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Soil-free sodic salt pans with local pH >10 have formed on loess and schist-derived clay surfaces in the semi-arid climate of Central Otago. This study identifies evaporative mineral distributions and associated variations in surface geochemistry, and these results are relevant to survival of rare halophytic plants on some of the pans. Evaporative salts on pan surfaces are dominated by NaCl from marine aerosols, and Na-sulphates and Na-carbonates. Some of the Na in evaporites was derived from alteration of schist-derived albite and Na-bearing smectite formed from albite. The high pH pan surfaces result from a combination of long-term (decades to millennia) albite alteration and short-term (minutes to months) Na-carbonate dissolution and recrystallisation. This muscovite is variably cemented by evaporative salts, and some crusts have abundant salt-dissolution voids. High pH has facilitated dissolution of aluminium and silica from albite and smectite, with evaporative precipitation of Al-oxyhydroxide contributing to crust cementation. Different dissolution rates of evaporative minerals during rain events and damp seasons results in differential mobility of salt components downslope. The salt pans are physically and chemically dynamic features that have formed on time scales of decades to centuries. [ABSTRACT FROM AUTHOR]

Evaporative mineral encrustations have formed on bare substrates at three different sites in the Maniototo basin. Two sites, at Patearoa and Belmont have salts derived primarily from marine aerosols. The Patearoa site has developed on clay-rich pans formed on a terrace eroded by the Taieri River into clay-altered schist basement, and dissolution of albite from altered schist has resulted in precipitation of sodium carbonate with halite, at pH∼10. The Belmont site lies on the flat distal part of an active alluvial fan, and halite-dominated salt encrustations develop on thin dry crusts through capillary action from below. At the nearby Hamiltons historic placer gold mine, excavations exposed clay-altered schist basement beneath Eocene auriferous sediments. Highly fractured and hydrothermally altered fault zones dominate basement, with secondary ankeritic carbonate and pyrite. Water-rock interaction of shallow groundwater in the basement leads to enrichment in dissolved Mg and sulphate that overshadows the marine aerosol components, and evaporative encrustations are dominated by Mg-rich minerals including brucite and epsomite. The halite-dominated sites host rare halophyte ecosystems, but progressive colonisation by adventive species will eventually cover the bare saline substrates. Similar colonisation at the Hamiltons site will result in natural rehabilitation of the abandoned mine site. [ABSTRACT FROM AUTHOR]

Soil-free saline substrates near Alexandra have developed from evaporated marine aerosols dominated by NaCl. The salts accumulate on impermeable substrates where there is minimal chemical interaction between water and rock. Most of the known saline sites occur on outcrops of clay-altered schist exposed immediately below the regional Miocene unconformity, and on clay-rich sediments eroded from altered schist. The unconformity zones that host these saline sites occur linearly along structurally controlled depressions. These unconformity zones have been major targets for placer gold mining, and most of the saline sites occur in abandoned mines where impermeable rocks were exposed by historic excavations. Native salt-tolerant plants (halophytes) have colonised parts of the saline substrates, especially on actively-forming clay pans derived from nearby steep mine cuts. Saline sites are ephemeral, as salt is readily removed in water, sediments can bury the surfaces, and incursion of adventive plant species encourages further sedimentation. Erosional disturbance can overcome these effects temporarily, and surface erosion to bare ground along the strike of the structurally controlled schist unconformity zones may also create new saline sites. [ABSTRACT FROM AUTHOR]