EBSD Basics ( ) Prior et al. 1999 Pyrite deformation in a...

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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

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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

.

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.

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