Metal Mobility During Metamorphism - MERC

Metal Mobility During Metamorphism:
A Review
D.K. Tinkham
Laurentian University
Mineral Exploration Research Centre
Metamorphic metal mobility
NEOMMS 2014
2
Metal mobility in the crust – the culprits
•  Fluids facilitate mobility
•  Melts facilitate mobility
•  Deformation facilitates mobility
•  They sometimes (usually) work as a team
•  Scale of mobility is variable:
•  Up to kilometer-scale in a fluid
•  Potential remobilization of metals out of a deposit into wall rocks or from
one part of a deposit to another
•  Reconstitution of ore and movement of metals within an ore-body via
deformation/recrystallization +- fluids (not discussed here; see Marshall et
al. 2000)
•  We’ll cover concepts related to low-T metallic melts and
metamorphic fluids
•  We’ll only discuss some of the more interesting metals
NEOMMS 2014
Metamorphic melts
3
Melting of Broken Hill ore (granulite facies)
Mavrogenes, MacIntosh, Ellis (2001):
•  Observations suggested Broken
Hill ore melted at granulite
conditions
•  Experiments indicate addition of Ag
reduces melting temperature in PbZn-F-Ag-S system
Melting in Fe-Zn-Pb-S system at 1 atm.
Mavrogenes, MacIntosh & Ellis (2001)
Frost, Mavrogenes & Tomkins (2002)
Metamorphic melts
NEOMMS 2014
Melting of Broken Hill ore
1. 
2. 
3. 
4. 
5. 
FeS-PbS-ZnS
FeS-PbS-ZnS + 1 wt.% Ag2S
Same as #2 with Fe0.96S
PbS-FeS-S(liq); Brett &
Kullerud (1967)
PbS-Ag2S; Urazov &
Sokolova (1941)
Mavrogenes, MacIntosh & Ellis (2001)
4
Metamorphic melts
NEOMMS 2014
Low-T melting in LMCE systems
Frost – Mavrogenes – Tomkins (2002) model:
•  Melting can occur at low-T in low-melting point chalcophile
metals (LMCE) systems: Ag, As, Au, Bi, Hg, Sb, Se, Sn, Tl, Te
•  Initial melts enriched in
LMCE elements
•  At higher T, progressive
melting leads to higher
Cu & Pb
5
Metamorphic melts
NEOMMS 2014
Low-T melting in LMCE systems
Frost – Mavrogenes – Tomkins (2002) model:
•  Melting can occur at low-T in low-melting point chalcophile
metals (LMCE) systems: Ag, As, Au, Bi, Hg, Sb, Se, Sn, Tl, Te
•  Initial melts enriched in
LMCE elements
•  At higher T, progressive
melting leads to higher
Cu & Pb
6
Metamorphic melts
NEOMMS 2014
Low-T melting during metamorphism
Frost – Mavrogenes – Tomkins (2002) model:
•  Arsenopyrite -> As-S melt at 500-600 oC
Clarke (1960); Frost, Mavrogenes & Tomkins (2002)
7
Metamorphic melts
NEOMMS 2014
8
Low-T melting in LMCE systems
Some features used to identify them (Frost et al., 2002):
•  Irregular enrichment in LMCE’s
•  Multi-phase sulfide inclusions in wall rock
enriched in LMCE’s
•  Included in phase while above melting point of
inclusion assemblage?
•  Low grain-boundary angles
•  LMCE assemblages filling fractures
(without evidence for fluid-related
alteration).. Difficult
•  Mn- or Ca-rich selvage (seen at Broken
Hill, Cannington, Aguilar)
Most importantly, understand the relative timing of metamorphism
and mineralization and determine peak metamorphic temperatures
Metamorphic melts
Low-T melting in LMCE systems
Potential occurrences (Frost et al., 2002):
NEOMMS 2014
9
Metamorphic melts
NEOMMS 2014
10
What does all of this have to do with real mobility?
1. Granulite facies mobility (high-T)
•  Immiscible polymetallic melts (with Au) mobilized with silicate
partial melts at Challenger deposit (Tomkins & Mavrogenes,
2002); remobilization within an existing deposit
2. Amphibolite facies mobility (medium-T)
•  Low-T polymetallic melts via arsenopyrite and stibnite melting
and mobilization into shear zones and into low-strain sites at
Hemlo (Tomkins, Pattison & Zaleski, 2004); remobilization
within an existing deposit
Granulite facies mobility (high-T)
NEOMMS 2014
1. Challenger deposit, South Australia
•  Archean gold deposit in metapelitic migmatites
•  Peak metamorphism at granulite conditions (800oC, 7.5 kbars;
Tomkins et al, 2002)
•  Au zones are lineation shoots with higher silicate-melt content;
strain partitioned into silicate melt-rich zones
Tomkins et al., 2002
11
Granulite facies mobility (high-T)
NEOMMS 2014
12
1. Challenger deposit, South Australia
•  Peak metamorphism at granulite conditions resulted in fluid-absent
silicate rock melting
•  Identification of fluid-absent (dehydration) melting important as it
indicates influx of H2O is unlikely (though not entirely impossible)
Tomkins et al., 2002
Granulite facies mobility (high-T)
NEOMMS 2014
13
1. Challenger deposit, South Australia
•  Initial Au mineralization in deposit is interpreted as pre-peak metamorphic
•  Garnet constrained to have grown at peak conditions during silicate
melting
•  Discrete spherical inclusions of arsenopyrite + Au interpreted to represent
garnet including As-S Au-bearing melt during silicate rock partial melting
•  Coexisting silicate melt and arsenopyrite-Au melt.
Tomkins et al., 2002
Granulite facies mobility (high-T)
NEOMMS 2014
14
1. Challenger deposit, South Australia
•  Po + Ccp +- Pnt in mesosomes, melanosomes and leucosomes;
concentrated in melanosomes
•  Polymetallic melts within leucosome and melanosomes
Au in annealled fracture in Qtz;
no fluid inclusions
Tomkins et al., 2002
Granulite facies mobility (high-T)
NEOMMS 2014
15
Challenger summary
•  Deposit is pre-peak metamorphic
•  Au shoots associated with silicate melts (anhydrous melting) and
As-rich envelope
•  Polymetallic LMCE melt coexisted with silicate melt; silicate melt
mobilized during deformation, and LMCE melt mobilized with it
•  This would represent internal remobilization in the classification
scheme of Marshall (2000) as Au was apparently not mobilized far
distances to outside of the initial deposit region
Amphibolite facies mobility (medium-T)
NEOMMS 2014
2. Hemlo deposit
•  Tomkins et al. (2004) documented metamorphic polymetallic melts
•  Archean gold deposit
•  Middle amphibolite facies (600-650oC) metamorphism after main
mineralization event
•  Arsenopyrite (FeAsS) and stibnite (Sb2S3) melt mobilized through
compressional zones (scavenging Au) and into dilational sites to
crystallize and concentrate Au
Polymetallic melt
collection sites
Tomkins et al., 2004
16
Amphibolite facies mobility (medium-T)
NEOMMS 2014
2. Requirements for FeAsS melting
•  Need relatively high sulfur fugacity
( fS2 ) with increasing T
•  If Py and Po present (on Py-Po
‘buffer’) fS2 increases with increasing
T, but…
•  Fe oxides/silicates and low-S fluid
fluxing will consume S generated by
Py breakdown.
•  If Py is completely consumed before
reaching high fS2, and arsenopyrite
is present, the path will traverse up T
to where arsenopyrite is converted
to löllingite (FeAs2) and Po
•  If Py not consumed, move to higher
fS2 and arsenopyrite can melt
•  Other more complicated paths are
possible
Tomkins et al., 2004
17
NEOMMS 2014
Amphibolite facies mobility (medium-T)
2. Hemlo deposit, summary
•  Schematic melt fields in
LMCE section envisioned for
Hemlo polymetallic melts at
600-650oC (Tomkins, 2004
and earlier experimental
workers)
•  Remobilization and
concentration of metals within
an existing ore deposit
•  Again, facilitated by
deformation that played an
important role in the actual
metal mobility
Tomkins et al., 2004; others
18
Metamorphic fluids and metals
Metamorphic fluids and metals
NEOMMS 2014
19
NEOMMS 2014
Metamorphic fluids and metals
1 kbar, 1.5 m NaCl, 0.5 m KCl
Kfs-Ms-Qtz buffered pH
Tot S = 0.01m; Hem-Mt
Au solubility and speciation
•  Au speciation dependent on T & P,
salinity, pH, oxidation.
•  These show AuCl2- as the most
relevant species at high T (for the
given salinity of diagrams); if there is
less Cl in your fluid the diagrams are
not of use to you; other species will
be the dominant transporter of Au
Fields of predominance (HS-, etc.)
And Au solubility
1 kbar, 1.5 m NaCl, 0.5 m KCl
Kfs-Ms-Qtz buffered pH
Py-Po-Mt (max S = 0.1m)
Williams-Jones, 2009
20
Metamorphic fluids and metals
NEOMMS 2014
21
Metamorphic devolatilization model (Phillips & Powell, 2010)
•  Arguably, the model was formulated over decades of research by many
people, but they have modelled the fluid production and chemistry
•  Could be applicable to some ‘Au-only’ deposits in Archean greenstone
belts
•  Timmins, Red Lake and Hemlo goldfields cited as prime Canadian
examples
•  Basics of model:
•  Mid-crustal, carbonate-bearing hydrated mafic rocks with 1-4 ppb Au
•  During prograde heating, major pulses of fluid are produced via dehydration with a
range of H2O-CO2 compositions (dominantly around the greenschist-amphibolite
facies boundary)
•  This fluid incorporates S and Au from the source rocks themselves
•  The fluid is eventually mobilized upwards, partly in structurally controlled zones
•  The fluid is rock-buffered initially but during much of the upward migration the fluid is
not buffered by the host rock
•  eventually precipitates Au and can results in Fe and carbonate alteration
Metamorphic fluids and metals
NEOMMS 2014
22
Source zone – generating H2O-CO2-H2S fluid
•  Across the greenschist-amphibolite facies transition, a generic
reaction for fluid production from mafic rocks would be:
pyrite + chlorite + epidote + ankerite/dolomite + albite + quartz = amphibole
+ plagioclase + fluid + pyrrhotite
-  Cl-bearing
phases are of
low
abundance;
fluid is low
salinity
-  Fluid is rock
buffered such
that the fluid
composition is
dictated by the
source rocks
Phillips & Powell, 2010
Metamorphic fluids and metals
NEOMMS 2014
23
Incorporating Au
•  Thermodynamic model
- 
- 
- 
- 
Up to 5 wt% of fluid produced from 440-520 oC
0.15 million t fluid per cubic km
Au (soft cation) takes advantage of reduced S in fluid (which it prefers anyway)
Since Cl is low, base metals in fluid will be low
Phillips & Powell, 2010
Metamorphic fluids and metals
NEOMMS 2014
Incorporating Au
•  Thermodynamic model
Au Complex formation equilibria:
At constant S content, maximum solubility results when activities of H2S and
HS- are equal. This is a function of pH:
As the fluid will be rock-buffered during migration, it is advantageous to have
an internal fluid buffering capacity to resist major changes in pH in the early
stages of mobility:
Phillips & Powell, 2010
24
Metamorphic fluids and metals
NEOMMS 2014
25
Maintaining Au in solution
As the fluid will be rock-buffered during migration, it is advantageous to
have an internal fluid buffering capacity to resist major changes in pH in
the early stages of mobility. Due to the amount of CO2 in the fluid, the
aqueous species H2CO3 and HCO3- should be approximately equal, and
the predominance field boundary is at nearly the same pH as the optimal
H2S = HS- boundary.
Their calculations
predict 3-15 t of Au
will be liberated from
each cubic km of
source rock
Phillips & Powell, 2010
Metamorphic fluids and metals
NEOMMS 2014
26
Effectiveness of Au scavenging
Phillips & Powell argue that their dehydration model is very effective at
scavenging nearly all Au during dehydration due to:
-  Au is incorporated at the site of dehydration; nearly the entire rock is
reconstituted in the sequence of reactions, exposing all Au to the fluid
-  Models requiring the fluid to episodically travel through Au-bearing
rocks at a stage later than generation of the fluid is less efficient; fluid
in that scenario is unlikely to come into contact with all pre-existing Au
in the rocks through which it passes
Cox, 2005
Metamorphic fluids and metals
NEOMMS 2014
27
Au deposition – metamorphic devolatilization model
•  Fluid mobilizes Au (and other soft cations) upward 3-10 km
(starting at 500-520 oC; ending near 300 oC)
•  Suggest three common mechanisms for Au deposition are most
likely:
Decrease oxygen fugacity by interacting with reduced carbon horizons
(decreases pH, depositing Au)
2.  Decrease total sulfur in fluid (wallrock sulfidation)
3.  Increase oxygen fugacity to sulfate field (interact with hematite, or more
likely with magnetite:
1. 
magnetite + CO2(fluid) = siderite + hematite
Melts, fluids, metals,
NEOMMS 2014
28
Conclusions
•  Low-T LMCE melting in the metamorphic environment,
popularized by Tomkins, coworkers, and authors over the last
12 years, results in local mobilization of metals within existing
deposits or to nearby host rocks. Deformation plays a
fundamental role in this process, potentially upgrading a
deposit.
•  Mobility of some metals in metamorphic fluids potentially
operates over the km-scale. Again, deformation plays a
fundamental role.
•  Metamorphic metal mobility via a fluid phases is likely at the
deposit scale. Again, deformation could play a fundamental
role in promoting delivery of metals to the fluid phase.
NEOMMS 2014
29
References
•  Cox, S. F., 2005, Coupling between deformation, fluid pressures, and fluid flow in ore-producing hydrothermal
systems at depth in the crust: Economic Geology, v. 100th Anniversary Volume, p. 39-75.
•  Frost, B. R., Mavrogenes, J. A., and Tomkins, A. G., 2002, Partial melting of sulfide ore deposits during medium-
and high-grade metamorphism: Canadian Mineralogist, v. 40, no. 1, p. 1-18.
•  Marshall, B., Vokes, F.M., and Larocque, A.C.L. 2000, Regional metamorphic remobilization: upgrading and
formation of ore deposits, in Metamorphosed and Metamorphogenic Ore Deposits, eds. Spry, P.G., Marshall, B.,
and Vokes, F.M., 310pp.
•  Mavrogenes, J. A., MaciIntosh, I. W., and Ellis, D. J., 2001, Partial melting of the Broken Hill galena-sphalerite ore:
Experimental studies in the system PbS-FeS-ZnS-(Ag2S): Economic Geology, v. 96, p. 205-210.
•  Phillips, G. N., and Powell, R., 2010, Formation of gold deposits: a metamorphic devolatilization model: Journal of
Metamorphic Geology, v. 28, no. 6, p. 689-718.
•  Tomkins, A. G., and Mavrogenes, J. A., 2002, Mobilization of Gold as a polymetallic melt during pelite anatexis at
the Challenger deposit, South Australia: A metamorphosed Archean Gold deposit: Economic Geology, v. 97, p.
1249-1271.
•  Tomkins, A. G., Pattison, D. R. M., and Zaleski, E., 2004, The Hemlo Gold Deposit, Ontario: An example of melting
and mobilization of a precious metal-sulfosalt assemblage during amphibolite facies metamorphism amd
deformation: Economic Geology, v. 99, no. 6, p. 1063-1084.
•  Williams-Jones, A. E., Bowell, R. J., and Migdisov, A. A., 2009, Gold in Solution: Elements, v. 5, no. 5, p. 281-287.

Download Report