WHOLE-ROCK ARSENIC AND URANIUM CONCENTRATIONS FROM SELECTED UNITS WITHIN THE AUGUSTA 1:100,000 MAP SHEET, MAINE

In association with detailed bedrock mapping and local petrologic studies over the past several years in the Augusta 1:100,000 map sheet, Maine, 87 whole-rock major- and trace-element geochemical analyses have been obtained. Of these, 58 are from metamorphic rocks such as mica schist, sulfidic schist, quartzofeldspathic gneiss, and amphibolite; 29 are from plutonic rocks. Each analysis includes both arsenic and uranium concentrations. A review of these data shows the following.

Measured U concentrations range from below the detection limit (0.2 ppm) to 21.3 ppm. U concentrations in the metamorphic rocks are low, with an average of 3.2 ppm, and only one analysis is above 10 ppm. U concentrations in the plutonic rocks are generally higher, with an average of 6.8 ppm. In particular, alkaline plutonic rocks of the Lincoln Syenite have the highest U contents (avg of 14 analyses = 11.8 ppm).

Measured As concentrations range from below the detection limit (0.2 ppm) to 1161 ppm. As concentrations in the plutonic rocks analyzed are all below 15 ppm and thus these rocks do not seem to be a significant source of arsenic. Low levels (< 10 ppm) were also found in the quartzofeldspathic gneisses and amphibolites. Sulfide-rich, rusty-weathering rock types show considerable variability – even within a single map unit. Two relatively thin (< 120 m), discontinuous, Fe- and Mn-rich rock units show anomalously high As concentrations. Grunerite-garnet gneisses of the Wilson Cove Member of the Cushing Formation (Razorville 7.5' quad) have As concentrations that range from 7 to 1161 ppm (avg of 16 analyses = 293 ppm). As concentrations in grunerite-garnet gneisses of the Muzzy Ridge unit (Searsmont 7.5' quad) range from 105 to 504 ppm (avg of 5 analyses = 277 ppm). Microprobe analyses indicate As is contained within arsenopyrite, cobaltite, and loellingite in these two rock units. Importantly, it should be noted that multiple analyses from several other rusty-weathering rock units reveal negligible As. Thus, not all sulfide-rich rock units in the Augusta sheet contain elevated concentrations of As.

This review suggests that in the Augusta 1:100,000 sheet, U levels are more predictably related to rock type than are As levels. This should be considered in future detailed studies which may attempt to relate drinking water quality to bedrock chemistry.