Epidote in altered basalt, South Range, Douglas County, WI. Vertical field of view is about 3 cm. (Photo by W. Cordua.)

Formula: Ca2(Al2Fe3+)(Si2O7)(SiO4)O(OH)2 Monoclinic


Epidote is a common mineral formed during low grades of metamorphism and hydrothermal activity. It is particular common in metamorphosed basalts and gabbros where it replaces plagioclase, pyroxene and olivine. It can also be found in schists and marbles. In addition, it is frequently found as vug or vein fillings in these rocks. It is commonly associated with chlorite, actinolite and quartz. Epidote occurs frequently in igneous and metamorphic rocks of the state’s Precambrian shield. Most of the Keweenawan volcanic rocks in the northwestern part of the state in Polk, Ashland, Bayfield, Douglas, and Iron counties contain abundant epidote. When it occurs as green radiating crystals with quartz in veins or filling amygdules it can make attractive specimens.

ASHLAND COUNTY: Epidote is common in the metamorphosed Keweenawan basalts in T.44N R.4W where it occurs with actinolite, biotite, chlorite and albite in groundmass and as amygdule fillings (Leighton, 1954). Some localities near Marengo are SW NW Sec. 20 and SW SW Sec. 34 T.45N R.4N (Lapoint, 1976).
— Epidote occurs in chlorite schist along the Potato River south of Penokee (Van Hise and Irving, 1892).

BAYFIELD COUNTY: Epidote is widespread as a groundmass and amygdule filling in Keweenawan metabasalts near Grandview, such as at NE SW Sec. 2 T.44N R.5W, SE NW Sec.1 T.44N R.6W and SE NE Sec. 4 T.4N R.6W (LaPoint, 1976; WSC Field notes 1990).
— Epidote is common with actinolite, biotite, chlorite and albite in metamorphosed basalts in many outcrops in T.44N R.5W and R.6W (Leighton, 1954).
— Epidotized basalts are common at the Montrose Copper Prospect in the NW 1/4 NW 1/4 Sec. 12 T.44N R.9W. Associated minerals are prehnite, chlorite, laumontite, quartz and calcite (Grant, 1901).
— Epidote is abundant in the metamorphosed basalts west of Drummond where it fills amygdules and replaces original igneous minerals. Some outcrops rich in epidote are in SE NE Sec. 9 T.44N R.6W; NW NE Sec. 29 T.44N R.7W and NW NW Sec. 36 T.45N R.7W (Ali, 1982).

DOUGLAS COUNTY: Epidote occurs throughout the county in the abundant outcrops of metamorphosed Keweenawan basaltic rocks. Only a few of the many outcrops are included here.
— Epidote is abundant in the dumps of the F shaft of the Weyerhauser Mine, SE SE Sec. 11 T.43N R.10W where it occurs with copper, calcite, chlorite and quartz (Smith, 1947, W.S.C. field notes, 1990).
— Epidote is abundant in the dumps of the main shaft of the Weyerhauser copper mine, NW SE Sec. 12 T.43N R.10W. It occurs as individual crystals over 1 cm. long and in radiating groups. Associated minerals are prehnite, chlorite, quartz, K feldspar and copper (Smith, 1947, W.S.C. field notes, 1990).
— Epidote occurs in veins and amygdules in Keweenawan basalt at the Chippewa Mine, SW SW Sec. 3 T.47N R.12W near Middle River. Associated minerals are prehnite, chlorite, quartz, copper and laumontite (Holliday, 1955).
— Ali (1982) noted that epidote was abundant throughout the eastern part of Douglas County, such as in the NE SW 1/4 Sec. 18 T.42N R.10W and NW SE Sec. 12 T.43N R.10W.
— Epidote occurs as attractive crystals lining cavities in basalt at the South Range Quarry, NW NW Sec. 1 T.47N R.13W. Associated minerals are chlorite, pink K feldspar, quartz (white, blue and agate), prehnite and chalcopyrite (Cordua, 1990).
— Epidote occurs in the SW Sec. 6 T.44N R.13W in calcite veins with copper (Irving, 1883). Irving also reports epidote and copper in calcite veins in the SW Sec. 6 T.44N R.13W. Grant (1901) refers to this same occurrence, noting it is in outcrop along the west bank of Moose River.
— Epidote occurs in veins with quartz, pyrite and malachite in basalts in NE Sec. 28 T.48N R.10W (Irving, 1883).
— Epidote is abundant in basalt outcrops along Crotty Brook in the NW Sec. 7 T.43N R.13W. IT is also found with copper in 3 old pits near the Copper Mine Dam in the SW Sec. 6 T.43N R.13W (Grant, 1901).
— Epidote is common in masses with prehnite, laumontite and copper at the Percival copper prospect in the N 1/2 SW NE Sec. 27 T.48N R.10W (Grant, 1901).

FOREST COUNTY: Epidote occurs in the metavolcanic rocks of and post-mineralization plutons at the Crandon deposit near Little Sand Lake. (Lambe and Rowe, 1989).

IRON COUNTY: Epidote occurs as a secondary mineral in the hornblende-rich schists and gneisses along the west branch of the Montreal River (Van Hise and Irving, 1892).

MARINETTE COUNTY: Epidote occurs in the groundmass and in amygdules of metamorphosed pillow lavas of the Quinnesec Formation quarried at Kremlin. Calcite, actinolite and stilpnomelane are also found here (Depangher, 1982).

MARATHON COUNTY: Epidote occurs with actinolite in metabasalts near Artus Creek (NW NE Sec. 29 T.29N R.6E), south of Rozellville (NE SE Sec. 29 T.29N R.4E) and along Troy Avenue (SE SE Sec. 12 T.29N R.7E) (LaBerge and Myers, 1983).

OCONTO COUNTY: Epidote occurs with actinolite, chlorite and clinozoisite in the meta-volcanic rocks near Mountain. (Sims et al., 1991).

ONEIDA COUNTY: Epidote is found with chlorite, actinolite and quartz in the Pelican River massive sulfide deposit (Bowden, 1978).
— Epidote occurs as crystals filling amygdules in pillowed andesite lava flows north of Monico at NE NE Sec. 30 T.36N R.11E (Mudrey, 1979).

PRICE COUNTY: Epidote occurs in the metavolcanic rocks of the Ritchie Creek massive sulfide deposit (DeMatties, 1990).

POLK COUNTY: Epidote is abundant in numerous outcrops of Keweenawan metabasalts throughout the county. Only a few of the many outcrops are noted here.
— Abundant outcrops containing epidote occur in the Frederick- Clam Lake area. Radiating crystals can be found, for example, large amygdules in the NW NW Sec. 35 T.37N R.16W (Hansell, 1928b).
— Epidote is found in veins, radiating crystals and as massive replacements of metabasalt in the Dresser Trap Rock quarries in Sec. 5 and 6 T.33N R.18W. Associated minerals are chlorite, actinolite and quartz (Cordua, 1989b).
— Epidote is abundant in the rocks exposed in Interstate Park and adjacent areas around St. Croix Falls (Cordua, 1979, field notes 1973-1993).

WASHBURN COUNTY: Epidote is found as pods and masses at the Mudge Copper exploration in the NE Sec. 5 T.42N R.10W. Copper, prehnite, malachite., quartz and calcite are also found here (Grant, 1901). Strong (1880) notes epidote is found with quartz crystals. in fissures in Sec. 13 T.42N R.10W.

WOOD COUNTY: Epidote occurs in veins with quartz and pyrite cutting amphibolite at the Turner Quarry, NW SE Sec. 36 T.23N R.2E, about 6.5 km. west of Pittsville (Maas and Van Schmus, 1981).
— Epidote occurs as xls. in quartz veins cutting granite in the quarry on the east side of County highway B in sec. 25 T.24N. R.2E near Cary Bluff (T. Buchholz, personal communication)
Epidote, both as masses and in crystals, occurs in the Tork and Haessley Quarries near Wisconsin Rapids (Buchholz, 1996).