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origin of the acidity is likely sulfuric acid, with subsequent production of hydrochloric The Yilgarn Craton contains granite- GSA TODAY | www.geosociety.org/gsatoday
and bromic acids (Benison and Bowen, 2013). gneiss complexes and greenstone belts,
which are deformed, faulted, and show a
The acid saline lakes and groundwaters are hosted by Archean rocks of the Yilgarn range of metamorphic alteration and
Craton and a thin, laterally discontinuous cover of sediment and sedimentary rock. various degrees of physical and chemical
weathering. These rocks are mined locally
Figure 2. Photos of acid brine-influenced landscapes, rocks, and sediments in southern Western for gold, iron, nickel, copper, lead, zinc,
Australia. (A) Highly weathered outcrop near Leonora. (B) Altered granite in Hyden containing yellow aluminum, uranium, and rare earth
and orange jarosite iron oxide and sulfate minerals. (C) Dry lake and surrounding sand flat east of elements, and some ore concentrations are
Norseman. (D) Acid brine lake with white halite and yellow water near Salmon Gums. (E) Cross section related to migration of the acid brines
of shallow sand flat sediment adjacent to acid brine lake southwest of Norseman. Note mottles of iron (Lawrance, 2001). Archean outcrops are
oxides, jarosite, and alunite. (F) Cross section of sand dune composed of iron oxide–coated gypsum found under and adjacent to some lakes
grains near Merredin. (G) Cross section of acid sulfate soil near Toodyay. (H) Cross section of iron oxide and in direct contact with some modern
concretions in recent sandstone near Kellerberrin. lake water. Localized “regolith” (i.e., highly
weathered Archean rock) has preserved
some igneous and metamorphic textures
from the precursor rock. However, its
mineralogy includes Fe-oxides, kaolinite,
and quartz, which are suggestive of altera-
tion by acid saline waters (Fig. 2) (Bowen
et al., 2013). Thin Eocene–Quaternary
sandstones, lignites, and rare carbonates
are found up to 100 m deep in paleoc­ hannel
basins (Clarke, 1993, 1994; Clarke et al.,
1996). Thin recent sandstones and iron-
stones are found along the shorelines of
some acid saline lakes. Modern clastic
sediments include quartz sand and
reworked evaporites. Acid lake waters
precipitate halite, gypsum, hematite,
kaolinite/halloysite, and rare opaline silica
(Fig. 2). Acid groundwaters precipitate a
suite of displacive crystals and early cements
in the surface and shallow subsurface sedi-
ments. These very early digenetic minerals
include halite, gypsum, hematite, goethite,
jarosite, alunite, rozenite, gibbsite, kaolinite/
halloysite, dickite, and hydrobasaluminute
(Fig. 2) (Benison et al., 2007; Bowen et al.,
2012; Story et al., 2010).

Neutral saline lakes in southern Western
Australia are the anomaly here (Fig. 1).
Some also have adjacent neutral ground-
waters and overlie paleochannels,
suggesting buffering of regional acid
groundwaters by rare Eocene limestones at
depth. Other neutral lakes are underlain
by acid or moderately acid groundwaters.
The larger neutral lakes may have a greater
ratio of meteoric runoff relative to acid
groundwaters. Other neutral lake waters
are separated by mud from acid ground­
waters, effectively making the neutral lake
a perched meteoric water table.

Acid saline groundwater is widespread
and seeps into most lakes to contribute to
their unusual chemistry. The overarching
question is: How did the extreme acidity
form here?

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