Formation of oreshoots in mesothermal gold-quartz vein deposits: examples from Queensland, Australia

Ore Geology Reviews, 8 ( 1993 ) 277-301 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

277

Formation of oreshoots in mesothermal gold-quartz vein deposits: examples from Queensland, Australia Stephen G. Peters* Central Norseman Gold Corporation Ltd., P.O. Box 56, Norseman, ~:A. 6443, Australia (Received April 3, 1989; revised and accepted March 20, 1991 )

ABSTRACT Peters, S.G., 1993. Formation of oreshoots in mesothermal gold-quartz vein deposits: examples from Queensland, Australia. In: S.J. Haynes (Editor), Vein-type Ore Deposits. Ore Geol. Rev., 8: 277-301. Mesothermal gold/quartz vein deposits in granitoid-hosted and melange/sediment-hosted goldfields, contain oreshoots in areas of structural dilation. The environment of formation is localized at depths of 2-5 km in brittle to brittle-ductile shear zones which have stick-slip seismic movement. Lithologic contacts and fissure intersections affect the shape and geometry of the oreshoots. Mineralizing fluids in these shear zones range between 3 and 10 wt% NaCI equiv, with c~80 values of about 5-8%0. Fluids are locally CO2-bearing with formation temperatures of between 220 ° and 350°C. Fluid source is either of deep magmatic or metamorphic origin and can rarely be correlated directly with nearby plutonism. Fluid transport is via single pass flow in high permeability conduits, such as mylonites in granitoids or melange cleavage in sediments. Complex mixtures of mineralized quartz, gouge, fault rocks and altered wall-rock reflect the environment of formation. Comb, ribbon, buck and breccia quartz and microscopic textures indicate different processes and stages of oreshoot formation. Quartz deposition is due to changes in silica solubility resulting from temperature and pressure fluctuations. Local pressure changes are due to reduced velocity of the fluid in dilated zones according to Bernoulli's equation. At restricted or dilated portions of the fissure, throttling and adiabatic cooling are common and result in quartz deposition and channel choking which lead to pressure and temperature build-ups and faulting. The long dip-lengths in many mesothermal oreshoots may also account for substantial pressure and temperature reductions in the fluid from bottom to top. The venturi effect in connecting fissures may also affect quartz deposition. Microscopic textures indicate that multiple generations of quartz have been involved in cracking, stress corrosion and dissolution due to porosity changes and dilation prior to and during faulting, Gouge and clay seams stabilize fault movement and may have acted as impermeable barriers and pressure seals which channelled fluid flow, and if hydrated, may have expelled concentrated brines when compressed by faulting. Hydrothermal alteration indicates early diffusive transport and chemical and pH gradients away from the oreshoots. Four main stages of oreshoot formation occur from the outside to the inside of oreshoots as: ( 1 ) ground preparation and nucleation, (2) reinjection and sheeting, ( 3 ) major fault movement, channeling of fluid flow, local dissolution and cracking, and (4) consolidation and oreshoot growth, including fluid stagnation and ponding.

Introduction

The gold-quartz vein deposits considered here are those which occur below the volcanic environment in brittle shear-zones. They are similar to the gold-quartz vein deposits of the *Present address: U.S. Geological Survey, Reno Field Ofrice, Mackay School of Mines, Reno NV 89507-0047 USA.

0169-1368/93/$6.00

California and Victoria type as classified by Lindgren (1933), or more recently by Berger (1986). These deposits are characterized by multiple quartz veins, with free gold averaging 800 fine and 5 to 10% sulphide in simple assemblages including pyrite, galena, sphalerite, arsenopyrite and chalcopyrite, with rare tetrahedrite-tennantite, tellurides and scheelite. Wall-rock alteration in granitoid-hosted de-

© 1993 Elsevier Science Publishers B.V. All rights reserved.

2 78

S.G. PETERS

ins (1984) as forming at greater depths than epithermal deposits and by Nesbitt et al. (1986) as products of meteoric water in deep fault-zones. Although the term "mesothermal gold-quartz vein deposits" implies formation at intermediate depths, recent models of the origin of these deposits vary as to the fluid source (cf. Ridge, 1968; Shelton, 1988; Kerrich, 1989), such that magmatic, metamorphic, and meteoric fluids are all suggested as possibilities for quartz and metal transport and deposition.

posits such as Charters Towers (Peters and Golding, 1989 ) consists of selvages around the veins, but melange/sediment-hosted (slate or turbidite-hosted), such as the Hodgkinson goldfied (Peters et al., 1990) show little alteration (Peters, 1987c). Conceptually, these deposits form at greater depths than near-surface epithermal gold-quartz vein deposits (Henley, 1985), but not as deep as most Archean (hypothermal) gold-quartz deposits (Kerrich, 1983; Colvine et al., 1988). Many of the gold-quartz veins classified by Lindgren (1933) as mesothermal, especially those occurring in turbidites or slates, are considered as originating from metamorphic or orogenic processes (Fyfe and Kerrich, 1984; Bohlke and Kistler, 1986; Cameron, 1989). Granodiorite- or batholith-hosted quartz-vein deposits in this class are interpreted by Sawk-

Oreshoot shape and size characteristics Host fissures and shear zones in mesothermal gold-quartz vein deposits are characterized by relatively uniform strike ( > 1000 m) and dip ( 100-500 m) lengths. Within this host

fissure

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Fig. 1. Anatomy and nomenclature of oreshoots in an hypothetical intermediate depth, fissure-related (mesothermal), gold-quartz vein system. Hatched areas represent disturbed and altered zones around the oreshoot. Dark areas represent quartz-rich portions.

FORMATION

OF ORESHOOTS

IN M E S O T H E R M A L

GOLD-QUARTZ

fissure are segregated oreshoots (Fig. 1 ). The oreshoots have higher gold values than the adjacent parts of the shear zones and may be identified in old districts by stope outlines (Fig. 1 ). Size of oreshoots usually ranges between 1 × 106 and 2 × 104 tonnes and the grade of the material is usually around 35 g/t. There is a tendency for oreshoots to be thicker and richer in the center, in a lobe or along one side, rather than to have uniform-grade distributions. Oreshoots may terminate abruptly or may taper to assay cut-offs. Usually the cut-offs are geologic, such as shear zone, quartz or alteration boundaries. Oreshoots are composed of complex quartzpods and gouge. Vugs and open-space filling textures are not dominant or diagnostic constituents. Between oreshoots, more simple combinations of quartz, alteration and gouge define barren or lower-grade portions of the veins and fissures (Fig. 1 ). Oreshoots are usually hosted in shear, fissure or fault zones which connect to form vein systems. Multiple oreshoots in hangingwall and footwall fissures have plunges which are related to the symmetry and geometry of the whole vein system (Fig. 1 ). Oreshoots average 0.25-1.75 m wide, but anomalous bulges up to 15 m thick also occur locally. The characteristic feature of many oreshoots is complexity; internal textures usually reflect multiple episodes of faulting and quartzmetal deposition and typically several different types and ages of quartz and gouge are present. Environment of o r e s h o o t formation

Deformation environment The fault rock-types and minimal wall-rock disturbance in mesothermal gold-quartz vein deposits are similar to those found in brittle to brittle-ductile shear zones (Ramsay, 1980b) and typical of stick-slip seismic failure (Byerlee, 1970; Sibson, 1977; 1982). Stick-slip faults typically have strain rates of 10-100 ms-~ or

279

VEIN DEPOSITS

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Fig. 2. Diagrammatic section showing elements of deep, intermediate and shallow portions of gold-quartz vein systems. Modified from Graton (1968), Henley et al. (1976), Sibson, ( 1977, 1982), Etheridge el al. (1983), Fyfe and Kerrich (1984), Wood and Walther ( 1986 ), and Cox et al. (1987).

more (Fyfe et al., 1978) and fault movement takes place at intermediate depths (3-5 k m ) , where m o v e m e n t is typically jerky, local, intermittent and is implemented by cracking, grinding and plucking of the sidewalls. Movement of all sizes is preceded by thermal softening, which results from high fluid-pressure gradients, and stress. Movement is translated into heat production, fluid compression and mass transport (Sibson, 1977, 1982; Rudnicki and Chen, 1988). More uniform movement producing brecciation, occurs at shallower depths (Fig. 2 ) and ductile m o v e m e n t is more

280 common in deeper regions of the crust (Scholz, 1988). Intermittant, jerky movement in stick-slip faulting is stabilized by both gouge and high fluid-pressure which lead to more uniform and lower strain-rates (Byerlee and Brace, 1972). Gouge aids and stabilizes fault movement by providing areas with less frictional resistance to sliding (Engelder, 1974). The amounts of gouge in oreshoots may indicate the relative amounts of movements in that area of the fault plane (Robertson, 1983; Hull, 1988; Walsh and Watterson, 1988, 1989), and may also indicate specific areas of shearing or compressional zones within the fault plane. If gouge indicates compressional zones, rather than dilated portions of the fault, gouge should be more common outside oreshoots, which is generally the case (cf. Moore et al., 1989). Intermingling of gouge and quartz within the oreshoots suggests that these areas are intermittantly both dilational and compressional, compatible with the rapid strain-rates. The importance of stick-slip faulting to oreshoot development is the intermittant nature of movement. This movement allows different portions of the fault plane to move faster than others, because of the influence of gouge and fluid pressure in local areas. This causes different gouge types and fluid pressures to develop in spatially separate areas. Once zones of stability and instability are created along the fault plane, further strain enhances these differences, causing the same zones of the fault to move faster than the others. This results in puckered areas in the fault planes. At shallower or deeper regions, more uniform strain-rates are present and do not provide this enhanced irregularity along the moving fault.

Fluid environment Origin and chemistry Hydrothermal fluids in many gold-quartz deposits have been interpreted as generated during regional metamorphism and cleavage

S.G.PETERS development (Henley et al., 1976; Norris and Henley, 1976; Fyfe and Kerrich, 1984; Cox et al., 1987 and Sibson et al., 1988). In addition silica-rich fluids may also be generated by distal magmatic-systems (Cameron and Hattori, 1987) that would have many similar characteristics to metamorphic fluids. These would include temperature of formation (200 °350 ° C ), low to moderate fluid salinities ( < 10 wt% NaC1 equiv.), and oxygen isotopes ( ~ ~80 = 5-8%o ) that are characteristic of either metamorphic (orogenic) or distal magmaticfluids (cf. Taylor, 1979), that have equilibriated with the host rocks (Peters and Golding, 1987, 1989). The fluid is locally CO2bearing ( e.g. Patterson, 1982; Wall et al., 1983; Goldfarb et al., 1986; Peters, 1987c). Fluid temperatures are estimated from fluid inclusions in quartz and from oxygen-isotope fractionation equilibria of vein minerals. Cooling is not common in the oreshoot environment due to rapid uplift, because isotherms within the rock mass surrounding the host fissure are raised faster that the rock mass can cool (Craw, 1988; Craw and Koons, 1988 ).

Fluid transport Fault rocks help to define the fluid environment in mesothermal gold-quartz vein deposits and typically indicate brittle processes that have occurred at 3-5 km depths (Sibson, 1977; Rudnicki and Chen, 1988), where shallow earthquakes are generated. These earthquakes develop dilatant zones by opening extension cracks which attract fluid and rapidly redistribute it following seismic faulting, on the order on 10 ~° liters per event (Sibson et al., 1975). Etheridge et al. (1983), and Kerrich ( 1986 ) have suggested that transport of fluids in this environment is facilitated by high permeability conduits. These conduits would be represented as mylonites in granitoid-hosted goldfields (O'Hara, 1988; Bell and Cuff, 1989), or shear zones and cleavage in melange/sediment-hosted goldfields (Fig. 2 ). The fluids might emanate from depths of 5-10 km,

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

281

at 400°-500°C and 1-2 kb, (Kerrich, 1983; Colvine et al., 1988). Flow is upward and the fluid would have undergone cooling and decompression as it reached depths of < 3-4 km.

Single-pass flow According to Wood and Walther (1986), a zone of single-pass flow lies below about 3 km, at which depth fluid pressure increases, due to the rapid decline in porosity. As PIa20/Plith approaches 1.0 (Fyfe et al., 1978) convection does not take place. The flow is therefore steadily upward, due to the buoyancy force that is created because the fluid has a lower density than the surrounding rock (Henley, 1973). Above 2-3 km depth in the epithermal environment, fluid pressure is usually less than or equal to hydrostatic pressure (Cathles, 1977; Norton and Knight, 1977), and therefore downward convection takes place only at shallower depths (Fig. 2). Although, several conditions that might allow meteoric water to penetrate to the formation depths are imaginable, isotopic evidence usually supports single-pass flow. Further evidence of a single-pass flow regime is the restriction of alteration to narrow zones around fissures, rather than the more pervasive alteration patterns found in circulating systems. The lack of indicators of meteoric fluid, such as low salinity and lack of boiling, coupled with the 6180 values, point to a single-pass flow regime at intermediate depths for mesothermal quartz-vein deposits, rather than a convecting system at shallower depths.

Relationship offluid to faulting Fluid flow below 2-3 km in brittle and brittle-ductile shear zones is largely confined to fractures in and around the fault zone (Brace, 1980). Fluid in the fault zone plays a major role in many faulting mechanisms, (Hubert and Rubey, 1959) by mechanically reducing friction, weakening quartz and raising the total effective stress (Secor, 1965; Phillips, 1972, 1986; Rudnicki and Hsu, 1988). This also re-

6 •

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Normal solution

-D r e s s u r s

Fig. 3, Mohr diagram of normal stress plotted against shear stress, showing the effect of increased P,.~a on faulting. Circle A represents stress in stable fault zones at zero pore pressure. Circle B represents stress in unstable rock if total stresses are the same at A but if pore pressure is raised by an amount P~, represented by the bars. This raises effective stress to the point where it encounters the Mohr envelope and rupture occurs. Modified after Secor (1965) and Phillips (1972).

duces the shear stress required for brittle failure and faulting (Fig. 3). Permeability and fluid flow are enhanced by high stress and fluid pressure which cause local cracking and dilation. This suggests that dilated zones along a fault, that have attracted relatively larger volumes of fluid, would be the sites of multiple fault-movement (cf. Byedee and Brace, 1972 ). Constraints on the environment of mesothermal gold-quartz veins in fault zones at intermediate depths suggest that the fluids are either of metamorphic or magmatic derivation, and have most likely travelled and are transported upward by seismic pumping and the buoyancy force in single-pass flow. Furthermore, there is a likelihood that a strong interplay exists between the transport of the fluid in the fault and the movement of the fault. Internal constituents

The main internal constituents in oreshoots are quartz, gouge and shear zone fill, and altered wall-rock (Fig. 4 ). These constituents are usually mixed with each other in complex pat-

2 82

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Fig. 4. Sketch of an idealized vein showing multiple episodes of quartz veining and gouge formation. Quartz events are (1) early quartz, now fractured and veined with secondary quartz; (!I) second major vein quartz, (III) brecciated vein quartz (IV) silicification quartz assimilating footwall; and (V) hangingwall quartz vein which may or may not be related to earlier quartz types. Modified from Peters ( 1988 ).

terns, the relationships between which may be used to interpret the dynamics of oreshoot formation.

Quartz The main quartz textures present in oreshoots of mesothermal gold-quartz deposits are comb, ribbon, assimilation and breccia quartz. In addition microscopic secondary veinlets occur in each of these textural types. Banded, crustiform quartz, typical of the epithermal environment (Berger and Bethke, 1985; Bodnar et al., 1985), or fibrous quartz, characteristic of high fluid-pressure and crack-seal mechanisms (Beach, 1977; Ramsay, 1980b), are not present in these goldfields as part of the mineralizing stage. Similarly, sheeting and pervasive alteration typical of the porphyry environment is not present (cf. Adams, 1920; Dowling, 1989).

Comb quartz Comb quartz occurs early as simple, 0.2-40 cm thick veinlets and in vugs. It also occurs as late cross-cutting veins and veinlets within larger complex oreshoots. It typically consists

of milky, stubby 1-5 m m long, 0.25 m m diameter, well-knit, euhedral to subhedral crystals which are either parallel, or perpendicular to the vein walls or randomly oriented. Denser finer-grained varieties are also c o m m o n (Fig. 5A). Comb quartz is most c o m m o n in granitoid-hosted deposits (Johnston, 1940; Peters, 1987a), but also occurs in melange/sedimenthosted goldfields (Haynes, 1987; Peters, 1987b ). Several generations of comb quartz are usually evident in oreshoots. Sulphides and gold are precipitated within comb quartz adjacent to the vein wall or in the center of the vein between quartz euhedra.

Ribbon quartz Ribbon quartz is characterized by alternating 0.5-10 m bands of milky, dense or slightly comby quartz, and by 0.25-3 m m thick planar, darker, bands of wall rock (Fig. 5B). Ribbon quartz occurs in 3-20 cm thick individual veins or in similar widths along the margins of more massive or complex quartz. The dark bands are commonly altered to muscovite and may contain carbonaceous material. They are sites of sulphide and gold deposition, microbrecciation, slip, annealing and stylolitic textures.

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

283

Fig. 5. Photographs of quartz types in granitoid- and melange/sediment-hosted gold-quartz veins; (A) multiple, milimetre- and centimetre-scale comb quartz, Charters Towers, overprinting mylonite. Some of the early veining (I) may represent remnant (mylonitic) quartz. The second vein (II) is dense and sugary compared to many more open, thicker comb quartz veins. (B) ribbon quartz, Hodgkinson. Dark layers may comprise wall-rock carbonaceous material, stylolites, microbrecciation zones or sulphides. (C) breccia quartz, Charters Towers. These breccia fragments are shattered, dense comb-quartz (similar now to buck quartz) and have been cemented by later comb-quartz. Similar material occurs in the Hodgkinson goldfield. ( D ) assimilation quartz in carbonaceous sediment, Hodgkinson goldfield (E) breccia quartz due to assimilation of wall-rock fragments, Hodgkinson goldfield: a more advanced stage of that portrayed in d. (F) microphotograph of pointed termination (left, arrow) in assimilation quartz injecting sandstone, Hodgkinson goldfield.

Ribbon quartz is more common in melange/ sediment-hosted deposits (McKinstry and Ohle, 1949; Read and Meinert, 1986; Haynes, 1987; Peters, 1987b ).

Assimilation-silicification Assimilation-silicification quartz occurs in a number of settings: ( 1 ) multiple injection of

veinlets (Fig. 5D); (2) replacement of wallrock rafts around or within large vein-areas (Fig. 5E); and ( 3 ) incipient quartz growth due to silicification in areas of intense alteration. The distinguishing features of assimilation quartz are 0.25-2.0 m thick veins or masses, and gradational contacts, rather than the planar contacts of other quartz types (Fig. 5D, E and

284

F). Assimilation quartz is also found on larger scales in complex oreshoots, cementing older quartz wall-rock inclusions and gouge together. Cross-cutting relationships between veins suggest that most assimilation quartz may be precipitated throughout the life of oreshoot formation and may be closely related to ribbon and comb quartz.

Buck (bull) Buck (bull) is used here to define white, relatively barren, massive and milky quartz which is common in oreshoots of both granitoid- and melange/sediment-hosted gold-quartz vein deposits. Buck quartz is usually coarse grained with 1-5 mm anhedral crystals, few voids and numerous < 1/tm single-phase inclusions that dust and cloud the crystals in thin section, (background quartz in Fig. 6A, B, and E). It is commonly cracked, sheeted or truncated by small slips and joints. Buck quartz may occupy isolated 0.5-2 m thick tensional areas, or may occur in shear zones as elliptical pods which are surrounded by clay seams and gouge. The term buck quartz may be used to represent several quartz types, such as remnant quartz from host mylonite, pre-gold, dense equivalents of comb or ribbon quartz, or crushed and annealed varieties of quartz.

Breccia quartz Breccia quartz may consist of either comb, ribbon or buck quartz that has been changed into a network texture. The most common type consists of interlocking fractures which outline centrimetre-scale elliptical shapes (Fig. 5C) with sulphide, wall-rock, and gouge fragments along < 1 mm anastomosing surfaces of the fractures. In thin section, the millimetre-scale quartz grains in breccia quartz contain few fluid inclusions, are anhedral, ragged edged, locally welded and strained. Smaller 0.1 m m 30 #m crystals occupy annealed slip-planes which separate microscopic domains (Fig.

S.G. PETERS

6C). More brittle, non-annealed textures occur with matrix which consists of sulphide-gold or alteration minerals (Fig. 6A and B). Breccia textures also result from assimilation (Fig. 5E). Breccia quartz is usually spatially distinct from other quartz types.

Secondary microscopic quartz Secondary microscopic quartz cross-cuts comb, ribbon, buck, breccia and assimilation quartz as five types of millimetre-scale veinlets (Fig. 6C to F): ( 1 ) Veinlets with breccia components, typified by granulated and annealed textures, smears of wall-rock, stylolite textures, or by 1030/tm wide cracks with feathery clear illite, sulphides and gold (Fig. 6C ). (2) 0,1-3 mm thick clear veinlets (Fig. 5D), whicti have optical continuity with the dusty surrounding quartz and display no evidence of injection. ( 3 ) Fillings of millimetre- and micron-scale cracks and voids in the host quartz, locally accompanied by sulphides and gold, with similar fluid-inclusion data to the host-quartz types. (4) Stylolites in ribbon or buck quartz which may commonly connect with sulphide-gold aggregates (Fig. 6E). (5) Late quartz veinlets, commonly crosscutting the alteration selvages, associated with typical late-stage minerals such as calcite, chlorite and hematite (Fig. 6F). Quartz types found in most mesothermal gold-quartz vein goldfields show similar timerelationships and indicate several stages of quartz precipitation. Comb and ribbon quartz types provide the basic building units of oreshoots in each goldfield, and may be precipitated throughout the development of oreshoots. Breccia, buck and secondary quartz veinlets tend to overprint these main quartz types. Multiple-quartz textural types, mixed or separated by gouge, indicate that quartz deposition was repetitious and probably contemporaneous with fault movement.

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

285

Fig. 6. Photographs of late quartz types in granitoid- and melange/sediment-hosted quartz veins. (A) microphotograph of breccia quartz with sulphide matrix, Charters Towers. (B) breccia quartz similar to a, but with secondary veinlets of gouge and fine grained quartz (arrow), which cross-cut the matrix, Charters Towers. (C) microphotograph of breccia quartz showing at least three generations of quartz: 1=early comb preserved on the right which has been brecciated by fractures; 2= secondary, denser, more anhedral comb quartz; 3= 3 mm wide zones of microbrecciation and annealing invading the two earlier quartz types. (D) secondary microveinlets which have optical continuity with background dusty buck quartz; when crossing calcite, the veinlets contain fine grained muscovite. ( E ) stylolite crossing dusty ribbon (buck) quartz. Arrow shows site of sulphide and gold aggregates, Hodgkinson. (F) secondary (late) calcite sheeting, cross-cutting early phyllic alteration of granodiorite, Charters Towers.

Gouge and shear-zone fill C r u s h e d a n d b r e c c i a t e d rock, gouge, phyllonite a n d clay seams are m i x e d with q u a r t z a n d altered rocks in the o r e s h o o t s in m e s o t h e r m a l g o l d - q u a r t z vein deposits (Fig. 4 ) . C r u s h e d a n d b r e c c i a t e d r o c k is m o s t c o m m o n

in g r a n i t o i d a n d s a n d s t o n e - h o s t rock. It consists o f locally c e m e n t e d g r a n u l a t e d wall-rock a n d q u a r t z in metre-scale irregular shapes along the fissures, similar to the quartz gouge described by E n g e l d e r ( 1 9 7 4 ) . In the m o r e pelitic rocks in m e l a n g e / s e d i m e n t - h o s t e d goldfields, m o n t m o r i l l o n i t e , illite a n d m u s c o v i t e

286

S.G. PETERS

phyllonite is a particularly common constituent of the host shear-zones. This phyllonite occurs as anastomosing 1 m thick zones, which are truncated by 1-20 cm thick clay seams (Fig. 4 ). Typical gouge may be laminated with heterogeneous dark colours which range from grey to black to white to dark green, and may m

35

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Hydrothermal alteration

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represent different fault events. Gold values in some oreshoots are similar in the quartz and in the surrounding gouge. Gold values in the gouge result from early mineralized quartz that has been crushed and incorporated in the gouge (Fig. 7 ). Multiple zones of gouge mixed with quartz suggest the same as the hand specimen and microscopic quartz-textures: that multiple fault movement was contemporaneous with and punctuated by quartz deposition.

5

6

7

AU g / t

Fig. 7. Histograms of gold values from the Hodgkinson goldfield collected from 25 to 0.5 m channel samples for this study and analyzed by atomic absorption in (a) quartz veins, and (b) gouge and shear zone fill. Values in ppm Au. Mean values are similar. Gold occurs in ground and shattered quartz in the gouge, indicating that gold mineralization and quartz veining probably occurred before and after fault movement (see Fig. 4); from Peters (1987b).

In granitoid-hosted gold-quartz goldfields, 1 cm-1 m wide zoned selvages surround centimetre-scale quartz veinlets. These selvages are similar in style to hydrothermal alteration around veins in other granodiorite rocks described by Lindgren (1896), Bonorino (1959) and Meyer et al. ( 1968 ). Phyllic assemblages (dominated by muscovite) occur next to the vein and give way to propylitic to intermediate argillic-assemblages (dominated by montmorillonite and illite) zoned outward from the vein (Fig. 8). Sheeted networks of veins develop over-printed, mixed zones, with a central vein as the site of intense phyllic alteration and silicification. Williams (1974), Patterson (1982), Goldfarb et al. (1986), and Peters (1987b) report that alteration is generally poorly developed in many melange/sedimenthosted gold-quartz deposits, although, internal portions of the original shear-zone and local wall-rock may have adjacent phyllic alteration-assemblages in these deposits (cf. Tomlinson et al., 1988). Alteration zoning on an oreshoot scale in granitoid-hosted deposits defines the dilated zones which have attracted and concentrated fluid flow. A central-elliptical phyllic core-zone is surrounded by a propylitic selvage (Fig. 8 ). Zoning on a vein system scale ( 100-1000 m) occurs as productive phyllic-alteration near the oreshoots and barren propylitic to intermediate argillic-alteration along dip or strike of the oreshoots on the same vein system. The re-

287

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZVEIN DEPOSITS lOOm

I

PLAN VIEW /

/

/

,/

ORESHOOT PHYLLIC ALTERATION

(muscovite) PROPYLITIC ALTERATION (montorillonite) QUARTZ VEIN

//

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Fig. 8. Diagrammatic model for lateral hydrothermal alteration zoning around a granitoid-hosted mesothermal goldquartz oreshoot. The alteration pattern roughly indicates the zone of dilation. The alteration acts as early ground preparation to soften an elliptical area and produces gougeand shear zone material. Alteration zoning also indicates temperature and chemical gradients may have existed from inside to outside the oreshoot, and that fluid flow was enhanced in this area (from Peters, 1988). striction of the alteration assemblages to narrow selvages implies that fluid flow was restricted to high-permeability zones within the host fissure. Broader alteration envelopes around oreshoots, suggest that the oreshoots were the sites of m a x i m u m fluid-flow in the fissure plane, and they were also the sites of greatest porosity and greatest dispersion into the wall-rock.

Controls of oreshoot location and shape Vein systems o f gold-quartz deposits m a y be localized on older ductile structures which are oriented subparallel to the differential stress, so that they were coincidentally reactivated and dilated by stick-slip faulting. Oreshoots are most c o m m o n in dilatant zones caused by changes in strike, splays, lithologic contacts and fissure intersections (cf. Hulin, 1929; McKinstry, 1955; Bursnall, 1989; Hodgson, 1989). Changes in strike and dip of a fault have been shown to be favourable loci for dilation and are usually attributed to reverse m o v e m e n t on S

symmetry kinks in the fissure plane (Newhouse, 1940; Emmons, 1948 ). Dilation has also been recognized in tensional openings within shear zones in Archean gold deposits (Kerrich and Allison, 1978; G u h a et al., 1983). Oreshoots also occur in dilated shear settings or in stressed areas of complex structures, such as pre-existing folds or ductile pre-gold shearzones (Fig. 9a). Splaying (secondary faulting) has also been shown to generate tensional fields and to localize oreshoots (Fig. 9a). Local stress reduction at the points of splay is indicated to be as high as 20% by Chinnery ( 1966a and b ), and Segall and Pollard (1980) suggest that splays m a y be focal points for seismicity, dilation and heat flow. Geometric complexity and multiple m o v e m e n t in splaying areas (Lajtai, 1969), may encourage fluid pressure and stress gradients to develop through the fault network which would enhance permeability, thus channelling fluid-flow through the splayed portions of faults. Many oreshoots occur on one side or the other of lithologic contacts (Fig. 9b), and the

288

S.G. PETERS

7

-

a .~

~ ~l~j'~(iv)

(iii)

¢ ,.,~) ~ ++

CONTACTS /

( ....

(x)

[INTERSECTIONS Fig. 9. Controls of oreshoot location. (a) Dilation. (i) change in dip or strike, (ii) splaying, (iii) gash vein between shear veins, (iv) gash veins between ends of fauhs; (b) Contacts. (v) shoot control of hangingwall and footwall due to offset, (vi) refraction through contact and dilation on one side; (c) Intersections. relationships of oreshoot plunge (dark circle) to fissure intersections portrayed on equal angle stereonets. Solid lines are dilated host shear zones. Dashed lines are non-dilated structures such as mylonites, dikes, and planar contacts: (vii) Tyrconnel oreshoot in Hodgkinson goldfield and Golden Gate oreshoot in Queen Vein system, Charters Towers goldfield, (viii) Scandinavian and Swedenborg lodes, Charters Towers goldfields, (ix) Day Dawn vein system, Charters Towers, (x) Brilliant vein system, Charters Towers. (modified from Peters, 1988). plunge of the oreshoot m a y coincide with the intersection o f a host fissure and the contact. This is also similar to p h e n o m e n a found in vein deposits in general (Knopf, 1929; McKinstry, 1955; Reid et al., 1975 ). When lithologic contacts in a layered rock-sequence are offset by a perpendicular fault, they develop complex relationships between the oreshoot and the wall-

rock (Fig. 9b). Lithologic contacts represent zones of contrasting competency, chemistry, thermal conductance and porosity. Intersections of two mineralized fissures result in the plunge of the oreshoot within one or both of the fissures (Penrose, 1910). The geometry, such as X, T, or Y, and the angle of intersection also influences the hydrothermal alteration-pattern and the development o f the oreshoot. Barren cross-faults and non-dilated fissures near oreshoots form intersections which may also be coplanar with the oreshoot plunge (Fig. 9c). Intersections increase permeability by providing larger surface areas and by increasing fracture density in a localized area to provide a zone where fluids of slightly different temperature, density, pressure and chemistry may mix. Control of large oreshoots is complex and several causes of dilation are usually present. Different aspects of control may dominate different portions of the same oreshoot, may become interrelated, or may change with time as an oreshoot develops. In areas of multiple faulting the distinction between splaying, lowangle intersections, shearing, tension regimes, and attitude changes may not be apparent. Changes with time may progress from early broad dilation, to fault m o v e m e n t and quartz deposition, to more complex interconnected conduit networks. Although locally complex, oreshoot controls all point to oreshoot development in zones of dilation which focus fluid in a moving fault. I n t e r p r e t a t i o n o f t e x t u r a l v a r i a t i o n in oreshoots

Constituents of oreshoots and their textural relationships reflect different stages in the development and growth of oreshoots. Hydrothermal alteration is typically an early event in the oreshoots accompanied by high fluid-pressure. Fault movement, gouge development, and increased permeability within the host fissure, cause reduction in wall-rock fluid interactions

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

by insulating and channelling the fluid. Quartz deposition in a dilated zone accounts for many of the textural varieties of quartz found in oreshoots (Fig. 10). Assimilation and silicification quartz are usually related to alteration. Most of the other quartz textural-types are more diagnostic of specific locations within a dilated zone. Quartz lextures Comb quartz in granitoid-hosted and ribbon quartz in melange/sediment-hosted deposits (Figs. 5A and B) are the basic building blocks oforeshoots, especially in central dilated zones. The margins or growth areas of many oreshoots are most likely to be represented by

i

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289

breccia quartz. (cf. McKinstry and Ohle, 1949). Fault movement compresses portions of the fluid reservoir within dilated areas of the fault network which causes pressure to rise in the connected reservoir, according to Pascals Principle. If such pressure changes were accompanied by increase in temperature due to faulting, local solubility in SiO2 in the fluid would be affected. Stylolitic textures in ribbon quartz (Fig. 5E) may be a result of dissolution and compression (cf. McClay, 1977; points d and e in Fig. 10). Dissolution and stylolite development would also be likely at the margins of the oreshoots where quartz deposition seals the conduit and results in pressure and temperature increases. Heat produced during fault movement promotes overpressuring within fluid inclusions, causing explosive decrepitation and grain shattering (Kerrich, 1976; Sibson, 1977 ); this produces annealed uniform, milky textures by Fig. 10. Theoretical P-Tconditions in an hypothetical oreshoot and their effect on quartz deposition. A rising fluid starting at point a at 10-15 km depth cools and drops in pressure to point b, decreasing as much as 20 wt% SiO2 in H20, according to the data of Kennedy (1950) and Holland and Malinin (1979). Inset shows an hypothetical PT path of the fluid after entering and exiting a dilated chamber. According to Bernoulli's equation, the pressure (P~) at point b (Area A~) increases at point cfand area A2, as a result of the decrease in velocity - if (h2-h~) is small. Temperature is also lowered due to adiabatic expansion, resulting a slight increase in S i Q solubility (point c, see inset). Exiting of the fluid through a restriction to smaller area A3 causes a decrease in pressure and temperature resulting in reduced SiO2 solubility and precipitation at the roof of the chamber (point d). This precipitation eventually seals the chamber, and causes pressure and temperature and pressure build-up (point e). The pressure build-up enhances effective normal stress and leads to faulting, heat generation and a sudden release of pressure (point f inset). The fluid then passes up the conduit as it cools (point g). Because of the great dip length in many oreshoots, pressure reduction is usually greater due to release of hydrostatic pressure up dip, more than from change in velocity of the fluid. Local fluctuations may occur, however, where h = 0 such as in local pockets along the fault plane or in the areas such as Aa and A5 which have different pressures due to venturi effects (modified from Peters, 1988).

290 creating numerous < 1 pm single-phase fluid inclusions. Buck quartz formed in this way may be former comb and massive ribbon quartz that has responded to stress by decrepitation, accompanied by intergranular creep (McClay, 1977) and annealing. Clear microveinlets which wash out < 5/~m inclusions but retain optical continuity with the host quartz (Fig. 6d), could also form by rapid influxes of heat along the fault plane causing intergranular recrystallization. The most common mechanism of static fatigue in quartz is by water-induced stress corrosion or cracking (Scholz, 1972; Atkinson and Meredith, 1981), which is dependent upon stress, temperature and water pressure (Griggs, 1967; Jones, 1975 ). Most crack growth in faults occurs prior to faulting as a result of large increases in porosity and dilation before failure (Brace and Bombolakis, 1963). Cracks in quartz associated with high PH2o conditions, develop in jumps with sudden stress drops (Martin, 1972 ), and such cracking in the presence of a silica-rich fluid, may explain many microscopic breccia and secondary quartz veinlets observed in oreshoots (Fig. 6A, B and C). These secondary quartz veinlets could result from the lowering of silica solubility in mineralizing fluids travelling through cracks which are subjected to temperature and pressure fluctuations during the faulting process.

History of fault movement Quartz and metal deposition in gold-quartz vein oreshoots takes place in dilated zones within faults due to reductions in pressure and temperature as the fluid rises up the host conduit. Local conditions, such as channel size, compression during faulting, or sealing of the conduit, may also affect pressure and temperature within the fault zone and lead to quartz deposition. Most quartz textures observed in oreshoots can be explained by sequential processes which involve a fluid entering and exit-

S.G.PETERS ing complex dilated zones in a moving faultsystem. The history and conditions of fault movement are reflected in the relationship of the internal constituents (quartz types, gouge and alteration) in the shear zone. The first event is usually alteration, particularly in granitoidhosted goldfields (Peters, 1987a), and assimilation quartz accompanies this early stage in melange/sediment-hosted deposits (Peters, 1987b). Alteration is followed by comb and ribbon quartz veinlets. Cracking, crushing and annealing of these early quartz types by fault movement produces complex shapes and mixtures of gouge and quartz (Fig. 4). For this reason breccia quartz and many secondary microscopic-quartz veinlets post-date most other quartz textural types. Each goldfield contains mixtures, quartz textures, and alteration styles which reflect unique local formation conditions and wall-rock type. Oreshoots in most mesothermal gold-quartz vein deposits share many of the same internal constituents, because similar processes have operated during oreshoot formation.

Relationship of quartz and metal deposition to faulting The solubility of quartz in aqueous solutions is relatively independent of fluid salinity and pH but is particularly sensitive to changes in pressure above 350°C as shown by Kennedy (1950). Holland and Malinin (1979) have calculated that 1 g SiO2 would precipitate from 1000 g of solution at similar pressure and temperature gradients. For a very large oreshoot of 106 tonnes of quartz, this would require about 109 tonnes of fluid (cf. Fyfe and Kerrich, 1984).

Bottom and center of oreshoot Figure 10 illustrates the change in terms of solubility of SiO2 and P - T conditions as a silica-rich fluid enters the oreshoot environment. Fluid travelling from a 5-10 km depth to inter-

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

mediate depths in the brittle deformation (mesothermal) environment (Fig. 2 ) would cool up to 150°C and drop about 550 bars in pressure in the process (points a-b, Fig. 10). Once at that elevation in the crust (point b, Fig. 10), stick-slip strain rates in a fluid-filled fissure could cause rapid local volume expansions due to dilation along the fissure of to 102103 m 3 in size. Faulting that could cause these expansions would be preceded by pressure build-up (pont c, Fig. 10). Adiabatic expansion and throttling of fluid into these zones along steep temperature and pressure gradients (point d, Fig. 10) would reduce SiO2 solubility and quartz deposition, through adiabatic cooling (Barton and Toulmin, 1961; Sharp, 1965; Toulmin and Clark, 1967). Under these conditions, quartz deposition takes place near the entrance to the dilated zones (Fig. 10). Once within the dilated zone, changes in fluid temperature and pressure are related to changes in fluid velocity which are dictated by the size of the channelway. Narrow portions of the channel might be on the order of 0.25 m wide (cf. Pyrak-Nolte et al., 1988) with strike lengths of 100 m (A~=25 m 2, Fig. 10). Dilated zones might be up to 1.5 m wide (A2= 150 m 2, Fig. 10). Fluid channelled into such dilated zones at rates of about 1 m / s e c 2 (V1) would slow to about 0.17 m / s e c 2 (V2), according to the continuity eq.:

V2 = (AI/A2) V~

(1)

and the pressure would increase in the larger channel volume according to Bernoulli's eq.:

Pi+l/2V~+h~pg=P2+l/2V~+h2pg

(2)

where p = t h e density of the fluid and g = t h e gravitational constant.

Top ofore shoot The reverse happens when the fluid exits the dilated zone, and pressure on the fluid drops. In addition, hydrostatic pressure decreases from the bottom to the top (h3-hl) of the di-

291

lated zone. This decrease in pressure lowers SiO2 solubility and quartz precipitates at the roof of the chamber (points c-d, Fig. 10). Quartz deposition at the roof of the dilated zone plugs the conduit and chokes the fluid (Fournier, 1985). The buoyancy force builds pressure under the seal which causes heat buildup because the fluid takes longer to cool (points d-e, Fig. 10). This rise in P-T increases the effective stress (Fig. 3) and contributes to fault movement and cracking, which breaks the seal and causes the fluid to escape from the dilated zone into a more narrow conduit. Above this point, fluid pressure also increases causing a decrease in quartz solubility (points e and f, Fig. 10). Precipitation of additional quartz above the developing oreshoot leads to further plugging of the dilated zone and causes repetitious P-T fluctuations (throttling) involving more faulting, quartz precipitation and cracking at the top of the dilated zone.

Effect of dip length and splays Elevation differences ( h 3 - h l ) in many oreshoots may be on the order of several hundred metres (Fig. 10 ) and therefore the effect of dip length on the pressure within the host shearzone can be calculated by combining elements from eq. (2):

P2=PI +I/2 ( V 2 t - V ~ ) - p g ( h 2 - h l )

(3)

Such an elevation change affects AP more than the change in velocity (Ah>AV). As fluid enters or exits a dilated zone, (Ah>AV), so that long dip-lengths result in general precipitation of quartz due to decreasing temperatures and pressure gradients (points f-g, Fig. 10), more: than change in channel size. Local instances where (A V< Ah) occur however, are usually at entrances and exits of dilated zones or in dilated zones with small dip-lengths (Fig. 10, and Laffite, 1962). Pressure and temperature changes in these cases tend to cause quartz deposition at the top and sides of an oreshoot area

292

and result in inward growth. Pressure differences, due to fluid flow to intersecting fissures in complex vein-systems, may also affect quartz deposition. For example, the elevation of areas A4 and A5 on Fig. 10 are equal (Ah = 0 ) , but A5 has a higher pressure than A4 due to the venturi effect. This illustrates the complex pressure and mixing regimes that may develop in the oreshoot environment.

Causes of fluid variation Mesothermal gold-quartz goldfields have uniform • 180quartz_whol e rock values of 6.5-7.0°/0o and relatively uniform quartz fluid-inclusion homogenization temperature and salinity, suggesting a single homogeneous fluid. There are no large heating and cooling trends through-out deposition due to pulsing different or evolving fluids. Deposition of quartz and metals results from variation in the fluid, caused by local conditions within the fault zone. The complex oreshoot environment provides many causes of fluid variation, such as wall-rock interaction, hydrothermal alteration, fluid immiscibility, dehydration of fault gouge and P - T changes, which may be responsible for most fluid variation in the fissure-hosted intermediate environment.

Fluid-wall-rock interaction Fluid-wall-rock interaction would be one way to affect fluid flow and chemistry and cause fluid variation along the fault plane. Chemical gradients between different rock types might affect ascending fluids at lithologic contacts in gold-quartz vein deposits (Bohlke, 1982; Fyfe and Kerrich, 1984 ). Fluid in fissures which cross lithologic contacts might also be affected by thermal gradients, caused by heat capacity-differences between rock types. Variation in the fluid, may also be due to direct chemical relationships between gold deposition and carbonaceous wall-rock (Wall and Ceplecha, 1976; Glasson and Keays, 1978 ). Chemical activity, pH and temperature

S.G. PETERS

gradients may result from alteration zoning around the quartz vein as the fluid flows away from the vein into the wall-rock (Rose and Burt, 1979). If fluid is channelled into separate structural-zones or wall-rock types, different streams of fluid tend to evolve independently (Fig. 11 ).

Hydrothermal alteration Hydrothermal alteration may affect fluid variation directly, by causing mechanical and chemical changes in areas of high fluid-flow (dilated areas) in the host fault-zones. These changes produce localized ground softening ("ground preparation" ). Prior to faulting, fluid transport within alteration selvages would initially be due to diffusion where chemical-concentration gradients dictate fluid flow (Frantz and Weisbrod, 1974; Watson and Brennan, 1987 ). Brittle faulting and gouge development following hydrothermal alteration produces cracking and increases porosity in developing oreshoots. Fluid transport after porosity increases would be due by infiltration, which is more dependent upon pressure gradients (Rose and Burt, 1979). Muscovite alteration liberates silica into the passing fluid (Coveny, 1981) and adjacent phyllic alteration assemblages often develop into zones of assimilation and silicification quartz. Alteration may also add heat to the system as an exothermic reaction (Cathles, 1977 ). These effects - ground softening, fault enhancement, silica introduction, and heat generation - suggest that alteration plays a physical as well as chemical role in oreshoot development.

Fluid-immiscibility processes Fluid-immiscibility processes may also be a source of fluid variation due to the physical properties of the fluid. Mixtures of aqueous brine and compounds of C, S, and N may be immiscible at temperatures below 440°C (Sterner and Bodnar, 1984). Fluids may become physically separated and further segre-

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

293

Fig. 11. Model of fluid flow and cause of district variation. Sketch of penetrative shear zones or cleavagein pelites which act as conduits that channel mineralizing fluid. Perpendicular planes (hatched) represent F2 axial planes or other shear zones in adjacent sandstone. As fluid enters these penetrative fabrics, separation takes place. This allows for separate evolution of a once-homogenousfluid due to site-specificwall-rockinteraction and localized thermal regimes. Post-gold tectonic and metallogenicevents may preferentially remobilize only one fabric and thereby overprint one structural style of early mineralization, but not the other. gated by their differential adsorption properties (Crawford and Hollister, 1986). In these cases, water-rich fluid may coat the surface of vein channelways and cracks, whereas sulphurous and gaseous components in the fluid moves along the centers. Similarly, density differences in immiscible water-brine and w a t e r CO2 fluids may result in different flow rates, for different components, thus further segregating fluid types according to their constituents (Watson and Brennan, 1987). Fluid immiscibility would therefore also account for local heterogeneity found in some fluid-inclusion clusters and in irregular micro-textures of quartz.

Dehydration of fault gouge Dehydration of fault gouge causes fluid variation during faulting episodes. Montmorillonitic fault gouge becomes impermeable at highstrain rates (Wang et al., 1979) and this impermeable gouge plays an important role in blocking or channelling fluid flow within the shear zone by acting as a local pressure seal. If

the micas or clays in the gouge are hydrated, compression reduces internal pore-pressure and releases saline fluid into the wall-rock or fault zone during fault m o v e m e n t (Wang and Mao, 1979). Such properties of gouge aid quartz and metal deposition by affecting the physical and chemical properties of the mineralizing fluid. Fluid travelling and separated into gouge-filled (compressive) conduits may evolve separately to fluid channelled into tensional conduits with lesser quantities of gouge (Fig. 11 ).

Changes in pressure~temperature Changes in pressure/temperature also cause fluid variation and may affect the solubility of quartz in aqueous solutions (Fig. 10). The largest effect on quartz precipitation is due to cooling upon ascent. A fluid travelling from 5 to 10 km at 1.5 kb at about 450°C would cool about 150 ° C so that the wt% solubility of SiO2 in HzO would decrease about 20% (Figs. 2 and 10), according to the data of Kennedy ( 1950 ) and Holland and Malinin ( 1979 ). In addition,

294

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FORMATION OF ORESHOOTS 1N MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

local changes in the oreshoot environment may affect SiO2 solubility in H20 on the order of 15% as silica-rich fluid cools, due to adiabatic expansion into dilated zones (Fournier, 1985 ). Pressure increases as the fluid enters these dilated zones and decreases as the fluid exits, according to Bernoulli's equation. These conditions tend to locally reduce S i O 2 solubility in the fluid, causing precipitation and plugging of the conduit at the top and at exits of the dilated zone (Fig. 10 ). Choking of the exits leads to temperature and pressure build-ups which initiate faulting (Fig. 3), and lead to additional flow, which in turn leads to repetitive precipitation, sealing of the exits, and throttling. The stable phyllic-alteration assemblages surrounding the gold-quartz veins are due to neutral pH fluids (Hemley and Jones, 1964; Meyer et al., 1968). Such fluids at moderate temperatures and low salinities transport gold via thiocomplexes (Seward, 1973). Decreases in fluid pH decrease the solubility of gold (Seward, 1984) and pyrite, galena and sphalerite are temperature as well as pH dependent (Barton and Skinner, 1979). Deposition of these sulphides lowers the activity of sulphur in the fluid which also lowers gold solubility (Seward, 1984) and allows for the deposition of these minerals with gold. Alteration zoning (Fig. 8 ) around the oreshoots indicates possible pH gradients which would allow precipitation of metals within the oreshoots. Quartz textural-types, gouge and shear-zone

295

fill, alteration style, and isotopic and fluid-inclusion data, all point to a relatively weakly to moderately saline homogeneous-fluid which ascended and cooled along discrete channelways in single-pass flow. Evidence for heterogeneous fluids due to pulsing or mixing is generally lacking in gold-quartz vein deposits, although later influx of meteoric water in a waning, uplifted collapsing system is possible. Changing conditions within the fault system such as wall-rock reactions, and P - T changes are probably responsible for fluid variation and can be used to explain most quartz and metal deposition.

Sequence of oreshoot growth Oreshoots that were formed at intermediate (3-5 km) depths usually occur in zones of dilation and high fluid-flow along intermittently (stick-slip) moving faults. The internal constituents within an oreshoot, such as quartz, gouge and hydrothermal alteration, commonly indicate a complex history of development. Indicators of fluid parameters suggest that a single homogenous, cooling fluid ascended along the host fault system and that a fluid-fault/ wall-rock interplay was responsible for variation in and evolution of the fluid. This caused quartz and metal deposition in spatially isolated oreshoots along the fault network. Models for oreshoot formation in these intermediate depth, fissure-related (mesothermal) gold-quartz veins are portrayed for gran-

Fig. 12. Models of oreshoot growth. The four steps show the sequential development of an oreshoot from a single pass, relatively homogeneous fluid. (a) Idealized steps of oreshoot growth in melange/sediment-hosted gold-quartz deposits: ( 1 ) single fissure, simple quartz (I) where Pnuid> Plith; (2) reinjection of two generations of ribbon quartz (11 and lll) along fissure and early fault movement; (3) major fault movement, gouge development, brecciation and segregation of quartz pods with fluid flow restricted to permeable cracks and sheeting in quartz pods; (4) consolidation and growth of mature oreshoot, with pulverizing of early quartz material, which is segregated by clay seams and growth of late quartz over growths (IV) and new veins (V). (b) Idealized steps of oreshoot growth in granitoid-hosted gold-quartz deposits: ( 1 ) simple comb quartz vein (I) with zoned inner phyllic core and outer propylitic halo; (2) reinjection of comb quartz veins (H) and alteration halos with silicification and choking of main conduit (III): (3) early faulting with gouge and clay seam development, isolating and cracking early quartz types (IV); (4) major faulting and mature oreshoot development with breccia quartz and micro-veinlets (V), late buck quartz (VI) and additional silicification and thick gouge. (c) Diagrammatic plan view, looking down dip of a exaggerated oreshoot. The numbers refer to oreshoot growth stages in Figs. 12a and 12b and are located in approximately the position that these internal features occupy in an oreshoot.

296

itoid-hosted and melange/sediment-hosted deposits in Fig. 12. These models are based on the integration and interpretation of quartz textural-types, structural controls and timing relationships between oreshoot components. Although ground preparation and deformation differ in each host-rock type, both oreshoot growth models have four main development steps: (1) ground softening and nucleation, (2) overprinting and reinjection, ( 3 ) fault movement and (4) consolidation. I Ground softening and nucleation The first stage of oreshoot formation involves ground softening, by hydrothermal alteration, with high fluid-pressure and diffusion fluid transport away from a central joint or fissure. Centimetre-scale comb quartz veinlets usually form in granitoid-hosted deposits and are surrounded by symmetrical alteration selvages (Fig. 12a). In areas of higher fluid pressures, typically in melange/sedimenthosted deposits, assimilation quartz with pointed terminations (Fig. 5f) invades wallrock adjacent to early-formed fissures (Fig. 12b). These early veinlets in both host-rock types may be focused on zones of earlier weakness, such as existing mylonites, dykes, cleavage or igneous apophyses. Simple features which are diagnostic of this stage would be most common at the peripheries of oreshoots and are not usually associated with commercial mineralization (Fig. 12c). H Overprinting and reinjection The second stage of oreshoot formation involves reinjection, overprinting, assimilation, and development of a lode zone. Major chemical transformation of the lode zone from fresh rock to altered rock is accomplished during this stage and the gross alteration-zoning pattern is established. Alteration may locally develop into silicification (assimilation-quartz) within some fissures and sheeted zones (Fig. 12b).

S.G. PETERS

Secondary comb quartz veins develop in granitoid-hosted deposits, but larger 10-20 cm thick ribbon quartz develops in melange/sediment-hosted deposits. At this stage, quartz precipitation begins to choke the conduits in the dilated zone and leads to early pressure build-ups which enhance fault movement. This movement develops along focal slip-planes, thin clay seams, and cracks in early-quartz pods and leads to complex fluid-flow paths and fluid transport by infiltration. The internal constituents that are diagnostic of this stage are also common in the transition between oreshoot and adjacent mineralized fissure along strike of the oreshoot (Fig. 12c).

III Fault movement

The third stage of oreshoot formation is typified by major fault movement within the earlier prepared lode zone. Gouge, clay seams and pods of breccia quartz are common and are usually mixed together. Fluid flow may be blocked by gouge and then be channelled through higher-permeability areas in the cracked-quartz pods. Micro-sheeting develops along planar weaknesses in ribbon quartz and local temperature and pressure gradients produce dissolution and stylolitic textures. Microscopic secondary veinlets (Tf=220-280°C) develop in the cracks and sheeted zones, especially near the peripheries of the oreshoots (Fig. 12c). Repeated movement and quartz deposition lead to metre-scale cuspate quartz shapes in the developing oreshoots that locally gape when off-set by fault movement, forming open pockets for further quartz precipitation, similar to mechanisms proposed by Johnston (1940). These isolated open pockets aid fluid ponding and promote fluid stagnation. Additional fault movement may cause local pulsation and streaming of these fluids along the fault zone, due to compression and local pressure gradients.

FORMATION OF ORESHOOTS IN MESOTHERMAL GOLD-QUARTZ VEIN DEPOSITS

IV Consolidation The fourth stage of oreshoot formation involves consolidation and growth of several mixed gouge and quartz zones, which are joined together within and along the fault plane into larger mature oreshoots. Plucking of wallrock and old quartz-vein material, rotation, brecciation and gouge development are diagnostic of this stage and responsible for complex shapes (Fig. 12a and b). Fluid flow is channelled selectively throughout the oreshoot at this time leading to local secondary-quartz and metal deposition along fractures and leads to local pressure build-ups and fracture. Late ribbon and comb quartz, microveinlets, breccia quartz, clay seams and gouge are well developed and mixed, and may develop centrally in the oreshoot (Fig. 12a and b). Complex separate pockets or pods along a lode zone or vein network may connect to form larger oreshoots, as consolidation and assimilation develops. Fluid composition changes and cools to less than 250°C due to ultimate diversion and cessation of fluid flow and due to local stagnation and ponding. Fluid influx would gradually decrease and late stage usually barren mineral assemblages such as calcite and chlorite, develop during late events. As the oreshoots develop, temperatures generally decrease from 350 ° to 220°C, overall fluid pressure drops, and chemical transport shifts from diffusion to infiltration. Gold and sulphide mineralization can be continuous throughout the four stages of oreshoot growth or may be confined to one stage only, if the system is sensitive to gold and metal deposition or transport. Complex oreshoots usually display multiple episodes of mineralization, grow to a relatively large size, and have the chance to develop high metal-contents.

Conclusions (1) Oreshoots in mesothermal gold-quartz vein granitoid- and melange/sediment-hosted

297

deposits contain several quartz textural-types which are mixed with gouge, shear-zone fill and alteration. (2) The nature of fault rocks associated with the oreshoots suggest that oreshoots develop in brittle to brittle-ductile stick-slip fault zones at 2-5 km depth and at formation temperatures between 220 ° and 350°C. (3) Fluid is most likely single-pass flow in these deposit types and may have been generated from either deep magmatic or metamorphic sources. The common elements between many granitoid- and melange/sediment-hosted deposits is derivation of a homogenous fluid from depth rather than local emanation of a fluid from or influenced by a nearby plutonic heat-source. (4) Oreshoots develop as fault movement and fluid flow produce pressure and chemical gradients in localized areas within the host fissures which, in turn, effect quartz and metal deposition. ( 5 ) The oreshoot areas develop physical and chemical properties that attract and channel fluid flow and cause additional brittle deformation and quartz and metal deposition.

Acknowledgements This paper resulted from PhD research at James Cook University of North Queensland on gold-quartz veins, as part of an Australian Industry Research Association, sponsored project on Gold Deposits in Northeastern Queensland. Funding was provided by a JCUNQ Research Scholarship. The concepts of oreshoot formation gained clarity from discussion with Greg Morrison. Early drafts of the manuscript were also read and improved by Chris Cuff, Peter Pollard, Roger Taylor and two anonymous reviewers. Kim Dowling provided useful discussions regarding quartz classification and texture in gold veins. Typing was gratefully completed by Barbara Kemp of Western Mining Corporation Ltd.

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