Structural controls on the base-metal vein deposits of the Northampton Complex, Western Australia

Ore Geology Reviews, 8 (1993) 89--115 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

89

Structural controls on the base-metal vein deposits of the Northampton Complex, Western Australia David R. Byrne and Lyal B. Harris Department of Geology, The University of Western Australia, Nedlands, WA 6009, Australia (Received February 3, 1992; revised version accepted June 26, 1992)

ABSTRACT Byrne, D.R. and Harris, L.B., 1993. Structural controls on the base-metal vein deposits of the Northampton Complex, Western Australia. In: D.I. Groves and J.M. Bennett (Editors), Structural Setting and Controls on Mineral Deposits. OreGeol. Rev., 8:89-115. The Northampton Complex is a Proterozoic inlier near the west coast of Western Australia, 450 km north of Perth. The copper, lead and zinc vein-type deposits occupy dilational sites along brittle-ductile shear zones. There are four types of structural settings of the ore deposits: ( 1) lens structures, (2) shear-breccia link-shear structures (extensional strike-slip duplexes), (3) intersecting brittle-ductile shears, and (4) curved brittle-ductile shears. A model involvingN-S dextral wrenching along the Darling Mobile Belt best explains the various features of the deposits. In this model, Proterozoic granulite-facies paragneisses of the Northampton Complex were fractured with the development of predominantly extensional fractures and normal faults and, to a lesser extent, Riedel (both R and R' ), restraint and principal displacement shears. Since previous isotopic dating has not resolved the age of mineralisation, crosscutting relationships between mineralisation, the older dolerite dykes (650-800 Ma) and younger 330 °-trending sinistral shears (500-650 Ma) are used to constrain the timing to between 650 Ma and 800 Ma. NE-SW shortening across the Paterson and King Leopold Mobile Belts, between 650 Ma and 800 Ma, may also have been associated with the dextral movements along the Darling Mobile Belt.

Introduction

Since 1848, more than one hundred copper, lead and zinc mines have been worked sporadically in the Northampton Mineral Field, Western Australia (Figs. 1-3) which, altogether, have produced at least 4268 t of copper (Marston, 1979), 77,700 t of lead, 42.3 t of zinc and 212.3 kg of silver (Blockley, 1971 ). The base-metal deposits occur as vein and fault-breccia fillings, associated with brittleductile shear zones (traditionally referred to as lodes). While the importance of a structural Correspondence to: D.R. Byrne, Department of Geology, The University of Western Australia, Nedlands, W.A. 6009, Australia.

control for these deposits was recognised by the miners and geologists of the nineteenth century (e.g., Brown, 1871; Woodward, 1895; Maitland, 1898 ), the first attempt at applying structural theory to many of the mines of the field was by Campbell ( 1952 ). Campbell's observations are very informative, although developments in structural geology over the last forty years necessitate modernisation of his models. Mauger (1978) undertook a statistical lineament analysis, using aerial photographs and satellite images, in an attempt to relate density and orientation of the lineaments to the locations of the deposits; but the results were inconclusive. This paper examines structural controls of the deposits in the light of current theories based on experimental

0169-1368/93/$6.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

90

D.R. BYRNEANDL.B.HARRIS

fault and shear zone models (Tchalenko, 1968, 1970; Wilcox et al., 1973; Bartlett et al., 1981; Naylor et al., 1986; Tron and Brun, 1991 ), and studies of mineralisation within wrench systems (Harris, 1987; Mueller et al., 1988; Hodgson, 1989). A discussion concerning the origin or source of the fluids from which the deposits precipitated is beyond the scope of this paper, although a possible timing for mineralisation is discussed in a tectonic context. While there is currently no underground access to the mines, some data were obtained from shallow opencuts and the collars of shafts. Campbell (1952) had access to many of the mines, and his descriptions are most useful to this investigation. Other sources of data, such as orientations of the brittle-ductile shears, include Maitland ( 1903 ) and Blockley ( 1971 ). Despite only a small number of plans (of variable quality) existing for some of the mines, much information about the style of mineral-

isation, such as the pitch of high-grade ore shoots, can be obtained from them. Most of the mine plans were compiled and redrafted to aid comparison. Unfortunately, there is a scarcity of information about the copper mines at Northampton, since most were worked during the nineteenth century when very little geological information was recorded; however, Campbell (1952) concluded that both the copper and lead deposits occupy the same type of structures. Because of the metallurgical problems in the past, there has been little mining of the zinc ore and, consequently, little is known about the sphalerite ore bodies. To help assess the structural setting of the deposits, interpretation of aerial photographs and digitally 'processed aeromagnetic data was undertaken along with structural analysis of outcropping areas. The locations of the mines are shown in Fig. 3, and samples referred to in the text are stored by the Geological Museum, University of

David Byrne graduated from the University of Melbourne, Australia, in 1984. From 1984 to 1989 he worked for the Geological Survey of Victoria and various mining companies exploring for gold in the central Victorian gold fields. In 1987 he obtained a Diploma of Education from the Bendigo College of Advanced Education. He is currently studying for a Ph.D. at the University of Western Australia on the structural and metamorphic history of the Northampton Complex and controls on base-metal mineralisation. He has also undertaken part-time teaching of undergraduates in structural geology and consulting projects in Au and base-metal mineralisation in Archaean and Proterozoic terrains.

Following studies majoring in geophysics and structural geology at the University of Melbourne (graduated in 1976), Lyal Harris was employed as a geophysicist/seismic interpreter for Shell Development, Perth. This was followed by postgraduate studies in structural geology and tectonics at the University of Montpellier, France. He then taught structural geology and tectonics at the University of Rennes, France, for four years, undertaking research towards his doctorate in structural analysis of Alpine Corsica along with experimental modelling of structures within ductile shear zones. Lyal Harris has been lecturer in structural geology at the University of Western Australia since 1984. During this time he has consulted in structural and regional tectonic analysis and aeromagnetic interpretation related to gold, base-metal and iron-ore mineralisation. Continuing themes in his research include structural analysis of shear zones, structural interpretation of regional aeromagnetic data, regional tectonic syntheses, structural controls on mineralisation, and analogue modelling.

STRUCTURALCONTROLSON THE BASE-METALVEINDEPOSITS,NORTHAMPTONCOMPLEX,WESTERNAUSTRALIA

Fig. 1. Geological summary map of western Western Australia showing the location and setting of the Northampton Complex (NC). The Darling Mobile Belt extends beneath the Perth and Carnarvon Basins and is exposed in the Northampton Complex (NC), Mullingarra Complex (MC) and the Leeuwin Complex (LC).

Western Australia. Nomenclature for wrench systems used in this paper is based mostly on Tchalenko ( 1968 ), with the term Xshears used in the sense of Bartlett et al. ( 1981 ). The principal stress directions are defined such that O'1~O'2~O"3 with the stresses positive in compression.

91

Darling Mobile Belt (Glikson and Lambert, 1973; Mathur and Shaw, 1982; Harris, 1987). The Darling Mobile Belt, which extends N-S along the western margin of the Archaean Yilgarn Craton, once separated the Yilgarn Craton from Greater India (Veevers et al., 1975) and underlies the Phanerozoic Perth and Carnarvon Basins (Fig. l; Trendall and Peers, 1975; Fletcher et al., 1985; Myers, 1990a). The Northampton Complex consists of granulite-facies paragneisses metamorphosed at around 1050 Ma (Wilson et al., 1960; Compston and Arriens, 1968; Richards et al., 1985). The gneisses have been intruded by a 1000-Ma (Richards et al., 1985 ) granitoid, and a 650-800-Ma (Embleton and Schmidt, 1985; Schmidt and Hamilton, 1990) tholeiitic dolerite dyke-swarm (Warren, 1973; Gibson, 1974; Michael, 1977). Sedimentation in the Perth and Carnarvon Basins began with deposition of the Tumblagooda Sandstone which has been dated by paleomagnetic methods as Ordovician (Schmidt and Hamilton, 1990). The age of mineralisation has not been precisely dated (discussed below ), but must post-date dolerite dyke intrusion (Feldtmann, 1921; Campbell, 1952 ). Richards et al. ( 1985 ) list reasons as to why they consider the mineralisation to predate sedimentation of the Tumblagooda Sandstone. The geographical distribution of the mines is shown in Fig. 3. All of the mines are situated in areas where the gneisses form the bulk of the country rock, whereas none occur where the granitoid outcrops along the eastern margin of the complex. The Lady Samson mine, 19 km southeast of Nabawa (Fig. 3 ), which is near to the granitoid, is actually situated in an area of gneiss just to the east of the granitoid.

Geological setting

Brittle-ductile shear zones associated with the base-metal deposits

The Northampton Mineral Field covers most of the Northampton Complex (Myers, 1990a), a partly fault-bounded inlier of the Proterozoic

The term "lode" has been used by Western Australian geologists over the last 1O0 years to refer to the brittle-ductile shear zones contain-

92

D.R. BYRNEAND L.B.HARRIS

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Fig. 2. Locality map of the Northampton Complex (Proterozoic) showing the mining centres of Galena, Northampton and Nabawa, and the general geology (modified after Playford et al., 1970; and Hocking et al., 1982).

93

S T R U C T U R A L C O N T R O L S ON THE BASE-METAL VEIN DEPOSITS, N O R T H A M P T O N COMPLEX, WESTERN AUSTRALIA

lodes as occurring along zones of intense shearing and brecciation, which vary from 0.310 m wide, and extend along strike for several kilometres. In the Narra Tarra mine, the lode was about 1.2 m wide and was almost totally occupied by galena (Maitland, 1903). Figure 4 shows how the lead ore was distributed in

ing the N o r t h a m p t o n base-metal deposits. T h e lodes have been described as " b a n d s " of crushed country rock with small quartz reefs containing disseminated copper and lead minerals, with payable (high-grade) shoots alongside the quartz reefs (Simpson and Gibson, 1907 ). F e l d t m a n n ( 192 l, 1922 ) described the

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Fig. 3. Maps showing the locations of the mines of the Northampton Mineral Field in the (a) Northampton-Nabawa district and (b) Galena district. Mines cited in the text are listed.

94

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~alena .ode )neiss m (approx.)

Fig. 3 (continued). Galena district.

Fig. 4. Cross-section of the "lode" (brittle-ductile shear) in the Narra Tarra East mine showing the distribution of galena as pinch-and-swell or boudinaged lenses (after Talbot, 1914). The attitude of the foliation in the gneiss is not known.

pinch-and-swell, to possibly boudinaged, lenses within the brittle-ductile shear in the Narra Tarra East mine, suggesting post-ore ductile deformation along ore-controlling shear zones. Feldtmann ( 1922 ) described the lead ore in the Baddera mine as occurring as discontinuous lenses within the lode which varied greatly in width and length (Fig. 5 ). Mineralisation is not everywhere confined to the brittle-ductile shears, but may extend into the country rock, with the shears either in the hangingwall or footwall of the ore body (Feldtmann, 1921 ). Commonly occurring within or along the margins of the lode are narrow puggy faults ( < 25 cm in width) containing clay gouge and breccia, which crosscut both the ore and silicified fault breccias (e.g., as seen in the surface workings of the Narra Tarra East mine ). Such puggy faults indicate post-ore brittle-ductile deformation.

The ore-controlling brittle-ductile shear zones commonly contain brecciated wallrock (gneiss, pegmatite, dolerite) cemented by massive sulphides or quartz, usually with disseminated sulphides. Woodward ( 1895 ) first described these breccias as "grits" and "conglomerates". In some cases there has been partial replacement of the wallrock fragments by quartz (Feldtmann, 1921 ). The quartz infilling the breccias is commonly vughy, in places containing octahedral galena (as at the North Geraldine and Narra Tarra mines), euhedral quartz and marcasite crystals. These features suggest that this type of mineralisation took place as open-space filling in dilatant zones (Feldtmann, 1921, 1922) where the void spaces were supported by competent wallrocks during dilation (Swanson, 1990). Some breccias are not always clast-supported, as shown by two samples ofbrecciated dolerite (samples

......... ~

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STRUCTURAL CONTROLS ON THE BASE-METAL VEIN DEPOSITS, NORTHAMPTON COMPLEX, WESTERN AUSTRALIA

95

N

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Massive Galena

Fig. 5. Composite level plan of the Baddera mine showingthe distribution of the lead ore in the bottom three levels and the largebody of ore at the intersection of the Main and Branch lodes (after Feldtmann, 1922). 116631 and 116632), which are supported by quartz and massive sphalerite (with minor galena and chalcopyrite) cement respectively (Figs. 6a and b). Cement-supported breccias attest to syntectonic mineralisation. Mineralisation also occurs in thin syntectonic quartz veins within dolerite (Figs. 6c-e ) and gneiss. An antitaxial shear vein was found to contain disseminated pyrite and galena (Sample 116634). These shear veins show secondary shear band development indicative of

ductile shearing (Fig. 6c). Microstructures, as shown in Fig. 6d, suggest that the galena and the surrounding quartz both appear to have undergone recrystallisation associated with shearing (unfortunately no shear sense can be described as the core is unoriented). In contrast, Fig. 6e shows galena in a syntaxial extension vein which is intergrown with quartz fibres elongate perpendicular to the vein walls, producing a cockade texture. Despite differences in texture, both types of veins were formed

96

D.R. BYRNEAND LB. HARRIS

|

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Fig. 6. Cement-supported brecciated dolerite with (a) quartz cement from the Lady Florence mine (Sample 116631 ) and (b) massive sphalerite cement with minor galena and chalcopyrite from the Ethel Maud mine (Sample 116632). (c) Syntaxial extension vein (Ev) adjacent to an antitaxial shear vein (Sv) in dolerite; both contain galena (Sample 116633 ). (d) Galena (Gn) in an antitaxial shear vein (Sample 116634). (e) Cockade galena (Gn) in the extension vein shown in (c). Samples 116633 and 116634 are from the Mary Springs mine. For ( a ) - ( c ), scale bars are graduated in millimetres.

during deformation and, consequently, the galena within these veins must also be syntectonic with respect to shearing and extension. The relative timing of these veins can be seen in sample 116633 (Fig. 6c), where a syntaxial extensional quartz vein occurs adjacent to, and includes fragments of, an earlier antitaxial shear vein (consisting mostly of carbonate with

minor quartz). Galena occurs in both veins. Stylolites, with the stylolite peaks oriented perpendicular to the vein walls, were also seen in the shear vein but are truncated by the later extension vein. The deposits usually have wallrock alteration haloes which involve silieification ___ kaolinisation + chloritisation, of gneiss, pegma-

STRUCTURAL CONTROLS ON THE BASE-METAL VEIN DEPOSITS, NORTHAMPTON COMPLEX, WESTERN AUSTRALIA

tire and dolerite (Berliat, 1954; Blocldey, 1971; Richards et al., 1985 ). Kaolinisation involved alteration of the feldspars in the gneisses, pegmatites and dolerites, while chloritisation has altered the ferromagnesian minerals in the dolerites and sometimes the garnets and biotites in the gneisses. In some places carbonation has bleached the dolerites via the formation of magnesite. Campbell (1952, 1965) showed that kaolinisation extended 2 to 3 m into the wallrock, and is less intense in the copper mines than in the lead mines. Bleaching of the dolerite dyke adjacent to the ore in the Geraldine Copper mine is only about 1 m wide. The silicified wallrocks and breccias are useful in exploration for locating the brittle-ductile shear zones through which mineralising fluids have passed since they are comparatively resistent to weathering (as at the Cow Rock mine where the silicified breccia forms a wall 1.5-2 m high). Silicified zones commonly occur on the surface as lines of float parallel to the underlying brittle-ductile shear zones.

Timing of mineralisation relative to deformation At least some of the base-metal vein-type mineralisation was originally syntectonic, as indicated by the occurrence of sulphides in non-clast-supported breccias and antitaxial shear veins. A complex history is indicated in sample 116633 involving (a) ductile shearing with precipitation of galena + carbonate + quartz, (b) compression perpendicular to the vein, and (c) extension perpendicular to the vein also accompanied by galena deposition. The intermediate stage of stylolite development suggests that two separate mineralising events possibly occurred. The galena in the extension vein may have been either remobilised from galena in the shear vein, or possibly involved in a different pulse of mineralising fluid. The boudinaging of galena mineralisation in the Narra Tarra East and Baddera mines suggests post-ore ductile shearing. Such intense

97

ductile deformation of ore is also supported by the occurrence of fine-grained schistose galena at the Grand Junction, Narra Tarra East, Normans Well, Protheroe and Three Sisters mines. Post-mineralisation faulting was also inferred by Feldtmann ( 192 l, 1922) from the occurrence of the narrow puggy faults, and from curved cleavage faces on galena crystals (Blockley, 1971). However, it is uncertain whether this post-mineralisation shearing and faulting were produced during continuation of the same deformation event associated with formation of the ore-controlling brittle-ductile shears, or were the result of later deformation events.

Orientations of the brittle-ductile shear zones The dominant 038 ° strike of the brittleductile shear zones was first noted by Woodward ( 1895 ) and later by Maitland ( 1900, 1903). Feldtmann ( 1921 ) also observed that the strikes of the lodes may vary from that of the 030°-trending dolerite dykes (see below), and listed five mines where the brittle-ductile shears have a northerly strike. The orientations of the brittle-ductile shear zones from 74 mines (shown in Fig. 7), were measured where possible, or else taken from Maitland (1903) and Blockley ( 1971 ). While the dominant strike of the brittle-ductile shear zones is 038 °, Fig. 7b shows that there is an asymmetric spread about this maximum from NNW to ENE. Clustering of poles within this distribution is moderately developed, producing smaller modal maxima and shoulders to the dominant maximum (Figs. 7b and c). The overlap between clusters may be due to slight rotation at shear intersections, or local anisotropies (e.g., gneissic layering). The cluster averages are summarised in Fig. 7d, which shows that all, but two of the great circles to the cluster averages, intersect at a common point (approximately 71 ° -, 307 ° ). No consistent trend has been recognised in the variation of strikes from one area to an-

D.RB.YRNAEND L.BH.ARRIS

98

015/80E

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~ ~ " 060/76N ~'~ 46/78W 360/733W

Fig. 7. Stereographic projections of the poles of the brittle-ductile shears from 74 mines ( n -- 86 ): (a) scatter diagram, (b) contoured at l, 3, 5, 7, 9, I I , 13, 15%/l%area. (c) Moving average rose diagram showing the variation in the strikes of the brittle-ductile shears ( n = 86, interval = 5 °, circle radius = 10% ), (d) Summary of cluster averages, represented by great circles to the cluster averages, estimated from (b).

other such that the strikes appear to be randomly distributed in a geographical sense. Movement indicators associated with brittleductile shear zones

Little attention has been paid in the past to the movement sense of the brittle-ductile shear zones localising mineralisation. Feldtmann (1921) noted that slickenside striae on the shear planes in the Wheal Ellen mine are in places vertical and elsewhere horizontal, but no detailed data concerning orientations were given. Campbell (1965) determined the movement sense on some NE-trending faults to be normal, presumably from slickenside steps where the striae pitched at about 90 ° (e.g., as at the Protheroe mine). Unfortunately, due to limited outcrop, shear sense indicators (stepped slickensides) were only detected at two mines, where they were associated with the narrow puggy faults. At the

Narra Tarra East mine, the fault orientation is 047/70W and the slickenside striae pitch 88 °N, whereas at the Three Sisters mine they pitch 88°S in the fault plane oriented 037/ 71W. Both were determined to have a normal sense of movement. The sense of movement along a brittle-ductile shear zone was determined at one outcrop not associated with any mines. At a road cutting along the North West Coastal Highway (AMG coordinates: 270750E 6887700N) about 26 km north of Northampton, a fault, oriented 011/75W and partially encrusted with jarosite, contained quartz slickenside striae with variable subhorizontal pitches (ranging 25°N-17°S). Steps in the slickensides indicate dominant dextral movement. Compar&on o f the brittle-ductile shear zone orientations and movement senses with wrench models

The geometry of the brittle-ductile shear zones illustrated in Fig. 7 bears little resemblance to distributions associated with models involving coaxial deformation, such as paired conjugate fault sets or orthorhombic fault sets (Reches, 1983; Reches and Dieterich, 1983; Krantz, 1988), but is similar to distributions of shears formed in wrench systems (Harris, 1987; Hodgson, 1989). These similarities are outlined as follows: (1) Continuous distribution of poles to shears, which may be formed by overlapping clusters along a great circle over a length of about 90 °, such as shown in Fig. 7b. (2) As a consequence of the distribution of the poles lying along a great circle, when the shears are plotted as great circles they intersect about a common point, such as 71 ° ---,307 ° in Fig. 7d. (3) The angles between the respective shears (P, D, R, etc.) are reasonably consistent for similar types of materials (Fig. 8a), barring variations due to local anisotropies (Tchalenko, 1968, 1970; Wilcox et al., 1973; Naylor

S T R U C T U R A L C O N T R O L S ON THE BASE-METAL VEIN DEPOSITS, N O R T H A M P T O N COMPLEX, WESTERN AUSTRALIA

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Fig. 8. (a) Stereographic projection of an idealised N-S dextral wrench system using angles commonly observed in experiments (after Bartlett et al., 1981; Hodgson, 1989 ). (b) Great circles to the cluster averages with their corresponding interpreted wrench system names, interplane angles (where the grey great circle is the plane perpendicular to the point 71 °--,307 ° ), and the inferred principal stresses. Stereographic projections showing relationships of (c) extension veins and (d) normal faults to the principal stresses, and the effect of permutation between at and a2. Nomenclature after Tchalenko ( 1968 ). Principal stresses are defined as at/> 0"2>t 0"3with compression as positive. In (d) U and D represent upthrown and downthrown blocks, respectively.

et al., 1986; Tron and Brun, 1991 ). Variations in these angles are, however, controlled by: ( 1 ) the angle between the maximum compressive stress and the principal displacement shear (i.e., in cases of transtension or transpression: Wilcox et al., 1973; Sanderson and Marchini, 1984; Richard, 1989; Tron and Brun, 1991 ), and (2) the angle of internal friction and other mechanical properties of the rock (Dresen, 1991 ). The two most acute angles shown in Fig. 8b correspond well to those involving P - D and D - R shears (Fig. 8a). The other two adjacent angles (22 o and 21 o ) are somewhat lower than those observed in experiments (Fig. 8a), although they are still symmetrical about the

99

dominant maximum (c. f., symmetry of Riedel shears about T shears). The common point of intersection of the cluster averages (71 °--,307 ° ), suggests that most of the brittle-ductile shears formed during the same deformation event. This coeval timing of the shears may explain an observation made by Tomich (1955) in the Paringa Wheal Fortune mine, where in one of the drives a set of faults striking 050 ° displaces ore in the main 030 o_040 °-trending shears, while in one of the stopes the 050 ° faults are displaced by the main ore-bearing shear. With regards to the sense of movement expected in wrench systems, P and R shears are synthetic with the principal displacement (D) shears, while R' and X shears are antithetic: i.e., in a dextral wrench system, P, D, and R shears are dextral while R' and X shears are sinistral. Structures striking in the T orientation may be either extension fractures or normal faults (Wilcox et al., 1973; Sanderson and Marchini, 1984). The latter are formed when a~ is vertical due to a permutation between o"1 and a2 in the wrench system (Angelier and Bergerat, 1983). On a stereographic projection, great circles of extension fractures pass through the common point of intersection (Fig. 8c), whereas normal faults can form in either of two orientations which are symmetrically disposed about the common point of intersection by an angle of about 30 ° (Fig. 8d). Shears striking in the S orientation are represented by reverse faults or thrusts. Assuming that the narrow puggy faults formed during the same event as the brittleductile shears, in a wrench model, the normal NE-trending faults in the Protheroe, Narra Tarra East and Three Sisters mines would be in the T orientation, while the dextral shear trending NNE 26 km north of Northampton may have been an R shear. The two great circles of the cluster averages which do not intersect at the common point, 71 o ~ 307 o, can also be explained in terms of wrench geometry (Fig. 7d). The 045/77E great

100

D.R. BYRNE AND L.B. HARRIS

circle has a similar strike to the 038/70W great circle (interpreted T shear), but differs in the angle of dip by 33 o. This relationship to the interpreted extension fracture suggests that the 045°/77°E great circle may correspond to a normal fault set (c.f., Figs. 8c and d). The 015/ 80E great circle has a strike close to that of the 014 o/73 oW great circle (interpreted R shear). The difference in dip between the great circles may be analogous to the difference in dip of helicoidal R shears either side of a principal displacement (D) shear (Naylor et al., 1986). Based on the similarities between the brittleductile shears and wrench geometries, the ability of the wrench model to account for seemingly anomalous features, such as the 045°/ 77 oE and 015 o/80 oE great circles, and with the support of the limited number of movement sense indicators obtained, the brittle-ductile shears possibly formed during N-S dextral wrenching (where N-S is the strike of the major displacement or D shear). Figure 8b shows which cluster average would correspond to which type of shear, and the inferred principal stress directions prevailing during formation of the brittle-ductile shears (also listed in Table 1).

Relationship of the mineralised brittle-ductile shears to dolerite dykes Numerous dolerite dykes constitute a swarm across the Northampton Complex (Fig. 2). The dykes average about 15 m in width, and range up to at least 28 km in length. Spacing of the dykes is irregular, varying from 60 to 2000

m, with a modal average of 375 m: Examination of aeromagnetic data and aerial photographs shows that, while the dolerite dykes generally strike 030 ° , smaller splays and changes in strike of the dykes also trend NNE to ENE (also shown in Fig. 9a). Comparison of Figs. 9a and 7c shows that the dominant 038°-trending brittle-ductile shear zones are generally parallel to the 030 °-trending dolerite dykes, although the dykes do not exhibit the same degree of variation in strike as the brittleductile shears. The bisecting nature and larger widths of the 030°-trending dykes (average about 15 m) compared to the splays ( < 5 m) suggests that the 030 O_trendingdykes intruded into extensional fractures, while the NNE and ENE splays may represent intrusion into dilational shear fractures. While some authors have recognised a structural association between the lodes and the dolerite dykes (Brown, 1871; Maitland, 1898, 1900, 1903; Feldtmann, 1921; Berliat, 1954), others have proposed a genetic relationship between the two, largely on the basis of their spatial association (Montgomery, 1908; Campbell, 1952, 1965; Prider, 1958; Jones and Noldart, 1962; Blockley, 1971, 1975). The extent of the association between the ore deposits and the dolerite dykes is illustrated in Table 2, which shows that less than half of the mines 030

°

..^^

TABLEI List of principal stress orientations, determined from Fig. 9b, associated with the N-S dextral wrench model Principle stress

Plunge and azimuth

o'~ ~2 a3

20---,038° 71°--'307° 17°--,130 °

Fig. 9. Moving average rose diagrams of the strikes of (a) dolerite dykes measured from aerial photograph interpretation maps (n = 444, interval = 5 °, circle radius = 15%), and (b) the 330 °- and 089°-trending shears, which sinistrally and dextrally offset the dolerite dykes, respectively, as measured from aeromagnetic images (n=65, interval= 5 °, circle radius= 10%).

STRUCTURAL CONTROLS ON THE BASE-METALVEIN DEPOSITS, NORTHAMPTON COMPLEX,WESTERN AUSTRALIA TABLE 2 The number of mines spatially associated with the dolerite dykes and various positions of the ore deposits relative to the dykes Association Yes

No Uncertain Total

Number 40

Position

Number

Within Hangingwall Footwall Both walls Uncertain

8 4 17 2 9

59 17 116

contain dolerite dykes. Montgomery (1908) and Campbell ( 1952 ) noted that the five largest mines in the Northampton Mineral Field (Narra Tarra, Baddera, Protheroe, Geraldine and Wheal Fortune mines), with depth of workings > 90 m, are not associated with dolerite dykes, although the strike of the lodes in four of these mines is NE; i.e., sub-parallel to the dolerite dykes. Thus, as Maitland (1900, 1903 ) and Feldtmann ( 1921 ) concluded, the ore deposits appear to occupy fractures which were reactivated after intrusion of the dolerite dykes into these fractures. It was noted by Feldtmann (1921) that many of the brittle-ductile shears occur along the margins of the dolerite dykes. At the Lady Tilly mine the brittle-ductile shear, while parallel to the dolerite dyke, is located in the gneiss a few metres away from the dyke margin. Table 2 lists the number of ore deposits occurring in various positions relative to the dykes, and indicates that the footwall margin of the dykes was a more favourable site for the development of brittle-ductile shears and deposition of ore than the hangingwall of the dykes. Relationship o f the brittle-ductile shear zones to 330 °-trending shear zones

Shear zones trending about 330 ° , which sinistrally offset the dolerite dykes by up to

101

1000 m, were first noted by Campbell (1952) and later mapped on aerial photographs by Prider ( 1958 ). Aeromagnetic images also show an 089°-trending set of structures that dextrally offset the dykes (i.e., portraying a conjugate relationship to the 330°-trending sinistral shears). The 330°-trending shears attain lengths of up to 20 km, and are spaced 2-5 km apart, whereas the 089 °-trending set is usually shorter (<6.5 km in length). The 330 ° shears bifurcate and, in places, change strike by up to 30 ° , whereas the 089 ° shears do not exhibit such characteristics (Fig. 9b). Unfortunately, these 330 °- and 089 °-trending shears tend not to crop out, although at two locations northeast of Northampton, there are mylonite zones where the position of a 330 °trending shear is projected. The mylonite zones consist of mylonitised gneiss and pegmatite up to at least 35 m wide, and have been described by Maitland ( 1903 ) as "sheeted zones of mica schist". The mylonite zones generally strike parallel to the 330 ° shears. The presence of almandine garnet, with biotite pressure shadows, suggests that the metamorphic grade during development of the mylonites could have been upper greenschist to amphibolite facies conditions. Shear sense indicators, such as a porphyroclasts and C-C' relationships, indicate vertical movement, usually west block down, in contrast to the sinistral and dextral offsets for the 330 ° and 089 ° shears, respectively, as shown on aeromagnetic images. Campbell (1952) assumed that the 330 °trending shears had a normal sense of movement and, using the dolerite dykes as markers, estimated that 1500 m of down-dip movement had occurred on one of the shears. In areas where three to four of these 330°-trending shears are spaced across the outcropping area of the complex, such as around Northampton and Galena (Fig. 2 ), the cumulative displacement in such a model would be as great as 4-6 km. Given the relative uniformity of metamorphic grade across such areas, inferred from the consistency of petrographic descriptions of

": ,i~iii;;"....."

v

Copper ore (stopes)

•

j

Lead Ore (stopes)

I L 7

NE

,

167.6mLevel

137.2mLevel

Level

)6.7m

!mLevel

l 50.3mLevel

!1~ k~.~.ault

, 50m

Level

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(~%~

Fig. 10. Longitudinal section of the Narra Tarra mine showing the adjacent copper and lead ore bodies (after Wilson, 1927b and Campbell,

1952).

~ii!iiiiiiii!iiiiiiiiii~

.

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STRUCTURAL CONTROLS ON THE BASE-METAL VEIN DEPOSITS, NORTHAMPTON COMPLEX, WESTERN AUSTRALIA

the gneisses (Prider, 1958; Jones and Noldart, 1962; Peers, 1971; Warren, 1973; Byrne and Harris, 1991 ), such large vertical displacements for the 330°-trending shears seem unlikely. The discrepancy in the sense of movement between the mylonite zones and that for the 330 °-trending shears suggests that the normal mylonite zones may have been reactivated, after emplacement of the dolerite dykes, as sinistral faults within a coaxial deformation r6gime involving subhorizontal maximum and minimum principal stresses oriented 300 ° and 030 °, respectively. The 330°-trending shears were believed by Campbell (1952) to have played a major r61e in controlling the locations of the ore deposits, such that the ore deposits occur at the intersection between the 330 °-trending shears and the NE-trending brittle-ductile shears (lodes). Campbell (1952), however, did not actually describe any mines where both types of shears occurred together. While there are numerous intersections between these two types of shears, only four are coincident with any of the mines: Surprise, Kathleen Hope, MC 367 and Wheal Fortune mines. The case involving the Surprise mine is presented below, where the movement along the NNW-trending shear associated with the deposits is inferred to be dextral, rather than sinistral. The brittle-ductile shear hosting the lead ore deposit in the Kathleen Hope mine is oriented about 025/77E (Blockley, 1971), and no mention has ever been made by miners or geologists of 330 °trending shears in the mine. Unfortunately, there is no information regarding the MC 367 mine. Maitland (1903) and Wilson (1927a) noted the occurrence of a NNW-trending shear associated with a mylonite zone in the Wheal Fortune mine, which truncates and "interferes" with the ore deposit. This interference is interpreted to indicate post-mineralisation movement. No ore was observed by Maitland (1903) or Wilson (1927a) along the NNWtrending shear or in the mylonite. The lack of evidence supporting Campbell's (1952) hy-

103

pothesis makes it doubtful that the 330 °trending shears were involved in localising the ore deposits, and from Maitland's and Wilson's observations, the 330°-trending shears appear to post-date mineralisation. Ore shoots The ore shoots are bodies of high-grade ore, which approximately correspond to the stopes shown on longitudinal mine sections. At Northampton, they commonly have an elongate shape, and pitch at various angles within the brittle-ductile shears. The lead and copper deposits tend to occur separately when in the same brittle-ductile shear (Campbell, 1965 ), with only eight mines producing both copper and lead. Figure 10 shows that, in the Narra Tarra mine, the adjacent copper and lead shoots were still distinct bodies (Wilson, 1927b; Campbell, 1952 ). Sphalerite appears to be associated more with the lead deposits than with the copper mines, but the degree to which it is mixed with the galena ore is uncertain. It has been suggested that ore shoots occur where faults intersect a lithology of contrasting rheology compared to the gneisses, such as pegmatites or quartzites (Blockley, 1971 ). In such a situation, the shoot would be expected to be of similar size and orientation (in the plane of the brittle-ductile shear) as the lithological body of contrasting rheology. Tomich (1955) suggested this in the case of the Paringa Wheal Fortune mine, but he noticed that rich shoots were also located where this competency contrast did not occur. Further evidence against this suggestion involves a pegmatite in the hangingwall in the Protheroe mine (Fig. 11 ), where the pegmatite pitches in the opposite direction to the lead ore-shoot, and the shoot is very much larger than the pegmatite's cross-sectional area. Two types of shoots have been recognised, both of which appear to be, at least partly, controlled in their setting and orientation by structural features. The first type of shoot is

104

D.R. BYRNEANDL.B.HARRIS

Y

SW

NE

....:::.::::~..~:::~:..::.::...,:,~:::.,:~::~::~ innff

50m

j

Lead Ore (stoped before 1956)

Lead Ore (reserves at May 1956)

Fig. 11. Longitudinal section of the Protheroe mine showing the shallow southwesterly pitch of the ore shoot (stopes), and its almost blind setting. Also shown are the relative size difference between the shoot and the cross-sectional area of the pegmatite body in the wall of the lode, and the opposite pitch directions of the shoot and the pegmatite (after Campbell, 1952).

best illustrated by the lead deposits, whereas the second type mostly involves the copper mines. Ore shoots in the lead deposits The ore mineralogy of the lead deposits consists mostly of galena, which has commonly been found in surface outcrop, including the first deposit, discovered in 1848 in the bed of the Murchison River, which became the Geraldine mine (Gregory, 1848 ). Weathering appears to have only affected the lead deposits to very shallow depths ( < 20 m), and has produced secondary minerals such as cerussite and pyromorphite. Generally, there has been little secondary enrichment, and the galena shoots are considered to represent primary deposits. Campbell ( 1952 ) divided the deposits into two types: (1) lens structures, and (2) shearbreccia link-shear structures. Two additional types are proposed here involving (3) intersecting lodes, and (4) curved lodes. The following describes these four types of structural

settings, with an explanation in terms of the wrench model described above. ( 1) The lens structure involves bifurcation of the brittle-ductile shear which, in cross section, produces a phacoidal body of country rock bounded by two shears (Fig. 12a). The lens may also be internally fractured and brecciated, which apparently produced favourable sites for high-grade deposits such as in the Protheroe mine (Fig. 12b). The internal fractures are more steeply dipping than the bounding shears, and may be subvertical. The shoots associated with such a structure would be expected to have a subhorizontal plunge parallel to the intersection of the two bounding shears, and that of the internal fractures with the bounding shears (e.g., Protheroe mine; Fig. 11). This type of structure was considered by Campbell ( 1952, 1965 ) to occur in the Protheroe and Narra Tarra Mines, both of which have brittle-ductile shears generally striking NE, but with opposite dip directions. Campbell (1952, 1965) concluded that the lens

STRUCTURALCONTROLSON THE BASE-METALVEINDEPOSITS,NORTHAMPTONCOMPLEX,WESTERNAUSTRALIA

-//- -//-

oow"

tag~2 Cross

b.

Plan

Sections

t J SE

NW Collar Protheroe Main Shaft

vel

Breccia 129.5m Level

,

40m

,

m Fault III LeadOre Fig. 12. (a) Campbell's ( 1952) interpretation of the lens structure. (b) Cross section of the Protheroe mine, 152 m southwest of the Main shaft, showing part of a lens structure with an internal subvertical breccia containing lead ore (after Campbell, 1952).

structures in the Protheroe and Narra Tarra mines developed during normal movement along the bounding shears. This would be expected in the proposed wrench model since the normal NE-trending brittle-ductile shears strike in the T shear orientation. (2) The shear-breccia link-shear structure

105

occurs where two en relais shear zones are linked near their ends by oblique shears or fractures (Fig. 13a). As the zone between the two main shears was under extension (as shown in Fig. 13a), then the linking fractures dilate, producing a suitable site for precipitation of ore (Sibson, 1986). In such a case the ore shoot would be expected to plunge parallel to the intersection between the linking and main shears. The Surprise, Galena and Surprise South mines are thought to be situated in such structures along a 345 °-trending brittle-ductile shear zone (Campbell, 1952, 1965), with linking shears trending N-S to NNE-SSW (Fig. 13b). A notable difference between Campbell's interpretation (shown in Fig. 13a), and the Surprise mine (in Fig. 13b), is the angle between the links and the shears, which is more acute in the case of the Surprise mine. Campbell's interpretation corresponds to a dilational jog (Sibson, 1986; Hodgson, 1989) where the angle between the links and shears is about 45 ° , while the Surprise mine is more similar to a strike-slip extensional duplex (Woodcock and Fischer, 1986; Swanson, 1990). In either case, assuming that the structure is extensional, movement on the shear zone is inferred to be dextral in agreement with the proposed wrench model (i.e., strike-slip extensional duplex along a P shear). The cross section of the Surprise mine (Fig. 13c) shows the West, West Branch and Main lodes dipping westward into the vertically dipping Model lode. This structural relationship is similar to that shown by Woodcock and Fischer ( 1986, fig. 12 ) for a strike-slip duplex flower structure which, in the case of an extensional duplex, would be a negative flower structure. Figure 13d shows that the ore shoots pitch steeply at about 70 ° S, and that grouping of the three mines along the NNW brittle-ductile shear indicates that the structure may be repeated along strike (in this case by distances of 160 and 130 m).

106

D.R. BYRNE AND L.B. HARRIS

al

N

1 EastLode Shear

ii

re~cia

I ,

i PennasLode

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,

WestBranch ::ii ~ Lode I :!i SurpriseMine Known Shears Inferred S h e a r s 1

Shaft Extent of stoping along drives

i 1

==.:H,. IRi :~

.0.,...,ll

SurpriseSouth Mine~

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, om, I~

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Surprise South Mine

Galena Mine

~e~' ir--/r-,i

GalenaMine

~ ,~'

Surprise Mine

~e~

,r -~r--jmW--]p,

, -"-~'F-I "F~.5,.L~,~t 28.omL~~t 57.9mLev,l Lead ore (stopes)

t

~'/ ,#

- ii;lu~:~ iii[imlmg--

____,

33.SmL~,et \2.0mwidth@ I0.5%Pb 50m

j

%,d>~

m'wt_

61"Oml'evdlU[

7~r

91.4p.L ~ a ""

Fig. 13. (a) Campbell's (1952) interpretation of the shear-breccia link-shear structure (sense of movement added by authors). (b) The Surprise group of mines are situated in shear-breccia link-shear structures along a NNW-trending brittle-ductile shear. (c) Cross section of the Surprise mine showing the width of the lodes and the manner in which the West, West Branch and Main lodes dip into the Model Lode. (d) Longitudinal section of the Surprise group of mines showing the steep southerly pitch of the ore shoots (stopes) (after Campbell, 1952 ).

STRUCTURAL CONTROLS ON THE BASE-METAL VEIN DEPOSITS, NORTHAMPTON COMPLEX, WESTERN AUSTRALIA

(3) One of the most common structural sites for mineralisation is at the intersection of two shears, where breccias commonly develop (Feldtmann, 1922). Examples include the Block Seven mine where a N-S-trending shear intersects a NE-trending shear, the Grand Junction and Baddera (Fig. 5 ) mines where N S-trending shears intersect ENE-trending shears, and Deebles mine where a NE-trending shear intersects ENE-trending shear. The ore shoots associated with intersecting shears would be expected to be steeply pitching parallel to the intersection between the two shears. This type of structure reflects the coeval timing of the differently striking shears in agreement with the proposed wrench model. (4) A much rarer structure involves a brittle-ductile shear which curves, with the ore deposit being situated on the curved section of the shear. This occurs at the Nooka mine (Fig. 14a) where the brittle-ductile shear changes strike from NE to NNE to N-S over a distance of about 100 m (length of the mine). In the proposed wrench model this corresponds to a shear curving from a T to an R to a D orientation. The ore shoot in this case, as indicated by stoping shown in Fig. 14b, pitched 90 °, parallel to the axis of curvature. The lodes may also be curved in cross section where the dip of brittle-ductile shear zones, usually trending NE, shallow or steepen with increasing depth. Grades and width of ore were noted by Woodward ( 1895 ) to be poorer along the shallowly dipping sections than along the steeper sections. An example occurs at the Mary Springs mine where the dip of the brittleductile shears shallow with depth, and the ore grades and widths decrease correspondingly from about 80 m.% Pb to 5 m.% Pb. This situation reflects how dilation varied along NEtrending faults with variable dips in the Northampton Complex, and would be expected for normal faults with curved fault planes and strikes parallel to the T orientation.

107

Ore shoots in the copper deposits Whereas the lead shoots are composed predominantly of hypogene galena, most of the copper shoots consist of supergene azurite, malachite, chalcocite and covellite above and around the water table (Feldtmann, 1921, 1922), which ranges from 3-15 m below the surface. In the Wanerenooka mine, malachite and azurite occurred above the water table, while below, there was chalcopyrite with pockets of up to 1000 t of chalcocite and covellite (Woodward, 1895 ). The supergene nature of the copper deposits is reflected in Fig. 15, where 71% of the copper mines are less than 25 m in depth. The difference in copper content between the primary and secondary copper minerals is believed to be, at least partly, responsible for the past cessation of mining of the copper deposits deeper than the supergeneenriched zone. At the Narra Tarra East mine, malachite and azurite occur in vughs, along grain boundaries, and in the narrow puggy faults within the brittle-ductile shears. One feature these sites have in common is that they are permeable conduits for fluids, such as meteoric water. Based on Campbell's (1952) conclusion that both the copper and lead deposits occupy the same types of structures, it is proposed that the location and orientation of the chalcopyrite deposits may have been controlled by the same structures which controlled the galena shoots. However, the occurrence of chalcopyrite, which has partly altered to azurite and malachite, indicates that the supergene-enriched copper shoots were probably controlled by permeable structures within the brittle-ductile shears in the vicinity of the chalcopyrite deposits. Thus, the copper shoots would be expected to have a subhorizontal pitch parallel to the intersection between the permeable structures and the water table. Lack of information regarding shoots in the copper mines prevents confirmation of this orientation.

108

D.R. BYRNEAND L.B.HARRIS

a.

bo

J

S --Ii

N

i

i)

evel

86.6m Level

4

I

: •

: : ~1,

4Om

I

Shaft

Lead ore (stopes)

~? Opencut/ ...

i

Collapsed Stope Lode at Surface

40m

Fig. 14. (a) Composite level plan of the Nooka mine showing curvature of the lode. (b) Longitudinal section of the Nooka mine showing vertical pitch of the ore shoot (stopes) (compiled from plans by the Geological Survey of Western Australia).

0-20 20-40 40-60 E t-

60-80 80-100

/ m

100-120

[] [] []

120-140 ~1 140-160 160-180 a

Cu Mines Pb Mines Cu & Pb Mines

I

I

10

20 Number of m i n e s

30

Fig. 15. Stackedbar graph showingthe knownmaximum depth of workings for mines which produced copper (n = 14), lead (n = 51 ) and both copperand lead (n = 6 ).

Discussion and tectonic implications Structural controls on ore deposits

The internal textures of the brittle-ductile shear zones, the variation in their orientations,

the movement indications as determined in the field or inferred from mine plans and the various orientations and structural settings of the ore shoots all have to be explained in any model which accounts for the structural settings of the base-metal deposits of the Northampton Complex. The simplest model which best explains these features is one where the Northampton Complex has been deformed in a N-S dextral wrench r6gime resulting from NE-SW compression and NW-SE extension. Antitaxial shear veins and cement-supported breccias containing sulphides support syn-deformational mineralisation. The vughy breccias and the four types of structural settings of the ore shoots, including lens and shear-breccia linkshear structures and brittle-ductile shear zones which intersect or have curved strikes, consistently indicate that dilational sites associated

STRUCTURAL CONTROLS ON THE BASE-METALVEIN DEPOSITS, NORTHAMPTON COMPLEX, WESTERN AUSTRALIA

with the wrench model were favourable for ore deposition. There is little support for genetic associations between the ore deposits and the dolerite dykes or the 330 °-trending shears. The spatial association with the dolerite dykes appears to be largely due to reactivation of fractures associated with dolerite dyke intrusion. One feature, commonly associated with wrench systems, missing from the Northampton Complex are major, through-going principal displacement (D) shears. In the proposed model, such a major shear zone would be expected to strike N-S. The geographical distribution of the mines (Fig. 3 ) shows no arrangements of the mines which might suggest the existence of a major D shear in the Northampton Complex, and neither are the dolerite dykes confined to stepping within defined corridors. However, the N-S-trending Darling Fault that forms the eastern boundary to the Perth Basin is located 80 km east of Northampton. The Darling Fault represents the Mesozoic manifestation of a major crustal discontinuity (the Precambrian Proto-Dading Fault Zone) wfi.~ch has been active since the Archaean (Blight et al., 1981; Bretan, 1985; Harris, 1987). Faulting in the Northampton Complex may, therefore, be subsidiary to transcurrent movements focused along the Proto-Darling Fault Zone and/or parallel structures in the Darling Mobile Belt beneath the Perth Basin which acted as principal displacement shears. As basement structures have been instrumental in controlling fault patterns in the Perth Basin (Byrne and Harris, 1992) the N-S-trending Yandi Fault (Fig. 2 ) could be the reactivated Palaeozoic expression of such a D shear. In this model the Northampton Complex may have behaved as a rigid body undergoing internal fracturing while the large displacements were taken up by major displacement shears around the complex reactivating existing ductile shear zones on the western margin of the Yilgarn Craton. An alternative explanation for the lack of major D shears in the Northampton complex involves the developmental sequence of shears

109

during wrenching, which has been noted by many of the experimentalists (Tchalenko, 1968; Wilcox et al., 1973; Bartlett et al., 1981; Naylor et al., 1986 ). In experiments involving sand and clay, extensional T shears form during the initial stages of wrenching (Wilcox et al., 1973). As wrenching, proceeds the conjugate Riedel shears (R and R') develop followed by linking P shears. Only after relatively large displacement of the basement fault do D shears develop, forming a major through-going transcurrent shear zone. Bartlett et al. ( 1981 ) observed that, in rock under high confining pressures, R and P shears form first followed by X and R' shears and finally D shears. In both cases the D shears are the last to form, and Naylor et al. (1986) state that in many field cases wrenching does not proceed beyond the development of the first set of Riedel shears. In this alternative explanation, no large dextral displacements need be invoked along the Proto-Darling Fault Zone.

Age of mineralisation Several attempts have been made to date the age of the mineralisation in the Northampton Complex (Prider, 1958; Michael, 1977; Michael and Groves, 1977; Richards et al., 1985 ). The lead isotope date of 500 Ma quoted by Prider (1958 ) was considered by Richards et al. (1985) to have very large uncertainties, and their own attempt at lead isotope dating confirmed that the lead ore is not amenable to direct chronological interpretation. Michael (1977) and Michael and Groves (1977) attempted to date the age of mineralisation by comparison of barite sulphur isotope data with the sulphur isotope age curve of Holster and Kaplan (1966). Michael (1977) obtained a value of 18.5o/oofor barite ~34S, and chose a Siluro-Devonian age for the mineralisation. However, with reference to the ~ 3 4 S of Holster and Kaplan ( 1966 ), other possibilities

1 10

that equally explain this value include 630 Ma, Early Carboniferous, Triassic and Early Cretaceous. The multitude of possibilities indicates that this method can not be used to unequivocally date the age ofmineralisation in the Northampton Complex. Rubidium/strontium dating of hydrothermally altered dolerite was attempted by Richards et al. (1985) on samples with differing degrees of alteration, obtained from the Mary Springs mine (drill core). The high degree of scatter about the regression line, which gave an age of 434 +_16 Ma (Ordovician-Silurian), was believed by Richards et al. ( 1985 ) to be due to the lack of isotopic equilibrium. This lack of equilibrium may be partly due to the low temperature associated with mineralisation, estimated by Michael (1977) to be about 150 °200°C, or remobilisation of lead during a younger, superposed event (as suggested on textural evidence shown in Fig. 6 ). Richards et al. (1985) also mention the possibilities of heterogeneous dolerite and hydrothermal fluid (with respect to Rb and Sr), and varying proportions between the dolerite and fluid as causes of the scatter about the regression line. Schmidt and Hamilton (1990) considered that both the degree of scatter and the lack of isotopic equilibrium preclude the use of Richards et al.'s (1985) data for establishing a reliable age of mineralisation. Schmidt and Hamilton (1990) also recognised that the data appears to consist of two groups which do not conform to the same systematics, with the least altered group giving an isochron of 674 _+17 Ma. Another cause of the scatter could be partial resetting associated with reactivation of NEtrending brittle-ductile shears after deposition of the Tumblagooda Sandstone, during the early stages of development of the Perth Basin. Byrne and Harris ( 1992 ) have recognised an event involving NW-SE extension which has down-faulted and tilted the Tumblagooda Sandstone. This mostly involved NE-trending faults, such as the Hardabut Fault, although the N-S-trending Yandi Fault also tilts the Tum-

D.R. BYRNE AND L.B. HARRIS

blagooda Sandstone. The two major differences between this event and the N-S dextral wrench model, is the almost non-existent variation in fault orientation, and the more brittle nature of the faults in the Tumblagooda Sandstone. It seems unlikely that the post-Ordovician event involved N-S dextral wrenching associated with mineralisation, but reactivation of some of the NE-trending shears formed during the latter event probably occurred (Byrne and Harris, 1992). The timing of mineralisation is essentially constrained by the age of older dolerite dykes, which are about 650-800 Ma (Embleton and Schmidt, 1985; Schmidt and Hamilton, 1990), and the younger 330°-trending shears, which sinistrally offset the dolerite dykes and disrupt the mineralisation in the Wheal Fortune mine (Maitland, 1903; Wilson, 1927a). The sinistral 330 °- and dextral 089°-trending shears appear to be a conjugate set formed by WNWESE compression and NNE-SSW extension. These stress directions are similar to those expected for a Late Proterozoic to Early Cambrian event which produced N-S sinistral movement along the Darling Mobile Belt (Bretan, 1985; Harris, 1987; Harris et al., 1989a,b). As such conjugate shears do not appear to affect the Tumblagooda Sandstone, these structures are most probably pre-Ordovician. Consequently, the age of the mineralisation in the Northampton Complex is probably between 650 Ma (the oldest age attributed to sinistral movements along the Proto-Darling Fault Zone) and 800 Ma (the oldest age of the dolerite dykes), soon after intrusion of the dolerite dykes.

Relationships to regional tectonic models Similar stress orientations to those determined for the brittle-ductile shears (Table 1 ) may be inferred as controlling emplacement of the dolerite dykes, where the larger 030 °trending set suggests NW-SE extension, and the conjugate nature of the smaller NNE- and

STRUCTURAL CONTROLS ON THE BASE-METAL VEIN DEPOSITS, N O R T H A M P T O N COMPLEX, WESTERN AUSTRALIA

ENE-trending sets indicates N E - S W maxim u m compressive stress. Fletcher et al. ( 1985 ) believed that the dykes were associated with a pre-rift arch related to an aulacogen connected with the formation of the Tethys Sea. However, rifting environments sometimes display alternating permutations of the principal stresses (Angelier and Bergerat, 1983). Stress

t

500 km

11l

permutations can happen without involvem e n t of any transcurrent m o v e m e n t , in which case the dykes may have intruded into a coaxial N W - S E extensional rifting situation where at and tr2 permutated. While this coaxial extension model m a y be feasible for the dolerite dykes, it does not satisfactorily explain the

King Leopold Mobile Belt '

650-800Ma

Paterson Mobile Belt Suspect Terranes

| Northern Australian

Pcate

%

a

Legend A

•

Thrust fault Normal fault

Northampton Complex

Wrench fault

Mullingarra - Complex I I

.., |

West

Australian

|

,

Leeuwin _ . ~ 1 Complex .~

P Ia t e

Thrust direction of hangingwall block

ml,.(l 1 Inferred stress field n ~ (y3 ....

State border

Fig. 16. Map of Western Australia showing NE-SW shortening across the King Leopold and Paterson Mobile Belts, and the dextral sense of movement along the Darling Mobile Belt at around 650-800 Ma (modified from Myers, 1990b). The normal fault between the Northampton and Mullingarra Complexesis hypothetical.

112

various features of the structures controlling the base-metal deposits. During the period of emplacement of the dolerite dykes in the Northampton Complex (between 650 and 800 Ma), the Paterson Mobile Belt was accreted to the northeastern margin of the Yilgarn Craton (Fig. 16), and underwent NE-SW shortening resulting in the formation of a thrust and nappe complex (Myers, 1990b). NE-SW shortening across the King Leopold Mobile Belt (Fig. 16) also occurred around this time (Tyler and Griffen, 1990; Myers, 1990b). Such events involving NE-SW crustal shortening may have induced dextral movement along the Darling Mobile Belt since it was a pre-existing zone of weakness. To date, no evidence for Late Proterozoic N S dextral movement along the Proto-Darling Fault or Darling Mobile Belt has been confirmed outside of the Northampton Complex. There has been no work performed on the Mullingarra Complex to comment on, while the Leeuwin Complex developed mainly during the 500-650 Ma event within a sinistral wrench regime (Harris, 1987; Harris et al., 1989a,b). While N-S dextral movement along the southern portion of the Proto-Darling Fault was determined by Bretan ( 1985 ) from shear sense indicators in mylonites, he believed this to have resulted from an Archaean event. Further detailed geochronology is required to establish the tectonic history of the Darling Mobile Belt. The lack of evidence for N-S dextral movement along the southern portion of the ProtoDarling Fault may be explained by invoking a hypothetical NE-trending normal fault or rift zone between the Northampton and Mullingarra Complexes, as shown in Fig. 16. Such a feature would divide the Australian portion of the Darling Mobile Belt into two halves, such that only the northern half would be affected by dextral movement, while dextral movement along the southern half would have been transferred to the west of Australia (i.e., in

D.R. BYRNE AND L.B. HARRIS

Greater India). Whether the NE-trending fault or rift extended for hundreds of kilometres, or joined another N-S-trending shear zone to the west of the Leeuwin Complex, in the manner of a dilational jog, is uncertain. The suggested location of the hypothetical fault shown in Fig. 16, between the Northampton and Mullingarra Complexes, reflects a significant difference between the two complexes. The former, with the dolerite dyke swarm, is believed to have been situated within the rift, whereas the Mullingarra Complex, in which no dolerite dykes have been recorded (Playford et al., 1976), would have been outside of the rift zone, in the southern portion of the Darling Mobile Belt. While no major NE-trending structures have yet been recognised south of the Northampton Complex, the northwestern margin of the complex is bounded by a NE-trending major crustal fault (Hardabut Fault, Fig. 2) associated with a gravity gradient (Playford et al., 1976). Although normal movement along this fault is known to have occurred since the Ordovician, it is possible that this movement represents reactivation of a Late Proterozoic structure (Byrne and Harris, 1992 ). Conclusions

The base-metal vein-type ore deposits of the Northampton Mineral Field appear to have been deposited in various dilational sites during deformation along brittle-ductile shear zones. Structural criteria, ore textures and regional correlations show that models involving post-Tumblagooda Sandstone primary mineralisation are not tenable, although minor remobilisation since the Ordovician is still a possibility. The four main types of structural settings of the primary ore shoots include the lens and shear-breccia link-shear structures, and intersecting and curved shears. A model involving the formation of brittle-ductile shears in a N-S dextral wrench system, which operated between 650-800 Ma, best explains the

STRUCTURALCONTROLSON THE BASE-METALVEINDEPOSITS,NORTHAMPTONCOMPLEX,WESTERNAUSTRALIA

various structural characteristics of the basemetal deposits of the Northampton Complex. The dolerite dykes, which pre-date the mineralisation but are probably still within the 650-800 Ma bracket, may have been emplaced into fractures formed either during N S wrenching, or coaxial NW-SE extensional rifting. Some of the fractures occupied by the dolerite dykes were subsequently reactivated at the time of mineralisation, particularly along the margins of the dykes. The dextral movement that possibly occurred along the Darling Mobile Belt between 650 and 800 Ma, may have resulted under the same regional stress field due to accretion of the Paterson Mobile Belt on to the Yilgarn Craton, and NE-SW shortening across the King Leopold Mobile Belt at that time. This event was followed by a 500-650 Ma event which involved sinistral movement along the Darling Mobile Belt, and produced 330 °and 089 °-trending conjugate shears that offset the dolerite dykes and disrupted mineralisation in the Northampton Complex. Deposition of the Tumblagooda Sandstone in the Ordovician and subsequent development of the Perth and Carnarvon Basins followed, during which some of the ore-bearing structures were possibly reactivated. Remobilisation into late extension veins may have occurred at this time. Acknowledgements This project was funded by West Australian Metals N.L., and is part of an Australian Research Council funded project concerning "Pan-African" events on the margin of the Yilgarn Craton. The aeromagnetic survey providing data used for interpretation was flown by Aerodata Holdings Pty Ltd and digitally processed by J. Ashley at Southern Geosciences Ltd. The authors wish to thank John Clema, formerly of West Australian Metals N.L., for arrangement and support of the project, and George MacDonald, formerly of West Australian Metals N.L., whose local knowledge

113

proved invaluable. John Blockley and David Groves are also acknowledged for their constructive comments on the manuscript.

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