Abstract: On the Helen claim in Homansville Canyon, East Tintic district, a small deposit of kaolinic clay typical of argillic alteration, together with hematite and manganese, occurs at the intersection of a minor north-south fissure with a steep east-trending contact of Tertiary quartz latite lava and dolomitized Cambrian Herkimer limestone. The fissure ends at a small, completely argillized monzonite plug about 100 yd north.
Argillic alteration and other types of hydrothermal alteration are common near the ore deposits and many of the intrusive bodies that are scattered through the roots of the Eocene volcanic field in which the East Tintic and Tintic districts lie. In most areas of argillic alteration abundant pyrite or alunite and barite suggest that the alteration was caused largely or in part by sulfur acids, but in the Helen claim these minerals are virtually lacking and in contrast fluorite, hematite, and manganese and aluminum oxides are abundant.
A mineral zonation parallel to the lava-dolomite contact is conspicuous. The zones in the lava outward from the contact are: (L 1) a silicic zone 1–5 in. wide with some kaolinic clays, mixed-layer clay and micaceous minerals; (L 2) a strongly argillized zone about 3 ft wide having much montmorillonite and a moderate amount of kaolinic and micaceous minerals, but in which the kaolinic minerals decrease away from zone L 1; (L 3) transition zone 2–5 ft wide in which montmorillonite diminishes, micaceous minerals increase, and kaolinic minerals disappear as altered rock grades into normal country rock. The bulk density of the rock decreases with the intensity of argillic alteration and nearly all chemical constituents decrease in amount per unit volume; magnesium, water, manganese, and fluorine, however, increase conspicuously.
The zones in the altered dolomite away from the lava contact are (D 1) a hematite-quartz zone 5–8 ft wide, which contains some minor sericite and manganese oxides; (D 2) a manganiferous zone 1–2 ft thick containing abundant manganese oxides and hematite, and minor clay, quartz, and kaolinic minerals; (D 3) a discontinuous zone about 8 in. thick of halloysite and kaolinite; (D 4) a fluorite-kaolin zone with minor diaspore about 1 ft thick; (D 5) a diaspor-fluorite zone about 4½ ft wide in which the kaolin minerals diminish away from zone D 4; (D 6) a diaspore-kaolinite zone about 1 ft wide with minor fluorite and mixed-layer clays; (D 7) a kaolin and mixed-layer clay zone about 3 in. thick with very minor manganese oxide, fluorite, and diaspore; (D 8) a “sanded dolomite” zone about 10 ft wide, consisting chiefly of weakly bonded granular dolomite with some kaolinite and mica; (D 9) hard fresh hydrothermal dolomite extending outward for several hundred foot. Zone D 8 grades imperceptibly into D 9.
The mineral zones suggest reaction of dolomite with hot halogen-rich acid emanations carrying halides of iron, aluminum, and silicon; the precipitation of the oxides of iron, manganese, silicon, and aluminum, and of fluorite, is appropriate to zonation responsive to the increasing pH caused by reaction with the carbonate wall rock.