The geology of aluminium phosphates and sulphates of the alunite group minerals: a review

Aluminium phosphates and sulphates of the alunite supergroup (APS minerals) occur in a wide range of environments of formation covering the metamorphic, igneous and sedimentary realms. Supergene processes, including mineral dressing and dumping when sulphide ores are mined, as well as hypogene alteration are also responsible for the precipitation of APS minerals. In these environments, complex solid solution series (s.s.s.) can form. The general formula of these alunite minerals is AB, (XO,),(OH),, where A is a large cation (Na, U, K, Ag, NH4, Pb, Ca, Ba, Sr, REE). B sites are occupied by cations of the elements Al, Fe, Cu and Zn. In nature, the anion (XO4)(x-) is dominated by P and S. Mineral dressing and identification of APS minerals often needs a combination of highly sophisticated measures including Atterberg settling methods, XRD, DTA, TGA, TEM-EDX, SEM, EMPA and XRF. In sedimentary rocks APS minerals occur in various rocks and environments of deposition: calcareous, phosphorite-bearing, argillaceous-carbonaceous, arenaceous, coal-bearing environments, in soils and paleosols, in saprolite (bauxites, laterites) and in calcareous-argillaceous sequences hosting Carlin-type SHDG deposits. In igneous rocks, APS minerals may be encountered mainly in acidic through intermediate pyroclastic, volcanic and subvolcanic rocks. They occur in barren volcanic rocks and porphyry-type intrusions that have sparked epithermal Au-Ag-base metal deposits, Au-Sb mineralization, APS-bearing argillite and alunite deposits in their immediate surroundings. Granitic and pegmatitic rocks are rarely host of supergene APS mineralization. During low-grade stage regional metamorphism, peraluminous parent rocks originating from a sedimentary or igneous protolith may also give rise to APS mineralization. Peraluminous parent rocks enriched in S and/or P are a prerequisite for the formation of APS minerals that are stable up to a temperature of 400 degreesC at moderately high fluid pressure of up to 1 kbar. The various APS mineralizations in nature occur in three zones; a fourth zone may be singled out when the enrichment of APS compounds in waste dumps, in acid mine drainage and during alum production is considered as part of the story. Zone I encompasses APS mineralization observed in metamorphic rocks of lowermost greenschist facies and igneous rocks that have undergone advanced argillic alteration with or without hydrothermal ore and non-metal mineralizations. Zone II following vertically upward in the earth crust is characterised by steam-heated, connate water- and ground water-related APS mineralization. This sort of APS mineralization forms close to the boundary between the vadose and phreatic hydraulic stockwork. The overlying zone III is confined to the topmost part of the vadose or infiltration zone. Its mineralization originated from meteoric waters and may be called supergene in the strict sense. Results obtained from the study of this kind of mineralization can directly be applied to artificial accumulation of APS compounds at the present-day surface. A combination of chemical measures such as S-O-H isotopes and REE variation together with experimentally based mineralogical data may be instrumental in the distinction of supergene and hypogene APS mineralization. This set of data may also assist in the assignment of APS mineralization either to hydrothermal magmatic or to steam-heated acid sulphate mineralizations. The K-bearing end members of the APS s.s.s, have proven to be an efficient tool to determine the age of formation of magmatic, weathering, diagenetic and alteration processes. APS minerals, however, can locally provide much more information to the origin of host and parent rocks than the rock-forming minerals themselves, which make up the host rocks of APS mineralization. (C) 2001 Elsevier Science B.V. All rights reserved.