Economic Geoh;gy Vol. 92, 1997. pp. 569 577 An Exsolution Origin for Low-Temperature Sulfides at the Hemlo Gold Deposit, Ontario, Canada WAYNE G. POWELL • AND DAVID R. M. PATTISON Department of Geology and Geophysics, University of Calganj, Calganj, Alberta, Canada T2N1N4 Abstract Many aspects ofthe timing and genesis ofthe Archean Hemlo gold deposit are unresolved; pre-, syn- and postmetamorphic models have been proposed. The presence of abundant low-temperature sulfide minerals udthin highly deformed, sillimanite-grade host rocks has been used tosupport apostmetamorphic origin for the deposit. However, thepresent Sb-As-Hg oreassemblage, including cinnabar-realgar-orpiment assemblages, developed through a sequence of exsolution events during postpeak metamorphic cooling. At peak metamor- phisre, most ofthe As and Hg,along with T1, Cu, Zn,and minor Fe,was incorporated into a high-temperature antimonian sulfosalt. In the lower amphibolite facies or upper greenschist facies, impure (Hg, Zn)S exsolved fi'om the high-temperature phase. An impure chaleopyrite and native antimony exsolved from the host sulfosalt soon after, leaving behind anSb2S•-As2S3 solid solution. Further cooling resulted in the exsolution ofcinnabar and mercurian sphalerite from the (Hg,Zn)S. Impurities in thecinnabar and ehalcopyrite exsolved to form aktashite, arsenopyrite and native antimony. Eventually, orpiment exsolved from thearsenic-bearing stibnite. Late, retrograde events converted the free orpiment torealgar and remobilized the realgar over short distances, along with cinnabar and native antimony. The history ofsequential sulfide exsolution, along with the deformed nature of theSb-As-Hg-bearing veins, and their metamorphosed alteration selvages, suggests that themetals were introduced prior to, or during, peak metamorphism. Introduction WITH its estimated 20 million ounces (Moz) of gold, the Henrio gold deposit of northwestern Ontario is the largest gold deposit currently being mined in Canada. In addition to itseconomic importance, Hemlo is of interest because of its atypical alteration style and metal associations, including Mo, As,Hg, andSb. The highly deformed nature of the deposit and the high metamorphic grade of the host rocks has ob- scured many relationships at the mine site andhas resulted in a broad spectrum of genetic models being proposed for the origin of the deposit. Currently discussed ideas for the genesis ofHemlo include premetamorphie porphyry mineral- ization (Kuhns et al.,1994; Johnston, 1996), syn- topostmeta- morphic shear zone-related mineralization (Hugon, 1986; Corfu andMuir, 1989), and postmetamorphic multistage re- placement mineralization (Pan andFleet, 1995). The relative timing of metamorphism and oredeposition isan important butunresolved aspect ofthedeposit's history. There isanapparent mismateh oflow- to moderate-tempera- ture sulfide minerals, such as realgar, orpiment, cinnabar, and stibnite, withhost rocks of sillimanite grade. The estimated peak metamorphic temperature of600øC exceeds the melting temperatures of these minerals: 307øC for realgar (Clark, 1960), 315øC for orpiment (Kirkinskiy et al., 1967), <345øC for cinnabar (Dickson and Tunell, 1959), and 556øCfor stib- nite (Pettit, 1964). This paper addresses theorigin of Sb-As-Hg-mineraliza- tionin the Hemlodeposit andits relationship to regional metamorphism. The distribution and textures of sulfide as- semblages, along with associated alteration, provide con- Corresponding author: email, wpoxvell@telusplanet.net straints onthe timing of gold mineralization at the Hemlo deposit. General Geology The Hemlo gold deposit is located inthe Hemlo-Sehreiber greenstone beltof theSuperior province, northwestern On- tario. Since production began in 1985, thethree mines that exploit thedeposit (Williams mine, Golden Giant mine, and DavidBell mine)have produced 9.5 million ounces (Moz) ofgold and account forone-quarter ofCanada's gold produc- tion. The main ore zone lieson the edge of a quartz-feldspar porphyritie rock, interpreted tobeintrusive in origin (Moose Lake porphyry; Kuhns et al.,1994; Johnston, 1996), within a highly deformed sequence of metasedimentary and metavol- eanie strata. Felsie volcanic rocks have been dated at 2772 _ 2 Ma (Corfu and Muir, 1989a).In the mine area,the strata are oriented at an average of 115 ø to 65 ø N. Granodioritie plutons intruded the supracrustal rocks between 2688and 2678 Ma (Corfu and Muir, 1989). Metamorphosed alteration associated with the deposit includes a centralzone of microcline, commonly barian, surrounded by a zone of muscovite. A more extensive zone of aluminum enrichment is defined by an increased abun- dance of aluminosilieates withrespect to the regional, unal- tered metasedimentary rocks (Kuhns et al.,1986; Johnston, 1996). Gold is spatially associated withmicrocline alteration, and Augrades correlate strongly with Mo. In general, thedeposit is also enriched in Sb,As,Hg, T1,Ba,V, andB. However, metals arebroadly zoned across the deposit. Arsenic, mer- cury, and thallium are all confined to thecentral part of the deposit. These metals are nearly absent from thenorthwest downplunge extension of theWilliams mine, where there is 0361-0128/97/1935/569-956.00 569