The mineralogy and geochemistry of the Hemlo Gold Deposit, Ontario

Abstract
The Hemlo gold deposit, discovered in 1982, is located near the northeast shore of Lake Superior, 35 km east of Marathon, Ontario, adjacent to Trans-Canada Highway i7. The deposit is of Archean age and occurs at the contact of felsic metavolcanics and pelitic metasediments. it consists of several mineralized zones, of which the main zone extends for a length of 2900 m, for a distance of 2500 m down-dip and ranges in thickness ji·om 3 to 45 m. This zone contains at least 80 million tonnes of ore with an average grade of 7. 7 glt Au. Three properties covering portions of the deposit are in production, i.e. from east to west, the David Bell mine, the Golden Giant mine and the Page-Williams mine. The gold ore is substantially enriched in Mo, V, As, Sb, Hg, Tl and Ba and contains a diverse assemblage of minerals. A total of 8i minerals have been identified in the deposit, of which three are new species, named criddleite, vaughanite and hemloite. Also, the Hemlo deposit is the first Canadian locality for several other minerals. Native gold is the principal gold mineral. it occurs as free grains, between 1 and 20 J1m, in gangue and is mercury-rich, with as much as 26.9 wt. % Hg. The ore zone is pyrite-rich and is characterized by the presence ofmolybdenite, green vanadian muscovite, a V-Sb-W-bearing rutile. barian microcline and in places, major barite. The principal mercury minerals, cinnabar and aktashite; the thallium minerals routhierite and parapierrotite and the arsenic mineral realgar are restricted to the central portion of the deposit. Telluride minerals are rare. Other common ore minerals are stibnite, tetrahedrite-tennantite, mercury-bearing sphalerite (up to 29.5 wt.% Hg), several sulphosalts of the lead sulphantimonide variety and arsenopyrite. Tomichite, a rare oxide mineral previously known only in the "green leader" gold lodes at Kalgoorlie, Western Australia is a spar~e mineral in the ore. Barite is a major constituent in both the Golden Giant and Page-Williams orebodies, with some parts containing up to 70%. In some barite-rich sections, angular fragments of ore are contained within the baritic matrix and only where sufficient ore fragments are present is the rock of ore grade. Barian microcline is the second major barium mineral. Potassic alteration is evident throughout the deposit and has produced abundant barian microcline, and white mica that is most abundant in the felsic rocks, but which is also developed in the sedimentary rocks adjacent to the ore. Structural studies by other investigators have suggested that the gold-bearing rocks occupy the most intensely deformed, central portion of a large scale, wide ductile zone of oblique thrusting closely related to the Lake Superior shear. The ore minerals were formed from hydrothermal fluids that may in part be related to the shear zone.

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