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

descending metal- and sulfate-rich solutions will form secondary sulfides in the saturated zone below the water table where free oxygen is almost absent ( p O 2 ~ below 10−40 atm.) (see Fig. 3). Formation of secondary sulfides occurs by replacement of Fe by Cu in the hypogene sulfides [pyrite (FeS 2 ), chalcopyrite (CuFeS 2 ), and bornite (Cu 5 FeS 4 )]. The secondary chalcocite (Cu 2 S) occurs on top, where Cu 2+ /HS - is high, while covellite (CuS) precipitates below, where Cu 2+ /HS - is lower. Enriched Cu sulfide zones in porphyry copper deposits are usually tens to hundreds of meters thick and can contain more than 1.5 gigatons of ore with 0.4–1.7 wt% Cu, invariably reaching higher Cu grades than those found in the primary (hypogene) orebodies (Reich and Vasconcelos 2015). 3. Supergene Potassium-Bearing Minerals Alunite [KAl 3 (SO 4 ) 2 (OH) 6 ] and jarosite [KFe 3+ 3 (OH) 6 (SO 4 ) 2 ] are present in weathering profiles encompassing a large range of geological environments. They are common in the oxidation zones of ore deposits, paleosols, silcretes, cave deposits, and weathered river or marine terraces, or deltaic deposits (Vasconcelos 1999a). Supergene alunite and jarosite commonly occur intergrown with primary and other supergene minerals in the weathered zones of orebodies, and as precipitates in veins and cavities (Alpers and Brimhall 1998; Sillitoe and McKee 1996). The most important parameters controlling the distribution of alunite-jarosite in natural environments are pH, Eh, (SO 4 2- ), (K + ), (Fe 3+ ), (Al 3+ ) [where the symbol ( ) denotes activity of the aqueous species], H 2 O fugacity, and temperature. In alunite-jarosite systems the chemical reactions which are dated by the K-Ar or 40 Ar/ 39 Ar methods are the simple precipitation of jarosite or alunite from aqueous solution (equations 1 and 4 in Table 1) or more complex mineral transformations (equations 2, 3, 5, 6, and 7 in Table 1). During weathering processes, manganese is mobile as the cation Mn 2+ , which predominates for most of the range of conditions characteristic of natural-water systems (Vasconcelos 1999a). At pH > 10.5, the complex MnOH + becomes the predominant form, and in solutions with high concentrations of HCO 3 - and SO 4 2- the complexes MnHCO 3 + and MnSO 4 (aqueous) may be important. Organic acids also control manganese solubility in the surficial environment. Mn 3+ species may occur in strongly acid or organic- rich solutions. However, the tendency for the Mn 3+ species to disproportionate indicates that it does not play a major role in the solution chemistry of manganese under surface conditions. The chemical oxidation ofMn 2+ andMn 3+ and the disproportionation of Mn 3+ in aerated surface water lead to the precipitation of Mn 4+ oxides. These oxides further catalyze the oxidation process. Thus, Mn oxide precipitation tends to occur on previously deposited oxides, forming accretionary growth bands. Supergene Mn oxides also catalyze the precipitation of other cations from solution (Bi, Ni, Co, Cu, K, Pb, and more), leading to the formation of complex supergene phases (Vasconcelos 1999a). Hollandite [Ba, K(Mn 4+ 6 Mn 3+ 2 )O 16 ], coronadite, cryptomelane, birnessite, romanéchite, todorokite, and vernalize are some of the complex Mn oxides commonly found in soils and weathering profiles. Similar to jarosite-alunite systems, the K-Ar or 40 Ar/ 39 Ar dating of supergene Mn oxides may determine the direct precipitation of these oxides from aqueous solutions (equation 8 in Table 1) or determine the timing of more complex metasomatic reactions in the weathering environment (equations 9, 10, and 11 in Table 1). When multiple precipitation-dissolution-reprecipitation reactions occur, geochronology of supergene Mn oxides may date the influx of solutions, promoting the dissolution of the previously precipitated Mn oxides, the transport of aqueous Mn 2+ elsewhere in the system, and the reprecipitation of new generations of Mn oxides in suitable sites. Because the most significant aqueous manganese species in the surficial environment is Mn 2+ , the partial dissolution of previously precipitated Mn oxides implies the influx of acid and/or reducing solutions (equations 12, 13 and 14 in Table 1). The aqueous Mn 2+ species generated by reactions 12, 13 and 14 (Table 1)must be reprecipitated nearbywithin theweathering profile if these reactions are to be identified and dated. The kinetics of the oxidation of Mn 2+ aqueous species and the precipitation of Mn 4+ oxides is generally slow in neutral to slightly acidic oxygenated weathering solutions. In alkaline conditions (pH > 8) these reactions proceed more rapidly. The reactions are also catalyzed by bacteria and by mineral surfaces in the natural environment. The identification of the exact reaction controlling the precipitation of datable Mn oxides is desirable if the ages obtained are to be used to interpret paleoclimatic conditions. Snelling ◀ Flood/post-Flood boundary ▶ 2018 ICC 565 Figure 3. Graph of redox (Eh) and the partial oxygen pressure ( p O 2 ) versus pH showing the stability of dissolved copper species and copper minerals in the supergene environment (after Sillitoe 2005). The diagram maps out possible occurrence of stable phases under particular redox and pH conditions along a supergene profile. Conditions shift vertically from the more reducing, saturated zone in gray at the bottom (below the water table), to the more oxidizing conditions towards the top of the profile (vadose zone, where the soil and rock pores contain air as well as water).