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the limited presence of geochemical “buffers,” such as carbonate
Figure 4. Brine evolution model for closed basins, featuring the Long-Lyons-
Hines (2009) model for acid saline Lake Tyrrell spring zone waters, and based
on the Eugster-Hardie-Jones brine evolution concept (Eugster, 1980; Eugster
and Hardie, 1978; Eugster and Jones, 1979). In this variation of the model,
abundance of HCO3 determines starting path. Several evolution pathways have
been documented for specific closed lakes with neutral alkaline conditions,
especially those in Africa and the western U.S.; in a complete model, these
would appear to the right of the detailed Lake Tyrrell path. The more extreme
and variable acid brines in southern Western Australia would appear as several tant players (e.g., Eugster, 1980; Eugster and Hardie, 1978; Eugster
complex evolution pathways to the left of the Lake Tyrrell path.
Climate and weather also play a role in acidity. In the semi-arid
climate of southern Western Australia, evaporation greatly effects
lake waters and, to a lesser extent, shallow groundwaters.
Evaporation drives off water from the acid saline brines, resulting
in more concentrated liquids. This evapo-concentration decreases
pH and increases salinity. During dry times, lake waters have pH
levels ~1–2 units lower than when the lakes are flooded. Likewise,
shallow groundwaters have their lowest pH when the lakes are
desiccated and shallow groundwater is evaporating. We have
observed these lowest pHs in lakes during late stages of evapo-
concentration (Benison et al., 2007; Bowen and Benison, 2009).
In addition, laboratory experiments show that pH of moderately
acidic water decreases several pH units upon evaporation (Foster
and Benison, 2006; Long et al., 1992).
Recycling of acid waters may occur as part of flooding–evapo-
concentration–desiccation cycles (Lowenstein and Hardie, 1985;
Benison et al., 2007). When halite and gypsum grow in the acid
lakes, they trap abundant acid fluid inclusions, which may
compose up to ~30%–40% by volume halite and ~10%–20% by
volume gypsum. When lakes flood and the halite and gypsum
dissolve (halite at a much greater rate than gypsum), the acid fluid
inclusions are released. Although this may be a small addition by
volume to the water, it likely contributes to the lowering of the
water pH after flooding.
The high degree of weathering of Archean rocks of the Yilgarn
Craton has resulted in little buffering capacity. The result may be
minerals and feldspars (Bowen et al., 2013; Long and Lyons, 1990,
1992). This has allowed low pH produced by acidification
processes to be maintained.
The extensive weathering of Archean rocks not only limits buff-
ering capacity but also may yield other acids. For example, chem-
ical weathering dissolves many minerals and yield waters rich in
ions, such as Cl, Br, and Fe. The high amounts of Cl− and Br− in
the acid waters suggest that hydrochloric and bromic acids exist
here in addition to sulfuric acid.
EXPANDING THE BRINE EVOLUTION MODEL GSA TODAY | www.geosociety.org/gsatoday
Hardie and Eugster (1970) and Eugster (1970) pioneered the use
of evaporite minerals to trace past brine evolution and the history
of evaporite sediments in closed basins. This brine evolution
model has evolved through the decades to incorporate the details
of chemical pathways for various neutral-alkaline brines, many of
which include carbon species and carbonate minerals as impor-
and Jones, 1979) (Fig. 3). Long et al. (2009) added the first low pH
branch to the geochemical model (Fig. 4). There are likely varia-
tions of acid brine evolution, just as variations in neutral and
alkaline brines exist. More work from the range of different acid
brine environments is required to build a brine evolution model
that fully encompasses the compositional and mineralogical range
produced by natural brines.
The Long-Lyons-Hines (2009) brine evolution model was based
on the acid spring zones along the shorelines of Lake Tyrrell in
northwestern Victoria. Their model suggests a geochemical evolu-
tion starting with waters enriched in Ca and Mg, but depleted in
HCO3. These waters precipitate gypsum (CaSO4 • 2H2O) and
halite (NaCl), undergo sulfide oxidation and ferrolysis to become
acidified, and precipitate Fe-oxides, jarosite [KFe3(SO4)2(OH)6], and
alunite [KAl3(SO4)2(OH)6]. The ending waters are Na-Mg-Cl-SO4
acid brines (Long et al., 2009) (Fig. 4). We note that the general
water evolution is the same in southern Western Australia, as well
as, perhaps, throughout the continent. However, the Yilgarn
Craton acid brines present more complex compositions. For
example, some lakes have Al + Si + Fe > Ca + Mg + K (Bowen and
Benison, 2009). Some acid brines have unusually high Al and/or
Si (up to 8,017 ppm Al and up to 13,300 ppm Si), but low Fe (0–10
ppm), and some have high Fe (up to 459 ppm), but lower Al and/
or Si. There are also temporal and spatial variations in major ions
at individual lakes. Because of this complexity and variability of
water chemistry and mineral precipitation and dissolution, no one
brine evolution pathway can be designated for the Western
Australian acid brines (Fig. 4). Figure 5 presents a flow chart that
depicts our general understanding of the geological and geochem-
ical evolution for the Yilgarn Craton of Western Australia, based
on observations published in Benison et al. (2007), Benison and
Bowen (2013), Bowen and Benison (2009), and Bowen et al.
(2013). We hypothesize that a transition from a warm and wet to a
warm and dry climate promoted the early chemical weathering
and late evaporation, oxidation, and acidification. Warm and wet
weathering in the Tertiary would have greatly decreased the buff-
ering minerals (Long and Lyons, 1992). Later arid climate would
have decreased the water:rock ratio, as well as enhanced concen-
tration of solutes, increasing the salinity. Laboratory experiments
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