201306-30 skarn lima seg student chapter

8/13/2019 201306-30 Skarn Lima Seg Student Chapter

1/31

Short course for the SEG student

chapter at Universidad Nacionalde Ingenieria, Lima, Peru

Skarn deposits


2/31

Universidad Nacional de Ingenieria, Lima, Peru

30 June 2013

Skarn deposits

Zhaoshan Chang

EGRU, [email protected]

Skarn deposits

1. Introduction, definition and mineralogy

2. Classification and terminology

3. Skarn-forming processes & evolution stages

4. Zonation in skarn systems

5. Factors affecting the formation of skarns andzonation patterns

6. Exploration (quiz)

Why skarns?Au, Cu, Sn, W, Pb, Zn, Mo, Fe, minor Ag, B, Be, Bi, Co, F, REE and

U

Common: >1400skarn deposits described in literatures Major source of W and Sn Significant source of base metals and Au, e.g., Antamina, Peru

(1521 Mt @ 0.9% Cu, 0.5% Zn, 0.02% Mo, 10.9g/t Ag); Ertsbergdistrict, Indonesia (716 Mt @ 1.2% Cu, 1.2 g/t Au)

High grade sweetener in porphyry deposits

Antamina, Peru. Photo courtesy of Cam McCuaig

Why skarns?


3/31

What is a skarn?

Defined by mineralogy: Ca-rich garnet and/or pyroxene

Contact betweenintrusions and

carbonatefavorable but notnecessary; notpresent at typelocality

Skarn mineralogy

Garnet A32+B23+C34+O12 Pyroxene A12+B12+C24+O6 Olivine A12+B14+O4 Pyroxenoid A12+B14+O3 Epidote A22+B33+C34+O12(OH)2 Amphibole A11+B22+C52+D84+O22(OH)2 Mica A1B2-3C4O10(OH, F)2 Carbonate A12+CO3 Others

Spessartine Mn3Al2(SiO4)3

Almandine Fe3Al2(SiO4)3Pyrope Mg3Al2(SiO4)3

Subcalcic

garnet

Skarn mineralogy

Garnet A32+B2

3+C34+O12

Grossular Ca3Al2(SiO4)3Andradite Ca3Fe2(SiO4)3

Grandite

Pyroxene A12+B1

2+C24+O6

Diopside CaMgSi2O6Hedenbergite CaFe Si2O6Johannsenite CaMnSi2O6

Salite

Garnet and pyroxene composition diagrams


4/31

Inappropriate pyroxene composition diagram Skarn mineralogy

Pyroxenoid A12+B1

4+O3

Ferrosilite FeSiO3

Rhodonite MnSiO3

Wollastonite CaSiO3

Pyromangite

Bustmite

Epidote A22+B3

3+C34+O12(OH)2

Epidote Ca2FeAl2Si3O12(OH)Clinozoisite Ca2AlAl2Si3O12(OH)

Classification of skarns

Major economic metal:

- Fe, Cu, Pb-Zn, Au, W, Sn, Mo

Magnetite is abundant in many types:

- Cu, Au, Sn

Pb-Zn skarns mostly polymetallic (e.g., with Ag)


Endoskarn vs. exoskarn:

Exoskarn is typically much more abundant than endoskarn:


5/31

Endoskarnat Mt Colin


Ca-skarn: protolith = limestone- Garnet, pyroxene, wollastonite, scapolite, Ca-rich plagioclase,

vesuvianite, epidote, amphibole, ilvaite, chlorite, prehnite, biotite,

quartz, carbonate

Mg-skarn: protolith = dolomitic- Olivine, pyroxene, humite, clinohumite, periclase, amphibole,

phlogopite, chlorite, serpentine, talc, burcite

Mn-skarn:- Johannsenite, spessartine, rhodonite, pyromangite, bustmite

Distal locations; Pb-Zn skarn;a subgroup of Ca-skarn

Magmas contain volatiles(H2O, HCl, SO2, CO2, HF,B, ) and metals

White Island, New Zealand:~300 t Au & ~1 Mt Cu discharged toatmosphere in the past ~10,000 yrs Courtesy of Jeff Hedenquist

Skarn-forming processes Water solubility and exsolution

Burnham (1979, 1981, 1997)

Water in aqueousphase - MagmaticHydrothermal fluid

Water dissolvedin melt

Solubility curve


6/31

Water solubility and exsolution

Burnham (1979, 1981, 1997)


Water in aqueousphase - Magmatic

Hydrothermal fluid

Secondboiling

First boiling

Rapid ascent;adiabaticdepressurisation

Cooling /crystallisationin a chamber

Slow ascendingaccompanied bycooling

Burnham (1979, 1981, 1997)

Second boiling

First boiling

Rapid ascending;adiabatic depres-

surisation

Cooling /crystallisation in achamber

Slow ascendingaccompanied by

cooling

Water solubility and exsolution



Hydrothermal fluids

Water exsolution

Burnham (1979, 1981, 1997)

1. Water slowly exsolves;

P increases; water not

released from chamber

yet due to solid carapace

Quartz USTs, NorthParkes, NSW

Buoyant hydrothermal fluid migrates upwards through the meltand accumulates in the roof of the inwardly-crystallizing magma

Bands of distinctly prismatic crystals (e.g., qz, mt) growdownwards from the roof of the volatile pocket

Volatile accumulationand UST growth

UST quartz form here.

Coalesced waterbubbles. If caught inmelt, miarolitic cavitieswill form.

Slide from Dave Cooke


7/31

Miarolitic cavities

Candela and Blevin (1995); Candela (1997)

Coarser grained domain; crystals terminated in or project into void Pockets of magmatic hydrothermal fluids; inward growth Maybe interconnected!water migration channel in magma

Field of view = 10cm

Field of view = 2.5 cm

Cavity

USTquartz

Lowenstern and Sinclair (1996)

Water exsolution

Burnham (1979, 1981, 1997)

2. Internal P > (lithostatic P + carapace

tensile strength, ~100 bar), !Hydro-

fracturing; P drops by ~75%. Form

magmatic hydrothermal breccia and

quartz veins/networks.

Burnham (1979, 1981, 1997)

3. Pressure drop causes decrease

of water solubility !rapid

degassing; Water takes away a lot

heat, causing quenching (pressure

quenching); Form porphyry texture

Waterexsolution


8/31

Water exsolution

Burnham (1979, 1981, 1997)

4. Seal; Magma at

depth continues to

crystallise; another

cycle

4. Seal; Magma at

depth continues to

crystallise; another

cycle

Water exsolution

Burnham (1979,1981, 1997)

5. Water exsolves from

deep seated magma

through second boiling

5. Wall rock is ductile atdepth; no explosion, only

expansion; pegmatite

Transitional stage physicalprocesses

Burnham (1997)

FWS: H2O solubility as mass

fractionation

PtVt: max. mechanical energyreleased upon complete crystallisation

MTS: max. tensile strength of rock,equiv. 100 bar

For Bingham quartzmonzonite with 2.7 wt% initial water,hydrofracturing occursat depth < ~ 4-5km

Transitional stage chemical processes

Burnham (1981, 1997)

Crystal + melt + aqueous fluid

"Aqueous fluid extracts metals and volatiles from melt"Interaction between water and crystals (e.g., Chang and

Meinert, 2004)"Crystallisation continues from melt


9/31

Transitional stage chemical processes

Burnham (1981, 1997)

Elemental partitioning between aqueous phase andmelt

"Cl: coefficient = 40:1; Cl in melt: ~0.03 of H2O wt% in melt!NaCl wt% in aqueous fluid = 2 x H

2O wt% in melt

!if 3-6 wt% H2O in melt, then ~6-12 wt% NaClinaqueous fluid

"S (SO2vs. H2S; redox state), CO2"F: 5:1"Most metals prefers aqueous phase, e.g., Cu: 9:1 (Candellaand Holland, 1984)

Timing of water exsolution

Burnham (1981, 1997)

Proportion of melt crystallised at water exsolution

Starting conditions: Dioritic to granitic melt, 2 wt% H2O,

Pressure Depth H2O solubility % of crystalli. at water saturation

5000 bars 20 km 10-11 wt% 80%

2000 bars 8 km 6.3 wt% 68%

500 bars 2 km 3.0 wt% 33%

Water exsolution may occur at much highertemperatures than magma solidus temperatures

Burnham (1981, 1997)

Timing of exsolution affects metal concentration inaqueous fluids

"Some elements can be taken into igneous minerals, e.g.,Fe, and some Cu !early exsolution favourable

"Some trace elements may only become enriched enoughat the end of crystallisation (high degree of crystalli.), e.g.,Be !late exsolution favourable L+V

V L

Single phase

H+V

H+L

L+H+V

80% 20%

Evolution of magmatic fluids


10/31

Magmaticfluid

At shallow depths (< 4 km)aqueous magmatic fluids

split into two separate phases

GAS PHASElow density

high SO2, HCl

low NaCl, metals

LIQUID PHASEhigher density

low SO2, HCl

high NaCl, metals

Partitioning changeswith changes in P, T

Magmatic vapor Water solubility and exsolution

Burnham (1979, 1981, 1997)

Water dissolved inmelt


Hydrothermal fluid

Carbonate-rich area:magma assimilatescarbonate!CO2increases!H2Osolubility decreases!exsolution occurs

earlier (Meinert, 1995)

Hypogene, up to 850 C

vapors, with HCl, SO2,

CO2, H2S

White Island, New Zealand; Courtesy of Jeff Hedenquist

High temperature degassing

" Such magmatic hydrothermal fluids rich in Si and Fe" Al typically has low solubility, except when acidity is

high or F content is high

Sn-associated Mole granite;Audetat et al., 2000

Fe up to 21.6 wt% Empire Cu-Zn skarn;Chang and Meinert, 2004

Skarn-forming processes


11/31

Evolution ofskarns - 1

Thermal metamorphism:

- Isochemical

Meinert, 1992

Before magmatic wateris released out frommagma chamber; nofluid flow

Meinert, 1992

Metamorphism:Thermal metamorphism

Calcareous: containing carbonateCarbonaceous: containing organic C

Bimetasomatism:

Calc-silicatehornfels

Biotitehornfels

Pyroxenehornfels

Biotitehornfels

"Very fine grained" Low permeability;

block fluid flow;none to weaklymineralized

" Garnet rich in Al

Meinert, 1992

Metamorphism:Thermal metamorphism

Bimetasomatism:


12/31

Reaction skarn

Zoned; fine-grained; low permeability; garnet Al-rich; miner. poor

Marble

Garnet

Pyroxene

Garnet

Marble

Reaction skarnZoned; fine-grained; low permeability; garnet Al-rich; miner. poor

Reaction skarn


13/31

Evolution ofskarns - 1b

Skarnoid- Fine-grained,- High Al, low Fe

Meinert, 1992

Small scale fluidflow along fractures,promoting minormass exchange

Skarnoid at smallscale

Reaction skarn and skarnoid

Metasomatic skarn,prograde

- Garnet, pyroxene,wollastonite,

- Olivine, pyroxene,

periclase,

Meinert, 1992

- Large scale fluid flow- Massive; protolith texture

obliterated- Higher Fe in skarn minerals- Zoned- May over print calc-silicate

hornfels, reaction skarns


- Protolith texture mostly obliterated- Ground preparation

Massive garnet skarn

0.5 cm

Calcite to garnet:CO2lost; density increased; mineral tends to be granular !mineral volume decreased, open space created,permeability and porosity increased


14/31

Mt Colin

Massivepyroxene

skarn

Elaine

Replacement of hornfels


Retrograde skarn andmineralisation

- ep, act/tr, qtz, + mt/hm

- chl, qtz, cal, + sulfides- cal

Mostly in skarn but may gobeyond

Meinert, 1992

ep: epidote; act: actinolite; tr: tremolite; cal: calcite; qtz:quartz; chl: chlorite; mt: magnetite; hm: hematite

" Metals dissolved as complexes (e.g., with Cl);" High solubility at higher temperatures;" Not deposited until lower temperatures or losing the ligands

(e.g., by dilution)

" Deposition when solubility decreases dramaticallyMagnetite

Chalcopyrite SphaleriteGalena

500-400C ? 400-250C?

300-200C ?


15/31

Williams-Jones et al. (2010)pH

Base metals in hydrothermal fluids

- Transported as Cl complexes(e.g., Wood and Samson, 1998)- Deposition causes acidity increaseZnCl4

2-+ H2S = ZnS + 4Cl-

+ 2H+

Concentrated vs.dispersed: wall rock

important

Carbonate-rich regiongood for base metaldeposition !higher grade

Retrograde alteration and mineralization

Garnet

EpidoteQuartz

Epidote replaces and pseudomorphs garnet

Retrograde alterationand mineralization

Pyroxene replaced by ilvaite, quartz, and siderite


16/31

Ilvaite replaced by siderite+quartz

Pyroxene replacedby quartz, siderite,and sphalerite

Magnetite is later than prograde skarn;

Sulfides later than oxides

Retrograde alteration andmineralization

Garnet

Chl + mt

Qtz-cal-py-cpy

GarnetGarnet

GarnetMt + chl

Py + cpyPy + cpy

Retrograde alteration and mineralization


17/31

Note thin chlorite between sulfides and garnet Black chlorite selectively replaces bands ofgarnet. Note sulfides are associated withchlorite-quartz-carbonate.

Ga: garnet; Cc: calcite; Qz: quartz; Chl: chlorite; Cpy: chalcopyrite;Py: pyrite; Bn: bornite

Quartz-calcite-sulfides replace some

bands of garnet and fill the interstices

CpyCpy

Cpy

Cpy+ChlCpy+Chl

Qz+Cc

Retrograde alteration andmineralization

Chang and Meinert, 2008

Retrograde alteration andmineralization may bepresent beyond skarn

Chl

Po

Cpy+Py+Au

Retrograde alteration & mineralization

Chl: chlorite; Cpy: chalcopyrite; Py: pyrite; Po: pyrrhotite

Cpy

Graniteporphyry

Hornfels


18/31

Skarns are typically zoned

Transfer of heat and mass from intrusions or fluid conduitsIntrusion Calcareous wall rock

Endoskarn Massive/metasomatic skarn

Bleached

marbl

e

Marble Limestone

Hornfels

Fluid escape structures

Garnet/pyroxene ratio decrease

Garnet colour lighter

Pyroxene colour darker

Cu Zn-Pb

Exoskarn is typically much more abundant than endoskarn

Reaction skarn

The closest intrusion is not necessarily the causative intrusion!

Zonation in a Cu skarn

gar: garnet; pyx: pyroxene; cp: chalcopyrite; wo: wollastonite;ves: vesuvianite; bn: bornite; po: pyrrhotite; py: pyrite

Carr Fork, Bingham, USA

Atkinson andEinaudi, 1978

38.3m Endoskarn.Dark red garnet 45.2m Red garnet 46.3m Brown garnet 48.2m Yellow garnet

54.2m Light yellowgarnet

Zoning away from intrusion in a Cu skarn


19/31

Zonation in a Cu skarn

Carr Fork, Bingham, Cu skarn

Zonation in a Zn skarn

Marble

Groundhog,USA;

Meinert,

1987

Gar: garnet; Pyx: pyroxene; Hd: hedenbergite; Jo: Johannsenite

Zonation in a Au skarn Fortitude, USA

Meyer and Meinert, 1991

Zonation at marble front in a Au skarn, Mexico

Darkmarble

Bleached marbleWollastoniteGarnet

Fluid escape structures distalfeatures beyond skarn

Meinert et al., 2005

Pyroxene

Garnet

Veins

Ve

ins

Garnet

Garnet veinsGarnet veins


20/31


50-100m away from skarn


200m away from skarn

Stylolite

Calcite vein offset by stylolite fluid escape structure

Meinert et al., 2005 Meinert et al., 2005


21/31

Fluid escape structures distalfeatures beyond skarn Fluid escape structures distal features beyond skarn

Beyond bleached marble, there are white, bleachedveins with diffusive sulfide veins in the middle. The veindensity and width decrease away from skarn

Dark marble

Diffusive veins(po-cpyMo)Bleached

halo

Fluid escape structures distal features beyond skarn Fluid escape structures distal features beyond skarn

Diffusive veins(po-cpyMo)


22/31

Factors affecting the formation of skarns

Redox state gradient between magma and wall rock

Causative magmaVolatilesDegree of fractionationRedox state

Wall rockCompositionRedox statePermeability

Depth of formation

Distance from magma

Grossular Ca3Al2(SiO4)3Andradite Ca3Fe2(SiO4)3

Diopside CaMgSi2O6Hedenbergite CaFeSi2O6

Redox state gradient

Oxidizing Reducing

Fe3+!garnet Fe2+!pyroxene

Intrusion Calcareous wall rock

Endoskarn Massive/metasomatic skarn

Bleached

marble

Marble Limestone

Hornfels

Fluid escape structures

Garnet/pyroxene ratio decrease

Garnet colour lighter

Pyroxene colour darker

The zoning pattern is based on:

If both the magma and the wall rocks are reducing

Ettlinger, 1990; Ray et al., 1996

Ilmenite-bearing

Fe2O3/(Fe2O3+FeO)= 0.15

Nickle Plate ,Hedley District,Canada

If both the magma and the wall rocks are oxidizing

A Cu skarn prospect, Philippines

High inmagnetite

DioritePorphyry Conglomeratemarble


23/31

38.3m Endoskarn.Dark red garnet 45.2m Red garnet 46.3m Brown garnet 48.2m Yellow garnet

54.2m Light yellowgarnet

Specularite

If both the magma and the wall rocks are oxidizing Effect of magmatic volatiles - F

Empire Cu-Zn skarn, USA

Chang and Meinert, 2004, 2008

Unusual features: 1) Abundant endoskarn, > exoskarn2) Proximal Zn minearlisation

Effect of magmatic volatiles - F

High F content in the magmatic-hydrothermal system as indicated by:

Empire Cu-Zn skarn mine, USA


1.53-2.46 wt% F in magmatichornblende

1.43-3.87 wt% F in magmatic biotiteFluorite as igneous accessory mineralFluorite as daughter mineral in fluid

inclusions

1.29-2.42 wt% F in hydrothermalvesuvianite

Fluorite in skarns

Effect of magmatic volatiles - F

Reasons for these unusual features:

- F greatly facilitates the dissolution of silicates- F decreases the solidus temperatures ofmagmas. When the late-stage fluids exsolved

from them, the fluids were already at lowtemperatures, therefore only shorttransportation distance was needed for thefluids to be cool enough to deposit sphalerite

Empire Cu-Zn skarn mine, USA



24/31

Textures indicating high F


Effect of causative magma compositions

Intrusions associated with

Fe and Au skarns: moreprimitive

Associated with Mo and Snskarns: more evolved and/orcrustally contaminated

Mostly calc-alkaline;tholeiitic and alkaline notcommon

Meinert, 1995

Meinert, 1995

Intrusions associated with Sn skarns: peraluminous

Sn and Au-only skarns: more reduced causative intrusionsCu, Zn, and Mo skarns: more oxidized causative intrusions

Meinert, 1995

Effect of causative magma compositions


25/31

Sn skarns

Intrusive rocks peraluminous, more evolved and/or with morecrustal contamination, and more reduced

Suite of trace elements: Sn, F, B, Be, Li, W, Mo, and Rb

Strong greisen alteration

Sn can be incorporated into silicate minerals, e.g., garnet (up to 6wt%), vesuvianite, and titanite; extensive retrograde or greisenalteration needed to release such Sn; most attractive ore bodiesdistal

Economic Sn in cassiterite, stannite, vonsenite (4FeOFe2O3B2O3),etc.

Plane-polarized

Pyroxene

Vonsenite

Plane-polarized

Hornblende

Garnet

Calcite

A Sn skarn in western Tasmania, Australia

GarnetCalcite

Hornblende

VonseniteVonsenite

Residual Hb

Fe-rich hb withbio replacement

Bio withresidual hb

Cassiteritein this band

Biotite

CassiteriteCassiterite

Biotite

Fluorite

Pyrrhotite

A Sn skarn in western Tasmania, Australia

Redox

W

W

W

Composition Ca skarn vs. Mg skarn

Porosity, composition

CaCO3+ SiO2(aq)= CaSiO3+ CO2

W: wollastonite P: pyroxene G: garnet

C + O2= CO2

Effect of wall rocks


26/31

Geometry

- Massive/irregular vs. stratiform


Geometry Massive/irregular vs.

stratiform


Chang et al., 2007

Stratiform skarn

Chang et al., 2007


Meinert, 1992

Depth of formation

-Ambient temperature- Metamorphism- Retrograde alteration

-PermeabilityW skarn vs. Cu skarn


27/31

W skarn

Musc - hm - chl - qtz

Musc - hm - chl - qtz

Bio - sill - alm - ilm cord ksp qtz

Granodiorite

Bio - sill - ilm cord

Bio - adl - sill - ilm

Wskarn

Wskarn

Granodioriteporphyry

Skarn

Cuskarn


28/31

Distance from causative intrusion

Fluid temperatures

Distal skarnsare rich in Mn

Johannsenite,rhodonite,pyromangite,

bustmite

Meinert, 1992

Zonation in skarns - summary


29/31

QuizA granodiorite intrusion contains ilmenite but no magnetiteFe2O3/(Fe2O3+FeO) < 0.2

The wall rocks ~800m awayfrom the intrusion are dark incolour

Sandstone

Shale

Limestone

Quiz

If there are skarns between such an intrusion andwall rocks, how wide would the skarn zone likelybe?

1) A few meters

2) Tens of meters3) Hundreds of meters

Quiz

In the zoning pattern, which zone would be themajor one?

1) Garnet > pyroxene zone2) Garnet pyroxene zone

3) Garnet < pyroxene zone

Quiz


30/31

Around the intrusion some biotite hornfels containingdisseminated pyrrhotite were found. What kind ofeconomic metal(s) may the skarn contain?

1) Fe 2) Cu 3) Au

4) Pb-Zn 5) W 6) Sn

Biotitehornfels

Pyroxenehornfels

Quiz

In the intrusion there are some garnet veins with greenpyroxene halo close to contact, but there is no greisenalteration. With this additional information, please estimate

again what economic metal(s) the skarn may contain:

1) Fe 2) Cu 3) Au4) Pb-Zn 5) W 6) Sn

Quiz

Estimate the composition of the garnet. Is it

1) Rich in Al (grossularitic), or

2) Rich in Fe (andraditic), or3) Rich in Mn (spessartine)

Quiz

Narrow garnetmargins; majority

grossularitic

Garnet composition


31/31

1

2

34

Where wouldyou drill?Why?

Quiz

134

Quiz

Is the graniteporphyry thecausative

intrusion forthe skarns?Why or whynot?

Quiz

201306-30 skarn lima seg student chapter

Documents