Brecciated Silver Ore from the Alma District

Rounded fragments are held by a cementing matrix

One of our our study members found this unsual ore specimen that reveals brecciation.  Note the small, rounded clasts.

Breccia is a rock composed of fragments of minerals or rocks in a matrix (cementing material). The fragments in this specimen are different from the composition of the matrix. This may be a hydrothermal breccia. This type of breccia  is formed in a void along a fault underground. The void draws in hot water that violently boils--like an underground geyser. Rock at sides of the fault fall inwards and the broken rock gets caught up in a churning mixture of rock, steam and boiling water. The rock fragments hit each other and sides of the fault which quickly rounds the once angular breccia fragments.