Petrology and Geochemistry of the Tuffaceous Footwall Rocks of the Williams Ore Zone, Hemlo Area, Ontario

Abstract

<p> The recent discovery of ore grade gold in the Hemlo area of Northern Ontario has provided the initiative for extensive exploration in that region. This study concentrates on rocks from the footwall to the ore zone on the Williams property of Long Lac Mineral Exploration. </p> <p> The footwall rocks consist of fine grained, finely laminated tuffs. The suite has undergone low grade regional metamorphism. Retrograde metamorphic effects are also present but appear to be confined to certain bands. Some of the minerals may have been introduced by the action of metasomatic fluids. </p> <p> The tuffs are quite felsic and many have the composition of a rhyolite. Samples tested for gold have up to 200 times the background concentration of a rhyolite . The gold is associated with high Sio2 and K20 and low Al2O3, CaO, Na2O, Fe2O3 and MgO contents. </p> <p> The ore deposit is similar in many respects to the Bousquet deposit of Northern Quebec. The Bousquet deposit has been interpreted as syngenetic by some authors and therefore it is quite possible that the Hemlo deposit is syngenetic as well. However the metamorphism and possible metasomatism that these rocks have undergone suggests that it may be an epigenetic deposit. Due to the limited number of samples in this study it could not conclusively be determined which hypothesis is correct. </p>