Cretaceous (91 Ma) Tombstone Suite monzonite and quartz monzonite intrudes Paleozoic Earn and Road River Groups lithologies as a series of semi-conformable sills along a 15km strike length defining the Brewery Creek Reserve Trend.  Younger, Tombstone Suite syenite and biotite monzonite occur locally in the south-central portion of the property.  All compositional phases of the Tombstone Suite intrusives are known to host gold mineralization.

Sill emplacement is primarily controlled by a tectonized, graphitic argillite at the contact between the Earn and Road River Groups.  This contact is also the locus of NNE-directed thrust faulting that has placed thin (<150 m thick) sequences of Silurian siltstone against Devonian siliciclastic rocks.  The age of faulting is probably related to earlier Mesozoic compression along the Dawson, Tombstone and Robert Service Thrust Faults and the closing of the Selwyn Basin.

Brewery Creek deposits exhibit characteristics of both epithermal type and intrusive-related gold systems. It is generally considered to be an alkalic intrusion-associated gold deposit, as most of the mineralization is concentrated within or proximal to the monzonites of the Cretaceous Tombstone Suite.  Gold mineralization occurs in fracture-controlled quartz stockwork in both siliciclastic and intrusive rocks along an east-northeast striking, moderately south dipping structural trend known as the Brewery Creek Reserve Trend. 

Altered intrusive rocks are typically the preferred host for gold mineralization, however gold mineralization at the Pacific deposit exhibits a strong preference for a siltstone host, and in other deposits into adjacent intrusive rocks.  Major ore-controlling structures in intrusive rocks are related to a post Tombstone age, NNW compressional event that produced ESE and NE striking conjugate shears and ENE listric normal faulting localized along graphitic argillite/intrusive sill contacts. Approximately 85% of the mined ore was hosted by the various Cretaceous-aged quartz monzonite sills with the balance contained in silicified and brecciated Earn Group sediments.