"Rock of the Month # 95, posted May 2009" ---

A sheared gold ore rich in distinctive green mica, from the large Hemlo gold deposit, located north of Lake Superior, beside the Trans-Canada Highway (Hwy. 17) in northwest Ontario. Sample 1607, a biotite- green mica schist, from the eastern part of the Hemlo orebody, was found in the David Bell mine (5 level, H3 stope, collected in situ, 23 March 1994). A spectacular biotite schist with abundant green mica, with well-developed crenulation cleavage. Note the "foliation fish" composed of brilliantly colourful green mica. The rock appears to be composed largely of quartz and feldspars, biotite, the green mica, pyrite, and possibly unseen barite, given a significant "heft" to the sample. Tom Muir (Ontario Geological Survey, pers.commun., 2009) notes that this sample displays exceptionally fine "green muscovite foliation fish... Given the appearance of the specimen and the presence of a crenulation cleavage, it strongly suggests that it is related to the `D3' dextral shear event (following the) `D2' sinistral event" (see Muir, 2002, 2003 ; also Lin, 2001 and Davis and Lin, 2003).

Indicators for mineralization at Hemlo include green micas, molybdenite, barite and realgar (see, e.g., Harris, 1989). Pyrite is the predominant sulphide. A pyritic ore from the Williams mine, on the west end of the orebody, was featured as Rock of the Month 80. The references that follow include a partial update of those quoted in R.O.M. 80.

Hemlo ore-zone green micas have been age dated at 2671 Ma, probably the event in which isotopic equilibrium was established between S in pyrite and barite, and O in quartz and muscovite, at about 500°C (Crocket et al., 1994). Green mica rich in V characterizes ore-grade mica schists. The V content of Hemlo muscovites varies from electron microprobe detection limits up to at least 17.6 wt.% V₂O₃ in roscoelite (Pan and Fleet, 1992a,b; Fleet and Pan, 1998). The principal substitution is evidently of trivalent vanadium for aluminium. The host schist was probably derived mainly from quartz-oligoclase porphyry. The high V and Cr contents probably stem from alteration of mafic-ultramafic rocks, and these metals became fixed in green mica and oxides during waning regional metamorphism. Later hydrothermal alteration then mobilized V anew, and generated the V-rich calc-silicates.

Schnieders and Smyk (1994) reviewed the history and development of the Hemlo mining camp, more or less at the peak of the mines' productivity, with annual production and tonnes/day milled for the 3 mines, from west to east, standing at ~450,000 oz / 6,000 T/day [Williams], 440,000 oz / 3,000 T/day [Golden Giant] and 200,000 oz / 1,000 T/day [David Bell]. The mines supported 1,200 direct jobs, and many more indirectly across the region. Today, although the Golden Giant has closed, the good news from the Williams mine lies with new resources delineated, which should expand the mine life at least until 2013. The earliest known exploration in the area involved Moses Peekongay and Donald McKellar in the 1860s and 1870s (Schnieders and Smyk, 1994). In the early 1930s J.E. Thomson recognized the potential of the Hemlo area. Trevor Page explored the area in late 1940s. Ken Fenwick of the Ontario Department of Mines suggested remapping in 1975. However, mining success at Hemlo was hard to achieve, underscoring the difficulties of exploration. From an early gold discovery in 1944 until the main ore zone was found in 1981, over 20 companies and individuals explored the area, in work which included 9 drill programs and over 130 drill holes. The pivotal discovery in May 91 came in hole 76 of the drill program, a case study of perseverance.

SOME RECENT REFERENCES (2004-2008) AND
CITED ARTICLES ON THE HEMLO GOLD DEPOSIT

Beakhouse,GP (2007) Structurally Controlled, Magmatic Hydrothermal Model for Archean Lode Gold Deposits: A Working Hypothesis. OGS OFR 6193, 133pp.

Beakhouse,GP and Lin,S (2006) Tectonic significance of the Pukaskwa batholith and its contact relationships with the Hemlo and Mishibishu greenstone belts. OGS OFR 6192, 7-1- 7-7.

Crocket,J, Thode,H and Ding,T (1994) Sulphur isotope studies of the Hemlo gold mineralization: sources of sulphur and implications for the mineralization process. Abs. Hemlo Geology Workshop, at OGS Symposium, Toronto, 31pp., 13-14.

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.

Fleet,ME and Pan,Y (1998) Arsenic in pyrite at Carlin, V in green mica at Hemlo, and Cl/REE/U-Pb in apatite of mafic/ultramafic intrusions. GAC/MAC Abs. 23, 56, Quebec.

Fralick,P, Purdon,RH and Davis,DW (2006) Neoarchean trans-subprovince sediment transport in southwestern Superior province: sedimentological, geochemical, and geochronological evidence. CJES 43, 1055-1070.

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

Heiligmann,M, Williams-Jones,AE and Clark,JR (2008) The role of sulfate-sulfide-oxide-silicate equilibria in the metamorphism of hydrothermal alteration at the Hemlo gold deposit, Ontario. Econ.Geol. 103, 335-351.

Lin,S (2001) Stratigraphic and structural setting of the Hemlo gold deposit, Ontario, Canada, Econ.Geol. 96, 477-507.

Mavrogenes,J (2005) The Hemlo gold deposit, Ontario: an example of melting and mobilization of a precious metal-sulfosalt assemblage during amphibolite facies metamorphism and deformation. SEG Newsletter 60, 34-35.

Muir,TL (2002) The Hemlo gold deposit, Ontario, Canada: principal deposit characteristics and constraints on mineralization. Ore Geology Reviews 21, 1-66.

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.

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

Schnieders,BR and Smyk,MC (1994) Hemlo then and now: an historical overview. Abs. Hemlo Geology Workshop, at OGS Symposium, Toronto, 31pp., 7-8.

Schnieders,BR, Scott,JF, Magee,MA, Muir,TL and Komar,C (2005) Thunder Bay South regional resident geologist report: Thunder Bay South district. OGS OFR 6148, 46pp.

Scott,JF, Campbell,DA, Hinz,P and Komar,CL (2008) Report of activities, 2007, resident geologist program, Thunder Bay South Resident Geologist Report. OGS OFR 6218, 50pp.

Smyk,MC and Magee,A (2004) Silver threads and golden needles: geological milestones in northwestern Ontario. Abs. 50th Annual Meeting, Institute on Lake Superior Geology, vol. 50 part 1, 161pp., 149, Duluth, Minnesota.