Abstract

Examination of the unique lithologies of the Ammonoosuc Volcanics at the Ore Hill mine (i.e., a basal, pillowed, epidotized amphibolite, a cordierite-kyanite footwall schist, a muscovite schist host rock, and a tremolite-phlogopite cap rock) suggests that the sequence represents a deformed and metamorphosed alteration zone associated with sea-floor precipitation of massive ore. The essentially stratiform and strata-bound nature of the Zn-Pb-Cu orebody suggests syngenetic massive sulfide genesis. Based on mining records, a reverse ore zonation is documented, in which chalcopyrite appears to be concentrated in the upper sections of the orebody, and sphalerite and galena increase toward the base. Evidence is presented, however, to support a lateral as opposed to a vertical metallic zonation scheme.Major element trends, associated with alteration in footwall mafic metavolcanic rocks (SiO 2 , FeO, MgO, Na 2 O, and K 2 O depletions), suggest sea-floor metasomatism of the local mine sequence. Similarly, electron microprobe analyses of hanging-wall biotites document an increase in magnesium content (ca. 25%), with proximity to the orebody. Trace element analyses of ore samples reveal anomalously high thallium (up to 42 ppm) and barium (up to 31,000 ppm) which may reflect ore fluid compositions and the associated ore-forming environment. Sulfides from Ore Hill have light delta 34 S values (between 0 and -5 degrees ) which are atypical, relative to other Ordovician massive sulfide deposits, and may not be explained in terms of simple inorganic or organic reduction of seawater sulfate. The high manganese contents of the garnets (up to 57% spessartine component) are believed responsible for the anomalously low temperatures (470 degrees -480 degrees C) derived from garnet-biotite geothermometry for the upper medium-grade regional metamorphism. Phase equilibria of the immediate footwall assemblage, however, provides a P-T range of 4.5 to 6.5 kb and 550 degrees to 620 degrees C, consistent with metamorphic conditions estimated by other workers in the region.

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