Adularia-sericite
epithermal ore deposits have well-documented alteration mineral zonation
patterns with distinctive geochemical signatures. Proper use of whole-rock
geochemistry can provide useful vectors for exploration and can enhance efforts
to characterise and quantify the mineralogy and intensity of hydrothermal
alteration.

Analyses of Au and Ag
provide the most direct vectors to orebodies, and pathfinder elements such as
As, Sb, Hg, Zn, Pb and Se are similarly zoned around at least some orebodies.
However, the minerals that contain these elements occur almost exclusively in
veins, and therefore identifying geochemical vectors related to wallrock
alteration that is zoned outward from orebodies is crucial for effective
exploration.

Distal alteration zones
contain quartz-illite-chlorite-carbonate-pyrite, whereas samples proximal to
veins contain quartz-adularia-pyrite, with lesser though variable
illite-chlorite-carbonate. Therefore, adularia-sericite epithermal ore deposits
show K alteration in their cores. The resulting K-metasomatism can be evaluated
in several ways. Areas that have been most intensely altered are commonly
characterised by highest K contents, but these results can be complicated by
variations in rock type across the prospective area. Potassium mass balance
results provide a robust and rigorous assessment of K-metasomatism, however,
knowledge about the composition
of
fresh rocks and correct identification of fresh precursor compositions of
altered rocks are required. Three alteration indices provide effective
evaluation of K-metasomatism:
1.
the K number, which is the ratio
K/(2Ca+K+Na) (molar);
2.
Barrie values [(K2O * 100) / Sr]; and
3.
the Rb/Sr ratio. In most places
these provide similar vectors, although K numbers can be complicated by the
appearance of hydrothermal calcite and/or albite.

Plots of K/Al vs (2 Ca
+ K + Na)/Al (molar) allow estimation of the intensity of K-metasomatism and its
mineralogical products, thereby complementing traditional XRD and petrographic
studies of alteration minerals.