxidized and locally mineralized samples from Barewood were collected from a drillhole 1 m apart down to 18 m, and four fresh samples were taken down to 28 m. These samples show little deviation from typical schist values (Fig. 4). The most altered zone near the unconformity coincides with the most hydrothermally mineralized zone, and some of these samples show minor

Table 2 X-ray fluorescence analyses of altered Otago schist beneath the regional unconformity at localities indicated in Fig. 1 Locality: Pennyweight Hill (4) Alteration: nonoxidized Alt. degree: weak Sample: DC UG6 SiO 2 TiO 2 A120 3 Fe20 3 MnO MgO CaO Na 20 K20 P205 LOI Total

69.52 0.52 14.99 3.36 0.05 1.51 1.79 4.02 2.00 0.14 2.09 100.00

St Bathans

Pennyweight Hill (4) nonoxidized strong DC UG4

Alexandra

Alexandra

nonoxidized moderate DC UG9

Wedderburn (3) nonoxidized strong DC UG3

oxidized weak DC UG15

71.25 0.59 15.27 3.49 0.04 1.27 0.14 2.47 1.89 0.02 3.51 99.95

66.39 0.77 17.36 5.18 0.08 1.87 0.06 0.36 3.00 0.01 5.07 100.16

68.29 0.63 17.69 2.42 0.03 1.23 0.39 1.18 3.19 0.17 4.60 99.83

62.83 0.86 18.51 6.98 0.05 0.92 0.11 0.32 3.45 0.07 5.75 99.86

All iron is calculated as ferric in oxidized rocks, and ferrous in non-oxidized rocks.

oxidized moderate DCUG16

Bannock burn oxidized strong DC UG22

Bannock burn oxidized strong DC UG20

64.69 0.62 20.78 3.39 0.10 0.73 0.2 0.65 2.39 0.07 6.82 100.43

63.91 1.32 22.73 1.55 0.01 0.83 0.49 0.21 1.66 0.00 7.58 100.29

60.63 1.16 22.42 5.09 0.05 0.56 0.14 0.44 1.24 0.03 8.37 100.14

D. Craw/Sedimentary Geology 92 (1994) 17-30

depletion of alkalis (Fig. 4). This alkali depletion is partly due to passive dilution by silica flooding, and partly due to leaching during kaolinitization as in the oxidized alteration zones described above.

9. Comparison of oxidizing and non-oxidizing alteration

The most obvious difference in outcrop between these different alteration zones is the preservation of ferrous iron in chlorite, pyrite (or pyrrhotite) and smectite in grey non-oxidized rock, whereas oxidized rock has only ferric iron in brown chlorite and goethite. The other main diflOSS o n 8

[]

ignition, %

oxidized/_

/ 6

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=

.

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Barewood Au •

2

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| 4

I 6

K20,

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J

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~ I 12

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%

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2 [] • [] 1

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non-oxidized I

I 2

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10. Chemistry of alteration

e., ""

3qa2 0 ,

"'-" 1

ference is the preservation of some albite and partial degradation of muscovite in most nonoxidized rock, whereas in oxidized rock the albite is fully kaolinitized under incipient alteration and muscovite is kaolinitized only after intense alteration. The differences with respect to muscovite and albite degradation are shown by comparative X-ray diffraction patterns. Ratios of peak areas were obtained for muscovite (002), albite (002), and kaolinite (001) peaks in diffractograms of non-oxidized and oxidized samples (Fig. 5). AIbite/kaolinite ratios for non-oxidized rocks are sensitive to lithology, as shown by the separation of pelitic and psammitic schist points (Fig. 5) from a single sample (from Pennyweight Hill), but this sensitivity does not alter the alteration trend. These data show the two distinct alteration paths, which merge for the most altered rocks. These different alteration paths are difficult to distinguish by whole-rock analysis, except for alkalis (see above). Smectite is present in incipiently altered oxidized zones in small amounts, and with further alteration this smectite breaks down to kaolinite and possibly calcite. Smectite and calcite are absent from highly kaolinitized oxidized rock. In contrast, smectite is a common mineral in the most altered non-oxidized zones. Since chlorite is generally unaltered in the non-oxidized zones (Fig. 3b), smectite is presumed to have been derived from kaolinitization of muscovite which has a significant phengite content (Fig. 3a; Brown, 1967). Smectite-vermiculite has formed by alteration of chlorite in some localities (above).

•.,

-==~....

~'oxidized

25

I

I 4

•

% I

I

I

5

Fig. 4. W h o l e - r o c k geochemical plots f o r non-oxidized (dots)

and oxidized (open squares) samples (Table 2), for loss-onignition vs AI20 3 (upper figure) and alkalis (lower figure). Analyses from drill core from the Barewood gold mine (Mackenzie, 1990) are also plotted (diamonds). Fields for typical host schist (data from Palmer et al., 1992) are shown for comparison. Arrows indicate direction of increasing alteration.

Some estimate of the difference in chemistry of the alteration fluids, other than the obvious oxygen content, can be obtained from the differences in mineralogy of the altered zones. Relationships of relevant minerals are depicted in Fig. 6 in terms of common dissolved ions Na ÷, K ÷, and H ÷. The composition of typical near-surface fluid is approximated by river water (Fig. 6) which lies in the kaolinite field. Oxidizing alteration involves degradation of albite and partial degra-

D. Craw/Sedimentary Geology 92 (1994) 17-30

0 : non-oxidized zone • : oxidized zone 0

oxidizing

=: r I .~ r n j

o

I-1 0

,.,

I:

pelltlc

Pennyimlght Hill

==iP" OL,p tO

p~tlc

"

non-oxidizing alteration

,,

•

III

30

0. t

I

I elloite (002)

o

d2071

0.8 I

I

1.2 I

I

/ keotlnlte (001)

g. 5. Plot of ratios of X-ray diffractogram peak areas for muscovite, kaolinite and albite for oxidized (squares) and non-oxidized rcles) alteration zones. Data from the bottom and top of the oxidized altered zone in drillhole DL21 (Fig. 1) are indicated.

~tion of muscovite, and alteration fluid composi9n presumably evolved towards these minerals it remained in the kaolinite field (Fig. 6). Nonquartz saturated

log [Ha "] / [H"1

/

Alblte

8

/ non-oxidizing

~

4

P~tasslum

river

water m

xidizing

2 Kaolinite 2 I

Idspar

log [K +] / [H+J

4 I

6 18 !

I

g. 6. Plot of ionic ratios of Na +, K ÷, and H ÷ in solution at °C showing mineral stability fields in relation to river waters ~ack square) (after S t u m m and Morgan, 1981). Diagram is awn for quartz saturation; i.e. log activity of aqueous silica - 4 . Inferred fluid compositional changes associated with eration are indicated with arrows (see text).

oxidizing alteration leaves albite largely intact, but results in muscovite degradation to kaolinite, so the fluid composition during intense alteration lay near the albite/kaolinite boundary (Fig. 6). Preservation of albite requires a fluid which is either more alkaline, or more sodium-rich (or both) than typical surface fluid. The different alteration paths can also be expressed in terms of oxidation and pH change on an E h - p H diagram drawn principally for iron minerals (Fig. 7). Non-oxidizing alteration occurred within the stability field of pyrite and ferrous-iron-bearing phyllosilicates, and involved alkaline fluid. Oxidizing alteration involved replacement of iron-bearing minerals by goethite and kaolinite, initially with iron-bearing carbonate which was later redissolved. Oxidation of sulphides in the gold-bearing deposits would have resulted in localized acidification during alteration (Kwong et al., 1982), but ultimately this alteration ends with deposition of scorodite under near-neutral pH conditions (Craw and Mackenzie, 1992). The effect of sulphides would not be significant in unmineralized schist because

D. Craw~Sedimentary Geology 92 (1994) 17-30

of the very low sulphide content. Hence, three distinct alteration paths can be delineated (Fig. 7).

11. Alteration zones: weathering or post-sedimentation products? The alteration zones beneath the unconformity have traditionally been interpreted as sub-aerial weathering zones formed during Miocene erosion prior to sedimentation and burial (Benson, 1935; Stirling, 1991). However, relationships (discussed below) between alteration zones and overlying sediments suggest that alteration post-dates sedimentation on the unconformity. Also, the alteration zones lack soil structures (FitzPatrick, 1980; Retallack, 1988), and original rock textures can be seen in even the most altered rocks. Preservation of primary albite and chlorite while muscovite is kaolinitized is atypical of soil development processes (Nesbitt and Young, 1989).

27

11.1. Distinctive minerals in sediment and alteration zone

The 12 ,A mineral which occurs in altered schist and sediment at Bannockburn is unusual and distinctive. It is unlikely that this mineral could be present in both basement and cover without being formed simultaneously, i.e., postdepositionally. Likewise, the distinctive green ferrous-iron-bearing smectite-vermiculite is found in basement and cover at Roxburgh, and in two different ages of overlying sediments at Matakanui. We suggest that these unusual minerals were formed by similar processes operating on all rock types, and that these processes operated after deposition of the sediments on the unconformity. The Matakanui occurrence of the smectitevermiculite resulted from mineral growth during or after deformation of the sediment along the Dunstan Range front (Fig. 1). 11.2. Channelized zones

--•l •dissolved -+0.6 Fo3+

ito

~

---+0.2 dissolvod \ Fe2+ N rO.O " ~

ethite

pyrite

ilicates

"'0124 ~

- -0.6

2 I

4 I

6 I

pH

8 I

10 I

Fig. 7. E h - p H diagram for iron minerals at 25°C and 1 bar (from Garrels and Christ, 1965; Hem, 1977; Craw and MacKenzie, 1992), showing three inferred alteration trends based on minerals observed in non-oxidized schist (A), oxidized schist (B), and oxidized gold deposit (C) alteration zones. Iron phyllosilicate stability field (stippled) is generalized after Garrels and Christ (1965). Log total dissolved sulphur, arsenic, and iron is - 6; log total dissolved carbonate is 0.

Sediments filling channels cut into basement are locally coarse-grained (clasts up to 5 cm), implying substantial water velocities during channel filling. Such rapid velocities, with accompanying bed load must have had an erosive effect on channel walls and floors prior to deposition. If the channel margins were originally made of soft weathered rock, they would have been readily eroded to fresh hard rock by the fast-flowing streams. The channel margins are now extremely soft and friable, so we infer that this softening is a result of kaolinitic alteration after deposition of the sediments. Clasts of basement schist or greywacke occur in channel sediments near to the unconformity, and these clasts are commonly altered in a similar manner to the underlying basement. For example, schist clasts are found in a small channel above the oxidizing alteration zone near Bannockburn (Fig. 1), and semischist clasts occur near the base of the large St Bathans palaeochannel at St Bathans (Fig. 1). The Bannockburn clasts have no albite, while the St Bathans clasts do have albite preserved, matching the alteration mineralogy of the underlying rocks. The clasts are

28

D. Craw/Sedimentary Geology 92 (1994) 17-30

rounded or subrounded and have survived some sedimentary transport. Now, however, the clasts are kaolinitized and extremely friable, so that they disintegrate when touched. We infer that the clasts were altered after deposition of the sediments.

12. Discussion and conclusions

Arguments presented in the previous section suggest that alteration at the unconformity occurred after deposition of the overlying sediments, and this alteration requires considerable fluid flow through the rocks. Regional scale fluid flow in sedimentary basins is now well documented (Hitchon et al., 1971; Frape et al., 1989), and penetration of this fluid into basement rocks has also been demonstrated (Gascoyne et al., 1989). This type of fluid flow occurs due to gravity-driven migration of fluids from higher altitudes on sedimentary basin margins (Garven, 1989). The Otago sedimentary sequences are not as thick or extensive as the continent-scale sequences in which basinal fluid flow has been examined. However, the same principles apply to all basins where rocks are sufficiently permeable to transmit fluid, provided there is sufficient hydraulic head (Garven, 1989). The basal sediments in Otago are dominated by high porosity sands and gravels which readily allow passage of groundwater. The hydraulic head was initially small as the sediments accumulated on an erosion surface of low relief. Late Cenozoic uplift resulted in dramatic changes to the relief of the unconformity and overlying sediments, with widespread uplift of ranges across central Otago (Fig. 1). This uplift must inevitably have mobilized formation waters in the sediments. Crystallization of distinctive authigenic clay minerals in sediments and sub-unconformity alteration zones suggests that the formation waters also penetrated into the basement. The difference between oxidizing and nonoxidizing alteration is presumed to be due to differing fluid compositions (Figs. 6, 7) in different parts of the sedimentary basins. In particular, the non-oxidizing alteration may be due to pas-

sage of fluids whose compositions were controlled by interaction with, or origin from, the overlying coal seams (Craw, 1984). The coal-related fluids may have been a more regional manifestation of altering fluids which have been documented on a local scale in the pedogenic environment of coal measures (Staub and Cohen, 1978; Rimmer and Eberl, 1982). Similar regional non-oxidizing diagenetic kaolinitization has been described by Merin and Segal (1989), who suggested that oilrelated fluids were responsible. Diagenetic alteration of sediments and underlying basement may be a more common phenomenon than previously recognized, and extensive non-oxidizing alteration may be the most distinctive feature of the process. Oxidizing alteration of gold-bearing vein deposits which intersect the unconformity has resulted in localized gold mobility due to decay of sulphides and associated formation of sulphur complexes (Webster and Mann, 1984; Craw and MacKenzie, 1992). It is also possible that gold was transported in solution from alluvial accumulations above the unconformity to be deposited in the oxidized zone in the basement. Chemical mobilization of detrital gold in sediments overlying the unconformity can occur in sulphur-bearing fluids under reducing conditions, derived from sulphur-bearing (0.2%) coal measures (Clough and Craw, 1989), so the non-oxidizing alteration fluid, which locally mobilizes pyrite (above), may therefore be capable of transporting gold. Lignite interlayered with the auriferous sediments in central Otago has a comparable sulphur content (0.2-0.5%; Douglas, 1986) to that discussed by Clough and Craw (1989). Minor gold deposition has occurred along the cleavages of authigenic ferrous smectite-vermiculite (Youngson and Craw, 1993), which also links gold mobility with non-oxidizing alteration. There are no known examples in Otago of gold deposits which have undergone non-oxidizing alteration at the unconformity. This may be because of the comparative rarity of non-oxidizing alteration, which perhaps never coincided with a gold deposit. Alternatively, gold vein deposits which have undergone non-oxidizing alteration may not yet have been discovered because of lack

D. Craw~Sedimentary Geology 92 (1994) 17-30

of the gold grain size enhancement which provided early prospectors of oxidized deposits with easier targets.

Acknowledgements Financial support for this study was provided by the University of Otago. The study has benefitted from extensive discussions with B.J. Douglas, who also provided information on some of the localities and facilitated access to drill core. Discussions with C.A. Landis, J.K. Lindqvist and J.H. Youngson improved ideas and presentation. Constructive reviews by D. Eberl, R. Fiichtbauer, D. Riedel, and an anonymous referee improved the manuscript. Valuable technical help was provided by J.M. Pillidge who made excellent thin sections of extremely friable altered rocks.

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