Structural Openings and the Localization of Ore BodiesEric P. Nelson1, Leandro Echavarria1, and Jonathan Saul Caine2 1 2 Dept. of Geology/Geological Engineering, Colorado School of Mines 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 (σ2). 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 σ1-σ2. 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. to be refracted from their initial path. For example, brittle layers preferentially develop vein arrays and boudin necks, and can cause developing structures that transect them 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 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 fracturebounded 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. 1 brittle crust where macroscopic fault and fracture systems are common. Because structural openings are important in the formation of many ore deposits. We hope our proposal will lead to a more comprehensive and useful classification system. 2001). 2 . especially in Cordilleran-type deposits formed in the upper crust. and splays (see fig. and how structural openings localize hydrothermal ore deposits. Structural permeability typically forms in the upper. Such permeability is paramount in the development of many ore deposits that are produced by focused flow of relatively large volumes of hydrothermal fluids. including breccia zones. (2001). Australia (fig. 1.. Lastly. 2). but also can form in the more ductile deeper crust in hydraulic fracture arrays. 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. Fluid flow in faults and shear zones is localized in areas of highest fracture aperture and fracture density. and commonly form in core and damage zones associated with fault jogs. Structural Permeability Structural permeability refers to transient enhancement of fluid flux through strain-related openings (Sibson. 1996). we briefly discuss structural methods that can aid the explorationist in predicting the location and orientation of ore shoots formed in structural openings.Breccias also can have a wide range of geometries. Modified from Cox et al. bends. Cox et al. but linear and tabular geometries are most common. Because past classifications of veins and other structural openings contain many old terms and ambiguous mixtures of categories. Geometry of contractional and dilatant jogs (a) and contractional and dilational splays at fault tips (b). Figure 1. we review and focus this paper on how stress and the mechanical properties of rock control the formation of structural openings. if fluid pressures are high enough. Victoria. Fluid flow in folded sequences may be concentrated in hinge zones. as illustrated by the saddle reef veins in the Bendigo gold fields. fluid flow occurs only in active structures where permeability is repeatedly renewed or held open by the local stress field (Sibson et al.Mineralization related to hydrothermal fluid flow occurs by mineral precipitation in structural openings and/or by replacement in wall rocks adjacent to structural openings. Repeated fracturing may occur with fluctuations in fluid pressure related to seismic cycles along faults and shear zones (Sibson. 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. proximity to intrusive bodies. from fluid inclusion studies (e. and are inferred from banded vein textures (fig. The far-field stress is not always what produces structures in the near field. Fluid pressure reduces the normal stress on planes and thus acts to promote sliding and opening of fractures. or mineral filled cracks. For example. These stresses may be far-field stresses (regional scale) or near-field stresses (district. and most breccias represent the physical evidence of past hydrothermal fluid flow in rocks. 1975.g.. This is 3 . and orientation of structural openings are strongly influenced by the orientations and relative magnitudes of mechanical stresses in hydrothermal systems. 3) in volcanic-hosted vein deposits which typically form above buried. Stress controls on structural openings The localization. Magmatic fluid pressure fluctuations have been interpreted from unidirectional solidification textures (Shannon et al. The types of fractures that form in hydrothermal systems can be controlled by the magnitude of the differential stress (σ1-σ3) and by the effect of fluid pressure (fig.. Cox et al. 2001). Veins. etc. Australia (modified from Cox et al. or mesoscopic scale). sustained Figure 2.. Schematic cross section representing goldquartz vein systems associated with upright fold structures in the Victoria gold fields. 1991). Because mineral filling of fractures can be rapid relative to the life span of hydrothermal systems.. 1982). regional stress fields may be locally rotated by the effects of non-planar faults.. bedding rotation on fold limbs. 4). geometry. 2001) or from fluctuations in fluid pressure related to crystallization and venting of magma chambers. Graney & Kessler. 1986). episodically-venting plutons. mine. Perú. D. and three classes of macroscopic fractures may form (fig. or wing cracks (fig. such as along dilational fault jogs and horsetail splays. Caylloma deposit. fault veins form along faults where the orientation (strike and dip) of the fault locally approaches the σ1-σ2 plane. or by slickenlines on the vein margin. Cox et al. 4. Perú. Martha Hill gold deposit. However. Ares deposit. Such veins can be considered “fault veins” and can be recognized either by their presence in faults. A high differential stress leads to formation of shear fractures. Examples of banded veins in epithermal deposits.Figure 3. two or three of these fracture types typically form together in some form of fault-fracture mesh (fig. 5). and thus have a component of opening strain locally. In isotropic homogeneous rock. B. lens cap for scale lower center. In fact. 4 . 1). illustrated by a negative shift of the state-of-stress circle on a Mohr diagram towards tangency with the Mohr-Coulomb failure envelop. an intermediate differential stress leads to formation of hybrid extension-shear fractures.. many faults contain veins. A. Sibson. as shown by Sibson (2001). C. Typically. Martha Hill deposit. stress controls the orientation and type of fractures formed during brittle deformation. 2001. 2001). and a low differential stress leads to formation of extension fractures. New Zealand. Faults have shear couple arrows. Orientation relative to principal stress axes of conjugate sets of shear and hybrid fractures. σ1 approximately horizontal. Davis & Reynolds.). Pure extension fractures may be recognized if crystal fibers filling veins are perpendicular to the vein wall.). near Banff. and of extensional fractures is shown to the right (modified from Cox et al. or because they are oriented in a manner that is mechanically compatible with the master fault 5 . 1982) and can be recognized by displacement across the fault or by slickenlines on the fault surface. 8). Types of stress-controlled vein meshes in relation to triaxial stress field. Hybrid fractures may be recognized if crystal fibers filling veins are oblique to the vein wall (fig. stylolites are horizontal wiggly lines.Figure 4. but these usually form in a zone of shear (figs. Vein mesh consisting of conjugate en echelon quartz veins and faults (lower right). 2001). hybrid extensionalshear fractures (e-s. Wilcox et al. and shear fractures (sh. 7b). 1996. From Sibson (2001). p. 2001 & Sibson. 6. Mohr diagram showing three possible states of stress leading to extensional fractures (ext. B. 7a. 1973. En echelon vein arrays are also a form of hybrid fractures as individual veins in the array are extension fractures. 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. A.. Not all veins have fibrous crystals however..367). 7d. Shear fractures are essentially faults or small-displacement faults (Marshak et al..). Canada. Figure 5. extension and hybrid extension-shear fractures have hatched. In this deposit ore sulfides are preferentially concentrated in traps where physical irregularities and changes in magma conduit morphology favored the precipitation. 1994. R’. Riedel model of subsidiary fractures associated with sinistral shear zone. Caine & Forster. Hayes & Titley. 1999). Another example of structural control on magma emplacement comes from the Voisey’s Bay magmatic Ni-Cu-Co sulfide deposit in Labrador in which mineralization occurred along structures that focused magmatic flow (Evans-Lamswood. 1995. that slickenlines on fault veins may form during a post-mineralization stress field unrelated to the stress field active during mineralization. stress may control the orientation of stockwork vein systems that comprise porphyry mineralized deposits (fig. Such plutons ultimately become the hosts for porphyry-style mineralization. 1980. 9. symmetric pattern. 6 . 2000). and intersections (Tosdal & Richards. capture. bends.zone (Sibson. and also are the Figure 6. and sills. fault jogs. Examples include many of the Arizona porphyries such as Morenci. 7c). Note. Modified from Davis & Reynolds (1996). 1997). Stress. centroExamples include San Juan copper porphyry deposit (Safford district). Silver Bell. 1982). On a relatively large scale. R. 2001) and some of the Arizona copper porphyries (Rehrig & Hendrick. On a smaller scale (deposit scale). control as well emplacement of igneous plutons. Hendrick & Titley.. and can therefore also structural the as openings. and preservation of sulfides as a result of changes in the velocity and viscosity of the magma. Arizona (fig. dikes. Ajo. T = extension fracture. source of hydrothermal fluids required for formation of associated vein systems. Examples include the structurally-controlled porphyry systems. Coe & Nelson. stress may control the location and orientation of plutons along 2001). Colorado (Coe. 1972. Hendrick & Titley. and the Henderson Mo-porphyry deposit. radial and concentric veins form in an intrusion-centered. however. associated veins. In other cases. 1982). 1982). such as Chuquimata in Chile (Ossandon et al. and those in the Superior-Globe-Miami district (Hendrick & Titley. and P are shear fractures. In some cases veins have a preferred orientation related to the regional or far field stress. 7 . Ireland. calcite-iron oxide veins in stockwork. A. 2001). 10. Silver Bell mine. 11. Chile. 12). extension veins formed in dilational fault jog along sinistral fault-vein. 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. This vein system formed in normal faults. B. E. C. Such ore shoots form due to non-planar fault geometry where the orientation of the fault plane approaches parallelism with the plane containing σ1-σ2. quartz crystal fibers oblique to vein wall in en echelon hybrid shear-extension vein array. When stress controls the orientation and kinematics of faults and fractures in the absence of pre-existing weaknesses.Figure 7. permeability anisotropy can develop with the direction of maximum permeability (kmax) parallel to the intermediate principal stress direction (σ2) (fig. 10. D. Arizona. In this case ore shoot orientation is perpendicular to the fault slip vector and can be predicted by kinematic analysis. Sibson. en echelon extension veins formed along margin of normal fault. en echelon quartz vein array cutting older quartz vein. Tierra del Fuego. An example comes from the Arcata volcanic-hosted epithermal Ag-Au vein system in Perú. Vein types and features. New Zealand. New Zealand. ore shoot orientation cannot always be predicted by kinematic analysis. 8 . When structural openings form along pre-existing weaknesses or due to fault undulations unrelated to the stress field (fig. In this case. 13). Arizona illustrating a centrosymmetric pattern of mineralized veins surrounding the San Juan pluton (modified from Hendrick & Titley. and not by the orientation of the slip line. 1982). Note slickenlines may form on individual vein walls through book-shelf sliding. San Juan mine area. Geometry and features of en echelon vein arrays.Figure 8. and offset of individual veins is opposite that of the overall array. Figure 9. Safford district. the orientation of the ore shoot is controlled by the orientation of pre-existing undulations on the fault surface. 9 . by the inherent structural weaknesses in rocks (bedding. cleavage and foliation. Note that the long axis of the ore shoot (parallel to σ2) is perpendicular to the slip line. Rock rheology is affected by the bulk composition of rocks.Figure 10. but are not easily identified or mapped. Slip line indicated by shear couple arrows. Curved arrows illustrate path of hydrothermal fluids. Fluid pressure variations also can affect rheological behavior and promote brittle failure. En echelon vein array illustrating kinematic control on ore shoot rake. Geometry of stress-controlled plumbing conduits relative to fault slip line (shown by shear couple arrows) and principal stress axes. and pre-existing fractures and faults). Figure 11. and is also affected by temperature and confining pressure conditions at the time of mineralization. Rheological controls on structural openings Rock rheology is a critical control on where some structural openings develop. Undulations on fault plane before displacement. Lower hemisphere equal area nets showing vein and slickenline data from Arcata system. Formation of ore shoots along pre-existing undulations on fault plane. Ore shoot formation along undulations after fault displacement. A. and summary of modeled stress axes (1=σ1. Echavarria et al. temperature gradients. Fault slip line (slickenlines) is not perpendicular to ore shoot. Modified from Guilbert & Park (1986). and can cause developing structures that transect them to be refracted from their initial path (Ferrill & Morris. As pointed out by McKinstry (1955) the relative competency of the rock sequences is important in localizing open space. B. and pre-existing weaknesses. Variations in mechanical properties of rock undergoing deformation cause variations in rheological response due to lithological changes. 2003). A. Brittle layers preferentially develop vein arrays and boudin necks.Figure 12. Figure 13. and 3=σ3). Longitudinal profile of Baja vein in the Arcata vein system. One 10 . Perú. 2=σ2. modeled and contoured tension (T) and compression (P) axes. showing orientation of slip line and long axis of ore shoot. and ore shoot orientation cannot be predicted by kinematic analysis of fault. B. (2003). From Echavarria & Nelson (2002). Charlotte mine are even more restricted to the granophyric Unit 8 of the Golden Mile dolerite (Clout.. Marshall & Oliver. 2001). and carried the highest gold grades in Victoria (Solomon & Groves. A second example comes from gold deposits in the Kalgoorlie district. Figure 14. Australia.. productive veins are concentrated within the Precambrian Idaho Springs-Ralston shear zone. A third example comes from the Morning Star mine in Victoria. Mineralization (Cu. In this case. Australia.691). where quartz-gold veins formed during reverse faulting are restricted to the Wood’s Point dike. p. 2001). Solomon & Groves. where altered and mineralized rock formed in veins and boudin necks in strain shadows adjacent to competent. 2000. low permeability metadolerite and metagranite bodies surrounded by weak calcsilicate rocks during syn-metamorphic fluid flow at amphibolite grade P-T conditions (fig.example comes from the Mary Kathleen fold belt in the central Mt. Pre-existing weaknesses may be utilized as openings. and quartz vein arrays in the Mt. Oliver et al. U-REE) in the district is spatially related to the alteration. 2000). Beach. Isa block. 14.. Veins and breccias in the shear-zone hosted Golden Mile deposit are best developed within the Golden Mile dolerite. 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.. 2000. Schematic 3D geology of the Mary Kathleen fold belt. et al. 2003). 2001. One example comes from the Idaho Springs district in Colorado. Au. where foliation in Precambrian basement rocks was reactivated in shear and extensional mode during Tertiary precious and base metal vein mineralization (fig. 1990. a differentiated tholeiitic sill. Australia. Nelson et al. Western Australia. 15. which contains foliations and folds that trend 11 . 1). map of productive veins within and northwest of IRS. basemap from Moench & Drake. Numerous attempts have been made to classify veins and structurally-controlled ore deposits (e. his type 4. Bateman’s classification of open space deposits (table 1) includes categories for fissure vein. dashed = synform (data from Beach. Guilbert & Park. and Lovering & Goddard (1950) and many others. 2000. Colorado. As with Bateman’s 1981 classification of open space deposits (table 1. solid = antiform.g. lines with arrows are fold axial plane traces. 1981.. Lower hemisphere equal area nets and map of structural elements in the Idaho Springs mining district. density contour of poles to veins in the Idaho Springs district (outlined on the map). Bateman. E. D.parallel to the shear zone boundary (fig. Figure 15. most classifications of structural openings contain an ambiguous mixture of terms and categories. C. rose diagram of trend of Precambrian folds within and northwest of IRS. For example. 1986). McKinstry (1948. Density contour of poles to foliation within and northwest of Idaho SpringsRalston (IRS) shear zone.A. 1955). and no economic veins are present. 1966). rose diagram of trend of Precambrian folds southeast of IRS. 15). Southeast of the shear zone foliation and folds trend nearly perpendicular to the shear zone boundary. tension-crack 12 . 1933. 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). Lindgren. A. B. breccia fillings. and shear-zone deposits. However. fissure veins and tension cracks are likely the same in origin. P. 13 . Cross sections on right illustrate the angular relationships of reverse and normal fault models. Also. or 90° from the slip line in the fault plane. R’. Figure 16. 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. Stereographic construction of ore shoot orientation from intersection of fault plane with various Riedel fractures (R. Therefore. and for north-striking vertical wrench fault with dextral shear sense. Left column: examples for north-striking reverse and normal faults with 45° dip to east. 1993).fillings. numerous categories could describe the same ore deposit. and the classification is thus confusing and its usefulness diminished. and breccias may fill any of the other ore types (Taylor & Pollard. much ore in shear-zone hosted deposits is present in what Bateman refers to as fissure or tension veins. and T). and strike slip). solution-cavity fillings I. unrelated set of criteria. shear-zone deposits C. Figure 17. normal. etc.A. ladder veins E.). simple complex. a ‘simple’ vein is defined as mineralization of a single. Axis of ore shoot formed in fault jog perpendicular to slip line (slickenlines) for three primary types of faults (reverse. classes in the proposed classification represent the types of open-space strain features that develop in association with the principal structures. stockworks D. The sub- dikes. pore-space fillings J. The primary classes of principal geologic structures that host ore bodies include fault and shear zones. Note that ore shoot forms in flat sections of reverse faults and steep sections of normal faults. stratigraphic or pluton contact. strain shadows. Because past classifications of veins and other structural openings contain many old terms and ambiguous mixtures of categories. and are generally easy to recognize in the field during exploration projects. Bateman’s (1981) classification of open space deposits formed by hydrothermal processes. folds. igneous-related structures. each category is based on a different. vesicular fillings Table 1. Also. However. tension-crack fillings G. breccia fillings (volcanic. tectonic. In another example. structural intersections. and collapse structures. classification. collapse) H. and a ‘complex’ vein contains multiple laminae. many single fault veins are laminated. (2001). The proposed classification is designed to assist explorationists in formulating structuralmineralization models for exploration projects. Guilbert & Park (1986) Although classify this veins seems as a simple. and to guide the explorationist in recognizing 14 . and ‘anastomosing’ veins as having a braided pattern. individual veins in an anastomosing array could have variable thickness. this structural type is included under the fault/shear zone category. ‘irregular’ veins are defined as having variable thickness. Modified from Cox et al. we propose a new working classification of structural openings (table 2). and anastomosing. However. irregular. Because structural intersections mostly involve intersection of a fault or shear zone with some other contact (another fault. saddle-reefs F. fissure veins B. simple fault fissure. For example. associations of structural features in the field. Proposed working classification of structural openings related to localization of ore bodies. dikes. Fold Saddle reefs Limb Faults Folding (shortening and extension) Bendigo (Australia) Orogenic gold (Au) Phillips & Hughes (1996) Cox et al. (1990) Golden Mile. such that practical improvements can be made and the classification can become more robust and useful in exploration. 15 . Strain shadow Boudin necks Vein arrays in brittle layers Local extension normal to layering Mount Isa (Australia) FeOx-Cu-Au-U Perkins (1984) Findlay (1982) Oliver et al. (2001) Clout et al. (1991) C. Kalgoorlie Orogenic gold (Au-Ag) (Australia) Table 2. It is hoped that this proposed working classification will generate discussion within the exploration community. (2003) Lovering & Goddard (1950) Lovering & Goddard (1950) Corbett & Leach (1996) Miller & Nelson (2002) Corbett & Leach (1996) Echavarria (2002) Hoeve & Sibbald (1978) Rytuba (1994) Dilatant bends Clyde Tungsten mine (Colorado) Bell Mine (Colorado) Fault splays (including wing veins) En echelon fractures Intersections (other faults. Fault and shear zone Dilatant jogs Process Shearing Example Ohio Creek (New Zealand) Porgera (Papua New Guinea) Arcata (Peru) Type of deposit Porphyry Copper (Cu-Au) Epithermal (Ag-Au-Pb-Zn) Epithermal intermediate sulfidation (Ag-Pb-Zn-Au) Epithermal (W) Epithermal (Ag-Pb-Zn) Epithermal (porphyry related) (Ag-Au-Pb-Zn) MVT (Zn-Pb) High sulfidation Epithermal (Au-Cu) Epithermal Intermediate sulfidation (Ag-Pb-Zn-Au) Unconformity-type uranium deposits (U) Epithermal (Au-Ag) Reference Corbett & Leach (1996) Corbett & Leach (1995) Echavarria & Nelson (2002) Echavarria et al. Classes A. unconformities) Wau (Papua New Guinea) Pillara (Australia) Mount Kasi (Fiji) Caylloma (Peru) Fault-Fault Rabbit Lake (Canada) Fault-Unconformity Jefferson Canyon (Nevada) Fault-Caldera margin B. (1991) Porphyry stockwork Flow related breccia (flows and domes) Cripple Creek Epithermal (Au-Ag) Chuquicamata. (2001) Padilla Garza et al. (2001) Titley (1982) Cunningham et al. 16 . in fold-related openings. with consideration of all the mesoscopic-scale structures that might be associated with the principal structure. Therefore. As with past classifications. fold. fluid overpressure. although structures in each of the four categories generally form by a single strain-related process. 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. Silver Bell. igneous related structures. (1985) Ossandon et al. Collapse related structures Dissolution Ohle (1985) Table 2. igneous-related structures may form by a number of possible processes. Faults can form in association with folds. (1996) Russell & Kesler (1991) Thompson et al. Igneousrelated structures Process Fracturing and brecciation due to intrusion. Buick Mine (Missouri) MVT (Zn-Pb) E. the fifth category label is modified with a rock-type association (igneous-related structures). For example. and collapse structures. cont. Ajo (Arizona) Todos Santos. and in collapse-related ore deposits. Breccias can form in dilational fault jogs and bends. In addition. collapse structure).Classes D. in diatremes and igneous breccia pipes. thermal expansion. La Porphyry deposits (Cu-Au) Escondida. or flow brecciation Example Type of deposit Reference Diatremes and breccia pipes Balatoc. Baguio (Phillipines) Pueblo Viejo (Rep. our proposed classification is not without problems. during exploration the structures in the principal categories must be viewed in a larger context. can be associated with any of the primary categories. El Salvador (Chile) Morenci. strain shadow. Epithermal (Ag) Carangas (Bolivia) Fracturing due to collapse Jefferson City Mine (Tennessee). veins and vein arrays (such as en echelon arrays). Although four of the main categories are labeled with structural types (fault. Dominicana) Epithermal (Au-Ag) Epithermal (Au-Ag) Sillitoe & Bonham (1984) Cooke et al. 17). and shearrelated folds. C-planes represent spaced shear planes. subsidiary fractures related to fault slip (such as those predicted by the Riedel model. Figure 18. p. Stereonet shows method of determining the ore shoot orientation from the intersection of S. Leyshon & Lisle. shear fabrics in fault rocks such as S-C fabric (e.81. figs. Structural features include slickenlines and fault corrugations. such as bedding and a dike (Marshak & Mitra. Slickenlines. 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. 6. Stereographic methods also can be used to construct the slip vector. 1996. S-C shear fabric in granitic rock from Altar district. Ramsay & Huber. fig. S-C type fabrics consist of two sets of anastomosing foliations (Berthe et al. 17 . Mexico. knowledge of orientation of the fault slip vector is extremely important for modeling the orientation of ore shoots. and S-planes represent generally penetrative planes of flattening. The orientation of the slip line can be determined using a number of structural features and techniques. S-plane and Cplane orientations shown in upper right.. as well as in fault zones formed by brittle cataclastic flow. p. 1988. In this case ore shoot orientation is perpendicular to the fault slip line (Fig. ore shoots form where the orientation of the fault zone changes and approaches parallelism with the plane containing σ1-σ2 (for example in fault bends and jogs. 1987.and C-planes. Sonora..16). 56).g. 10).Structural methods in exploration for structural openings A number of structural methods are useful in the exploration for ore deposits in structural openings. 1. When stress controls the orientation and kinematics of faults and fractures in the absence of pre-existing weaknesses. As many hydrothermal deposits are associated directly or indirectly with faults or shear zones. 1979) and can form in ductile shear zones formed by plastic deformation mechanisms. p. 632). Yellow is surface representing contour of 3% Zn grade.or fault striations. it can often be assumed that fault vein striations of this type are parallel to the slip line. B. A. Cross section of Eastern fault showing near-vertical splays off of fault where fault steepens. as many fault veins form by repeated movements and fluid flow events during one tectonic cycle. are common on the walls of many fault veins. 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 18 . Longitudinal cross section of Eastern fault showing near horizontal intersection between main fault plane and more vertical second-order fractures (veins). if the striations formed by mechanical wear during shearing (fault movement). the slip line at the time of fault vein formation. and may not be parallel to the slip line during fault vein formation. 2002). However. then they formed parallel to Figure 19. C. Gocad™ 3D computer model of Pillara Mississippi Valley-type Zn-Pb deposit in Western Australia (Miller & Nelson. Ramsay & Huber. If the striations formed by crystal fiber growth (termed slickenfibers. then they must have formed after the time of fault vein formation. Nonetheless. North-looking perspective of 3D model showing simple graben-bounding faults offsetting colored stratigraphic contacts. 1987). 1979). Conclusions Structural openings in rocks constitute the primary permeability network for the formation of most hydrothermal ore deposits. 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. For example.. ore shoot orientation is perpendicular to the fault slip vector and can be predicted by kinematic analysis. 18). Hoover & L. Modern 3-D computer modeling techniques also can be very useful in predicting ore shoot orientation. using a Gocad™ 3D model (fig.. 1912): New York.H. exploration for hydrothermal ore deposits should include analysis of geologic structure and evolution. density. 19). If ore is related to shear folds. References Agricola G. rather than being controlled by stratigraphy. geometry.model (fig. 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. The line of intersection between the faults and the T-fractures is nearly horizontal as demanded by kinematics of graben formation. Hoover. If ore is related to a fault or shear zone with S-C type shear fabric (Berthé et al. showed that Zn ore in the Pillara mine in Western Australia is concentrated along graben-bounding faults and within splay veins (extension.C. 16). Dover Publications Inc. The distribution. However. Further.. the slip line can be constructed by stereographically analyzing fold hinge orientation and fold asymmetry by the Hansen method (Hansen. ore shoot orientation is predicted to be parallel to the intersection of the S-planes and the C-planes (Fig. when structural openings form along pre-existing weaknesses. 1950.De Re Metallica (English translation by H. Miller & Nelson (2002). rheological variations. and fault kinematics to evaluate structural controls on localization of ore bodies. 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Report "Structural Openings and the Localization of Ore Bodies"