Source of lead and mineralizing brines for rossie-type Pb-Zn veins in the Frontenac axis area, New York ( USA).

Abstract

Veins composed mainly of calcite and some galena, sphalerite, fluorite, and other minerals are widespread in the Frontenac axis area of New York and Ontario. In New York, the veins (Fig. 1) occur mainly west and northwest of Gouverneur (Brown, 1983). The veins, mined in the 1800s for lead, were first described by Emmons (1838). Smyth (1903) called these the "Rossie lead veins," a name now used locally for all veins of this type. Similar veins in southeastern Ontario were described by Uglow (1916) and Sangster (1970). The steeply dipping veins are aligned along extensional fractures in metamorphic rocks of the Grenville Complex and in Paleozoic sedimentary rocks, including the Potsdam Sandstone of Late Cambrian age, and limestones as young as Middle Ordovician. However, the age of emplacement of Rossie veins is unknown. The veins are mineralogically simple, consisting of calcite (~95%) with minor and approximately equal amounts of sphalerite and galena, and sparse fluorite, barite, celestite, and chalcopyrite (Brown, 1983). Paragenetically early minerals are crushed and granulated; later undeformed minerals fill open spaces (Fig. 2). Sandstone dikes that originated from either the overlying Potsdam Sandstone or stratigraphically higher arenaceous rocks cut into the Rossie veins and occupy all available space. Their unstratified nature suggests that sand was intruded under high hydraulic pressure during tectonic events that reopened the veins. Most veins are vertical and narrow (1-3 cm wide) and those that were mined are as much as 1.7 m wide. The veins trend mainly northwest-southeast but have a local east-west orientation. The fractures and veins show evidence of strike-slip faulting with a right-lateral offset due to late reactivation of north-east-trending faults that originally formed during the Proterozoic Grenville orogeny. Relatively rare inclined veins, termed gash veins (Brown, 1983), occur in proximity to some of the regional northeast-trending faults. These veins contain open spaces lined with large, undeformed crystals of calcite (~90%) and green fluorite (~5%). The minerals of the northeast-trending gash veins, which dip to the northwest, are believed to be paragenetically late results of the same mineralization and tectonic event that produced the Rossie veins. For example, the green fluorite in the gash veins is similar in color and occurrence to small fluorite crystals in vugs in the vertical Rossie veins. Associated with the fluorite in both occurrences is coarse crystalline calcite containing tiny tetrahedra of chalcopyrite aligned along crystallographic planes. The present study of fluid inclusions and lead isotopes was done to understand better the nature and possible source of the mineralizing solutions for both vertical and gash veins and the possible origin of the lead and zinc mineralization. Large deposits of essentially syngenetic sphalerite with minor to trace galena occur in the Proterozoic rocks of this region (e.g., Balmat-Edwards, New York). Although none of these deposits are known in the immediate vicinity of Rossie veins, the possibility that Rossie veins contain remobilized metals from these older deposits was considered.