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