Gordon R. Yule HBSc thesis abstract

A genetic model for the localization of uranium mineralization related to the Proterozoic rocks of the Thunder Bay District was proposed by Franklin (1978).  This model was investigated in detailed examination of a breccia-type uranium occurrence referred to as the Innes Lake Radioactive Breccia.  The occurrence is situated in the Good Morning Lake fault which transects a late Archean quartz monzonite.

Observations are:

1. STRUCTURAL - The Good Morning Lake Fault strikes 330°.  It is characteristic of the regional early Keweenawan faulting and fracture systems proposed by McIlwaine et al (1974).  Detailed structural analysis indicates that this fault zone pre-dates late Keweenawan fractures which strike 60°.  These tensional fractures host the lead-zine-barite veins which are also found within the district (Franklin and Mitchell, 1977).
2. VEIN SYSTEM- The fault zone comprises fine to coarse grained, altered wallrock and siliceous vein fragments within a very fine-grained hematitic groundmass.  Fragments indicate multiple stages of vein formation including: injection of siliceous fluids, crystallization, and subsequent brecciation.  Common vein structures include comb-structure, and miarolitic cavities.  Tourmaline formed at an early stage in vein development along with milky quartz.  A cherty breccia and a hematitic breccia are late stage events which are post-dated by uranium-bearing fluorapatite mineralization.
3. MINERALIZATION - Detailed ground radiometric surveys indicates high uranium: thorium ratios.  Autoradiographs have delineated radioactive concentrations within hematite-enriched fractures within the hematitic breccia.  These uranium-bearing fractures crosscut this breccia, thereby suggesting that uranium introduction was a late stage event.  Anomalously high P₂O₅ values (greater than 3%) and x-ray analysis of the mineralization suggest that the uranium mineralization is mineralogically associated with phosphate within the apatite species.
4. ALTERATION - Alteration zones are pervasive in the quartz monzonite wallrock which envelopes the breccia and consist of sericitization, chloritization, and hematitization.  The alteration is associated with the early stage tourmaline mineralization.  Geochemical analyses for uranium in the wallrock suggest that the metal is not derived from the alteration zones.

The anomalous P₂O₅ values associated with the uranium mineralization stage suggest that local apatite-bearing uraniferous Archean pegmatites are the probable source rocks as proposed by Franklin (1978).  The role which the nearby Archean/Helikian unconformity at the base of the Sibley Group played with regard to this mineralized fracture is unclear but Franklin (1970) has suggested that other types of vein mineralization within the district are the result of precipitation from circulating groundwaters along the unconformity.