.2222 [4.2222222222222022 2.22 ‘ .2225; 935' 2222:)...222321‘3223 '22‘23EE: 2:223 I. :3 \l EFJLT : Cgig f3; ‘..’ - . 'I"'.!‘ I" 213' '3'3'33'.‘ 3.3“ 03 z 2 <- - 12.2..{. 225 2 £.- [.3 Ls THC 2.... 2:222: . 222.2.222’22 2 2222 2372.. 2222242 hr 2 22% 2’2: (x :2V21Hn2rw‘vfi ‘5; 2 v r: ‘k.’ Qfl“! 3431335531; 222222....”2'. 1" 2': 23:22:; 2322223 Deg 2223 23222 222. 2 22222222232222 322.22% 2322222222222 2.226 22 W 8222222 1968 " IIIIEHHI"1ITHTIWiielll‘i'iflvflflfififlJil'flfifl'flfllm ' ‘5’” i 5: Michiga.. ' CC ' University ww THESlQ AEH 413% MW—C. i.- m ABSTRACT AN EXAMINATION OF THE PETROGRAPHIC TYPES OF THE ELY GREENSTONE OCCURRING IN THE AREA OF TWIN LAKES, MINNESOTA by LeRoy W. Smith The Ely greenstone is an eight to fifteen mile wide strip of Early Precambrian rocks located in north central Minnesota just south of the Minnesota-Ontario boundary. This study is an attempt to evaluate the petrographic types of the Ely greenstone found in the area of Twin Lakes, Minnesota. In the field the greenstone was subdivided into extrusive, metadiabasic, foliated, and massive greenstone. Felsic porphyry, diabase, and iron formation were found associated with the greenstone. On the basis of field relations and thin section study the felsic porphyry was called a dacite. Spectrochemical analyses of seventy—eight samples of greenstone were performed to determine their chromium, vanadium, zirconium, nickel, and cobalt content. Ele- mental concentrations of samples grouped on the basis of petrographic field appearance, apparent trends in trace element concentrations, and enrichment in elemental con— centration failed to produce any unequivocal information about the greenstone. LeRoy W. Smith Thin sections of thirty—six samples of greenstone were studied in an attempt to obtain more information about individual samples. Although all samples carried a high percentage of secondary minerals, eight distinctive thin sections of greenstone were found that had apparently originated as the following diverse rock types: diabase, porphyritic basalt, monzonite, and lamprophyre. This diversity of original rock type was not evident in the field. Trace element concentrations in these distinctive greenstones were not greatly different from published values of unaltered rocks. AN EXAMINATION OF THE PETROGRAPHIC TYPES OF THE ELY GREENSTOKE OCCURRING IN THE AREA OF TWIN LAKES, MINNESOTA By if: 11 LeRoy W.VSmith A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology 1968 ACKNOWLEDGMENTS The writer acknowledges the assistance of Drs. H. B. Stonehouse, J. Trow, and S. Romberger who critically read this thesis. Special thanks are due to Dr. Harold B. Stonehouse who served as chairman of the thesis committee. My wife, Helen Jane, deserves a special acknowledg— ment for her assistance in the typing of the rough draft of this thesis. Acknowledgment is made to Harvey J. Hakala of United States Steel who loaned thin sections that were utilized in this study to the writer. ii TABLE OF CONTENTS Page ACKNOWLEDGMENTS . . . . . . . . . . . ii LIST OF TABLES . . . . . . . . . . . v LIST OF FIGURES O O O O C O O O O O 0 Vi INTRODUCTION . . . . . . . . . . . . 1 Problem. . . . . . . . . . . . . 1 Geographic Setting . . . . . . . . . 1 Geologic Setting. . . . . . . . . . 5 Previous Work. . . . . . . . . . . 5 Method of Attack. . . . . . . . . . lO Petrographic Methods . . . . . . . . lO Spectroscopic Analyses. . . . . . . . l2 STRATIGRAPHY OF THE ELY GREENSTONE. . . . . l5 Extrusive Greenstone . . . . . . . . 15 Metadiabasic Greenstone . . . . . . . 17 Iron Formation . . . . . . . . . . l7 Foliated Greenstone. . . . . . . . . 19 Diabase. . . . . . . . . . . . . 20 Massive Greenstone . . . . . . . . . 2O DaCite O O O O O O O O 0 O O O 0 2O TRACE ELEMENT GEOCHEMISTRY . . . . . . . 25 Vanadium . . . . . . . . . . . . 25 Zirconium . . . . . . . . . . . . 26 Chronium . . . . . . . . . . . . 26 Nickel . . . . . . . . . . . . . 27 Cobalt O I O O O O O O 0 O O O O 28 Trace Element Concentrations in Rocks. . . 28 iii Page SPECTROCHEMICAL ANALYSES OF THE ELY GREENSTONE . 3A MICROSCOPICALLY DISTINCTIVE SAMPLES OF GREENSTONE . . . . . . . . . . . . 39 Metadiabase. . . . . . . . . . . 39 Porphyritic Basalt . . . . . . . . . A0 Acidic Greenstone. . . . . . . . . . Al Lamprophyre. . . . . . . . A2 Undifferentiated Greenstone . . . . . . A2 CONCLUSIONS. . . . . . . . . . . . . AA RECOMMENDATIONS FOR FURTHER WORK. . . . . . A6 APPENDIX. . . . . . . . . . . . . . A8 REFERENCES . . . . . . . . . . . . . 53 iv Table LIST OF TABLES Stratigraphic Succession and Geochronology of the Precambrian of Minnesota . . Trace Element Concentrations in Rocks . Trace Element Concentrations in Shales and Basalts. . . . . . . . . Trends in Trace Element Concentrations. Page 30 31 36 Figure \N C\ KO 10. LIST OF FIGURES Location of Study Area . Geologic Map of Twin Lakes, Minnesota Sample Locations . . . . . . . Extrusive Greenstone Outcrop . . . Close-up of Extrusive Greenstone. . Close-up of Extrusive Greenstone—Meta- diabasic Greenstone Contact . . Diabase Dike Contact with Massive Greenstone . . . . . . . Breccia Composed of Greenstone Fragments Breccia—Dacite Contact . . . . Range in Trace Element Concentration of Petrographic Field Types of Greenstone vi Page 11 16 16 18 18 22 U) U“! INTRODUCTION m The most outstanding characteristics of the Ely greenstone are the amygdaloidal, spherulitic, and ellipsoidal structures that are recognizable in some rock exposures in the field. Other exposures of the greenstone differ in the absence of these structures, color, and tex— ture. This study is an attempt to evaluate all of the petrographic types of the Ely greenstone found in the area of Twin Lakes, Minnesota. Field observations, thin section examination, and spectrochemical analyses were utilized in this study. Geographic Setting The Ely greenstone is an eight to fifteen mile wide strip of Keewatin (Early Precambrian) rocks that extends from the vicinity of the town of Tower, Minnesota on the west to the Minnesota-Ontario boundary on the east. Figure 1 shows the general location of the Ely greenstone, and the location of the area within the greenstone selected for this study. The setting of this study is shown in greater detail in Figure 2. Centered around Twin Lakes, Minnesota, this .ao.< run.» .0 cotouod .. 0.30: l (a.r\‘/\\~/W\/u. I\ (,. ,z\ fan), WNMLMWWWWWLWMWMNWNQANWW l>>>>>\/\<<<\ «— .Bnfiu :: . . . tr . x...n..\\\. .\ \x .\\\,.,.x.\.\. . .\\ W\\\\\\ . .\\ I o._4050 I \\\..‘ ‘ KN‘NV‘ \wa \.\.\M.\wx §F .4 . x§ . .S \\,\.\. . . I \\\x\x\fi . . \Gr \\.. .' ‘ I II I I I - y ' \- - hr! - . Ila PPOII... . . . \\WW ‘I I . \ . x“ a I I I \.\ \Hs.\\\ I . . L \\\\\\M.\ . \\ \ \\\. ..x >oam Io mpficmso mwcmm mucmfio r0 meQEoo cpSHSQ mxoop pmnfio pocfl>flpcz va mcH£OH£oozou oCOpwcome mam :mfipmoco macaw cflomsoox cmfipoewompm mflpmm coHmepoulcopfl cmosom CNfipcmeHgmaHooooo.............~ApHEeHOrHCOOCD.-cu.ea....-o.....o......o........-o.c.co Ash-D WV ©m©H>HUCD macaw mxmq omficx :mfimemHEHB Ten TS cmEowHomnmuHHHampm mficflwha> . cmmxocmm...O.Ol0O.a...I.IothECHOPHCOOCDIOOOOOOOOOOOOOOOCIIOOIC......OOOOOO.......OOOOOOOOOOOOOOOOOI. va mofiuppmSG xSOfim on...noooo-onso.aothEhOMCOOCDoooco-ooaago-o-ooona-oooo-o owczzxosm . Q30hw mfiHH>copo poUH>HGCD oacmoao> opocm nopoz cm3mcom3mx cmfipnfiwoopm mama 2.2.: ....I300......II..zpflELogCOOCDOOOOOOOOO.......0.000000... oncomocmm omq so pcom mcoumpcmm mmaxocfim A.%.D ©.Ov .........-0......IhpfiE-HOIHEOOCDOIDOOOOCO.0.o...-000.000..........CIOOOIIOOOOIOIOOIOIOIIIOIOC cmfipoemo oaouooamm . . Emuwmm Amummw oav mxoom o>Hw5pucH acowopo cofiumchom oocozuom acnmz loofigom mama H.Muomm::HE no cmfihoEmomLQ on» do mwoaocoacoomw com coammooosm aficomuwHuMLumtt.H mqm<9 This series of green rocks, including the associated Soudan iron formation, had originally been named the Kawisiwin Series by Winchell (1899). Since Clements' (1903) work is the only comprehensive study of the green- stone ever completed and his nomenclature was that advo— cated by the United States Geological Survey, the terms Ely and Soudan have been generally adOpted. Clements (1903) gave particular emphasis to three macrosc0pic structures found within the Ely greenstone. He felt that the amygdaloidal, spherulitic, and ellipsoidal structures indicated strongly that the Ely greenstone origi- nated as surface or submarine flows. He also recognized the presence of intrusives but considered that most of these were the result of magma that penetrated contemporaneous flows as dikes. Clements (1903) found the ophitic texture to be most common in the coarse—grained greenstone. Ophitic, micro— phitic, intersertal, pilotaxitic, flowage, and spherulitic textures were noted in the fine—grained rocks. The original textures of many thin sections of the Ely greenstone were found completely altered to a confused aggregate by Schwartz (1924). He observed that nineteen out of fifty thin sections he examined showed remnants of diabasic texture. Schwartz noted that porphyritic texture with feldspar as phenocrysts is common and felt that these rocks were a later contact intrusive phase of the green— stone. Gruner (19Ul) made a study of the structure of the area around Knife Lake, Minnesota which extends to a point three miles east of the present study area. On the basis of structures within the Knife Lake sediments, Gruner delineated major longitudinal faults dividing the district as a whole into long segments or belts. Each one of these belts was thought to be distinct in itself but im— possible to connect stratigraphically with any others. Gruner (1991) observed a whitish-weathering acidic rock in the greenstone most of which consisted of a felsic groundmass and small quartz or feldspar phenOCrysts. He set the age of these felsic porphyries as Laurentian since they were not found in the Knife Lake sediments. Gruner (19Ul) felt that the greenstone found in the Knife Lake area was not in tight, isoclinal folds charac— teristic of the Knife Lake rocks. He believed that instead they may have been flexed, sheared, and tipped on edge in response to the same forces that folded the Knife Lake sediments. He cited as proof for this belief the vertical and straight attitudes of the felsic prophyry dikes. His rationale was that if the greenstones had been folded they would have shown defects of deformation due to folding. Klinger (1956) found large amounts of elastic, tuffaceous, and sedimentary materials within the Ely green— stone in the vicinity of Tower, Minnesota. In one outcrOp within the greenstone Klinger found sediments showing graded bedding indicating the presence of a sedimentary sequence below the Ely greenstone. In his comprehensive work on the geochronology of northern Minnesota, Goldich (1961) stated that the presence of a sedimentary formation below the Ely greenstone is difficult to demonstrate. Goldich's stratigraphic column for the Precambrian of the study area is reproduced as Table 1. Post metamorphic hydrothermal solutions were re- sponsible for the iron ore deposit within the Ely green- stone according to Machammer (196“). In his study of the Zenith Mine located at Ely, Minnesota he found mineralogical zoning within the ore body grading outward from the ore body through kaolinite, muscovite, and chlorite zones into un- altered greenstones. The localization of iron deposition within jasperlite was thought to be due to its brecciated condition. Two recent maps of the area covered by this study are available. An aeromagnetic map by Meuschke and others "1963) recorded two long, narrow, high anomalies, one on 1he north end and one on the south end of the present study area. An egg—shaped high anomolous area appeared in the center of the present study area. Iron formation was located in the area of all three of these anomalies. J. C. Green (1966) prepared the portion of the geologic map of the Gabbro Lake quadrangle that covers the 10 present study area. He differentiated the rocks within the map area into seven rock units. His structural inter- pretation followed closely that of Gruner (1941). A portion of Green's map was used for Figure 2. Method of Attack Field work was conducted with two objectives: (1) obtaining a representative sample of individual petro— graphic types for subsequent laboratory examination, and (2) establishment and examination of contacts between petrographic varieties within the greenstone. The first objective was attempted during the 1966 field season by randomly picking points within the study area on aerial photos and then visiting these points to collect samples. These sample locations are numbered from 71 to 93 on Figure 3. The second objective was accomplished during the 1967 field season by sampling along six traverse lines. These traverses were run perpendicular to the regional strike of the area and were laid out in the field by pacing with a solar compass using aerial photos for control. The locations of these samples are noted by the dashed numbers in Figure 3. Petrographic Methods Thin sections from throughout the area of the Ely greenstone, loaned by United States Steel, were examined. 53$ 18$“. fl 2 a . [63 N. TRIANGLE b LAKE > .goo~. » FIGURE 3 SAMPlI [OCAHONS ll Random Sample :3 0-. Trevor}. Scale L ”4 Mill 0 Sample 12 These together with a review of Schwartz's (192“) paper allowed the writer to obtain a preliminary idea of some of the rock types occurring in the Ely greenstone. Thin sections of rocks from the area were studied, some commercially ground and some prepared by the writer. Sawed sections were compared with thin sections. Comments on individual samples will be presented later. In general, thin section examination had to be confined to the identifi- cation of major minerals and textures present because of the highly altered character of the rock. Spectroscopic Analyses Representative 15 gram samples were ground to coarse sand size between two rocks obtained from the same sample locations. Final grinding was then done in a Spex Mixer Mill fitted with a high-alumina ceramic vial and ball. This grinding procedure produced a powder, the bulk of which was less than 200 mesh. Graphite powder (National Carbon Co., Grade Sp-2) was mixed with the sample to assure a smoother, more com- plete burn on the spectrograph. Spectrographically pure palladium was also added to the sample to act as an in— ternal standard as recommended by Ahrens (1960). The pro- portions of the various constituents were as follows: Sample—-9l parts, graphite-—183.1 parts, and palladium-— :01 parts. 13 Analytical conditions were as follows: Excitation: Interrupted arc, 30 ohms, “0 mfd, 360 mh, 15 sec burn, sample positive. Slit: 20 microns wide, 3 mm long. Transmittance: 60% Electrodes: .2“2-inch graphite electrodes. Plates: Eastman Kodak Spectrum Analysis No. 3. Processing: Developer D—19 3% min., Stop 3% acetic acid 30 sec., fixer Kodak fixer 10 min. Photometry: Jarrel-Ash Microphotometer. Calculations: Seidal calculating board, cali— bration curve from iron spectrum by two step filter method. Analysis Lines: CR 28“3.2 V 3185.3“ Zr 3391.8 Ni 3“1“ Pd 3“21 Co 3“53.5 External standards were established as recommended by Dolwell (1967). Standard rock samples, granite G-1 and diabase W—l, were used to obtain a working curve over a limited range and this curve was extended for a wider range of concentrations utilizing the slope obtained from synthetic mixtures. All values are based on duplicate analyses. Sample 2—7 was analyzed 29 times to obtain an estimate of the reproducibility of the data. The standard deviations for these 29 determinations were as follows: 1“ Ni H- 8.8% Cr i l“.6% Zr H- 16.7% Co H- 17.5% V + 22.8% The precision of the vanadium value was considered poor. It was also later observed that the vanadium values obtained for all samples were consistently about twice the expected values. Although the vanadium line read was that recommended by Ahrens (1960) and numerous other sources, a search was made of the literature to determine if any mention had ever been made of the unreliability of the 3185.3“ angstrom line of vanadium. This search of the literature revealed that Shaw (1958) had observed that this vanadium line is coincident with an unlisted calcium line. Shaw maintained that V 3185 angstrom could be used only if the sample contained less than 2% Ca0. Since the lowest published analysis of CaO in the Ely greenstone is “.“6% (Schwartz, 192“), the values for vanadium determined in this study were undoubtedly high. No other suitable vanadium lines were present. STRATIGRAPHY OF THE ELY GREENSTONE Study of Green's (1966) geologic map and Meuschke's (1963) aeromagnetic map of the area revealed that the iron formation occurrences tend to strike roughly east—west to north 700 west. Green frequently shows dacite units and extrusive greenstone units roughly conformable with the iron formation. From this information it was decided that traverses run perpendicular to iron formation would be most likely to maximize the number of petrographic types encountered. The following rock types were recoge nizable in the field. Extrusive Greenstone The most distinctive rock type found within the greenstone is the spherulitic, ellipsoidal, and amygdaloidal greenstones. Rock outcroppings showing one or more of these extrusive characteristics were designated extrusive. The most outstanding feature of this rock type in the field is its irregularly weathered surface. Figure “ shows the broken nature of an outcrop of extrusive green- stone. Close inspection of this rock type often reveals a surface with many gash-like indentations as illustrated in Figure 5. 15 l6 FIGURE “.—-Extrusive greenstone outcrop showing irregular weathered surface. Sledge hammer is 32 inches in length. Location is 600 feet east of sample location 6-2 in the NE l/“ of section 13. FIGURE 5.--Close-up of gash-like weathering of extru- sive greenstone. Pencil is 5 inches in length. Location is 100 feet east of sample location “-7 in the SE l/“ of section 2. 17 Thin section examination of this rock type revealed an almost Opaque mass of green. However, the radiating texture seen frequently suggests that zeolites were origi— nally present. In one thin section dull brown, tangled fibers were observed. These probably represent chalcedony or zeolites. Metadiabasic Greenstone Almost always occurring within or adjacent to green— stone showing extrusive features is an extremely fine- grained light green rock type. This rock characteristically contains many small calcite veinlets. Its specific gravity is frequently slightly less than 2.9. In several places where the contact of this rock with extrusive greenstone could be observed it appeared to have originated as a sill. Figure 7 shows this relation— ship. Iron Formation The iron formation occurring in the area is really three different rock types. A gray—black massive chert was found at the north end of traverse four. Banded chert- magnetite was found in traverses two, five, and six. Lastly, a well—banded chert and siderite rock was round in the roadcut 300 feet to the east of station “—5 in traverse four. l8 FIGURE 6.--An extrusive greenstone-metadiabasic greenstone contact occurs between the two sample bags. Amygdules can be seen in the extrusive greenstone on the left. The sample bags are eight inches in length. Loca- tion is 200 feet east of sample location 3-8 in the SE l/“ of section 2. FIGURE 7.--Diabase dike contact with massive green- stone. Location is in the NE l/“ of section 13. 19 It has been previously noted that the iron formation tends to occur adjacent to the extrusive greenstone and dacite. Another distinctive feature of contacts of the iron formation with the greenstone was the tendency of the greenstone to become lighter in color and more mottled in appearance within one hundred feet of the greenstone—iron formation contact. This was observed in traverses three, five, and six. Foliated Greenstone Occurring just to the south of extrusive greenstone in traverse four was a 200—foot wide belt of crudely foliated rocks. Megascopic examination of these rocks seemed to indicate that their foliation was due to parallelism of green hornblende and/or chlorite flakes. Field examination of this rock type failed to reveal any definite criteria to establish whether its texture was due to a sedimentary origin or to subsequent shearing. Although the occurrence cited was adjacent to extrusive greenstone, other occurrences were observed that did not appear to be associated with extrusive greenstone. Thin section examination failed to produce any helpful information on this rock type. 2O Diabase A relatively fresh diabase dike was observed in the NE% of section 13. This dike was shown on Green's (1966) map. Directly across the small bay from the location shown by Green the chilled contact of the dike shown in Figure 7 was observed. This was considered noteworthy because the other rock units observed in the field failed to show recognizable chill zones. Massive Greenstone This rock type represents what remains after the greenstones showing more distinct field characteristics have been described. Its weathered surface is smooth; it is medium to dark green in color; and its grain size is usually that of a basalt. Sawed sections of this rock usually reveal two or three shades of green that give it a mottled appearance. Sawed sections revealed a curious variety of tex- tures which indicated that this rock type actually con— tained several distinct types. Thin sections were ob- tained of several of these types and trace element analyses were determined for comparison with the extrusive green- stone and the foliated greenstone. Dacite Frequently occurring adjacent to iron formation or extrsusive greenstone was a light feldspathic rock with 21 quartz or feldspar phenocrysts. Green (1966) designated this rock a dacite, and that designation will be used here. Unfortunately this rock type is extremely limited in eXposure, being frequently found on the edge of swamps or lakes or on islands in the swamps and lakes. In megascopic appearance and the location of field occurrences, this rock type appeared to match the felsite porphyry described by Gruner (19“1) as Laurentian dikes. Proof that the dacite was intrusive into the greenstone was extremely difficult to find in the study area due to the limited exposure of the dacite. One contact of the dacite that was observed at the north end of traverse three was so interesting and proble- matic that it deserved detailed description. On the north side of the mouth of the small bay labeled A in Figure 2 is found the breccia pictures in Figure 8. It is com— posed of angular greenstone fragments in a carbonate matrix. The long axes of these fragments are roughly parallel. This breccia contacts the dacite a few feet from the water's edge. This contact can be seen in Figure 9. Is this breccia eruptive, flow, or contact? The calcite matrix would tend to indicate that it was not a flow or eruptive breccia since these are usually cemented witli fine basaltic or a felsic matrix. However, the aliggned fragments are not what would be eXpected 22 FIGURE 8.-—Breccia composed of greenstone fragments set in a carbonate matrix. Location is on the east shore of Jasper Lake in the NE l/“ of section 2. Exact location is marked with an A in Figure 2. FIGURE 9.--Breccia-dacite contact located four feet north of Figure 8. 23 from a contact breccia formed by the shattering of wall rock. Thin section examination of this breccia revealed that the greenstone fragments do not have sharp contacts with the calcite. This seemed to suggest that the calcite has replaced a basaltic matrix that had corroded the frag- ments. Therefore, it would seem probable that this breccia was originally a flow breccia and likely that the dacite overlying it was not intrusive. Thin section examination of the dacite reveals extremely large phenocrysts (up to “ mm. in diameter) set in a fine—grained quartz, feldspar, and calcite matrix. The following outstanding features were noted in the dacite: (1) an extreme excess of calcite occurs in the ground mass over that expected in a dacite, (2) quartz grains present occur in a great variety of sizes, (3) some abnormally large crystals of both quartz and feldspar are present, (“) although most quartz was clear, some contained inclusions that appeared identical to the ground mass of the rock, and (5) embayment that is present suggests en— croachment. All these features of the dacite would indicate that the "phenocrysts" in the dacite are in part phyro- blasts. These criteria are those suggested by Grout (19“1) and for the development of igneous looking rocks by metasomatism. 2“ One outstanding refutation of this View would be an explanation of the previously mentioned breccia adjacent to this dacite as a contact breccia. This was earlier considered unlikely. The development of this "dacite" is then believed to be due largely to meta- somatic replacement. It is suggested that the original rock originally contained ferromagnesium minerals which have been replaced and whose volume is now occupied by quartz, feldspar, and calcite. TRACE ELEMENT GEOCHEMISTRY~ The behavior of many trace elements in a solidifying silicate melt is fairly well understood. Goldschmidt (1937, 19““) felt that an element's ionic structure, atomic structure, and ionization potential determined the affinity of each element between the various possible mineral structures. Work since Goldschmidt's has re- vealed the importance of other factors especially for non-ionic bonds. This recent work has been summarized by Taylor (1965). If the concentrations of these elements were due only to one or more solidifying silicate melts, the following factors would govern their distributions in minerals. Vanadium In magmas vanadium is probably present as Va3+. Vanadium occurs in magnetite, pyroxenes, amphiboles, and biotite. Ringwood (1955) has linked the appearance of V in residual melts to the fact that its high ionic poten- tial is sufficient to cause a complex to form in volatile— rich magmas. 25 26 Nockolds and Mitchell (19“8) gave the vanadium distribution as follows in a diorite: ppm Plagioclase 20 Hypersthene 100 Augite 200 Biotite “00 Zirconium The high charge and large radius of zirconium make it difficult for zirconium to enter any of the common rock forming silicates. It has been found to enter early Skaergaard pyroxenes and late apatites but was not found in plagioclase or olivine. The majority of zirconium in igneous rocks is found in the mineral zircon. Chromium Chromium is present in magma as the Cr3+; its radius is very close to that of Fe3+. However, the smaller electronegativity of Cr3+ as compared with Fe3+ leads to chromium's preferential removal at an early stage of fractional crystallization of a basic magma. Chromium is found principally in the mineral chromite. It is also found in pyroxenes. Wager and Mitchell (19“8, 1951) eXplained chromium's limited entry into Skaergaard 2+ olivines and ilmenites by its inability to occupy Fe positions due to the charge difference. 27 The occurrence of chromium in a diorite is given by Nockolds and Mitchell (19“8) as follows: ppm Plagioclase -—- Hypersthene 200 Augite 1500 Biotite 800 Nickel The radius of nickel is nearly identical to that of magnesium. In early basic rocks nickel occurs concen- trated and apparently is substituting for magnesium. In later formed rocks the ratio NizMg decreases indicating that perhaps the bonding character of Ni causes its preferential concentration in early crystallates. In igneous rocks nickel is found in silicates, sulfides, oxides, and occasionally as iron—nickel alloys. In silicates nickel is usually found concentrated in early formed olivine. The pyroxenes and to a smaller degree the amphioboles also contain some nickel. Nickel was found in augite, hypersthene, and bio- tite in a norite by Nockolds and Mitchell (19“8) as follows: ppm Plagioclase --- Hypersthene 300 Augite 200 Biotite 150 28 Cobalt In basic rocks, Ni and Co enter the olivine structure because of their similar size and charge. The slightly larger size of Co2+ does restrict the acceptance of cobalt into magnesium positions to a greater degree than for nickel. The relationship between the depletion of Ni and Co has been shown by Taylor (1965). Because of the faster depletion of nickel, the Ni:Co ratio falls during fractionation. Nockolds and Mitchell (19“8) found cobalt distri- buted in the following manner in a diorite: ppm Plagioclase l5 Hypersthene 100 Augite 70 Biotite 50 Trace Element Concentrations in Rocks Since trace elements are preferentially accepted in specific minerals, rocks with a given mineral assemblage usually have similar trace element concentrations. Trace element analyses of basic igneous rocks usually show high concentrations of nickel, cobalt, vanadium, and chromium due largely to their high percentage of ferro— magnesian minerals. Zirconium in contrast tends to be concentrated in acidic igneous rocks due to its selective 29 occurrence in the mineral zircon. Table 2 lists the trace element concentrations of several suites of igneous rocks. Argillaceous sediments show less characteristic trace element concentration due to their more variable mineralogy. In general, the Cr, Co, and Ni concentrations in these rocks tend to be lower than in basic igneous rocks. A comparison of some published trace element values for basalts and argillaceous sediments is pre— sented in Table 3. In the field, the only evidence available suggests that the Ely greenstone, with the exception of the dacite, was all originally a basic igneous rock. It would then seem a reasonable hypothesis to test whether the trace element composition of the Ely greenstone is close to that of a basic igneous rock. Consistent trends in trace element concentrations could perhaps indicate differ— entiation in the original basic rock. Since pelites derived from argillaceous rocks could easily be mistaken for metamorphosed basic igneous rocks, their trace element concentrations must also be con— sidered. This approach to trace element data assumes that no chemical migration or enrichment has occurred in the rock since it was formed. 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