Sillitoe, 2010 - Avala Resources Ltd.

Anatomy of a hydrothermal
breccia
In general, hydrothermal breccia show systema@c facies.
There is a core of highly brecciated material. Where
fluidiza@on has been intense, the matrix consists of rock flour
(finely milled rock) with clasts of varying sizes. In a single
stage breccia, clast sizes increase in size and angularity
toward the margin. Rocks immediately outside of the breccia
are crackled, that is fractured in place and essen@ally can be
fit together as a jig-­‐saw. Inter-­‐clast displacement and
rounding increases toward the breccia.
Mul@-­‐stage breccia have complex overprin@ng facies from the
subsequent events.
Breccia nucleate at depth and propagate upward un@l the
energy is dissipated, thereby forming conical body. The roof
rocks are cut by fingers of crackle breccia to rock flour breccia
which represent the upward propaga@on of the larger body.
Areas of maximum permeability are along breccia margins;
complex when mul@ple breccia bodies present.
Where they breach the surface, they form dis@nc@ve
volcanic-­‐like features (see next slide)