Porphyry and Epithermal 101
!"#$%&"' ") % *"#*+,#, -. /0*"123 4 !"#$%&'() +$#,$$" -(.-(/0 ("1 2)1%3#2$%-(' &%30$44$4
Hydrothermal alteration in silicate rocks Examples from Reko Diq porphyry Cu-Au, H15 complex, Pakistan
Richard M. Tosdal, Ph.D., PGeo PicachoEx LLC (U.S.A)
[email protected]
SEG student chapter, UNI, Lima, Peru, October 2015
Summary of Igneous Processes • Magma compositions are variable, & dictate ore metal suite • Trace element contents are also unique to each magma • As magma crystallizes, it saturates in water-rich fluid rich in chlorine & sulfur , and this forms hydrothermal ore fluid. • During crystallization a common suite of incompatible elements is concentrated in last melts, & can then be incorporated into the HT ore fluid. • So, in the “porphyry copper” hydrothermal footprint, a similar suite of ore metals and “trace metals” are transported --- can be potentially used for vectoring
531!6$1 7%3- 8!''$49 :;<=
!"#$"%& ()*+,-. / 01-2)34, 5*+, +- 67+$"7+)8 9 :;< / =)> ?&@)27+,4A +- ,2$+B"BC "B+2B +B $2-C 41-2)34, @8,72C@47$") 5*+,D &2$62-+B# *6 C2 :E $2)")D &2$$2B)8 F>GHD I*C J C2 KE LCMH N"=)< / (> $"8 I4 4O*"))8 +$627C"BC +B PBD !2 #7"B+C4-D 64#$"%C4-< / 9P>D 9P;QJ>D =;F:> "74 -*I27,+B"C4M / >'?!14 %$'$(4$1 7%3- #)&!0(' -$#('?-!"3?4 .%("!#$4 (%$ 02(%.$ +('("0$1@ / C2C") =)> R 9P> R 9P;Q> S 9R N"R R TR R :MF?(4R: R (4RFA R :="R: R :!#R: R :!BR:M / !"#$"%& =)>7+&@ -2)*%2B- "74 -)+#@C)8 "&+,+& ?9=)AM
Typical Pathfinder Elements in Magmas
Metals in magma & fluids
-#0%&"' %'/ /013#.5&"' ") *0#$0%62723, *0#$0%62723,
Metal
Continental Crustal Behavior in Hydrothermal Complexation Background(ppm) # Silicate Melt Chloride Sulfide __________________________________________________________________________ ChalcophileMetals Ag (silver) 0.08 Incompatible XX X As (arsenic) 1 Incompatible X X Au (gold) 0.002 Incompatible XX XX Bi (bismuth) 0.06 Incompatible X X Hg (mercury) 0.08 Incompatible X XX Co (cobalt) 10 Compat in Olivine XX -Cr (chromium) 140 Compat in FeMag XX -75 Incompatible XXX X? Cu(copper) Mn (manganese) 1400 Compat. in FeMags XXX -Mo (molybdenum) 1 Incompatible --Ni (nickel) 20 Compat in Olivine XX -Pb (lead) 8 Compat. in Feldspar XXX -Sb (antimony) 0.2 Incompatible X X? Se (selenium) 0.05 Incompatible X X? Te (tellurium) 0.001 Incompatible X -V (vanadium) 190 Compat. In FeMag & Ox XX -Zn (zinc) 80 Compat. in FeMags XXX --
98,72C@47$") ")C47"%2B &74"C4- 647$4"I+)+C8 C@72*#@ @8,72U7"&C*7+B# @8,72U7"&C*7+B# "B, ,4-C728- 647$4"I+)+C8 C@72*#@ $+B47") 674&+6+C"%2BM
Compatible – can stay in magma Incompatible – can go into fluid Many trace elements chloride complexed, & so easily transported
Chalcophile Oxyanion Alkalis
OxyanionMetals B (boron) Sn+4 (tin) W+6 (tungsten)
10 2.5 1
Incompatible Incompatible Incompatible
-XX XX
V464%%34 672&4-- L+C@ 7464"C4, 5*+, 6*)-4-
----
01C47B") 5*+, +BC47"&CL+C@ $"#$"%&> @8,72C@47$") 5*+, "C +BC47U"&4MM WBC47"&%2B 3+" ")C47"%2B $+B47")2#8
Alkalis & Alkali Earths Ba+2 (barium) 340 Compat in Kspar XXX -Cs+ (cesium) 1 Compat in Kspar XXX -+ Li (lithium) 13 Compat in FeMag XXX -Rb+ (rubidium) 60 Compat in Kspar & Bi XXX -Sr (strontium) 360 Compat in Plagio XXX - Tl+ (thallium) 0.36 Incompatible XX -__________________________________________________________________________ #
Average crustal abundances S.R. Taylor, S.M. McLennan, 1985
Evolution of porphyry hydrothermal system Main factors controlling metal deposition and mineral alteration • oxidation state of fluid - control on oxide, sulfate and sulfide deposition
• disproportionation of SO2 with decreasing T - largely temperature controlled, increases ratio of H 2S to SO2 and favors sulfide deposition [4SO2 + 4H2O H2S + 3HSO4 + 3H+]
• Interaction of SO2 gas with plagioclase or mafic silicates (sulfidation reaction) increases ratio of H2S to SO 2
• fluid acidity
Seedorff et al., 2005
- controls alteration patterns, affects
• host rock composition and external fluid
aH2S. Acidity increases (pH lowers) with deceasing T°C as acids disassociates
composition – dictates mineral phases being precipiated
Weis, Driesner & Heinrich, 2012, Science
Common alteration assemblages Possible Mineral Assemblage
Process / composition
Generitic Term
Kaolinite-montmorillonite (± illite- chlorite)
K, Ca, Mg-metasomatism
Argillic
Andalusite – pyrophyllite - kaolinite te (± quartz – muscovite / illite)
K, Ca, Mg-metasomatism
Advanced argillic
Chlorite-sercite (± montmorillonite-illite-smectitecalcite-epidote)
K, Ca, Mg-metasomatism
Intermediate argillic
Muscovite (sericite) – quartz -pyrite ± chlorite
K, Na, Ca, Mg-metasomatism
Phyllic / Sericitic
Albite ± epidote-chlorite-hematite
Na, Ca, Mg-metasomatism
Sodic
K-feldspar ± biotite-quartz-sericite-albite-anhydriteteepidote / actinolite / pyroxene / garnet
K-metasomatism
Potassic / K Silicate (Calc-postasic)
Biotite ± K-feldspar-magnetite-quartz-albite-an K-feldspar-magnetite-quartz-albite-anhydrite hydrite
K-metasomatism
Potassic / K Silicate
Garnet-epidote-actinolite-chlorite-carbonate± magnetite-hematite
Ca-, Na-metasomatism
Calc-silicate
Chlorite-epidote-albite ± carbonate-sericitemontmorillonite-pyrite-hematite
Ca-Mg-metasomatism
Propylitic
Actinolite-chlorite-albite ± epidote
Ca, Na-metasomatism
Sodic - Calcic
Evolution of porphyry hydrothermal system
Depth influence on disproportionation reaction 8' 9"#*+,#, -. 1,130$1: $"13 ") 3+0 ; 21 2'2&%77, 2' %' "<2/2=0/ )"#$> ;?@
Decreasing T increases dissociation of salts & acids (K+ and H+ increase relative to KCl & HCl) ! dissociation of acids (HSO4-, HCl, H2CO3) leads to real acidity increase and sulfide deposition
!"#$"%& P;: ,+-672627%2B"C4- C@72*#@ C@4 C4$6 L+B,2L U72$ QKE C2 FKE 2 QP;: R Q9:; S F9:P;Q R 9:P
Important alteration types Hydrolytic (acidic), H+ = K + Alkali exchange, Na+ = K + Precipitation/dissolution, quartz Oxidation/reduction (S2, O2)
Greater fluid rock interaction ! neutralizes acidity (H )
X@+- #4B47"C4- 74,*&4, P C2 66C -*)Y,4-D "B, "&+, C@"C &2B347CU4),-6"7 C2 -47+&+C4M
Depth constraint on reaction may provide an explanation for abundance or lack of magnetite in porphyry Cu deposits J.H. Dilles, unpublished 2010
After Seedorff et al. (2005)
A: B C D E02'1 57%112F5%&"' ?Z*-C"U-2B [ 9*BCD J\]KA>> )+B^+B# 34+B- C2 ")C47"%2B V4)"%2B C2 _"))>72&^ ")C47"%2B [ 274 JM `ab "6)+%& O*"7Cc 34+B?=68 d eB d !"#A "--2&+"C4, L+C@ T>-+)+&"C4 ")C"%2B :M `eb 3B- ?OCc>!2P :A L+C@ &4BC47)+B4D -C"I)4 +B T> -+)+&"C4 FM `fb 34+B- ?g8 27 =68>g8 27 g8>eBA L+C@ L")) 72&^ ,4-C7*&%34 -47+&+%& ")C47"%2BM WB " #+34B 668 +BC7*-+2B &4BC47D a 3B- "74 &*C I8 e 34+B- &*C I8 f 34+B?X4$647"C*74 ,4&74"-4A
A730#%&"' ;07E%G0 !"#$%&"B> ()*+, $234- ")2B# 34+B ?",34&%34 C7"B-627CAD "B, ,+h*-4- +BC2 72&^ +B 6274 5*+,
• Wedge of hydrothermally altered rock forms with time and advances • Pervasive alteration = overlapping vein selvages Geiger et al.,2001
_")) 72&^D 5*+, &2$62-+%2BD "B, C4$647"C*74 &2BC72)L@+&@ @8,72C@47$") $+B47") 674&+6+C"C4• 150°C • illite + chl ± kaol • illite + chl ± Ksp • 300°C • Musc(Phen) + Chl ± Ksp or • Bi + Ksp • Pyroph (AA)
Hydrothermal mineral assemblages and association
2 ) +
H ( /
+ 2
g M f o s e i t i v i t c a g o L
• 400°C • Ksp-Musc-Bio Log activities of K+/H+
!07/1*%# *#052*23%&"' C@4 72)4 2U &22)+B# 347-*- @4"%B# 5*+,• Alkali exchange KAlSi3O8 + Na+ = NaAlSi3O8 + K+
Silica (Qz) Solubility--All SiO2 is in solution in condensed fluid (not vapor) Magmatic Fluids A-B vein Qtz in hi-T cooling Dissolution ~450°C
Cooling of magmatichydrothermal — potassic alteration
Heating of circulating connate or groundwater — sodic alteration
Seedorff et al., 2005
D veins Qtz <400°C
Circulating non-mag fluids Dissolve silica on heating to 400°C Precipitation of Qtz causes sealing, so cannot access high-T areas
K+/Na+
Brines can access up to ~500°C
CL image Butte qz (Rusk&Reed)
Temperature effects of metal sulfide precipitation 01647+$4BC- +B 2B4>6@"-4 Y4), 2U L"C47>N"=) V2&^>I*h474, "--4$I)"#4 iCc>T-6>g)"#>!*-&>g8
Sequence of sulfide minerals depends upon Fe/Cu ratio in fluid •
Early hypogene with high Fe/Cu
•
Sericitic alteration removes Fe, leading to higher S state in superposed minerals
=*>(4>P 300°C
Pb+2 + H2S = PbS + 2H +
Arrow in direction of increased fS2
016)"+B- 647+6@47") #42&@4$+&") c2B+B# "B, )"&^ 2U gI>jB +B 6276@878 =* &274 Courtesy of John Dilles
Porphyry intrusion and hydrothermal evolution
Successive porphyry intrusions rise to level of neutral buoyance following breach of carapace of underlying magma chamber – follow fractures generated by exsolved fluid Begins to crystallize, increasing fluid pressure until exceeds load, leading to catastrophic fracturing, transitions to hydrostatic pressures, and quench crystallization of magma
Porphyry characteristics • Genetic association with porphyritic granitoid rocks that both – are porphyritic have a sugary (micro-aplitic), fine-grained groundmass – are multiphase, most commonly with decreasing phenocryst abundance and size with decreasing age
Calcalkalic PCD
Calc-alkalic PCD
Hnbd-bt granodiorite porphyry, Reko Diq, Pakistan
Hnbd-bt granodiorite Porphyry, Quellaveco, Peru
Alkalic PCD Alkalic PCD
A3313..3"$ 1!4#%!0#9 BC C("(1(
Bingham, Utah
Biotite Quartz Monzonite, North Parkes, NSW
Biotite syenite porphyry, Galore Creek, BC
Multiple Igneous Events are Associated with Multiple Hydrothermal Events Porphyry dikes are Accompanied by K-silicate & Sericitic alteration & are Followed by Sodic-Calcic, Propylitic-Act, & Related Chlorite-Sericite-Hematite±Mag ± Cu-Au
Early stage porphyry emplacement, fluids unmix to brine + vapor; vapor ascends to near-surface
Dilles et al, in revision
Intermediate stage porphyry emplacement, main porphyry Cu-Mo (-Au) ore stage from central plume exsolved from magma; entrainment of non-magmatic water on flanks of system
Dilles et al, in revision
Late stage, deeper-sourced magmatic fluids penetrate hightemperature system at lower temperature; external fluid traverses through the porphyry Cu deposit
Dilles et al, in revision
D213#26.&"' ") %730#%&"' )%5201 %'/ $2'0#%7 %110$67%G01 2' *"#*+,#, -. 1,130$
Magmatic to hydrothermal transitions — Collection of volatiles at tops of porphyry intrusions
Metal-rich volatiles exsolve from the magma during crystallization
Dilles, Tosdal, Halley, 2011
Examples of magmatic-hydrothermal transition
Examples of magmatic-hydrothermal transition
Growth direction Quartz UST with late cpy Uni-directional solidification textures
k4+B > ,+^4
=68>eB>!"# Y))4, 64#$"C2+, &"3+C8
=68 ,726)4C
• mineral growth from the roof and walls of a magma body that is crystallizing downward and inward • Common beneath areas of high grade
Multiple UST
Quartz veins record loss of volatiles upward from horizons marked by quartz UST layers Sample oriented so that quartz grains in UST layers are pointing downward and inward
H2G+ 30$*0#%3.#0 %730#%&"' 4 J 12725%30 %'/ *0#2*+0#%7 *#"*,772&5 D$.!3"(' ("1 E$!" 03"#%3''$1
Many veins at top of sample; few veins at bottom of sample Offsetting vein
e r s U S T l a y
G r o w t h d i r e c ti o n
B%3. H2I%.
Circles mark sites of fluid release events: earlier in lighter colors, later in darker colors
Fluids emanate from below sample
From Garwin, 2000
Lab 3/4 sample MP-B, Mineral Park, Arizona; courtesy of Eric Seedorff
High temperature pervasive K-silicate (potassic) alteration Qtz-bt-Kspar bn-cpy
Pakistan
Kspar-bt-mag
British Columbia
Chl after bt-cpy
Papua New Guinea
Mag-bt-py
Serbia
Early high temperature veins (not common in shallowly emplaced systems Early mus + bornite + cpy ± Kspar
Valley, High land Valley, Canada
Early dark micaceous veins W/ biot + sericite + Kspar; Abundant Cp + Mag + Py
Anaconda Dome, Butte, USA
Early dark micaceous vein with biot + chlorite + bornite + cpy + mag
Relincho, Chile
Irregular qtz-bn-mag
High temperature veins
Sheeted quartz-magnetite veins common at tops of porphyry intrusion
A range of type and mineral infill
Sheeted qtz-cpy
Butte USA
Irregular sugary quartz with K spar halos
Qtz & anh & Kspar vns
Calcic-potassic alteration
Sulfide occurrences Disseminated
Center line
Sulfide only
Qtz-bt-Kspar bn-cpy
– – – – – –
Oyu Tolgoi, Mongolia
Reko Diq, Pakistan
SW Oyu Tolgoi, Mongolia
• Occur mostly in syenitic and certain monzonitic porphyry copper deposits • Calc-silicate minerals and ore minerals occur in igneous rocks • Mineralogy
Batu Hijau, Indonesia
K-feldspar Biotite Anhydrite Garnet Diopside Actinolite
K%#7, %'/ /00* L% %730#%&"' • 0"7)8 ")I+C4 746)"&4$4BC 2U +#B42*U4),-6"7 L@+&@ +- C@4B 234767+BC4, I8 C86+&") 62C"--+& ")C47"%2B $+B47") "--4$I)"#4• =2$$2B)8 )2##4, "- 62C"--+& ")C47"%2B • g227)8 *B,47-C22, ")C47"%2B C864 L+C@ 67262-") C2 I4 $"#$"%& 27 -")+B4 41C47B") 5*+,• !"8 I4 $274 L+,4-674", C@"B &*774BC)8 "&&46C4,
Sericitic (phyllic) alteration — H+ metasomatism •
Forms at <350-500°C (lower temperatures ge nerally require slightly higher pH, acidic conditions).
•
Typical assemblage is sericite – quartz – pyrite ± chlorite
•
In mafic rocks where there is a paucity of K in the protolith get replacement of Na and Ca by H+, but high (Fe+Mg)/K results in abundant chlorite rather than sericite Pervasive sericite-pyrite-qtz, Red Chris
906670: M;A F(". ("1 G%$.3%)9 HI:H
G%$.3%)9 HI:J
D-vein, Bethsaida, Highland Valley
District-scale distribution of hydrolytic alteration Sericite - chlorite (green sericite)
Transition stage between K-silicate and sericite alteration Characterized by intergrown chlorite and phengitic muscovite (sericite)
Simuku, PNG Sillitoe, 2010
Advanced argillic alteration (Lithocap environment)
N%30 A730#%&"'
• X86+&"))8 ,434)264, +B )+C@2&"6 4B3+72B$4BCD U27 41"$6)4 O*"7Cc>")*B+C4 46+C@47$") -8-C4$- "B, "I234 6276@878 ,462-+C• (27$- 2347 " L+,4 C4$647"C*74 7"B#4< lJEE>GEEm= • a&+,+C8 2U 5*+,- +- ^48 &2BC72) 2B ")C47"%2B • f2$+B"C4, I8 O*"7Cc 6)*- &2$I+B"%2B- 2U 68726@8))+C4D ,+&^+C4D ^"2)+B+C4D ")*B+C4D 4C& Barren lithocap at Cerro Casale, Chile
Barren lithocap at Yerington district, U.S.A
Vertical zoning in PCD system
Veins external to the mineralized porphyry system
Leached cap
• Vertically rising buoyant plume cools, creating sericite alteration immediately above the porphyry, overlain by advanced argillic • Late D-style veins can escape laterally, traveling up to 5 km, and at Butte up to 10 km laterally PCD
• The most widespread and common selvage minerals are sericite or fine-grained muscovite replacements of feldspar and mafics, and chlorite replacements of mafic minerals. Chlorite occurs in upper and outer exposures of the HT alteration zones and where mafic content of rock increases.
• At shallow depths above porphyry system, specularite may be present due to the lack of sulfur in the hydrothermal fluid Fluid saturated carapace
Lateral D-vein form swarms or discrete veins, sometimes only mm thick, and can be mineralized
Vertical, concentric D vein selvages at Ann-Mason, Yerington, Nevada (USA) Shallow (<1 km depth)
Courtesy of John Dilles
Deep, Ore Zone
Note, in the S-poor and upper parts of the system, specular hematite is stable in the SC zones, and locally in the S zones rather than pyrite
.
At transition to advanced argillic; quartz replaces wall rock adjacent to fluid channel
B
Central Morococha district: mine level 400 - 4375 m.a.s.l.
Manto Italia
N-8719000
n"C47") C2 $"B8 I*C B2C ")) 6276@878 =* &4BC47- "74 c2B4, +BC47$4,+"C4 C4$647"C*74 34+B-M
SierraNevada
Ag - Pb
BrechaRosita
Montero basalt Duvaz
Zn-Pb-Ag Huamachuco
Sericite-advanced argillic transition
Central quartz vein is rare in the sericite zone; most of alteration is conversion of rock to white mica; phengitic muscovite at depth but muscovite at shallower levels
Codiciada composite stock
?
Zn - Cu Ri queza 100 or e body
N-8718000
Ombla, lower
BrechaRiqueza
Ombla, upper
Cu Sulfurosa pyritebodies
Gertrudis porphyry
SanFrancisco porphyry
Toromocho
Churruca
Sulfurosa R e ct ifi c ad o ra
Legend
N-8717000
fluidinclusion sample location base metalvein
SanFrancisco
E - 3 7 5 0 0 0
?
massive base metal mineralization massive pyrite replacement bodies
Toromocho composite stock
porphyryintrusions (dioritic- granodioritic) Triassic-JurassicPucará carbonaterocks,skarn
A
View to the San Francisco porphyry center, Morococha district
? N-8716000
Triassic-JurassicPucará carbonate rocks 0 TriassicMituvolcanicrocks
?
0.5
E - 3 7 6 Km 0 0 0
E - 3 7 7 0 0 0
1 km
Manuelita
Codiciada composite stock
SSW
E - 3 7 8 0 0 0
NNE 4800 m
Anticona intrusion
1
Anticona intrusion
2 3
1 6
A
1
4
Lithologies
Sericite
Sericite
4375 m
5
quartz feldspar porphyry Toromocho pipe breccia composite porphyry intrusions stock (dioritic - granodioritic) ? Anticona diorite Triassic-Jurassic Pucará carbonate rocks, skarn Jurassic Montero basalt Triassic-Jurassic Pucará carbonate rocks, silicified Triassic-Jurassic Pucará carbonate rocks Triassic Mitu volcanic rocks
. .
X@4-4 "74 f>C864 34+B- L+C@ @8,72)8%& ")C47"%2B 2U L")) 72&^-
B
base metalsulfide bodies pyrite-quartz bodies
2
1 6
1 porphyry-type mineralization 2 contact skarn 3 pyrite bodies
3
4 skarn-hosted sulfide bodies (Manto Italia)
San Francisco intrusion
4000 m 5 tube-like replacement bodies 6 Cordilleran polymetallic veins
="C&@62)4 , .4C . . .")M :EJK . 1
–1
,
,
.
.,
Evolution of porphyry hydrothermal system Late, low T stage, <~250°C (mainly post-ore) • Fluids cool while reacting with host rocks, • Fluids become increasingly rock buffered —H+ in the fluid is consumed in exchange for cations in the rock as the rock passes into the intermediate argillic alteration (smectite-illite-kaolinite with K-feldspar stable). • Magmatic fluid is generally mixing with an external fluid source • Characterized by stability of K-feldspar; biotite and plagioclase replaced
Propyllitic alteration
Intermediate Argillic Alteration
672#74--+34 @8,7"%2B ?32)"%)4 ",,+%2BA Illite - chlorite over late porphry, Red Chris
• (27$- "C C@4 -"$4 %$4 "- #4C @+#@>X 5*+,- &2$+B# +B &274 2U -8-C4$ • (27$- +B &2*BC78 72&^• WBC47"&C- L+C@ 9+#@>X 5*+, ")2B# $"7#+B- 2U 6)*$4 • =+7&*)"%2B 2U C@4 5*+, +- ,7+34B I8 C@4 C@47$") 4B47#8 2U C@4 &"*-"%34 I"C@2)+C@ • =@"7"&C47+c4, I8 &2$I+B"%2B 2U 46+,2C4D ")I+C4D &")&+C4D 687+C4D &@)27+C4D +))+%& &)"8D "B, "C @+#@47 X I8 "I*B,"B&4 2U 46+,2C4 "B, "&%B2)+C4
Kaolinite overprint on plagioclase outside D vn, Highland Valley
Illite overprint on biotite, Reko Diq Courtesy of David Cooke
Propylitic alteration
Distal alteration from the thermally driven circulation Temperature increase toward core Changes include: 1. Mineralogy – increase in epidote; at depth transition from epidote to actinolite/ tremolite 2. Chemistry – Zn halo 3. Mineral chemistry (e.g. Fe3+ in epidote)
1 km
From Garwin, 2000 Map from Norman et al, 1991
Na-Ca alteration Influx of brines into porphyry system Removes most elements and precipitates Na-rich plagioclase and Ca-bearing minerals (actinolite at higher T and epidote at lower T)
K0/"3'!#$L#%$-3'!#$ M ('+!#$ N $&!13#$O 02'3%!#$
Alteration probably not higher temperature than 400° C
Na-Ca alteration may destroy grade
Aerial Photo of Blue Hill Fault Block -- the faulted western half of Ann-
View (looking south) of East Jersey (left pit) and Jersey open pits
Mason, Yerington District, Nevada
Magmatic upflow N">=" ")C474, 72&^-
Regional circulation
Buried PCD
Tremolite-albite over cpy-py veins
Spatially related systems can lead to large overlapping hydrothermal alteration
Hydrothermal alteration distribution of Reko Diq porphyry cluster
Propylitic
Sericitic (phyllic) 1 km
Transitional ser-chl (clay) 4 km
Intense potassic
CN
Superposition of epithermal over porphyry (telescoping)
9"#*+,#, -. /0*"1231 %#0 5#.13%7 15%70 $%G$%&5 +,/#"3+0#$%7 1,130$1
Superposition of higher Cu-containing minerals formed at lower temperatures (~300±°C), commonly in veins over older higher temperature (>400°C) hypogene sulfide minerals associated with K-silicate alteration •
Described at Butte (Brimhall, 1979), but recognized as a common feature of many porphyry Cu-Mo systems (Chuqui, El Salvador, Escondida, Collahuasi)
•
Formed late in the hydrothermal evolution of porphyry, locally a couple of Ma after the K-silicate phase (Escondida – Padilla-Garza et al., 2001; perhaps Chuqui – Ossandon et al., 2001)
•
Commonly forms veins characterized by high-sulfidation type minerals (bornite, chalcocite, covellite)
•
Usually present in rocks dominated by sericitic (hydrolytic) alteration
•
Referred to as Cordilleran base-metal lodes in Peru (Bendezu et al., 2009) and considered by many to be “epithermal” in character
Yerington cross-section from Dilles et al. (2000)
Sequence of events at Butte
O%2' 13%G0 E02'1
Houston et al., in press
66 Ma
•
Formed two lower grade (~0.3-0.4% Cu) centers
•
Dominated by Ksilicate alteration and extensive EDM type veins
64 Ma 65 Ma
Simplified geology of Butte (Rusk et al., 2008)
~62-63 Ma ~350° !275°C
Houston et al., in press
• Area of intense hydrolytic (muscovite – quartz) alteration formed last, lying between porphyry centers
•
•
• Associated with pyrite •
Houston et al., in press
Fixed available Fe in rock
Main Stage Metal Zoning in East-West long section (Proffett, 1979, 1999, ms)—note intrepretation of 20° tilt to west Interpretation: deep sericitic alteration zone is feeder to Main Stage
Replacement of lower S state (cpy) minerals by higher (bn – cc – cov – en) Leaching and oxidation of cpy from older PCD inferred to be source of Cu in high sulfidation Cu-Fe±As sulfides in Main Stage veins
Chuquicamata Cu-Mo porphyry deposit Late qtz-enargite-covellitechalcocite-bornite veins superposed on lower grade chalcopyrite-bornite±K silicate altered rocks Early qtz-Mo veins
Pre-Main Stage Cu-Mo
Complex and probably superposed hydrothemal events Cu veins general NE to NW strikes (inherited from Pz fabrics?) (Lindsay et al, 1995; Ossandon et al., 2001)
enargite
covellite
chalcocite
pyrite
Ossandon et al., 2001
Chuquicamata cross sections
What happened? Fluid composition has evolved with decreasing temperature, increased fluid:rock ratio, and time to higher sulfidation state, thus enhancing the precipitation of late bornitechalcocitecovellite-enargite
Ossandon et al., 2001
Sequence of minerals depends upon Fe/Cu ratio in fluid •
Early hypogene with high Fe/Cu
•
Sericitic alteration removes Fe, leading to higher S state in superposed minerals
=*>(4>P 300°C
Arrow in direction of increased fS2 Courtesy of John Dilles
Telescoping of epithermal on PCD
Exhumation brings porphyry environment closer to surface, changing physiochemical environment of ore deposition
However in some deposits the host rocks dictate the hypogene mineralogy, which may be similar to that characterizing hypogene upgrading (telescoping)
Q0R" D2S -"$*70<
Example from Reko Diq, Balochistan, Pakistan
P0"7"G, 01C4B-+34 JE 1 JE ^$D 472,4, 32)&"B+& Y4), L+C@ &)*-C47 2U Jo 6276@878 =*>a* ,462-+C-
O2"50'0 *"#*+,#, 2'30#E%71 :FMF p ::MQdEMQ !" ?X"Bq44) R P"+B,"^A JoMQd:MK p JQM:GdEMJ\ !" 9I) 6@"-4 " JFModEM: p JJM]FdEMEQ !" a* 7+&@ 6@"-4 " JJMQdEM: p JEMKdEM: !" =*>a* 6@"-4 " "
Q01".#501 PHII;Q 9JQ>JK S QJEE !C rEMKEH=* [ EMFE#sC a* X"Bq44) S :J Q !C rEMGH P*647#4B4 =*
H-14
Diamond core drilling at Reko Diq, 2009
H-15
Section from Razique, in prep
Supergene modification (oxidation and upgrading)
H-15 – sericite-chalcopyrite-covellite
H-15 – sericite-pyrite-bornite-covellite
Removes: Cu, Zn, Fe, Mn, K, +/- As, Au Remaining: Mo, Pb, Ag, Mn, Fe
Supergene alteration accompanied by argillic alteration. Argillic alteration superposed over higher temperature, which can be seen texturally preserved Low temperature conversion of minerals to acid stable phases (e.g. K-spar to illite / kaolinite) Toquepala, 1976
Commonly confused with sericitic alteration, the higher temperature alteration phase
Supergene Cu & Fe Chemistry A) Source-0)Pyrite-rich rocks 0)Transport by acids from pyrite
n o i t a d i x O
B) Precipitation 1) Reduction on pyrite 2) Neutralization on feldspar
But, can be distinguished due to presence of phyllosilicate crystals in the sericite alteration and lack in the argillic
Variation in sulfide character at the Toki cluster, Chile
Supergene enrichment profile Cuajone, Peru g2-C>$+B47") 9*"8)+))"- ($
g2-C>$+B47") =@*BC"&"))" ($M
94$"%C4 21+,"%2B 2U &@")&2&+C4 c2B4
Higher pyrite produced better sulfide enrichment P*647#4B4 &@")&2&+C4
vB21+,+c4, -47+&+C4> 687+C4> &@")&2687+C4
t"72-+C4 R #24C@+C4 "u47 -47+&+C4>687+C4 72&^
Development of oxidation profiles: Cu + Fe + (Mn,Al) mobility during oxidative destruction of sulfides Fe+++ + H2O
Effect of sulfide content on supergene enrichment and oxidation
Fe(OH)3 + 3H+(aq)
Fe(OH)3
FeOOH(s) + H2O
Low pyrite leads to in situ oxidation with little Cu transport and upgrading of Cu content Sulfide content affects acid generation capacity and ability to form supergene or oxide Cu deposit
Rivera et al., 2010
1. Supergene enrichment is a function of pyrite content 2. Low pyrite yields poor enrichment due to low acid generation capacity 3. High pyrite yields potential for enrichment 4. Enrichment requires somewhat stable erosion / exhumation 5. Semi-arid climate best (e.g. Andes, SW US) 6. Too high of erosion rates and wet climates exhume the deposits too fast for significant enrichment to form, thus a mixed sulfide content 7. Mo and Au will be residual in leached cap 8. Au may be enriched to higher grade due simply to mass removal digenite Cu9S5
Chalcocite Cu2S
Porphyry Cu deposits have: 1. More or less predictable distribution of alteration assemblages 2. Alteration assemblages form from the interplay between exsolved magmatic-hydrothermal fluid and an external thermally driven fluid 3. Grade correlates with vein density and with sequence of porphyry intrusions 4. Supergene enrichment enhances the economics of deposits (most extensive zones probably been found and mined unfortunately) 5. Represent the prize for base metal companies as the giant deposits have >100 year mine life
Toquepala, 2005
