Mainland Belt silver deposits of the Thunder Bay District are hosted in steeply dipping normal faults that cut Aphebian Rove Shale and Neohelikian Logan diabase sills. The Mainland Belt is located in the southern province of the Canadian Shield.
This study examines the Creswel silver deposits which occur in and around a group of diabase sills 1.7 km west of Rabbit Mountain. The term, Creswel silver deposits includes the following mines and prospects: the Badger Mine, Porcupine Mine, Porcupine Jr. Mine, Keystone Veins, Eschweiller Veins, Animike Mines Vein 22 and the West Beaver Mining Property (Little Pig Vein and Vein No. 2).
The veins usually have a common mineralogy consisting of quartz, calcite, fluorite, and variable amounts of sphalerite, galena, pyrite, chalcopyrite, argentite and silver. Silver bearing veins show evidence of secondary fracture, which crosscuts pre-existing vein development. Silver bearing vein material is emplaced after secondary fracture and is richer in sulphides than pre-existing vein material. Furthermore, calcite after secondary fracturing is coarser than calcite of the primary vein development. Secondary pulses of vein material may also be more voluminous and calcite rich than the pre-existing vein.
Four controls may have been responsible for the development of silver bearing veins, these are:
- A geochemical control, in that silver veins are commonly associated with lower argillite member of Rove Shale.
- A structural control apparently exists such that secondary fracturing and a subsequent secondary pulse of vein material is necessary for the development of silver bearing veins.
- Silver may be deposited at the intersection of vein systems. This theory is somewhat tenuous and more work must be done to prove or refute this supposition.
- A second sill is known to occur beneath some of the Creswel silver deposits. The importance of the sill is, as yet, uncertain but it may have acted as a permeability barrier effectively trapping fluids between the two sills. Furthermore, the added heat and concentrated circulation may have enhanced the "scavenging " ability of the fluids thus producing silver bearing veins.
At any rate, the common theory of genesis states that, heat from the intrusive diabase sills stimulated water circulation in the metal rich lower argillite unit of the Rove Shale. Soon after diabase had crystallized a faulting event occurred and structural dilatent zones opened and vein material was deposited.