Geology of the Lake Mary quadrangle, Iron County, Michigan

Abstract

The Lake Mary quadrangle is in eastern Iron County, in the west part of the Upper Peninsula of Michigan. The quadrangle is underlain by Lower and Middle Precambrian rocks, formerly designated Archean and Algonkian rocks, and is extensively covered by Pleistocene glacial deposits. A few Upper Precambrian (Keweenawan) diabase dikes and two remnants of sandstone and dolomite of early Paleozoic age are also found in the area. The major structural feature is the Holmes Lake anticline, the axis of which strikes northwest through the northeast part of the quadrangle. Most of the quadrangle, therefore, is underlain by rock of the west limb of the anticline. To the northwest along the fold axis, the Holmes Lake anticline is separated from the Amasa oval by a saddle of transverse folds in the vicinity of Michigamme Mountain in the Kiernan quadrangle. The Lower Precambrian rocks are represented by the Dickinson group and by porphyritic red granite whose relation to the Dickinson group is uncertain, but which may be older. The rocks of the Dickinson group are chiefly green to black metavolcanic schist and red felsite, some of the latter metarhyolite. The dark schist is commonly magnetic. The Dickinson group underlies the core area of the Holmes Lake anticline, which is flanked by steeply dipping Middle Precambrian formations of the Animikie series. A major unconformity separates the Lower Precambrian rocks from the overlying Middle Precambrian rocks. In ascending order the formations of the Middle Precambrian are the Randville dolomite, the Hemlock formation, which includes the Mansfield iron-bearing slate member, and the Michigamme slate. An unconformity occurs between the Hemlock formation and Michigamme slate. The post-Hemlock unconformity is thought to be represented in the Lake Mary quadrangle by the absence of iron-formation of the Amasa formation, which is known to lie between the Hemlock and the Michigamme to the northwest of the Lake Mary quadrangle in the Crystal Falls quadrangle. Post-Hemlock erosion may account also for the absence of iron-formation of the Fence River formation on the east limb of the Holmes Lake anticline within the Lake Mary quadrangle. The Randville dolomite is not exposed and is known only from diamond drilling in the northeast part of the area where it occurs in the east and west limbs of the Holmes Lake anticline. The formation has a maximum thickness of about 2,100 feet; this includes a lower arkosic phase, some of which is quartz pebble conglomerate, a medial dolomitic phase, and an upper slate phase. The triad is gradational. Included within the formation are a few beds of chloritic schist thought to be of volcanic origin. An unconformity between the Randville and the succeeding Hemlock is not indicated in the quadrangle, but is probably present. The Hemlock formation is best exposed in the northwest and south-central parts of the area. The apparent thickness of the formation is 10,000- 17,000 feet. It is composed mainly of mafic metavolcanic rocks and intercalated slate and iron-formation. In the north part of the quadrangle the volcanic rocks are greenstone, which includes altered basaltic flow rocks, volcanic breccia, tuff, and slate. Pillow structures are common in the metabasalt. It is not certain if any Hemlock rocks are present in the east limb of the Holmes Lake anticline. In the south part of the quadrangle, the rocks of the Hemlock are chiefly chlorite and hornblende schist and hornfels. Pyroxene hornfels is sparingly present. At least two sedimentary slate belts are included in the Hemlock formation. One of these, the Mansfield iron-bearing slate member, includes in its upper part an altered chert-siderite iron-formation 30 to over 150 feet thick from which iron ore has been mined at the Mansfield location. The position of the iron-bearing rocks has been determined magnetically, and past explorations for iron ore are discussed. Though probably; unconformable, the contact between the Hemlock and the Michigamme formations appears conformable. The Michigamme slate consists of at least 4,000 feet of interbedded mica schist and granulite, the altered equivalents of the slate and graywacke characteristic of the Michigamme in adjacent areas. The Michigamme rocks are best exposed in the south part of the quadrangle in the vicinity of Peavy Pond. Two periods of regional metamorphism have resulted in the alteration of almost all of the rocks of the quadrangle. The Lower Precambrian rocks underwent at least one period of metamorphism, uplift, and erosion before the deposition of the Randville dolomite. After the deposition of the Michigamme slate, a post-Middle Precambrian period of regional metamorphism occurred with attending deformation and igneous intrusion. The grade of metamorphism rises toward the south in the area. The rocks in the northern two-thirds of the quadrangle are representative of greenschist facies of regional metamorphism, whereas the rocks in the southern onethird of the quadrangle are representative of the albite-epidote-amphibolite, the amphibolite, and the pyroxene hornfels facies, the metamorphic node centering about the intrusive Peavy Pond complex in the Peavy Pond area. The Precambrian sedimentary and volcanic rocks are cut by intrusive igneous rocks of different types and several different ages. Gabbroic sills and dikes invaded the Hemlock rocks at some time after the Hemlock was deposited and before the post-Middle Precambrian orogeny and metamorphism. Some contact metamorphism attended the intrusion of the major sills. One of the sills, the West Kiernan sill, is well differentiated. A syntectonic igneous body, composed of gabbro and minor ultramafic parts and fringed with intermediate and felsic differentiates and hybrids, the Peavy; Pond complex, was intruded into the Hemlock and Michigamme formations during the post-Middle Precambrian orogeny. The complex is situated in the Peavy Pond area at the crest of the regional metamorphic node. Contact-altered sedimentary and volcanic rocks margin the complex. The effects of regional metamorphism have been superposed on the contact metamorphic rocks peripheral to the complex and on the igneous rocks of the complex as well. The mafic augite-bearing rocks of the complex emplaced early in the orogeny were deformed by granulation at the peak of the deformation and subsequently metamorphosed to hornblende rocks. Some of the intermediate and felsic rocks of the complex were foliated by the deformation, while the more fluid, felsic parts of the complex were intruded under orogenic stress and crystallized after the peak of deformation. The deformation culminated in major faulting during which the formations were dislocated, and some of the granite of the complex was extremely brecciated. A few diabase dikes, probably of Keweenawan age, have intruded the deformed and altered Animikie rocks. The only known metallic resource is iron ore. The Mansfield mine produced 1¥2 million tons of high-grade iron ore between the years 1890 and 1913. Sporadic exploration since 1913 has failed to reveal other ore deposits of economic importance.