Seismic pumping a hydrothermal fluid transport mechanism

R . H . S I B S O N , J . M e M . M O O R E & A. H . R A N K I N

SUMMARY

A consequence of the dilatancy/fluid-diffusion mechanism for shallow earthquakes is that considerable volumes of fluid are rapidly redistributed in the crust following seismic faulting. This is borne out by the outpourings of warm groundwater which have been observed along fault traces following some moderate (M5-M7) earthquakes. The quanti-

ties of fluid involved are such that significant hydrothermal mineralisation may result from each seismically induced fluid pulse, and the mechanism provides an explanation for the textures of hydrothermal vein deposits associ- ated with ancient faults, which almost invari- ably indicate that mineralisation was episodic.

HYDROTHERMAL vein deposits, sometimes of economic importance, are often found in the upper, brittle regions of ancient fault zones. I t is a characteristic of these deposits that their textures usually indicate that mineralisation took place episodically. To date, little attention has been paid to the transport mechanism needed to move hydrothermal fluids rapidly and intermittently along fault zones, so that they attain the requisite state of disequilibrium in the upper crust. Current ideas on the source mechanism for shallow earthquakes (Frank i965, Nur I972, Scholz et al. I973) which invoke fluid transport on a large scale in and around fault zones, provide an answer to this problem.

I. Textures of hydrothermal vein deposits

The formation of hydrothermal vein ores involves the transport of material in aqueous solutions from a source region to a zone of deposition. Mineral veins then form by replacement processes, or by crystal growth in rock cavities and open fractures. Many vein systems are associated with faults, and their ore textures generally indicate that mineral deposition was discontinuous (Bateman i95o ). Veins in faults commonly consist of cemented breccias resulting from several periods of movement, in which each episode of shattering was accompanied or followed by growth of gangue and ore mineral crystals. Vein ores in extension fractures which are adjacent to faults, but which themselves have suffered no shear displacement, are texturally distinct, often consisting of laminated aggre- gates of various minerals with crystal growth zoning and wall rock alteration, deposited during a series of mineralising episodes. Fig. I illustrates one such extensional vein in which ten generations of various minerals and several hundreds of distinct crystal growth zones formed. In the course of development, the vein- filling was separated from its wall rocks and split internally by lateral extension several times as new episodes of deposition were accommodated. More complex vein textures may develop from a succession of interspersed shear and extensional displacements on fractures during mineral deposition.

Jl geol. Soc. Lond. vol. x3x , I975, pp. 653-659 , 4 figs. Printed in N. Ireland

654 R. H. Sibson, J. McM. Moore & A. H. Rankin

Two inferences may be drawn. First, that incremental deposition of minerals occurred when pulses, rather than a steady flow of hydrothermal fluid passed through the vein fractures, and second, that the pulses of fluid were associated in time with increments of shear and extensional displacement on fracture systems. Any proposed transport mechansim for hydrothermal fluids in and around fault zones must explain these allied phenomena.

2. The dilatancy/fluid-diffusion model for shallow earthquakes

In recent years, the dilatancy/fluid-diffusion model for energy release in shallow earthquakes has received increasing attention because of its potential for explaining a number of observed earthquake precursors (Scholz et al. I973). In its simplest form the model (Fig. 2) supposes that prior to seismic shear failure along an existing fault, the region for some considerable distance around the focus of the subsequent earthquake dilates in response to rising tectonic shear stress (r) by the opening of extension cracks and fractures normal to the least principal compressive stress (a~). The development of this fracture porosity causes the fluid pressure (p) in the dilatant zone to decrease, inducing a slow inwards migration of fluid from the surrounding crust. At the onset of dilatancy, the drop in fluid pressure causes a rise in the frictional resistance to shear along the fault (r/ = / , ( a ~ - - p ) , where is the coefficient of static friction and a, is the normal stress across the fault). As the migrating fluid fills the cracks, fluid pressure rises again and frictional resist- ance decreases. Seismic failure eventually occurs when the rising shear stress equals the frictional resistance. According to an empirically derived relationship, the precursory dilatant periods for earthquakes of magnitude M5, M6, M 7 and M8 are 0.3, 2, z 3 and 83 years respectively (Whitcomb et al. I973).

The rapid partial relief of shear stress which accompanies earthquake faulting allows the cracks within the dilatant zone to relax (Scholz & Kranz z974), and the fluids they contain must be expelled rapidly upwards in the direction of

Fissure Vein Cross Section

H ~" H X I X?I

W . . ~ : W I ~ . . . . . . . . . : .r" .. . . . . . " ,--~'~-~: I ( A A :'".'....~~~.,:~:~ . . . . ~ : : :, ~: " ..... ~::~::.;, ~ C

L "" ~ " : :~i "" '" " ' " ' " " ~ ~ : ' - : r . " ,~ : ~';" ".'- ~.~" :~' , . . . . . . . ~'~' . .V, ' . " - . . . . . " ~. : .~ , ' . ' . ' . ' . ' . ' , • " ~ " " / ; ~ " ;" "~" " : ~ : " k " "

R "-'-?.:; ~ ' : ' . ~ . '~ '~ ' i ' ' . ; . ' . ' . " ~ . . . . . ~, , . ~_ :~. v..~:: .~. R • " - . . . . . .. ~-::- " " : • . . - . . . .> ..-: ~. : ' ,~: - : : :L.~- 0 0 I" ". ' . ' . ' ." ~. ~---. • ;. -. ~ . . :,. -~ ..,'. "-~:,.

C :.~.~'~::. . : . ; . - . : . ~ ~ • .: g:f,,.~ ~ .:~.~,.:::~?: C K : - ' : : : " f , ~ : y ) ~ : : :" ".." ." " "

~ '~-----~ ~ - . f l l l ~ l l l L - - , ~ - ~ - ~ ' - ~ '2 4, S ~ 8 9 8~65 4 2 3 I I 10

7 7

~ LimoNtic Jasper ~ Crystallk~ ~ Cryptocrystalline Quartz Silica

Scale

cent imetres

Fzo. z. Hydrothermal quartz-jasperoid mineralisation in a fissure vein from Wheal Crowns, Cornwall. I-V--major episodes of extensional opening; z-xo---separate generations of mineral deposition.

Seismic pumping 655

easiest pressure relief (Fig. 3). This latter effect should be especially marked with wrench and normal faults, where a3 is horizontal and the cracks may be expected to lie in vertical planes. Note, though, that the generally lower differential stresses associated with normal faulting (Sibson i974) may reduce the extent to which dilatancy occurs. Upflow from the collapsing dilatant zone must take place through the fault and adjacent fractures, with flow rates decreasing as the source region of the earthquake returns to its pre-dilatant state. It should also be noted here that individual seismic events usually occur as localised shear dislocations Which do not lead to displacements over the entire surface of an existing geological fault. Thus, following any one earthquake, both resheared and unsheared areas of the fault surface may serve as passageways for flow. Irregularities in the fault surface and intersections with minor fractures will act as channels, concentrating flow and increasing the effective permeability of the whole fault zone.

Substantial transient outflows of water along spring-lines in the vicinity of faults are not infrequently observed following moderate shallow earthquakes (Briggs & Troxell i955, Tchalenko i973). The Matsushiro earthquake swarm, which was energetically equivalent to a single event of magnitude M6. 3 (Hagiwara & Iwate 1968), resulted from strike-slip movements about a buried wrench fault c. i o km in length (Kasahara i97o ). It was accompanied by the surface expulsion in one year of about io 1° litres of warm, Na-Ca-C1 brine, saturated with carbon dioxide (Tsuneishi & Nakamura i97o ). This outflow has been interpreted as resulting from the collapse of a focal region with a dilatant strain of about 10 -4 (Nur 1974).

T f-

T_

p.

Fluid f l o w around dilatant zone

Bui ld -up of Onset of Fluid f i l l s EQ Collapse tectonic shear dilatancy dilatant cracks of dilatant stress zone

I I

~ Inflow

~' Outflow

~ T

f F~G. 2. Synoptic diagram of the seismic pumping process (modified from Scholz

et al. 1973).

656 R. H. Sibson, dr. MeM. Moore ~ A. H. Rankin

3. Hydrothermal mineralisation by seismic pumping The dilatancy/fluid-diffusion model provides an explanation for the intermittent flow of hydrothermal fluids in and around fault zones, and suggests that in effect, seismic faulting acts as a pumping mechanism whereby individual earthquakes are capable of moving significant quantities of mineralising fluid rapidly from one crustal environment to another.

If the mechanism is to promote the formation of metalliferous hydrothermal vein deposits, it is of crucial importance that the fault intersects a suitable source region such as a pile of volcanic rocks, sediments undergoing lithification or metamorphism, or a granitic pluton. Potential metallogenetic situations then arise when the dilatant zone coincides with such a source region. When the zone

Fault ~ 01 ~ x,,~(l,.t

I [ " ."" : " : ' : " ." " : i~ l Mineral

I

I

. . . . - - . . .

/ /

/ I

I h. I ~ - ~

I , A

\ \ X

• \ ~ • \

Limit of shear dislocationJ

Fluid migration direction (after fault movement)

~ Shear displacement vectors

• Earthquake focus

% ~ . . , - a ~ - - d ~ : ~ - ~ "

.9=,,,-./

/ . •

"%:--::::

Dilation 'Z one

Fx o . 3. F lu id expuls ion following the collapse o f a d i l a t an t zone a r o u n d a w r e n c h fault . Fo r s implici W the shear dis locat ion is t aken to ex tend over the cross-sectional a rea o f a spher ical d i l a t an t zone.

Seismic pumping 6 5 7

collapses after an earthquake, fluids carrying material dissolved from the source rocks are transported over periods of days to months to different physico-chemical environments which may bring about mineral deposition. Hydrothermal de- posits may develop on the fault itself in both resheared and unsheared regions, and in connected extension fissures including those created by hydraulic fracturing. Fluids ejected from the fault plane enter accessible extension fractures across which there is a lower normal stress.

To demonstrate the viability of the seismic pumping mechansim for the emplace- ment ofhydrothermal deposits, we consider the potential of a single M6 earthquake for transporting quartz in solution from a focal depth of x o km to depositional sites at higher levels. Quartz is chosen because its solubility in water is dependent largely on temperature and pressure, with chemical factors playing only a minor role (Holland 1967). An earthquake fault dimension (L) of IO km and a mean slip of I O-IOO cm are reasonable parameters for an event of this magnitude (Wyss & Brune 1968 ). Assuming, conservatively, that the dilatant volume (V) is a sphere (the symmetrical updoming at Matsushiro suggests that this is reasonable (Hagiwara 1972)) of radius L[2, and that the dilatant strain (AV/V) is IO -s (Scholz et al. I97~3, Nur 1974) , the volume of fluid released after the earthquake (AV ~-, lO -5 . L8/2) is about 5 × IO9 litres.

The solubility of quartz in pure water along a geothermobaric gradient of 35°C and 3oo bars per kilometre is shown in Fig. 4. At a depth of lO km, 5 × lO9 litres of water can potentially dissolve ~.~iot0 g of quartz in the dilatant zone. From the solubility curve it is clear that by the time the expelled fluid cools to 1oo°C during ascent, more than 95% of this quartz will be precipitated. The volume of quartz deposited is equivalent to a sheet vein of approximate dimensions 1 cm × 4o m × IO km, extending along the length of the earthquake fault, but channel flow will tend to concentrate this. A fault with a finite displacement of I km could have produced from i o 8 to to 4 such mineralising fluid pulses.

Q 71

Depth ( k i n )

2 4 6 8 10 12 14 I I "1 i I I I

1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 Temperature (°C)

FIG. 4"

Quartz solubility, Q, (g.SiOs/kg.solution) in pure water along a gcothcrmobaric gradient of 35°C and 3oo bars per kilomctrc (after Holland, x967).

658 R. H. Sibson, J. McM. Moore & A. H. Rankin

4. D i s c u s s i o n

Most previous workers have suggested that faults play a passive role in the emplace- ment of high-level hydrothermal deposits, acting either as permeable conduits for percolating fluids, or in some eases as impermeable barriers which impound migrating waters. Here, we suggest that the mechanics of seismic faulting plays a key role in the intermittent transport of hydrothermal fluids. The calculations in the previous section show that the potential for hydrothermal ore formation is very great, provided suitable material is available for solution in the zone of dilatation, and physico-chemical changes can be effected to bring about the deposition of minerals during ascent. Because their composition is suited to the transport of a wide variety of gangue and ore minerals (Roedder 1972), the saline fluids expelled at Matsushiro are of particular interest in this regard. Interest- ingly, a study of recent metalliferous sedimentation in the Red Sea (Bignell 1975) draws attention to the episodic appearance of metal-bearing brines, and to the association of brine-filled deeps with regions where the median rift is offset along transform faults. As it is these sections of the transform faults which should be seismically active, it seems possible that the intermittent faulting accompanying sea-floor spreading could provide a mechanism for the sporadic discharge of metal-bearing brines. We would also suggest that the quantities of fluid involved in the seismic pumping process are such (lO 1° litres ~ 6 × I o ~ barrels) that the mechanism may substantially assist the migration of hydrocarbon fluids in tectoni- tally active areas.

ACKNOWI.~DO~NTS. We thank Dr N. J. Price, Dr D. J. Shearman, Professor Janet Watson and Professor G. R. Davis for critical reading of the manuscript.

5 - R e f e r e n c e s

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Seismic pumping 659

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Received I6 June I975; revised typescript received 23 July I975.

Richard Hugh Sibson, John McMahon Moore & Andrew Hugh Rankin, Department of Geology, Royal School of Mines, Imperial College, London SW7 2BP