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Textural and mineralogical studies of the Bald Mountain Cu-Zn-Au-Ag massive sulfide deposit, northern Maine, document a well-preserved premetamorphic hydrothermal evolution involving both exhalative and subsea-floor replacement processes. The 30-million metric tonne (Mt) Bald Mountain deposit forms a thick bowl-shaped accumulation of sulfides up to 215 m thick within a synvolcanic sea-floor graben of Early Ordovician age. Five principal stages (facies) of mineralization are recognized. Stage I mainly developed Fe sulfide mounds composed of fine-grained pyrite (As- and Sb-rich) and probably marcasite, with locally abundant sphalerite, sparse galena, and silica. Framboidal pyrite and colloform pyrite ± sphalerite ± galena are present locally, near the stratigraphic top of the deposit.

During late stage I mineralization partial collapse of the sulfide mounds took place, probably due to dissolution of matrix anhydrite, producing thin to very thick (up to 20 m) accumulations of pyrite-quartz breccias. Following this mound collapse, stage I resumed with exhalative mineralization that filled the graben to its rim. Related mineralization formed volumetrically minor replacements of rhyolite ignimbrite. Stage I massive sulfides have a geochemical signature marked by generally high contents of Zn, Pb, As, Sb, Ag, Au, Hg, and Tl. This mineralization was succeeded, mostly within the graben structure, by the precipitation of low-temperature deposits of silica and Fe oxyhydroxides (now hematitic chert) that cover stage I deposits to depths of up to 28 m, as a hydrothermal cap to the massive sulfides below.

Stage II formed mainly pyrrhotite-chalcopyrite replacements of stage I sulfides in the deep subsurface at temperatures of ca. 340° to 400°C based on arsenopyrite geothermometry. Stage II developed mainly after precipitation of the exhalative ferruginous silica cap, which may have sealed in the system thermally and chemically against shallow seawater entrainment. In addition to high Cu, stage II deposits contain abundant Co and Se. During this and subsequent stages of mineralization, older stage I sulfides underwent extensive recrystallization and zone refining at ∼250° to 325°C, accompanied by the replacement of pyrite by sphalerite ± galena, formation of euhedral quartz, arsenopyrite, and pyrite ± electrum, and remobilization of galena into small veins. Geometric relationships involving mineral assemblages and whole-rock (massive sulfide) geochemical data, together with textural information, suggest that Zn, Pb, As, Sb, Hg, and Tl in stage I deposits were dissolved and transported both upward and laterally by the zone refining for at least 150 m, resulting in very low contents of these elements within underlying stage II deposits. Gold was also remobilized and locally concentrated by the zone refining.

Stage III deposits consist of wavy quartz ± chalcopyrite veins and replacements, reflecting their emplacement into unlithified massive sulfide mounds. Coeval to younger stage IV mineralization produced a complex assemblage of coarse pyrite with major amounts of chalcopyrite, magnetite, and greenalite; siderite, quartz, minnesotaite, ferropyrosmalite, and sphalerite are generally minor. Like stage II deposits, those of stage IV are significantly enriched in Co and Se, relative to stage I deposits. Stage IV also developed by subsea-floor zone refining, superimposed on older stages as veins and replacements, preferentially in the lower part of the deposit. Epigenetic hematization and silicification of fine-grained hanging-wall sediments (now argillites) between andesite flows, and of overlying fine-grained rhyolite ignimbrites, may have occurred when stage IV fluids breeched the ferruginous silica cap. Later, stage V mineralization formed siderite-rich veins with variable amounts of quartz, pyrite, marcasite, pyrrhotite, sphalerite, greenalite, magnetite, hematite, and calcite, both in the massive sulfide body and the stringer zone.

Occurrences of very Fe rich silicates in stages II, IV, and V, and of siderite in stages IV and V, contrast with the absence of Mg-bearing silicates and carbonates throughout the deposit (excluding the footwall stringer zone). These compositions record the involvement of end-member Fe-rich hydrothermal fluids during formation of late sulfide veins and replacements, without appreciable amounts of shallow entrained (unreacted) seawater. Stage IV and V deposits precipitated from CO2-rich fluids based on their abundant siderite gangue. Stage IV formed mainly by the oxidation of stage II pyrrhotite, producing assemblages of pyrite ± magnetite (and rare magnetite without pyrite) together with locally prominent Fe3+-bearing greenalite; this relatively high fO2 environment continued during stage V mineralization, forming greenalite and hematite. Oxidation may have resulted from fluid boiling due to breaching of the ferruginous silica cap and consequent lowering of pressure in the hydrothermal system.

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