FIELD EVIDENCE FROM ST. LAWRENCE COUNTY, NEW YORK, FOR THE TRANSPORT OF SILICA AND PRECIPITATION OF QUARTZ BY ORGANIC GEOLOGICAL MECHANISMS
Three occurrences of quartz in Precambrian Grenville marble may be the result of the transport of silica by organic molecules formed in the soil zone (e.g. humic acids) and the precipitation of quartz in fractures. The source of the silica appears to be the breakdown of metamorphic silicates in the Grenville marble in contact with the soil zone. The quartz has formed in fractures from 2 to 4 m below the soil zone.
All three localities are roadcuts. The Oxbow roadcut is on the east side of St. Lawrence Co. Rt. 3, 3.2 km north of the intersection of the village of Oxbow. The Yellow Lake roadcut, is on the west side of County Rt. 10, 6.2 km north of the intersection with Rt. 3. The Beaman Road roadcut is on the north side of Beaman road just west of its intersection with Peabody Road.
The accessory minerals include calcite, dolomite, and iron sulfides. Barite is also present at the Beaman Road occurrence. The quartz occurs as euhedral prismatic crystals that are typically a translucent gray color, sometimes almost transparent. At Beaman Road and Yellow Lake, they are small. At Oxbow, the quartz crystals reach 5 cm or more in clusters up to 10 or 15 cm across. Graphite rosettes as inclusions reveal the origin of the quartz. These have weathered out of the overlying marble, been carried down fractures by descending groundwaters and become embedded in the quartz. These graphite rosettes suggest that the precipitation of quartz is ongoing. Organic molecules from the soil zone may in some cases be a significant mechanism for moving silica under surface conditions.