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Transcript of M05 Life Cycle
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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
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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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The Life-Cycle of Hydrothermal Systems 2
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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The Life-Cycle of Hydrothermal Systems 3
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.
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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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The Life-Cycle of Hydrothermal Systems 4
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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The Life-Cycle of Hydrothermal Systems 5
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)
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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
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The Life-Cycle of Hydrothermal Systems 6
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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The Life-Cycle of Hydrothermal Systems 7
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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The Life-Cycle of Hydrothermal Systems 8
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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The Life-Cycle of Hydrothermal Systems 9
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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The Life-Cycle of Hydrothermal Systems 10
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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The Life-Cycle of Hydrothermal Systems 11
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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