Sulfides, Hydrothermal Minerals and Native Elements Chapter 19

Sulfides and Sulfosalts Most common ore source of metals M p X r X may be S (sulfides) S+As (sulfarsenides), and As (arsenides) M= metal or semimetal: Fe, Cu, Zn, Pb, Ni, Co, Hg, Mo Sulfosalts, As, Sb, and Bi take the place of metals (M) Magmatic and hydrothermal origin

Electronegativity of S (2.5) is less than that of O (3.5) hence the bonding is only 10-20% ionic Recall from Chapter 3: Ionic Bond: High difference in electronegativity between anion and cation Covalent and metallic: low or zero difference in electronegativity Pauling (1960): empirical mathematical relationship % ionic character = 1- e -0.25(x a -x c )2 X 100 (x a -x c )=difference in electronegativity Of the eight most abundant elements in the crust: O: electronegativity = 3.5 always from anion, the other seven: (Si, Al, Fe, Ca, Na, K, Mg) all form cations when bonding with O and have electronegativity differences from 1.7 (Si) = 50% ionic to 2.7(K)=80% ionic No easy or simple classification Minerals with metal, semimetal ( As, Sb, Bi) and S = sulfosalts

Hydrothermal circulation in a continental setting – Minerals form in hydrothermal veins, disseminated deposits and grade into pegmatites. Hydrothermal Minerals When hot magma resides in the crust, water in the surrounding rock is heated and begins to convect. As it travels, the hot water dissolves materials (ions) from the surrounding rock and carries them to new locations. If the conditions of the water change (temperature, pressure, pH, oxygen content), new minerals will precipitate.

A hot spring on the surface is a sign of deep hydrothermal circulation.

Seawater circulates through the ridge basalt. Most minerals form when the hot water exits into the cold deep ocean water. Sulfide minerals form “black smokers”, sulfates form “white smokers”. Minerals are typically volcanogenic massive sulfides. Hydrothermal circulation on the ocean floor.

Hydrothermal veins

The dark colored layers are chalcopyrite, sphalerite and galena – valuable ore minerals The white layers are quartz – a useless “gangue” mineral which must be removed

Porphyry copper replacement deposit – copper minerals are deposited in fractured, altered igneous rock

See Table 19.2, page 417 The diagram shows some of the main categories of sulfide mineral associations in continental settings: A.Porphyry copper – chalcopyrite, other copper sulfides and molybdenite, near the top of a felsic igneous intrusion B. Hydrothermal vein with chalcopyrite, galena and sphalerite C.Galena and sphalerite in limestones, typically with dolomite D.Low temperature (epithermal) gold, silver, cinnabar vein E.Low temperature (epithermal gold, cinnabar deposit) Fig. 19.1, page 415

Fluid inclusions record mineral and fluid temperature. Minerals entrap fluid as they grow. When the minerals cool, the fluid contracts, forming a bubble of gas. By heating the mineral until the bubble disappears (until the fluid reaches its original volume) you can estimate the temperature of entrapment. The trapped liquid can be analyzed to find the chemistry of hydrothermal fluids Sulfides form mostly from NaCl rich brines – the Cl in NaCl forms soluble metal salts (CuCl, PbCl, AgCl etc)

Hydrothermal circulation can produce zones of altered rocks around an intrusion (or in the intrusion itself). A common pattern observed : The silicates in igneous rocks (feldspar, hornblende and micas) are altered by reaction with hydrothermal solutions to form characteristic alteration minerals: Propylite (chlorite and epidote form) Argillite (clay minerals form) Sericite (mica forms from clays) Potassic: K-Feldspar crystallizes w/ or w/o recrystallized biotite or sericite Alteration zones can be from a few centimeters around a vein to several kilometers around a large intrusion. Fig. 19.2, page 416. Secondary or Supergene Hydrothermal Minerals

Figure 19.3, page 418 Sulfide minerals are unstable in the presence of oxygenated groundwater. Primary (hypogene) sulfides react to form secondary (supergene) sulfides, and then supergene oxygen-bearing minerals such as oxides, carbonates, sulfates, and phosphates, depending on the anions that are available in the groundwater. At the surface, red/orange colored iron oxides (gossan) are left behind and become a marker for sulfide mineral prospecting. See Table 19.3, page 382 for names of some minerals in the oxidized part of the supergene zone.

Supergene Processes: Oxygenated ground water (derived from rain or snow melt) reacts with pyrite: Pyrite + Oxygen + water = Goethite + sulfuric acid 4FeS O H 2 O  4FeOOH + 8 H 2 SO 4 Chalcopyrite CuFeS 2 + CO 2 + 2H 2 O  Cuprite CuO+ FeCO3 + 2H 2 SO 4 Copper carbonates, malachite and azurite can form in a similar fashion. So near the surface sulfides are oxidized to form oxides, hydroxides carbonates and sulfates  this near surface zone is known as the Oxidized Zone and the new minerals are called Supergene or secondary minerals (see Table 19.3) Because Pyrite is the most common sulfide, oxidation of pyrite leaves a rust, yellow and red colored surficial zone of iron oxides and hydroxides – this zone is called the gossan The released sulfuric acid helps dissolving sulfide minerals e.g., 2Cu 2 S + 4H 2 SO 4 + 5O 2 -- > 4CuSO 4 + H 2 O When Meteoric water containing dissolved metals encounters the ground water table – it gets diluted and causes large scale precipitation of dissolved metals by reversing the above reaction Pyrite also reacts with the sulfate solution to precipitate copper sulfide This extensive reprecipitation forms a zone of rich ore – this process is called Supergene enrichment and the zone is called the reduced zone; by this time the meteoric water has exhausted it’s oxygen –occurs at or just below the water table.

The stability of supergene hydrothermal minerals is typically shown on a plot of Eh (a measure of the availability of oxygen) versus pH (concentration of hydrogen ions, or acidity). The concept of this phase diagram is the same as that of a plot of pressure versus temperature. Minerals shown are: Chalcocite Native copper Covellite Cuprite Malachite Which would you expect to form in alkaline, highly oxidized waters? Oxygen rich environments Oxygen poor environments

Cu-Fe Sulfide Minerals Py Po Cp Bn Cc Dg Cv

Other Common Sulfide Minerals Galena PbS - dense, cubic cleavage Sphalerite (Zn,Fe)S – submetallic black to resinous yellow, brown luster Pentlandite (Fe,Ni) 9 S 8 – yellow-bronze; w/ Cp and Po in magmatic ores Cinnabar HgS – vermilion-red color, dense Molybdenite MoS 2 – silver metallic sheets Pt Galena Zn>>Fe Zn>Fe Zn

Arsenosulfides, Arsenides and Sulfosalts Cobaltite (Co,Fe)AsS – silver white metallic Arsenopyrite FeAsS – silver white metallic Realgar AsS (red) - Orpiment As 2 S 3 (yellow) Stibnite Sb 2 S 3 silver-gray prisms Enargite Cu 3 AsS 4 – striated metallic columns and blades – a sulfosalt Skutterudite (Co,Ni)As 3 silver-gray cubes

Mineral Classifications Principally by dominant anion or anionic group Secondarily by internal mineral structure Native Element Sulfides (S) Sulfosalts (AsS) Oxides (O) Hydroxides (OH) Halides (Cl, F, Br, I) Carbonates (CO 3 ) Sulfates (SO 4 ) Phosphates (PO 4 ) Nitrates, Borates, Tungstates, Molybdates, Arsenates, Vanadates... Silicates Nesosilicates Nesosilicates Sorosilicates Sorosilicates Cyclosilicates Cyclosilicates Inosilicates Inosilicates Phyllosilicates Phyllosilicates Tectosilicates Tectosilicates

Native Elements Metals – Gold, Silver, Copper, Platinum, Palladium, Osmium, Iridium, Iron, Fe- nickel Semi-metals – Arsenic, Bismuth, Antimony Non-metals – Sulfur, Diamond, Graphite

Native Metals metallic bonding dense, cubic close packing properties: soft, malleable, ductile, sectile, good heat and electrical conductors variable melting points: low-Au, Ag, Cu; high-PGEs Au AgCuPt

Native Non-metals SulfurDiamondGraphite S 8 ring molecules bonded by weak van der Waals forces C 6 ring molecules bonded by weak van der Waals forces; good electrical conductivity StrongCovalentBonds Synthetic (industrial) Diamonds

Native Element Occurrences Gold – Hydrothermal fluids related to magmatism; commonly occurs in veins quartz and pyrite; may form detrital grains to produce placer deposits; Rarely occurs alloyed with other elements. Silver – Hydrothermal ore deposits rich in sulfide, arsenides, and bismithides; also commonly associated native copper. Copper – Sulfide-poor hydrothermal ore deposits or secondary oxidation of Cu- sulfide minerals; most abundant occurrence is the native copper deposits of the Keweenawan Peninsula of Upper Michigan where it occurs in lava flows and interflow conglomerates. Platinum – Occurs as primary deposits in mafic intrusions and as secondary placer deposits. Diamond – Occurs in mantle-derived kimberlite pipes with other high temperature/high pressure minerals Sulfur – Precipitates near volcanic vents from volcanic gasses and secondarily by oxidation of sulfide minerals.