![Richards (2003)
Inflation +
lateral
propagation
Cupolas –
fractionated, volatile
(fluid)-rich dacitic
magmas forming sub-
volcanic stocks/dykes
Batholith dynamics
• Porphyry deposits generally in and around tall,
narrow stocks/dikes (~<2km2), emplaced at shallow
depths (1-4km)
- stocks/dikes develop in cupolas/apophyses off deeper
batholiths (6-10km)
- Insufficient Cu in shallow stocks/dikes to form Cu deposits;
• must derive from deeper magma chambers
• need a way to concentrate Cu from this magma chamber
! exsolved volatile(fluid) phase!
Average Cu in andesitic magma ~60ppm
Total Cu in giant deposit ~10Mt
10Mt/60ppm = 1.7x1011 t of magma
Volume of magma = (1.7x1011 t)/(2.7t/m3) ~60km3
[@ 100% efficiency Cu removal]
volatile exsolution
cupolas
Comb quartz layers in intra-
mineral monzonite,
Ridgeway, NSW
Courtesy of David Cooke
hydrothermal fluids](https://image.slidesharecdn.com/07porphyryii-250202062855-61f301b2/85/07-Porphyry-II-system-to-mining-exploration-2-320.jpg)
![From Heinrich (2005)
Cu-rich fluid pooled at
top of cupola.... Poised
to make a porphyry
deposit
Genesis of primary mineralization
•build-up of volatile-rich
magma at top of cupola causes
increase in Pfluid owing to
- volatile exsolution and volume
expansion
•solid carapace fractures and
fluid is released to react with
surrounding rock
- can happen in multiple smaller
events (stockwork veining)
- or as a more catastophic
process (breccias)
•pressure release (Pf changes
from Plith to Phydro) induces
exsolution from melt
- residual magma crystallizes
rapidly
- forms porphyry groundmass
Genesis of primary mineralization
After Robb, 2005
Genesis of primary mineralization
•Primary control on
porphyry evolution is
generation of a hydrous
fluid from the melt
- overall metal budget
- magma composition
- Cl- content
- H2O content
- oxidation state of magma
- sulfur content
•Exsolution history of fluid
from melt is CRITICAL
factor for metal content
- depth of crystallization
- relative extent of 1st and
2nd boiling
•Fluid release into
crystallized parts of cupola
region is necessary to
produce actual deposits
•Can understand main factors controlling Au & Cu deposition
from an understanding of:
- fluid cooling
• control on metal solubility
- fluid oxidation state
• control on oxide, sulfate and sulfide deposition
- disproportionation of SO2 (in fluid) with decreasing T
• largely temperature controlled
• increases ratio of H2S to SO2
• favors sulfide deposition [4SO2 + 4H2O = H2S + 3HSO4
-
+
3H+
]
- fluid acidity
• controls alteration patterns, affects [H2S]
• acidity increases with deceasing temperature as acids
disassociate (HCl, H2SO4, H2S, H2CO3)
! strongly influences alteration patterns.
Genesis of primary mineralization](https://image.slidesharecdn.com/07porphyryii-250202062855-61f301b2/85/07-Porphyry-II-system-to-mining-exploration-7-320.jpg)
07 Porphyry II system to mining exploration
- 1. Porphyry Deposits II • Cu > Mo ~ Au > Pb, Zn, Rh, W, etc - Copper used in construction, currency, electronics - Molybdenum used in high strength alloy and high-T steel; aircraft parts, paints and lubricants Porphyry deposit minerals chalcopyrite CuFeS2 digenite Cu9S5 chalcocite Cu2S bornite Cu5FeS4 enargite Cu3AsS4 molybdenite MoS2 source of metal, S, Cl, water Dehydration (or melting) of slab generates Cu- and S-bearing fluid phase (largely oxidized from interaction with seawater) Hedenquist & Lowenstern, 1994 transport of metal, S, Cl, water Richards (2003) A: Fertile Magma Production • Partial melting in migmatitic zone at base of crust • Melts transferred to upper crust along dykes in shear zones B: Volatile Exsolution • Magma ascends to neutral buoyancy level • Need to restrict volcanism? • Volatiles exsolved during fractional crystallisation (mafic magma involvement?)
- 2. Richards (2003) Inflation + lateral propagation Cupolas – fractionated, volatile (fluid)-rich dacitic magmas forming sub- volcanic stocks/dykes Batholith dynamics • Porphyry deposits generally in and around tall, narrow stocks/dikes (~<2km2), emplaced at shallow depths (1-4km) - stocks/dikes develop in cupolas/apophyses off deeper batholiths (6-10km) - Insufficient Cu in shallow stocks/dikes to form Cu deposits; • must derive from deeper magma chambers • need a way to concentrate Cu from this magma chamber ! exsolved volatile(fluid) phase! Average Cu in andesitic magma ~60ppm Total Cu in giant deposit ~10Mt 10Mt/60ppm = 1.7x1011 t of magma Volume of magma = (1.7x1011 t)/(2.7t/m3) ~60km3 [@ 100% efficiency Cu removal] volatile exsolution cupolas Comb quartz layers in intra- mineral monzonite, Ridgeway, NSW Courtesy of David Cooke hydrothermal fluids
- 3. hydrothermal fluids North Parkes, NSW (Lickfold et al. 2003) UST’s Vein-dykes Courtesy of David Cooke Bajo de la Alumbrera: Harris et al. 2003 At low degrees of crystallization: At high degrees of crystallization: • vapor bubble formation • coalesce • tubule formation • volatiles drain upwards Miarolitic cavities Courtesy of David Cooke Fluid filled zone volatile exsolution typical volatile exsolution •Pressure drops as magma rises - reduces solubility of H2O in the melt •Eventually melt reaches H2O saturation - water is exsolved as separate phase: 1st boiling •Near surface, magma may gain enough buoyancy from fluid phase that it leads to volcanism •8% H2O magma = saturation at ~3kb (9-12km) •4% H2O magma = saturation at ~1kb (3-4km) •2% H2O magma = saturation at ~0.25kb (1-2km) •(After Cloos, 2001) porphyry volatile exsolution • Hydrothermal fluid is largely derived from 2nd boiling of larger batholithic intrusion beneath hypabyssal porphyry - 2nd boiling = due to crystallization - Drier magmas need crystallization to achieve H2O saturation at depths of the upper crustal magma chambers • The generation of a fluid phase depends on... - initial H2O content - pressure - degree of crystallization
- 4. fluid composition • Magma composition, P-T-t cooling history constrain composition of the exsolved fluid - mostly H2O - Salt components: NaCl, KCl, FeCl2 • ~5-10wt% total salinity in fluid • Oxidized, so SO2 > H2S - sulfur actually in melt as SO4 2- (S6+) - no sulfides form • Cl- partitions preferentially into fluid phase - strongly pressure-sensitive up to ~6km (~2kb) • but unusual to have much fluid at that depth • deeper magmas (than ~6km) exsolve a Cl- rich (briny) (and hence metal-rich) fluid - shallower exsolve Cl-poor fluid • Thus, generation of Cu- rich fluids must occur deeper than level of stocks/dikes (1-4km) hosting most of the ore fluid composition (Cl) • Metals partition into aqueous fluids depending strongly on Cl concentration fluid composition (metals) D Zn,fluid-melt Need to get bubbles from here To here First boiling Second boiling
- 5. Cloos, 2002 At low levels of crystallization, fluid saturation occurs by 1st boiling at shallow depths in cupola. Low Cu owing to low salinity of fluids Zoom in here Early magma chamber Cloos, 2002 •Bubbles in magma that rise from great depth are copper-rich - become large enough near the surface to rise on their own and separate from melt - fluid can accumulate beneath the cupola - partially degassed magma then sinks Convection moves lots of melt through this process! Later magma Cloos, 2002 Late bubbling magma chamber As cupola cools, crystallization and fluid saturation reaches main chamber where pressures are high enough for saline Cu-rich fluids to form. This fluid rises up through crystal mush wall and in crystal rich suspension zone Yerrington evolution
- 6. From Heinrich (2005) Cu-rich fluid pooled at top of cupola.... Poised to make a porphyry deposit BUT – if have volcanic release of exsolving fluid phase ! no build up of Cu- rich fluid and NO porphyry deposit Why not more deposits? • Source (largely orthomagmatic) - The source of the metals, sulfur, Cl, and water is the metasomatized (fluid-altered) mantle melts above dehydrating subducting slabs • Transport (magmatic and hydrothermal) - The metals, sulfur and water are transported together in the intermediate composition magma rising to upper crustal magma chambers - 2nd boiling leads to exsolution of metal rich fluid phase that actually carries metals to site of precipitation of sulfide precipitation and ore formation Porphyry-Cu STT model Formation of mid-crustal magma chamber and exsolution and trapping of hydrothermal fluid in apical zones of chamber is a critical first step in the porphyry magmatic-hydrothermal ore system • Transport (magmatic and hydrothermal) - The metals, sulfur and water are transported together in the intermediate composition magma rising to upper crustal magma chambers - 2nd boiling lead to exsolution of metal rich fluid phase that actually carries metals to site of precipitation of sulfide precipitation and ore formation • Trap (hydrothermal) - Getting metal out of fluid and into sulfide Porphyry-Cu STT model •Main ore minerals: chalcopyrite, bornite, gold, molybdenite - hosted in veins or breccias •Gangue: qz, or, anh, mt, bt ± ser ± py •Within deposit zonation: - low pyrite, Cu-rich core, on outer edge of potassic alteration zone - outer pyrite-rich halo in phyllic alteration zone •Some deposits have Cu- Au rich cores inside an intermediate Mo-rich annulus & outer pyrite halo 11,000E 9,600RL 9,450RL 9,800RL 10,000RL 10,200RL E26 - Cu & Au Grades 10,800E 10,600E 11,200E 200 m + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + >2 g/t Au 1 - 2 g/t Au 0.5 - 1 g/t Au >2 % Cu 1- 2 % Cu 0.5 – 1 % Cu House, 1994 Free Au in qz-mt vein, Ridgeway, NSW Primary Mineralization
- 7. From Heinrich (2005) Cu-rich fluid pooled at top of cupola.... Poised to make a porphyry deposit Genesis of primary mineralization •build-up of volatile-rich magma at top of cupola causes increase in Pfluid owing to - volatile exsolution and volume expansion •solid carapace fractures and fluid is released to react with surrounding rock - can happen in multiple smaller events (stockwork veining) - or as a more catastophic process (breccias) •pressure release (Pf changes from Plith to Phydro) induces exsolution from melt - residual magma crystallizes rapidly - forms porphyry groundmass Genesis of primary mineralization After Robb, 2005 Genesis of primary mineralization •Primary control on porphyry evolution is generation of a hydrous fluid from the melt - overall metal budget - magma composition - Cl- content - H2O content - oxidation state of magma - sulfur content •Exsolution history of fluid from melt is CRITICAL factor for metal content - depth of crystallization - relative extent of 1st and 2nd boiling •Fluid release into crystallized parts of cupola region is necessary to produce actual deposits •Can understand main factors controlling Au & Cu deposition from an understanding of: - fluid cooling • control on metal solubility - fluid oxidation state • control on oxide, sulfate and sulfide deposition - disproportionation of SO2 (in fluid) with decreasing T • largely temperature controlled • increases ratio of H2S to SO2 • favors sulfide deposition [4SO2 + 4H2O = H2S + 3HSO4 - + 3H+ ] - fluid acidity • controls alteration patterns, affects [H2S] • acidity increases with deceasing temperature as acids disassociate (HCl, H2SO4, H2S, H2CO3) ! strongly influences alteration patterns. Genesis of primary mineralization
- 8. • Cloos, M., 2001. Bubbling magma chambers, cupolas, and porphyry copper deposits, International Geology Review 43, pp. 285-311. • Dilles, J.H., 1987. Petrology of the Yerington Batholith, Nevada - Evidence for Evolution of Porphyry Copper Ore Fluids, Economic Geology 82, pp. 1750-1789. • Hedenquist, J.W., Lowenstern, J.B., 1994. The role of magmas in the formation of hydrothermal ore deposits, Nature 370, pp. 519-527. • Richards, J.P., 2003. Tectono- magmatic precursors for porphyry Cu- (Mo-Au) deposit formation, Economic Geology 98, pp. 1515-1533. • Seedorff, E., Barton, M.D., Stavast, W.J.A., Maher, D.J., 2008. Root Zones of Porphyry Systems: Extending the Porphyry Model to Depth, Economic Geology 103, pp. 939-956. • Seedorff, E., Dilles, J.H., Proffett, J.M., Einaudi, M.T., Zurcher, L., Stavast, W.J.A., Johnson, D.A., Barton, M.D., 2005, Porphyry deposits: Characteristics and origin of hypogene features, in: J.W. Hedenquist, J.H.F. Thompson, R.J. Goldfarb, J.P. Richards, (Eds.), Economic Geology One Hundredth Anniversary Volume, Society of Economic Geologists, Littleton, Colorado, pp. 251-298. • Shinohara, H., Hedenquist, J.W., 1997. Constraints on magma degassing beneath the far southeast porphyry Cu-Au deposit, Philippines, Journal of Petrology 38, pp. 1741-1752. Bibliography