Contact metamorphism of the Blue Hill mine area Blue Hill, Maine

Abstract

Sulfide-rich rocks in the Blue Hill mine area are restricted to the hornblende-hornfels facies (zone I) except for the Lower Second Pond copper ores. Lower Second Pond copper ores contain both andalusite and fibrolitic sillimanite with abundant potassium feldspar, and are distinctly lacking in chlorite. The presence of fibrolite and more abundant K-feldspar may indicate closer proximity of the Lower Second Pond ores to an igneous body. These are assigned to zone II which is in the orthopyroxene subfacies of the K-feldspar-cordierite-hornfels facies of Winkler. Gahnite was noted in a single sample from zone I. Restriction to zone I is considered a function of differences in original chemical composition: ores in zone I, for example, are consistently sphalerite-richer than those in zone II. The ore minerals in zone I are texturally similar to those of zone II. Retrograde phenomena are much more abundant in zone I than zone II but are perhaps the most striking feature associated with all these sulfide-rich rocks.