► DEPOSITIONAL TEXTURES

Textures vary among ore deposits, depending upon the nature of the mineralizing fluids, the physical and chemical characteristics of the host rocks, and the mode of emplacement. Textural interpretation can assist greatly in determining the time relationships of successive mineral assemblages in a rock, the overall environment of formation, and the manner of deposition. In epigenetic deposits, textures can help define the sequence and nature of events as in the complex vein shown in Figure 3-11, and they permit the distinction of mechanical and chemical controls as mentioned earlier in this chapter. In syngenetic deposits—such as sedimentary, volcanic-hosted, and mafic igneous ones—textures may reveal accumulation rates and styles, cooling histories, and the nature of precipitation. Resorption of early formed crystals may produce peculiar textures in magmatic segregation ores (Figures 4-2, 9-3, and 9-4), and diagenetic changes may modify the primary

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textures of sedimentary deposits. The textures of hydrothermal ores tend to be diverse.

Replacement

As defined by Lindgren (1S33), replacement, or metasomatism, is “. . . the process of practically simultaneous capillary solution and deposition by which a new mineral of partly or wholly differing chemical composition may grow in the body of an old mineral or mineral aggregate.” Re-placement ordinarily implies little or no change in the volume of the replaced rock, although in some rocks considerable shrinkage or expansion takes place. The process of metasomatism is of great significance in the emplacement of epigenetic ore deposits; many ores are deposited almost entirely in this manner, and nearly all ores show some evidence of replacement of preexisting materials. The process is especially characteristic of deposits formed at high temperatures and high pressures where open spaces are scarce and where communication with the surface is impeded.

The efficacy of replacement is often astounding. The intimate preservation of plant cell or growth-ring morphology in petrified wood is well known, as is the fact that wood—a fibrous substance composed of carbon, hydrogen, oxygen, nitrogen, and minor elements—can be replaced by silica, even though there is no apparent similarity between the two substances. The replacement of one mineral or rock by another may be striking and clear; fossils, sedimentary textures, and folded structures are commonly preserved in faithful molecule-by-molecule detail. A compilation of the minerals that can replace one another indicates that there is practically no limit to the direction of metasomatism. As a bold generalization, it might be stated that given the proper conditions, any mineral can replace any other mineral, though natural processes usually result in favored, commonly recurring reactions. The important factor seems to be the chemical difference between the mineral or rock being replaced and the replacement medium, be it liquid, gas, or a wave of diffusing ions. Hence merely because quartz is stable at the Earth’s surface, we cannot conclude that quartz will resist metasomatism. In fact, quartz and the silicates very commonly undergo replacement. In contrast, a fluid that reacts with and replaces limestone may be inert to quartz, or vice versa. As a result, selective replacement may be of the most detailed character. Bastin and his colleagues (1931) proposed the following general rules for replacement: (1) sulfides, arsenides, tellundes, and sulfosalts can replace any rock, gangue, or ore mineral, (2) gangue minerals replace rock and other gangue minerals, but do not commonly replace sulfides, arsenides, tellurides, and sulfosalts, (3) oxides replace all rock and gangue minerals but are rarely replaced by gangue minerals, and (4) oxides rarely replace sulfides, arsenides, tellurides, and sulfosalts. These statements reflect the “normal course” of ore deposition,

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whereby fluids carrying ore components typically invade and replace rocks dominated by silicates. Statements (2) and (4) are correct because the gan-gue and oxide minerals referred to are part of the ore deposition event and are commingled with it rather than replacing it.

Although replacement has been generally recognized and described for many years, the means by which the actual transfer of materials takes place has been the subject of debate. One fundamental question is how the tremendous volumes of the “old” replaced materials are removed by the same solution that deposits the “new” minerals. Presumably the dissolved materials are transported back from a replacement front through the aqueous ore solutions by diffusion, as in wall rock alteration. In other words, different ions may be diffusing down chemical gradients in quite different directions. The fact that replacement generally appears to take place volume for volume (without increasing porosity) raises another major problem: How can we write chemical equations representing electrically and molec-ularly balanced reactions with equal volumes of solid materials on each side? Ridge (1949, 1961) attempted to illustrate replacement with equations that balanced molecularly, volumetrically, and electrically. He found that he could write equations acknowledging near-surface chemistry and geology but could develop only rough approximations for deep-seated reactions.

In fact, the “budget” approach to metasomatism requires that atomic structure, space, and valence all be conserved, but there is no real evidence that all of those requirements must be met. Metasomatism can be molecule for molecule, but it may also involve one structure growing while another decays, with components being added and subtracted diffusionally from outside as required by the relative rates of destruction and construction. In other words, there may be no molecular-level structural inheritance at all. Barnes (1979) describes the replacement of limestone by sphalerite in a brine medium as involving etching and dissolution of calcite, especially along grain boundaries, merely with the concomitant precipitation of sphalerite and iron sulfides in the space thus created. The reactions were probably

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with the H+ generated in Equations (4-6) and (4-7) aiding in the dissolution expressed by Equation (4-5). Ridge (1961) showed the same reactions summarized as

CaC03 + Zn+2 + S= -* ZnS + Ca+2 + (C03) = (4-8)

and was able to balance it to within 0.01 vol % as

20CaCOs + 31Zn+2 + 31S= 31ZnS + 20Ca+2 + 20(CO3) = (4-9)

Volume = 1220.6 cubic angstroms —* 1220.47 cubic angstroms

Such careful treatment shows that volume for volume space can be conserved.

Limestone replacement at low temperature and pressure was studied by Garrels and Dreyer (1952). They were able to produce replacement textures similar to natural ones under controlled laboratory conditions. Many variables were examined, and it was concluded" that the major control of replacement was the pH of the mineralizing fluid, which in turn controlled the solubility of the carbonate host rock. Garrels and Dreyer suggested that dissolving limestone changes ore-bearing solutions in the direction of ore-mineral precipitation. Accordingly, they consider the solubility of the host to be the key factor in metasomatism. Ames (1961) proposed that the principal factor is the solubility of the replacement product relative to the solubility of the host rock in the same solution, rather than merely the solubility of the host. In any case, the importance of simultaneous dissolving action and precipitation from a single solution is well established.

Garrels and Dreyer also found that numerous small, closely spaced openings and a slightly higher secondary permeability create ideal conditions for replacement. They concluded that the mineralizing solutions are carried along these zones of secondary permeability, but that the solutions move from the channels to the replacement front mainly by diffusion rather than by mechanical flow. It seems clear that replacement is far more important as a mechanism at higher temperatures, and that high pressure inhibits replacement by closure of permeability.

Interpretations of mineral textures tc give unique and positive information are difficult. In spite of the great amount of work done on mineral textures and structures, the causes of many textural relationships are poorly understood. Although many textures are clear and unequivocal enough for positive statements, others may have several interpretations. For example, textures such as the one shown as Figure 3-5 have been attributed to the process of exsolution (Schwartz, 1931), replacement (Loughlin and Kosch-mann, 1942), eutectic crystallization, or, if the two minerals are chalcopyrite and bornite, the removal of part of the iron from chalcopyrite by hot water or steam (Park, 1931). Brett (1964) found that many exsolution textures were deceptive because they look as if they were formed by replacement.

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He was able synthetically to produce replacement-type textures by exsolution, and vice versa. Barton and Skinner (1967) showed that textural equilibration times for many sulfide pairs were so short that homogenization of minerals—or at least modification of primary depositional textures in most sulfide ore deposits—is inescapable. However, ores in most types of ore deposits do show repetitive, characteristic, and reasonably interpretable textures. Most microscopists and mappers place reliance upon a broad spectrum of textural interrelationships, all the while recognizing the need for cautious interpretation of some of them.

Excellent treatises on the textures of ore minerals are available and should be studied by serious students of ore deposits (Van der Veen, 1925; Schneiderhohn and Ramdohr, 1931; Schouten, 1934; Bastin 1950; Edwards, 1952, 1954; Ramdohr, 1955, 1980; Craig and Vaughan, 1981; and Picot and Johan, 1982). Textures may be studied megascopically in mine openings or in the field, in place or in hand specimens in the laboratory; they can be evaluated mesoscopically in rough, sawed, or polished pieces with binocular microscope or hand lens; or they can be examined microscopically with finely polished specimens in reflected light.

As established earlier, ores may be deposited by either open-space filling or replacement. Ores deposited in openings probably followed dominantly textural or structural controls; those that replaced preexisting rocks were probably guided structurally and then controlled chemically. Accordingly it is of fundamental importance for the understanding, evaluation, and development of an ore deposit to ascertain whether the minerals originated by replacement or by open-space filling. It should be recalled that a deposit formed by either mechanism alone would be an exception—open-space filling is likely to be accompanied by some replacement, and vice versa. These two major types of textures—replacement and open-space filling—are discussed separately below, but the student must be aware of their interaction.

Replacement Textures

A great deal has been written about textural criteria for recognizing replacement. Pseudomorphs and relict textures are considered diagnostic, but most others are only suggestive and may be formed in other ways. Except for a few diagnostic criteria, it is unwise to base conclusions upon single indications; confidence in any determination is directly proportional to the number of criteria available. A list of 19 of the more reliable criteria is given below. Although the scale of the accompanying illustrations is microscopic, most relationships can also be used megascopically in the field ( Bastin et al., 1931; Schouten, 1934).

1. Pseudomorphs (Figure 4-6). If the form of a preexisting mineral is preserved, especially if the internal structure is also discernible, a replacement origin is indisputable. The preservation of original structures

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(a) Carbonate

(.b)

Figure 4-6. (ft) Pseudomorphism. Bementite (crosshatched) has replaced most of a rhomb of preexisting carbonate. Bementite, a manganese silicate, is monoclinic and could not form a rhombohedron by itself. Olympic Peninsula, Washington. 60 x. (6) Fluorite ha£ replaced a fossil (circled) in Doctores Formation limestone at the Las Cuevas fluorite deposit, Mexico. The original form of the fossil is obliterated where replaced by fluorite. Replacement is irrefutable since no CaF 2

shells are known. (From Ruiz et ah, 1980.)

and textures in sedimentary, igneous, or metamorphic rocks, as well as organic remains, may be pseudomorphic. For example, preore fold structures or oolites now pseudomorphed by sulfides are conclusive evidence of replacement.

2. Widening of a fracture filling to an irregular mass where a fracture crosses a chemically reactive mineral grain or rock (Figure 4-7). If a

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Figure 4-7. Widening of a fracture filling to an irregular mass where the fracture crosses chemically reactive mineral grains or rock layers. Here digenite in the fracture that cuts eovellite (cv) and chalcopyrite (cp) has expanded into the eovellite and left the chalcopyrite almost unreplaced. Replacement of the eovellite is indicated. Cananea, Mexico. 40 x. See also Figure 4-30.

veinlet widens across only one variety of mineral, it suggests that this mineral was receptive to replacement and that replacement occurred. Mineralization spreading laterally into a single limestone unit in a sequence of varied sedimentary layers is a large-scale example.

3. Irregular or vermicular intergrowths at wide places along fractures or at grain boundaries not related to crystallographic directions (Figure 4-8). The vermicular intergrowths represent a leading edge or front of replacement, not completed. Replacement is not the only mechanism by wdiich vermicular intergrowths are formed; they also develop during crystal growth in a eutectic mixture and by exsolution during the slow cooling of some solid solutions, and they are typically related to crystallographic directions. Only the nonoriented irregular intergrowths pictured can be considered criteria of replacement.

4. Islands of unreplaced host mineral or wall rock (Figure 4-9). Isolated, seemingly suspended relict bits of one material in another, especially if

Figure 4-8. Vermicular irregular intergrowths at wide places along fractures or at grain boundaries; not related to crystallographic directions. Skutterudite (sk) and niccolite (ni) have replaced native silver (Ag). The vermicular diffuse intergrowths of skutterudite extending irregularly into the silver are evidence of replacement. The niccolite is probably an intermediate reaction product. The entire field once was presumably all silver. Cobalt, Ontario, Canada. 30 x. (After an unpublished photo by D. E. Eberlein.)

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Figure 4-9. (a) Oriented islands of one mineral in another. In this texture, pyrite relict fragments form an atoll structure, a common texture in Cu-Fe-S vein ores. An early pyrite cube, the outline of which is preserved, has been almost completely replaced by bomite. Chalcopyrite is an intermediate reaction product. Bisbee, Arizona. 40 x. (6) Photo-micrographic example of oriented relict islands of a replaced mineral. Clusters of pyrite relicts (white) in chalcopyrite (light gray, within pyrite) and bomite (medium gray, external to pyrite) clearly reflect earlier grain forms. Dark gray is quartz; black is holes in polished surface. Butte, Montana. (Photo by Geological Research Laboratory, Anaconda Company.)

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an earlier form or common orientation is perceptible, require a replacement relationship, one that did not carry to completion.

5. Concave surfaces into the host, the “cusp and caries” texture (Figure 4-10). The diffusion of ions at the replacement front goes on at different rates, so that some parts of the front form concave reentrants as if the replacing mineral had bitten into the host. The cusp and caries designation is a dental analogy—caries are cavities in teeth, clearly

Figure 4-10. (a) Concave surfaces of one mineral into another, the cusp and carie texture. Chalcopyrite (cp) has replaced tetrahedrite (tt). By analogy with dental effects, the “bites” are caries, the points between them cusps. Coeur d’Alene, Idaho.

45 x. (b) Excellent example of cusp and carie textures. Hessite (h) (AgTe2) is replacing altaite (a) (PbTe). Ben Butler mi»e, Red Cliff, Colorado.

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later than the enamel; the cusps are relict protuberances between cavities. The texture is a familiar one to microscopists.

6. Nonmatching walls or borders of a fracture (Figure 4-11). If replacement works outward from a central fissure, the opposite fronts of replacement will almost never match in detail and may differ radically.

7. Rims of one mineral penetrating another along its crystallographic directions (Figure 4-12). Replacement may work outward from any small fissures or from grain margins and boundaries, but has advanced preferentially along cleavages. For example, galena may be replaced by covellite or cerussite that penetrate it preferentially along directions that are obviously parallel to its cleavages (see also Figure 3-3).

8. Oriented unsupported fragments (Figure 4-13). If a piece of one mineral is completely surrounded by another mineral and still maintains its

Figure 4-11. Nonmatching walls or borders of a fracture. The original fracture extended between the arrows. Introduced galena (gn), tetrahedrite (tt), and ehalcopyrite (cp) have replaced sphalerite (si). Note the caries of those minerals—the metasomes—

Arrows show the veinlet centerline. Cananea, Mexico. 30 x. See also Figure 4-10.

Figure 4-12. Rims of one mineral penetrating another along crystallographic directions. Here the metasome bornite (bn) protrudes from fairly continuous rims aiong preferred directions in the ehalcopyrite (cp) host. Replacement along the fracture echoes the preference. The replacement proceeded from the volume now filled by gangue. Cananea, Mexico. 40 x.

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Figure 4-13. Oriented unsupported fragments. If cleavage, anisotropism, or other optical characteristics, trains of inclusions, or any other criteria establish that the “island” was once continuous with the “mainland,” then replacement—here tetrahedrite (tt) by chalcopyrite i,cp)—is indicated. Coeur d’Alene, Idaho. 45 x. See also Figure 4-9.

orientation with respect to more of the same material on the outside, the texture is practically diagnostic of replacement at any scale. The fragment may be any size, and the orientation may be established by crystallographic directions, cleavage, bedding, or foliation.

9. Selective association (Figure 4-14). Since replacement is a chemical process, specific selective associations of pairs or combinations of minerals can be expected. If the chemistry of a system changes, it will more likely reflect a changed ratio of related constituents than a com plete switch to different ones. Chalcopyrite is more likely to replace bornite by virtue of a changed Cu/Fe ratio or a different /(S2) than it is to replace quartz. Thus consanguinity of assemblages is part of the diagnosis of replacement.

10. A. younger mineral that transects older structures (Figure 4-15). The presence of a crystal—a metablast, for example—that interrupts bedding or foliation requires that the structure is older. If the crystal had

Figure 4-14. Selective association. Galena (gn) (PbS) rather than calcite or quartz is invaded by gratonite (gt) (Pb^AsjS^) when the activity of arsenic in the fluids increased. Adapted from a specimen from Darwin, California. 45 x.

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Figure 4-15. Metablasts of pyrite that cut foliation in schist from the Mother Lode district, California. The pyrite cubes grew in the rock but do not bend or displace the foliation. They must, then, have replaced the material that occupied their space. About 0.9 x.

grown by any process other than replacement, it would have displaced or deformed the structure.

11. A young phase deposited in obvious relation to microfractures, cleavage planes, or grain boundaries (Figure 4-16). Ore-forming fluids can be introduced along small fractures. If grains of a young phase grow athwart such fractures and protrude into the walls, replacement is indicated.

12. Disparity in size of one mineral in another. Large crystals in a finegrained groundmass, or vice versa, suggest that the two grew independently or by different processes that might include replacement.

13. A mineral deposited along what was clearly an advancing alteration- reaction front. If deposition took place by open-space filling, ore minerals should stop abruptly at the wall of a fissure. Conversely, replacement is indicated by the gradual enlargement and merging of meta- somes along a replacement front. Such a zone should also be evident

Figure 4-16. Young phase deposited in obvious relation to microfractures, cleavage planes, or grain boundaries. Pyrite (py) lies along an obvious contact between siderite (sid) and galena (gn) showing triangular etch pits. The pyrite must have replaced both galena and siderite. Adapted from a Pribram, Czechoslovakia, specimen. 150 x .

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from the progressive increase in size of metacrysts and from the gradation of replacement from the wall rock into the vein. Figure 3-4 is an excellent example.

14. The presence of a depositional sequence in which minerals become progressively richer in one constituent (Figure 4-17). For example, if polybasite (Ag16Sb2Sn) has been acted upon by more silver-rich solutions, the replacement material should show a progressive enrichment in silver. A crystal or fragment of polybasite may grade first to acan- thite (Ag2S) and then eventually to native silver, reflecting the gradual displacement of antimony and sulfur. An intermediate-stage specimen might show relict cores of polybasite rimmed in sequence by acanthite and native silver.

15. Doubly terminated crystals (Figure 4-18). If a crystal grows within an open cavity, it is normally attached to a wall and develops crystal faces only at the free end. This restriction does not affect crystals growing by replacement, so that doubly terminated crystals may indicate a replacement origin. This criterion is limited because doubly terminated crystals may also develop within magmas and, exceptionally, by unusual attachment in open spaces.

16. Gradational boundaries. Replacement processes may produce either abrupt or gradational contacts between the host rock and the orebody. Since open-cavity filling usually forms abrupt contacts, at least on a microscopic scale, a gradational boundary commonly indicates advancing replacement. (See also Figure 3-4.)

Figure 4-17. Depositional sequence in which minerals become progressively richer in one constituent. The intermediate phase commonly is a reaction rim. Hypogene chalcocite always has a selvage of bomite between itself and the chalcopyrite that it replaces. The sequence of CuFeS2-Cu6FeS4-Cu2S, in which the copper-to-iron ratio is 1-5-00 and the —eial-to-sulfur ratio 1-1.5-2. Adapted from a Tsumeb, Namibia, specimen. 45 x. See also Figure 4-8.

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Figure 4-18. Doubly terminated quartz crystal from the Stibnite Vein, Wolf Creek, Fairbanks district, Alaska, containing stibnite crystals. The quartz is said to be a replacement of gangue minerals in the vein, and probably overgrew and enclosed the stibnite granules. 18x. (Collected by P. 0. Sandvik; photo by W. J. Crook.)

17. Residual resistant minerals. Some minerals are refractory to most mineralizing solutions and may be preserved even after the surrounding minerals have been replaced. For example, zircon or corundum found in the same proportions in the sulfides of an orebody as in a nearby schist would support the argument that the ore had replaced some of the schist. The resistant minerals are special types of islands or unreplaced fragments of host rock (criterion 4).

18. No offset of an intersected linear fracture (Figure 4-19). When a vein is opened laterally, any intersecting feature is also translated. But if a linear or planar feature projects straight across a vein filling, there can have been no dilation, and filling must be the result of replacement.

19. No offset along the intersection of fractures (Figure 4-20). Movement along a fissure offsets any planar or linear feature that it intersects obliquely, but does not commonly offset an intersecting fracture formed

Figure 4-19. No offset of an intersected linear feature. A veinlet of native silver (Ag) flanked by skutterudite (sk) cuts across a straight joint (jt) in calcite (cal). Were the silver and skutterudite open-space fillings in a dilatant opening, the joint would have been spread from A to B and continue from B to C. Replacement, or fortuitous movement, is required. Cobalt, Ontario. 30 x. See also Figure 4-26.

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Figure 4-20. Intersecting fractures showing no offset at the intersection. The fractures were probably part of a cognate joint set and show enlargement by replacement rather than by dilational opening. Tetrahedrite (tt), chalcopyrite (cp), and siderite (sid). Coeur d’Alene, Idaho. 30 x.

at the same time. Both fractures may be enlarged by replacement of their walls and tend to cross one another without any change in course, but only a coincidence of displacement would permit a rematch of intersecting veins that had been spread apart and open-space-filled.

Exsolution

A “near neighbor” to replacement is the process of exsolution. In both phenomena, migration of one or more components, commonly involving diffusion, results in the supplanting of a preexisting phase by a younger one. Exsolution textures and their evaluation are normally only seen by the mineragrapher (the ore microscopist). It is true that exsolution is not strictly speaking a depositional feature, and perhaps should not be included in this chapter. But there are enough situations in which exsolution textures and distributions can be mistaken for those of replacement that their description is justified.

Reexamine Figure 3-5. Were the process of exsolution to be carried further, the miniscule blebs of chalcopyrite would migrate to cleavages and crystallographic planes and collect there, leaving impoverished zones behind. That process can be seen to have begun especially on the right side of Figure 3-5. Taken even further, the chalcopyrite—now essentially all on planes—might resemble a phenomenon of chalcopyrite from outside invading and replacing the sphalerite. Taken to completion, the lowest free- energy state is for the exsolved phase to form separate grains occupying texturally equivalent positions in a mosaic with the original host phase. For many years, veinlets and grains of pentlandite t(Fe,Ni)9S8] in pyrrhotite (Fe^a-S) in the Sudbury Complex (Chapter 9) were perplexing. Some considered them to be of replacement origin, with pentlandite invading from “outside” the pyrrhotite. Others thought them to be the product of well- developed exsolution. A careful textural study of scores of polished pieces of ore sulfides has verified the latter explanation. It matters because it

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means that pentlandite, a principal contributor of nickel to the ores, can be expected virtually wherever pyrrhotite is found, and pyrrhotite occurs extensively along the floor of the complex. Were the pentlandite not integral to the pyrrhotite, its distribution would almost certainly be more erratically subject to unknown external controls. In general, replacement veinlets widen at intersections, and exsolution veinlets become narrower. In the former case, replacement attacks corners formed by two veinlets from two sides, replacement is more complete, and the zone widens. In the latter, the host contributes the exsolved phase into the veinlet. Close to a comer the exsolved ions are migrating both ways, and the intersection becomes starved. Indeed, some microscopic exsolution patterns involve exsolved crosses the arms of which narrow to a pinpoint at the intersection.

Excellent lists of criteria for distinguishing exsolution and replacement are available in Edwards (1965), Ramdohr (1980), and Craig and Vaughan (1981), but need not be reviewed here.

Open-Space Filling

Open-space filling is common in shallow zones where brittle rocks yield by breaking rather than by plastic flow. The openings in these zones tend to remain open because of low pressure transmitted through surrounding rock. At shallow depths, ore-bearing fluids have relatively free circulation, and their open connection with the surface permits deposition to be brought about by abrupt pressure and temperature changes—including retrograde boiling—as opposed to prolonged contact with surrounding rocks which undergo slow chemical changes in deeper environments. Although ores de-posited in vugs and open cavities are generally readily distinguishable from replacement ores, criteria associated with open-space deposition must nevertheless be used with caution because they are sometimes inconclusive (Kutina and Sedlackova, 1961). As stated earlier, it should also be stressed that many ore types show evidences of both replacement and open-space filling and thus an overlap of the two processes. A list of common criteria by which open-space filling may be recognized is given below.

Open-Space Filling Textures

1. Many vugs and cavities (Figure 4-21). If veins, breccias, and other partly-filled openings contain many vugs and cavities that can be interpreted as spaces left by incomplete filling of larger open spaces, then open-space filling is indicated. The inward growth of ore and gangue minerals in an open fissure stops when opposite walls meet. Since growth is not uniform along a fissure, the progressively impeded circulation of ore-bearing solutions normally leaves unfilled pockets. These crystal- lined centerline openings are called vugs.

2. Fine-grained minerals on the walls of a cavity with coarser minerals

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Figure 4-21. Quartz-crystal-lined vugs left by incomplete vein filling. Locality unknown, but typical of scores of epithermal veins around the world. 0.9 x. See also Figures 4-22 and 4-23.

in the center (Figure 4-22). The first crystals that form along the sides of an open vein are usually fine-grained, probably because of heat loss to the wall rocks and consequent rapid crystallization. Conversely, crystals that form in the centers of such veins are inclined to be coarse and probably form from more dilute, cooler solutions late in the vein history. Whatever the reasons, the phenomenon is common enough to be distinctive of open-space filling.

3. Crustification (Figure 4-23). As ore-bearing solutions change in composition, or as environmental factors change, different minerals are deposited along the walls of a vein or cavity. Early formed crystals become encrusted with later minerals. The presence of small euhedral dolomite crystals on top of large euhedral fluorite crystals is a common example. Some veins have a banded appearance owing to crustification.

4. Comb structure (Figure 4-23). Along the junction of crystals that have grown from opposite walls of a fissure, there is generally an interdigi- tated vuggy zone due to the merging of euhedral prismatic crystals. Because this jagged zone of juncture resembles the outline of a rooster’s comb, it is known as comb structure.

5. Symmetrical banding (Figure 4-24). Crystals deposited along a cavity or fissure ordinarily grow symmetrically toward the center, in which case the orientations, morphologies, and compositions of crystals on opposite sides of a vein are mirror images. As the ore fluid or environ -ment changes, minerals deposited will differ in composition, forming crustification in a bilaterally symmetrical pattern inward from vein walls. Such symmetrical banding may involve mineralogical and color changes, and can produce a banded vein diagnostic of open-space filling.

E. Matching walls. If an open fissure has been filled without replace-

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Figure 4-22. Fine-grained quartz grading into coarser crystals in a vug. Locality unknown. See also Figure 4-23.

ment, the outlines of opposite walls should match, that is, if the vein material were removed, the unreacted wall rocks on opposite sides should fit together like mated pieces of a jigsaw puzzle. The fit across many veins is readily apparent.

7. Cockade structure (Figure 4-25). Mineralization within the open spaces of a breccia, or any other fragmental rock, commonly produces a special pattern of symmetrical banding or erustincation known as cockade structure. Each opening is a receptacle for sequential deposition. The overall rock pattern is one of host-rock fragments coated with layers of inward-radiating crystals, with triangular patterns predominating.

8. Offset oblique structures (Figure 4-26). If a planar or linear feature is cut obliquely by an open fissure, it is offset at right angles to the vein walls because it was spread apart as the fissure opened. Replacement along a fracture would cause no offset along such a preexisting structure; it would project straight across. (See also Figure 4-19.)

9. Colloform structures. Colloform banding, composed of fine onionskinlike successively deposited layers (Figures 4-27 and 4-28 and next section), can only form in open space.

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Figure 4-23. Crustification, comb structure, and centerline vugs, (a) Veinlet of aragonite through limestone and shale. Locality unknown. (b) Vein from latite porphyry (top and bottom) through amethyst (medium gray) to centerline inward-facing chalcedonic quartz. Commonwealth mine, Pearce, Arizona. The bar scale is in centimeters. (Photo of b by W. K. Bilodeau.)

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Figure 4-24. Sketch of part of the reopened, symmetrically banded complex Matkobozska Vein, Pribram, Czechoslovakia. An early opening (A) was filled with “ore”—an intergrowth of chalcedony and siderite with Ag-Pb-Zn suifosalts— then quartz. A second opening (£>) was filled with minor sphalerite, then calcite, siderite, and finally ankerite. See also Figure 3-11. (From Kutina, 1957.)

Figure 4-25. Cockade structure of quartz infilling the cavities in a breccia. Locality unknown, but a typical epithermal structure.

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Figure 4-26. (a) Offset of an oblique structure. A and B were adjacent before the vein opened; the two segments no longer lie in a single plane, and the intervening material is open-space filling. Lateral wall movement can of course complicate relationships. Compare with Figure 4-19. (6) Specimen from Silver Bell, Arizona, in which the white quartz vein is 1 cm wide. (Photo by W. K. Bilodeau.)

Colloidal-Colloform Textures

Amorphous minerals, such as opal, neotocite, “copper pitch,” “wood tin,” and some garnierite, used to be thought to have been deposited from colloidal suspensions. Moreover, it was believed that many cryptocrystal- line minerals—chalcedony, some manganese oxides, pyrite, marcasite, and pitchblende, and the oxidation products of copper, lead, and zinc, such as malachite, azurite, chrysocolla, anglesite, cerussite, and smithsonite—were carried and deposited as colloids that aggregated and crystallized shortly after deposition. Some geologists think that gold, especially in shallow low- temperature deposits, travels short distances in the colloidal state. Most geochemists now consider the existence of colloids in hydrothermal systems highly unlikely.

A colloidal system consists of two phases, of which one, the dispersed phase, is diffused in the other, the dispersion medium.. Colloidal particles range in size between ions in true solution and particles in coarse suspension, the general limits being defined at 10-7 and 10-3 cm. The colloidal material may be solid, liquid, or gas, and may be dispersed in a medium of any of these same states. In the study of ore transport, however, we are concerned essentially with solids dispersed in liquid media. A colloidal system consisting of solid particles dispersed in a liquid is called a sol. A given kind of colloidal particle may adsorb cations and behave as a positively charged body, or it may adsorb anions and become negatively charged. Since the particles of a sol all have the same charge, they repel each other and prevent coagulation. If an electrolyte is added to the sol, the colloidal particles become neutralized and flocculate, or deposit. The result can be a

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banded or finely layered, typically botryoidal or reniform aggregate of the type that characterizes African malachite, for example, and is common in some sphalerite from low-temperature deposits (Figure 4-27). However, we saw in Chapters 2 and 3 that most hydrothermal fluids are indeed brines, or at least saline enough to be the electrolyte that is anathema to colloids and colloidal transport. We now perceive that at most temperatures and in most hydrothermal fluids, at least, colloidal transport and deposition cannot be important. At low temperature and in nonsaline groundwater, colloids can and do persist.

As a result of his study of colloform sphalerite specimens from many of the world’s major geological districts, Roedder (1968) decided that most if not all of these ores grew directly as minute druses of continuously eu-hedral crystals that projected into an ore fluid that was a true solution, not a colloidal dispersion. He found that all of the textural features believed to be diagnostic of colloidal deposition were ambiguous, inapplicable, and therefore invalid as criteria for the samples he studied, and perhaps of most other colloform mineral samples as well. However, Roedder did not explain the fact that such colloform minerals as the manganese oxides, garnierite, and opal at times do not reveal any structural pattern even upon X-ray study; they are truly noncrystalline.

In a thoughtful discussion of Roedder’s paper, Haranczyk (1969) concluded from his extensive studies in the Silesian-Cracovian lead and zinc deposits of Poland that colloform textures might form from both true solutions and sols. He proposed the term hemicolloids for solutions that are intermediate between true and colloidal ones. The cryptocrystalline, white, claylike form of sphalerite known as brunciie (Figures 20-12 and 20-13) was almost certainly deposited as a colloid. Haranczyk later (1971) clearly showed that some of the second-generation minerals in the Silesian-Cracovian ore deposits of Poland could be of colloidal origin. He states that most of the second-generation sulfides show hemicolloidal textures. This conclusion clashes

Figure 4-27. Colloform texture. Globular sphalerite with concentric banding, considered to be of colloidal origin. Orzel Bialy mine, Katowice, Poland. Natural size. Also see Figures 20-9 and 20-10. (From, Kutina, 1957.)

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with the modern concept that these deposits were formed from warm electrolyte brines; clearly, we need more experimentation and morphological data.

The terms colloidal texture and colloform texture should be used carefully. The former should be used only where deposition from a true colloidal system is demonstrable; colloform suggests a texture that resembles, and could be, colloidal in origin, but cannot be determined to be so. A truly colloidal origin can be postulated if the following are observed: (1) infinitesimal grain sizes in banded textures as in some malachite (Figure 4-28) and most wad, (2) shrinkage or dehydration cracks, as in opal and some limonites, (3) diffusion bands, or Liesegang rings (Figure 4-29), (4) chaotic, amorphous noncrystalline structure of component compounds, (5) the presence of unpredicted, possibly originally adsorbed ions, such as barium in psilomelane, and (6) continuous spherulitic textures demonstrably controlled by surface tension. Interrupted bands normally deny the role of

Figure 4-28. Colloform banding in malachite from Kolwezi, Zaire. Some of the bands can be traced through the whole specimen, but some lobes interfere with others, perhaps as stalagmite-stalactite masses were filled in. Deposition can have been either truly colloidal or as banded microcrystalline accumulations. The bar scale is in centimeters. (Photo by W. K. Bilodeau.)

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Figure 4-29. Probable diffusion banding in an agate, quite possibly developed while the silica was a colloidal “silica gel.” Note also the cockade pattern of the later quartz filling in the central vug. Locality unknown. 1.7 x. (Photo by W. J. Crook.)

colloids (Figures 20-9 and 20-10). Clearly, all colloform textures are open-space fillings; they may or may not have involved colloids.

Even though the part played by colloids in ore deposition may be trivial, it seems likely that under near-surface conditions and in dilute watery solutions at comparatively low temperatures the role may be appreciable. Many deposits have been described for which a colloidal origin is probable or in which colloids have played a supporting role.

An unusual deposit of colloform magnetite was found in an igneous metamorphic environment at the northern end of Vancouver Island, Canada (Stevenson and Jeffery, 1964). The colloform magnetite possessed the textural features usually ascribed to colloidal deposition, and the deposits ap-

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peaned to have been formed by a type of gel metasomatism of the limestone. The iron, carried in acid solution, replaced limestone and appears to have been precipitated colloidally during an intermediate state of aggregation between ionic solution and the final precipitate.

Hosking (1964) pointed out the presence of opaline gels that are apparently forming in the deposits of Cornwall, England (Chapter 6). He described the tin lodes of St. Agnes where almost perfect concentrically zoned spherules of “wood tin”—ultrafine-grained colloform cassiterite— are suspended in quartz. These spherules are covered at places with acicular crystals of cassiterite whose long axes are normal to the spherule surfaces. He considers these spherules to have developed as the result of centripetal migration of tin-bearing fluids through a silica gel.

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