1
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-Pedrosa 1 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 Africa 3. Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA 4. Robertson's Research, Llandudno, U.K. 3 200 West Bench 200 South 224 West Bench Upper Northwest North Zone 5250 ore (Fred) Maki Fault Zone Central zone South zone Scale 0 300m Southwest ore West ore East ore Northwest ore Victoria, B.C. CANADA USA Admiralty Island 0 20 40 km Juneau Hoonah Sitka Angoon Juneau Eagle Peak Young Bay Hawk Inlet Greens Creek Mine 10 km 0 Hawk Point Geological setting at least three stages of folding two brittle deformational events Colloform low strain zones fold hinges Fold limbs The Greens Creek deposit is a Triassic-age, polymetallic Zn-Pb-Ag-Sb-As-Hg-Mo-Tl- (Cu-Au) massive sulphide deposit formed in a low temperature, shallow-water ore- forming environment in an evolving intra-arc rift. (Taylor et al. 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-fill sedimentary sequence intercalated with bimodal volcanic rocks and intruded by mafic-ultramafic dikes and sills (Taylor et al. 2000) It has experienced and other primary textures are locally preserved in , 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 obliterated original sulphide textures . ! ! ! greenschist facies metamorphism (c. 325 °C, 2-4.5 kbar, ) H i g h - a n g l e f a u l t Lower Southwest thrust system L o w e r S o u t h w e s t f a u l t F2 F2 F3 F3 F2 F2 F2 F2 F2 F2 F2 F3 F3 PHYLLITE SERPENTINE CHLORITE CHERT ARGILLITE SILICEOUS ROCK SILICEOUS SLATEY ARGILLITE ARGILLITE F3 F3 300 ft 250 ft 200 ft 300 ft 250 ft 200 ft Scale 0 100 ft ? ? ? ? ? ? ? Massive sulfide C a r b o n a te F2 0 30 m 134-10b 224W5-38B 215-03 215-21 Geological Section through Greens Creek Ore Deposit along line XS 2500. S ocations of 215-03 and 215-21 are shown ample l Location and ore body maps Aims 1. To demonstrate what can be seen in pyrite using EBSD/SEM techniques 2. To discuss an example of inherited crystallographic orientations: To discuss an example of imposed crystallographic orientations: colloform pyrite from a fold hinge (215-03) deformed pyrite blasts from a fold limb (215-11) 3. Pyrite deformation in a fold-limb shear-zone BSE (A) and reflected light (E) images showing asymmetric sphalerite strain shadows around pyrite, indicating a top to left shear. Close up BSE (B), orientation contrast (C) and reflected light (F) images of two impinging pyrites demonstrate contrasting information available from each. OC image (D) shows relationship between distributed variations in crystallographic orientation and morphology of the pyrite contact. Concentric domains in upper pyrite are located above a grain-boundary ramp-like feature. Upper hemisphere, equal angle pole figures for <100> axes indicate deformation can be explained as rotations mainly around one <100> axis (circled). The core-to-rim lattice rotation sense (~18º in the upper pyrite, ~20º in the lower pyrite) is shown by the arrows. Cox (1987) attributes this type of “ lattice misorientation to dislocation creep involving the development of twist boundaries by the knitting together of screw dislocations to form a network structure. Dislocation creep should not be a dominant process at lower greenschist facies conditions... rotation about an axis” A D C B <100> Y Z <100> Y Z E F Transitional coble creep B r i t n t i l e c - d u t l e tra si tio n Deformation mechanism map for (approx. polycrystalline pyrite (McClay & Ellis 1983). The boundaries are transitional and the vertical axis is not to scale. Contours are 100 micron) Dislocation creep results from dislocation glide along {100} planes, coupled with dislocation climb, and is normally associated with higher temperatures than implied by the Greens Creek metamorphic conditions. One possibility is that the contact between the two pyrites has localised strain resulting in frictional heating and a localised higher temperature during the deformation. Alternatively, it may be that the dislocation creep field opposite needs to move into lower temperature. Colloform summary 1. Radiating pyrite crystallites grain sizes vary CPO varies arranged in concentric layers with or without galena seams between. 2. Crystallite between layers 3. Crystallite between layers ! ! Tendency towards <100> CPO in finer grained layers Tendency towards rotation about <110> CPO in coarser grained layers C D E F G H I 50 m m n=32 n=27 n=9 n=11 n=24 n=33 n=11 A B Reflected light & forescatter orientation contrast images of the same colloform. “F” and “C” labels refer to fine and coarse grained layers, respectively Comparisons with Existing Work on Framboidal Pyrite Microcrysts 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) = more oxic water column? Smaller crystallites (higher D/d) in the colloforms studied here tend to be better ordered than larger crystallites (lower D/d)better ordered at high D/d less ordered at lower D/d Do colloforms have potential as records of redox change, even after 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 framboids formed beneath than water columns (Wilkin et al. 1996 GCA). D/d has potential as a palaeoenvironmental indicator ! ! oxic anoxic “Disordered” microcrysts in a framboid. What’s in a Colloform pyrite? Orientation maps and associated pole figures for automatic EBSD. White areas in maps gave no data. colloform pyrite analysed by OC images and associated pole figures for EBSD. Black ellipse represent <110> direction about which many data are “rotated”. colloform pyrite analysed by manual EBSD Basics ( ) Prior et al. 1999 References: Summary: 1. 3. Pyrite deforms plastically to significantly lower temperatures than generally accepted. EBSD and orientation contrast imaging provide simple tools to reveal the inner workings of pyrite. 2. Colloform textures in pyrite may have potential as records of palaeo-environmental variation with time.

EBSD Basics ( ) Prior et al. 1999 Pyrite deformation in a ...pcapboyle/home/2003-Greens-Creek-Fermor.pdf · Alan P. Boyle , Katja Freitag , ... deposit formed in a low temperature,

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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

�� ����

����

�����

Northwestore

�� ��������

�����

�� ���

�������

������

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

.

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.

������ ������ ������

������

�����

�����

������

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.