2.- Yacimientos en Skarn (Conceptos, Terminología, Clasifica

8/9/2019 2.- Yacimientos en Skarn (Conceptos, Terminologa, Clasifica

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Skarn Deposits


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Skarn definition

Granular rock consisting of Ca-silicates (e.g.,

garnet, pyroxene, amphibole); does not imply

presence or absence of mineralization

f mineralization --! skarn deposit


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Introduccin

Trmino usado originalmente por los mineros en

Suecia para referirse a la ganga de grano

grueso, muy dura, compuesta por calcosilicatosasociados a la mena de Fe (Geijer & Magnusson,

19!"# $ncluye una amplia %ariedad de rocas con

minerales calcosilicatados ricos en a, Mg, Fe, 'l

y Mn, al margen de si estn asociados aminerales de inters econ)mico o no#


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Skarn general features"ormed during contact or regionalmetamorphism by metasomatic processes

in#ol#ing magmatic, metamorphic, meteoric

and$or marine fluids

"ound close to intrusions, along faults andshear-zones, in shallo% geothermal systems,

at the bottom of the oceans, and deep in the

continental crust

&ll these skarns are linked by their

mineralogy, %hich is characterized by the

presence of a #ariety of Ca-silicates, especially

garnets and pyroxenes


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Skarn general features "ound in all continents and in rocks of all

ages

"ound mostly in se'uences containing

limestone

Can form also %here limestones are absent,#irtually in any kind of rock sandstone,

siltstone, granite, basalt, komatiite


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Ambiente geolgico

&cti#idad magmtico-hidrotermal en

cinturones orog*nicos (cerca a

intrusiones, metamorfismo regional,zonas de cizalla y fallas, sistemas

epitermales y geot*rmicos, fondos

marinos)

Continuum con p+rfidos, greisens y #etas


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Metals recovered

from skarns

"e, , Cu, n, b, /o, &g, &u, 0n 1 2,

344, ", 5

2ntil 6788 one of the most important

sources of Cu and "e

0till the most important source of

(98: of the %orld-production)

3ecently exploited mostly for &u


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Ways to form skarns/etamorphic recrystallization (isochemical%ith loss of C


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Types of skarnformation


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&) sochemical metamorphism in#ol#es recrystallization and changes

in mineral stability %ithout significant mass transfer. 5) 3eaction

skarn results from metamorphism of interlayered lithologies, such as

shale and limestone, %ith mass transfer bet%een layers on a small

scale (bimetasomatism).


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0karnoid results from

metamorphism of impure

lithologies %ith some mass

transfer by small scale fluid

mo#ement.

"luid-controlled metasomatic skarn

typically is coarse grained and does

not closely reflect the composition or

texture of the protolith. onation of

most skarns reflects the geometry ofthe pluton contact and fluid flo%.


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Skarnoid

0karnoid results from metamorphism of

impure lithologies %ith some masstransfer by small scale fluid mo#ement.

@he fluid-controlled metasomatic skarn

typically is coarser grained and does notas closely reflect the composition or

texture of the immediately surrounding

rocks


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Infiltration skarns

ntrusion-related --! contact skarns

/etamorphic --! regional skarns (mostly -&u-

skarns e.g., 5indal, Aor%ay; 5lacklite rospect,

Ae% /exico; Aa#achab, Aamibia)

roximal (B88 m from intrusion)

Distal (!88 m from intrusion)


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Contact skarn

formation & evolution Dynamic process in time and spaceEEE

0karn fluid chemistry and mineralogy changein space and, at a fixed point, also in time

5ound to magmatic e#olution of the intrusionin time and to intrusion-host rock interaction

in time and space


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Magmaevolution

T decrease

Fluid phase exsolution during

crystallization


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Magma!ost rock

interaction =eating of host rocks and their fluids --! @

gradient around intrusion

Folatilization of host rock components (mostly

C


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Skarn formation

stages

/etamorphic

rograde

3etrograde


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Metamorp!ic skarn

stage

Folatiliztion CaC--! Ca C


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0karn

4#olution


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0karn alteration of the country rock, generallycarbonate

/ay be classified according to the dominantmineralogy

/ost of the %orldMs maNor skarn deposits arein calcic exoskarns

2sually sho% a zoning of both silicate and

#$oskarn


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0karn alteration of intrusi#e rock

n classic carbonate intrusi#e contact, this %ould be theintrusi#e rock altered to calc-silicate

/ay sho% zoning from potassic minerals (primaryO) tocalcic at contact

5iotite -! amphibole -! pyroxene -! garnet (to%ardsmarble)

#ndoskarn


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Skarn Zonation

Abkrzungen: Ac = Actinolith; And = Andradit; Bio = sekundrer Biotit; Ca = Kalzit; Di= Diopsid; K-fs = sekundrer Kalifeldspat; Non = Nontronit (Schichtsilikat); Sap =

Saponit (Schichtsilikat); Sid = Siderit; Tc = Talk; Tr = Tremolit; Wo = Wollastonit.


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"rograde stage

mineral %oningCu skarnsproximal distal

G&3A4@bro%n green-yello%

P3Q4A4light color

(high-/g)

dark color

(high-"e)

D k d b t k


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Dark red-bro%n garnet skarn

from the est Cananea

Deposit (/einert, 67R


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"rograde stage

mineral %oning

=ost rock composition"luid composition

0ource composition

-@

-@-Q


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etrograde skarn stageCooling, in#ol#ement of meteoric fluids, oxidizing, C


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Continental /ine (Ae% /exico, 20&). ntense retrogradeen#elopes on #eins cutting through prograde garnet (red-

bro%n color) and pyroxene (light tan color) skarn. n more

%idespread retrograde alteration, the #ein en#elopes

%ould coalesce and completely obliterate the prograde

garnet and pyroxene.

3etrograde &lteration


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etrograde skarn stage

@BI88LC

0alinities B


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Skarn types

Calcic skarnsCa- and Ca-"e(-/g)-silicates

(%ollastonite, #esu#ianite, andradite, hedenbergite)

/agnesian skarns/g- and Ca-/g(-"e)-silicates

and oxides$hydroxides (forsterite, diopside, talc,

brucite, periclase, magnetite)


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educed vs' o$idi%ed

skarns

rganic matter

a*(s" (s" Fe*(a6" !Si*!(a6" *!//0 aFeSi!*+(s" *!(g" 2eden3ergite

.a*(s" !Fe*(a6" .Si*!(a6" 14!*!//0 a.Fe!(Si*5".(s" andradite

/agma chemistry


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Magma f()

f*!

-ematite

Titanite

Magnetite

$lmenite

/ost porphyry-

Cu plutons

*(+ATI(+ A+D ,(ST (C-S


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Bio hfelsPx hfels

Calcareous

siltstone

limestone

dolomite

gar

gar

woa

mp

h px

px

ol ol

se

rp

se

rp

Ca-sil (gar)

Pxh

fels

3educed en#ironment xidized en#ironment

*(+ATI(+ A+D ,(ST (C-S


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SpaceTime %onation

/ineralogy

Gar$px ratio Gar color

composition

x color composition Distal pyroxenoid-#esu#ianite

/etal ratios

roximal Cu1/o1 Distal /n-b-n-&g-

&u


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resent in almost all skarns

Garnet

yroxene

&mphibole

Tuartz

ther Common

li#ine

ollastonite

4pidote

&ctinolite

/ore Uocal

0capolite

eolite

plagioclase

Skarn Mineralogy


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Skarns and depth of formation

W

ZnCu


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Skarn types and factors

=o

strock

ox

idation

state

depth

luton oxidation state

Cu

&u

/o px!gar

gar!px"e gar

/g-rich px


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Skarn deposits

0karn deposits are only formed in association %ithdiffusion and infiltration processes not %ith

isochemical processes

/aNority of skarn deposits is associated %ithinfiltration skarns

@emperature H88LVH98LC but lo%er in b-n skarns

ressure one to se#eral kilometers


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/odern trend is to adopt descripti#e classificationsbased on the dominant economic metal.

0e#en maNor skarn types

&u,

Cu,

"e,

/o,

0n,,

n-b

thers include ", C, 5&, t, 2, 344

#conomic Classification of Skarns


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(re stages in skarn

rograde stage scheelite, magnetite

(magnesian skarns), sulfides (after

scheelite) CaCC =


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Uargest economic skarns (most common)

0ome exceed 988 /t %ith more than 88 /t of contained

"e/agnetite skarn important source of iron

Consists dominantly of magnetite %ith minor silicate

gangue

Calcic-iron skarns V ceanic slands

2sually associated %ith gabbro through diorite to syenite/agnetite %ith garnet and pyroxene

/agnesian-iron skarns2sually associated %ith granites or granodiorites/agnetite

Iron Skarns

M tit i


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Magnetite in

magnesian skarns/agnesian skarns4conomic magnetite in

prograde stage

.Fe*(a6" !Si*!(a6" aMg(*."! 14!*!//0 Fe.*5 aMgSi!*+ !*!

dolomite magnetite diopside


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Magnetite in calcic

skarnsDoes not form

"e(a')


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/any (associated %ith porphyry copper deposits)

Calc-alkaline

Granodiorite to monzogranite stocks

Continental margins/esozoic and tertiary %estern &mericas

Carboniferous arcs in %estern 3ussia

sland arcsTuartz diorite to monzogranite

Copper Skarns


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Cu skarn %onation

. ld Sk d d


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9-69 g$t &u (&u V 5i V @e V &s geochemical association)

2sually in clastic-rich protoliths

0karn alteration of dikes, sills, and #olcanoclastic units

3elati#ely fine-grained skarn

Dominated by iron-rich pyroxene (!=d98)

roximal zones contain garnet

4arly $ Distal zones ha#e biotite 1 W-feldspar hornfels

W-feldspar, scapolite, #esu#ianite, apatite, amphibole

.old Skarns/ reduced

d d . ld Sk 1 tit d +3


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Cu-skarn to gold skarn

KK t &u from 68.7 /t ore (K.6 g$t &u)

roximal skarn garnet-rich %ith minor pyxroxene

&u V 5i V @e V &s association

Copper Canyon granodiorite

"e


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.old skarn %onation 41ortitude5

, l A d d Sk


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678I V 6779 6.I /t X 9. g$t

0karn in clastic rocks spacially associated %ith =edleydiorite

"e


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,edley Au skarn


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($idi%ed gold skarns =igh garnetpyroxene ratios, relati#ely "e-poor garnet and pyroxene

Uo% total sulfides pyrite!pyrrhotite

/inor but ubi'uitous chalcopyrite, sphalerite, and galena

=ighest gold grades not in prograde garnet-pyroxene, but %ith later retrogradealteration including abundant W-feldspar (adularia) and 'uartz

0ome are transitional to other types of gold mineralization such as epithermaldeposits in %hich phase separation (boiling) can be an important precipitationmechanism

4xamples /cCoy (Ae#ada), AambiNa (4cuador)


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Skarn e$amples

http://pangea.stanford.edu/ODEX/kurts-images/micrographs/skarnhand.jpg


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Skarn examples


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-eden3ergite (2d", grossular

garnet (gt", magnetite (mt", 6uart7 (67" s8arn assem3lage

http://www.newcastle.edu.au/discipline/geology/research/talawang/slideA1.htm

Palaeozoic magnetite skarn atTallawang, NSW, Australia


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f


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Salite


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Sk t t i tti


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Skarn geotectonic setting

Documents

2.- Yacimientos en Skarn (Conceptos, Terminología, Clasifica