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Ore formation by hydrothermal fluids

Most ore deposit on earth are formed or modified by hydrothermal fluids. These warm to very high
temperature fluids are mostly composed of water and range in salt content and acidity. With the help of
a heat source, they move through the tiny, empty pores and structures within rocks to mobilize and
deposit valuable metals and other elements.
Alteration and exploration
When companies talk about alteration in their press releases, they are usually referring to the end
result of the alteration process – the minerals and collections of minerals (known as assemblages)
resulting from certain types of alteration.
Individual drill holes, exploration properties and even large mines are often mapped by geologists
according to the alteration style. Alteration is a key aspect of core logging, together with colour,
texture, and rock
type.

Porphyry copper
deposit model (GSA)
Finding certain types
of alteration in drill
core or field mapping
can indicate certain
types of
mineralization and
direct explorers to
ore zones. Alteration
can enrich an
orebody, or expand its
footprint, making it
easier to find.
Although alteration
can be a huge arrow
pointing the way to a
new ore deposit, it can
also be a very
confusing patchwork
of unusual minerals, seemingly designed to confuse geologists, young and old.

Introducing the –ics
Here are the –ic terms most often associated with the term alteration in a company media release, and
the ore deposits they are associated with.
Potassic
Potassic alteration involves the formation of new potassium feldspar minerals and possible some
biotite. There may also be small amounts of sericite, chlorite and quartz. This type of alteration is
typically found at the core of porphyry copper deposits, the result of alteration by very high temperature
potassium-rich fluids.
Phyllic (AKA sericitic)
Phyllic alteration is associated with porphyry copper deposits too, but also epithermal and volcanic
massive sulphide deposits. It forms over a wide range of temperatures and is a very common alteration
style. The resulting minerals include mostly a fine-grained white mica known as sericite – it is
sometimes referred to as sericitic alteration – and some quartz, chlorite and pyrite.
Propylitic

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