HYDROTHERMAL METAL SOLUBILITY AND SPECIATION HYDROTHERMAL MINERAL DEPOSITS

HYDROTHERMAL METAL SOLUBILITY AND SPECIATION

HYDROTHERMAL MINERAL DEPOSITS Epithermal Au-Ag Porphyry Cu-Mo Epithermal Au-Ag

THE WATER MOLECULE Oxygen Hydrogen Electrons from hydrogen Water is a polar molecule, which explains why it is such an excellent solvent, It can form positively directed shells around anions and negatively directed shells around cations.

SIMPLE SOLVATION Simple ion solvation (hydration)

SOLVATION AND THE HYDROGEN BOND - - + + + - + - - + H-bond. Ice crystals + + + - + + + Hydrogen bonds impart structure to water and ice.

THE DIELECTRIC CONSTANT Ameasure of the capacity of a solvent to solvate ions - high values equal high solvation capacity. External field (capacitor) Internal field (dipoles) δδ+ + + + + δ- δ+ - Uncharged, disoriented dipoles Charged, oriented dipoles Determined by creating an electrical field between two capacitor plates and measuring the voltage. The oriented dipoles create an internal field that opposes the external field. The dielectric constant is the ratio of the voltage required to orient the dipoles in a vacuum over that in water.

DIELECTRIC CONSTANT( ) OF WATER The dielectric constant of water decreases with increasing temperature and decreasing pressure, leading to a corresponding decrease in its power to solvate ions 4 90 Kbar 70 50 30 20 2 L+V 200 10 5 Cp 400 T OC 600 Eugster (1986)

COMPLEXATION As the dielectric constant of water decreases, the water molecule shells around metal ions and anions break down facilitating the formation of complex molecules involving these species, e. g. , Cu. Cl 42 -. These complexes will be surrounded by weaker solvation (hydration) shells. O H H H O H H Complex molecule solvation (hydration) H O H H H O O

ELECTRONEGATIVITY AND THE PERIODIC TABLE

IONIC (HARD) BONDING Transfer of electrons – electrostatic interaction + _

COVALENT (SOFT) BONDING Sharing of electrons - polarisability n Individual atoms with spherical electron clouds n Protons attract electron clouds and polarise each other n Covalent bond

PEARSON’S HSAB PRINCIPLES AND AQUEOUS METAL COMPLEXES Hard acids (large Z/r) bond with hard bases (ionic bonding) and soft acids (small Z/r) with soft bases (covalent bonding). Hard Borderline Soft Acids H+, Na+>K+ Al 3+>Ga 3+ Y 3+, REE 3+ (Lu>La) Mo+6, W+6, U+6 Zr 4+, Nb 5+ Fe 2+, Mn 2+, Cu 2+ Zn 2+>Pb 2+, Sn 2+, As 3+>Sb 3+=Bi 3+ Au+>Ag+>Cu+ Hg 2+>Cd 2+ Pt 2+>Pd 2+ Bases F-, OH-, CO 32 - >HCO 3 SO 42 - >HSO 4 PO 43 - Cl- HS->H 2 S CN-, I->Br. Pearson (1963)

THE PERIODIC TABLE

PREDOMINANCE DIAGRAM FOR THE SYSTEM H-S-O H 2 S =HS- + H+ Log K = -p. H H 2 S + 2 O 2 = HSO 4 - + H+ Log K = -p. H – 2 log f. O 2 H 2 S + 2 O 2 = SO 42 - + 2 H+ Log K = -2 p. H – 2 log f. O 2 HS- + 2 O 2 = SO 42 - + H+ Log K = -p. H – 2 log f. O 2

COPPER SPECIATION 1 m Na. Cl

MOLYBDENUM SPECIATION Unlike most other metals, Mo, which occurs in hydrothermal fluids as Mo 6+, is so hard that it reacts with water molecules to form covalently bonded, negatively charged molybdate (Mo. O 42 -) species. The same is also true of W.

MODELLING Cu-Mo ZONING Porphyry Cu-Mo deposits are characterized by high temperature Mo (Mo. S 2) at depth and shallower Cu (Cu. Fe. S 2). The question is why. HMo. O 4 - + H+ + 2 H 2 S = Mo. S 2 + 2 H 2 O +O 2 Cu. Cl 2 o + Fe. Cl 2 o + 2 H 2 S = Cu. Fe. S 2 + 4 H+ + 4 Cl. Cp Mo Aqueous fluid containing 2 m Na. Cl, 0. 5 m KCl, 4000 ppm Cu and 1000 ppm Mo in equilibrium with Kfeldspar, muscovite and quartz.

ZINC SPECIATION 1 m Na. Cl

Au-Ag SPECIATION Epithermal Au-Ag deposits may be goldrich or silver-rich or even zoned from gold to silver. This behaviour is determined by the complexation of the two metals with silver preferring chloride complexation for a wide range of f O 2 -p. H conditions and gold, bisulphide complexation. 1 m Na. Cl

THE PERIODIC TABLE

REE FLUORIDE/CHLORIDE COMPLEXES Dashed lines, theoretical extrapolations from ambient temperature data. Solid lines (Migdisov et al. , 2009) experimental determinations. Note 1: REE fluoride complexes three orders of magnitude more stable than REE chloride complexes Note 2: Above 150 o. C LREE complexes more stable than HREE complexes.

MODELLING REE SOLUBILITY IN A F-BEARING Na. Cl BRINE 10 wt. % Na. Cl, 500 ppm F, 200 ppm Nd The REE are transported dominantly as chloride complexes despite the greater stability of REE fluoride complexes, because HF is a weak acid and REE fluoride is relatively insoluble. Migdisov and Williams-Jones (2014)

ION-PAIRING / LIGAND AVAILABILITY HCl is a strong acid and easily dissociates, whereas HF is a weak acid Log K HF = H+ + F- T C Log K Cl- ions are freely available whereas F- ions are not. HCl = H+ + Cl-

HYDROTHERMAL FRACTIONATION OF THE REE LREE are mobilised (as chloride complexes) relative to the HREE; REE are deposited as monazite. Fluid contains 10 wt. %Na. Cl, 500 ppm F, and 50 ppm of each REE. Rock contains 100 ppm P. Williams-Jones et al. (2012)

PREFERENTIAL MOBILISATION OF La (LREE) RELATIVE TO Lu (HREE) La/Lu ratio (monazite) increases away from the deposit, showing that the LREE were preferentially mobilised relative to the HREE, thereby providing an exploration vector. UMS Chl transition 18. 8 8. 2 Chl zone Fault K-rich zone Up flow Unconformity-type U deposits can contain up to 0. 3 wt% REE; 90% HREE 2. 1 Namambu Lower Mine Sequence (LMS) 250 m Ranger U deposit, Australia Fisher et al. , 2013

METAL TRANSPORT BY VAPOUR Georgius Agricola (1494 -1555) “Metal exhalations are drawn up from the depths, and the rising fumes pass into the veins and stringers and are united through the effects of the planets and made into ore” De Re Metallica (1556) “On the Nature of Metals”

METALLIC SUBLIMATES /VOLCANIC GASES Sir Humphry Davy (1778 -1829) Johann Dahl The 1820 eruption of Vesuvius “On the 6 th of January (1820)…. I collected sublimations. . . one specimen with large saline crystals with a slight tint of purple, proved to be common salt with a minute portion of muriate of cobalt……On January the 26 th. . the appearance of the sublimations was considerably changed: those near the aperture were coloured green and blue by salts of copper” Philosophical Transactions (1828)

EXPERIMENTAL SUPPORT FOR ORE FORMATION Auguste Daubrée (1814 -1896) Daubrée, in trying to understand the origin of Cornish and other tin vein deposits, synthesised cassiterite by reacting gaseous Sn. Cl 4 with water vapour. Tin(IV) chloride boils at 114 C Sn. Cl 4 g + 2 H 2 Og = Sn. O 2 + 4 HCl Daubrée (1849)

THE CHANGING TIDE: THE USGS RULES Waldemar Lindgren (1860 -1939) Director of the USGS - champions aqueous liquid ore fluids “In ore deposits there are vast quantities of non-volatile material, particularly silica, which can hardly have been emitted in gaseous form. In spite of what the geophysicists say the magmatic emanations ascended in liquid form” Econ. Geol. (1927) Louis Graton (1880 -1970) Succeeds Lindgren as Director of the USGS – firmly establishes in his treatise “The nature of the ore-forming fluid” that the ore fluids are aqueous liquids. Econ. Geol. (1940) “It is improbable that an important gas phase exists in the normal magma chamber. But even if existing, a gas is shown far inferior to a liquid for transporting mineral solutes”.

EXPERIMENTALISTS FIGHT BACK George Morey (1888 -1965), Bowen, Fenner (Geophysical Laboratory), defend vapour transport model Morey conducts experiments on the solubility of solids in steam The issue of silica (quartz veins) “approximately 50 grams of quartz had been deposited near the top of the bomb. The silica was derived from the refractory tubes in the bomb and could not have been carried except through the gaseous water - a solubility of 2/3 to 1 wt. % would be indicated at this slight pressure” Econ. Geol. (1940)

VAOUR TRANSPORT OF METALS FINALLY REJECTED Konrad Krauskopf (1911– 2003) Calculated the solubility of ore metals in the vapour phase “The very low volatilities of copper, silver and gold… show that no hypothesis of simple vapor transport can satisfactorily account for the origin of ore deposits. ” Econ. Geol. (1957) Vapour pressure of metalliferous solids (300 o. C) Cu. Cl solid = Cu. Cl gas Ag. Cl solid = Ag. Cl gas Au. Cl Solid = Au. Cl gas log X Cu. Cl = -17 log X Ag. Cl = -12 log X Au. Cl = -36

THE PHOENIX RISES FROM ITS ASHES Christoph Heinrich Metal concentrations in vapour Alumbrera Grasberg 20μm Dvap/brine 10 As Vapour Au Cu 1 Sb Sn 0. 1 Mo Fe Zn Pb Ag Bi Brine Au, ppm <0. 53 10. 17 Ag, ppm <40 100 Mo, ppm <300 60 Cu, wt% 3. 3 1. 2 Mn, wt% 0. 14 0. 2 Zn, wt% 0. 12 0. 15 Pb, wt% 0. 02 0. 04 Audetat, et al. (1998); Ulrich et al. (1999)

SOLUBILITY OF Ag. Cl IN HCl-H 2 O VAPOUR Ag. Cl solubility independent of p HCl; the complex is Ag. Cl solubility increases with increasing f H 2 O; hydration Migdisov and Williams-Jones (2013)

SOLUBILITY OF Cu. Cl IN HCl-H 2 O VAPOUR Cu. Cl solubility independent of p HCl; the complex is Cu. Cl solubility increases with increasing f H 2 O; hydration Migdisov et al. (2014)

SOLUBILITY OF SILVER IN HCl-H 2 O VAPOUR Silver solubility increases with hydration number Ag. Cl + n H 2 O = Ag. Cl·(H 2 O)n log K = log f (Ag. Cl) – n log f (H 2 O) Migdisov and Williams-Jones (2013)

EXTRACTING THERMODYNAMIC DATA The linear relationship between ΔG and reciprocal temperature enables extrapolation to high temperature Log K = -ΔG/RT Migdisov and Williams-Jones (2013)

METAL SPECIATION IN WATER VAPOUR Rather than constituting widely dispersed molecules, water vapour comprises clusters of hydrogen-bonded water molecules. Metal species, which are uncharged, dissolve in water vapour by attaching to clusters of water molecules via hydrogen-bonding. Molecular dynamic simulation of solvation (hydration) in water vapour.

SO WHAT DID KRAUSKOPF IGNORE? The effects of complexation and particularly solvation by H 2 O clusters make heavy metals volatile Reaction Hydration Cl + Au Au Cl

SOLUBILITY OF SILVER IN HCl-H 2 O Vapour Hydration increases with increasing H 2 O pressure or density but decreases with increasing temperature Solubility increases with increasing temperature but reaches a maximum because of the effect of decreasing hydration Migdisov and Williams-Jones (2013)

SOLUBILITY OF COPPER IN HCl-H 2 O Vapour Cu is transported dominantly as Cu. Cl·H 2 O, with subordinate Cu. Cl 2 and Cu. Cl at high temperature Note the very high concentrations of Cu in the vapour, > 1. 8 wt. % Cu Migdisov et al. (2014)

EPITHERMAL GOLD ORE FORMATION Vapour-dominated hydrothermal plume rises from magma transporting Au and depositing it as temperature drops below 400 C Hurtig and Williams-Jones (2014)

LADOLAM, PAPUA NEW GUINEA; EPITHERMAL GOLD DEPOSITING NOW Ladolam is an epithermal gold deposit containing 1, 300 tons of gold forming on the flanks of Luise volcano (active), Lihir Island; the average grade is 10 ppm Au, and the highest grade 120 ppm Au. Gold saturates due to boiling of a fluid initially containing 14 ppb Au. The annual fluid flux leads to deposition of 24 kg Au; 1, 300 tons Au in 55, 000 yr. Simmons and Browne (2006)