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

could also be present in pre-Flood strata sequences. However, the supergene weathering of many ore deposits at today’s earth surface may still offer an additional criterion for determining the location of the Flood/post-Flood boundary in the geologic record. This is because other criteria can be used to establish that the primary ore deposits were indeed formed during the Flood or even pre-Flood. For example, most porphyry copper deposits are hosted by granite plutons which intrude into fossil- bearing, Flood-deposited sedimentary layers. On the other hand, Precambrian banded iron formations (BIFs) were largely formed before the Flood and may even date back to the Creation Week (Snelling 2009) but were only exposed at the earth’s surface as a result of the erosion at the end of the Flood as the waters retreated. Furthermore, lateritic iron and manganese deposits were deposited as sedimentary layers on top of or between fossil-bearing, Flood- deposited sedimentary layers. Of course, some of these porphyry copper and lateritic iron and manganese deposits may also have formed in the first decades after the Flood due to the declining residual catastrophism. Some consideration needs to also be given to the chronology of the Flood, and especially the timing and actions of the Flood waters when they retreated. This is highly relevant to the question of when the primary ore deposits were exposed by erosion to weathering, which then initiated the formation of the supergene minerals. For example, the granite plutons hosting the porphyry copper deposits were intruded some 2-5 km below the ground surface at that time, so that 2-5 km of overlying host rocks had to first be eroded away to expose the primary copper ore to weathering and formation of the supergene minerals that have been radioisotope dated. The timing of this weathering relative to the end of the Flood is crucial to considering whether the radioisotope ages derived for the supergene minerals can pinpoint where the Flood/post-Flood boundary should be placed in the geologic record. SUPERGENE WEATHERING OF ORE DEPOSITS Supergene metallic ore deposits form when common rock types or deeply buried primary metallic ore bodies are exposed at or within 1000 m of the earth’s surface. It is necessary to avoid confusion here by defining the term supergene. Supergene is “said of a mineral deposit or enrichment formed near the surface, commonly by descending solutions” (Neuendorf et al. 2005, p.645). Thus, supergene alteration of primary ore deposits, which is due to the downward passage of weathering and oxygenated groundwaters and occurs at in situ ground temperatures, should not be confused with hypogene or hydrothermal alteration of primary ore deposits. Hypogene is “said of a mineral deposit formed by ascending solutions” (Neuendorf et al. 2005, p.315). Hydrothermal alteration generally produces a different set of minerals from those produced by supergene alteration. And hydrothermal alteration usually occurs at depths >1 km, whereas supergene alteration occurs at <1 km depth and is related to groundwater fluctuations and oxygen in the atmosphere penetrating downwards from the land surface. Rocks and ore deposits that were formed at high temperatures and high pressures when exposed at and near the earth’s surface have their equilibrium disturbed. This causes their mineral constituents to react and undergo transformations so as to adjust to the new lower temperatures, pressures, and higher oxygen concentrations and moisture conditions. They undergo chemical weathering, which promotes the oxidation, dissolution, remobilization, re- precipitation, and re-concentration of metals of economic interest. Recurrent dissolution, transport, and re-deposition of metals through time has created a chemically stratified weathering profile (Reich and Vasconcelos 2015) that contains a comprehensive record of these chemical reactions which have occurred at the earth’s surface. Today’s rates of these reactions are invariably climate-dependent, reflecting ambient temperature, availability of liquid water (involving rainfall intensity and seasonality), evapotranspiration rates, and biological and microbiological activity (Vasconcelos 1999a). The phenomenon known as supergene enrichment refers to the secondary, in situ, accumulation of metals (for example, Cu, Zn, Ag, Au, Ni, or U) as a result of three essential processes: 1)The electrochemical oxidation of primary sulfides, oxides or native metals (for example, native copper Cu 0 to Cu 2+ ); 2)The transport of the released metals as soluble metal species (for example, CuSO 4 0 , AuCl 4 1- ); and 3)The reprecipitation of the metals by reduction (for example, Cu 2+ to native copper Cu 0 ), by supersaturation (for example, Mg 2+ in magnesite deposits), or by cation-exchange (for example, Ni 2+ exchange for Mg 2+ in smectite- or serpentine-group minerals). In particular, oxidation processes leading to mineral leaching are commonly catalyzed by specialized Fe- and S-oxidizing bacteria. Oxidation processes are also active in the surficial vadose zone and the capillary fringe above the water table. Leaching processes also respond to changes in physicochemical properties, such as the partial pressure of oxygen ( p O 2 ), and its effect on the redox potential (Eh), as well as the activity of H + of descending aqueous solutions (Reich et al. 2009; Sillitoe 2005). Weathering profiles hosting supergene ore deposits may extend down to 1000 m below the surface, but they are mostly inaccessible to scientific investigation. When these systems are drilled for their mineral potential and eventually exposed by open-pit mining operations – some open pits may be 5–6 km wide and more than 1 km deep – they provide access to paleoclimatic records that are otherwise unavailable. Minerals found in supergene ore deposits record information about chemical reactions and geochemical (and paleoclimatic) conditions prevailing during the formation of the deposits. Detailed descriptions of these chemical reactions which form the main minerals produced by supergene weathering of ore deposits are provided in the Appendix. DATING OF SUPERGENE MINERALS Supergene metal deposits host a comprehensive record of climate- driven geochemical reactions that may span the entire Cenozoic. Products of these reactions can be dated by a variety of radiogenic isotopic methods, such as 40 Ar/ 39 Ar, (U–Th)/He, U–Pb, and U-series. High spatial resolution methods have distinct advantages when dating minerals from soils and weathering profiles that contain complex assemblages of intimately intergrown minerals precipitated at distinctly different times. The most commonly Snelling ◀ Flood/post-Flood boundary ▶ 2018 ICC 554