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The Life-Cycle of Hydrothermal Systems

Module 5


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The Life-Cycle of Hydrothermal Systems Contents i

Contents

1. Introduction 1

2. Single Cycle 2

2.1 Cooling of Plutons 2

2.2 Changes in The Primary Fluid 4

2.3 Changes Due to Secondary Fluids 4

2.4 Erosion 4

3. Rejuvenation of Hydrothermal Systems 6

3.1 Lifespan of Hydrothermal Activity 6

3.2 Influence of Magmatism on Mineralisation 63.3 Evidence For Rejuvenation in Active and Fossil Systems 7

4. Practical Exercise 9

/var/www/apps/conversion/tmp/scratch_6/128593686.doc


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The Life-Cycle of Hydrothermal Systems 1

1. Introduction

In this section we will first describe typical changes that a hydrothermal system can go

through in a single cycle, and then look at the possibilities of more complicated historieswhen hydrothermal systems become rejuvenated. These are not just theoretical

considerations. They are based on observations in real systems, some of which are linked

to known economic deposits. For example, in the Creede deposit in Colorado, a great deal

of fluid inclusion work has been done, and a stratigraphy has been erected based on

colour banding in the abundant sphalerite in veins, which permits good time control

(Figure 1). Twenty separate episodes of sphalerite deposition can be recognised. While

these overallshow a sequence of decreasing salinity and temperature with time, as would

be expected with progressive dilution and cooling of a initial magmatic fluid, they also

reveal two major cycles of rejuvenation plus some minor fluctuations.

- 2

- 4

- 6

- 8

FreezingTem

perature(C)

K:geo\lec\min\98min\mod16\fig1.cdr

200 220 240 260

Homogenisation Temperature (C)

11.8

3.4

6.5

9.4

NaClEquiv

alent(wt%)

Figure 1:

Plot of Th versus Tm ice (and estimated salinity) for 221 primary

inclusions in a 50 mm band of zoned sphalerite from Creede

(after Roedder 1984)



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2. Single Cycle

Within a single life-cycle of a hydrothermal system, the effects that will produce

evolutionary changes are:

Physical changes as a result of cooling

Chemical evolution due to changes in the primary fluid

Chemical evolution due to changes in the secondary fluid

Erosion

These are of course all interrelated, but it is convenient to consider them one at a time:

2.1Cooling of Plutons

The simplest model of a hydrothermal system is that it is induced by the emplacement of a

pluton, which then interacts with groundwater, and starts to cool. Convection of water is

established, which cools the pluton more rapidly. With time, thermal fracturing of the

pluton occurs, so that the water can penetrate the pluton, extracting heat and leaching

minerals (Figure 2).

AI30/16.2M

Early

Low

Late

Medium

High

Figure 2:

Fluid flow lines and isotherms about a cooling pluton in

uniformly permeable wall rocks. Flowlines, shown by arrows,

are for early circulation when the central pluton is unfractured

(impermeable), and later circulation when the stock becomes

permeable by fracturing. The dotted extension of the "low"

temperature isotherm depicts possible spreading due to sealing

by an impermeable cap (after Beane 1983)



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This model is probably an over-simplification for any real system. But even for this simple

model, mathematical modelling of the cooling process shows that any point within the

surrounding host rock may undergo quite a complex thermal and chemical history due to

the different physical properties of water at different temperatures, boiling, and the

physical effects of groundwater encroachment to the pluton. It is not as simple astemperatures rising to a peak and then declining (Figure 3), since the change of

permeability with time as thermal fracturing spreads accelerates convection. Factors such

as the exothermic nature of the chemical reactions during alteration will further complicate

the picture.

Note that the diagrams below are based on some early work by Cathles which was updated

by Cathles and Erendi (1997), but the conclusions remain valid.

Figure 3:

A: Examples of modelled fluid pathlines over 200,000 years in a

hydrothermal system, representing the redistribution of fluids

caused by the thermal anomaly. Arrows indicate direction offluid motion.

B: Pressure-temperature path of fluid packets that circulate

along paths in an evolving hydrothermal system, as above.


K:Geo\Lecs\99Min\Mod17\fig3.ppt

2

3

2

1

5

24

7

41

4

6 3 8

7

4

3

7

5

8

2

400 800Temperature (C)

Pressure(bars)

BA

400

800

Depth(km)

Distance (km)


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2.2Changes in The Primary Fluid

When a pluton is first emplaced, it will be partially solid and partially liquid. As

crystallisation progresses, the more volatile magmatic component will be concentrated in

the residual liquid. As we have discussed for porphyry mineralisation, these magmaticvolatiles are mobile and reactive. They will be released into the convective hydrothermal

fluid at a greater or lesser rate. With time, the magmatic volatiles will tend to become

depleted, and there will be a greater proportion of groundwater. Thus the hydrothermal

system at a shallower depth will evolve from a more acidic, high-sulphidation type system

to a less acidic, lower-sulphidation type.

On a more localised scale, consider the common pseudomorphing of platy calcite by quartz

in epithermal veins. This is simply due to the relative solubilities in a progressively cooling

fluid: if a fluid which deposits calcite cools, the calcite becomes more soluble and

dissolves, whereas quartz becomes less soluble. Thus this is the normal evolutionary

sequence: it does not imply two successive fluids of different composition. Where platy

carbonates are preserved, it is a sign that there has been a relatively abrupt cessation of

fluid throughput.

2.3Changes Due to Secondary Fluids

A common sequence is for the accumulation of geothermal gases and their oxidation to

build up a zone of acid, sulphate- and/or bicarbonate-rich fluids above and around the

margins of a hydrothermal system. As the heat source declines, convection will wane and

the pressure gradient may reverse, causing these fluids to encroach back into the systems.

This is sometimes called the "thermal collapse" of the system.

In fossil systems this occurrence can often be identified as a late-stage acid-bicarbonate

overprint on the hydrothermal alteration, perhaps producing a carbonate-kaolinite

assemblage. The importance of pH shifts to gold deposition means that this process can be

associated with mineralisation. If the system is rejuvenated, the process can be repeated,

perhaps several times.

2.4Erosion

The degree of erosion during the lifetime of a hydrothermal system may be considerable,

especially in the island-arc setting. Tectonism may accentuate or diminish the effects, as

may the constructional effects of volcanism. But in general the effect will be to shift

isotherms down with time in relation to the rock, in a sort of conveyor-belt process. This

will once again cause overprinting of alteration zones in an apparently retrograde fashion,

since the temperature at the surface cannot exceed 100C in a stable situation. Examples

of the differences between systems with little erosion and overprinting and those with

extensive erosion and overprinting are given by Sillitoe (1994a) (Figure 4).



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Figure 4:

A: Non-telescoped porphyry Cu-Mo system at Red Mountain,

Arizona, showing 600 m separation between K-silicate core and

advanced argillic-sericite zone. Undocumented thickness of

advanced argillic alteration and surficial acid-leached zone have

been eroded.

B: Telescoped porphyry Au system at Marte, Chile, showing

advanced argillic zone juxtaposed with Au-bearing quartz veinlet

stockwork (of K-silicate parentage) developed in dioriteporphyry stock. Acid-leached rock generated above paleo-water

table is nearby.

C: Extremely telescoped porphyry Cu-Mo-Au system at

Ladolam, Lihir Island, PNG, showing overprinting of K-silicate

altered monzonitic intrusion and tabular zone of low-

sulphidation epithermal Au mineralisation. Acid leaching,

locally still active, affects intrusive rocks, and is believed to

overprint Au mineralisation (redrawn from Sillitoe 1994a)


Lithology

Alteration

Intrusive rocksVolcanic rocks

Acid leaching

Advanced argillicSericitic

AL

++

AS

Intermediate argillicoverprinting K-silicate

IA

K-silicateK

Metresasl

A. Red Mountain

0

WNW2000

1500

1000

500

0

Main porphyry Cu-Momineralisation

1000Metres

SA

K

++ +++

SA SA

+

SASA

SASA SA

ESE

KK

K KK K ++ + +

+ ++

++ + +

+K

K

+

++

SA

+

+K

KK

K

Hydrothermal breccia-hostedCu-Mo mineralisation

Metresasl Reconstructedvolcanicpaleosurface

B. Marte

SW NE5000

4500

4000 Metres 10000

AL

Porphyry Au ore (>1g/t)

IA

A A A AAA A??AL

AL AL ALAL

C. LadolamSSE NNW

+K

AL

-200

-400

200

0

Metres

Metresasl

5000

+

++

+

+ + ++

+++

K

MINIFIE AREA

K KKK+ +K K

+ + +++KK K +

ALAL

KKK

++K +++ +

AL

Epithermal Au ore (>1g/t)

KAPIT AREA

LIENETZ AREA

AL

k:geo\lec\min\98min\mod17\fig4.ppt


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3. Rejuvenation of Hydrothermal Systems

3.1Lifespan of Hydrothermal Activity

There is good geological evidence from certain active systems that they are long-lived, in

the range of 250,000 to 500,000 years. This raises an immediate problem as to the heat

source. A simple energy flux calculation shows that a quite unreasonable amount of

magma is required to support activity for that period of time. For example, for the

Wairakei system in New Zealand, Grindley (1965) calculated that 3750 km3 of magma

would have to be emplaced and cool to provide the current heat output for the lifetime of

the system. Given the size of the upflow zone (a few km2), it is not possible that this

amount of magma could have been emplaced within the crust beneath it.

Similarly Cathles and Erendi (1997) concluded that a single intrusion could fuel

hydrothermal activity for up to 800,000 years, but only under special circumstances andwith large, hot ultramafic sills. This can be compared to recent empirical evidence for

quite short-lived hydrothermal activity based on Ar-Ar dating at Round Mountain (Henry

et al. 1997), possibly as little as 50,000 years. Silberman et al. (1979) interpreted that

hydrothermal activity at Steamboat Springs took place over 3 million years, but

intermittently.

The conclusion must be that the heat output of the system has fluctuated with time. Thus

the steady-state models as used by geothermal reservoir engineers are not applicable in the

longer term. And, given the importance of physico-chemical changes to mineralisation, it

is likely that the periods of change are more important than the steady-state intervals.

3.2Influence of Magmatism on Mineralisation

Having concluded that the heat output of a hydrothermal system varies with time, and must

be rejuvenated, the next step is to look for a probable mechanism. The most obvious is

intermittent dyke injection. Hydrothermal systems generally occupy zones of structural

weakness, where repeated magmatism is to be expected. Estimates of the total heat flux

with time in large volcano-tectonic structures such as the Taupo Volcanic Zone show that

about half of the heat flux is directly due to volcanism, and the remainder to convective

heat transfer through hydrothermal activity (Hochstein et al. 1994; Lawless et al1995).

The heat output of a hydrothermal system can be expected to vary intermittently on two

time cycles: a short-scale cycle in response to hydrothermal eruptions and self-sealing, and

a longer time-scale related to magma intrusion (Figure 5).



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0

50

100

150

200

250

Hydrofracturing and eruptions

100 150 200 250

Episodes of mineralisation

Time, 103 years

Temperature,

C

Major Intrusion

encroachmentCollapse & groundwater300

350 Minor Intrusions

AI31/16.5M50

Figure 5:

Temperatures with time in a periodically rejuvenated hydrothermal system

Both of these processes can be related to mineralisation. The type of regularly-repeated,

very finely-laminated rhythmic veins that we see in epithermal deposits are probably due to

the first process, whereas the larger hydrothermal breccia zones and cross-cutting veins are

due to the second process.

The episodes of magma intrusion can have much greater effects on the hydrothermal

system than do the episodic "normal" hydrothermal eruptions, even if the net energy input

is not very great. It is easy to imagine a hydrothermal system which with time has built up

a convective temperature gradient sitting just on boiling-point for depth. Any input ofenergy to the bottom of the system (in contrast to a pressure release at the top), can trigger

a large release of stored energy as well as the energy actually input to the system by the

magma. Events of this type could in some cases be much more important for gold

mineralisation than the steady-state processes.

There is also a synergistic effect from the magmatic volatiles, in terms of mobilising gold

and base metals and dropping the pH of the fluid.

3.3Evidence For Rejuvenation in Active and Fossil Systems

In a number of both active and fossil systems, there is good evidence that pressure and

temperature conditions have fluctuated, other than through a simple cooling sequence

(Lawless 1988). For active systems, such evidence includes:

Changes in thermal activity, especially the occurrence of very large

hydrothermal eruptions.

The occurrence of alteration mineralogy out of step with the current physico-

chemical conditions (other than a simple cooling sequence).



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Evidence for fluctuating water level in the system, other than simply in

response to erosion.

The occurrence of surficial features such as sinter aprons which are not in

equilibrium with current fluids.

For fossil systems, evidence for rejuvenation includes:

The occurrence of cross-cutting veins with varying mineralogy.

Changes in fluid inclusion data with time, especially where an increase in

salinity and temperature with time is indicated, e.g. Kelian, Creede.

Overprinting by high-temperature mineralogy, i.e. prograde overprinting.

Note that this does not apply to VHMS deposits, in which prograde

overprinting is common and normal.

Evidence for short-lived temperature transients e.g. lack of replacement of

bladed carbonates by silica.

Late-stage dykes cutting across alteration zoning.

Late-stage pebble-dykes or large, poorly cemented bodies of breccia.

Correct application of geological principles to exploration strategy requires that these

situations be recognised. If there has been rejuvenation within a hydrothermal system,mineralisation may be have been associated with only one stage and the other episodes of

veining and alteration may be just a "distraction" from the principal economic target.



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4. Practical Exercise

A At an early stage in the life of a certain hydrothermal system, the water level is atthe surface. Using Table 1, what is the maximum stable temperature at a depth of

1004 m ?

......................... C

B Refer to Table 2. Which of the minerals listed are stable at this temperature ?

.......................................................

C The erosion rate is 1 m per 100 years. How much will the surface drop in 90,000

years ?

......................... m

D What is now the depth of a rock which was previously at the point in (a) above ?

......................... m

E Refer back to Table 1. What is now the maximum temperature of this rock ?

......................... C

F Which of the minerals listed above are no longer stable at this temperature ?

.......................................................

G What new minerals (from Table 2) are now stable at this temperature ?

.......................................................



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Table 1 Boiling Point for Depth Relationship for Pure Water

Temperature

(C)

Pressure

(Bars)

Depth

(m)

100 1.01 0105 1.21 2

110 1.43 4

115 1.69 7

120 1.99 10

125 2.32 14

130 2.70 18

135 3.13 23

140 3.61 28

145 4.16 34

150 4.76 41

155 5.43 48

160 6.18 57165 7.01 66

170 7.92 76

175 8.92 88

180 10.03 101

185 11.23 114

190 12.55 130

195 13.99 147

200 15.55 165

205 17.24 185

210 19.08 207

215 21.06 231

220 23.20 256

225 25.50 284

230 27.98 315

235 30.63 348

240 33.48 383

245 36.52 422

250 39.78 463

255 43.25 507

260 46.94 555

265 50.88 607

270 55.06 662

275 59.50 721280 64.20 785

285 69.19 853

290 74.46 926

295 80.04 1004

300 85.93 1088

305 92.14 1178



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Table 2: Mineral Stability Ranges

Mineral Abbreviation Temperature

(C)

pH Other

Smectite

Illite-Smectite

Illite

Sericite

Halloysite

Kaolinite

Dickite

Chlorite

Chlorite-Smectite

Corrensite

Anhydrite

Gypsum

Marcasite

Alunite

Chalcedony

Cristobalite

Opal

Albite

Adularia

OrthoclaseStibnite

Wairakite

Epidote

Garnet

Biotite

Actinolite

Magnetite

Calcite

Dolomite

Ankerite

SideriteKutnahorite

Sm

I-Sm

I

Ser

Hal

Ka

Dic

Chl

Chl-Sm

Cor

Ah

Gyp

Mar

Alu

Chal

Cris

Op

Ab

Ad

OrStib

Wai

Ep

Ga

Bt

Act

Mt

Cc

Dol

Ank

SidKut

270

280

>300

wide range

low?

low?

low?low-mod.?

near neutral

near neutral

near neutral

near neutral

neutral-acid

neutral-acid

neutral acid

neutral

neutral

near-neutral

acid

acid

acid?

acid

often acid

maybe acid

neutral

neutral

near-neutral

neutral

neutral

near-neutral

near-neutral

near-neutral

neutral-acid

mixing?

boiling?

boiling?

high K

mixing

mixing?

Note: This table is simplified and should not be used for accurate work.


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