The Proceedings of the Eighth International Conference on Creationism (2018)

Supergene Weathering of Ore Deposits 1. Iron Oxides The oxidation of magnetite (Fe 3 O 4 ) to hematite (Fe 2 O 3 ) and the hydration of magnetite or hematite to goethite (FeOOH) are the product of chemical reactions (identifiable through ore microscopy) within supergene blankets overlying lateritic iron deposits. These chemical reactions are illustrated and labeled with blue numbered arrows in the Eh–pH diagram in Fig. 2 representing the thermodynamically favored pathways, and are: Reaction (1) 2Fe 3 O 4 (magnetite) + ½O 2 (dissolved) ↔ 3Fe 2 O 3 (hematite) Reaction (2) Fe 2 O 3 (hematite) + H 2 O (liquid) ↔ 2FeOOH (goethite) As reaction (1) indicates, the direct oxidation of magnetite to hematite is thermodynamically favored (ΔG 0 reaction = −49.375 kcal) when rocks, which formed under reducing conditions, enter the oxidation zone near the earth’s surface. This process can be dated by the (U–Th)/He analysis of supergene hematite. On the other hand, the direct hydration of supergene hematite (martite) to goethite (reaction 2) is not thermodynamically favored (ΔG 0 reaction = +0.567 kcal). Yet, goethite formed after hematite is abundant in duricrusts overlying supergene iron deposits. It appears that the hydration of the primary ore oxides involves a two-stage process involving reactions thermodynamically favorable in the supergene zone of ore deposits. First, hematite is reductively dissolved to soluble Fe 2+ by carboxylic acids (for example, acetic acid, a common organic acid in weathering profiles) (ΔG 0 reaction = −96.99 kcal): Reaction (2a) 4Fe 2 O 3 (hematite) + CH 3 COOH (aqueous) ↔ 8Fe 2+ (aqueous) + 2CO 2 (dissolved) + 10H 2 O (liquid) This is followed by the subsequent re-oxidation of Fe 2+ by dissolved O 2 in weathering solutions and precipitation of goethite (ΔG 0 reaction = −104.286 kcal): Reaction (2b) 2Fe 2+ (aqueous) + ½O 2 (dissolved) + 4OH - (aqueous) ↔ 2FeOOH (goethite) + H 2 O (liquid) These reaction pathways are shown as labeled blue arrows in Fig. 2. Microscopic evidence also suggests close links between these reactions and microorganisms (Monteiro et al. 2014). 2. Supergene Copper Minerals Supergene metal deposits contribute significantly to the world’s supply of selected base metals (Cu, Zn, Ni, Co) and structural metals (Al, Fe, Ni, V). Giant porphyry copper deposits contribute about 70% of the total global copper inventory and have median grades at ~0.4 wt% Cu (with hypogene and supergene Cu grades often being reported together). Yet supergene grades in most deposits are higher than hypogene grades. In most porphyry copper orebodies, the stepwise hydrolysis and oxidation of primary pyrite-bearing assemblages leads to a decrease in the pH of descending groundwaters and the liberation of oxidized sulfur as SO 4 2- anions. There is a simultaneous breakdown of chalcopyrite (CuFeS 2 ), which produces soluble Cu 2+ ions that are transported downwards, encountering progressively greater reducing conditions deep into the profile. This process is accompanied by “capping,” the precipitation of iron oxyhydroxides in the leached zone from which Cu is removed. The thickness of the leached cap is highly variable but can reach several hundred meters in porphyry copper deposits, particularly when the water table was deep enough during the supergene oxidation and enrichment phase (Reich and Vasconcelos 2015). Copper is concentrated within the subjacent oxide zone, which forms laterally extensive deposits composed of assemblages called “green oxides” or “copper oxides”. This mineralogically and compositionally complex layer is composed of copper minerals including oxides, sulfates, hydroxy-chlorides, carbonates, silicates, and native copper. Among the copper minerals that may be encountered are cuprite (Cu 2 O), tenorite (CuO), brochantite [Cu 4 SO 4 (OH) 6 ], chalcanthite [CuSO 4 .5H 2 O], antlerite [Cu 3 SO 4 (OH) 4 ], malachite [Cu 2 CO 3 (OH) 2 ], azurite [Cu 3 (CO 3 ) 2 (OH) 2 ], atacamite [Cu 2 Cl(OH) 3 ], turquoise [CuAl 6 (PO 4 ) 4 (OH) 8 .4H 2 O], native copper (Cu 0 ), and chrysocolla (Cu 2-x Al x )H 2-x Si 2 O 5 (OH) 4 .nH 2 O, among many others. The precipitation of “green oxide” mineral assemblages in the vadose zone is largely controlled by the enclosing rock type and pH (Fig. 3), forming thick (<200–300 m) layers containing ore with >1 wt% Cu grade. Furthermore, Cu and other metals dissolved in groundwater can migrate laterally when hydraulic conditions are favorable and form large “exotic-type” Cu-oxide deposits in gravel sequences that are far (distal) from the source. Under more reducing conditions, the remaining Cu in the Snelling ◀ Flood/post-Flood boundary ▶ 2018 ICC 564 Figure 2. Eh–pH diagram for the Fe–O 2 –H 2 O system. Blue arrows illustrate the thermodynamically favored pathways for the oxidation (1), dissolution (2a), and precipitation (2b) reactions.