Structural Openings and the Localization of Ore Bodies

Eric P. Nelson1, Leandro Echavarria1, and Jonathan Saul Caine2

1 Dept. of Geology/Geological Engineering, Colorado School of Mines 2 U.S. Geological Survey, Denver, Colorado Abstract

Structural openings form in rocks by brittle, dilatant deformation over numerous scales and

many types of ore deposits are localized by the flow of hydrothermal fluids through such

openings in structurally permeable rocks. Structural permeability refers to transient

enhancement of fluid flux through strain-related openings. The distribution, geometry, density,

and orientation of ore bodies in structural openings are controlled by two primary factors: the far

and near field stresses (i.e., the far stress field is not always what produces structures in the

near field) and the mechanical properties of rock. Other driving forces include localized

changes in stress related to fluid pressure and heat transfer within a deforming rock mass. We

discuss how stress and internal mechanical properties control the formation of structural

openings, and present a classification of geological structures that host ore bodies as a result of

the interactions between stress, strain, fluid flow, heat transfer, and chemical reaction.

When stress controls the orientation and kinematics of faults and fractures in the absence of

pre-existing weaknesses, permeability anisotropy can develop with kmax parallel to the

intermediate principal stress direction (2). In this case ore shoot orientation is perpendicular to the fault slip vector and can be predicted by kinematic analysis. Such ore shoots form due to

non-planar fault geometry where the orientation of the fault zone approaches parallelism with

the plane containing 1-2. However, when structural openings form along pre-existing weaknesses, ore shoot orientation cannot always be predicted by kinematic analysis.

Variations in mechanical properties of rock undergoing deformation cause variations in

rheological response due to lithological changes, temperature gradients, and pre-existing

weaknesses such as faults, joints, and bedding. For example, brittle layers preferentially

develop vein arrays and boudin necks, and can cause developing structures that transect them

to be refracted from their initial path. Fluid pressure variations also can affect rheological

behavior and promote brittle failure, but are not easily identified or mapped.

The principal geologic structures that host ore bodies include fault and shear zones, joints and

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joint arrays, structural intersections, folds, strain shadows, igneous-related structures, and

collapse structures. Further, the geometry of these principal structures can range from linear to

planar to irregular, and can include en echelon, conjugate, intersecting, and stockwork

geometries. Because we can classify ore bodies based on the types of geological structures

discussed above and the variations that naturally occur from material property heterogeneity,

exploration for hydrothermal ore deposits should include analysis of geologic structure and

evolution, rheological variations, pre-existing weakness, and fault kinematics to evaluate

structural controls on localization of ore bodies.

Introduction

The knowledge that geological structures host metal deposits is very old, and dates back to the

time of the ancient Greeks (Agricola, 1556). This knowledge has aided mineral exploration for

many years in a general sense and, as Guilbert & Park (1986) noted, Detailed studies of

structure are essential in exploration, and they unquestionably have led to more discoveries of

ore than any other approach. Structural openings formed from crustal deformation control

hydrothermal fluid flow during and after deformation coupled with mineralization, and thus can

control the location, geometry and orientation of many ore deposits.

Structural openings form in rocks by brittle, dilatant deformation over numerous scales and

many types of ore deposits are localized by the flow of hydrothermal fluids through such

openings in structurally permeable rocks. The distribution, geometry, density, and orientation of

ore bodies in structural openings are controlled by two primary factors: the far and near field

stresses and the mechanical properties of rock. Other driving forces include localized changes

in stress related to fluid pressure and heat transfer within a deforming rock mass.

Examples of structurally controlled upper crustal ore deposits are epithermal precious metal and

polymetallic vein systems and porphyry systems (Cu Au, Mo) (McKinstry, 1955; Richards &

Tosdal, 2001). Examples of structurally controlled deposits formed at deeper crustal levels

include the mesothermal (or orogenic) shear-hosted deposits typical of the Archean cratons,

such as those in the Abitibi belt, Canada (Robert & Poulsen, 1997) and in the Kalgoorlie district,

Western Australia (Solomon & Groves, 2000), and the Bendigo saddle reef deposit in Victoria,

Australia (Cox et al., 1991). Common expressions of ore in structural openings are veins and

breccias. Veins are mineral-filled fractures, and breccias are fractured rock in which fracture-

bounded blocks have rotated. The geometry of veins can range from linear to planar to

irregular, and can include en echelon, conjugate, intersecting, and stockwork geometries.

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Breccias also can have a wide range of geometries, but linear and tabular geometries are most

common.

Because structural openings are important in the formation of many ore deposits, especially in

Cordilleran-type deposits formed in the upper crust, we review and focus this paper on how

stress and the mechanical properties of rock control the formation of structural openings, and

how structural openings localize hydrothermal ore deposits. Because past classifications of

veins and other structural openings contain many old terms and ambiguous mixtures of

categories, we propose a new working classification of structural openings that is based on

specific geologic structures that can be linked to stress and rock properties. We hope our

proposal will lead to a more comprehensive and useful classification system. Lastly, we briefly

discuss structural methods that can aid the explorationist in predicting the location and

orientation of ore shoots formed in structural openings.

Structural Permeability

Structural permeability refers to transient enhancement of fluid flux through strain-related

openings (Sibson, 1996). Such permeability is paramount in the development of many ore

deposits that are produced by focused flow of relatively large volumes of hydrothermal fluids.

Structural permeability typically forms in the upper, brittle crust where macroscopic fault and

fracture systems are common, but also can form in the more ductile deeper crust in hydraulic

fracture arrays, if fluid pressures are high enough. Fluid flow in faults and shear zones is

localized in areas of highest fracture aperture and fracture density, including breccia zones, and

commonly form in core and damage zones associated with fault jogs, bends, and splays (see

fig. 1; Cox et al., 2001). Fluid flow in folded sequences may be concentrated in hinge zones, as

illustrated by the saddle reef veins in the Bendigo gold fields, Victoria, Australia (fig. 2).

Figure 1. Geometry of contractional and dilatant jogs (a) and contractional and dilational splays at fault tips (b). Modified from Cox et al. (2001).

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Mineralization related to hydrothermal

fluid flow occurs by mineral precipitation in

structural openings and/or by replacement

in wall rocks adjacent to structural

openings. Veins, or mineral filled cracks,

and most breccias represent the physical

evidence of past hydrothermal fluid flow

in rocks. Because mineral filling of

fractures can be rapid relative to the life

span of hydrothermal systems, sustained

fluid flow occurs only in active structures

where permeability is repeatedly renewed

or held open by the local stress field

(Sibson et al., 1975; Cox et al., 2001). Repeated fracturing may occur with fluctuations in fluid

pressure related to seismic cycles along faults and shear zones (Sibson, 2001) or from

fluctuations in fluid pressure related to crystallization and venting of magma chambers. Sibson

(2001) has shown that cyclic accumulation and release of shear stress on seismogenic

structures leads to significant fluid redistribution during both coseismic and aftershock phases.

Magmatic fluid pressure fluctuations have been interpreted from unidirectional solidification

textures (Shannon et al., 1982), from fluid inclusion studies (e.g., Graney & Kessler, 1986), and

are inferred from banded vein textures (fig. 3) in volcanic-hosted vein deposits which typically

form above buried, episodically-venting plutons.

Stress controls on structural openings

The localization, geometry, and orientation of structural openings are strongly influenced by the

orientations and relative magnitudes of mechanical stresses in hydrothermal systems. These

stresses may be far-field stresses (regional scale) or near-field stresses (district, mine, or

mesoscopic scale). The far-field stress is not always what produces structures in the near field.

For example, regional stress fields may be locally rotated by the effects of non-planar faults,

proximity to intrusive bodies, bedding rotation on fold limbs, etc.

The types of fractures that form in hydrothermal systems can be controlled by the magnitude of

the differential stress (1-3) and by the effect of fluid pressure (fig. 4). Fluid pressure reduces the normal stress on planes and thus acts to promote sliding and opening of fractures. This is

Figure 2. Schematic cross section representing gold-quartz vein systems associated with upright fold structures in the Victoria gold fields, Australia (modified from Cox et al., 1991).

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Figure 3. Examples of banded veins in epithermal deposits. A. Martha Hill gold deposit, New Zealand; B. Martha Hill deposit, lens cap for scale lower center; C. Ares deposit, Per; D. Caylloma deposit, Per. illustrated by a negative shift of the state-of-stress circle on a Mohr diagram towards tangency

with the Mohr-Coulomb failure envelop. In isotropic homogeneous rock, stress controls the

orientation and type of fractures formed during brittle deformation, and three classes of

macroscopic fractures may form (fig. 4; Sibson, 2001, Cox et al., 2001). A high differential

stress leads to formation of shear fractures, an intermediate differential stress leads to formation

of hybrid extension-shear fractures, and a low differential stress leads to formation of extension

fractures. However, as shown by Sibson (2001), two or three of these fracture types typically

form together in some form of fault-fracture mesh (fig. 5). In fact, many faults contain veins, and

thus have a component of opening strain locally. Such veins can be considered fault veins

and can be recognized either by their presence in faults, or by slickenlines on the vein margin.

Typically, fault veins form along faults where the orientation (strike and dip) of the fault locally

approaches the 1-2 plane, such as along dilational fault jogs and horsetail splays, or wing cracks (fig. 1).

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Figure 4. Mohr diagram showing three possible states of stress leading to extensional fractures (ext.), hybrid extensional-shear fractures (e-s.), and shear fractures (sh.). Orientation relative to principal stress axes of conjugate sets of shear and hybrid fractures, and of extensional fractures is shown to the right (modified from Cox et al., 2001 & Sibson, 2001).

Pure extension fractures may be recognized if crystal fibers filling veins are perpendicular to the

vein wall. Not all veins have fibrous crystals however. Pure extension fractures may also be

inferred if they bear an angular relationship (generally between 30-45) to a mechanically related fault as predicted by the Riedel model (fig. 6, 7a; Wilcox et al., 1973; Davis & Reynolds,

1996, p.367). Hybrid fractures may be recognized if crystal fibers filling veins are oblique to the

vein wall (fig. 7b). En echelon vein arrays are also a form of hybrid fractures as individual veins

in the array are extension fractures, but these usually form in a zone of shear (figs. 7d, 8).

Figure 5. A. Vein mesh consisting of conjugate en echelon quartz veins and faults (lower right), near Banff, Canada. 1 approximately horizontal. B. Types of stress-controlled vein meshes in relation to triaxial stress field. Faults have shear couple arrows, extension and hybrid extension-shear fractures have hatched, stylolites are horizontal wiggly lines. From Sibson (2001). Shear fractures are essentially faults or small-displacement faults (Marshak et al., 1982) and

can be recognized by displacement across the fault or by slickenlines on the fault surface, or

because they are oriented in a manner that is mechanically compatible with the master fault

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zone (Sibson, 1994; Caine & Forster, 1999). Note, however, that slickenlines on fault veins may

form during a post-mineralization stress field unrelated to the stress field active during

mineralization.

Stress, and therefore structural

openings, can also control the

emplacement of igneous plutons,

dikes, and sills, as well as

associated veins. On a relatively

large scale, stress may control the

location and orientation of plutons

along fault jogs, bends, and

intersections (Tosdal & Richards,

2001). Such plutons ultimately

become the hosts for porphyry-style

mineralization, and also are the

source of hydrothermal fluids

required for formation of associated

vein systems. Examples include the structurally-controlled porphyry systems, such as

Chuquimata in Chile (Ossandon et al., 2001) and some of the Arizona copper porphyries

(Rehrig & Hendrick, 1972; Hayes & Titley, 1980; Hendrick & Titley, 1982). On a smaller scale

(deposit scale), stress may control the orientation of stockwork vein systems that comprise

porphyry mineralized deposits (fig. 7c). In some cases veins have a preferred orientation

related to the regional or far field stress. Examples include many of the Arizona porphyries such

as Morenci, Silver Bell, Ajo, and those in the Superior-Globe-Miami district (Hendrick & Titley,

1982). In other cases, radial and concentric veins form in an intrusion-centered, centro-

symmetric pattern. Examples include San Juan copper porphyry deposit (Safford district),

Arizona (fig. 9; Hendrick & Titley, 1982), and the Henderson Mo-porphyry deposit, Colorado

(Coe, 1995; Coe & Nelson, 1997).

Another example of structural control on magma emplacement comes from the Voiseys Bay

magmatic Ni-Cu-Co sulfide deposit in Labrador in which mineralization occurred along

structures that focused magmatic flow (Evans-Lamswood, 2000). In this deposit ore sulfides

are preferentially concentrated in traps where physical irregularities and changes in magma

conduit morphology favored the precipitation, capture, and preservation of sulfides as a result of

changes in the velocity and viscosity of the magma.

Figure 6. Riedel model of subsidiary fractures associated with sinistral shear zone. T = extension fracture, R, R, and P are shear fractures. Modified from Davis & Reynolds (1996).

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Figure 7. Vein types and features. A. en echelon extension veins formed along margin of normal fault, New Zealand; B. quartz crystal fibers oblique to vein wall in en echelon hybrid shear-extension vein array, Ireland; C. calcite-iron oxide veins in stockwork, Silver Bell mine, Arizona; D. en echelon quartz vein array cutting older quartz vein, Tierra del Fuego, Chile; E. extension veins formed in dilational fault jog along sinistral fault-vein, New Zealand. When stress controls the orientation and kinematics of faults and fractures in the absence of

pre-existing weaknesses, permeability anisotropy can develop with the direction of maximum

permeability (kmax) parallel to the intermediate principal stress direction (2) (fig. 10, 11; Sibson, 2001). In this case ore shoot orientation is perpendicular to the fault slip vector and can be

predicted by kinematic analysis. Such ore shoots form due to non-planar fault geometry where

the orientation of the fault plane approaches parallelism with the plane containing 1-2. An example comes from the Arcata volcanic-hosted epithermal Ag-Au vein system in Per. This

vein system formed in normal faults, and slickenline data were used by Echavarria & Nelson

(2002) to model the principal stress directions and to show that the slip line is essentially

perpendicular to the ore shoot (fig. 10, 12).

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Figure 8. Geometry and features of en echelon vein arrays. Note slickenlines may form on individual vein walls through book-shelf sliding, and offset of individual veins is opposite that of the overall array. When structural openings form along pre-existing weaknesses or due to fault undulations

unrelated to the stress field (fig. 13), ore shoot orientation cannot always be predicted by

kinematic analysis. In this case, the orientation of the ore shoot is controlled by the orientation

of pre-existing undulations on the fault surface, and not by the orientation of the slip line. Figure 9. San Juan mine area, Safford district, Arizona illustrating a centro-symmetric pattern of mineralized veins surrounding the San Juan pluton (modified from Hendrick & Titley, 1982).

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Rheological controls on structural openings

Rock rheology is a critical control on where some structural openings develop. Rock rheology is

affected by the bulk composition of rocks, by the inherent structural weaknesses in rocks

(bedding, cleavage and foliation, and pre-existing fractures and faults), and is also affected by

temperature and confining pressure conditions at the time of mineralization. Fluid pressure

variations also can affect rheological behavior and promote brittle failure, but are not easily

identified or mapped.

Figure 10. Geometry of stress-controlled plumbing conduits relative to fault slip line (shown by shear couple arrows) and principal stress axes. Curved arrows illustrate path of hydrothermal fluids.

Figure 11. En echelon vein array illustrating kinematic control on ore shoot rake. Slip line indicated by shear couple arrows. Note that the long axis of the ore shoot (parallel to 2) is perpendicular to the slip line.

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Figure 12. A. Lower hemisphere equal area nets showing vein and slickenline data from Arcata system, modeled and contoured tension (T) and compression (P) axes, and summary of modeled stress axes (1=1, 2=2, and 3=3). B. Longitudinal profile of Baja vein in the Arcata vein system, Per, showing orientation of slip line and long axis of ore shoot. From Echavarria & Nelson (2002), Echavarria et al. (2003).

Variations in mechanical

properties of rock undergoing

deformation cause variations

in rheological response due to

lithological changes,

temperature gradients, and

pre-existing weaknesses. As

pointed out by McKinstry

(1955) the relative competency of the rock sequences is important in localizing open space.

Brittle layers preferentially develop vein arrays and boudin necks, and can cause developing

structures that transect them to be refracted from their initial path (Ferrill & Morris, 2003). One

Figure 13. Formation of ore shoots along pre-existing undulations on fault plane. A. Undulations on fault plane before displacement; B. Ore shoot formation along undulations after fault displacement. Fault slip line (slickenlines) is not perpendicular to ore shoot, and ore shoot orientation cannot be predicted by kinematic analysis of fault. Modified from Guilbert & Park (1986).

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example comes from the Mary Kathleen fold belt in the central Mt. Isa block, Australia, where

altered and mineralized rock formed in veins and boudin necks in strain shadows adjacent to

competent, low permeability metadolerite and metagranite bodies surrounded by weak calc-

silicate rocks during syn-metamorphic fluid flow at amphibolite grade P-T conditions (fig. 14;

Marshall & Oliver, 2001, Oliver et al., 2001). Mineralization (Cu, Au, U-REE) in the district is

spatially related to the alteration. A second example comes from gold deposits in the Kalgoorlie

district, Western Australia. Veins and breccias in the shear-zone hosted Golden Mile deposit are

best developed within the Golden Mile dolerite, a differentiated tholeiitic sill, and quartz vein

arrays in the Mt. Charlotte mine are even more restricted to the granophyric Unit 8 of the Golden

Mile dolerite (Clout, et al., 1990; Solomon & Groves, 2000). A third example comes from the

Morning Star mine in Victoria, Australia, where quartz-gold veins formed during reverse faulting

are restricted to the Woods Point dike, and carried the highest gold grades in Victoria (Solomon

& Groves, 2000, p.691).

Figure 14. Schematic 3D geology of the Mary Kathleen fold belt, Australia, showing how distribution of altered and mineralized rock (black) was controlled by deformation (E-W shortening) of competent metadolerite bodies in calc-silicate host rocks (modified from Oliver et al., 2001).

Pre-existing weaknesses may be utilized as openings. One example comes from the Idaho

Springs district in Colorado, where foliation in Precambrian basement rocks was reactivated in

shear and extensional mode during Tertiary precious and base metal vein mineralization (fig.

15; Beach, 2000; Nelson et al., 2003). In this case, productive veins are concentrated within the

Precambrian Idaho Springs-Ralston shear zone, which contains foliations and folds that trend

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parallel to the shear zone boundary (fig. 15). Southeast of the shear zone foliation and folds

trend nearly perpendicular to the shear zone boundary, and no economic veins are present.

Figure 15. Lower hemisphere equal area nets and map of structural elements in the Idaho Springs mining district, Colorado. A. Density contour of poles to foliation within and northwest of Idaho Springs-Ralston (IRS) shear zone; B. density contour of poles to veins in the Idaho Springs district (outlined on the map); C. rose diagram of trend of Precambrian folds within and northwest of IRS; D. rose diagram of trend of Precambrian folds southeast of IRS; E. map of productive veins within and northwest of IRS; lines with arrows are fold axial plane traces, solid = antiform, dashed = synform (data from Beach, 2000; basemap from Moench & Drake, 1966).

Proposed working classification of structural openings related to localization of ore deposits

Early descriptions of some structural controls on mineralization were summarized in works by

Newhouse (1942), McKinstry (1948, 1955), and Lovering & Goddard (1950) and many others.

Numerous attempts have been made to classify veins and structurally-controlled ore deposits

(e.g., Lindgren, 1933; Bateman, 1981; Guilbert & Park, 1986). As with Batemans 1981

classification of open space deposits (table 1; his type 4.A.1), most classifications of structural

openings contain an ambiguous mixture of terms and categories. For example, Batemans

classification of open space deposits (table 1) includes categories for fissure vein, tension-crack

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fillings, breccia fillings, and shear-zone deposits. However, fissure veins and tension cracks are

likely the same in origin, and breccias may fill any of the other ore types (Taylor & Pollard,

1993). Also, much ore in shear-zone hosted deposits is present in what Bateman refers to as

fissure or tension veins. Therefore, numerous categories could describe the same ore deposit,

and the classification is thus confusing and its usefulness diminished.

Figure 16. Stereographic construction of ore shoot orientation from intersection of fault plane with various Riedel fractures (R, R, P, and T), or 90 from the slip line in the fault plane. Left column: examples for north-striking reverse and normal faults with 45 dip to east, and for north-striking vertical wrench fault with dextral shear sense. Cross sections on right illustrate the angular relationships of reverse and normal fault models. Lower right stereonet illustrates a more general case in which the fault is an oblique slip fault and the ore shoot rakes 26 in the fault plane.

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In another example, Guilbert & Park

(1986) classify veins as simple,

complex, irregular, and anastomosing.

Although this seems a simple

classification, each category is based on

a different, unrelated set of criteria. For

example, a simple vein is defined as

mineralization of a single, simple fault

fissure, and a complex vein contains multiple laminae. However, many single fault veins are

laminated. Also, irregular veins are defined as having variable thickness, and anastomosing

veins as having a braided pattern. However, individual veins in an anastomosing array could

have variable thickness.

Because past classifications of veins

and other structural openings contain

many old terms and ambiguous

mixtures of categories, we propose a

new working classification of structural

openings (table 2). The primary

classes of principal geologic structures

that host ore bodies include fault and

shear zones, structural intersections,

folds, strain shadows, igneous-related

structures, and collapse structures.

Because structural intersections mostly

involve intersection of a fault or shear

zone with some other contact (another

fault, stratigraphic or pluton contact,

dikes, etc.), this structural type is included under the fault/shear zone category. The sub-

classes in the proposed classification represent the types of open-space strain features that

develop in association with the principal structures, and are generally easy to recognize in the

field during exploration projects.

The proposed classification is designed to assist explorationists in formulating structural-

mineralization models for exploration projects, and to guide the explorationist in recognizing

A. fissure veins B. shear-zone deposits C. stockworks D. ladder veins E. saddle-reefs F. tension-crack fillings G. breccia fillings (volcanic, tectonic, collapse) H. solution-cavity fillings I. pore-space fillings J. vesicular fillings

Table 1. Batemans (1981) classification of open space deposits formed by hydrothermal processes.

Figure 17. Axis of ore shoot formed in fault jog perpendicular to slip line (slickenlines) for three primary types of faults (reverse, normal, and strike slip). Note that ore shoot forms in flat sections of reverse faults and steep sections of normal faults. Modified from Cox et al. (2001).

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associations of structural features in the field. It is hoped that this proposed working

classification will generate discussion within the exploration community, such that practical

improvements can be made and the classification can become more robust and useful in

exploration.

Classes Process Example Type of deposit Reference A. Fault and shear zone

Shearing

Dilatant jogs Ohio Creek (New Zealand) Porgera (Papua New Guinea)

Porphyry Copper (Cu-Au) Epithermal (Ag-Au-Pb-Zn)

Corbett & Leach (1996) Corbett & Leach (1995)

Dilatant bends Arcata (Peru) Clyde Tungsten mine (Colorado) Bell Mine (Colorado)

Epithermal intermediate sulfidation (Ag-Pb-Zn-Au) Epithermal (W) Epithermal (Ag-Pb-Zn)

Echavarria & Nelson (2002) Echavarria et al. (2003) Lovering & Goddard (1950) Lovering & Goddard (1950)

Fault splays (including wing veins)

Wau (Papua New Guinea) Pillara (Australia)

Epithermal (porphyry related) (Ag-Au-Pb-Zn) MVT (Zn-Pb)

Corbett & Leach (1996) Miller & Nelson (2002)

En echelon fractures

Mount Kasi (Fiji) High sulfidation Epithermal (Au-Cu)

Corbett & Leach (1996)

Intersections (other faults, dikes, unconformities)

Caylloma (Peru) Fault-Fault Rabbit Lake (Canada) Fault-Unconformity Jefferson Canyon (Nevada) Fault-Caldera margin

Epithermal Intermediate sulfidation (Ag-Pb-Zn-Au) Unconformity-type uranium deposits (U) Epithermal (Au-Ag)

Echavarria (2002) Hoeve & Sibbald (1978) Rytuba (1994)

B. Fold Folding (shortening and extension)

Saddle reefs Bendigo (Australia) Orogenic gold (Au) Phillips & Hughes (1996) Cox et al. (1991)

Limb Faults C. Strain shadow

Local extension normal to layering

Boudin necks Mount Isa (Australia) FeOx-Cu-Au-U Perkins (1984) Findlay (1982) Oliver et al. (2001)

Vein arrays in brittle layers

Golden Mile, Kalgoorlie (Australia)

Orogenic gold (Au-Ag) Clout et al. (1990)

Table 2. Proposed working classification of structural openings related to localization of ore bodies.

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Classes Process Example Type of deposit Reference D. Igneous-related structures

Fracturing and brecciation due to intrusion, thermal expansion, fluid overpressure, or flow brecciation

Diatremes and breccia pipes

Balatoc, Baguio (Phillipines) Pueblo Viejo (Rep. Dominicana) Cripple Creek

Epithermal (Au-Ag) Epithermal (Au-Ag) Epithermal (Au-Ag)

Sillitoe & Bonham (1984) Cooke et al. (1996) Russell & Kesler (1991) Thompson et al. (1985)

Porphyry stockwork

Chuquicamata, La Escondida, El Salvador (Chile) Morenci, Silver Bell, Ajo (Arizona)

Porphyry deposits (Cu-Au) Ossandon et al. (2001) Padilla Garza et al. (2001) Titley (1982)

Flow related breccia (flows and domes)

Todos Santos, Carangas (Bolivia)

Epithermal (Ag) Cunningham et al. (1991)

E. Collapse related structures

Fracturing due to collapse

Dissolution Jefferson City Mine (Tennessee), Buick Mine (Missouri)

MVT (Zn-Pb) Ohle (1985)

Table 2. cont.

As with past classifications, our proposed classification is not without problems. Although four

of the main categories are labeled with structural types (fault, fold, strain shadow, collapse

structure), the fifth category label is modified with a rock-type association (igneous-related

structures). In addition, although structures in each of the four categories generally form by a

single strain-related process, igneous-related structures may form by a number of possible

processes. Another practical problem with this classification is that a number of mesoscopic

structural features may be associated with large-scale structures in more than one category.

For example, veins and vein arrays (such as en echelon arrays), can be associated with any of

the primary categories. Faults can form in association with folds, igneous related structures,

and collapse structures. Breccias can form in dilational fault jogs and bends, in fold-related

openings, in diatremes and igneous breccia pipes, and in collapse-related ore deposits.

Therefore, during exploration the structures in the principal categories must be viewed in a

larger context, with consideration of all the mesoscopic-scale structures that might be

associated with the principal structure.

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Structural methods in exploration for structural openings A number of structural methods are useful in the exploration for ore deposits in structural

openings. As many hydrothermal deposits are associated directly or indirectly with faults or

shear zones, knowledge of orientation of the fault slip vector is extremely important for modeling

the orientation of ore shoots. When stress controls the orientation and kinematics of faults and

fractures in the absence of pre-existing weaknesses, ore shoots form where the orientation of

the fault zone changes and approaches parallelism with the plane containing 1-2 (for example in fault bends and jogs; figs. 1, 10). In this case ore shoot orientation is perpendicular to the fault

slip line (Fig. 17). The orientation of the slip line can be determined using a number of structural

features and techniques. Structural features include slickenlines and fault corrugations,

subsidiary fractures related to fault slip (such as those predicted by the Riedel model, fig. 6,16),

shear fabrics in fault rocks such as S-C fabric (e.g., Ramsay & Huber, 1987, p. 632), and shear-

related folds. S-C type fabrics consist of two sets of anastomosing foliations (Berthe et al.,

1979) and can form in ductile shear zones formed by plastic deformation mechanisms, as well

as in fault zones formed by brittle cataclastic flow. C-planes represent spaced shear planes,

and S-planes represent generally penetrative planes of flattening.

Figure 18. S-C shear fabric in granitic rock from Altar district, Sonora, Mexico. S-plane and C-plane orientations shown in upper right. Stereonet shows method of determining the ore shoot orientation from the intersection of S- and C-planes.

Orthographic and combined orthographic/stereographic methods can be used to determine the

orientation of the slip line using piercing point analysis if the fault offsets two differently oriented

structural planes, such as bedding and a dike (Marshak & Mitra, 1988, p.81; Leyshon & Lisle,

1996, p. 56). Stereographic methods also can be used to construct the slip vector. Slickenlines,

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or fault striations, are common on the walls of many fault veins. If the striations formed by

crystal fiber growth (termed slickenfibers; Ramsay & Huber, 1987), then they formed parallel to

the slip line at the time of fault vein formation. However, if the striations formed by mechanical

wear during shearing (fault movement), then they must have formed after the time of fault vein

formation, and may not be parallel to the slip line during fault vein formation. Nonetheless, as

many fault veins form by repeated movements and fluid flow events during one tectonic cycle, it

can often be assumed that fault vein striations of this type are parallel to the slip line. If ore is

related to a fault or shear zone with subsidiary fractures ore shoot orientation is predicted to be

parallel to the intersection of the fault with any of the subsidiary fractures predicted by the Riedel

Figure 19. Gocad 3D computer model of Pillara Mississippi Valley-type Zn-Pb deposit in Western Australia (Miller & Nelson, 2002). A. North-looking perspective of 3D model showing simple graben-bounding faults offsetting colored stratigraphic contacts. Yellow is surface representing contour of 3% Zn grade. B. Cross section of Eastern fault showing near-vertical splays off of fault where fault steepens. C. Longitudinal cross section of Eastern fault showing near horizontal intersection between main fault plane and more vertical second-order fractures (veins).

19

model (fig. 16). If ore is related to a fault or shear zone with S-C type shear fabric (Berth et al.,

1979), ore shoot orientation is predicted to be parallel to the intersection of the S-planes and the

C-planes (Fig. 18). If ore is related to shear folds, the slip line can be constructed by

stereographically analyzing fold hinge orientation and fold asymmetry by the Hansen method

(Hansen, 1967).

Modern 3-D computer modeling techniques also can be very useful in predicting ore shoot

orientation. For example, Miller & Nelson (2002), using a Gocad 3D model (fig. 19), showed

that Zn ore in the Pillara mine in Western Australia is concentrated along graben-bounding

faults and within splay veins (extension, or T fractures). The line of intersection between the

faults and the T-fractures is nearly horizontal as demanded by kinematics of graben formation,

rather than being controlled by stratigraphy.

Conclusions

Structural openings in rocks constitute the primary permeability network for the formation of

most hydrothermal ore deposits. The distribution, geometry, density, and orientation of ore

bodies in structural openings are controlled by two primary factors: the far and near field

stresses and the mechanical properties of rock. When stress controls the orientation and

kinematics of faults and fractures in the absence of pre-existing weaknesses, ore shoot

orientation is perpendicular to the fault slip vector and can be predicted by kinematic analysis.

However, when structural openings form along pre-existing weaknesses, ore shoot orientation

cannot always be predicted by kinematic analysis.

Because we can classify ore bodies based on the types of geological structures discussed

above and the variations that naturally occur from material property heterogeneity, exploration

for hydrothermal ore deposits should include analysis of geologic structure and evolution,

rheological variations, pre-existing weakness, and fault kinematics to evaluate structural

controls on localization of ore bodies. Further, use of modern structural geologic analyses

allows for the prediction of the orientation and possibly the location of unsampled ore bodies

associated with various structural openings in rocks.

References

Agricola G. (1556).- De Re Metallica (English translation by H.C. Hoover & L.H. Hoover,

1912): New York, Dover Publications Inc., 1950.

20

Bateman A. M. ( 1981).- Economic Mineral Deposits, New York, Wiley, 916p.

Beach S. T. (2000).- Kinematic analysis of mineralized veins in the Idaho Springs mining

district, central Colorado. Masters thesis, Colorado School of Mines, 160 p.

Berthe D., Choukroune P. & Jegouzo P. (1979).- Orthogneiss, mylonite and non coaxial

deformation of granites; the example of South Armorican shear zone. Journal of Structural

Geology, 1, 31-42.

Bursnall J. T. (1989).- Mineralization and Shear Zones, Geological Association of Canada,

Short Course Notes, 6, 299 p.

Caine J. S. & Forster C. B. (1999).- Fault zone architecture and fluid flow: Insights from

field data and numerical modeling. In: Haneberg W. C., Mozley P. S., Moore J. C. &

Goodwin L. B. (eds); Faults and sub-surface fluid flow in the shallow crust: AGU

Geophysical Monograph, 113, 101-127.

Clappison R. J. S. (1953).- The Morning Star mine, Woods Point. In: A. B. Edwards (ed);

Geology of Australian Ore Deposits, 5th Empire Min. Met. Congr., 1, 1077-1081.

Clout J. M. F., Cleghorn J. H. & Eaton P.C. (1990).- Geology of the Kalgoorlie gold field. In:

F.E. Hughes (ed); Geology of the Mineral Deposits of Australia and Papua New Guinea,

411-431.

Coe J. A. & Nelson E. P. (1997).- Characterization of fracture networks using close-range

photogrammetric mapping and GIS analysis. In: Hoak T. E., Klawitter A. L. & Blomquist

P.K. (eds.); Fractured Reservoirs: Characterization and Modeling, Rocky Mountain

Association of Geologists, 1997 Guidebook, 43-55.

Coe J. A. (1995).- Close-range photogrammetric geologic mapping and structural analysis,

Henderson Mine, Empire, Colorado, Masters Thesis, Colorado School of Mines, 192p.

Cooke D., McPhail D. & Bloom M. (1996).- Epithermal gold mineralization, Acupan, Baguio

District, Philippines; geology, mineralization, alteration, and the thermochemical

environment of ore deposition. Economic Geology, 91, 243-272.

Corbett G. & Leach T. (1995).- Porgera gold deposit; structure, alteration and

mineralisation. In: Mauk J. L. & St. George J. D. (eds); Proceedings of the 1995 PACRIM

Congress: Exploring the Rim, 151-156.

Corbett G. & Leach T. (1996).- Southwest Pacific Rim Au-Cu Systems: Structure, Alteration

and Mineralization. Exploration workshop presented for SEG/SME at Phoenix, Arizona,

March, 1996. 178 pp.

Cox S. F., Knackstedt M. A. & Braun J. (2001).- Principles of structural control on

21

permeability and fluid flow in hydrothermal systems. In: Richards J. P. & Tosdal R. M.

(eds.); Structural Controls on Ore Genesis, Society of Economic Geologists, Reviews, 14,

1-24.

Cox S. F., Wall V. J., Etheridge M. A. & Potter T. F. (1991).- Deformational and

metamorphic processes in the formation of mesothermal vein-hosted gold deposits

examples from the Lachlan fold belt in central Victoria, Australia, Ore Geology Reviews, 6,

p.391-423.

Cunningham C., McNamee J., Pinto Vasquez J. & Ericksen G. (1991).- A model of volcanic

dome-hosted precious metal deposits in Bolivia. Economic Geology, 86, 415-421.

Davis G. H. & Reynolds S. J. (1996).- Structural Geology of Rocks and Regions, 2nd

edition, John Wiley & Sons, New York, 776p.

Echavarria L. (2002).- Control estratigrfico y estructural de la mineralizacin en el Distrito

de Caylloma, Peru. Unpublished report Mauricio Hochschild Mining Co.

Echavarria L. & Nelson E. (2002).- Structural controls on the Arcata epithermal vein

system, Per, Geological Society of America, Abstracts,

Echavarria L., Yagua T., Nelson E. & Benavides J. (2003). Sistema epitermal de Arcata,

Sur de Peru. ProExplo 2003, Lima, Peru, this congress.

Evans-Lamswood D. M., Butt D. P., Jackson R. S., Lee D. B., Muggridge M. G., Wheeler

R. I. & Wilton D. H. C. (2000).- Physical controls associated with the distribution of sulfides

in the Voiseys Bay Ni-Cu-Co deposit, Labrador. Economic Geology, 95, 749-769.

Ferrill D. A. & Morris A. P. (2003).- Dilational normal faults. Journal of Structural Geology,

25, 183-196.

Findlay D. (1982).- Necking, a structural control in the location of ore deposits. Global

Tectonics and Metallogeny, 1 (4), 304-324.

Graney J. R. & Kessler S. E. (1986).- Role of magmatic gas in adularia-sericite epithermal

vein deposits. GSA 28th annual meeting, Denver, Co. Abstracts w/ programs, GSA 28 (7),

402.

Guilbert J. M. & Park C. F. Jr. (1986).- The Geology of Ore Deposits, W.H. Freeman & Co.,

New York, 985 p.

Hansen E. (1967).- Methods of deducing slip-line orientations from the geometry of folds.

Year Book, Carnegie Institution of Washington, Washington, DC, United States, 387-405.

Hayes F. M. & Titley S. R. (1980).- The evolution of fracture-related permeability within the

Ruby Star granodiorite, Sierrita porphyry copper deposit, Pima County, Arizona. Economic

22

Geology, 75, 673-683.

Hendrick T. L. & Titley S. R. (1982).- Fracture and dike patterns in Laramide plutons and

their structural and tectonic implications, American Southwest. In: Titley S. R. (ed.);

Advances in Geology of the Porphyry Copper Deposits, Southwestern North America:

Tuscon, Univ. Arizona Press, 73-91.

Hoeve J. & Sibbald T. I. I. (1978).- On the genesis of Rabbit Lake and other unconformity-

type Uranium deposits in northern Saskatchewan, Canada. Economic Geology, 73, 1450-

1473.

Leyshon P. R. & Lisle R. J. (1996).- Stereographic Projection Techniques in Structural

Geology, Butterworth-Heinemann Ltd., Oxford, 104p.

Lindgren W. (1933).- Mineral Deposits, 4th ed., New York, McGraw-Hill, 930 p.

Lovering T. S. & Goddard E. N. (1950).- Geology and Ore Deposits of the Front Range

Colorado, U.S. Geological Survey Professional Paper 223, 319 p.

Marshak S. & Mitra G. (1988).- Basic Methods of Structural Geology, Englewood Cliffs,

N.J., Prentice Hall, 466p.

Marshak S., Geiser P. A., Alvarez W. & Engelder T. (1982).- Mesoscopic fault array of the

northern Umbrian Apennine fold belt, Italy; geometry of conjugate shear by pressure-

solution slip. Geological Society of America Bulletin, 93, 1013-1022.

Marshall L. J. & Oliver N. H. S. (2001).- Mechanical controls on fluid flow and brecciation in

the regional host rocks for Eastern Fold Belt ironstone-Cu-Au deposits, Mt. Isa Block. In:

Mark G., Oliver N. H. S. & Foster D. R. W. (eds.); Mineralisation, alteration and magmatism

in the Eastern Fold Belt, Mount Isa Block, Australia: Geological Review and Field Guide,

Geological Society of Australia, Specialist Group in Economic Geology, Publication No. 5,

30-45.

McKinstry H. E. (1948).- Mining Geology, Prentice Hall, Englewood Cliffs, N.J., 680 p.

McKinstry H. E. (1955).- Structure of hydrothermal ore deposits, Economic Geology,

Fiftieth Anniversary Volume, 170-225.

Miller J. McL. & Nelson E. P. (2002).- Three-dimensional strain during basin formation

orthorhombic fault patterns and associated MVT mineralization, Lennard shelf, Western

Australia, Geological Society of America, Abstracts.

Moench R. H. & Drake A. A. Jr. (1966).- Economic geology of the Idaho Springs district,

Clear Creek and Gilpin counties, Colorado. U. S. Geological Survey Bulletin 1208.

Nelson E. P., Beach S. T. & Layer P. W. (2003).- Laramide dextral movement on the

23

Colorado Mineral Belt interpreted from structural analysis of veins in the Idaho Springs

Mining District. Geological Society of America, Rocky Mountain Section annual meeting,

Abstracts.

Newhouse W. H. (1942).- Ore Deposits as Related to Structural Features, Hafner Publishing

Co., New York, 280 p.

Ohle E. (1985).- Breccias in Mississippi Valley type deposits. Economic Geology, 80, 1736-

1752.

Oliver N. H. S., Ord A., Valenta R. K. & Upton P. (2001).- Deformation, fluid flow, and ore

genesis in heterogeneous rocks, with examples and numerical models from the Mount Isa

district, Australia. In: Richards J. P. & Tosdal R. M. (eds.); Structural Controls on Ore Genesis,

Society of Economic Geologists, Reviews in Economic Geology, 14, 51-74.

Ossandon G., Freraut R., Gustafson L., Lindsay D. & Zentilli M. (2001).- Geology of the

Chuquicamata Mine: a progress report. Economic Geology, 96, 249-270.

Padilla Garza R., Titley S. & Pimentel F. (2001).- Geology of the Escondida Porphyry Copper

deposit, Antofagasta Region, Chile. Economic Geology, 96, 307-324.

Perkins W. (1984).- Mount Isa silica dolomite and copper orebodies; the result of a syntectonic

hydrothermal alteration system. Economic Geology, 79, 601-637.

Phillips G. & Hughes M. (1996).- The geology and gold deposits of the Victorian gold province.

Ore Geology Reviews, 11, 255-302.

Ramsay J. G. & Huber M. I. (1987).- The Techniques of Modern Structural Geology, Volume 2,

London, New York, Academic Press, 311-700.

Rehrig W. A. & Hendirck T. L. (1972).- Regional fracturing in Laramide stocks of Arizona and its

relationship to porphyry copper mineralization. Economic Geology, 67, 198-213.

Richards J. P. & Tosdal R. M. (2001).- Structural Controls on Ore Genesis. Society of Economic

Geologists, Reviews in Economic Geology, 14, 181p.

Robert F. & Poulsen K. H. (1997).- World-class Archaean gold deposits in Canada: An

overview. Australian Journal of Earth Sciences, 44, 329-351.

Robert F. & Poulsen K. H. (2001).- Vein formation and deformation in greenstone gold deposits.

In: Richards J. P. & Tosdal R. M. (eds.); Structural Controls on Ore Genesis. Society of

Economic Geologists, Reviews in Economic Geology, 14, 111-156.

Russell N. & Kesler S. (1991).- Geology of the maar-diatreme complex hosting precious metal

mineralization at Pueblo Viejo, Dominican Republic. In: Mann P., Draper G., Lewis J. (eds);

Geologic and tectonic development of the North America-Caribbean Plate boundary in

Hispaniola.. Geological S.ociety of America Special Paper 262, 203-215.

24

Rytuba J. (1994).- Evolution of volcanic and tectonic features in caldera settings and their

importance in the localization of ore deposits. Economic Geology, 89, 1687-1696.

Shannon J. R., Walker B. M., Carten R. B., Geraghty E. P. (1982).- Unidirectional solidification

textures and their significance in determining relative ages of intrusions at the Henderson Mine,

Colorado. Geology, 10, 293-297.

Sibson R. (2001).- Seismogenic framework for hydrothermal transport and ore deposition. In:

Richards J. P. & Tosdal R. M. (eds.); Structural Controls on Ore Genesis. Society of Economic

Geologists, Reviews in Economic Geology, 14, 25-50.

Sibson R. H. (1994).- Crustal stress, faulting, and fluid flow. In: Parnell J. (ed.); Geofluids:

Origin, migration, and evolution of fluids in sedimentary basins. Special publications of the

Geological Society of London, 78, 69-84.

Sibson R. H. (1996).- Structural permeability of fluid-driven fault-fracture meshes: Journal of

Structural Geology, 18, 1031-1042.

Sibson R. H., Moore J. M. & Rankin A. H. (1975).- Seismic pumping A hydrothermal fluid

transport mechanism. Journal of the Geological Society of London, 131, 653-659.

Sillitoe R. & Bonham H. Jr. (1984).- Volcanic landforms and ore deposits. Economic Geology,

79, 1286-1298.

Solomon M. & Groves D. I. (2000).- The lode gold deposits of the Western Australian Shield. In:

The Geology and Origin of Australias Mineral Deposits, Centre for Ore Deposit Research, Univ.

Tasmania and Centre for Global Metallogeny, Univ. Western Australia, 54-84.

Taylor R. G. & Pollard P. J. (1993).- Mineralized Breccia Systems: Methods of Recognition and

Interpretation. Economic Geology Research Unit, Key Centre in Economic Geology, James

Cook University, Contribution 46, 31p.

Thompson T., Triple A. & Dwelley P. (1985).- Mineralized veins and breccias of the Cripple

Creek District, Colorado. Economic Geology, 80, 1669-1688.

Titley S. R. (1982).- Advances in Geology of the Porphyry Copper Deposits, Southwestern

North America: Tuscon, Univ. Arizona Press, 560p.

Tosdal R. M. & Richards J. P. (2001).- Magmatic and structural controls on the development of

porphyry Cu Mo Au deposits. In: Richards J. P. & Tosdal R. M. (eds.); Structural Controls on

Ore Genesis. Society of Economic Geologists, Reviews in Economic Geology , 14, 157-181.

Wilcox R. E., Harding T. P. & Seely D. R. (1973).- Basic wrench tectonics. American

Association of Petroleum Geologists Bulletin, 57, 74-96.