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Insights into pyrite microstructural development from using electron
backscatter diffraction (EBSD) to investigate the Greens Creek
(Alaska) sulphide deposit.Alan P. Boyle , Katja Freitag , Eric Nelson , Murray Hitzman , James Churchill , Magdalena Lopez-Pedrosa1 2 3 4 1
1. Department of Earth & Ocean Sciences, University of Liverpool, Brownlow Street, Liverpool, L69 3GP, U.K.2. Private Bag X1013, Suite 14, Palaborwa 1390, South Africa3. Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA4. Robertson's Research, Llandudno, U.K.
3
200 WestBench
200 South
224 WestBench
UpperNorthwest
NorthZone
5250 ore(Fred)
MakiF
ault
Zone
Centralzone
Southzone
Scale
0 300mSouthwestore
�������
Westore
East ore
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����
�����
Northwestore
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Victoria, B.C.
CA
NA
DA
US
A
Adm
iralty
Island
0 20 40 km
Juneau
Hoonah
Sitka
Angoon
Juneau
EaglePeak
Young Bay
Hawk Inlet
Greens Creek Mine
10 km0
HawkPoint
Geological setting
at least three stages of folding
two brittle deformational events
Colloformlow strain zones fold hinges
Fold limbs
The Greens Creek deposit is a Triassic-age, polymetallicZn-Pb-Ag-Sb-As-Hg-Mo-Tl- (Cu-Au) massive sulphidedeposit formed in a low temperature, shallow-water ore-forming environment in an evolving intra-arc rift. (Taylor etal. 1999; Freitag 2000; Taylor et al. 2000).
The ores are hosted by a discontinuously exposed, 600-km-long belt of rocks comprising a 200-800 m thick rift-fillsedimentary sequence intercalated with bimodal volcanicrocks and intruded by mafic-ultramafic dikes and sills(Taylor et al. 2000)
It has experienced
and other primary textures are locally preservedin , especially large-scale .
are often strongly sheared
Himmelberg et al. 1994; Himmelberg et al. 1995, which have locally
overturned parts of the deposit and largely obliteratedoriginal sulphide textures
.
�
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greenschist facies metamorphism (c. 325 °C, 2-4.5kbar, )
Hig
h-a
ng
lefa
ult
Lower Southwestthrust system
Lo
wer
So
uth
wes
tfa
ult
F2
F2
F3
F3
F2
F2F2
F2
F2 F2
F2
F3F3
PHYLLITE
SERPENTINECHLORITE
CHERT
ARGILLITE
SILICEOUSROCK
SILICEOUSSLATEYARGILLITE
ARGILLITE
F3F3
300 ft
250 ft
200 ft
300 ft
250 ft
200 ft
Scale0 100 ft
?
? ?
?
?
?
?
Massivesulfide
Carbonate
F2
0 30 m
134-10b
224W5-38B
215-03
215-21
Geological Section throughGreens Creek Ore Deposit alongline XS 2500. S ocations of
215-03 and 215-21 are shownample l
Location and ore body maps
Aims
1. To demonstrate what can be seen in pyrite using EBSD/SEM techniques2. To discuss an example of inherited crystallographic orientations:
To discuss an example of imposed crystallographic orientations:
colloformpyrite from a fold hinge (215-03)
deformed pyriteblasts from a fold limb (215-11)
3.
Pyrite deformation in a fold-limb shear-zoneBSE (A) and reflected light (E) images showing asymmetric sphaleritestrain shadows around pyrite, indicating a top to left shear.
Close up BSE (B), orientation contrast (C) and reflected light (F) imagesof two impinging pyrites demonstrate contrasting information availablefrom each.
OC image (D) shows relationship between distributed variations incrystallographic orientation and morphology of the pyrite contact.Concentric domains in upper pyrite are located above a grain-boundaryramp-like feature.
Upper hemisphere, equal angle pole figures for <100> axes indicatedeformation can be explained as rotations mainly around one <100>axis (circled). The core-to-rim lattice rotation sense (~18º in the upperpyrite, ~20º in the lower pyrite) is shown by the arrows.
Cox (1987) attributes this type of “ latticemisorientation to dislocation creep involving the development of twistboundaries by the knitting together of screw dislocations to form anetwork structure.
Dislocation creep should not be a dominant process at lower greenschistfacies conditions...
rotation about an axis”
A
D
CB
<100>
Y
Z
��
��
��
��
��
��
��
��
��
��
<100>
Y
Z
�
E
F
Transitionalcoblecreep
Brit
n
t
i
le
c- du
t le tra sitionDeformationmechanism map for(approx.polycrystalline pyrite(McClay & Ellis1983). Theboundaries aretransitional and thevertical axis is not toscale. Contours are
100 micron)
Dislocation creep results from dislocation glide along {100} planes, coupled withdislocation climb, and is normally associated with higher temperatures than impliedby the Greens Creek metamorphic conditions. One possibility is that the contactbetween the two pyrites has localised strain resulting in frictional heating and alocalised higher temperature during the deformation. Alternatively, it may be thatthe dislocation creep field opposite needs to move into lower temperature.
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Colloform summary
1. Radiating pyrite crystallites
grain sizes vary
CPO varies
arranged inconcentric layers with or without galena seamsbetween.
2. Crystallite between layers
3. Crystallite between layers
�
�
Tendency towards <100> CPO in finer grained layers
Tendency towards rotation about <110> CPO in coarsergrained layers
C
D
E FG
HI
50 � m
n=32n=27
n=9 n=11
n=24
n=33
n=11
A
B
Reflected light & forescatterorientation contrast images of the
same colloform. “F” and “C” labelsrefer to fine and coarse grained
layers, respectively
Comparisons with Existing Work on Framboidal PyriteMicrocrysts may be variably ordered within framboids (see below,Ohfuji & Akai 2002), with framboids and
(Ohfuji et al. 2002)
More ordered relatively smaller crystallites (higher D/d) = moreoxic water column?
Smaller crystallites (higher D/d) in the colloforms studied heretend to be better ordered than larger crystallites (lower D/d)…
better ordered at high D/dless ordered at lower D/d
Do colloforms have potential as records of redox change, evenafter greenschist metamorphism?
Might “disordered” framboid microcrysts be ordered about <110>?
Framboid D/d graphs
D/d is higher
for
Dead Sea (a)Great Salt Marsh (b)
for framboidsformed beneath than
water columns(Wilkin et al. 1996 GCA).
D/d has potential as apalaeoenvironmentalindicator
�
�
oxicanoxic
“Disordered”microcrysts ina framboid.
What’s in a Colloform pyrite?
Orientation maps and associated pole figures forautomatic EBSD. White areas in maps gave no data.
colloform pyriteanalysed by
OC images and associated pole figures forEBSD. Black ellipse represent <110>
direction about which many data are “rotated”.
colloform pyriteanalysed by manual
EBSD Basics ( )Prior et al. 1999
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Summary:
1.
3. Pyrite deforms plastically to significantly lowertemperatures than generally accepted.
EBSD and orientation contrast imaging provide simpletools to reveal the inner workings of pyrite.
2. Colloform textures in pyrite may have potential asrecords of palaeo-environmental variation with time.