Abstract T he U nderground tour will examine Proterozoic Zn sulfide deposits of the Balmat Zn mine, described by deLorraine and Dill (1982) and in this volume by deLorraine (2001). The evening preceding the tour will feature a talk on Balmat geology. This talk will involve a slide show presentation with maps, cross sections, and mine models made available for inspection. The latest structural interpretations will be reviewed and there will be in-depth discussions regarding the response of massive sulfides to high-grade, near granulite- facies metamorphism in a sequence of interlayered silicated dolomitic marbles and pure marble units of extremely high ductility contrast. We will demonstrate that ore was tectonically remobilized 600 m and more laterally, by over 2000 m in the down-plunge dimension. This happened in several parent-daughter pairs. The degree and extent of remobilization of massive sulfide at Balmat appears to be unique anywhere in the world.

Abstract Zinc orebodies of the Balmat-Edwards-Pierrepont Zinc mining district in the northwest Adirondacks of New York State rank as near world-class deposits having nearly 40.8 million metric tonnes (Mt) of past production plus reserves grading 9.4% Zn. Orebodies occur in Mesoproterozoic siliceous dolomitic marble that was metamorphosed and polydeformed during the Grenvillian orogeny about 1.1 Ga. Remobilization of inferred syngenetic massive ore during high-grade metamorphism has been locally extensive depending on the stratigraphic unit from which a particular orebody originated. Two remobilized orebodies, the Loomis and Lower Gleason at No. 3 Mine, show clear linkage to the Upper Gleason, a massive, high-grade parent lens of ore. The extent to which ore was remobilized from the Upper Gleason has only recently been discovered. External remobilization is over 500 m laterally discordant to stratigraphy and over 2000 m in the down-plunge dimension. Daughter orebodies or remobilized daughter ore zones within orebodies are well documented elsewhere at Balmat. There also exist clearly discordant, externally remobilized, daughter ore-bodies for which the source or parent bed can be inferred but which have not yet been located. The Upper Fowler and Mud Pond orebodies are two such “orphans” whose existence depends upon a third, parent orebody hitherto unsuspected but now predicted to reside in unit 6. The extent to which ore is dissipated to tectonic slides is limited by the presence of thick, bedded anhydrite layers in the section. Tectonic slides die out in anhydrite so that sulfide migration is constrained by the impingement of faults into those strata. In the Fowler orebody, distances of remobilization less than several hundred meters are observed because the structure was encased by flowage of anhydrite very early on in its evolution, effectively sealing off any further sulfide migration. Plastic flow, and to some extent fluid-aided plastic flow, along ductile-brittle synmetamorphic faults or tectonic slides is thought to have been the dominant transport mechanism. There is a clear association between rock type and ore thickness. Ore is thicker where it follows the trends formed by the intersection of competent, silicated units with mineralized fault or tectonic slide surfaces, and thinner where following intersection trends of ductile rocks with fault surfaces. Substantial revisions are made here to the number of ore-forming cycles in the Balmat section from six or more, down to three. Evaporite deposition in the form of anhydrite is tied closely to sulfide deposition so that three “evaporite-ore” events are recognized in the Balmat section corresponding to source horizons in units 6, 11, and 14. Any ore in contact with other units in the Balmat section is “in transit” and may have been remobilized significant distances from its source bed.

Abstract The zinc mines, located six miles southeast of the Village of Gouverneur in St. Lawrence County, New York, are a part of the world-class Balmat-Edwards zinc mining district which extends for 10 miles from Balmat northeasterly to Edwards. Occurrences of zinc and lead on the John D. Balmat farm were noted as early as 1838, but it was not until1927 that a core drilling program by what was then St. Joseph Lead Company resulted in a major discovery of high-grade zinc mineralization which later developed into the No.2 Mine. Subsequent discoveries in 1946 and 1965led to the development of the No.3 and No. 4 Mines. In 1979 core drilling intersected high-grade zinc mineralization at Pierrepont leading to the development of the Pierrepont mine. Near the northeast end of the district, St. Joe and predecessors and now ZCA have operated the Edwards mine from 1915 to 1980 and the Hyatt mine sporadically since 1918, but neither deposit has approached the size of the Balmat deposits. Balmat production has increased in stages from an initial 500 tons per day in 1930 through a major expansion completed in 1972 which effected a milling capacity of 4300 short tons per day. Current milling rate is on the order of 700 thousand tons annually. Production to date has been on the order of 37 million tons of ore at 9.4% Zn and 0.5%Pb.

A belt of pyrite deposits about 40 miles long occurs in the Grenville Series of the northwest Adirondacks, New York. The pyrite deposits are conformable and are restricted to certain stratigraphic zones. Graphite is a consistent accessory. The trace element characteristics of the pyrite, very low tellurium and a high nickel to cobalt ratio, are typical of pyrite from known sedimentary occurrences. The massive pyritic zones contain isotopically light sulfur (~ - 10 % 0 average) and exhibit similar sulfur isotope characteristics to the Wabana, Newfoundland, and Nairne, Australia, deposits, both of inferred sedimentary origin. The pyrite sulfur probably underwent bacterial reduction from aqueous sulfate. On the basis of the stratigraphic, geochemical, and isotopic evidence, the Adirondack pyrite deposits are inferred to have been initially bacterial, sedimentary-diagenetic in origin. The pyrite-bearing rocks have been strongly metamorphosed, on a regional scale, in the upper almandine-amphibolite facies and hornblende granulite subfacies. Most of the host rocks are migmatitic gneisses. In the vicinity of the pyritic zones, the mineralogy and textures of the regionally metamorphosed host rocks have been altered by subsequent hydrothermal activity. Dravite is present in concentrations of up to several percent in some of the pyrite deposits; texturally, the dravite exhibits replacement relationships with the host rock silicate minerals. The ground-mass of the pyrite concentrations consists largely of quartz and very line-grained nearly isotropic chlorite; sericite is abundant locally. Massive, coarsely crystalline pyrite aggregates and nodular pyrite are common rock fabric elements. The nodular structure consists of concentric spherical shells, entirely of pyrite, or largely of chlorite and quartz. Chemically, the groundmasses of the pyritic ores are high in H 2 0, Mg0, and C (graphite) and low in Na 2 0, Ca0, and K 2 0 in comparison to the regional gneisses. The pyritic deposits thus have undergone at least a three-stage history: sedimentation-diagenesis; high grade metamorphism; and localized hydro-thermal metamorphism. Pyrite and ore sulfides from the pyritic-zinc deposit at Balmat, New York, contain isotopically heavy sulfur (~ + 15 % 0 ). Mt. Isa, Australia, Pine Point, Canada, and Meggen, Germany, are similar to Balmat in their stratabound characteristics, abundance of pyrite with zinc or lead or both, and isotopically heavy sulfur. On the basis of published data, circulating thermal brines coupled with bacterial or organic chemical reduction of sulfate could be a source of sulfide sulfur with a relatively heavy and uniform isotopic composition. Sulfur isotope data on the Lockatong (Pennsylvania), Wabana (Newfoundland), and Wauseca (Michigan) pyrite concentrations were determined for this study and given for comparison.