"Rock of the Month # 80, posted for February 2008" ---

A pyrite-rich feldspathic Au ore from the large, late Archean Hemlo gold deposit, a modern-day gold find containing more than 600 tonnes of the precious metal. This ore is from the Williams mine, the westernmost of three mines developed on the major orebody. Loose in 5 x-cut, 9215 E level. Sample 1590, collected 14 March 1994. The 9215 level displayed a wide range of host-rock and ore types, including biotite, sericite and green-mica schists; veinlets of quartz-carbonate and of pyrite; felsic porphyries with assorted colours and degrees of deformation; banded feldspathic rocks; calc-silicate rocks; quartz lenses with tourmaline-rich selvages, and pyritic and baritic gold ores. This is a striking banded specimen with abundant sulphide (circa 12%) in foliae 2-10 mm thick. Traces of unusual bright green sheet silicate (barian mica?) occur in the siliceous matrix. Probably a relatively high-grade ore (several grams/tonne gold?).

The Hemlo deposit has attracted much attention from industry and academics alike. There are numerous reasons for this. It is a "world-class" deposit, and as cynics like to observe, such discoveries tend to generate world-class lawsuits. It was found adjacent to Highway 17, the southern arm of the Trans-Canada Highway, and thus many geologists had travelled unknowing beside and over it. The geology was such that a raft of mutually incompatible ideas was floated to account for the deposit's origin. Research on unusual aspects of the chemistry and mineralogy of the ore fuelled the debate. A measure of consensus was eventually achieved. Rather than write many more pages on the deposit, a selective bibliography is appended, less than 10 percent of MINLIB records on Hemlo. The choices include articles which are variously representative, timely, detailed and influential in their coverage of aspects of the Hemlo gold camp and its ore types.

The scale and pace of mining have increased enormously since the discovery of other top-rank hard-rock gold camps, such as Timmins (Porcupine) and Kalgoorlie. Thus it is rather disconcerting for some of us in the industry to see the decline of the mining camp, like an old friend, having seen it rise, from the initial excitement of the discovery, to the status of a major mining centre, and now to see the inevitable approach of mine closure. The central Golden Giant mine was closed in January 2006, and the David Bell and Williams mines are scheduled to close in 2009 and 2011, respectively.

Selective Chronological Bibliography, Research on the Hemlo Area and its Ores

Pre-1980

Coleman,AP (1899) Copper regions of the upper lakes. OBM Ann.Rep. 8 pt.2, 121-174.

Thomson,JE (1933) Geology of the Heron Bay-White Lake area. ODM Ann.Rep. 41 part 6, 1932, 34-47 plus map.

Springer,JS (1977) White River sheet. OGS Mineral Potential Map P1519, 1:250,000 scale.

1980-1989

Muir,TL (1982a) Geology of the Hemlo area, District of Thunder Bay. OGS Rep. 217, 65pp.

Muir,TL (1982b) Geology of the Heron Bay area, District of Thunder Bay. OGS Rep. 218, 89pp.

Anon (1984) Did Hemlo start like New Zealand hydrothermal field? Northern Miner 69 no.46, B34.

Wood,J and Wallace,H (1984) Major role of OGS in Hemlo discovery. Northern Miner 69 no.46, C7-8.

Brown,MR (1985) First gold from Hemlo. Northern Miner 71 no.7, 1-2.

Goldie,R (1985) The sinters of the Ohaki and Champagne Pools, New Zealand: possible modern analogues of the Hemlo gold deposit, northern Ontario. Geoscience Canada 12, 60-64.

Phillips,GN (1985) Interpretation of Big Bell/Hemlo-type gold deposits: precursors, metamorphism, melting and genetic constraints. Trans.Geol.Soc.S.Afr. 88, 159-173.

Hugon,H (1986) The Hemlo gold deposit, Ontario, Canada: a central portion of a large scale, wide zone of heterogeneous ductile shear. Proc. "Gold '86" Symposium, Toronto (Macdonald,AJ editor), 517pp., 379-387.

Lefolii,K (1987) Claims: Adventures in the Gold Trade. Key Porter Books Ltd, 264pp.

Harris,DC (1989) The Mineralogy and Geochemistry of the Hemlo Gold Deposit, Ontario. GSC Econ.Geol.Rep. 38, 88pp.

Wyman,D and Kerrich,R (1989) Archean shoshonitic lamprophyres associated with Superior Province gold deposits: distribution, tectonic setting, noble metal abundances, and significance for gold mineralization. In "The Geology of Gold Deposits: the Perspective in 1988" (Keays,RR, Ramsay,WRH and Groves,DI editors), Econ.Geol.Monograph 6, 667pp., 651-667.

White,JC and Barnett,RL (1990) Microstructural signatures and glide twins in microcline, Hemlo, Ontario. Can.Mineral. 28, 757-769.

Svela,O (1991) Hemlo mines pass their peak but years of production to go. Northern Miner 77 no.14, 1,19.

Williams,HR, Stott,GM, Heather,KB, Muir,TL and Sage,RP (1991) Wawa subprovince. In "Geology of Ontario" (Thurston,PC, Williams,HR, Sutcliffe,RH and Stott,GM editors), OGS Spec.Vol. 4, part 1, 709pp., 484-539.

Pan,Y and Fleet,ME (1992) Calc-silicate alteration in the Hemlo gold deposit, Ontario: mineral assemblages, P-T-X constraints, and significance. Econ.Geol. 87, 1104-1120.

Kuhns,RJ, Sawkins,FJ and Ito,E (1994) Magmatism, metamorphism and deformation at Hemlo, Ontario, and the timing of Au-Mo mineralization in the Golden Giant mine. Econ.Geol. 89, 720-756.

Michibayashi,K (1995) Two phase syntectonic gold mineralization and barite remobilization within the main ore body of the Golden Giant mine, Hemlo, Ontario, Canada. Ore Geology Reviews 10, 31-50.

Muir,TL, Schnieders,BR and Smyk,MC (editors) (1995) Geology and Gold Deposits of the Hemlo Area, Revised Edition. Institute on Lake Superior Geology Field Trip Guidebook, 41st Annual Meeting, Marathon, Ontario, 120pp.

Pan,Y and Fleet,ME (1995) The late Archean Hemlo gold deposit, Ontario, Canada: a review and synthesis. Ore Geology Reviews 9, 455-488.

Muir,TL (1997) Hemlo Gold Deposit Area. OGS Report 289, 219pp. plus map folder.

Powell,WG and Pattison,DRM (1997) An exsolution origin for low-temperature sulfides at the Hemlo gold deposit, Ontario, Canada. Econ.Geol. 92, 569-577.

2000 onwards

Frost,BR, Mavrogenes,JA and Tomkins,AG (2002) Partial melting of sulfide ore deposits during medium- and high-grade metamorphism. Can.Mineral. 40, 1-18.

Davis,DW and Lin,S (2003) Unraveling the geologic history of the Hemlo Archean gold deposit, Superior province, Canada: a U-Pb geochronological study. Econ.Geol. 98, 51-67.

Muir,TL (2003) Structural evolution of the Hemlo greenstone belt in the vicinity of the world-class Hemlo gold deposit. Can.J.Earth Sci. 40, 395-430.

Tomkins,AG, Pattison,DRM and Zaleski,E (2004) The Hemlo gold deposit, Ontario: an example of melting and mobilization of a precious metal-sulfosalt assemblage during amphibolite facies metamorphism and deformation. Econ.Geol. 99, 1063-1084.