Nomenclature, concepts and classification of oreshoots in vein deposits

Ore Geology Reviews, 8 (1993) 3-22 3 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Nomenclature, concepts and classification of oreshoots in vein deposits

Stephen G. Peters Central Norseman Gold Corp. Ltd., P.O. Box 56, Norseman, WA 6443, Australia

(Received October 15, 1991; revised version accepted October 2, 1992 )

ABSTRACT

Peters, S.G., 1993. Nomenclature, concepts and classification of oreshoots in vein deposits. In: D.I. Groves and J.M. Bennett (Editors), Structural Setting and Controls on Mineral Deposits. Ore Geol. Rev., 8: 3-22.

Oreshoots are discrete hypogene masses usually hosted within a planar channel, surface, lode or conduit which may be either a shear zone, fissure, fault zone, or lithologic bed or unit such as a contact. Oreshoots are characterized by breadth, strike ( > 1000 m) and dip, and plunge ( 100-500 m) lengths and have higher metal contents than the adjacent parts of the host conduit. The mass of most oreshoots ranges between 1 X 104 and 2 X 104 tonnes. There is a tendency for oreshoots to be thicker and richer in the center, rather than to have uniform grade distributions. The thickness of the oreshoots may be between 0.25 and 1.75 m in shear-zone-hosted deposits, to up to 60 m in replacement deposits. Several conduits may connect to form vein systems. Vein systems have common fluid sources which result in general homogeneity of alteration, mineralization types and oreshoot control, and, therefore, commonly share the same plumbing system. The internal constituents usually reflect unique episodes relating to ore formation. The main intern constituents in oreshoots are mineralization, gangue and alteration. These constituents usually mix with each other in complex patterns, the relationships between which may be used to interpret the processes of oreshoot formation.

The term "ground preparation" represents the effect of various events in the geologic history of an ore district or oreshoot area that have assisted in enhancing the rocks so that oreshoots can preferentially form in certain areas or geometries. Several types of ground preparation can be recognized: ( 1 ) sequential deformation that produces a grain in the rock, (2) severe faulting and jointing which augments permeability and areas where ore minerals can precipitate, and (3) interplay between ore fluid and deformation to produce an oreshoot.

Controls of oreshoot location and shape are usually due to dilatant zones caused by changes in attitude, splays, lithologic contacts and intersections. In addition, conceptual parameters such as district fabric, magic distances and stacking are also used to describe the geometry of oreshoots. Controls in vein systems and the location and geometry of oreshoots within vein systems can be predicted by a number of qualitative concepts such as internal and external plunges, district plunge, district stacking, conduit classification, gradients and warps. These concepts have a practical and empirical application in most districts where they are useful in the exploration for ore, but are of such broad and general application that they can rarely be explained definitively.

Introduction

The knowledge of oreshoots in epigenetic deposits and the prediction of their location and plunges has always been a major concern

Correspondence to: S.G. Peters, United States Geological Survey, Reno Field Office, Mackay School of Mines, Reno, NV 89557-0047, USA.

of economic geologists (Penrose, 1910; Hulin, 1929; Blanchard, 1931; McKinstry, 1941 ). Be- cause each mining district or camp has its unique geologic characteristics and history, a diverse terminology evolved concerning the orebodies and their geometries around the world. However, because of common characteristics there are many simple names and concepts that are applicable in most vein deposits.

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

4 S.G. PETERS

The objectives of this paper are to catalogue, in increasing scale, the most commonly used names and concepts and to propose a nomenclature that can be used to bridge the gap between applications for practical mine geologists and the research of academicians. A common nomenclature facilitates the communication between upper management in mineral exploration and production where clear ideas are necessary to insure productive budget levels and to enable realistic business deci- sions. A unique nomenclature would also facil- itate the communication between and among corporations. This paper addresses and classi- fies the common empirical concepts and associated terminology in order to link the many areas of expertise in vein mining. Processes pertaining to these concepts are not addressed here; they are discussed in a companion paper by Peters (1991a). This paper focuses on epi-

genetic, vein-type mineralization in situations where oreshoots can be identified. Syngenetic, stratiform, and stratabound orebodies are not considered as they have their own nomenclature and concepts.

Orehoots in veins are discrete hypogene masses usually hosted within a planar structure which has acted as a conduit for the ore fluid. These structures are commonly referred to as a channel, surface, or lode which may be a shear zone, fissure, fault zone, or a lithologic boundary. The geometry of oreshoots is ex- pressed with reference to breadth and strike lengths (> 1000 m), and by dip (Lindgren, 1933; Jensen and Bateman, 1981 ) and plunge lengths ( 100 to 500 m; Fig. 1 ). Oreshoots are characterized by a higher metal content than the adjacent parts of the host conduit. The mass of most oreshoots containing precious metal veins ranges between 2 10 4 and 1 10 6

C,

CHANNEL, SURFACE LOD

OR CONDUIT

b ! RAKE \ C OR

~\~/ P ITCH

SPLAY

B,

GASH OR LADDER VEINS

Fig. 1. Sketch of anatomy and nomenclature of oreshoot elements. Stereo net depicts planes (upper case letters) and angles (lower case letters) in block diagram.

NOMENCLATURE, CONCEPTS AND CLASSIFICATION OF ORESHOOTS IN VEIN DEPOSITS 5

" 75 - \ \ , I ' , \

,-,~ ~\ ~- -~. "~

., \ x . \ \

I

Fig. 2. Examples of vein systems in some ore districts: (a) Cripple Creek, Colorado. Vein systems are clustered in and around a Tertiary circular sedimentary-volcanic basin within Precambrian crystalline rocks. Each vein system has distinct ore controls (Lindgren and Ransome, 1906; Koschmann, 1949 ). (b) Charters Towers, Northeastern Queensland. Several clusters of east-striking and shorter north-striking vein systems have ore controls affected by host plutons (outlined in thinner pen) in a composite batholith. Ore controls are also different in each cluster and sub-cluster (Peters, 1990). (c) Hodgkinson goldfield, Northeastern Queensland. Vein systems are clustered along structural domains. Local tungsten is restricted to the vein systems along domain boundaries (Peters et al., 1990). (d) Butte, Montana. Multiple overprinting hydrothermal events have selectively invaded this complex vein system (Meyer et al., 1968).

tonnes. Parts of the oreshoot and conduit systems are named as if they were parts of fault systems (cf., Ramsay and Huber, 19 8 7 ).

There is a tendency for oreshoots to be thicker and richer in the center, in a lobe, or along one side (cf., Dickinson, 1942), rather than to be uniform in grade distribution. Ore- shoots may terminate abruptly or may taper in thickness or grade to assay cut-offs. Usually the terminations are caused by geologic features, such as shear zones, quartz or alteration boundaries. The thickness of oreshoots is typically between 0.25 and 1.75 m in shear-zone- hosted precious-metal veins, and up to 60 m

thick in vein-associated replacement deposits. Subsidiary veins, which connect to the main oreshoot, are referred to as spur, cross or caunter veins (Finucane, 1948) or as links (O'Driscoll, 1953), and leg and neck reefs (Hodgson, 1989). Unique oreshoot configu- rations may take the form of ladder veins, gash veins, or even stockworks. The plunge of an oreshoot is defined as the direction and inclina- tion of its central (best-fit) axis. The rake or pitch of an oreshoot refers to the angle between its central axis and the horizontal within a planar surface, such as the host conduit or a projection plane (Fig. 1 ).

6 S.G. PETERS

The most characteristic feature of many oreshoots is its complexity. Internal constituents usually reflect unique episodes relating to ore formation. Oreshoots are composed of specific mineral assemblages that are distinct from assemblages and rock types outside the oreshoot. Gangue and alteration mineral assemblages and gouge between oreshoots define the barren or lower-grade portions of the host and conduit (Fig. 1 ).

Several oreshoots clustered together can be taken to define an orebody. Several conduits can connect to form vein systems. An individual vein system will typically represent a single, connected plumbing system with a common fluid source and, therefore, oreshoots within one vein system commonly show a sim- ilarity in alteration, mineralization and structural controls which distinguishes them from other vein systems. The plunges of individual oreshoots within a vein system may be related to the geometry of the entire vein system (cf., Fig. 2 ).

Internal constituents

The main internal constituents of an oreshoot are the products of ore mineralization, gangue minerals, and assemblages formed by hydrothermal alteration. These constituents are usually closely related with each other in complex patterns (cf., Robert and Brown, 1986a; Peters, 1991 a). Their relationships can be used to interpret processes of oreshoot formation and paragenesis (Ramdohr, 1969; Stanton, 1972). Recognition of zoning, growth types, intergrowths and contact rims, impurities and repetitions allow inferences concerning the geologic history of the oreshoot, as discussed by Peters ( 1988, 1991a).

Mineralization

Mineralization in oreshoots is composed of ore minerals such as sulfides, tellurides or na- tive gold. These ore minerals are usually asso-

ciated with one main gangue mineral such as quartz, a silicate, or a carbonate. Mineraliza- tion occurs within the vein, disseminated in wall-rock inclusions, or within the wall rock it- self, i.e., in the alteration zone surrounding the main conduit. Deposition of ore minerals may be simultaneous, successive or overlapping. Post-mineralizing events such as overprinting, decomposition and supergene enhancement must be filtered out of any interpretation prior to genetic modelling.

Gangue

The texture of gangue minerals helps the interpretation of the history of the oreshoot. Quartz, the most common gangue mineral in many kinds of veins, has genetically diagnostic aspects as discussed by Adams (1920) and Dowling and Morrison ( 1989 ). Comb, ribbon (laminated), assimilation and breccia quartz, together with microscopic secondary veinlets occur in mesothermal gold-quartz deposits (McKinstry and Ohle, 1949; Peters, 1991a). Banded, crustiform and open-space filling quartz are more typical of the epithermal environment (Bodnar et al., 1985; Berger and Bethke, 1985 ). Open-space filling textures are also observed in both extensional and shear veins in many ductile environments (Robert and Brown, 1986b). Fibrous quartz is characteristic of high fluid pressure and crack-seal mechanisms (Secor, 1965; Beach, 1977; Ram- say, 1980b), and is typical of more deeply formed deposits with high fluid pressures that may pump open the host rock, usually in a ductile environment (Poulsen and Robert, 1989; Sibson et al., 1988). Similarly, sheeted veins with pervasive alteration are typical of the porphyry environment. The term "buck or bull" quartz is 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 vari- eties of quartz.


Conduit petrology

Ore conduits are filled with a mixture of crushed and brecciated wall rock, gouge, phyllonite, clay seams and foliated rock mixed with gangue and altered wall rocks. These constituents are important indicators of the formation of an oreshoot in shear zones. In many cases, a fault rock type can be directly related to a wall rock type, as discussed by Sibson (1977). For instance, quartz gouge (Engelder, 1974) is most common in granitoid or sandstone host rocks, whereas pelitic rocks more commonly alter to montmorillonite, illite and muscovite- rich phyllonite and clay seams (fluchan).

The type, amount or thickness of gouge, proximal to or within a segment of the host conduit, may be proportional to the relative amounts of movement in that specific area of the fault plane (Hull, 1988; Walsh and Watter- son, 1988, 1989), and may also indicate specific areas of greater shear or compressional strain compared to dilated zones.

Hydrothermal alteration

Patterns of alteration zones on oreshoot scale define the dilated zones that have attracted and concentrated fluid flow (Rose and Burt, 1979; Robert and Brown, 1986b). Typically, a central elliptical core zone occurs in the wall rock as productive alteration adjacent to the oreshoots, and fringe zone alteration occurs outside, along dip or strike, of the oreshoots within the same vein system. Fringe zone alteration is chemically or thermally related to productive alteration and commonly contains low-grade mineralization. Barren alteration may also occur along the same conduit, and although part of the same mineralizing event, indicates thermal and chemical conditions where mineralization could not occur. Unrelated alteration is alteration that came before or after the mineralizing event (i.e., early- or mid-barren alteration of Lovering, 1949) and commonly lies outside the host conduit (Fig. 3). Magmatic

hydrothermal systems, such as porphyry or volcanic hot spring environments, usually have several distinct hydrothermal events overprinting each other and creating complex alteration styles.

The restriction of alteration assemblages to narrow selvages implies that fluid flow was restricted to high-permeability zones within a host conduit (Lindgren, 1896). Broader alteration envelopes around oreshoots suggest that the oreshoots were the sites of maximum fluid flow in the fissure plane, and were also the sites of greatest porosity and greatest lateral disper- sion in the wall rock. In addition, more "reactive" wall rocks lead to wider alteration haloes than lesser reactive rocks along the same conduit.

Hydrothermal alteration may directly affect the growth of oreshoots, by causing mechanical and chemical changes to the wall rocks in these areas of high fluid flow. These changes produce local ground softening or hardening (ground preparation). Ground softening (chloritization or pervasive clay alteration) will result in weaker zones which may control the localization of subsequent faulting and shearing. Ground hardening will lead to competency and will favor fracturing, brecciation and increased porosity. Chemical ground preparation also results from hydrothermal alteration. Muscovite alteration liberates silica into the passing fluid (Coveney, 1981) and phyllic alteration assemblages adjacent to the conduit often develop in zones of wall-rock assimilation and silicification. The alteration process may also add heat to the system if it involves exothermic reactions (Cathles, 1977 ). Potential effects such as ground preparation, fault enhancement, silica introduction, and heat generation, suggest that alteration plays both a physical and chemical role in oreshoot development.

Oreshoot textures

The textural relationship between gangue, ore minerals and wall-rock alteration assem-

8 S.G. PETERS

Fig. 3. Sketch of a plane or section of an hypothetical oreshoot at a contact between two rock types (+'s and v's). The barren pre-ore alteration may have contributed to early ground preparation. Productive alteration surrounds and is in direct contact with the oreshoot, which contains gangue and mineralization. Fringe-zone alteration occurs outside the productive alteration. Barren alteration may also be related to the same fluid or mineralizing event, but may signify different chemical or physical conditions. Post-ore alteration is usually unrelated but may locally overprint mineralization.

blage allow discrimination of the processes of oreshoot formation. Textures found in oreshoots may represent pre-ore, syn-ore and post- ore events in the host conduit. Multiple-fluid episodes, different fault movements and chemical replacement may all have specific textural signatures. Ore fluid is usually circu- lating in these areas of multiple episodes where high complexity and porosity may predate the main mineralization. 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 equa- tion. 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. Pressure and chemical aspects of the ore fluid may also have caused fault movement and dilation to further develop in specific areas that form into oreshoots. Skinner (1979) has suggested four main causes of mineral deposition: (1) de- crease in temperature, (2) boiling through de- crease in pressure, (3) chemical changes due to hydrothermal alteration, and (4) chemical changes due to fluid mixing.

Textural variation is common within, between and along oreshoots and conduits. For example, brecciated wall-rock material may be cemented by ore and gangue minerals, or brecciated ore and gangue minerals may be ce-


mented by later stages of mineralization. In other cases, banded and crustiform quartz of oreshoots in epithermal vein deposits may give way to chalcedony and agate within the host conduit at levels above the oreshoot (Berger and Eimon, 1982).

Interpretation of oreshoot features

As Bateman (1942) noted, the precise nature of oreshoot control and genesis is un- solved in the majority of cases. However, it is often possible to make an interpretation of the effects of ground preparation and the various stages of oreshoot growth from analysis of the internal constituents of an oreshoot.

Ground preparation

Ground preparation refers to local changes, which will later favor ore deposition that occur prior to the arrival of the ore-forming fluid. This concept is distinctly different from that of dynamic interaction of the ore fluid with the wall rock to promote oreshoot growth during mineralization. The difference between these two concepts is illustrated by the distinction proposed by Poulsen and Robert (1989) between geometric oreshoots, those that are the result of the intersection of the host conduit with favorable geologic elements, and kine- matic oreshoots, those related to active shear zones and vein development by conduit/fluid interaction.

Several types of ground preparation can be recognized: ( 1 ) physical preparation of the ore conduit to enhance permeability and prepare a trap site; (2) chemical preparation of a trap site or district; and (3) development of a unique district fabric and trap site due to events throughout the geologic history. An example of the first and second type is localized faulting or crackling, and alteration, at the top of a cupola.

Stages of oreshoot growth

Oreshoots usually occur in zones of dilation and high fluid flow. The internal constituents of an oreshoot (such as quartz, gouge and hydrothermal alteration minerals), commonly indicate a complex history of development (cf., Laffitte, 1962). Models for oreshoot formation in brittle-ductile shear-zone-hosted gold- quartz veins are provided by Lang (1979), Foster (1989), Bouchet et al., (1989), and illustrated by Peters ( 1988, 1991 a ) based on the integration and interpreted study of quartz textural types, structural controls and timing relationships between oreshoot components. Typical oreshoot growth sequences consist of: (1) ground preparation and nucleation, (2) overprinting and reinjection, (3) fault movement and (4) consolidation. Mineralization can be continuous throughout these four stages of oreshoot growth, or it may be confined to only one stage. Complex oreshoots that display multiple episodes of mineralization may grow to become relatively large, and, therefore, have the chance to develop high metal contents. The early ground preparation and nucleation stage involves hydrothermal alteration, at relatively high fluid pressure with resultant fluid diffu- sion away from the central conduit and its fo- cusing in zones of earlier weakness, such as existing mylonites, dikes, cleavage or igneous apophyses. Fluid pressure may also be lowered at releasing bends (Sibson, 1990 ) or dilational jogs along strike slip faults in the near-surface environment. In this case, fluid-pressure changes are a consequence of faulting rather than a trigger for faulting.

The overprinting and re-injection stage involves wall-rock assimilation and the development of a lode zone and is typified by a major chemical transformation of the conduit. At this stage, gangue and mineralization precipitation begins to choke the conduit in the dilated zones and leads to early pressure build- ups which enhance fault movement along slip planes.

10 S.G. PETERS

The fault movement stage is typified by major fault movement within the earlier prepared lode zone. This results in mechanical deformation of the internal constituents of the early stages. The amount of displacement along faults is not uniform but varies. There may be a tendency for those parts of the faults near oreshoots to have increased displacement and this results in gouge, clay seams and pods of brecciated gangue and mineralization mixed together. Microscopic secondary veinlets develop in the cracks and sheeted zones, espe- cially near the peripheries of the oreshoots. Repeated movement and deposition lead to meter-scale cuspate shapes in the developing oreshoots. These cuspate shapes locally gape where offset by prior fault movement, forming open pockets for further precipitation, similar to the mechanisms proposed for the formation of gold-bearing quartz veins at Grass Valley, California (Johnston, 1940 ).

The consolidation stage involves growth of several zones, which may be joined together within and along the conduit into larger, composite, mature oreshoots. Plucking of wall rock and old vein material, rotation, brecciation and gouge development are diagnostic of this stage and responsible for complex oreshoot shapes.

Controls of oreshoot location and shape

Oreshoots are most common in dilatant zones caused by changes in attitude, splays, lithologic contacts, and intersections (cf., Hu- lin, 1929; Hulin and Goddard, 1950; Mc- Kinstry, 1955; Bursnall, 1989; Hodgson, 1989). In addition, conceptual parameters such as district fabric, magic distances and stacking are also useful to describe the geometry of the oreshoots. Favorable sites are usually found in areas of low mean stress, of tensile or shear failure, within large strain zones, or in areas of tensile stress.

Changes in attitude

Changes in strike and dip of a host conduit have been shown to be favorable loci for dilation (Fig. 4) and, in a dip sense, are usually attributed to reverse or normal movement on properly oriented kinks with S or Z symmetry in the fissure plane (Newhouse, 1940; Em- mons, 1948; Garnet, 1966). Dilation due to attitude changes has also been recognized in tensional openings within shear zones in vein tungsten (Brown, 1957), silver veins (Lyons, 1988), and Archean gold deposits (Kerrich and Allison, 1978; Guha et al., 1983). Ore- shoots also occur in dilated shear settings or in disturbed areas of complex structures (Blan- chard, 1936 ), such as pre-existing folds or ductile pre-gold shear zones (Fig. 4 ). A special type

a

J

J \ d.

ilOOm I

CHANGES

IN

ATTITUDE

Fig. 4. Changes in attitude. Sketches of oreshoots related to local changes in strike and dip (rolls and warps in host conduit). (a) Oreshoot at short section of attitude change; (b) composite oreshoot in long portions of attitude change; (c) oreshoot in concave roll in vein; (d) oreshoot in convex roll in vein accompanied by intersection of an auxiliary fissure. (From Peters, 1987b.)

NOMENCLATURE, CONCEPTS AND CLASSIFICATION OF ORESHOOTS 1N VEIN DEPOSITS 1 1

of change in strike and dip is refraction where attitude change of the host conduit is coinci- dent with a change in the host-rock type as described by Knopf (1929) and Reid et al. ( 1975 ), and discussed by Treagus ( 1988 ) and Peters (1987c).

Splaying

Secondary faulting or horse tailing has also been shown to generate tensional fields and lo- calize oreshoots (Fig. 5 ). The local mean stress reduction at the points of splay is indicated to be as high as 20% by Chinnery ( 1966a,b ). Se- gall and Pollard (1980) suggest that splays may be focal points for seismicity, dilation and heat

a .

SPLAYS

b.

I C. Fig. 5. Splaying. Sketches of oreshoots related to splaying in host conduits. (a) Oreshoot on short hanging-walt splays sandstone; (b) and (c) oreshoots on footwall splays; (d) oreshoots located in complex splaying, intersection and change in strike; (e) oreshoots along multiple splays, and (f) oreshoot at splay intersection. (From Peters, 1987b.)

flow. Cymoid loops enclose cymoid lenses where two major veins are connected by shear splays to form a duplex structure (Sibson, 1990). Ore is common in the horse between two veins, or adjacent to bends, close to the closure of the duplex or in proximal tensional gashes (cf., Gemmel et al., 1988; Lyons, 1988; Harley and Charlesworth, 1990; Teagle et al., 1990). Geometric complexity and multiple movement in splaying areas (Lajtai, 1969) may result in local tensile stresses (Gamond, 1987 ). This may encourage fluid pressure and stress gradients to develop through the fault network, enhancing permeability and channel fluid flow through the splayed portions of the faults. Standard vein and fissure orientations based on models of Tchalenko (1968) can predict or differentiate tensional or compressional orientations in a complex array of faults or shear zones (cf., Mueller et al., 1988).

Lithologic contacts

Many oreshoots occur on one side or the other of lithologic contacts (Fig. 6), and the plunge of the oreshoot may coincide with the intersection of a host fissure and a lithologic contact (Knopf, 1929; McKinstry, 1955; Reid et al., 1975 ). When lithologic contacts in a layered rock sequence are offset by a perpendicu- lar fault, complex relationships between the oreshoot and the wall rocks may develop (Fig. 6 ). Lithologic contacts represent zones of con- trasting competency, chemistry, thermal con- ductance and porosity. Lithology may influence conduit and vein style, such as at the Arltunga goldfield, Northern Territory (Dirks and Wilson, 1991 ), where tensional veins are parallel to kink-zone boundaries in competent units, and tension gashes or dilational veins are formed in incompetent units. Special types of ore deposits in host structures interacting with lithologic contacts are represented by bedding- plane faults and saddle reefs, as described by Behre (1937), Cox et al. (1986), and Tomlin- son et al., (1988).

12 S.G. PETERS

ROCK TYPE

"~i~i~!~i iiiiiiiiii:~::%., I

i loo~ i

: :.....

~ - j r." t 5~ I

I aom I

I l oom I

Fig. 6. Sketches of the relationship of oreshoots to stratigraphy. (a) Oreshoots in siliceous shale at the refracted intersection with veined chert; (b) oreshoots at intersections of juxtaposed stratigraphic units; (c) oreshoot at refracted contacts of sandstone and carbonaceous shale; (d) local undulations within oreshoots due to juxtaposed shale bands; (e) oreshoots generally avoiding chert and forming in shale; (f) oreshoot along the intersection of shear zone and carbonaceous shale band. (From Peters, 1987b.)

Intersections

The intersection of two mineralized conduits commonly results in an oreshoot (Rick- ard, 1902 ) and the oreshoot will plunge within one or both of the conduits parallel to the intersection (Penrose, 1910). The geometry, such as X, T, or Y shapes, and the angle of intersection also influences the hydrothermal alteration pattern and the development of the oreshoot. Barren cross-faults and non-dilated fissures near oreshoots form intersections which may also be collinear with the oreshoot

plunge (Fig. 7 ). Intersections favor ore deposition and increase porosity 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.

District fabric

Each district contains a variety of rock types which have acquired fabrics due to their unique geologic history. Hydrothermal mineralization and oreshoot formation are most commonly superimposed on the existing fabrics (cf., White et al., 1986; Raybould, 1976). Oreshoot plunge, location and abundance can usually be directly related to unique geometries which ex- isted prior to mineralization. For example, a strong mineral or intersection lineation in the host rocks may be related to (Poulsen and Robert, 1989) or be used by later major host shear zones, or may also define the plunge of some oreshoots, such as in the Messina copper deposits, northern Transvaal (Fig. 8). In this way, the mineralizing solutions and associated deformation use the older fabrics. In some cases deformation events may overprint old shear zones and the plutonic rocks intruding them. This overprinting deformation may lead to reactivation of old shear zones and development of new faults in the plutonic rocks, providing new structural sites for mineralization in both the old shear zone and the younger faults.

In the Cripple Creek district, the myloni- tized contact between PreCambrian granite and the augen gneisses has been truncated by a Tertiary caldera development; the mylonite zone has been subsequently reactivated, pro- ducing new brittle faults and mineralization in both the mylonite zone and the caldera-fllling lithologies (Fig. 9). In other cases, vein systems of gold-quartz deposits may be localized on older ductile structures which are oriented subparallel to the differential stress, so that they


a b. C

INTERSECTIONS

Fig. 7. Sketches of mineralization related to intersections. (a) Intersection of early and late fissures to produce veins and shoots (at circles); (b) intersection of master and secondary shear zones with oreshoots forming at the boundaries on both sets; (c) oreshoots localized at the intersection of master fissures and cross-tensional fissures. (From Peters, 1987b. )

] PARAGNEISS ] GRANITE GNEISS

4,

I 1kin I b ~ ~

"1':"::: /

... ii.!!.

Fig. 8. Oreshoots controlled by district fabric at Messina, Northern Transvaal. Granulite-facies gramte gneiss and accom- panying paragenesis have been folded in two main ductile events. The brittle Messina Fault and F3 "warping" accompany hydrothermal copper mineralization, which is nucleated on the early fabrics. Stereo nets show contoured lineations and common oreshoot plunges (black dots): (a) Cambell Mine: (b) Harper Mine; (c) Messina Mine where breccia pipe plunges down conical fold fabric, and (d) Spence and Artonvilla Mines. (Adapted from Songe, 1946; Jacobsen, 1974; and Jacobsen and McCarthy, 1976. )

were coincidental ly react ivated and di lated by stick-slip faulting.

In another example involving complex district fabrics at Charters Towers, Queens land

(Fig. 10), oreshoots were formed preferentially in older mylonit ic rocks; the flat plunges of the oreshoots in the Day Dawn Lode mimic the mul l ion and elongation l ineation in the

14 S.G. PETERS

:!

500 metres

J Gold Structures

/.S Myionile

BEACON HILL DEPOSITS

Fig. 9. District fabric affecting location of lode structures at Cripple Creek, Colorado. A northeast-trending mylon- itized contact between PreCambrian granite and augen gneiss, at Beacon Hill, has been propagated within younger Tertiary rocks as brittle fissures. Gold mineralization has used both the new and the old fabrics. (Adapted from Lindgren and Ransome, 1906. )

mylonite, but along strike of the veins in un- deformed rocks, postdating the mylonite, a sympathetic fissure system has been propagated and oreshoot plunges are steep (Peters, 1990; Peters and Golding, 1987, 1989).

"Magic" distances

2

3

E - W A4YLONITE

ORDOV/CIAN GRANITOIDS

_ ~ NW MYLONffE

DEVON~AN GRANITOIDS

x PRESERVED MEGABLASTS

Fig. I0. Sketch of development of district fabric at Chart- ers Towers, Northeastern Queensland. East-striking mylonites ( 1 ) are preferentially preserved during batholithic development (2, 3) through various deformation and plutonic events in inliers. These early fabrics are preferentially oriented for post-batholithic hydrothermal gold mineralization, and where they are preserved, they served as channelways for mineralizing fluids and dilated traps for oreshoots. In this sketch two 2-3-km diameter circular megablasts preserve the east-striking fabrics (4). (Adapted from Peters, 1990. )

In some instances, oreshoots may form at specific empirically predictable distances away from geologic contacts or along conduits. There may also be spatial punctuation or periodicity between oreshoots and host conduits that define patterns (Petersen, 1990). Examples are in the gold-quartz oreshoots at Norseman, Western Australia (Campbell, 1990) which systematically lie about 1000-1500 m to the west of a banded iron formation. Gradients due to temperature, pressure or fluid chemistry can

be applied to the geometries found in oreshoots in terms of metal ratio contours to explain magic distances (Petersen et at., 1977; Loucks and Petersen, 1988). Other examples are where oreshoots lie a distinct distance from a contact along a host conduit or where veins lie certain distances apart as discussed by Ku- tina et al. (1967). Magic distances are tradi- tionally empiric qualitative concepts used in ore districts (Fig. 11 ).


"MAGIC DISTANCE"

ORE Y dW, ."0%

Fig. l 1. Sketch of magic distance in an hypothetical area with two rock types and two vein directions. Oreshoots tend to occur in sandstone (dots) typically a distance "d" in from the granite contact. The concept of magic distance is empiric and does not imply process.

Stacking

In many vein systems oreshoots tend to "stack up" or to be aligned along specific, predictable orientations from one host conduit to another, such as in the Kapunda Mine, South Australia (Dickinson, 1944). Cross faulting, folding or other geologic entities can also be aligned along the stacking direction; however, this direction, like magic distance, is also an empirical or qualitative concept and the link to geologic features may be weak or in some cases entirely lacking. Stacking can take place in more than one plane (Fig. 12). Periodicity or stacking of oreshoots within an ore conduit along the horizontal can be quantified as the development ratio, which is defined by the to- tal strike length of the conduit divided by the length of oreshoots along that horizon. A typical use of the development ratio is used in old districts to measure how much underground development might be necessary along a vein to expose a number of oreshoots, based on pre- vious production (drifts/stopes). The development ratio usually differs from one conduit to another within and between vein systems.

An example of stacking due to a district fabric has been described by Narayanaswami et

~ s J

Fig. 12. Sketch illustrating stacking direction in an area with a layered sequence of rocks with crossing shear zones. Ore zones cluster where these shear zones cross the sandstone (dots)-shale (dashes) contacts. The location of the oreshoots straddles the contact, however. For instance, point A would be a more likely location for the next oreshoot, rather than point B. Stacking is an empirical concept and may defy structural analysis. It is also three dimensional.

al. (1960) in the Kolar goldfield where veins have been localized on stratigraphic contacts between folded competent massive amphibolite and less competent amphibolite. The veins form en-echelon oreshoots in areas of dilation where veins cross-cut folds and the thickest veins occur at the crest and troughs of the folds. The multi-generational folding produces a variety of pattems and the oreshoots are "stacked" along a geometric fabric that has been prepared by the folding.

Controls in vein systems

Location and geometry of oreshoots within vein systems can be predicted by using a number of qualitative concepts such as internal and external plunges, district plunge, district stacking, conduit classification, gradients and warps. These concepts have practical application in most districts in exploration for and production of ore, but are of such broad and general application that they can rarely be explained definitively. These concepts are in many ways

16 S.G. PETERS

district-scale equivalents of those used on the oreshoot scale and describe different predictable ways in which entire clusters of oreshoots display geometric patterns.

Internal and external plunges

Within a single conduit or lode, oreshoots may cluster in such a way that individual oreshoots plunge in one direction (internal plunge) but the overall cluster may plunge in another direction (external plunge). For example, the oreshoots in the Mararoa lode at Norseman, Western Australia, plunge exter- nally to the north, controlled by a favorable bed, which host the oreshoots but have elongation internally to the southeast, controlled by cross shear zones or gabbroic dikes (Fig. 13a). In another case, oreshoots within the Bobtail lode at Cripple Creek tend to plunge as a cluster to the north along a breccia/granite contact, although the individual oreshoots have elongation directions plunging to the south (Fig. 13b ).

District plunge

Within individual vein systems, oreshoots or clusters of oreshoots may stack or plunge in predictable geometries, as with the oreshoots in the Independence vein system at Cripple Creek (Fig. 14). Another example is the tendency for most of the oreshoots at Norseman to occur consistently more deeply toward the north (Thomas et al., 1990). When similar geometries occur in a number of vein systems, a district plunge can be described and may point to a control of stratigraphy or indicate the source or pathway of ore fluids within the district. This concept can be used to predict blind (non-outcropping) undiscovered vein systems or repetition of vein systems at depth.

District stacking

When oreshoots cluster and plunge as a group, these groups may be repeated or stacked

- - z.--..]~

.... SURFAC~ ~

[] ....... . A . . - ~ L 1k in J

a MARAROA REEF

b BOBTAIL LODE

Fig. 13. Examples of longitudinal projections showing internal (small arrows ) and external (large arrows ) plunge: (a) Mararoa Reef Norseman, Western Australia. Four distinct oreshoots individually plunge to the south but all of the lodes tend to plunge as a clustered group in a predictable trend to the north. The envelope containing the external plunge represents the trace of a specific stratigraphic bed on the reef surface (adapted from Conolly, 1936; Cambell, 1990; and Thomas et al., 1990). (b) Bob- tail lode, Cripple Creek, Colorado. The main trend of mineralization is steeply to the north along the breccia-granite contact. Individual smaller oreshoots tend to plunge to the south. Two levels of stopes are depicted in black to illustrate how interpretation is made.

along predictable directions if the same conditions exist away from the original cluster (Fig. 14). An example of repetition of geologic conditions might be a second favorable host rock such as a sedimentary or volcanic unit, or a secondary cross fault, parallel to the known controlling cross fault. At Charters Towers vein systems stack and group with less than random patterns within plutons (Fig. 2b). At Norse-


N S

SURFACE +

] ORE ZONE ~ ~ + + +

~ l l STOPING D ISTR ICT PLUNGE

I 500m I (CRIPPLE CREEK)

Fig. 14. Sketch of a longitudinal projection along the Independence vein system at Cripple Creek, Colorado, illustrating district plunge. Numerous large oreshoots cluster and plunge as a group to the south as a shallow angle, similar to external plunge but at a larger scale. This concept allows speculations of repetitions of entire oreshoot clusters or postulation of fluid-flow pathways.

man, Western Australia major north-striking lodes are stacked along cross-cutting north- northwest cross faults (Campbell, 1990).

Deep conduits may guide fluid flow from depth to the oreshoot environment and act as high-permeability conduits as suggested by Ethridge et al. (1983). Stacking of different conduits or portions of conduits may represent areas of fluid flow along separate conduits that are connected at depth. General characteristics of mineralization are similar in each stacked portion of a vein system or oreshoot cluster, but differences in tenor and strength and oreshoot control between them may be due to local structural and chemical conditions with separate fluid evolution that could have devel- oped within each conduit. Fluid evolution rate would be dependent on the depth of separa- tion from a parent fluid and the disequili- brium of the fluid with the wall rock. These > 1-km-scale geometries allow different or later fluids to selectively enter the district through individual vein systems. This allows separate avenues of fluid flow within one ore district and accommodates and spatially partitions sepa-

rate metallogenic episodes if they are present ( Peters, 1987a).

Conduit classification

Conduits within vein systems may each have different characteristics, some of which are more conducive to oreshoot formation that others. Many vein systems contain long strike length, continuous master fissures which ex- hibit fabric and other signs of shear movement (cf. Ramsay, 1980a). These are accompanied by shorter strike length auxiliary conduits which contain more signs of extension or tension (Ramsay and Huber, 1983), such as the spur, link or caunter veins (Figs. 1 and 15). There is a tendency for oreshoots to occur within these more tensional structures where fluid flow and porosity is increased. Explora- tion and mapping techniques, such as low-level soil and rock geochemistry and alteration studies and joint density studies allow discrimination within a district or vein system of which areas or conduit orientation had the greatest fluid flow. These areas may be synonymous

18 S.G. PETERS

TENSION VE INS

I

~o I

60 45

I I

a. b. c .

S d. e .

50m I I

Fig. 15. Tension veins. Types of tension and spur veins in the Hodgkinson goldfield (Peters, 1987b). (a) Hanging wall and footwall spur veins; (b) complex spur veins associated with auxiliary shear zones; (c) gash veins filled with quartz; (d) large-scale parallel ladder veins filled with quartz; (e) section oforeshoot in gash vein between two splays; (f) gash veins between master shears.

with or are separate from traps or areas where the fluids precipitated concentrated mineralization in oreshoots.

Gradients

As fluids flow through or up a conduit they move from one geologic environment to another. Changes in fluid chemistry, temperature, or pressure are responsible for precipitation of ore and gangue minerals (cf., Edwards and Atkinson, pp. 161-163 ). These changes of environment can sometimes be directly corre- lated with lithologic contacts, metamorphic grade or other geologic parameters. Gradients in strain, porosity, temperature, pressure, and chemistry can be due to metamorphism (Hen-

ley et al., 1976; Kerrich, 1986), rock-type changes, faulting or shear zones, folding, mag- matism or other factors. The magnitude and spatial patterns of the gradients in relation to host conduits on a district or vein-system scale are important features in the distribution of oreshoots. Gradients may repeat within a district and be responsible for district plunges and district stacking.

Warps

Broad-scale gentle folding may locally be superimposed upon host rocks and vein systems on a district scale and may dilate large areas of the crust, e.g., kilometer-scale dilatant areas which trap oil and gas. These warps occur late in deformation sequences and usually follow older tighter folding or shear-zone development. Examples of warping occur at Messina, Northern Transvaal where F3 folding appears to bow a broad area which may have focused the ore fluids to form a district centered on a 15-km length of a master fissure (Fig. 8 ). Sim- ilar-scale warps can be interpreted in many districts by interpreting district rock distribution, contact shapes and fold symmetries. Warps may explain why one zone of a conduit is mineralized but another is not. The geometry of the dilation due to warping, such as dilation refraction or partitioning through lithologic or structural regimes can explain and predict district stacking.

Conclusions

The control of large oreshoots is complex, and several causes of dilation are usually present. One distinct, unambiguous set of names and concepts for different components of oreshoots, vein systems and their geometries allows the comparison of different districts and also allows clarifying, productive communication between geoscientists interested in study- ing or exploiting the mineralization. Different aspects of control of mineralization may dom-


inate different portions of the same oreshoot, may become interrelated, or may be interpreted to change with time as the oreshoot de- veloped. In areas of multiple faulting, the distinction between splaying and low-angle intersections, is not clear.The development of an oreshoot changes through time; it may progress from early broad dilation to fault movement and quartz deposition, and then to more complex interconnected conduit net- works. The anatomy of oreshoots is displayed, on a small scale, by the petrology and textural relationships of the internal constituents. At larger scales, oreshoot features are displayed by the geometric relationships between and within the vein systems. The systematic documenta- tion and analysis of oreshoots and vein-system components will, hopefully, result in the con- ceptualization of processes or empiric relationships which are applicable to district ap- praisal and ore discovery.

Acknowledgements

This paper was prepared with the knowledge and appreciation of pressing searches for oreshoots in the Messina, Cripple Creek and Norseman districts, and of studies of the Charters Towers and Hodgldnson districts. The ideas mentioned in this paper are formed in close association with geologists, engineers and prospectors in these districts, who also had an avid interest in understanding, finding, and mining oreshoots. Early drafts of this paper where read by W.C. Peters and K. Johnson who provided comments and suggested some changes and additions which improved the paper. The manuscript benefited from the reviews and comments of Francois Robert, and an anonymous reviewer of Ore Geology Re- views. Drafting was done by M. Kelly at Cen- tral Norseman Gold Corporation Ltd. and is gratefully acknowledged.

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Nomenclature, concepts and classification of oreshoots in vein deposits