Quartz Epithermal Textures - Dong Et Al 1995 (Econ Geol. p1841-p1856)

Economic Geology Vol 90, 1995, pp. 1841-1856

QUARTZ TEXTURES IN EPITHERMAL VEINS, QUEENSLAND--CLASSIFICATION, ORIGIN, AND IMPLICATION

GuoYI DONG, GREGG MORRISON*, AND SUBHASH JAIRETH** Department of Earth Sciences, James Cook University of North Queensland, Townsville 4811, Australia

Introduction

In hydrothermal veins, quartz is a dominant gangue mineral and is typically the only phase deposited throughout the life of the hydrothermal system. Therefore, the characteristics of quartz--its morphology, crystal structure, chemical composition, and physicochemical properties--might reflect dif- fering hydrothermal conditions during vein growth, including those which favor gold mineralization.

Many modern techniques are being used to characterize vein quartz and to distinguish mineralized quartz from barren quartz, e.g., fluid inclusions (Roedder, 1984; Sherlock etal., 1993), oxygen isotopes (Rye and Rye, 1974; Matsuhisa et al., 1985), electron paramagnetic resonance (Van Moort and Russell, 1987), cathodoluminescence (Nickel, 1978), thermoluminescence (Sankaran etal., 1983; Hochman etal., 1984), trace elements analyses (Anufriyev etal., 1973), mass spec- trometry of the thermally released gas (Barker and Robinson, 1984), infrared (Wu and Yu, 1987), and microstructural features (Stenina etal., 1989). In general, all these techniques have met with mixed success, and a few of the distinguishing features for mineralized and barren quartz have been defined. However, high cost, difficulty with interpreting data, and limi- tations in experimental equipment inhibit most of these techniques as effective and economical exploration tools.

There is a fundamental way' to characterize vein quartz, i.e., the morphology of quartz and its aggregates. Adams (1920) was the first to propose a detailed description of the common microscopic characteristics of vein quartz. His paper has been the most valuable base for subsequent studies on textures of vein quartz. The work of Spurr (1926), Shaub (1934), Stillwell (1950), Lovering (1972), Boyle (1979), Sander and Black (1988), and Saunders (1990), among others, also considered the character of quartz in specific environments. Recently, Dowling and Morrison (1990) undertook an investigation of quartz textures in various types of hydrothermal veins in north Queensland and developed a general quartz textural classification. Eleven textures were defined and grouped to evaluate four gold-mineralizing environments (i.e., epithermal, por- phyry, plutohie, and slate belt), each with a distinct quartz textural assemblage associated with gold mineralization.

In the light of this general success, more detailed work on quartz textures in epithermal veins, where there is a wide range of quartz textures, was carried out in the present study. The main themes of this paper are to develop a unified classification of common quartz textures in epithermal veins, to understand their possible origins in terms of the processes of

* Present address: Klondike Exploration Services, 7 Mary Street, Towns- ville, Queensland 4811, Australia.

** Present address: Mineral Resources Branch, Bureau of Mineral Re- sources Canberra, A.C.T. 2600, Australia.

formation and fluid conditions, and to explore the relationship between quartz textures and gold mineralization on a broad scale. A systematic evaluation of three-dimensional distribution of quartz textures and textural assemblages in selected epithermal systems and thereby the textural zoning model will be presented in another paper.

Most of the spedmens used in this study were collected from the late Paleozoie epithermal veins of north Queensland, Australia. Regional tectonic and metallogenic studies (Mor- rison, 1992a, b; Walshe etal., 1995) suggest most of the veins formed during destruction of a Carboniferous continental magmatic are. They are hosted largely in volcano-sedimentary and volcanic rocks of andesitie to rhyolitie composition. De- tailed work on representative deposits (Digweed, 1991; Tare et al., 1992; Bobis etal., 1995; Worsley, 1995) has defined a late Paleozoie epithermal province in north Queensland with many features similar to the Tertiary province of the western United States (Morrison, 1992a, b). All the deposits included in this study are of the adularia-serieite type in the classification of Heald etal. (1987).

Classification of Quartz Textures

A classification of quartz textures in epithermal veins is developed from a review of the available descriptive literature and observation of approximately 400 spedmens and 150 thin sections from more than 20 epithermal deposits and prospects. Thirteen quartz textures are defined (Fig. 1) on the basis of mutual geometrical relations among individual crystals, or crystal aggregates, and/or the internal features of individual grains. Most of the textures described are readily identified in hand specimens. A few, however, can only be viewed under the microscope. The majority of textural terms used in this classification are adopted from existing terminol- ogy with some modification where necessary.

This study deals only with low quartz (Phillips and Griffen, 1981). Based on the size of individual grains it can be subdi- vided into: (macro)crystalline, microcrystalline, and erypto- crystalline (Bates and Jackson, 1987). Chalcedony refers to eryptoerystalline quartz, either with fibrous or granular habit (Phillips and Griffen, 1981). These terms will be applied in the following description for some quartz textures. Massive

This is a general term that refers to quartz veins which have a more or less homogeneous appearance over wide areas and display an absence of banding, shear fractures, or similar features.

Crustiform

The term erustiform is analogous to erustifieation banding described by Adams (1920), Lindgren (1933), and Shaub (1934). This texture involves successive, narrow (up to a few

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1842 SCIENTIFIC COMMUNICATIONS

centimeters), and subparallel bands which are distinguished by differences in texture, mineral proportions, and/or color. Commonly, banding is symmetrically developed from both walls of a fissure (Fig. %).

Cockade: This is a subtype of erustiform texture, as described previously by Taber in Adams (199.0) and Spurt (199.6). In breeeias, concentric erustiform bands surrounding isolated fragments of wall rocks or early vein materials pro- duee cockade texture.

Colloform

This term was first proposed by Rogers (1917). In general, where the external surface of a mineral or mineral aggregate shows combined spherical, botryoidal, reniform, and mammillary forms, it is called eolloform. For silica minerals, this texture is a characteristic feature of ehaleedonie aggregates in fine rhythmic bands (Fig. 2b). Under the microscope, chalcedony in eolloform banding often has a microfibrous habit with sharp re-entrant angles between adjacent contacting spheroids. Moss

This texture has features similar to the "miero-botryoidal gel structure" described by Adams (1920). In hand specimens, silica aggregates display a heterogeneous turbid appearance, similar to moss vegetation (Fig. 2e). Under the microscope, groups of spheres (usually ranging from 0.1-1 mm in diam) are highlighted by the distribution of impurities within aggregates of silica minerals (Fig. 2d). Some spherical impurities also show an internal concentric or radiating pattern. Moss texture may gradate to eolloform texture if the spheres become interconnected. Comb

Comb texture refers to groups of parallel or subparallel quartz crystals which are oriented perpendicular to vein walls, thus resembling the teeth of a comb (Fig. 2e). Normally crystals display a uniform grain size and have euhedral terminations at their free ends. This texture is frequently described in the literature, including Adams (1920), Schieferdecker (1959), and Boyle (1979). Zonal

Zonal texture displays alternating clear and milky zones within individual quartz crystals (Fig. 2f). Milky zones are usually crowded with fluid or solid inclusions and are always parallel to crystal growth faces. Mosaic

Aggregates of microcrystalline or crystalline quartz crystals have highly irregular and interpenetrating grain boundaries (Fig. 3a). In hand specimens, the sample usual!y has a vitreous and tightly packed appearance. This texture is equivalent to a jigsaw texture which is one of most common microtextures in jasperold (Lovering, 1972) and is also noted in some epithermal deposits (Saunders, 1990).

Feathery Under the microscope with crossed polars, individual quartz

crystals display a splintery or feathery appearance seen only as

slight optical differences in maximum extinction positions. This texture is usually well developed on the margins of quartz crystals with a dear euheclral core (Fig. 3b) or as patches throughout quartz crystals (Fig. 3e). The term "feathery" is adopted from Adams (1920), and a simfiar texture has been reported by Sander and Black (1988), who called it "plumose."

Flamboyant This texture has been described by Adams (1920) and

Sander and Black (1988). The chief characteristic of this texture is the radial or flamboyant extinction of individual quartz crystals with a more or less rounded crystal outline. Similar to the feathery texture, it can either be developed in the rim of a quartz crystal with a dear euhedral core (Fig. 3d), or throughout the crystal (Fig. 3e).

Ghost sphere This texture commonly occurs within microcrystalline

quartz as cloudy spheres highlighted by the distribution of impurities (Fig. 3f). Ghost-sphere texture may be regarded as a special moss texture, because both textures have the same feature--spherical distribution of impurities within silica phases such as amorphous silica, chalcedony, or quartz. However, if the host is quartz, ghost-sphere texture is used to characterize the internal feature of quartz crystals. Ghost- sphere texture may gradate to mosaic texture where the impurities are gradually eliminated and crystal boundaries become interpenetrating. Some quartz crystals with ghost-sphere texture display radial extinction and therefore share the characteristic features of the flamboyant texture. Pseudobladed

Aggregates of quartz or chalcedony may be arranged in a bladed or platy form. Three subtypes are defined on the basis of the morphology of the aggregate of blades.

Lattice bladed: This texture is comparable with the "pseu- domorphic lameliar, platy, or tabular" quartz texture described by Lindgren (1899), Schrader (1912), and Morgan (1925). It displays a network of intersecting silica blades with polyhedral cavities partly filled with comb quartz crystals (Fig. 4a). In thin sections, each blade consists of a series of parallel seams separated by quartz crystals or crystallites which have grown symmetrically about the seams and perpendicular to them (Fig. 4b).

Ghost bladed: Blades are identified on the polished surfaces of hand specimens by concentrations of impurities. Commonly blades are dispersed randomly within quartz aggregates and lack cavities between the blades (Fig. 4e). Under the microscope, the blades are differentiated from the matrix by differences in grain size, shape, and/or outlines of impurities (Fig. 4d). The thick silica blades usually have a ragged shape with a set of parallel partings.

Parallel bladed: Silica blades are parallel within a group but adjacent groups may have different orientations. The outline of groups defines an overall granular pattern in hand specimens (Fig. 4e). The microscopic feature of the parallel texture is essentially similar to that of lattice-bladed texture: each group comprises a set of parallel seams, separated either by rectangular quartz crystals (Fig. 4f), or by prismatic crystals

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Texture Sketch of Texture

Type Grain Grain Internal Feature Morphology

Size Form of Individual of Crystal Crystal Aggregate

References

Primary Growth Textures

Massive

Crustiform

Cockade

Colloform

Moss

Comb

variable anhedral not applicable

variable variable not applicable

variable variable not applicable

free fibrous not applicable anhedral

[me variable not applicable

variable prismatic not applicable

variable prismatic zonal

FIe;. 1. Classification of quartz textures.

homogenous Smimov (1962) Bates &

Jackson (1987)

successive Adams (1920) banding Shaub (1934)

Lindgren (1933) Buchanan (1981)

concentric Adams (1920) banding Spurt (1926)

semi-spherical, Rogers (1917) reniform, Adams (1920) mammillary

spherical Adams (1920)

parallel- Adams (1920) orientated Schieferdecker (1959)

Boyle (1979)

not applicable Smirnov (1962)

and/or crystallites growing perpendicular to the seams. Adams (1920) described a texture called "lameliar quartz" which has features very similar to the parallel-bladed texture.

Pseudoacicular

The pseudoaeieular texture was first described by Lindgren and Bancroft (1914) from the Republic district, Washington, and was also reported by Adams (1920) and Schrader (1923). In hand specimens, aggregates of silica minerals, commonly associated with adularia or its weathering products (serieite or kaolinite), display a radial aeieular appearance (Fig. 5a). Under the microscope, this is indicated by linear arrange- ments of fine-grained, sometimes roughly rectangular, quartz eryst'als and/or by a linear distribution of adularia or its weathering products (Fig. 5b).

Saccharoidal

In this texture, loosely packed vitreous to milky, fine-grained quartz aggregates have the appearance of sugar in hand specimens (Fig. 5e). Under the microscope, abundant elongated subhedral crystals, some with double terminations, are randomly distributed in a matrix of smaller, anhedral grains (Fig. 5d). Locally there is an alignment of elongate crystals giving the impression of a crude mesh or network. This is the "retiform structure" described by Lindgren (1901), Adams (1920), and "retieulated texture" described by Lovering (1972).

Discussion on the textural classification

As discussed in some textbooks (e.g., Stairnov, 1962; Bates and Jackson, 1987), the term "texture" is used for the general


Texture Sketch of Texture

Type Grain Grain Internal Feature Morphology

Size Form of Individual of Crystal Crystal Aggregate

References

Recrystallization Textures

Mosaic • Feathery

Flamboyant

Ghost-sphere

Replacement Textures

Ghost-bladed

P0x'0llel-bladed • Pseudo-acicular

Saccharoidal

free anhedral not applicable interpenea'ating Lovering (1972) Saunders (1990)

variable prismatic plumose not applicable Adams (1920) Sander and

Black (1988)

variable round radial not applicable Adams (1920) Sander and

Black (1988)

f'me anhedml spherical not applicable Adams (1920)

free anhedml not applicable intersecting Lindgren (1899) to prismatic bladed Schrader (1912)

Morgan (1925)

free anhedml not applicable intersecting bladed

fine anhedml not applicable parallel Adams (1920) to rectangular bladed

free anhedral not applicable acicular Lindgren and to reckangular Bancroft (1914)

Adams (1920) Schrader (1923)

free anhedml not applicable interlocking Lindgren (1901) to prismatic Adams(1920)

Loveting(1972)

FIG. 1. (Cont.)

physical appearance or character of a rock, including the size and shape of, and the mutual relations among, its component minerals. Saeeharoidal and mosaic textures typically belong to this category. The term "structure" is generally used for the larger features of a rock and is determined by the spatial ar-

rangement of its mineral aggregates which differ from one another in shape, size, composition, and texture. This is best represented by erustiform structure. However, the two terms are often used interchangeably, and some textures may parallel major structural features. For instance, eolloform and comb


III ,

FIC. 2. Quartz textures I. a. Crustiform: alternating fine bands consisting of pink adularia, microcrystalline quartz, comb quartz, and chlorite, developed from both walls of a fissure. Central extended lode, Cracow, Queensland (department catalog no. 35322). b. Colloform-crustiform: classic examples of colloform (botryoidal) and crustiform (alternating) bands cohsidered characteristic of epithermal veins. McLaughlin, California (35324). c. Moss: silica aggregates display a heterogeneous turbid appearance, similar to moss vegetation. Pajingo, Queensland (35327). d. Moss: groups of spheres highlighted by the distribution of impurities within aggregates of sfiica minerals. Pajingo, Queensland (35327). Plane-polarized light. Scale bar = 0.2 mm. e. Comb: groups of parallel or subparallel quartz crystals oriented perpendicular to vein wall, resembling the teeth of a comb/White Hope lode, Cracow, Queensland (35329). f. Zonal: within individual quartz crystals, there are alternating clear and inclusion crowded zones. Quartz Hill, Queensland (35330). Metric scales = 1 cm.


F[c. 3. Quartz textures II. a. Mosaic: aggregates of microcrystalline quartz crystals with highly irregular and interpenetrating grain boundaries. Pajingo, Queensland (35331). Crossed polars. b. Feathery 1: a feathery appearance in the rims of the crystals with euhedral cores, seen only as slight optical differences in maximum extinction positions. In another position (e.g., bottom center) the quartz crystal displays a very similar interference color between the euhedral core and rims. Pajingo, Queensland (35332). Crossed polars. c. Feathery 2: a feathery appearance seen as patches throughout quartz crystals. Afarti, Queensland (35333). Crossed polars. d. Flamboyant 1: radial or flamboyant extinction of individual quartz crystals with more or less rounded crystal outline. In this sample, the flamboyant texture is well developed in the rims of crystalline quartz crystals with euhedral cores. Central extended lode, Cracow, Queensland (35334). Crossed polars. e. Flamboyant 2: flamboyant extinctions seen throughout the crystals with rounded surface in bands. Pajingo, north Queensland (35336). Crossed polars. f. Ghost sphere: solid and/or fluid inclusion defined spheres within microcrystalline quartz crystals. Central extended lode, Cracow, Queensland (35337). Crossed polars. Scale bars = 0.2 min.


ERIC

F•c. 4, Quartz textures III. a. Lattice bladed: a network of intersecting sfiica blades with polyhedral cavitiesi Bimurra, Queensland (35339)1 b. Lattice bladed: in thin section, each blade consists of a series of parallel seams separated by quartz crystals or crystallites which have grown symmetrically about the seams and perpendicular to them, Bimurra, Queensland (35339)1 Crossed polars. c. Ghost bladed: blades are identified on the polished surface of the hand specimens by the concentration of impurities. This texture commonly occurs in crustiform bands and lacks the cavities between bladesß Woolgar, Queensland (35340). d. Ghost bladed: aggregates of quartz crystals with superimposed bladed texture identified by outlines of impurities and finer grain size. Woolgar, Queensland (35340). Crossed polars. e. Parallel bladed: silica blades are parallel within each group but adjacent groups have different orientations. Bimurra, Queensland (35341). f. Parallel bladed: each group is composed of a set of parallel-oriented quartz crystals which have more or less rectangular shapesß Bimurra, Queensland (35341). Crossed polars. Scale bars = 0.2 mm, metric bars = 1 cm.


Fro. 5. Quartz textures IV. a. Pseudoacicular: aggregates of silica minerals, commonly associated with adularia or its weathered products (kaolinitc or illitc), display a radial acicular appearance, caused by differences in color and/or relief in hand specimens. Pajingo, Queensland (35342). b. Pseudoacicular: acicular appearance is indicated under the microscope by linear arrangement of fine-grained quartz crystals and linear distribution of day minerals. Pajingo, Queensland (35342). Crossed polars. c. Saccharoidal: loosely packed fine-grained quartz aggregate, having a sugary appearance in hand specimens. Rose's Pride lode, Cracow, Queensland (35343). d. Saccharoidal: under the microscope, slender subhedral crystals are randomly distributed in a matrix of smaller, anhedral grains. Locally there is alignment of elongated crystals giving the impression of a crude mesh texture. Rose's Pride lode, Cracow, Queensland (35343). Crossed polars. Scale bars = 0.2 ram, metric bars = I cm.

can be used as both textures and structures. Many people today prefer to group texture and structure together in regard to the general features of a rock or vein (Lovering, 1972; Craig and Vaughan, 1981; Augustithis, 1982; MacKenzie et al., 1982). This ,concept has been applied in the present study.

Since the criteria for this classification are defined by various parameters (such as the morphology of mineral aggregates, the internal feature of an individual crystal), a certain specimen could be described in several textural terms by using different criteria. For example, comb texture describes groups of quartz crystals sharing the same orientation; however, individual crystals in comb texture could also display zonal texture or feathery texture. Crustiform texture refers to the banded arrangement of mineral aggregates which differ from one other in texture and composition, it naturally in- eludes many other textures within each band. The way to deal with this problem is to name all textures observed, so that the characteristic of the sample can be illustrated entirely.

The Possible Origins of Quartz Textures

Interpretation of quartz textures is always a difficult subject, since it requires a substantial knowledge of the solubili- ties of silica minerals, various kinetic processes such as polymerization, coagulation, nucleation, crystallization, dissolution, and 'recrystallization of silica minerals, most of which are still not well understood, particularly in very complex hydrothermal systems. The following discussion attempts to provide some possible explanations toward the origins of quartz textures, based on comparative observation and literature review. Most of the interpretations remain at an empiri- cal stage.

Three major classes are considered from a genetic point of view: (1) primary growth textures which represent the morphologies formed during crystal growth or the deposition of amorphous silica; (2) recrystallization textures which result from the recrystallization of chalcedony, or crystallization and


subsequent recrystallization of amorphous silica to quartz; and (3) replacement textures which represent partial or complete pseudomorphs of other minerals by silica minerals within veins. Most of the quartz textures described above can be fitted into one of these three categories.

Primary growth textures In general, any process which causes changes in fluid condi-

tions may lead to the formation of simple crustiform bands. These processes include: cooling, mixing of two fluids, reactions between wall rocks and the fluid, and boiling (Buchanan, 1981). However, to produce complex crushform bands with numerous repetitive changes in mineral composition (e.g., quartz, adularia, sulfides) and/or textures, the hypothesis "epi- sodic pressure release" (Buchanan, 1981) is a feasible mechanism. The drops in the total confining pressure will allow the fluids to boil, resulting in loss of gases, cooling, pH rises, and precipitation of ore and gangue minerals. As minerals deposit, the near-surface veinlets become filled by these minerals, effectively forming a sealed cap to the fracture system. Once sealed, the pressure increases and boiling at depth ceases. Tectonism, or more likely hydrofracturing, can break the sealing cap to allow a second, and later, episode of boiling and mineralization, and again seal the system. In this manner, a repetitively banded crustiform texture is formed.

The separation of fragments in loose breccia by the force of growing crystals, proposed by Adams (1920), is a feasible explanation for the cockade texture observed in the majority of samples in the present study. These show quartz prisms bristling from all surfaces of fragments and sharp contacts between fragments and banded materials. Hydrothermal brecciation generally precedes the formation of a cockade texture, as brecciation allows deposition of silica minerals and other minerals around newly formed fragments.

Colloform and moss textures both have distinct rounded

forms, although one exhibits continuous bands and the other isolated spheres. Two processes, both indicative of a silica precursor, were proposed to explain the formation of rounded forms. The first one is that of the precipitation of silica gel in free space (Rogers, 1917; Adams, 1920). The controlling factor for this process is considered to be surface tension, a property of fluids caused by intermolecular forces near the surface, tending to reshape all nonspherical surfaces into a spherical, minimum free energy configuration (cf. Adamson, 1976). The second process is that of the segregation of impurities by crystallization from silica gel (Adams, 1920; Keith and Padden, 1963, 1964a, b; Oehler, 1976). The principal requirement for this process is a very slow rate of impurity diffusion compared with the rate of crystal growth, which typically occurs in viscous silica gel with impurities. The slightly different appearance between colloform and moss textures may be caused by the different occurrences of initial nuclei: those adhered on wall rock or early formed vein rock result in the formation of colloform texture; whereas those suspended in silica gel lead to the formation of moss texture.

To form comb texture, geometrical selection must proceed effectively (Grigor'ev, 1961, p. 190). Geometrical selection is a type of competition for space between adjacent crystals, which results in the growth of only those crystals where the

direction of maximum rate of growth is perpendicular to the growth surface. This requires relatively slow changing conditions in an open space during crystal growth.

Zonal texture is confined to quartz crystals that grow di- rectly from hydrothermal fluid. This requires the hydrothermal fluid to be only slightly saturated with respect to quartz, suggesting slow changing or very mildly fluctuating conditions during crystal growth (Fournier, 1985).

Recrystallization textures All silica minerals except quartz are metastable and have

a tendency to convert to quartz after deposition. The possible ways of forming various reerystallization textures are illustrated in Figure 6.

Close inspection of a feathery texture reveals that some individual domains of extinction resemble small subhedral-

euhedral quartz crystals (see Fig. 3b). This dosely resembles epitaxial growth of quartz (Rimstidt and Cole, 1983), i.e., small quartz crystals grow or accumulate on a large existing quartz crystal which acts as a surface favorable for nucleation and growth (Fig. 6-A1). Later these small crystals are reerys- tallized in approximate crystallographic continuity with the host quartz crystal. The original shape of small crystals is preserved as a slight difference in extinction which may be induced by dislocations along the boundary of adjacent small crystals during the reerystallization.

The final appearance of a feathery texture is controlled by the mutual relationships between small crystals and the host crystal. For example, if small crystals grow or accumulate on a euhedral quartz crystal, after reerystallization, feathery extinction is confined to the grain margins. If the host quartz crystal continues to grow together with or after the growth or accumulation of small crystals, eventually these small crystals will be enclosed by the host grain. After reerystallization, feathery extinction will be developed as patches or zones throughout the grain (Fig. 6-A2), with some individual domains of extinction even erossing euhedral growth zones of the crystal (see Fig. 3e).

The initial components of a flamboyant texture are likely to be aggregates of fibrous chalcedony with rounded external surfaces, which originate from silica gel, either as coatings on early formed quartz crystals or wall rock (Fig. 6-C2, refer to colloform texture), or as groups of spheres (Fig. 6-D2, refer to moss texture). When the recrystallized materials follow the crystallographic orientation of initial nuclei of each chalcedonic spheroid or that of the large crystal upon which they are coated, crystalline or microcrystalline quartz crystals with radiating extinction, possibly induced by the dislocation between adjacent chalcedonic fibers, are formed (Fig. 6-C3). This can be illustrated in a series of photographs (Fig. 7).

A ghost-sphere texture could be generated from recrystallization of amorphous silica or chalcedony with a moss texture (Fig. 6-D1 and D2), if original spherically distributed impurities are preserved in quartz crystals due to their low solubility (Fig. 6-D3).

A mosaic texture has been suggested as the product of recrystallization of massive chalcedony or amorphous silica (Lovering, 1972). A similar texture is commonly found in calcite marbles (Harker, 1950; Augustithis, 1985). It is possi-


Polymerization and aggregation __-- --

I- 5o.,icaio

Possible

silica precursors

/ / xcz • / • ,,a•xo2 to the quartz textures

Feathery Jigsaw Flamboyant

3

Ghost-sphere

Various quartz textures as indicated in figure 1

FIG. 6. The interpreted origin of recrystallization textures. The definition of polymerization, aggregation, crystallization, condensation, and solidification follows to Iler (1979). Various forms of opal: B1, C1, and D1; various forms of chalcedony: B2, C2, and D2; various forms of quartz: A1, A2, B3, C3, and D3.

ble that recrystallized coarse grains locally follow the shape of the original small grains, forming highly irregular and interpenetrating boundaries of the crystals.

Replacement textures

The possible processes of forming various replacement textures are demonstrated in a flow chart (Fig. 8).

Pseudobladed (lattice bladed, ghost bladed, and parallel bladed): Calcite and barite are the most common soluble phases that may be replaced by quartz in epithermal veins. In our sample collection, primary bladed barite usually has a spindlelike shape and is commonly dissolved, leaving spindlelike molds rather than being replaced by quartz. In con- trast, primary bladed calcite often displays very similar morphologies to those of lattice or ghost-bladed pseudomorphs (Fig. 9a).

Unlike barite, carbonate crystals usually contain numerous microscopic inclusions, dominated by iron hydroxides. After carbonate is replaced by quartz, these impurities are preserved due to their low solubility and usually still define original crystal outlines. Occasionally rhombic cleavage traces of original carbonate can also be preserved (Fig. 9b).

The lamellar parting, which is parallel to the basal pinaeoid of carbonate crystals, is the most distinctive morphological

feature of calcite (Fig. 9c and d). As noted by Adams (1920), replacement proceeds along these planes more easily than along rhombohedral cleavage planes. This selective replacement yields a set of parallel structures within lattice-or ghost- bladed pseudomorphs, which are defined under the microscope either by different grain sizes of quartz, by different contents of impurities, or by preferred orientation of quartz grains. Not surprisingly, parallel-bladed texture, which shows an overall granular outline for each group, is the product of selective replacement of massive granular calcite.

As illustrated in Figure 8-A2, quartz crystals start to replace bladed carbonate along the outline of the crystals and pina- coidal partings within the crystals. As the process goes further, it appears that the replacement front is at the boundary between the replaced mineral (carbonate) and the replacing mineral (quartz), and quartz crystals in the former layer keep growing simultaneously. Finally, as carbonate is totally re- moved, quartz crystals in every two adjacent layers merge into a seam, with crystals back to back against each other and with the grain-size of crystals increasing outward (Fig. 8- A3). This is the typical microscopic feature of lattice-bladed texture (see Fig. 4b).

If the starting material is quartz intergrown with bladed carbonate (Fig. 8-B1), the replacive quartz commonly grows


FIG. 7. An early stage of recrystallization, forming flamboyant texture. a. Colloform-banded, initial chalcedony coating on dear quartz crystals (plane- polarized light). b. In the area close to clear quartz crystals, initial chalcedony has recrystallized with the same optical orientation as the host quartz crystals (crossed polars). c. In the maximum extinction position, recrystallized materials showing flamboyant texture (crossed polars). McLaughlin, California (35344). Scale bars = 0.2 mm.

on existing quartz crystals with the same crystallographic orientation, and the original bladed form is only defined by concentrations of impurities (Fig. 8-B3, cf. Fig. 4d). After replacing massive granular carbonate, quartz crystals may de-

velop rectangular forms commonly seen in parallel bladed texture (Fig. 8-C3, also see Fig. 4f). This is possibly because replacement takes place more readily along the lameliar partings within carbonate crystals than transverse to them (Ad- ams, 1920).

A pseudoacicular texture is formed via the replacement of calcite by quartz and adularia along a set of radial acicular structures within calcite crystals (Fig. 8-D1), as suggested by Lindgren and Bancroft (1914), Adams (1920), and Schrader (1923). Partial replacement of calcite by quartz along this structure is also visible in some samples (Fig. 9e).

The presence of radial-acicular structure within calcite crystals is somewhat problematic, since this structure does not follow any consistent structural feature in calcite crystals such as rhombohedral dearage planes, twin planes, or basal pinaeoid planes. As shown by Nield and Heniseh (1969) and Gareia-Ruiz and Amoros (1981), calcite crystals grown in sfiiea gel are usually turbid with some spedfie but unusual morphologies such as radial fibers. Dissolution of turbid calcite crystals in acid leaves a residue of silica gel which has the same structure as the original growth medium. It is indicated from these results that the silica network which

constitutes the gel is incorporated into the growing calcite crystals more or less intact. In this way, an unusual radial- aeieular structure, composed of sfiiea, is formed within calcite crystals. Later replacement by quartz should preferentially follow this structure where there are sfiiea inclusions for nu-

cleation (Fig. 8-D3). A saeeharoidal texture has been generally interpreted as

the product of the replacement of calcite (Lindgren, 1901; Adams, 1920; Lovering, 1972). Presumably, diffusion of sfiiea- bearing fluid through randomly distributed crystallographic defects, rather than along the lameliar partings (e.g., forming parallel-bladed texture), within massive granular carbonate is responsible for the initial nucleation of quartz crystallites randomly dispersed in carbonate crystals (Fig. 8-E2). Further diffusion along the boundary between carbonate and quartz leads to the formation of slender subhedral-euhedral, or even doubly terminated, quartz crystals (Fig. 9f) which eventually interlock, forming saccharoidal texture (Fig. 8-E3). In some cases, remnants of calcite are found within the saccharoidal texture. They could later be replaced by finer grained aggregates of anhedral quartz, or be dissolved, giving a porous appearance which is commonly seen in the saccharoidal texture.

Implications of This Study 1. The most conspicuous quartz textures in epithermal

veins are those which record the presence of a silica gel precursor (e.g., colloform, moss, ghost sphere, flamboyant, and pseudoacicular). To form silica gel, the fluid needs to be highly supersaturated with respect to amorphous silica (Fournier, 1985). At low temperature (e.g., below 100øC), the solubility of amorphous silica is relatively low (e.g., 364 ppm at 100øC, cf. Fournier, 1985, appendix) and the rate of silica precipitation is very slow (Rimstidt and Barnes, 1980). Therefore, the fluid may easily reach equilibrium with amorphous silica at low temperatures. For example, cooling of a dilute fluid in equfiibrium with quartz from a hot reservoir

185'2 SCIENTIFIC COMMUNICATIONS

Lattice Bladed Ghost Bladed Parallel Bladed Pseudo-Acicular Saccharoidal

FIc.. 8. Interpretation of stages in the formation of various replacement quartz textures. The top row represents original forms of calcite; the middle row shows initial stage of replacement of calcite by quartz; the bottom row shows various quartz textures (indicated in Fig. 1) formed after complete replacement of calcite.

(e.g., at 230øC with silica concentration of 388 ppm) may yield a fluid supersaturated with respect to amorphous silica at about 100øC. This may occur when an ascending hydrothermal fluid rises fast enough and silica does not precipitate during the ascent. In fact, colloform and moss textures have been observed in a number of siliceous sinters in active geothermal systems (e.g., White et al., 1956; Herzig et al., 1988; Fournier et al., 1991). At higher temperatures, the fluid supersaturated with respect to amorphous silica should have a relatively high silica concentration (e.g., at 220øC the fluid saturated with respect to amorphous silica contains about 1,070 ppm silica, cf. Fournier, 1985, appendix), such a high silica concentration is not easily attained simply by cooling from a hot reservoir. However, where the fluid undergoes boiling, significant cooling due to adiabatic expansion (decreasing the solubility of silica minerals) and the loss of water to the vapor phase (increasing silica concentration in residual solution) can make the fluid highly supersaturated with respect to amorphous silica, even at relatively high temperatures. For example, assuming a reservoir has a temperature of 300øC and pressure of 200 bars, the water in equilibrium with quartz under these conditions should contain about 750 ppm SIO2. This will yield a fluid just saturated with amorphous silica at about 170øC (cf. Fournier, 1985, appendix). According to the enthalpy balance calculation (e.g., Henley, 1984), isoenthalpic boiling of a fluid from 300 ø to 200øC will cause 25 percent water loss. Thus if taking the steam loss into account (assuming 30% water loss as a maximum), the silica concentration in such a fluid may reach about 1,070 ppm, which is equivalent to the solubility of amorphous silica at about 220øC. Therefore, the presence of the quartz textures

inherited from silica gel, along with other geological, mineralogical, or fluid inclusion evidence of relatively high temperatures, is a good indicator of boiling in epithermal environments.

2. The recognition of a carbonate precursor is also important. In epithermal environments, the precipitation of vein calcite is most likely driven by the loss of CO2 due to boiling, and the subsequent generation of CO•- ions from the dissoci- ation of HCO• (Henley, 1985; Reed and Spycher, 1985). In addition, calcite may be precipitated where cooler marginal fluids come into contact with hotter rocks due to its retro-

grade solubility, but this process is commonly restricted to the margins and shallow parts of a system (Simmons and Christenson, 1994). The control of calcite morphology has been the subject of a vast geological and geochemical literature (e.g., Bischoff, 1968; Kirov et al., 1972; Folk, 1974; McCauley and Roy, 1974; Lahann, 1978; Given and Wikinson 1985). In general, crystal morphology of calcite is suggested to be controlled mainly by the rate of crystal growth, Ca'2+/ CO•- ratios in the fluid, and the presence of impurity ions such as Mg •+, Na +, and SO4 •-, but the mechanisms are still somewhat obscure and unverified by direct evidence. In many active geothermal systems, bladed calcite is commonly restricted to the boiling zone and may contain coexisting liquid- and vapor-rich inclusions (Browne, 1978; Keith et al., 1978; Tulloch, 1982; Simmons and Christenson, 1994). However, without clear genetic evidence, it is difficult to conclude that other forms of vein calcite (e.g., granular) cannot form in boiling environments, or that bladed calcite forms exclusively in boiling zones.

Calcite becomes more soluble with decreasing temperature


Fzc. 9. Primary calcite and partial replacement of calcite. a. Primary-bladed calcite: a network of intersecting bladed calcite separated by polyhedral cavities, identical morphology with lattice-bladed pseudomorphs in Figure 4a. Komata mine, Coromandel, New Zealand. b. Rhombic cleavage traces: after calcite is replaced by silica, rhombic cleavage traces are occasionally preserved by the distribution of brownish-colored impurities. Note no calcite is left in this sample. Barambah Creek, Queensland. Crossed polars. c. Partial replacement of bladed calcite: bladed calcite is being replaced by quartz along parallel partings, identical morphology with ghost-bladed pseudomorphs in Figure 4d. Red Dome, Queensland. Crossed polars. d. Partial replacement of granular calcite: granular calcite is being replaced by fine-grained quartz along parallel partings, identical with parallel-bladed pseudomorphs in Figure 4f. Yandan, Queensland. Crossed polars. e. Partial replacement of granular calcite: partial replacement of granular calcite by quartz along radial acicular structure in the crystal. Standard lode, Cracow, Queensland. Crossed polars. f. Partial replacement of granular calcite: granular calcite is partly replaced by randomly dispersed slender subhedral-euhedral quartz crystals. Rose's Pride lode, Cracow, Queensland. Crossed polars. Scale bars = 0.2 min.


but less soluble as the partial pressure of CO2 decreases (Ellis, 1959). Hence boiling may precipitate or dissolve calcite, depending on the composition of the hydrothermal solution, the pressure at which boiling is initiated, the drop in temperature and pressure and whether the system is open or closed. In gas-rich, open systems, boiling dramatically reduces the partial pressure of CO2. At relatively low temperatures, the loss of C02 will cause significant enrichment of CaC03 in the fluid due to the high solubility of calcite. Consequently, boiling initiated from gas-rich and open systems at relatively low temperatures favors precipitation of large amounts of calcite. If these early precipitated calcite crystals are not im- mediately segregated from residual fluids, they are readily dissolved and replaced by silica minerals when the fluid cools further, forming various replacement textures.

Thus, if a fluid undergoes boiling which results in the loss of CO2 without rapid cooling (e.g., isothermal boiling), or if a fluid is heated by hotter rocks, the fluid may be supersaturated with respect to calcite but undersaturated with respect to quartz. In this condition, calcite precipitates alone and is later replaced by silica minerals, forming lattice-bladed, parallel- bladed, or saccharoidal textures--depending on the morphology of carbonate precursors and the development of cleavages and fractures, as discussed previously. On the other hand, where the fluid undergoes boiling which induces both the loss of CO2 and rapid cooling (e.g., isoenthalpic or subisoenthalpic boiling, cf. Reed and Spycher, 1985), the fluid could be supersaturated with both calcite and quartz or even amorphous silica. In this case, calcite and quartz or amorphous silica precipitate simultaneously, forming ghost-bladed or pseudoacicular textures when calcite is later replaced by silica.

3. From the comparison between well-mineralized vein systems (such as Cracow, Pajingo, and Mount Coolon) and barren or poorly mineralized ones (e.g., Quartz Hill, Woolgar, and Bimurra) (cf. Digweed, 1991; Porter, 1991; Dong, 1993; Bobis et al., 1995; Worsley, 1995), it is apparent that textures inherited from silica gel (e.g., ghost-sphere, flamboyant, and pseudoacicular) are widely distributed in the former systems, whereas absent or poorly developed in the latter group. A possible explanation, as discussed earlier, is that the same fluid condition for forming silica gel (i.e., boiling) also favors the precipitation of gold. Alternatively or additionally, gold may be transported as colloidal particles which are protected by colloidal silica. If this is the case, more gold is able to be transported to preferential sites, forming rich orebodies. The idea that colloids are important in the formation of some gold deposits is not new. Lindgren (1936) suggested that gold may be present as colloidal particles in neutral to alkaline hydrothermal solutions and is apparently protected by colloidal silica. Frondel (1938) showed experimentally that colloidal gold is stable up to 350øC when colloidal silica is present. Goni et al. (1967), Boyle (1979), Fournier (1985), and McHugh (1988) all postulated from various aspects that gold colloids are important in gold mobility under surficial conditions or in natural waters. More recently, Saunders (1990) has explained the origin of the bonanza epithermal ore at the Sleeper mine by using the hypothesis of colloidal transport of gold and silica. Although there are many uncertainties

regarding the hypothesis of colloidal gold transport, it is a mechanism worthy of further investigation.

Conclusions

A descriptive classification of quartz textures has been developed from a review of the literature and the examination of many samples from epithermal veins in Queensland. Thirteen textural types have been defined, most of them have been interpreted genetically and grouped into three major classes: (1) primary growth textures, (2) reerystallization textures, and (3) replacement textures. It has been recognized that some specific quartz textures can be used as an indicator of boiling in epithermal environments, and there is a positive correlation between gold mineralization and the quartz textures indicative of silica gel precursors.

This study provides the groundwork for further systematic evaluation of the distribution of quartz textures and textural assemblages in selected epithermal systems. A textural zoning model, which can be used to determine the vertical position within an epithermal system and to predict the likely locus of gold mineralization, will be proposed in another paper.

Acknowledgments This study was a part of project P247 "Epithermal Gold

Deposits in Queensland" sponsored by Australian Mineral Industries Research Association. The support from the staff and students of the Earth Sciences Department at James Cook University, sponsor companies, and AMIRA is grate- fully acknowledged. Two Economic Geology reviewers are sincerely thanked for their constructive comments.

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Quartz Epithermal Textures - Dong Et Al 1995 (Econ Geol. p1841-p1856)

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