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Evolution and Style of Fracture Permeability in Intrusion-Centered Hydrothermal Systems SPENCER R. TITLEY University of Arizona ABSTRACT Deep and tall columns of permeable crustal rocks evolve as a consequence of fracturing during shallow emplacement and rapid cooling of water-bearing felsic magma. Rapid cooling of the magma and sudden pressure drops result in both the development of the characteristic porphyritic texture of rocks in these systems and overpressures of magmatic water; sudden release of the hydraulic energy built up in the melt is believed to have resulted in development of stockwork-style (reticulate) fracturing of the magma and its wallrock. Early stages of cooling of magmas considered here takes place through conduction, but subsequent cooling is dominated by development of the stockwork and consequent convective flow; rapid transfer of heat to wallrock is focused by the stockwork system and results in rapid local rise of temperatures and consequently pressures of pore water, resulting in further rock failure in the walls. The dominant character of flow porosity that evolves is that of an intricately interconnected three-dimensional network of planar fractures with large length-to-aperture ratios and continuities that range from microns to hundreds of meters. Field study and analysis of these systems reveal that they form from a complex process characterized by episodic rock failure and a consistent and predictable evolutionary course of hydrothermal reaction with walTrocks, usually in a thermal regime of declining walTrock temperatures. Studies of waters suggest changing provenances with magmatic waters dominating early hydrothermal systems, subsequent mixing with waters of different sources including pores, and, ulti- mately, dominance by meteoric water. 50
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY INTRODUCTION Emplacement and cooling of magma at shallow crustal levels are attended by thermal-mechanical stresses result- ing in the formation of extensive interconnected networks of closely spaced (centimeter) joints in large rock vol- umes. These volumes of densely fractured rocks with reticulate fracture patterns, informally referred to as stock- works, represent secondary permeability imposed on crus- tal rocks that underlie areas of up to hundreds of square kilometers, centered on the locus of magma emplacement. In most instances a volcanic center may be inferred as such a locus from regional or areal geology. These centers of areally small (kilometers), subvolcanic intrusions are the object of discussion in this chapter. The shape of the rock volume affected, which includes the pluton complex and many times its volume of wall- rocks, is crudely cylindrical and may have had a height that extended from a permeable volcanic superstructure to depths corresponding to the height of the cooled magma column. This height is believed to have been as great as 7 km at Yerington, Nevada (Dilles, 1987), and at least 3 km at San Manuel, Arizona, as scaled from Lowell (1968~. The permeability imposed on these systems in the form of joints provides a means for localized rapid cooling of magmas by convection of fluids of magmatic and meteoric provenances. The movement of these fluids from crustal wallrocks and magmatic sources results in the formation of metal-sulfur deposits and some centers of modem geo- thermal activity. The passage of fluids through joints centered on the igneous system also results in mineralogi- cal modification (hydrothermal alteration) of joint walls and filling of open space in the joint system by precipi- tated minerals. This chapter presents the results of detailed field and laboratory analyses of the geometrical and mineralogical ~ ~-~ ~'~t ;: PACIFIC J .\ o ,lND.-AUST~ '., J ~\_/ REPLATE A ~' Hi., 1~1 aNTwnTIc Pl ATE - 51 characteristics of these joints and their host rocks from a broad sampling of sites. These studies reveal information concerning the development of flow porosity in a specific but widespread geological phenomenon of modern as well as old tectonic regimes. Intrusive centers have been stud- ied in both island arc and continental settings, where they have been found to reveal generally consistent styles and histories of joint evolution as well as a consistent pattern of evolution of hydrothermal minerals; these phenomena are, in turn, consistently interpretable in a context that allows assessment of some aspects of geochemical and thermal evolution of the hydrothermal systems. This chapter addresses some relevant geological as- pects of the occurrence of these intrusive centers, relevant aspects of petrology, an overview of the characteristics of fracturejoint evolution, and through a synthesis, some of the implications of the results of these studies. GEOLOGICAL SETTING AND ENVIRONMENTS Volcanic and seismic activity are phenomena of mod- ern erogenic regions. The existence of volcanic rocks and young epizonal intrusions within erogenic belts attests to historical volcanic activity; the presence of the eroded tops of swarms or chains of epizonal intrusions, as well as the presence of erosional remnants of coeval volcanic rocks, trace and reveal the position of still older orogens. In Figure 3.1 regions of both older (Mesozoic and Cenozoic) erogenic belts and modern erogenic regions are shown. The belts of current activity may be seen to transgress both oceanic and continental lithosphere and are marked by the lines of seismic activity and volcanism. These settings are sites of plate convergence, where, apparently, subduction- related processes result in generation of magma and trans- fer of heat to the shallow crust. Widespread geological field evidence from the circum =~=~. fAMERICAN ~ ~ C:`,, PLATE PLATE ~ 6a FIGURE 3.1 Map of major plates and continental masses of the world, showing (black circles) important centers of stud- ied Mesozoic and younger pluton-centered hydrothermal activity. Shaded areas trace presently active regions of seismic and vol- canic activity where erogenic processes are taking place. Modified from Stanton (1978~.
52 Pacific erogenic belts indicates that extrusive volcanic activity and epizonal intrusion are related processes. Magmas rise in regions where subduction-related stress has resulted in localized sites of crust weakened by fault- ing, folding, or thinning or where extensive magma gen- eration at depth has resulted in deep regions of buoyant silicate melt. Within the cores of volcanic systems, bodies of magma are emplaced and ultimately cool. Some important characteristics of these systems merit comment. They occur in clusters of a few to many tens of intrusion centers in areas of 104 to 105 km2, and many were apparently formed at times of high subduction-related compression in both island arcs (Titley and Heidrick, 1978) and continental settings (Heidrick and Titley, 1982~. Individual pluton centers or clusters give rise to chains that trace the edges of cratons or lie on or adjacent to island arcs. In some locations pluton centers occur within crust that has been strongly deformed, and their settings have been described as mobile regions. Viewed at re- gional scale, the generally circular intrusion centers ordi- narily range from a kilometer in diameter to systems up to, but rarely more than, about 5 km across, but they have been localized beneath volcanic rocks covering areas an order of magnitude greater. In young Tertiary systems, volcanic rocks coeval with intrusions are commonly, although not invariably, closely adjacent; in older Tertiary or Mesozoic systems, similarly coeval volcanic rocks are present but distant, which is interpreted as representing remnants after weathering of a larger volcanic superstructure. The intrusive rocks com- prise porphyritic phases and are ordinarily members of felsic rock clans, ranging from quartz diorite through gran- ite. The suites of many intrusive centers are accurately described as complex, such as at Ajo, Arizona (Wadsworth, 1968), and Ray, Arizona (Banks et al., 1972~. Volcanic rocks are seldom more mafic than andesite and range through progressively more felsic rocks to rhyolite. Whereas many different textural varieties of igneous rocks are present in the complex rocks of a volcanic system, a young mass of porphyritic igneous rock is invariably at the core of the volume of fractured rock. NATURE OF SECONDARY POROSITY IN INTRUSION-CENTERED SYSTEMS Stockworks associated with porphyry intrusions are vertical columns of fractured rocks, kilometers in diameter and height. This volume of fractured rock includes not only rocks of the intrusion complex but also volumes of wallrock surrounding porphyry that are many times greater than the intrusions. Within the stockwork, the fractures that compose the porous volume vary widely in scale and style; they range from through-going veins with continuity SPENCER R. TITLEY of kilometers, through intersecting veins and veinlets of meters to centimeters of continuity, visible to the unaided eye (the mesoscopic veins of this chapter), to veinlets whose scale ranges downward in size to those resolvable only with special optics. Evidence from the field, based on the volume and distribution of hydrothermal alteration products, suggests that it is the fractures (joints) at mes- oscopic scale that dominate the properties of flow porosity (permeability) of these systems. The style of mesoscopic joints in the stockwork is shown in Figure 3.2. There it can be seen that the style of openings in the stockwork is dominated by a three-dimen- sional mesh of intersecting planar to curviplanar joints. The development of domains of variably fractured rock within stockworks is an important characteristic wherein, for reasons that are uncertain and that are not obviously related to the mechanical properties of Ethology, some volumes of rock manifest many more fractures than con- tiguous volumes; the flow porosity is thus anisotropic at the scale of tens of meters. Field studies reveal that there has been little or no rotation or transport of rock following the fracturing pro- cess in most porphyry-centered systems. Such a generali- zation excludes volumetrically insignificant masses of localized breccia that occur in some systems and clearly Joint Zone and Jl-Joint Set Sn~ooth- surfaced, continuous - planor; pronounced influcace oa local izing ~ ~ ~ arotion - m i nerolirof ion J2 ~ Joint Set s Roug-h -sur f oced, discontinuoua curvi ploner; rorely ollered or m i neroi i zed ;,l - ~; ';' `/ ! :' J3- Joint Sets Dikes,Veins ond Fault Ouite irrequlor, discontinuous veins ( Fv) microtroctures ond hoirlin. crocl`s i1 ~ ~ ~ lI y4, ('/ i`'`l/ Aplite dike ~ J3-Joint Se1 FIGURE 3.2 Schematic block diagrams illustrating typical and characteristic styles of fractures with stockwork systems. The several kinds of fractures successively overprint each other and result from episodic events. Reprinted from Heidrick and Titley (1982) with permission, University of Arizona Press, Tucson.
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY FIGURE 3.3 Photographs of thinned sections showing habit of microveins as used in this paper. Scale bar is 1 cm. The upper diagram shows apparently early microveins in relationship to thicker 0.5-cm quartz veins. In both the upper and lower dia- grams an apparent systematic orientation or set of orientations is shown that may be interpreted in the context of oriented stresses that control fracture directions, even down to this scale. excludes some systems such as Cerro Verde (Peru) and Cananea (Mexico), where large breccia masses are major constituents of the fractured rock volumes. Whereas the mass of hydrothermally derived minerals that occur within and adjacent to the joints may aggregate up to 10 percent of the total volume of fractured rock, field evidence does not indicate that this volume was produced instantane- ously. The walls of both steep and flat veins of stock- works are parallel over meters of length, regardless of vein thickness, revealing negligible rotation. Evidence of offset of old veins by younger fractures is extremely rare and 53 where present is at the scale of centimeters. Fracture asperity may locally result in pinching and swelling of vein walls, but such apparent roughness cannot be un- equivocally related to movement. Consequently, transport of fractured rock has been considered trivial, the dominant motion or displacement only that of true joints, normal to the fracture wall. The only reasonable explanation of the relatively high volume of crack-related alteration minerals is that fractures evolved episodically and appear to have been filled continuously through a time of cooling and consequent shrinking of the hot rocks. Within the domains of mesoscopic joints, smaller domains of microscopic fractures are present. These frac- tures, defined here as features less than 0.1 mm in width, are also interconnected but are discontinuous with persis- tence ordinarily on the scale of centimeters (Figure 3.31. Whereas they contribute significantly to the porosity of the rocks, by virtue of their discontinuous nature and apparent small widths, they do not appear to have contrib- uted to the permeability of the stockwork; filled with fluid nonetheless, they may be representative of fluid-rock reac- tions controlled more by diffusion than flow. ROLE OF MAGMATIC PROCESSES IN FORMATION OF JOINTS The occurrence of porphyry plutons associated with volcanic centers is significant. The intricately intercon- nected, closely spaced joints that compose the common and typical stockwork evolve in association with rocks of porphyritic textures; stockworks are not widely, if at all, recognized as having evolved in the course of the forma- tion of igneous intrusions with phaneritic textures. The evolution of porphyritic igneous rocks and their associated stockwork fractures appears to be integrally related, and the evolution of porphyries continues as a subject of fundamental petrological study. A generalized and simplified concept concerning the origin of these rocks explains that they represent multistage cooling of felsic silicate melts. Interruptions in uniform cooling, which commenced at great but unknown depths, and chilling of melt accompanied by drops in pressure, attend the rapid rise of magma to shallow levels of the crust, a phenome- non consistent with the apparently rapid emplacement of magmas in volcanic systems and the presence there of porphyritic igneous rocks. The characteristics of porphyry intrusions in the com- plexes considered here, as seen in the field, attest to this likelihood of rapid emplacement of magmas. Xenoliths of any sort are rare to absent within, at the margins, or in the caps of these porphyry plutons, and wallrocks are unde- formed, the implication being that magmas were emplaced as fluids with a minimum amount of sloping. Studies of
54 many porphyry stocks only rarely reveal the presence of recognizable foliation, flow lines, or other evidence of motion of magma along contacts. These characteristics of rock texture, foliation, and the absence of xenoliths have been ascribed to a process of "permissive" rather than "forceful" (Mutch and McGill, 1962) emplacement of the porphyry magmas. A "permissive" or unimpeded em- placement of such magmas is envisioned as possible, and even likely, in the active tectonic environment in which they occur, with tectonic stresses resulting in deeply pene- trating faults and consequent channelways. The most important result of emplacement of magmas at shallow depth is that of rapid cooling and quenching of melt that contained crystals of earlier-formed minerals. The porphyritic textures are thus formed in the walls and across some great but unknown depth of the cooled magma. Rapid cooling and crystallization of melt bring about changes in water pressure, the effects of which have been described by Burnham (1967, 1979~. Briefly stated, the exclusion of water from the silicate melt by crystalliza- tion, especially at the shallow depths (3 to 5 km) consid- ered here, results in overpressures of exsolved water that exceed the tensile strength of the rocks. Brittle failure under these conditions (hydraulic fracturing) ensues and evolves the fractures composing the stockwork. Emplaced magmas may cool initially by conduction of heat to their walls (Norton and Knight, 1977~. This pro- cess is believed to result in rock failure and to complement the highly energetic effects bringing about the hydraulic failure described above. Water contained in the wallrocks, chiefly in pores, becomes an agent of energy when heated; the effects of this heating have been described by Knapp and Knight (1977) and Knapp and Norton (19811. As wallrocks to porphyry plutons are heated by conduction or convection in the early stages of magma emplacement, contained pore water passes into the supercritical region and effective pressures exceed the tensile strength of the rock. The result is rock failure seen now in the presence of the large volumes of fractured wallrock surrounding por . . P nyry intrusions. NATURE OF JOINTS AND FRACTURE SYSTEMS At regional scale the intrusive centers may be seen in some terranes to be related to, and presumably controlled by, variations in the effects of stress in different domains within regional fault systems. Whereas such control is difficult to establish or to even propose in older terranes subjected to tectonic overprinting, it may be reasonably inferred from geological features in young terraces of is- land arcs. As revealed in the geology of the mobile belt of Papua New Guinea (Figure 3.4), batholiths and related centers of porphyry complexes lie within or adjacent to SPENCER R. TITLEY Probable Net Convergence Directions ({its) Ma cK:~n` ~P1~ro t ~ant IVe~V GlJ/4~£-A ~ L ~_'~_ CJ6f /£ ,£ 'I'm;, - -''a set r '\`2 t%_ 3L~ 1 ~290 ~4 KlLO~ETEltS ~ - FIGURE 3.4 Modified geologic map of Papua New Guinea showing trace of regional faults and location of principal batho- liths and centers of pluton emplacement. Black areas are those of mappable igneous bodies and centers. Modified from Titley and Heidrick (1978~. southward deflections of generally WNW-striking regional linear faults. Interpreted right-slip in this regional fault system result in areas of "low compression" at the south- ward deflections of their trace. Regional stresses that appear to have controlled positioning of these systems at such a scale are interpreted as having even more funda- mental importance in affecting properties of joints at kilo- meter to meter scale. Joint and Fracture Orientations The orientation of mesoscopic fractures in stockworks has been traditionally described as "random," and at cas- ual glance in outcrop the swarms of joints appear to mani- fest such characteristics. In detail, however, measure- ments of fracture orientation in numerous pluton- centered systems in cratonic and arc settings (Rehrig and Heidrick, 1976; Titley and Heidrick, 1978) reveal properties of align- ment and dip that are consistent with inferred orientations of the regional or local stress field, an association that is intuitively obvious, although it is not conspicuous or promi- nent at outcrop scale. Detailed studies of joint orientations in large systems reveal two extremes in the habits of orientation of mes- oscopic fractures, from each of which may be deduced the orientation of principal stresses. These extremes are shown in Figure 3.5, geologic sketch maps of fracture systems in two pluton-centered stockworks in Arizona. In Figure 3.5A fractures radial and concentric to a center of intru- sion reveal patterns evolved at high levels in pluton sys- tems where the maximum principal stress is vertical and intermediate and least principal stresses are hydrostatic, equal, and horizontal. The example of Figure 3.5A views
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY a Laramide intrusion complex very near its top with the orientations of fractures related to stress in a shallow epi- zonal environment. Stresses in this crustal region result in a pattern of centrosymmetric, concentric, and radial frac- tures. In Figure 3.5B (a map of a surface weathered into the deeper parts of a Laramide intrusion complex in south- e~n Arizona) a pattern of joint development is present that was controlled by stresses in deeper parts of the crust. In the example of Figure 3.5B, fluid inclusion data derived from study of quartz formed in the joints are interpreted to indicate that we are viewing the system at a depth at least 2 km beneath the original surface (Preece and Beane, 1982~. The fracture orientations of Figure 3.5B may be further interpreted as reflecting a horizontally oriented maximum principal stress, a roughly south to north ori- ented minimum principal stress and a vertical intermediate principal stress. The analysis of geometry and distribution of joints formed near the locus of intrusions, coupled with radio- metric age data that show fracturing to be essentially synchronous with intrusion, has led to interpretations of origin and orientations of regional stress. In Arizona the consistent, nearly monotonous ENE and NNW directions of fracturing seen in Laramide intrusion systems (Figure 3.6) correspond to the directions of stress inferred for the normal convergence directions of Laramide plate subduc- tion (Rehng and Heidr~ck, 1976; Heidr~ck and Titley, 1976, 19821. Miocene stress directions acting on eastern Papua New Guinea are seen to be consistent with the orientations of fractures surrounding centers of felsic plutons in the Eastern Highlands (Titley and Heidrick, 1978; Asami and Batten, 1980~. Field work completed in these regions, widely sepa- rated in space and in times of igneous activity, reveals without known exception joint geometries that are region- ally consistent within regions with the time and inferred \~ , *< in. · . N_ - ~, . 0 0.5 1 - ki iometers -I / v ,. . , ,; -, A 0 1 2 3 ki lometers 55 related tectonic style. A reasonable interpretation of the genesis of these fracture networks may be drawn from these numerous observations; the thermal and mechanical energetics of pluton emplacement and cooling bring about rock failure; the geometry of resulting joints is controlled by orientation of regional and local stresses. Alteration, Succession, and Distribution of Altered Fractures Three important physical and mineralogical properties of fractures that make up the volumes of stockwork have been studied in detail. The interrelationships of these features establish a basis for understanding the way in which joints and corresponding characteristics of flow porosity evolve. The first of these properties is that of the associated hydrothermal alteration products. A second is that of the succession of veins and alteration products as revealed in cross-cutting relationships. The third is the relative abun- dance, in space, of different sets of veins as defined by alteration products. Vein Alteration and Paragenesis The flow of fluids through the joint network results in alteration of the crack walls and deposition of hydrothermal minerals. Evidence from the field, from the study of many slabbed specimens, and from the results of mineralogical and fluid inclusion analyses reveals that this process is complex, takes place through many stages, and proceeds in the hydrothermal environment under declining temperatures. Viewed at outcrop scale at the surface (Figure 3.7) and on mine bench, the mesoscopic fractures of stockworks reveal heterogeneous characteristics of style and manner of alteration and of weathering. The heterogeneity in appearance stems from the style of joint that formed at a a/ // ,,,~(,, \/ / / ~ ' / \ FIGURE 3.5 Geologic sketch maps of structures and joint-fault-dike orientations in Laramide intrusion centers in Arizona. Figure 3.5A a map of the San Juan Pluton at Safford, Arizona, modified from Heid rick and Titley (1982), manifests patterns seen at high intrusive levels where-frac tures trace radial and concentric patterns with respect to the center of plutons. Fig ure 3.5B shows fracture pattern orienta tions revealed at a weathered depth of about 2 km into the Sierrita, Arizona, pluton B system (patterned areas). Pronounced orientation of fracture pattern believed to represent effects of region-wide compres sion during the Laramide.
56 particular time and the contrasting mineralogy formed by the hydrothermal solutions that passed through it. Within systems the habits of the altered joints that occur at spe- cific stages in fracture evolution are broadly consistent and predictable from place to place. They differ, however, between systems. Distinctive assemblages of alteration minerals develop within specific joint sets at different stages of the hydro- thermal process. Timing of stages is revealed from the cross-cutting relationships of joints with different altera- tion products and styles. A typical example of this ubiqui- tous habit and a common succession of hydrothermal products are shown on the polished and mapped slab of Figure 3.8, wherein veins and veinlets characterized by specific alteration assemblages reveal consistent cross- cutting relationships. The habit of intersection of cracks with different vein mineralogies bespeaks a complex and protracted process of fracture development in stockwork . evolution. The development of vein alteration assemblages has been described in reports spanning many decades of work, with increased levels of understanding by numerous au- thors (Schwartz, 1947; Creasey, 1966; Meyer and Hemley, 1967; Carson and Jambor, 1974; Brimhall, 1979; Rose and Burt, 1979; Beane and Titley 1981; Einaudi et al., 1981; Beane, 1982; Einaudi, 1982~. A widely described and generalized succession of vein alteration assemblages (Titley, 1982) from early to late in a quartz-2 feldspar- mafic wallrock is as follows: 1. The assemblages quartz, quartz-biotite-orthoclase, or quartz-biotite form in and adjacent to the thermal center and in some instances may be of large areal extent, proba- bly synchronous with assemblages 2 through 5. 2. The assemblage chlorite-epidote-carbonate-zeolites that evolves at the periphery of the system. 3. The assemblage quartz-orthoclase-(biotite)-sulfides near the thermal center. 4. The assemblage quartz-(orthoclase)-sulfides near the thermal center. 5. The assemblage quartz-sericite-pyrite developed within the system that overprints earlier stages. 6. Ultimate overprinting of stages 3, 4, and 5 by the mineralogy of stage 2 that collapses on the system as its center cools. This common succession of alteration types in potas- sium silicate rocks, as also shown in the slab of Figure 3.8, reveals important characteristics of the progress of frac- ture and chemical evolution in the systems. Because each mineral assemblage forms in its own set of fractures at a specific time, it must be interpreted that the development of fractures is episodic. Detailed fluid inclusion data from many hydrothermal systems such as these (e.g., Preece SPENCER R. TITLEY 5 o .,, o Ul I= _ , =- o~ m Z m~ _ a: :? · NTOUR INTERVAL OFF . r°, {~' ~:-4 ~ N ~ 2172 ~1~ s A _ ~CONTOUR INTERVAL / ~\ 10-1~'o r¢-4 ,b ~<ZLF \job~o-~ W: ~ ~ ~HE N.4912 _ ~N 5328 C FIGURE 3.6 Rose diagrams of fracture patterns from stocks and wall rocks in Laramide centers in Arizona. Equal-area, pole-to plane plots are shown for each of the fracture sets. N = number of observations. Reprinted from Heidr~ck and Titley (1982) with permission, University of Arizona Press, Tucson. and Beane, 1982) reveal that the succession of mineral assemblages evolves under declining temperatures of so lutions. A tentative conclusion that the fractures contain ing the specific minerals are also opening under a corre sponding lowering of temperature of the rock mass must follow. That early alteration assemblages are largely anhydrous and later ones hydrous suggests that either the
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY volume of solution increases or that certain ion activity ratios change, bringing about an effective increase in the concentration of H+. By stage 5 the manifestation of areally large volumes of rock, completely and pervasively converted to masses of quartz-sericite and pyrite, suggests profound H+ metasomatism, from which may be inferred a considerably enhanced volume of solution flow. The shift from anhydrous to hydrous phases also has been described in carbonate wallrocks where calc- and magnesium-sili- cate alteration minerals are formed (Einaudi, 1982; Johnson and Norton, 1985~. 57 / / \ ~ I ,l~q ki+mt ~it?-~<q-cpy u, cm ! FIGURE 3.7 Map of weathered outcrop in granitic wallrocks of a porphyry pluton in Papua New Guinea. Photograph scale is shown by hammer near bottom. The exposure is selected for these purposes as the weathering of sulfide-bearing veins de velops sufficient contrast to observe the different tones (from different alteration types) and cross-cutting nature of the dif ferently altered joint sets. Reprinted from d Titley (1982) with permission, University of Arizona Press, Tucson. FIGURE 3.8 Slabbed surface of wallrock sample from Siemta, Arizona, showing the cross-cutting habit of differently altered joints. q = quartz; cpy = cha~copyrite; hi = biotite; mt = magnetite; Ksp = orthoclase. Reprinted from Titley (1982) with permis sion, University of Arizona Press, Tucson. Vein Abundance and Distribution Systematic studies of the abundance and distribution of veins have been re- ported from the Mayflower stock in Utah (Villas and Norton, 1977~; Silver Bell, Arizona (Kanbergs, 1980; Norris, 1982~; Red Mountain, Arizona (Kistner, 1984~; and Sierrita, Arizona (Titley et al., 1986~. Results of reconnaissance studies have been reported from prospects in Papua New Guinea (Titley, 1978; Titley et al., 19781. A simple method of measurement of fracture abun- dance has been applied to outcrop, drill core, and selected samples in the study of the stockworks. Very simply, the
58 method involves measuring of fracture area in rock vol- umes or fracture length on surfaces and then dividing that value by, respectively, either the volume or area of the sample. The value obtained is per centimeter. For the fracture abundance present in the systems studied, values were determined from length/area relationships in 2500- cm2 circular or square sample areas. Area/volume values were determined on drill core and selected samples of slabbed rocks. The values obtained were used to deter- mine both the distribution of vein types and their abun- dance (Titley, 1978; Haynes and Titley, 1980; Titley et al., 1986). Recognition of the existence of vein successions as revealed by contrasts in alteration allows an assessment of the distribution and abundance of fractures that form at different stages in the process. Such a study for a system has been carried out in detail and complete scope only at Sierrita, Arizona. Data collected from a thousand sample sites at Sierrita, over an area of about 70 km2, have been treated by area-averaging techniques and contoured as shown in Figure 3.9, wherein isopleths of fracture density values, representing all fractures, close on the porphyry center (from Titley et al., 1986~. Separate maps showing the distribution and abundance of five different alteration types are integrated into separate curves of density versus distance (Figure 3.10~. Most of the measurements of fracture abundance at ,'" - V~/~} /~ '\ ~ ~ 't ~( ~\: · ~A- / ~ ( ~ , _ _ -\ ~ / isopleth values in cm~ ki lometers FIGURE 3.9 Vein/joint abundance map of the Sierrita system, Arizona. Isopleths show values for the total numbers of fractures (per centimeter) in outcrop and close on the porphyry center (patterned areas). Outer isopleth shows values nearly zero, but rare altered joints persist outward for another kilometer. Within the center of porphyry, fracture abundance values double and locally exceed values of 1.5 cow. Figure modified from Titley e! al. (1986) and data in Haynes and Titley (19801. SPENCER R. TITLEY O?5- . 0 15 0.10 SIERRITA 0.20 ~ CZ~ CHL-CPY-MO (ANH-~1M) of- PY ( OR-EP- MT) ~ \\ QZ-OR (Bl) 0.05 ~ )~ KM FIGURE 3.10 Plot of fracture abundance (vertical) as a function of vein-alteration type and distance (horizontal) from an as- sumed center within the porphyry mass at Sierrita. Oldest-to- youngest vein sets proceed from qz-or to the complex qz-or-chl- cpy veins that are the youngest. The values shown in Figure 3.9 include the values of the quartz-sulfide set and the quarcz-ortho- clase set. qz = quartz, or = orthoclase, chl = chlorite, cpy = chalcopyrite, mo = molybdenite, anh = anhydrite, hm = hematite, ep = epidote. Sierrita were made on the vertical faces of benches in the pit or walls of washes or on horizontal to subhorizontal surfaces of adjoining areas. Inspection of vertical faces across a pit depth of several hundred meters, as well as inspection of vertical faces of surrounding outcrops, indi- cates a relatively low (much less than 10 percent) propor- tion of flat (less than 45°) fractures at these depths in the system. Whereas flat fractures are present at higher levels of such systems (e.g., the young systems of Papua New Guinea; see also Knapp and Norton, 1981), at the deeper (2 km?) level exposed at Sierrita they are uncommon. In these circumstances the determination of length/area rela- tionships in fractured rocks is a close approximation of the area/volume value. Such a conversion is reasonable at Sierrita and otherwise comparable deeply exposed intru- sion centers. The information shown in Figure 3.10 is revealing in several important respects. It shows an episodic progres- sion of fracturing that commences with an early, wide- spread event of relatively uniformly low numbers of frac- tures, followed by progressively more centrally restricted episodes of fracturing. Hydrothermal fluids flowed through fractures following each episode, and each stage of frac- tures is altered in a unique and characteristic way with the most dense fracturing and subsequent alteration in a stock- work closely centered on the porphyry center. Studies of fluid inclusions from vein quartz from the different vein sets reveals further that fluids depositing quartz in each
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY subsequent event were cooler than those of each older stage of quartz formation. The most dense fracturing is the youngest, most constricted, and most centrally located of the joint sets. And where these stockworks are hosts to ore minerals such as chalcopyrite, molybdenite, or tin minerals, it is this centrally restricted set that localizes the most abundant sulfide mineralization in the porphyry-stock- work environment. From the data in Titley et al. (1986) and summarized and shown in Figures 3.9 and 3.10, it is possible to make estimates of rock volumes containing equal fracture areas for specific fracture sets and their measured fracture densities. Such an estimate is shown in Figure 3.1 1 where cylindrical volumes 1 km in height with each containing 100,000 km2 of fracture area are schemati- cally portrayed. At the low measured densities of frac- tures containing quartz and orthoclase (Figure 3.10), the relatively large volume shown is necessary, whereas at the much greater fracture densities measured in the orebody from the ore-sulfide-bearing quartz veins, a much smaller volume is required to contain the same area of fractures. Each of the fracture types shown in Figures 3.10 and 3.11 is overprinted in the central volume of rock by the younger vein stages. Viewed in two dimensions the progress ap- pears to evolve with development of early, widespread joints, each successive fracture-forming event resulting in more closely spaced joints progressively more closely focused on the center of the system. 4.0km- 25 km- ~ 1 /~km ~I \ QZ-OR QZ- S QZ- ClJ-MO VOLU MES OF 100,000 kn,2 OF FRACTURE AREA FIGURE 3.11 Generalized diagram, from data of Figure 3.10, showing relative cylindrical volumes of rock 1 km high, in the Siemta system, that would contain 100,000 km2 Of fracture area. Values were estimated by planimeter of plotted and interpolated isopleths of fracture abundance in the data of Figure 3.10 and conversion to area/volume relationships. Illustration of the frac- ture abundance data in this way reveals the apparent focusing effect of joint-evolving events, with the passage of time, progres- sively closer to the intrusion center. 59 SOURCE OF WATER It is appropriate to review the nature and origin of waters that have been so critically important in the evolu- tion of stockwork and its intrinsic secondary permeability. The foregoing has underscored the complex character of these systems in the context of apparent episodic develop- ment of joints and in the context of the implied changing patterns of chemical evolution. There is no question that water in some form and likely from different sources has been an important agent of change in the evolution of these systems. Interpretations of fluid types and source stem from the data developed from analyses of fluid inclu- sions and from studies of oxygen and hydrogen isotopes, generalized and summarized in Figure 3.12. Fluid Inclusion Data Numerous workers, cited in Roedder (1984), have stud- ied the fluid inclusions found in the hydrothermal quartz of porphyry-cored stockwork ore deposits. The solutions in these inclusions reveal both very high (e.g., 60+ wt.% NaCl equivalent) and low salinities (on the order of 2 to 10 wt.% NaCl equivalent). Although some exceptions to the generalization have been found, the high-salinity inclu- sions are those that are also interpreted to have been formed at high (more than 500°C) temperatures and the low salin- ity inclusions at lower (less than 350°C) temperatures. From these characteristics a common and widespread in- ference as to source has evolved that high-temperature, high-salinity inclusions derived from magmatic water and that the low-temperature, low-salinity solutions evolved from a meteoric source. This inference is strengthened from evidence of the isotopic character of water involved. Data from Isotopes Oxygen and hydrogen isotopes have been studied in a few porphyry stockwork systems and are reviewed in Sheppard et al. (1971) and Ohmoto (1986~. Viewed inde- pendently of the data from fluid inclusions, the isotopic character of early (high-temperature, high-salinity) solu- tions reveals the habit of inferred magmatic solutions; the isotopic character of solutions that formed late-stage al- teration minerals requires a component of meteoric water. Beyond identification of meteoric water by its isotopes, interpretations of other provenances of hydrothermal wa- ters in these systems, such as basin, pore waters, or meta- morphic waters (each likely), become clouded in uncer- tainty because of overlapping isotopic characteristics and uncertainties resulting from mixing of waters from differ- ent sources.
60 70~ ~ 60 x 4l `' 40 30 - :~ 2~) - O 10 In 0~ -40 ED ( /00) -80 -120 A at, ............. i. ... .... \ I m ~ (\,,, . ji A/ / / ~ at//,,,,) a/ ~ ~ // / I ,1//////~1 ~ll///llllll ~§l/////////} ~///~/l//// /// /~ ,/ 1/ / / / / // /// /lll//] (////~ .~//~/}, vll/llll/l/// (///l/l l/ll/, (/llll ill' C//////~J' i/ - 2< lo 300 4~ 560 6bo 7 50 Who Temperature of Homogenization (°C) B / SMaN / , %% Rito ~Q`f -20 -10 0 +10 +20 O(YOo) FIGURE 3.12 Generalized data from fluid inclusion measure- ments on quartz from porphyry-centered systems (Figure 3.12A), after Reynolds and Beane (1985~. The diagram plots homogeni- zation temperatures (Th) against salinities (wt.% equivalent NaCl + KCl) of fluid inclusions in hydrothermal minerals (mostly quartz) and reveals a general separation of fluids into three types, based on salinities and temperatures. Figure 3.12B shows the isotopic composition of water (~80 versus SD) from po~phyry- centered systems, as modified from Ohmoto (19861. In a general way that corresponds to the data from fluid inclusions, there is a gross bimodal nature of the data revealing likely magmatic char- acter to early waters, mixing, and ultimately meteoric character to the latest fluids. NATURE AND EVOLUTION OF FRACTURE- RELATED PERMEABILITY The dominating element of flow porosity or permeabil- ity in porphyry-pluton-centered systems is the fracture network. Because fracture networks are shown to evolve sequentially, however, the total number of cracks measur- able in rocks is not a measurement of characteristics useful in a determination of instantaneous permeability. Earlier- formed cracks are sealed, and, as part of a succession of formation of cracks, temporally intermediate stages of joint formation are succeeded by still younger joints that are successively sealed by alteration products. It is important SPENCER R. TITLEY to emphasize, again, that interpretations of data developed from fluid filling temperatures reveals that in most in- stances the early history of crack formation in wallrocks (cracks that formed under increasing wallrock tempera- ture) has not been recognized. Whereas it is likely that joints formed under these conditions, it is possible, indeed likely, that they remained closed or at least relatively impermeable through the time of rising wallrock tempera- tures and thermal expansion. The thermal and mechanical effects of this early stage of pluton emplacement and wallrock heating remain enigmatic and are an important object of both laboratory and field search. ~ Estimation of permeability may be made by evaluation of the expression nip · v~v k = -, (3.1 ) where k is permeability (cm2), n is fracture abundance (cm-l), and d is fracture aperture (cm) (Norton and Knapp, 1977~. Measurements of fracture abundance reported here have been made in the units of the expression. It may be seen, however, from the continuously varying value of fracture density for each fracture set (Figure 3.7) that the values of permeability at a constant value of aperture will vary in a similar way. The additional parameter necessary for calculation is that of joint aperture. This dimension, in the altered systems studied here, is not directly determin- able and has been estimated elsewhere in only a few re- ported instances. Inspection of the character of altered joints reveals that their width, as manifested by their filling and wallrock alteration selvages, is variable, most commonly between millimeters and a few centimeters. Although it is tempting to assign aperture values on the basis of such widths, the textural data indicate that such assessments must be made with caution and in most instances would be in error. Interpretations based on textural evidence and vein habits lead to the belief that the vein apertures were narrow (i.e., millimeters or less, rather than centimeters) in these sys- tems. Inspection of many altered joint sets from many sys- tems reveals that crack walls are parallel over distances commonly measured in meters. This habit is seen in three dimensions, wherein large blocks appear suspended in the network of parallel-walled, altered fractures. Whereas this characteristic may not establish a rigorous basis for inter- pretation of the evolution of narrow (millimeter-wide) veins, the case of centimeter-wide veins requires that the vein selvages "grow" (or apparently widen) at a rate suffi- ciently slowly to restrict rotation or transport of the af- fected rock masses. Inasmuch as fluid inclusion data indicate cooling during the life of crack formation and
EVOLUTION AND STYLE OF FRACTURE PERMEABILITY filling, it is reasonable to propose that the joints may originate with small apertures (millimeters or less) and maintain some degree of flow porosity and capacity to transmit fluids as a result of continuous thermal contrac- tion. Succinctly stated, wide (millimeters to centimeters) alteration selvages are viewed to be a result of a process of gradual crack widening during cooling and alteration, not the result of the instantaneous development of numerous wide (centimeter) joint openings. The flow porosity is not indicated by the width of vein alteration products. Further, the times at which a crack formed and at which it opened to the passage of fluid may well be different. Results of the microscopic study of textures of altera- tion minerals adjoining and within veins reveal that re- placement of walls is a dominating character of the joint- altering process. Open-space filling textures, such as in- ward and interpenetrative growth of quartz toward vein centers, is uncommonly rare. Further, there is rarely any indication of inward growth as might be seen in optical properties of quartz as viewed through the polarizing microscope. The habit of apparent replacement persists across all of the alteration sets studied from these systems; the process results, apparently, in nearly complete filling of the original space available. The microscope com- monly reveals that even in wide (centimeters) vein fill, the site of residual porosity remains largely in the central part of the filled-altered joint where, it is interpreted, the last fluids passed. Beyond the evidence from textures, addi- tional observations and inferences from the habit of joints and joint alteration lend support to the interpretation of small values of aperture. Young vein sets, representing different alteration char- acteristics from older sets, are imposed on older sets with- out their visible offset along the older veins (see Figure 3.6~. Such nonoffsetting fracturing would be likely to develop only in a rock mass in which the mechanical competence is maintained. From such characteristics and interpretation, Titley et al. (1986) proposed that the altera- tion of time-specific joint sets results in annealing of the rock and restoration of mechanical homogeneity. SYNTHESIS High pore pressures in the environment of emplace- ment of shallow plutons are a result of localized sources of heat acting on pore water. A consequence of this phe- nomenon is rock failure when its tensile strength is ex- ceeded by the pressures of contained water. Emplacement of small felsic plutons into shallow por- tions of the crust is a rapid process in active tectonic regimes where plutons appear to have followed the paths of earlier magmas of volcanic systems. The rapid em- placement results in marked thermal contrasts between 61 cool, shallow crust and magmas; this contrast results in episodic fracturing of porphyry and its wallrocks as cracks form and become annealed by alteration, and magma cooling retreats to progressively greater depths. Refer- ence volumes of wallrock in;close proximity to the por- phyries undergo heating and then cooling; in the process rocks fail by a process resulting in widespread jointing and, at shallow depths, small bodies of breccia (trans- ported rock fragments). Joints evolve in abundance near the centers of thermal-mechanical energy and diminish in number outward. Development of joints results in instantaneously im- posed permeability characterized by a network of fractures and fluid flow that ultimately results in cooling of the thermal center and synchronous alteration of fracture walls. Evidence of episodic breaking of rocks exists in the super- position of temporally and mineralogically distinct altera- tion assemblages in their own fracture sets in the same rock volumes; textural evidence permits interpretation that original joint walls continuously separate. Concomitantly, open space is apparently filled, inhibiting transport of blocks but maintaining minimal flow porosity from thermal con- traction until the joint space is completely filled by altera- tion products and deposition of hydrothermal minerals, throttling fluid flow. As the joint space becomes restricted, so does fluid flow, further resulting in episodic increases in pressure above deeper but still cooling magma. This process of hydrothermal flow and reaction brings about constriction of gradually widening vein apertures phenom- ena necessary to the process of intermittent but continuing rock failure. ACKNOWLEDGMENTS This review has been improved by suggestions from D. L. Norton, R. V. Kirkham, and an anonymous reviewer. Some of the research reported here was based on work supported by the National Science Foundation under grant EAR 78-22897. Figures 3.2, 3.4, 3.5A, 3.6, 3.7, and 3.8 are reproduced here by permission, from Advances in Geology of the Porphyry Copper Deposits: Southwestern North America, edited by S. R. Titley, University of Ari- zona Press, Tucson, 1982. REFERENCES Asami, N., and R. M. Britten (1980~. The porphyry copper deposits at the Frieda River Prospect, Papua New Guinea, Society of Mineralogists and Geologists of Japan; Geological Special Issue 8, 1 17-139. Banks, N. G., H. R. Cornwall, M. L. Silberman, S. C. Creasey, and R. G. Marvin (1972~. Chronology of intrusion and ore deposition at Ray, Arizona, Part I, K-Ar ages, Economic Geology 67, 864-878.
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