Formula: (Mg,Fe)2SiO4 Orthorhombic
Description: Olivine is here used to refer to a solid solution series of minerals ranging from the magnesium end member (forsterite) to the iron end member (fayalite). Olivine is widespread in basalts and gabbros, and is often the first mineral to form magmas of this composition. Most of the Keweenawan-age volcanic and plutonic rocks of Polk, Burnett, Douglas, Ashland, Bayfield and Iron counties originally contained olivine. Often, however, the olivine has been subsequently replaced by secondary minerals such as chlorite. Olivine is common in ultramafic rocks, such as those in Wood and Marathon counties. In most of these occurrences, it has also been replaced by secondary minerals such as serpentine and talc. Iron-rich olivine occurs in syenite and feldspathoid-bearing igneous rocks, such as those of the Stettin pluton near Wausau. Olivine may also form during high-temperature metamorphism, as it has in Ashland County where iron formation was intruded by gabbroic plutons. In most Wisconsin occurrences the olivine is inconspicuous and best seen in thin sections.
ASHLAND COUNTY: Olivine is widespread in gabbros, ferrodiorite and anorthosites of the Mineral Lake Intrusion northwest of Mellen, where it is associated with plagioclase and clinopyroxene (Klewin, et. al., 1989). Some localities listed by Klewin et al. (1989) include in gabbro exposed in the NE Sec. 14 T.44N R.3W; in coarse pegmatitic gabbro in the SE Sec. 11 T.44N R.3W; in anorthositic gabbro in the SE NE Sec. 11 T.44N R.4W and as iron-rich olivine in ferrodiorite in the NE NE SW Sec. 25 T.45N R.4W.
— Olivine is common in peridotite found at the base of the Rearing Pond Intrusion of the Mellen Complex, as is exposed along Spring Brook in the SE SW Sec. 26 T.45N R.4W, west of Mellen (Klewin et al., 1979).
— Fayalitic olivine is found in hornfels developed in the Ironwood Iron Formation near its contact with the gabbroic rocks of the Mellen complex. One such outcrop is in the SE NE Sec. 14 T.44N R.4W where fayalite is associated with magnetite, orthopyroxene, ferroaugite, pigeonite, apatite, grunerite and quartz (Laybourn, 1979).
BAYFIELD COUNTY: Olivine occurs in small amounts in the Keweenawan basalts exposed throughout the county. It is typically partly to completely replaced by chlorite and other secondary minerals.
— Ali (1982) reports olivine surviving replacement as tiny grains in basalts exposed in the NE NE Sec. 27 T.44N R.8W and in the NW NW Sec. 33 T.44N R.9W.
DOUGLAS COUNTY: Olivine, altered to secondary mineral, is a common in the troctolite in the Buckley Quarry in the NW Sec. 32 T.48N R.12W and nearby outcrops near Amnicon Falls (Dickas and Mudrey, 1991).
FOREST COUNTY: Olivine, partly altered to iddingsite, is a minor component of the Duck Lake gabbro, a post-ore intrusion into the Crandon massive sulfide deposit near Little Sand Lake (Lambe and Rowe, 1989).
IRON COUNTY: Olivine is widespread in small grains in the Keweenawan gabbroic and basaltic rocks in the county.
— Olivine occurs as grains up to 4 mm in diameter in the picrite of the Potato River Intrusion such as is exposed in outcrop in the SW SW Sec. 2 T.45N R.1W (Tabet and Mangham, 1978; Klewin et al., 1989), SE SE Sec. 30 and NE NE Sec. 31 T.46N R.1E (Klewin, 1987).
— Olivine is found as small grains in layered troctolite of the Potato River Intrusion with plagioclase and clinopyroxene in outcrops such as in the SW NW Sec. 32 T.46N R.1E (Klewin et. al., 1989), SW SW Sec. 2 T.45N R.1W and NW NW Sec. 32 T.46N R.1E (Klewin, 1987).
— Olivine occurs in the troctolite and gabbro (locally pegmatitic) of the Potato River Intrusion. Olivine bearing rocks are exposed in the NW SW Sec. 36 T.46N R.1W (Klewin, 1987) and the NW NW Sec. 32 T.46N R.1E (Klewin et al., 1989). Associated minerals are plagioclase, orthopyroxene and clinopyroxene.
— Olivine-bearing gabbroic rocks in the eastern portion of the Mellen intrusion complex are found in outcrops in T.45N R.1W such as in the SW Sec. 3, the NE SE Sec. 10 and the SE SE Sec. 17 (Tabet and Mangham, 1978).
LINCOLN COUNTY: Irving (1883) reports olivine, extensively replaced by serpentine, as large crystals with orthopyroxene in peridotite exposed “a short distance below the mouth of Copper River.”
MARATHON COUNTY: Iron-rich olivine (Fayalite) can be found in the alkaline intrusive rocks of the Stettin Pluton west of Wausau. It is also found in the form of fayalite as an accessory in some of the alkali granite, monzonite and pegmatite in the Nine Mile Pluton. (Falster et. al., 2000). – Olivine occurs as tiny grains in nepheline syenite in the core zone of the Stettin pluton in the SW Sec. 2 T.29N R.6E (Sood, Myers and Berlin, 1980). Similar outcrops occur in Sec. 1 and 12 (Weidman, 1907b).
— Olivine occurs in small amounts with nepheline, perthite, biotite, zircon and other minerals in the old zircon mine in the SE SE Sec. 22 T.29N R.6E (Sood, Myers and Berlin, 1980).
— Olivine is an accessory mineral in the pegmatitic nepheline syenite exposed in cuts along County Highway U, 0.5 miles east of the little Rib River in the SW SW Sec. 18 T.29N R.7E (Myers et al., 1984).
— Olivine is an accessory minerals in quartz syenite exposed at the old Technical Institute in Wausau (NE NE Sec. 35 T.29N R.7E). Here it is associated with K feldspar, hornblende, hedenbergite, fluorite, zircon, allanite and titanite (LaBerge and Myers, 1983).
RUSK COUNTY: Olivine occurs as a wall-rock alteration mineral (forsterite) and in an unmetamorphosed gabbro intrusion at the Eisenbrey massive sulfide deposit north of Ladysmith (May, 1996).
WOOD COUNTY: Fayalite has been tentatively identified as tiny green platelets in calcite veins cutting granite in the Haske Quarry, sec. 25 T.24N. R.2E. (Tom Buchholz, personal communication).