Hydrothermal Alteration

Hydrothermal Alteration

Module 11Contents

11.Introduction

2.Alteration Intensity13.Alteration Rank24.Alteration Pervasiveness45.Alteration Assemblages46.Overprinting66.1Prograde Overprinting76.2Retrograde Overprinting86.3Other Overprinting117.Practical Exercise12

1. Introduction

Hydrothermal fluids have various effects on the rocks they pass through: existing minerals can be removed, new minerals can be deposited in fractures and pores in the rocks, or the existing minerals can be replaced by other minerals in processes of hydrothermal alteration. All of these processes may occur together. The new minerals that are formed, whether by deposition in cavities or by replacement, are called secondary minerals. In this module we are looking at secondary minerals which form by the alteration (replacement) of other minerals. Mineral deposition (veining) is discussed in a separate module (Module 13), since although the mineralogy is similar, the textures are quite different.

Hydrothermal alteration is comparable to diagenesis, in which primary minerals are replaced by diagenetic phases upon burial and the consequent increase in temperature and pressure. Since the temperatures can be similar in both settings, the same minerals can occur (e.g. clays, zeolites), and it can be difficult to distinguish hydrothermal alteration from diagenesis in metasedimentary rocks.

The nature of the secondary minerals and the speed at which they form are controlled by factors such as the temperature, fluid composition, rock composition, and water:rock ratio. The last factor depends on the permeability of the rocks, the fluid flow rate and the length of time available. Alteration will be most intense in permeable rocks, where there is a high fluid flow, and in long-lived systems. Alteration can be described in terms of its intensity, rank, and pervasiveness.

2. Alteration Intensity

The intensity of alteration is an indication of how much a rock has been changed by the fluids passing through. Alteration intensity can be determined quite accurately from the proportion of secondary minerals relative to the proportion of primary minerals remaining, and whether the original texture is preserved. The intensity will vary among different lithologies according to the permeability of the rock and the reactivity of the primary components. Thus a highly permeable limestone or glassy tuff will be very susceptible to alteration, whereas a quartz sandstone will be very slow to react with hydrothermal fluids. The general sequence among common minerals, going from most reactive to least reactive is:

Carbonates

Volcanic glass

Mafic minerals

Plagioclase

K-feldspar

Apatite

Quartz

Zircon

There are exceptions; for example in some rocks the feldspars will be altered while the mafic minerals are preserved, but this gives an idea of the susceptibility of common minerals to alteration in the presence of hydrothermal fluids.

The alteration intensity may be reported as a percentage (e.g. 40% secondary minerals), or using descriptive categories. Our procedure (as outlined in Appendix 2) is to describe alteration as weak where secondary minerals make up less than 25% of the rock, moderate where they comprise 25-75%, and strong if they total more than 75%. Rocks with no secondary minerals are unaltered, whereas in rocks with few remaining primary minerals (except for resistant phases such as quartz, apatite and zircon) alteration is intense unless the primary textures are no longer visible, in which case it is total.

3. Alteration Rank

The alteration rank is an indication of the conditions (especially temperature) responsible for producing the secondary minerals within a rock. Thus, rocks which contain secondary minerals that formed at low temperature are described as being of low rank, whereas those with high temperature alteration minerals are high rank, and those between the two as intermediate rank.

The intensity and rank of alteration caused by hydrothermal fluids are controlled by two conflicting factors (Figure 1). On the one hand, most chemical reactions proceed more rapidly at high temperatures, and the fluids are more mobile. So more rapid alteration is expected at depth. In contrast, at near-magmatic temperatures the fluid, being basically of magmatic origin, is close to being in equilibrium with rocks of igneous origin. Furthermore, species such as HCl are much less dissociated at high temperatures, which means that the effective acidity (H+ activity) is less than the molar concentration of HCl would lead one to expect.

These effects were quantified for the reaction of carbonic acid with rhyolite by Bischoff and Rosenbauer (1996). They found that the degree of reaction was greatest at 150 -200C. The extent of alteration at 200C was 27 times as great as at 350C. At lower temperatures the rates of reaction were too slow to be effective (Table 1).

Table 1:Rates of reaction for carbonic acid/rhyoliteTemperature

CTime

300Minutes

200One day

1004000 days (~ 11 years)

501000 years

The time given is that required for reaction to proceed to 50% of saturation (after Bischoff and Rosenbauer 1996)

Figure 1:

Variations in alteration style and intensity with depth/temperature

in volcanic terrane

There is therefore limited potential for alteration of the intrusive that forms the heat source to a hydrothermal system, or igneous host rocks. Near-intrusive alteration is restricted to a certain amount of hydration and potassium and silica metasomatism, giving K-feldspar and biotite. (This is not of course the case if the host rocks have a different composition). As the fluid cools, becomes diluted with more groundwater, and becomes modified through reactions, it gets further and further out of equilibrium with the host rocks. So, while the rate of reaction may be slower, it can have a greater effect on the mineralogy of the rocks. In general, the process is one of removing ferromagnesian cations and replacing them with alkalis, along with silica addition, hydration, and a variable amount of carbonation and sulphidation. As the temperature drops, the Na/K ratio of the fluid rises, and that of the rock correspondingly falls. This process leads to the systematic zonation of alteration that we see in hydrothermal systems.

4. Alteration Pervasiveness

Alteration may not be uniform in intensity throughout a rock sample, drill core or outcrop. Instead, there may be parts that are intensely altered, and parts where alteration is weak or moderate, or where a different alteration assemblage is present. In extreme examples, alteration may be restricted to within a few millimetres of a vein, forming a visible halo around it. The variation of alteration intensity is known as pervasiveness; where alteration extends for many metres or kilometres it is pervasive, and where it is localised on a millimetre to centimetre scale it is localised. In either case it might be of any intensity or rank; here we are just describing how extensive it is. If alteration is localised, it might only occur close to permeable zones (e.g. along bedding in tuffs or within fractures), or it may have a more irregular distribution.

5. Alteration Assemblages

From studying secondary minerals, geologists have learned that certain minerals often occur together, forming distinctive mineral assemblages. Most assemblages are at thermodynamic equilibrium, so comprise an equilibrium assemblage. Disequilibrium assemblages may also occur; these contain minerals that should not occur together on chemical or thermodynamic grounds, and are particularly common where there has been rapid mineral deposition. This is especially common in assemblages formed from aggressive acid solutions. Many of the reactions which work to produce an equilibrium assemblage can be quite sluggish, and the original (primary or earlier-formed secondary) minerals can persist metastably, thus forming a disequilibrium assemblage. In this way, minerals from two or more different alteration assemblages may be preserved in a rock.

Although rank is a useful concept, in practice it is more common to refer to specific alteration assemblages. Several alteration assemblages have been defined, although slightly different definitions are used by different groups of workers. Others avoid all reference to assemblages, and instead refer individually to all of the secondary minerals that are present. We consider that the concept of alteration assemblages is a useful one, provided each assemblage is clearly defined. It is of particular benefit for rapidly describing groups of rocks, or comparing the alteration at different prospects, since the conditions of formation of a particular assemblage should be similar anywhere. However, as with any classification scheme, it is essential to first describe a rock and the minerals and textures within it before assigning it to an alteration assemblage. The standard definitions that we use are summarised below, and listed in Appendix 2.

Argillic:Clay-rich assemblages dominated by low-temperature clays such as kaolinite, smectite, and interlayered illite-smectite. These are formed by low temperature (290C) than propylitic assemblages.

Potassic:Major secondary minerals are biotite, orthoclase, quartz, and magnetite. Anhydrite is a common accessory, and minor albite and titanite or rutile can also develop. Potassic alteration is caused by near-intrusive, hot fluids (>300C) with a strong magmatic character and high salinity.

Advanced Argillic:Contain alunite, diaspore, and/or pyrophyllite, together with one or more of quartz, chalcedony, kaolinite, and dickite. Zunyite and phosphorus-bearing phases such as woodhouseite may also occur. These assemblages occur as tabular near-vertical zones formed from condensed acid magmatic vapours in the porphyry environment, and as near-horizontal blankets at shallow epithermal levels, where acid-sulphate fluids form from oxidised steam condensates.

Skarn:May contain garnet, clinopyroxene, vesuvianite, scapolite, wollastonite, epidote, amphibole, magnetite and calcite as major components. Minor amounts of biotite, K-feldspar, quartz and chlorite may also be present. Minerals present are similar to those found in potassic, high temperature propylitic and propylitic assemblages of porphyry systems, but with a wider range of cations. Developed in the presence of calcium-rich, high salinity fluids over a wide temperature range, with early anhydrous minerals forming in the range 300 - 700C. Occur near the contact between calcareous lithologies and intrusives.

Some rocks lack any diagnostic minerals, and so can not be readily assigned to an assemblage. This includes samples that consist largely or entirely of quartz, or calcite, adularia, zeolites, or sulphide or sulphate minerals. Even so, we can often still infer something about the fluid conditions (e.g. adularia = near neutral, zeolites = low CO2). In such cases, rather than trying to fit a rock into an assemblage, it may be best to refer to the alteration as silicification, or calcite-, adularia-, zeolite-, sulphide- or sulphate-alteration, as appropriate.

There can be extensive overlap among some of these assemblages, especially between phyllic and propylitic, and phyllic and argillic alteration. In effect, these are useful end-members, between which there may be a gradation. Some workers have attempted to define additional assemblages which are variants or mixtures of those listed above. For example, because of the overlap between phyllic, propylitic, and to a lesser extent argillic assemblages, these are sometimes combined into a single category. However, the propylitic assemblage differs from the other two in the extent of mass transfer (due to the degree of water-rock interaction), rather than any differences in temperature or fluid chemistry. The propylitic assemblage is in effect an intermediate step during formation of a phyllic assemblage (or argillic, depending on the temperature), and so such overlap is to be expected. We consider that there is benefit in distinguishing individual assemblages, and where there is overlap it is worth noting the nature and proportions of secondary minerals rather than simply allocating a sample to a single assemblage.

Some of the above terms have been used incorrectly. For example, the term potassic has been wrongly applied to phyllic or even argillic altered rocks. Although these rocks may have elevated potassium contents, they are quite different from the potassic alteration as defined above, in which there is biotite rather than illite or illite-smectite, and orthoclase rather than adularia.

6. Overprinting

Overprinting is the partial or complete replacement of one secondary mineral assemblage by a different assemblage, and must occur due to changes in physical and/or chemical conditions. In a stable hydrothermal system, provided there is sufficient permeability, reactions between fluids and rocks will eventually produce a secondary mineral assemblage that is in equilibrium both internally, and with the fluid, at the prevailing conditions. Yet hydrothermal systems are seldom stable for long (in the geological sense), and when conditions change, then a new set of reactions works to produce a new equilibrium assemblage. Such changes generally affect the fluid temperature, pH, or chemistry, or possibly all three, and may be associated with mineralisation.

Overprinting may be prograde (producing assemblages of higher rank), retrograde (producing lower rank assemblages) or neither (due to differences in fluid chemistry or state, rather than temperature or pressure). If overprinting reactions are sufficiently complete, there may be no remnants of the prior assemblage left. However, usually some minerals are preserved because they are resistant to alteration or they are enclosed in a resistant mineral, while others may be recognisable from crystal pseudomorphs. Prior generations of veining or fluid inclusion data might also indicate that overprinting has occurred; for example, there may be two populations of fluid inclusions (as distinct from mixed populations due to phase separation).

6.1 Prograde Overprinting

In hydrothermal systems, prograde overprinting commonly results from a rise in temperature. A rise in temperature may be due to heating in response to renewed magmatism (e.g. emplacement of a shallow intrusive body). If a system is already at or near a boiling point for depth profile, it will not sustain higher temperatures unless the pressures increase. This can occur if the water level is raised or permeability is sufficiently low for lithostatic pressures to be contained, even temporarily. If the water level is already near the surface, more rock must be added before the water level can be raised. This may occur by the gradual accumulation of material, as in submarine hydrothermal systems and VHMS deposits, or by sudden accumulation of material, as in caldera basins.

Renewed magmatism:With renewed magmatism, the boiling point for depth profile is commonly exceeded, so catastrophic phreatic eruptions occur at the surface, with hydrothermal brecciation and boiling at depth (Browne and Lawless, in prep). The sudden boiling and loss of gas result in destabilisation of bisulphide complexes, and deposition of gold and other metals at depth. Overprinting may follow such an event, but usually takes longer to achieve, occurring after the hydrothermal system has returned to a stable temperature profile at or below that of boiling point for depth conditions. This may require elevation of the water table and/or a decline in the heat flux into the system.

Magmatic activity has been documented in historical times at several active geothermal systems, including Rotomahana in New Zealand (1886), Suoh in Indonesia (1933), and Pinatubo in the Philippines (1991). The effects on the hydrothermal system are commonly overshadowed by the volcanic effects (e.g. Pinatubo), since the phreatic/ phreatomagmatic eruptions are generally smaller and less spectacular than volcanic eruptions (although at Rotomahana the reverse was true). Phreatomagmatic eruptions were observed at Suoh in 1933, and the system there still maintains a boiling point for depth profile today. Possible effects on the geothermal system are illustrated in Figure 2.

At Kelian (Indonesia), mineralisation coincided with prograde overprinting due to renewed magmatic activity at a waning hydrothermal system. There was little in the way of mineralogical change, because this event was short-lived. Evidence remains in the form of late stage, hot, saline fluid inclusions, a large phreatomagmatic breccia body, and a large gold deposit (Van Leeuwen et al. 1990). The same mechanism has probably occurred in many other mineralised systems, but the temperature transient may often be too temporary to cause significant recognisable overprinting.

Accumulation of material:Prograde overprinting may also arise from more gradual changes (Figure 2), such as where the land surface is subsiding, with accumulation of material during the course of hydrothermal activity, or where there is a rising water table (due to climatic change, tectonic activity, etc.). Examples are submarine hydrothermal systems, and systems located within structural basins or calderas in which epiclastic and/or pyroclastic deposits are accumulating. Although these changes may seem rapid from a human perspective, the consequent temperature increase is gradual, and will not in itself cause mineralisation.

Figure 2

Temperature - depth diagram, showing the likely effects of renewed magmatism and accumulation of material on a fixed point in a hydrothermal system. Also shown are hydrostatic boiling point for depth curves for pure water, and 10%NaCl and 4.4%CO2 solutions (from Henley 1985).

6.2 Retrograde Overprinting

Retrograde overprinting (also known as telescoping) generally results from a temperature decline. A drop in temperature may be due to gradual cooling of a hydrothermal system at it wanes. It is thus almost inevitable in fossil hydrothermal systems, and for this reason is more common than prograde overprinting. However, cooling at a given point can also occur following lowering of the water level. This may be due to climatic change, or the removal of a thickness of rock from the surface; either a gradual loss of material by erosion, or sudden loss due to volcanic eruption or sector collapse. Volcanic eruptions also have other effects on hydrothermal systems (including heating), but sector collapses can occur in the absence of recent volcanic activity.

Sector collapse:

Major sector collapses have been recognised at many active geothermal fields, including Papandayan in West Java, Indonesia (1772), Muria in Central Java, Indonesia (undated) and Lihir in Papua New Guinea. Sector collapse structures are often mistakenly identified as calderas. There are important differences between the two, both in terms of their origins, and their effects on a hydrothermal system.

A caldera is a large volcanic depression formed by the downward collapse of the ground surface above a magma chamber during an eruption. Calderas are typically kilometres to tens of kilometres across. Caldera-forming eruptions produce pyroclastic deposits, which are generally fine grained and distributed more or less evenly around the caldera (depending on wind direction and strength at the time of eruption). The caldera basin itself will be partially to completely infilled with eruption deposits and slumped material from the caldera walls, so that a point on the original land surface will be buried. Up to 50% of the erupted pyroclastic material may fall back into the caldera basin, especially in the largest calderas, where it can be as much as 5 km thick (Lipman 1984). As an example, the 1883 eruption of Krakatau (Indonesia) ejected some 18km3 of dacitic pyroclastics and produced a caldera measuring 6 km by 7 km.

Figure 3:

A: Cross sections of composite cones that have undergone sector collapse (from Siebert 1984), and B: Schematic cross sections through different types of calderas (from Wohletz and Heiken 1992, after Walker 1984).

A sector collapse is the lateral (outward) collapse of part of a volcano to form a large debris flow (Figure 3), and may or may not be associated with an eruption. Sector collapses may be up to several kilometres across. They produce coarse epiclastic sector collapse deposits that extend out as a fan on one side of a volcano.

Following sector collapse and lateral removal of the collapsed material, a point originally at depth will be exposed at the surface. For example, in 1980 a sector collapse removed 2.3 km3 of material during the eruption of Mt. St. Helens, leaving a large amphitheatre with a steep back slope (Voight et al. 1981).

A sector collapse above a hydrothermal system will suddenly reduce the confining pressure, which may result in hydrothermal brecciation, boiling and mineralisation. The effects are similar to those of renewed magmatism, but with a different mechanism; decreased confining pressure, rather than increased temperature (Figure 4). A sudden pressure decrease can have the same effect on a hydrothermal system as sudden heating, pushing the system beyond boiling point for depth conditions. However, in this case the temperature will decrease rather than increase (though more slowly than the pressure decreased), causing retrograde overprinting. In contrast, caldera collapse is likely to result in prograde overprinting.

Figure 4

Temperature - depth diagram, showing the likely effects on a hydrothermal system of sector collapse, caldera collapse, gradual cooling and erosion/cooling/tectonic activity.

An example of mineralisation that has been attributed to the effects of sector collapse and retrograde overprinting is the giant Ladolam gold deposit on Lihir Island (PNG). Here there was a porphyry system, with potassic alteration and incipient Au-Cu mineralisation forming at depth beneath a clay-altered volcanic pile. A massive sector collapse of the weak clay-altered material exposed the top of the porphyry system. The sudden large pressure drop that occurred during this collapse resulted in massive brecciation and boiling, so that epithermal mineralisation was superimposed on the porphyry system (Moyle et al. 1990). Several features indicate that Lihir is a sector collapse, rather than a caldera, including exposed monzonitic intrusives and high temperature overprinted by low temperature alteration within the caldera. Furthermore, rather than producing pyroclastic deposits, coarse epiclastic debris flow deposits extend tens of kilometres offshore to the northeast (Herzig et al. 1994).

Erosion, cooling or tectonic activity:

Many hydrothermal systems are located in areas of active uplift and tectonism, and it is possible for a system to be uplifted and eroded to considerable depth over its lifetime. Gradual changes due to slow uplift or declining temperatures in a waning hydrothermal system will produce retrograde overprinting, or telescoping, as discussed by Sillitoe (1994a) and described in further detail in Module 5. Such telescoping due to gradual uplift will generally not cause mineralisation, but sudden pressure changes accompanying tectonic activity might. Mineralisation and retrograde overprinting at Porgera (PNG) could result from repeated sudden pressure fluctuations associated with tectonic activity during uplift and erosion of a mesothermal system.

6.3 Other Overprinting

Overprinting may reflect changes in fluid chemistry, or changes from liquid-dominated to vapour-dominated conditions. However, these changes in themselves are not likely to cause mineralisation, except where they are associated with processes such as boiling or mixing of different fluids. Mineralisation in such systems is generally related to one or other state of the system, rather than to the process of overprinting. For example, at Masupa Ria (Indonesia), high and low-sulphidation epithermal systems are superimposed, but mineralisation is principally associated with the low-sulphidation event (Thompson et al. 1994).

7. Practical Exercise

How would you describe the alteration in the following samples? You should note the pervasiveness, intensity, alteration assemblage(s), and comment on overprinting relationships, if present.

ADark green porphyritic rock with abundant chlorite, cut by a 1 mm quartz vein that is surrounded by a 5 mm thick whitish clay-rich zone.

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BWhitish illite-rich rock with disseminated pyrite and a faint porphyritic texture, cut by 2 to 5 mm wide quartz-K-feldspar-magnetite-chalcopyrite veins, which in turn are cut by 1 to 2 mm quartz-zeolite veins.

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