Adam Fage's MSc thesis abstract

Thesis Title: 
Geology, Geochemistry and Geochronology of the Hemlo East Property, Schreiber-Hemlo Greenstone Belt, Ontario

The Hemlo East property is located approximately 50 kilometres east of the town of Marathon, Ontario and is situated within the Schreiber-Hemlo greenstone belt of the Wawa-Abitibi Terrane of the Archean Superior Province. Lithologies within the Hemlo East Property are broadly divided into mafic metavolcanic, felsic metavolcanic, metasedimentary, intrusive and metaintrusive rocks. Stratigraphic continuity within the study area is demonstrated by the large number of drill holes in the area. Rock staining indicates that the K-feldspar present in felsic metavolcanic rocks in the Gouda-Thor-Carroll area of the Hemlo East Property is a product of alteration likely associated with base metal and gold mineralization.

New major and trace element, Sm-Nd isotope, and geochronology data are presented for volcanic, plutonic, and metasedimentary rocks of the Hemlo East property to investigate their origins. Positive to negative εNd (-1.14 to 2.15) for tholeiitic basalts indicate that they were derived from depleted to moderately contaminated sources. FI rhyolites with steep trace element patterns (La/Ybn = 16 to 44) and fractionated HREE (Gd/Ybn = 2.1 to 7.5) are interpreted to have been derived from a deep mantle source. Metasedimentary rocks are geochemically similar to felsic metavolcanic rocks and are interpreted to be derived wholly, or in part, from a felsic source.

New U-Pb age dates include 2704.8 ±1.1 Ma and 2705.6 ±1 Ma from the Gouda and Thor metarhyolites, respectively; 2694.5 ±1 Ma from the DC Lake metadacite; 2691.6 ±1.1 Ma from the Upper Anomalous Zone volcaniclastic unit; 2693.1 ± 1 Ma from the Moose Lake Porphyry at Hemlo; 2683.4 ± 1.7 Ma from the Cedar Lake Pluton granodiorite; 2686.5 ±1.3 Ma from the White River Pluton monzogranite; and a maximum age of ~2696 Ma from the Frank Lake felsic metasedimentary unit. Results from the new and compiled U-Pb geochronological data indicate that there are two associations of felsic volcanism and two associations of plutonism, with a new association of felsic volcanism at 2705 Ma. This 2705 Ma phase represents the oldest felsic volcanic unit dated in the greenstone belt. This data points toward a northward younging of stratigraphy in the Gouda-Thor-Carroll area implying that the stratigraphy is right way up.

A long lived oceanic subduction zone produced the arc rhyolites and granitic intrusive rocks seen in the Hemlo East Property. There is no indication from this study as to what comprised the crust that the pre-tectonic plutons were intruded into. High La/Ybn ratios for the rhyolites suggest that their source magmas were derived at depth from the melting of the subducting slab. Island arc tholeiites (metabasalts) erupted from a primitive back-arc behind the island arc. The area between the arc and back-arc would have allowed for island arc tholeiites to be deposited alongside rhyolites in conjunction with sediments coming off of the volcanic arc to produce the stratigraphic associations seen in the Gouda-Thor-Carroll area. These supracrustal units were later intruded by TTG plutons.

A copy of the thesis can be downloaded here