U-Pb geochronology of monazite and xenotime in hematite ore deposits, Marquette Range, Michigan: Evidence for multiple episodes of iron mineralization

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1 1 GSA DATA REPOSITORY Appendix DR1. SHRIMP analytical procedures and data interpretation, concordia figures, and data tables U-Pb geochronology of monazite and xenotime in hematite ore deposits, Marquette Range, Michigan: Evidence for multiple episodes of iron mineralization By Birger Rasmussen, Jian-Wei Zi, Stephen Sheppard, Bryan Krapež, Janet R. Muhling SHRIMP U-Th-Pb ANALYSES Data acquisition Xenotime and monazite data acquisition followed established procedures (Foster et al., 2000; Fletcher et al., 2004) except that Y and Nd were also monitored in the monazite analyses (Fletcher et al., 2010). The data were collected in multiple analytical sessions, using na primary ion beams focused onto 10 µm spots. The post-collector retardation lens was activated to reduce stray ion arrivals. The mass resolution (M/ΔM at 1% peak height) was >5000 in all sessions. For element abundance determinations, Pb/U and Pb/Th calibration, and corrections for matrix effects and Pb/Pb fractionation, both xenotime and monazite made use of a suite of reference materials (see Fletcher et al., 2004; 2010) that were in a separate mount. The primary reference material for monazite was French ( 206 Pb/ 238 U age 514 Ma; 208 Pb/ 232 Th age 504 Ma), and Z2908 and Z2234 were the secondary reference materials used to monitor matrix effects. Z2908 ( 207 Pb/ 206 Pb age 1796 Ma) was also used to monitor and correct for instrumental mass fractionation of 207 Pb versus 206 Pb. The primary reference material for xenotime was Xeno-1 ( 207 Pb/ 206 Pb age 997 Ma; 206 Pb/ 238 U age 994 Ma), with MG-1 and BS-1 as secondary reference materials (see Fletcher et al., 2004). Data reduction All data were processed using Squid-2 software (Ludwig, 2009). Xenotime data were subjected to exponential calibrations for Pb/U, using 206 Pb + / 238 U + as the primary data ratio, and indirect calibrations of Pb/Th. For monazite, the primary ratios for Pb/U and Pb/Th were 206 Pb + / 270 [UO 2 ] + and 208 Pb + / 264 [ThO 2 ] +, both of which were subjected to 1-dimensional calibrations (Fletcher et al., 2010). Corrections for instrumental mass fractionation of Pb isotopes and for matrix effects in xenotime and monazite Pb/U and Pb/Th were made subsequently, using spreadsheet templates.

2 2 Results Jackson Mine Thirty-three analyses were obtained on monazite in sample (Table DR1). Fifteen analyses are discordant ( 10% discordance) or contain high common lead (f206 > 1%), therefore are considered not giving reliable ages. Five analyses from 4 monazite grains yielded 207 Pb/ 206 Pb dates between 2640 Ma and 1981 Ma. The remaining 14 analyses from 11 monazite grains have low U ( ppm, average 226 ppm) and Th ( ppm, average 1458 ppm), and yielded a weighted mean 207 Pb/ 206 Pb date of 1810 ± 18 Ma (MSWD = 0.6) (Fig. DR1). Republic Mine Monazite grains in sample from the Republic Mine are characterized by low U, high Th and high Th/U ratios (Table DR2). Twenty-three analyses were obtained. Excluding 10 analyses with high common lead (f206 > 1%) or large discordance ( 10% discordance), 13 spots on 7 monazite grains yielded a weighted mean 207 Pb/ 206 Pb date of 1796 ± 17 Ma (MSWD = 1.5) (Fig. DR2). A single xenotime analysis yielded a 207 Pb/ 206 Pb date of 2522 ± 7 Ma, which is interpreted to represent a detrital grain (Table DR3). Champion Mine Three samples of schistose hematite ore from the Champion Mine were dated, with U-Th-Pb results listed in Table DR4. Excluding discordant analyses ( 10% discordance) or those with high common lead (f206 > 1%), these samples yielded main age populations with weighted mean 207 Pb/ 206 Pb dates of 1758 ± 3 Ma (n = 20, MSWD = 1.4; Champion 1A), 1765 ± 6 Ma (n = 15, MSWD = 0.74; Champion 6), and 1773 ± 7 Ma (n = 15, MSWD = 1.4; Champion 7E2). These dates are identical within uncertainties and, combined together, they give a mean 207 Pb/ 206 Pb date of 1761 ± 3 Ma (n = 50, MSWD = 1.7) (Fig. DR3). There are two outlier analyses from sample Champion 1A, the c Ma grain is obviously of detrital origin, whereas the 1663 Ma grain is interpreted to represent one formed during younger metamorphic event. Humboldt Mine Four samples from the Humboldt Mine were analysed for monazite U-Th-Pb geochronology (Table DR5). Omitting a discordant analysis, 13 analyses from sample gave 207 Pb/ 206 Pb dates between 1822 Ma and 1739 Ma, with a weighted mean date of 1771 ± 10 Ma, but the MSWD of 4.7 suggests excessive scattering of the data. Excluding the oldest and the youngest analyses, the remaining 11 analyses yielded a mean 207 Pb/ 206 Pb date of 1771 ± 7 Ma (MSWD = 2.0). Eight concordant analyses from 5 monazite grains in sample yielded a weighted mean 207 Pb/ 206 Pb date of 1758 ± 15 Ma (MSWD = 2.4). Monazite analyses from sample yielded a main population at 1766 ± 6 Ma (n = 18, MSWD = 1.0), while another four analyses from 2 detrital monazite grains record ages of Ma. Results of main metamorphic ages from the three samples are indistinguishable within errors, combining analyses from all three samples results in a weighted mean age of 1766 ± 4 Ma (n = 37, MSWD = 1.7). Sample yielded a metamorphic age at 1794 ± 16 Ma (n = 4, MSWD = 0.99), while detrital monazite grains in this sample yielded ages between 2617 Ma and 2313 Ma (Fig. DR4). It is noteworthy that the ca Ma monazite grains have U and Th compositions analogous to the grains of similar age in sample from the Republic Mine (Table

3 3 DR2). Two xenotime analyses from a Humboldt Mine sample yielded ages of ca Ma (Table DR6). Saginaw Mine Nine samples (Saginaw 13B, 15, 16, 17, 19A, 20, 19-15, and A and B) from this mine were collected and analysed for xenotime geochronology. U-Th-Pb data from these samples are combined together and listed in Table DR7. Only concordant (discordance within 9%) and low common Pb (f206 <1%) analyses are considered in age calculation. Five analyses from 5 grains gave detrital ages between 2584 Ma and 2416 Ma. The remaining 19 analyses can be grouped into two populations, with an older cluster at 1779 ± 9 Ma (n = 10, MSWD = 1.9) and a younger one at 1681 ± 9 Ma (n = 9, MSWD = 4.6) although the younger group show excessive scattering as indicated by its large MSWD value (Fig. DR5). Mixture modelling (Sambridge and Compston, 1994, embedded in Isoplot 3) reveals two dominant components at 1779 ± 6 Ma (44%) and 1681 ± 10 Ma (56%), supporting the above grouping. Two analyses from a single monazite grain in a Saginaw Mine sample yielded 207 Pb/ 206 Pb dates of 1733 ± 16 Ma and 1692 ± 26 Ma (Table DR8). In general, monazite grains in samples from the Jackson Mine and Republic Mine gave ages at ca Ga, and are characterized by low U concentrations, whereas those in samples from the Champion Mine and Humboldt Mine show increasing U and yielded predominant ages of ca Ga (Fig DR6). REFERENCES Fletcher, I.R., McNaughton, N.J., Aleinikoff, J.A., Rasmussen, B., and Kamo, S.L., 2004, Improved calibration procedures and new standards for U Pb and Th Pb dating of Phanerozoic xenotime by ion microprobe: Chemical Geology, v. 209, p Fletcher, I.R., McNaughton, N.J., Davis, W.J., and Rasmussen, B., 2010, Matrix effects and calibration limitations in ion probe U Pb and Th Pb dating of monazite: Chemical Geology, v. 270, p Foster, G., Kinny, P., Vance, D., Prince, C., and Harris, N., 2000, The significance of monazite U-Th-Pb age data in metamorphic assemblages; a combined study of monazite and garnet chronometry: Earth and Planetary Science Letters, v. 181, p Ludwig, K.R., 2009, Squid 2; a user s manual. Berkeley Geochronology Center, 100p. Sambridge, M.S. and Compston, W., 1994, Mixture modelling of multi-component data sets with application to ion-probe zircon ages: Earth and Planetary Science Letters, v. 128, p Stacey, J.S., and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p

4 4 Figure DR1. U Pb concordia plot for monazite in sample from the Jackson Mine. Ellipses represent 1σ errors. Individual outliers are labelled. Rejected analyses (due to high common Pb or large discordance) are not displayed. Figure DR2. U Pb concordia plot for monazite in samples from the Republic Mine. Ellipses represent 1σ errors. Individual outliers are labelled. Rejected analyses (due to high common Pb or large discordance) are not displayed.

5 Figure DR3. U Pb concordia plot for monazite in samples from the Champion Mine. Ellipses represent 1σ errors. Individual outliers are labelled. Rejected analyses (due to high common Pb or large discordance) are not displayed. 5

6 Figure DR4. U Pb concordia plot for monazite in samples from the Humboldt Mine. Ellipses represent 1σ errors. Individual outliers are labelled. Rejected analyses (due to high common Pb or large discordance) are not displayed. 6

7 7 Figure DR5. U Pb concordia plot for xenotime in samples from the Saginaw Mine. Ellipses represent 1σ errors. Individual outliers are labelled. Rejected analyses (due to high common Pb or large discordance) are not displayed. Figure DR6. Variation of U concentrations with age revealed by monazite analyses from the dated samples from the Marquette Range. Ellipses represent 1σ errors. Individual outliers are labelled. Rejected analyses (due to high common Pb or large discordance) are not displayed.

8 8 Table DR1. SHRIMP U-Pb data for monazite in samples from the Jackson Mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Main group (1810 ± 18 Ma, n = 14, MSWD = 0.6) 1454H H C F F F I I H G F H H C Outliers 1454A F F D D Rejected 1453C C F

9 1453E I C C H D E B J Analysis identification is nnnn.p-q where nnnn is the SHRIMP mount number, p is the monazite/xenotime grain and q is the analysis site in the grain. Pb * indicate radiogenic Pb. All Pb isotope data have been corrected for common Pb, using a modelled Pb isotopic composition from Stacey-Kramers (1975) at the approximate age of the sample. f 206 is the proportion of common Pb in 206 Pb, determined using the measured 204 Pb/ 206 Pb. Disc. is apparent U-Pb discordance, defined as 100( t[ / 206 Pb*] - t[ 206 Pb*/ 238 U] ) / t[ / 206 Pb*]. Analyses are sorted by descending / 206 Pb*, except those not considered in age calculation due to high common Pb or large discordance. 9

10 10 Table DR2. SHRIMP U-Pb data for monazite in samples from the Republic mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Republic mine 9 Main group (1796 ± 17 Ma; n = 13, MSWD = 1.5) 1452C D D B D B D A A E B E E Rejected 1452D D D F F C D F D D Footnotes are the same as in Table DR1

11 11 Table DR3. SHRIMP U-Pb data for xenotime in samples from the Republic mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Republic mine 1323D Footnotes are the same as in Table DR1

12 12 Table DR4. SHRIMP U-Pb data for monazite in iron ore samples from the Champion mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Champion1A Main group (1758 ± 3 Ma; n = 20, MSWD = 1.4) 1321H E F A D F E F G D H H C D G D D H A B Outlier 1321I H Rejected analyses due to high common Pb or discordance 1321E H.1-1a H

13 1321D Champion 6 Main Group (1765 ± 6 Ma; n = 15, MSWD = 0.74) 1406A F C A F G G F J C G E D A I Rejected analyses due to high common Pb or discordance 1402J A J G G G I C G G I I D H F

14 1402J D J G F E E J A K F Champion 7E2 Main Group (1773 ± 7 Ma; n =15, MSWD = 1.4) 1404F J F B J J H C E J E C A C F.1-1a Rejected analyses due to high common Pb or discordance 1404A C F L Footnotes are the same as in Table DR1

15 15 Table DR5. SHRIMP U-Pb data for monazite in samples from the Humboldt Mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Main group (1771 ± 7 Ma; n = 11, MSWD = 2.0, excluding the oldest and youngest analyses) 1447F I B D C D J C I B F E G Rejected 1447F Main group (1758 ± 15 Ma, n = 8, MSWD = 2.4) 1458C C D C B C D C

16 Rejected 1458H I I H I I I E B Detrital 1459F K K F Main group (1766 ± 6 Ma; n = 18, MSWD = 1.0 ) 1459B H I G C J C D C E G C B E E I B

17 1450B Rejected 1459D E E B H B B A B B B Detrital 1457G E C B B A D Main group (1794 ± 16 Ma, n = 4, MSWD = 0.99) 1457I I I I Rejected 1457I I I I I

18 1457I I I I Footnotes are the same as in Table DR1 18

19 19 Table DR6. SHRIMP U-Pb data for xenotime in sample from the Humboldt mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Humboldt 1333B B Footnotes are the same as in Table DR1

20 20 Table DR7. SHRIMP U-Pb data for xenotime in samples from the Saginaw Mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Combined data from samples A & B, Saginaw 13B, 15, 16, 17, 19A, 20, and Detrital 1465A C I F H Group 1 (1779 ± 9 Ma; n = 10, MSWD = 1.9) 1445C F B F C G D F G F Group 2 (1681 ± 24 Ma; n = 9, MSWD = 4.6) 1445C E H H E D E E H

21 21 Rejected due to high common Pb or discordant 1445D I I K K A Footnotes are the same as in Table DR1

22 22 Table DR8. SHRIMP U-Pb data for monazite in samples from the Saginaw Mine, Marquette Analysis No. U (ppm) Th (ppm) Th/U f 206 (%) 207 Pb * / 206 Pb * ± 206 Pb* / 238 U ± / 235 U ± 208 Pb * / 232 Th ± Disc. (%) / 206 Pb* Age (Ma) ± (Ma) Saginaw 1457H H Footnotes are the same as in Table DR1