The origin and evolution of base metal mineralising … · The origin and evolution of base metal...
13
PII S0016-7037(01)00579-8 The origin and evolution of base metal mineralising brines and hydrothermal fluids, South Cornwall, UK S. A. GLEESON, 1, * ,² J. J. WILKINSON, 1 F. M. STUART, 2 and D. A. BANKS 3 1 T. H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK 2 Scottish Universities Environmental Research Centre, East Kilbride, Glasgow, G75 OQF, UK 3 School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK (Received May 30, 2000; accepted in revised form February 1, 2001) Abstract—A fluid inclusion geochemical study has been carried out on quartz from post-Variscan quartz 6 carbonate 6 base metal sulphide 6 anhydrite 6 fluorite veins hosted by Palaeozoic basement (Porthleven, Menheniot, Cornwall) and Permo-Triassic sediments (Western Approaches). Data indicate that the base metal mineralising fluids have a similar bulk chemical composition to the saline fluids found in the Permo-Triassic basinal sequence and support the hypothesis that these basins are the source of the mineralising fluids. Cl and Br systematics suggest that the brines were formed either by the evaporation of seawater or a seawater–meteoric water mixture past the point of halite precipitation. The major cation composition (Na, Ca, K, Mg) of the brines is not consistent solely with evaporation processes but may be explained by dolomiti- sation processes, albitisation processes, or both, which are recognised in the basinal sequences. The presence of seawater in the base metal mineralised veins suggests that the first marine incursions (Late Triassic) into the region must act as a lower age limit for the mineralisation. The halogen chemistry of a second, hotter (200°C), more dilute (0 –5 wt.%) fluid identified in fault-hosted E-W trending veins in the Porthleven area, suggests that the chlorinity of these fluids has a magmatic origin. Circulation of these fluids in post-Variscan extensional structures was driven by the local high-heat-producing Cornubian batholith. The local high-heat-producing granites provided fracture permeability and a heat source that heated the base metal mineralising fluids as they entered the horst block and the dilute fluids circulating around the granites. Petrographic evidence suggests that both palaeohydrologic systems were active contemporaneously. However, each flow system was isolated in differently orientated structures, and there is little evidence for fluid mixing. Copyright © 2001 Elsevier Science Ltd 1. INTRODUCTION Contemporary basinal brines, although variable in composi- tion, are typically moderate- to high-salinity Na-Ca-K-Fe-Cl fluids, which can contain significant concentrations of ore- forming metals such as Pb, Zn, and Cu (Hanor, 1994a,b). Several studies have demonstrated that analogous fluids are responsible for metal transport and deposition in many epige- netic, sediment-hosted base metal deposits (e.g., Sverjensky, 1984), and thus sedimentary basins have been suggested as the source of mineralising fluids in these deposits. It is difficult, however, to chemically link a particular deposit with a specific basin because the chemical makeup of mineralising palaeoflu- ids is a complex function of the basin chemistry, subsequent modifications due to water-rock interactions during migration out of the basin, and processes at the site of mineral precipita- tion. However, there is a possibility that characteristic geo- chemical indicators may be used to establish the missing link, tracing the geochemical evolution of a fluid from basin to deposit. The Cornubian Peninsula of southwest England hosts a se- ries of post-Variscan, north-south to northwest-southeast trend- ing Pb-Zn mineralised veins known locally as “crosscourses” (e.g., Jackson et al., 1989). Microthermometric studies have shown that the major element chemistry of these fluids is comparable with that of evolved oil-field brines, and fluids with similar microthermometric properties have been found in frac- tures in the offshore Permo-Triassic basins in the Western Approaches of the English Channel (Gleeson et al., 2000; Fig. 1). These basins have been suggested as potential source re- gions for these fluids (e.g., Scrivener et al., 1994; Gleeson et al., 2000). Southwest England therefore provides an ideal area for an investigation of the geochemical evolution and expulsion of basinal brines from proximal sedimentary basins into a neigh- bouring structural high. Here we report the results of a bulk fluid inclusion study that elucidates the origin and evolution of base metal mineralising brines in such a geologic setting. 2. BACKGROUND GEOLOGY The Upper Carboniferous/Lower Permian Cornubian batho- lith was intruded into a sequence of low-grade (prehnite- pumpellyite facies) meta-sediments in multiple intrusion events that lasted c. 30 Ma, each with its own associated phase of hydrothermal activity (Chesley et al., 1993; Clark et al., 1994). Mineralisation styles in the region vary (see Jackson et al., 1989, for a review), but most of the economic metal production came from swarms of E-W trending veins. Along with the Sn and W oxide bearing E-W veins, there is also a series of E-W polymetallic sulphide bearing veins. Fluid inclusion work on these structures (e.g., Jackson et al., 1982; Scrivener et al., *Author to whom correspondence should be addressed (sarah@earth. leeds.ac.uk). ² Present address: School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK. Pergamon Geochimica et Cosmochimica Acta, Vol. 65, No. 13, pp. 2067–2079, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 1 .00 2067
Transcript of The origin and evolution of base metal mineralising … · The origin and evolution of base metal...
PII S0016-7037(01)00579-8
The origin and evolution of base metal mineralising brines and hydrothermal fluids,South Cornwall, UK
S. A. GLEESON,1,* ,† J. J. WILKINSON,1 F. M. STUART,2 and D. A. BANKS3
1T. H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK2Scottish Universities Environmental Research Centre, East Kilbride, Glasgow, G75 OQF, UK
3School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
(Received May30, 2000;accepted in revised form February1, 2001)
Abstract—A fluid inclusion geochemical study has been carried out on quartz from post-Variscan quartz6carbonate6 base metal sulphide6 anhydrite6 fluorite veins hosted by Palaeozoic basement (Porthleven,Menheniot, Cornwall) and Permo-Triassic sediments (Western Approaches). Data indicate that the base metalmineralising fluids have a similar bulk chemical composition to the saline fluids found in the Permo-Triassicbasinal sequence and support the hypothesis that these basins are the source of the mineralising fluids.
Cl and Br systematics suggest that the brines were formed either by the evaporation of seawater or aseawater–meteoric water mixture past the point of halite precipitation. The major cation composition (Na, Ca,K, Mg) of the brines is not consistent solely with evaporation processes but may be explained by dolomiti-sation processes, albitisation processes, or both, which are recognised in the basinal sequences. The presenceof seawater in the base metal mineralised veins suggests that the first marine incursions (Late Triassic) intothe region must act as a lower age limit for the mineralisation.
The halogen chemistry of a second, hotter (200°C), more dilute (0–5 wt.%) fluid identified in fault-hostedE-W trending veins in the Porthleven area, suggests that the chlorinity of these fluids has a magmatic origin.Circulation of these fluids in post-Variscan extensional structures was driven by the local high-heat-producingCornubian batholith.
The local high-heat-producing granites provided fracture permeability and a heat source that heated the basemetal mineralising fluids as they entered the horst block and the dilute fluids circulating around the granites.Petrographic evidence suggests that both palaeohydrologic systems were active contemporaneously. However,each flow system was isolated in differently orientated structures, and there is little evidence for fluidmixing. Copyright © 2001 Elsevier Science Ltd
1. INTRODUCTION
Contemporary basinal brines, although variable in composi-tion, are typically moderate- to high-salinity Na-Ca-K-Fe-Clfluids, which can contain significant concentrations of ore-forming metals such as Pb, Zn, and Cu (Hanor, 1994a,b).Several studies have demonstrated that analogous fluids areresponsible for metal transport and deposition in many epige-netic, sediment-hosted base metal deposits (e.g., Sverjensky,1984), and thus sedimentary basins have been suggested as thesource of mineralising fluids in these deposits. It is difficult,however, to chemically link a particular deposit with a specificbasin because the chemical makeup of mineralising palaeoflu-ids is a complex function of the basin chemistry, subsequentmodifications due to water-rock interactions during migrationout of the basin, and processes at the site of mineral precipita-tion. However, there is a possibility that characteristic geo-chemical indicators may be used to establish the missing link,tracing the geochemical evolution of a fluid from basin todeposit.
The Cornubian Peninsula of southwest England hosts a se-ries of post-Variscan, north-south to northwest-southeast trend-ing Pb-Zn mineralised veins known locally as “crosscourses”
(e.g., Jackson et al., 1989). Microthermometric studies haveshown that the major element chemistry of these fluids iscomparable with that of evolved oil-field brines, and fluids withsimilar microthermometric properties have been found in frac-tures in the offshore Permo-Triassic basins in the WesternApproaches of the English Channel (Gleeson et al., 2000; Fig.1). These basins have been suggested as potential source re-gions for these fluids (e.g., Scrivener et al., 1994; Gleeson et al.,2000). Southwest England therefore provides an ideal area foran investigation of the geochemical evolution and expulsion ofbasinal brines from proximal sedimentary basins into a neigh-bouring structural high. Here we report the results of a bulkfluid inclusion study that elucidates the origin and evolution ofbase metal mineralising brines in such a geologic setting.
2. BACKGROUND GEOLOGY
The Upper Carboniferous/Lower Permian Cornubian batho-lith was intruded into a sequence of low-grade (prehnite-pumpellyite facies)meta-sediments in multiple intrusion eventsthat lasted c. 30 Ma, each with its own associated phase ofhydrothermal activity (Chesley et al., 1993; Clark et al., 1994).Mineralisation styles in the region vary (see Jackson et al.,1989, for a review), but most of the economic metal productioncame from swarms of E-W trending veins. Along with the Snand W oxide bearing E-W veins, there is also a series of E-Wpolymetallic sulphide bearing veins. Fluid inclusion work onthese structures (e.g., Jackson et al., 1982; Scrivener et al.,
*Author to whom correspondence should be addressed ([email protected]).† Present address:School of Earth Sciences, University of Leeds,Leeds LS2 9JT, UK.
Pergamon
Geochimica et Cosmochimica Acta, Vol. 65, No. 13, pp. 2067–2079, 2001Copyright © 2001 Elsevier Science LtdPrinted in the USA. All rights reserved
0016-7037/01 $20.001 .00
2067
1986, 1987) suggests the mineralising fluids were moderatelyhot (200–280°C) and variably saline (3–15wt.%).
The Cornubian Peninsula is also host to the north-south tonorthwest-southeast trending crosscourses. These veins cross-cut the main granite E-W lodes and postdate the economicgranite mineralisation (e.g., Dines, 1956). Early workers be-lieved that the crosscourses are genetically related to the gran-ite-related mineralisation (Dines, 1956). Subsequent fluid in-clusion work suggested that the base metal mineralising fluidswere low-temperature, high-salinity fluids similar to those re-corded in the northern Pennine orefields and from the Missis-sippi Valley Type deposits of the central United States(Sawkins, 1966; Alderton, 1978). Shepherd and Scrivener(1987) suggested that the fluids were basinal brines that had
been expelled from basins to the north and south of the Cor-nubian Peninsula (e.g., Fig. 1 and Gleeson et al., 2000) confirmthe presence of similar fluids in the basins to the south of thepeninsula.
3. STUDY AREA AND SAMPLE PETROGRAPHY
Porthleven in south Cornwall (Fig. 1) has several examplesof base metal mineralised crosscourses exposed on a coastalsection, and these veins are the focus of this study. A smallnumber of samples were also collected from Menheniot (Fig. 1)to facilitate a comparative study. Veins in Lower Palaeozoicrocks and Permo-Triassic sediments in one offshore well, lo-cated'70 km south of the Cornubian Peninsula were sampled(Fig. 1), although only one fracture (1697 m) in the Permo-
Fig. 1. Sedimentary basins in SW England and location of off-shore well 87/12-1A and the Porthleven and Menheniotstudy areas.
2068 S. A. Gleeson et al.
Triassic sediments yielded enough quartz for this study. Fi-nally, several nonsulphide mineralised E-W veins from Porth-leven also were sampled. A thorough description of the fieldrelationships, petrography, and the results of a microthermo-metric study on these veins can be found in Gleeson et al.(2000). For the purpose of this article, a brief review of the veinmineralogy and results of the inclusion study from only quartzin these veins is presented below (see also Fig. 2; See Table 1for a summary of the vein types found in this study).
3.1. Porthleven Crosscourses
The mineralogy of the base metal mineralised cross-courses in Porthleven is dominated by the alternate precip-
itation of quartz and siderite. Quartz crystals can be large(up to 10 cm) and are strongly growth zoned. Occasionally,thin bands of grey to brown-yellow chalcedonic silica andsmall ('100 mm) inclusions of anhydrite also have beenidentified in these veins. Sulphide mineralisation consists ofwall-rock hosted arsenopyrite, early chalcopyrite, andsphalerite, and the dominant ore mineral, galena. Pyriteoccurs rarely in crosscutting fractures and as fine-grainedrims on siderite. Host-rock alteration is minor and is domi-nated by silicification. Microthermometric studies of theseveins suggested that the base metal mineralising fluids werelow-temperature ('130°C), 24 to 27 wt.% NaCl1CaCl2bearing brines (Gleeson et al., 2000).
Fig. 2. Summary Th- Tmi plot of fluid inclusion data from all vein sets showing broadly two fluid types: (i) a saline,low-temperature brine (base metal mineralised crosscourses and in fractures in and under the Permo-Triassic basins to thesouth) and (ii) a moderately hot, dilute fluid found in E-W trending quartz veins.
Table 1. Summary of the paragenesis, orientation, and fluid characteristics of the vein types encountered in this study.
Vein type Vein name Orientation Paragenesis Fluid salinityHomogenisation
Temperature
Late-post Variscana Polymetallic lodes E-W Quartz1 Polymetallic sulphides 3–15 wt% NaCl equiv. 200–280°CPost-Variscan E-W veins E-W Quartz6 pyrite 0–5 wt% NaCl equiv. ModalTh ' 2208CPost-Variscan Crosscourses N-S, NNW-
SSEQuartz6 siderite6 galena6
sphalerite6 pyrite 6chalcopyrite
Approx. 26 wt% NaCl1CaCl2 equiv.
Modal Th ' 1308C
Post-Variscan Intrabasin vein Unknown Quartz, anhydrite, barite 20–27 wt% NaCl1CaCl2 equiv.
.110°C
a Data taken from the summary of Jackson et al. (1989).
2069Origin and evolution of mineralising brines
Tab
le2.
Sum
mar
yof
the
bulk
fluid
incl
usio
nan
alys
esan
dth
eca
lcul
ated
mol
aliti
esfo
rth
em
ajor
anio
nan
dca
tions
.
Vei
nty
peS
ampl
eno
.M
iner
alS
alin
itycC
a/N
aK
/Na
Mg/
Na
Cl/B
rC
lmol
alN
am
olal
CaC
l2
mol
alK
mol
alM
gm
olal
Br
mol
al
Por
thle
ven
Xco
urse
SP
9201
7/1
Qua
rtz
26.8
1.9E
201
5.3E
202
4.7E
203
3.3E
102
a6.
9E1
004.
7E1
009.
2E2
012.
5E2
011.
54E2
022.
1E2
02P
orth
leve
nX
cour
seS
P92
017/
2Q
uart
z26
.92.
3E2
015.
8E2
022.
8E2
034.
1E1
02a
7.0E
100
5.6E
100
1.1E
100
2.7E
201
8.60
E203
1.7E
202
Por
thle
ven
Xco
urse
SP
9201
7/3
Qua
rtz
27.1
1.5E
201
6.0E
202
3.7E
203
3.9E
102
a6.
8E1
005.
0E1
007.
3E2
013.
0E2
011.
39E2
021.
8E2
02P
orth
leve
nX
cour
seS
P92
017/
4Q
uart
z27
.01.
2E2
016.
3E2
021.
4E2
021.
9E1
02a
6.7E
100
5.0E
100
6.18
E201
3.2E
201
5.44
E202
3.5E
202
Por
thle
ven
Xco
urse
SP
9201
8/1
Qua
rtz
26.6
9.6E
202
6.5E
202
4.6E
203
4.4E
102
a6.
5E1
005.
1E1
004.
95E2
013.
4E2
011.
86E2
021.
5E2
02P
orth
leve
nX
cour
seS
P92
018/
2Q
uart
z26
.91.
8E2
014.
8E2
023.
6E2
034.
4E1
02a
6.8E
100
4.8E
100
8.79
E201
2.3E
201
1.24
E202
1.5E
202
Por
thle
ven
Xco
urse
SP
9201
8/3
Qua
rtz
27.4
1.9E
201
6.5E
202
5.2E
102
a7.
0E1
004.
8E1
009.
31E2
013.
2E2
010.
00E1
001.
3E2
02P
orth
leve
nX
cour
seS
P92
018/
4Q
uart
z26
.82.
2E2
016.
0E2
024.
4E1
02a
7.0E
100
4.6E
100
1.0E
100
2.8E
201
0.00
E100
1.6E
202
Por
thle
ven
Xco
urse
SP
9201
8/5
Qua
rtz
27.1
1.6E
201
6.4E
202
3.8E
203
6.7E
102
a6.
9E1
004.
9E1
008.
0E2
013.
2E2
011.
34E2
021.
0E2
02P
orth
leve
nX
cour
seJ1
31/1
Qua
rtz
26.9
9.2E2
025.
5E2
023.
0E2
034.
7E1
02a
6.6E
100
5.3E
100
4.9E
201
2.9E
201
1.28
E202
1.4E
202
Por
thle
ven
Xco
urse
SP
93L2
Qua
rtz
26.9
9.2E2
026.
3E2
022.
8E2
034.
3E1
02a
6.5E
100
5.2E
100
4.9E
201
3.3E
201
1.17
E202
8.4E
202
Por
thle
ven
Xco
urse
VL1
Qua
rtz
26.9
7.9E2
026.
3E2
024.
7E2
037.
8E1
02a
6.5E
100
5.2E
100
4.2E
201
3.3E
201
2.06
E202
1.7E
203
Por
thle
ven
E-W
SP
9200
1Q
uart
z6.
41.
6E201
6.4E
202
4.5E
203
1.9E
103
a1.
2E1
008.
7E2
011.
4E2
016.
0E2
024.
0E2
036.
5E2
04P
orth
leve
nE
-WS
P92
006/
1Q
uart
z0.
71.
4E201
1.0E
201
1.0E
201
1.0E
202
Por
thle
ven
E-W
SP
9200
6/2
Qua
rtz
1.1
1.2E2
015.
2E2
025.
7E2
033.
0E1
03a
2.0E
201
2.0E
201
2.0E
202
7.0E
203
8.0E
204
6.0E
205
Por
thle
ven
E-W
SP
9400
3Q
uart
z1.
16.
2E202
6.0E
202
6.3E
203
2.4E
103
a2.
0E2
012.
0E2
011.
0E2
029.
0E2
039.
0E2
046.
0E2
05P
orth
leve
nE
-WS
P93
003
Qua
rtz
1.1
7.1E2
025.
6E2
022.
1E2
031.
9E1
03a
2.0E
201
2.0E
201
1.0E
202
8.0E
203
3.0E
204
Por
thle
ven
E-W
SP
9300
4Q
uart
z0.
44.
5E202
6.5E
202
3.0E
203
1.0E
201
1.0E
201
2.0E
203
3.0E
203
2.0E
204
Men
heni
otX
cour
seS
M92
001
Qua
rtz
26.4
2.1E2
015.
2E2
026.
0E2
035.
3E1
02a
6.7E
100
4.6E
100
9.4E
201
2.4E
201
1.85
E202
1.3E
202
Men
heni
otX
cour
seS
M92
004
Qua
rtz
26.4
1.4E2
015.
9E2
026.
5E2
036.
4E1
004.
8E1
006.
5E2
012.
9E2
012.
34E2
02M
enhe
niot
Xco
urse
SM
9200
5Q
uart
z26
.42.
1E201
7.6E
202
5.8E
203
5.7E
102
a6.
6E1
004.
5E1
009.
4E2
013.
4E2
011.
69E2
021.
2E2
02M
enhe
niot
Xco
urse
SM
9200
6Q
uart
z26
.68.
8E202
5.2E
202
3.3E
203
6.5E
100
5.3E
100
4.6E
201
2.8E
201
1.17
E202
87/1
2-1A
1697
mQ
uart
z27
.52.
9E202
4.0E
202
9.4E
203
3.6E
102
b6.
2E1
005.
9E1
001.
7E2
012.
4E2
014.
48E2
021.
7E2
02S
eaw
ater4
2.2E
202
2.2E
202
1.1E
201
6.6E
102
6.0E
201
5.0E
201
1.0E
202
1.0E
202
5.5E
202
8.6E
204
Cat
ion
anal
yses
byth
ede
crep
itatio
n-lin
ked,
indu
ctiv
ely
coup
led
plas
ma
atom
icem
issi
onsp
ectr
omet
rym
etho
d.A
nion
anal
yses
bya
crus
h-le
ach
ICP
andbN
eutr
on-ir
radi
ated
nobl
ega
sm
ass
spec
trom
etry
.c
Sal
inity
for
Ca-
rich
fluid
sis
wt%
.N
aCl1
CaC
l 2eq
uiva
lent
asca
lcul
ated
from
the
phas
edi
agra
mof
Oak
eset
al.
(199
1).
For
NaC
l-dom
inat
edflu
ids,
salin
ityis
wt%
NaC
lequ
ival
ent
calc
ulat
edus
ing
the
equa
tion
ofB
odna
r(1
993)
.d
Sea
wat
eran
alys
esar
efr
omth
eco
mpi
latio
nof
Fon
tes
&M
atra
y(1
993a
).
2070 S. A. Gleeson et al.
3.2. Menheniot Crosscourses
The Menheniot crosscourse veins are characterised by mul-tiple phases of prismatic quartz and chalcedony with texturesvery similar to the Porthleven crosscourses. Yellow and greenfluorite occurs late in the paragenesis and is associated withgalena, chalcopyrite and minor sphalerite. Large fluorite cubes(1–4 cm) are commonly found in vugs. Pyrite and minormarcasite are occasionally present in late veinlets, often cross-cutting quartz and fluorite veins. Gleeson et al. (2000) reportedfluid inclusions in quartz with similar salinities to the Porth-leven crosscourses (20–27 wt.% NaCl1 CaCl2 equiv.) but atgenerally lower temperatures ('100°C).
3.3. Porthleven E-W Veins
At Porthleven, a series of E-W trending normal faults that arequartz mineralised are associated with late- to post-Variscandeformation. As with the crosscourse veins, the E-W structuresare dominated by multiple phases of banded quartz growth.Pyrite is associated with the quartz, particularly near the wallsof the vein, and there is rare, paragenetically late galenapresent. Microthermometric studies indicate that these veinswere formed from moderately hot (200°C), dilute (0–5 wt.%)fluids (Gleeson et al., 2000).
3.4. Intrabasin Vein
In well 87/12-1A, at approximately 1697 m below sea level,the Permo-Triassic sandstone is fractured and contains a min-eralised vug lined with euhedral quartz and bladed anhydritewith minor barite. Fluid inclusions in quartz suggest these
fluids were 20 to 27 wt.% NaCl1 CaCl2, low temperature(,110°C) brines (Gleeson et al., 2000).
4. ANALYTICAL TECHNIQUES
Slices from (1–2 cm) individual growth zones in quartz veinswere crushed using a tungsten carbide block and pestle andsieved to obtain a 1- to 2-mm fraction. These chips were thenboiled in 50% HNO3, rinsed in water, hand picked to removeimpurities, and electrolytically cleaned for 1 week followingthe procedure outlined by Roedder (1958). Bulk fluid inclusionanalyses were carried out on the cleaned quartz grains bydecrepitation-linked, inductively coupled plasma atomic emis-sion spectrometry (D-ICP), crush-leach ion chromatography(CL-IC), and neutron irradiation noble gas mass spectrometry(NI-NGMS). Because of space confines, detailed comparisonsbetween these different techniques found in Gleeson (1996, inpreparation) cannot be included here. For the purpose of thiscontribution, only data for the major cations, Mg, Cl, and Brwill be presented.
D-ICP analyses were carried out by placing 0.3 g of eachsample in a test tube connected to a Fisons ARL 3580Bsimultaneous/sequential ICP-AES and heated rapidly to 750°Cin an electric furnace. Standard solutions were analysed bynebulisation at the end of each run to calibrate the sensitivity ofthe system in the manner described by Thompson and Walsh(1983). Three to five replicate splits of each sample wereanalysed to assess precision. 90% of the samples analysed forthe major cations yielded residual standard errors of less than10%. Although some of the variations within samples may havebeen due to contamination of solid phases included in the
Fig. 3. Cl and Br systematics of all quartz veins. Absolute concentrations are calculated from bulk fluid analyses andmicrothermometric data using the technique described by Shepherd et al. (1985).
2071Origin and evolution of mineralising brines
quartz, some of the samples seemed to be heterogeneous innature on the scale of sampling, leading to larger errors (Glee-son, 1996). The data were processed using correlated back-ground correction that resulted in a lower detection limit thanwas obtained in previous analyses (Coles et al., 1995).
Inclusion fluids were extracted from the quartz samples bycrushing 2 g of sample in 3 mL of doubly distilled deionisedwater with an agate mortar and pestle. The leachate was cen-trifuged and analysed for halogens by ion chromatography atthe Natural History Museum and the University of Leeds. Ofthe 21 samples analysed using this technique, only 3 did notyield an acceptable charge balance (1.06 0.3; Shepherd et al.,1985) and these data are not included in Table 2.
A small number of irradiated quartz samples were analysedby the NI-NGMS technique using a procedure modified slightlyfrom that of Burgess and Parsons (1994). In six cases, repeatcrushes were performed to determine how sample inhomoge-neity might affect the measured ratios. Blanks were measuredat intervals between crushes by performing the crushing pro-cedure without samples.
5. MAJOR ELEMENT ANALYSES
Microthermometry has broadly identified two fluid types(Fig. 2): a saline Ca-bearing brine (in Porthleven and Menhe-niot crosscourses and in all the offshore veins) and a dilute fluid(in the E-W veins at Porthleven). Bulk fluid analyses were onlycarried out on quartz samples that were dominated by one fluid
inclusion population. Absolute concentrations of solutes areobtained by combining bulk fluid inclusion data and microther-mometric measurements to give molal concentrations (Shep-herd et al., 1985) and then converted to mg/L using values forbrine densities calculated by FLINCOR (Brown, 1989). Datafor all vein types are presented in Table 2.
5.1. Cl and Br in Mineralising Brines and Dilute Fluids
Much of the discussion of the origin of waters and solutes insedimentary basins relies on the use of solutes that behave asconservative tracers (i.e., components with initial concentra-tions that may only be modified by mixing of solutions).Chlorine, and particularly bromine, are often regarded as con-servative elements because they do not tend to partition intomineral phases readily under basinal conditions and thus mayreflect the origins of solutes in formation fluids (e.g., Ritten-house, 1967; Carpenter, 1978; Fontes and Matray, 1993a,b;Kesler et al., 1996). In particular, chlorine and bromine ratioscan be used to trace halogen contributions with distinct sourcessuch as seawater, evaporite interacted fluids, and magmaticfluids (e.g., Bo¨hlke and Irwin, 1992).
Absolute Br and Cl data for all samples analysed are pre-sented in Figure 3, together with the composition of seawaterand the position of the seawater evaporation trajectory (SET).Data from the Porthleven and Menheniot crosscourses and thered bed hosted quartz vein plot together close to the SET
Fig. 4. Plot of Cl/Br ratios of dilute fluids and brines in comparison with volcanic fumarole condensates and the St Austellgranite in Cornwall (after Bo¨hlke and Irwin, 1992).
2072 S. A. Gleeson et al.
beyond the point of halite precipitation. Although the bulk ofthese data do not sit directly on the SET, they are within 2% ofevaporated seawater values, which is considered to be within anacceptable interval of uncertainty (Walter et al., 1990). TheBr-rich nature of these fluids rules out the dissolution ofevaporites as a source of the fluid salinity and suggests thatthese fluids originated as seawater that was subaerially evapo-rated (e.g., Carpentar, 1978; Hanor, 1994a,b). Halite precipita-tion occurs when a fluid has been concentrated to;11 timesseawater. The most bromide-rich sample analysed in our studysuggests that some of the brines evolved by evaporation tonearly 40 times seawater concentration (i.e., nearly to the pointof K-Mg salt precipitation). It should be noted, however, ifseawater is diluted by a meteoric water that contains no halo-gens and sufficient evaporation occurs subsequently such thatthe salinity of the fluid exceeds seawater values, the resultantfluids will be indistinguishable from evaporated seawater (e.g.,Knauth and Beeunas, 1986). It is not possible to discriminatebetween these two scenarios on the basis of halogen analyses.Isotopic data suggest that there is a component of meteoricwater in the Menheniot mineralising fluids, but this componentcannot be identified in the Porthleven or intrabasin brines(Gleeson, 1996; Gleeson et al., 1999). Halogen systematicstherefore suggest that all the brines studied originated either asseawater or as a composite seawater-meteoric water that wasevaporated past the point of halite precipitation. These data alsosuggest that the brines were hydrologically isolated since for-mation and did not undergo significant mixing with either freshor seawater before precipitation of the veins.
In the dilute E-W fluids, the Cl and Br values sit to the left
of the SET on Figure 3. This, combined with the low salinities(0–5 wt.%) and chemical and isotopic characteristics (Gleeson,1996; Wilkinson et al., 1995) of these fluids suggests that it isunlikely that the chlorinity of these fluids has been acquired byhalite dissolution. Unlike the brines, the Cl/Br ratios of thesefluids have values that are more typical of halogens derivedfrom magmatic systems (Fig. 4). As discussed above, most ofthe economic oxide mineralisation in Cornwall was hosted byE-W trending vein systems and spans the age of the intrusion ofthe Cornubian batholith (Clark et al., 1993). Quartz veins atPorthleven are likewise hosted by E-W trending, high-angle,extensional faults, but the chemical and isotopic composition ofthe fluids suggest these fluids are meteoric waters that circu-lated around the high-heat-producing granites (Gleeson, 1996;Gleeson et al., 2000) and postdate the magmatism. It thereforeis improbable although not impossible that mixing between amagmatic fluid and meteoric fluid has taken place. Anotherpotential source for the chlorine and bromine in these fluids isinteraction of the hot waters with Cl-bearing magmatic mineralphases such as biotite (e.g., Edmunds et al., 1985). At themoment we cannot resolve between these two hypotheses.
5.2. Source of Major Cations in the Brines
The brines have molar Na/Br ratios of 515 or less and Cl/Brratios of 775 or less (Fig. 5). The bulk of the data broadly plotin the region expected for the composition of seawater that hasevaporated beyond the point of halite precipitation. However,the data are offset from this trend, demonstrating a Na depletionrelative to seawater. The crosscourse fluids have molar K/Br
Fig. 5. Na-Cl-Br systematics for Porthleven, Menheniot and intrabasin brines. Data points on Y-axis have no Naanalyses.
2073Origin and evolution of mineralising brines
ratios ranging from 5 to 36 (Fig. 6), which span the seawaterK/Br ratio. With the exception of two data, the K/Cl ratio ofthese fluids is approximately constant and bears no relationshipto the SET. Ca concentrations are higher than seawater for allbrine samples, including the intrabasin brine (Table 2). Mgconcentrations are well below seawater values, and there is nosystematic relationship between the two elements (Fig. 8).
Halogen systematics suggest that the crosscourse mineralis-ing fluids should sit on or close to the SET at halite precipita-tion (Fig. 4). The offset from this trend indicates that the fluidshave lost Na by exchange reactions such as albitisation ofplagioclase (e.g., Kesler et al., 1996). However, in the case ofCa and Na (Fig. 7), there is a marked negative correlation forboth for the Porthleven and Menheniot crosscourses. In manyformation waters, as the chlorinity increases there is an asso-ciated increase in the concentration of monovalent and divalentcations in solution. Pore fluid equilibrium with respect to spe-cific aluminosilicate mineral assemblages fixes the activityratios of the cationic species resulting in the activity of divalentions being twice that of monovalent ions (Hanor, 1994a,b).This suggests that as salinity increases, the ratio of Ca to Na insaline fluids also should increase (Yardley, 1997). However,once halite precipitates, Na concentrations decrease in the porefluid although salinity continues to increase. The inverse linearrelationships between Na and Ca from the Porthleven andMenheniot crosscourses suggest that the Ca/Na ratios of thesefluids were buffered in some way. The Menheniot fluids alsoappear to have a slope that could be consistent with a processsuch as albitisation (2:1 exchange of Na for Ca). Although no
textural evidence has been found in this study to suggest thealteration of plagioclase to albite, albite is present in nearly allthe Permo-Triassic red bed sequences sampled (Gleeson et al.,2000). The Palaeozoic Porthscatho Formation that may liealong the fluid expulsion flow path contains up to 36% feldspar(Floyd and Leveridge, 1987), predominantly albite-oligoclaseand K feldspar. Much of the albite is likely to be the result oflow-grade Variscan metamorphism; whether any subsequentalbitisation due to ingress of the mineralising fluids has oc-curred is unknown. The slope of the Porthleven data is notconsistent with such an exchange reaction and is problematic.Ca has been added to these fluids and Na has been lost, but theslope of the linear relationship between these two elementssuggests that simple mineral equilibria are not appropriate here.
The intrabasin sample has lower Ca and higher Na than thebase metal mineralising brines (Fig. 7), suggesting that, if theseare equivalent to the mineralising fluids, the addition of Ca tothe crosscourse fluids occurred at a relatively late stage of basinevolution or during expulsion of the fluids out of the basin.
The Cl-K-Br diagram is also difficult to interpret (Fig. 6).When seawater evaporates, the K/Br ratio remains constantuntil the precipitation of Mg and K salts. However, the K/Brratio appears to decrease throughout the period of halite pre-cipitation, from values approximately three times seawatervalues to values approximately half those of seawater. Thesamples that have K/Br values higher than those of seawatermust have had K added subsequent to evaporation. In manyPermo-Triassic basins, the rocks contain up to 5% K by weight(e.g., Burley, 1984; Jeans et al., 1994). The sandstone se-
Fig. 6. K-Cl-Br systematics for Porthleven, Menheniot and intrabasin brines. Most samples clearly have enriched Krelative to seawater contents.
2074 S. A. Gleeson et al.
quences sampled in this study contain K-feldspar that is beingreplaced by illite, suggesting that the release of K to porewatersby illitisation of feldspar is a likely mechanism for K enrich-ment. The Palaeozoic Porthscatho Formation also contains upto 5 wt.% K2O (Floyd and Leveridge, 1987), indicating that thebrines could have acquired K during interaction with K-bearingrocks during flow out of the basin.
The fluids encountered in this study have low concentrationsof Mg relative to seawater and must have lost a significantamount of Mg during water-rock interaction (Fig. 8). Althoughprecipitation of Mg-smectite or chlorite in the basinal sequenceis a possible mechanism for Mg depletion, such phases werenot identified in the petrologic study (Gleeson, 1996). Precip-itation of primary dolomite could explain this Mg loss. Thisobservation is supported by the presence of dolomitic cementsand fractures in the wells sampled (Gleeson et al., 2000).Primary dolomite precipitation, however, will also result in aremoval of Ca and, as discussed above, the brines are generallyenriched in Ca. The most likely mechanism for removal of Mgand addition of Ca is the dolomitisation of limestone; however,there is no evidence of any dolomitisation occurring in thesampled wells. It may be that the precipitation of primarydolomite may be responsible for Mg loss, but the associated Caloss is masked by other water-rock reactions.
6. DISCUSSION
6.1. Environment of Brine Generation
The fluid inclusion chemistry clearly shows the evaporiticorigin of the saline brines responsible for low-temperature base
metal mineralisation in southwest England. It is important,therefore, to examine when a suitable palaeoenvironment mayhave existed to produce the observed chemical trends to placeconstraints on environments of genesis of mineralising forma-tion waters. Subaerial evaporitic basins are hydrologicallyclosed or restricted (i.e., the rate of evaporation must exceedinflow), a condition that is met particularly in arid to semiaridclimates (Eugster, 1980). The tectonics and environment ofdeposition of the Permo-Triassic sequences in the WesternApproaches is suitable for the formation of evaporites (Evans,1990).
Most authors consider that the depositional environmentfrom the late Carboniferous to the Middle Triassic in SouthernEngland to be mainly continental-type sedimentation with amarine influenced evaporitic playa-type facies developing onlyin the Late Triassic (Ziegler, 1990). However, Evans (1990)stated that the basal parts of the Late Permian to Early TriassicAylesbeare Group represent continental alluvial fans, but thefiner portions of the sequence represent a slower rate of depo-sition that may reflect “short-lived, periodic marine incur-sions.” There is no supporting reference for this interpretation,and published and oil company logs, lithologies, and palaeo-environmental reconstructions suggest that the environment offormation of the Aylesbeare Group to be wholly nonmarine. Inthe absence of any evidence to the contrary, we subscribe to themajority view that the first marine incursions took place in theLate Triassic into an evaporitic coastal playa-type environment.
Coastal playa environments can be subject to storm floodingby seawater as well as meteoric inflow waters and are oftenassociated with the recycling of previously deposited salt bod-
Fig. 7. A plot of Ca against Na concentrations. Absolute data are calculated from bulk inclusion data, suggesting that theclear negative correlation seen in the Porthleven samples is not a function of closure due to calculations of Ca/Na ratios formicrothermometric data. Note that the intrabasin vein, although on the same trend, has significantly less Ca, suggesting thatCa acquisition may be on the flow path out of the basin.
2075Origin and evolution of mineralising brines
ies by dissolution (e.g., Pierre et al., 1984). Br and Cl relation-ships (Fig. 3) show that evaporite recycling is unlikely to be thesource of the salinity of the brines, but the evaporation of amixture of groundwaters with seawater or the evaporation ofmarine waters alone is consistent with the depositional envi-ronment.
6.2. Constraints on Timing of Vein Formation
This study has not attempted to directly date any of themineralised veins sampled. However, the marine or marine-meteoric origin of the crosscourse mineralising fluids providesa constraint on the timing of mineralisation. Radiometric agesof the crosscourse mineralisation are contentious. Scrivener etal. (1994) showed that Rb/Sr dating of fluid inclusions yields a236 6 3 Ma age for the Tamar Valley crosscourses; theseresearchers further suggested that these structures are MiddleTriassic in age and that the fluids are sourced from the domi-nantly Permian nonmarine Aylesbeare Group. The presence ofseawater in these veins indicates that the Porthleven and Men-heniot crosscourse fluids cannot have been sourced from sucha palaeoenvironment and must postdate the Late Triassic.These data suggest, therefore, that the Porthleven and Menhe-niot veins are younger than the Tamar Valley veins and thatthere are multiple mineralising events.
Although the barren E-W veins sampled in this study are inthe same orientation as some of the mainstage granite related
mineralisation, the composition of the fluids and the genesis ofthese veins is clearly very different. It is possible that thesefluids are genetically related to the fluids responsible for theformation of the polymetallic veins in Cornubia but mustrepresent a much younger “waning” of the system. The pres-ence of crosscourse fluids in secondary inclusion trails inPorthleven E-W veins, and vice versa, indicates that there wasalternating movement of E-W structures and N-S crosscourses,suggesting that there was some movement on the E-W struc-tures occurring into the Late Triassic (see Fig. 9). This modelis strongly supported by slickensides on the crosscourses,which are parallel to the intersection lineation defined by thecrosscourse and the E-W trending fault planes (Wilkinson,1990).
6.2. The Evolution of Two Fluid Systems
Modelling of hydrothermal circulation within and aroundhigh-heat-producing granites by Fehn et al. (1978) suggestedthat through time granites lose their heat relatively slowly byconduction but can lose heat much more rapidly by convectivecirculation, in response to increases in permeability. In otherwords, the release of heat from the granite is episodic andrelated to specific tectonic events. Fehn et al. (1978) suggestthat in Cornwall such a mechanism may explain the differentdates for uranium mineralisation, all significantly younger thanthe batholith, as given in Darnley et al. (1965). It is possible
Fig. 8. Mg-Cl-Br systematics show that all samples have Mg concentrations much less than seawater.
2076 S. A. Gleeson et al.
then that the release of heat from the granites also may haveaffected the base metal mineralising brines that were expelledalong structures into the Cornubian massif during Triassicextension. A role for high-heat-producing granites in someareas of base metal mineralisation has previously been sug-gested to explain the relatively high temperatures of the min-eralising fluids at shallow depths (e.g., Solomon and Heinrich,1992; Spirakis and Heyl, 1996). Our data suggest this is cer-tainly the case for low-temperature base metal mineralisation inCornwall.
In Cornwall we have good evidence of the presence of twoseparate palaeohydrologic systems in different orientated struc-tures that must have been active at the roughly the same time.Meteoric water was convecting around the high-heat-producingCornubian batholith in E-W fractures associated with the col-lapse of the Variscan orogeny. While this was still occurring,Permo-Triassic extension resulted in the formation of rift ba-
sins in a semiarid environment that initially underwent influxesof seawater in the Late Triassic (e.g., Fig. 10). This palaeoen-vironment allowed the generation of evaporitic brines at sur-face that were then buried and underwent chemical modifica-tion in the basinal sequences, resulting in the loss of Mg andsome Na and the enrichment of Ca and K. The fluids wereexpelled along faults that were active during basin formation(e.g., Coward and Trudgill, 1989).
At the site of deposition, both fluid types were isolated intheir individual structures. Both crosscourse and E-W veins setshave multiple phases of quartz growth and crack seal texturesthat are consistent with the episodic influx of fluids alongdilational structures and fluid pressure cycling, resulting inalternating movement on both fault sets. At structural intersec-tions, therefore, there are secondary trails of one fluid type inanother but there is little evidence of the presence of both fluidtypes being present at the one time or of significant mixingtaking place. Only a small number of inclusions record theintermediate salinities and temperatures that might be expectedfrom mixing of the two fluids (Wilkinson, 1990; Gleeson et al.,2000) suggesting that mixing was rare.
This work highlights the usefulness of fluid inclusion chem-istry in placing constraints on the processes effecting the chem-ical evolution of fluids that are spatially and temporally re-moved from their point of origin. The halogen analyses suggestthat similar to other low-temperature base metal mineralisedprovinces (e.g., Kesler et al., 1996; Viets et al., 1996), the roleof surficially evaporated seawater (and therefore palaeoenvi-ronment) is crucial in the generation of these Pb-Zn lodes.
7. CONCLUSION
Bulk fluid inclusion chemical analysis of quartz from post-Variscan quartz6 carbonate6 sulphide6 anhydrite6 fluoriteveins hosted by Palaeozoic basement (Porthleven, Menheniot,Cornwall) and Permo-Triassic sediments (Western Ap-proaches) indicate that the base metal mineralising fluids havesimilar compositions to vein forming fluids in the off-shorePermo-Triassic basinal sequences. This strongly supports thehypothesis that the crosscourse mineralising fluids are basinalfluids derived from the Western Approaches Trough.
Cl and Br systematics indicate that the crosscourse mineral-ising fluids originated as seawater or a mixture of seawater andmeteoric water that was evaporated past the point of haliteprecipitation. The composition of the mineralising brines sug-gest that Na, K, Ca, and Mg compositions were modified bywater-rock interactions such as albitisation and illitisation ei-ther in the basin or on the flow path out of the basin to the siteof deposition. The presence of seawater in all vein types sug-gests that the first marine incursions (Late Triassic) into theregion must act as a lower age limit for the base metal miner-alisation in the crosscourses studied.
These events are broadly contemporaneous with hot mete-oric water circulation around the high-heat-producing Cornu-bian batholith in E-W extensional faults. Halogen compositionsof the dilute E-W fluid indicate that these fluids are acquiringhalogens from the magmatic system, although whether this isoccurring directly or by interaction with magmatic mineralphases is unclear.
Previous work has shown that the local high-heat-producing
Fig. 9. Sketch of fluid inclusions and their microthermometric char-acteristics from a crosscourse vein (A) showing a secondary fracturecontaining the more dilute E-W fluid after Wilkinson (1990). Thissample also shows evidence of this secondary fluid refilling inclusionsin some growth zones. Sketch B shows an E-W vein with a secondaryfracture containing a low-temperature, high-salinity brine inclusionssimilar to those found in the crosscourses.
2077Origin and evolution of mineralising brines
granites provided fracture permeability and (?) periodic heatfluxes that heated both brines and dilute fluids. Petrographic,microthermometric, and field evidence suggests that there wassome temporal overlap between the two palaeohydrologic sys-tems that were active in differently orientated structures.
Acknowledgments—We would like to acknowledge the technical ex-pertise of B. J. Coles (Imperial College) and G. Jones and V. Din(Natural History Museum [NHM]) for bulk analyses. SG would alsolike to thank BP for access to core material and logs and M. Grady(NHM) for the use of her clean lab for the crush leach work. SG wassupported in the course of this work by Shell International Oil Co. JJWwas supported by a lectureship funded by Minorco Services U.K. Theauthors would also like to acknowledge Jay Gregg, J. K. Bo¨hlke, andan anonymous reviewer for their constructive reviews of this manu-script.
Associate editor:M. A. McKibben
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