Mineralogy of Chub Lake-Type Hematite Deposits in St. Lawrence County, NY

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Numerous localities of specular hematite have been found in the Grenville Province in St. Lawrence County, New York. Here, we focus on six of them: the Dodge mine, the Chub Lake prospect, the Toothaker Creek prospect, the Bowman prospect, the Whitton prospect, and the Toothaker Pond prospect. We used literature research, interviews, and personal observations to establish the history of each site as a source of mineral specimens. We examined extensive holdings of specimens from each site in the New York State Museum. We used sight identification, chemical tests, x-ray diffraction, and scanning electron microscopy with energy dispersive spectroscopy as necessary to identify all the mineral species present. We had determinations made of the stable oxygen isotope content of quartz, hematite, and calcite from the Chub Lake prospect, reported as 18O relative to Vienna Standard Mean Ocean Water (VSMOW). We conclude that these occurrences formed from groundwaters at a temperature of about 170 °C in areas of low topography on the surface of the Precambrian basement rocks. Two hypotheses for this process are presented and evaluated. Well-crystallized specimens of bladed specular hematite and Cumberland-habit quartz are the most common minerals found. Noteworthy accessory crystallized minerals include barite, calcite, and goethite. All six deposits are relatively free of sulfides, so that secondary goethite formed from weathering of iron-rich carbonates at some sites. It is likely that more such deposits will be discovered in this region in the future.