A magnetic anomaly near Bear Lake, Houghton County, Michigan

Wright, James C. · 1952 · Report

A large magnetic anomaly of unknown origin occurs about 1 1/2 miles east of Bear Lake, Houghton County, Michigan, in secs. 24 and 258 T. 56 N., R. 34 W.

This 1952 document, A magnetic anomaly near Bear Lake, Houghton County, Michigan, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

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Contents Page ntroduction 00 00010000110400000004100 GeolOgY 00000411001041 00000041100000000000000400 Measurement of the magnetic anomaly Significance of the magnetic anoma4

Further exploration loreeoelpoire.4 SUMMarr 400000001000011000004000000000000,0114100004 0 40 Illustrations Figure 1. Aeromagnetic map of an anomaly near Bear Lake, Houghton County, Michigan. 2. Dip-needle survey of a magnetic anam04rnmer Bear Lake, Houghton County, Michigan.

A =gnat:: anomaly near Bear La :3, Houghton County, Michigan by James C. Wright Introduction A large magnetic anomaly of unknown origin occurs about li miles east of Bear Lake, Houghton County, Michigan, in secs. 24 and 258 T. 56 N., R. 34 W. The occurrence is isolated in an area of very weakly magnetic rocks and has special geologic interest because it is adjacent to a non-magnetic rhyolite porphyry body that is atratigraphically higher than any other known igneous rock in the Keweenawan series. An aeromagnetic survey of the Michigan copper district by the Geophysics Branch of the U. S. Geological Survey in 1948 first revealed the anomaly (Fig. 1). The anomalous area has been more fully outlined by a dip-needle survey, and briefly discusses the geology of the area and possible significance of the anomaly. Geology The area surrounding the anomaly is underlain by rocks of the very thick Keweenawan series, which dip gently northwest toward Lake Superior. The formations recognized J in the vicinity of the J Lane, A. C., The Kewoenaw series of Mehlan: Michigan Geol. and Biol. Survey Pub. 6, vol. 1, pp. 41, 1911. Butler, B. S., and Burbank, W. S., et al, The copper deposits of Michigan: U. S. Geological Survey, Prof. Paper 1448 pp. 16-25, 1929.

anomaly include, in otratigraphic order: a thick group of copperbearing lava flows, the Copper Harbor conglomerate, the nonesuch shale, and the Freda sandstone. Post-glacial lake sands red clays mantle the Keweeaawan formations and farm a nearly level, partly swampy surface aver the annrutly. With the exception of the rhyolite porphyry described below and perhaps the rock responsible for the local magnotic ammuCly described in this report, all of the area shown in Figure 2 is probably under­ lain by northwestward-dipping Freda sandstone. The sandstone is not exposed in the map area, but the outcrops at Freda, Ilichiganj *dr Lane, op. cit., p. 40. (about 10 miles southwest of Bear Lake), are dominantly red arkosic sandstone with some interbedded red and green shale, and minor beds of grit and pebble conglomerate. The only local bedrock exposures are two small rhyolite porphyry outcrops (points "A" and "B" on Fig. 2). In 1917-1918 Butler and Burbank, op. cit., p. 47. this rhyolite porphyry was explored by six diamond drill holes and a forty-foot shaft J. The locations of the shaft and of the three J Weed, W. H., The mines handbook, vol. 14, p. 785, 1920. diamond-drill-hole collars found during the survey are shown in Figure 2.

The local, isolated occurrence of rhyolite porphyry in a sand­ stone area suggests that the rock is intrusive, but no definite conclusions about its structure can be made from the available data. The rock in both the surface exposures and the shaft dump is a typically flow-banded, essentially non-magnetic rhyolite porphyry that ranges from red to pinkish gray. Amalie feldspar predominates in the rock and is accompanied by accessory biotite, horrblendo, quartz, apatite, and iron oxides. The coarser-ground facies have a tine granitic texture (about 04'm:imams), while the finer-grained facies consist of an aggregate of grains (about O.0mm diameter) in a glassy natrix, with some ptenocrysts of apatite and orthoclase. A few rhyolito specimens collected at the shaft dmmp and from abandoned diamond-drill-core chips shove little green copper stain in joints and calcite seams. The exceedingly fine-grained original copper mineral, where megascopicall,y visible, appears to be native copper. Measurement of the magnetic anomaly The aeromagnetic survey revealed, but did not fully outline the anomaly which was crossed by only ono flight-line, number 87 (see Fig. 1). The ground survey 4th a Lake Superior model dip needle was made to complete the mapping of the anomaly. Three survey lines, originating at point "C" (see Fig. 2) on a watercourse near several old prospect pits, and radiating (1) irregularly northwest (down the watercourse, (2) due east, and (3) nearly due south, were run with stations spaced every 25 feet. The values for these points were fairly close to one another over

considerable distances so the readings along other lines were taken only every 50 feet. Minor irregularities in tho magnetic contours were smoothed by averaging three readings taken at successive 50­ foot intervals along the traverse lines. Spot values outside the anomaly area are averages of the reading at the point indicated on the map and four readings taken at symmet­ rically spaced intervals 50 feet from the point. An extreme anomaly, about 40 feet in diameter, occurs about 100 feet east of the outcrop at point qr. At the center of thin area (too small to be shown on the map) the dip-needle deflection was 21 degrees. 340Finance of the magnetic anomaly Either of two minerals, magnetite or pyrrhotites may cause the anomaly discussed here. Neither the Freda sandstone nor the rhyolite porphyry is known to contain either mineral in quantity. Local concentrations of magnetite or pyrrhotite might be contained in another igneous rock associated with the rhyolito, or in an adjacent contact-metamorphosed sandstone. There Jo nothing to indicate which type of concentration, if either, is present. No other igneous body-has been found intruding the Freda forma­ tion, so that the effect of contact metamorphiam on this sandstone can not be gauged.

However, at }Aunt Bohemia (about 30 miles northeast in Keweenaw County) there is a complex igneous intrusive consisting of two rock types cutting the Keweenawan series Di. The intrusive rocks would be 2/ Hubbard, L. L., Keweenaw Point with particular reference to the felsites and their associated rocks: Michigan Geol. Survey, vol. 6, pt. 2, 155 pp., maps 1898. Wright, F. Z., The intrusive rooks of Mount Bohemia= Mich.: Hichigeun Geol. Survey Rept. for 1908, pp. 355-402, 1909. classified according to the Johannes system by their analyses as bi Wright, op. cit., pp. 366.373. Broderick, T. M., Differentiation in lavas of the Michigan Kereenawan: Bull. Michigan Coll. Min. and Technology, vol. 8, p. 536, 1935. an aplite and a symodiorite. The symodiorite contains an unusu­ ally large amount (10.9%) 2/ of magnetite, and a vein along the 2/ Broderick, ibid. contact of the two facies bears minor amounts of pyrrhotite in association with other iron and copper sulfides 1/. A somewhat Wright, op. cit., p. 392. similar occurrence of either mineral may cause the magnetic anomaly near Bear Lake. Whether the rock causing this magnetic anomaly bears any minerals of economic value is not known. Rhyolite seen on the dump at the old shaft shows only a few insignificant. traces of copper, but as shown on Figure 2, the drill holes and shaft were not sunk

in the area of the maGnotic anomaly. Magnetite (Fe304), which most commonly causes magnetic anomalies, is an ore of iron, but is minable only Id= present in fairly high concentrations and large quantities. A rough analysis was made of the dip needle survey assuming j Balaley, J. R., personal communication. a sensitivity of 200 game per degree. Estimated depths to the source of the anomalies are on the order of 100 feet. The observed anomalies could be produced by a few. masses of 10,000 to 15,000 tons erlh consisting of 60% magnetite (by weight). If it is assumed that the sources are dike like, extending to great depth, the computed susceptibility compares to that of diabase. Pyrrhotitc (FeS)„ which may also cause magnetic anomalies, is in itself of little economic value, but is commonly associated with copper and nickel-bearing minerals. Further exploration Diamond drilling would be the most satisfactory method of further investigating this area of magnetic anomaly. The diamonddrill core would allow positive identification of the magnetic material, as wal as yield a measure of the form and extent of its occurrence. Trenching %pull be unsatisfactory unless the magnetic material happens to occur right at the bedrock surface. Electrical, gravimetric, seismic, and more-refined magnetic surveys might all add to the indirect knowledge of the physical characteristics of the anomalous material, and perhaps indicate its depth below surface.

The magnetic anomaly near Bear Lako, about one-half mile in diameter is spatially, and probably genetically, closely related to the adjacent rhyolite porphyry. It maybe caused either by magnetic igneous rock associated with the nonimmagnetic rhyolite porphyry, or by contactsametemorphic effects of the felaito or other intrusive rock on the surrounding Freda sandstone. Copper, iron, or nickel minerals might be included in the magnetically anomalous rock, but no evidence, non. available, indicates their presence or absence.

R.34W. R.33 W. 47°15" - rt 8 8° LAKE SUPERIOR JSevenm LAIN STATE_ RK 1.2 MIL. /852!'-' / 24 ,./ 23 )' r

T.56 N. /) , jii i, (P,01///' EAR LAKE 0 0,,y/ " -1.1--tr0.4 MIL. / / / / I 2378///,' 1 / l ' r / // I I l l ; / I I I 1 2499X/ // / / H ' 25,,'/x / / /

47u12.3o"LB- 64ciii-li.-S-.6.S. Hancock, Michigan, quadrangle AEROMAGNETIC MAP OF AN ANOMALY NEAR BEAR LAKE HOUGHTON COUNTY, MICHIGAN Magnetic contours from aeromognetic profiles by the U. S. Geological Survey. Contour interval 100 gammas, locally supplemented by dashed half-interval contours. Arbitrary magnetic datum. 1000 2000 3000 4000 5000 FEET 3 map is preliminary and hai-rret--1 ited or reviewed for conformity FIGURE I S. Geological Survey standards

4 7°15 — 88735' R .34W. R. 33 W. 8 8°32 . 30: LAKE SUPERIOR — 3.7 3,6 , ,CALUMET 5.0411. 3.8' '10 AN STATE — - $ep 3— 1K 1.2 MIL. / -ga 4.3.9 s 24', _c.4 .„2 / — 3 shaft, T. 56 N. o 4 A1 AR LAKE LAKE ,i;j3.0 2 rim 0.4 MIL /( ' drill holes ! 78 5 efr, EXPLANATION Survey lines Points referred to in text Center of several readings with average downward deflection in degrees of north end of dip needle. ---t-a ;, WAASA Magnetic contours 3' (Dashed where opproximotely located) Number indicates downward deflection in degrees of north end of dip needle. 47°12 . 30" Bose from U.S.G.S Hancock , Michigan, quadrangle DIP—NEEDLE SURVEY OF A MAGNETIC ANOMALY NEAR BEAR LAKE, HOUGHTON COUNTY, MICHIGAN Is nap is preliminary and b8 $t o , 5000 FEET 1000 2000 3000

dited or mil/lowed for oonforthity S. Geolugical Survey standards FIGURE 2 menolature.

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