Analytical methods Samples and results

Fig. 3. U-Pb concordia diagram of zircon analyses from sample OM95-27, the felsic metavolcanic. Numbers refer to analyses in Table 2. Error ellipses are 2s. on several characteristics, as follows. Attempts were made to select zircons that were clear, crack- free and lacking inclusions, cores and over- growths. However, the poor quality of samples often made selection of cracked and cloudy grains necessary. Titanite fragments were divided based on colour and selected for internal clarity and lack of cracking. Apatite grains are prismatic whereas the feldspars are generally anhedral. Both were selected for clarity and lack of cracking. Rutiles were chosen for crystal shape, colour and luster an indicator of freshness. Rutile aggre- gates were washed in HF for 15 min in an ultra- sonic cleaner to remove surrounding silicate minerals. Selected zircons were abraded Krogh, 1982, but many were left unabraded due to the possibility of shattering because of internal cracks. No other minerals were abraded. Large titanite, apatite and feldspar fractions were weighed, whereas weights for zircon, baddeleyite and rutile fractions were estimated by eye and are likely accurate to about 9 50. Zircon, baddeley- ite and rutile were digested in bombs, while titan- ite, apatite and feldspar were digested in Savillex capsules. For zircon, U and Pb were separated using HCl with 50 ml anion exchange columns following the method of Krogh 1973. Titanite, apatite, rutile and feldspar were passed through 500 ml anion exchange columns with HBr, follow- ing the method of Corfu 1988. Pb blanks are 1 pg for small column and 2 pg for large column chemistry. U blanks are taken to be 0.1 pg. U and Pb were loaded onto Re filaments using H 3 PO 4 and silica gel. Isotopic analysis was carried out on a VG354 mass spectrometer in peak jumping mode, with either a Faraday collector for large signal samples or Daly detector for small signal samples. The mass discrimination correction for the Daly detector is 0.40 per AMU and the thermal mass discrimination factor is 0.10 per AMU. Common lead in the apatite, titanite and rutile samples made ages variably dependent on the isotopic composition defined for the initial common Pb. Data from the feldspar from OM95- 2 were used as a common lead correction on all regressions for these minerals. Data are plotted with two sigma error ellipses on U-Pb and Pb-Pb diagrams Figs. 3 – 9. Regressions are calculated during pre-gold and gold-forming stages. Sul- phides are mostly pyrite, but also include minor sphalerite, galena, and chalcopyrite. Pyrite miner- alization occurred before, during and after gold deposition, as did silicification. Increased sulphi- dation and silicification correlate with increased veining and gold mineralization. A several hundred metres thick gabbro dyke, believed to be a member of the Avanavero Suite, cuts the mineralized pluton at a depth of : 200 – 300 m Bertoni et al., 1991. Members of this group of mafic rocks occur throughout the Guyana Shield in the form of sills, dykes and other irregular bodies and have been dated by K-Ar, Rb-Sr and Ar-Ar methods. Sidder and Mendoza 1995 estimated the age at between 1650 and 1850 Ma, based on compiled data from several sources.

4. Analytical methods

Sample sizes ranged from 6 to 25 kg before crushing. Heavy mineral separates from the Wi- lfley table were screened to − 70 mesh, and then separated using conventional magnetic and heavy liquid techniques. Grains were hand-picked based Fig. 4. Pb-Pb diagram showing feldspar analyses from OM95- 2, and hydrothermal titanite, apatite and rutile analyses from OM95-2, OM95-26 and OM95-28, respectively. The error el- lipses are highlighted by circles. The line represents a regres- sion of all analyses except 7 titanite and 29 feldspar. The Stacey and Kramers 1975 growth curve is also shown. using the program of Davis 1982. Resulting age errors are quoted at 95 confidence levels.

5. Samples and results

Mineralized and non-mineralized samples, showing varying degrees of hydrothermal alter- ation, were chosen in order to obtain ages on both igneous emplacement and hydrothermal crystallization Fig. 2. U-Pb analytical data for samples with highly to moderately radiogenic Pb are presented in Table 2. Pb-Pb data for low UPb titanite, apatite, rutile and feldspar fractions are presented in Table 3. Note that analyses 5 – 6 are given in both tables. 5 . 1 . OM 95 - 27 Meta6olcanic host rock Sample OM95-27 is an intermediate-felsic metavolcanicsubvolcanic rock recovered as drill core from 130 m depth under the Wenot Lake zone. It consists of plagioclase and minor quartz phenocrysts in a fine-grained quartz-feldspar ma- trix. Secondary carbonate, chlorite and sericite are abundant, with minor pyrite and accessory zircon. Zircons from this sample are elongate l:w 3:1, clear and colourless, with an average length of : 50 mm. Two abraded grains were run as separate analyses, along with one unabraded piece that contained longitudinal cracks. Data from the abraded fractions are concordant and, with the slightly discordant datum from the unabraded fraction, define an age of 2120 9 2 Ma Fig. 3. 5 . 2 . OM 95 - 2 Marginal hornblende diorite Three samples of hornblende-rich marginal phases were collected. OM95-2 is a porphyritic hornblende diorite taken from the north edge of the pit. The hornblendes in this unit are stubby crystals showing a sub-parallel orientation. The matrix consists of quartz and plagioclase with minor smaller amphiboles and homogeneously distributed magnetite crystals. Strong hydrother- mal alteration produced carbonate, sericite and epidote. Alteration of the hornblendes to chlorite and actinolite is pervasive. Accessory phases in- Fig. 5. U-Pb concordia diagram showing igneous titanite and apatite analyses from OM95-2 and OM95-4, and hydrother- mal titanite analyses from OM95-2. The line represents a regression of the igneous titanite and apatite results forced through the origin of concordia. C . Norcross et al . Precambrian Research 102 2000 69 – 86 Table 2 U-Pb data for zircon, baddeleyite, titanite and apatite a ThU Pb com pg 207 Pb 207 Pb 206 Pb Age 206 Pb 238 U 207 Pb 235 U Weight mg Disc Number U ppm Fraction Ma 204 Pb OM 95 - 27 meta6olcanic 746.9 0.3894 9 8 7.068 9 20 0.002 2120.0 9 2.2 98 1 Ab z, flat 1 0.28 0.9 355.4 0.3891 9 12 2 7.060 9 28 1 Ab z, round 2119.6 9 4.2 0.1 0.001 84 0.29 0.8 669.7 0.3794 9 8 6.877 9 18 2117.8 9 2.2 2.5 5.0 1 z, frag, cracked 0.6 169 0.006 3 OM 95 - 2 hornblende diorite 165.5 0.3816 9 8 6.820 9 18 4 2092.9 9 1.8 Brown titanite 0.5 0.477 61 3.83 560 49.38 0.3580 9 13 6.096 9 29 2007.5 9 5.1 2.0 36 5 3.13 9.5 0.050 colourless titanite 1.69 266 32.70 0.3520 9 8 5.965 9 28 1998.6 9 6.3 3.2 6 0.290 colourless titanite 6.3 43.09 0.3652 9 8 6.365 9 24 2048.3 9 4.4 2.4 243 11 7 colourless titanite 0.226 2.28 OM 95 - 4 bladed hornblendite 79.01 0.3844 9 9 6.875 9 21 8 2094.2 9 2.7 Brown titanite -0.1 0.303 79 2.53 1101 417.7 0.3812 9 8 6.822 9 18 2095.4 9 1.6 0.8 517 9 1.48 397 0.181 Dark brown titanite 179 Colourless titanite 65.77 0.3764 9 9 6.730 9 23 2093.9 9 3.0 2.0 0.288 11 2.12 10 22.53 0.3697 9 9 6.592 9 56 2089 9 13 3.5 528 1.39 Apatite 0.225 6.0 11 OM 95 - 11 quartz diorite 1.64 12.1 67.75 0.3279 9 10 5.843 9 24 2087.6 9 4.2 14.3 0.001 12 228 3 Ab z, fuzzy core 180.9 0.3134 9 8 5.562 9 26 13 2080.7 9 7.0 2 z, cracked, fuzzy core 17.7 0.003 230 1.3 10.8 174.6 0.2900 9 6 5.148 9 24 2081.1 9 7.4 23.9 5.9 195 1.62 14 1 z, v. cracked 0.002 4.2 1 z, cracked 372.7 0.3456 9 10 6.163 9 24 2088.9 9 3.8 9.7 0.006 88 1.01 15 1.14 69.1 61.95 0.2879 9 8 5.001 9 60 2043 9 21 22.8 16 3 z, cracked 0.005 308 OM 95 - 28 quartz diorite 31.02 0.3095 9 14 5.512 9 44 2087 9 11 19.0 17 1 Ab z, clear 0.0002 261 1.43 8.8 47.92 0.3204 9 10 5.725 9 30 2092.5 9 5.8 16.4 9.2 221 1.28 18 1 Ab z, clear 0.0005 11.9 3 Ab z, fuzzy cores 100.3 0.2793 9 8 4.988 9 16 2091.3 9 2.8 27.1 0.002 214 1.42 19 1.24 9.3 214.7 0.2432 9 6 4.301 9 12 2074.0 9 1.8 35.9 20 4 z, cracked 0.003 304 OM 95 - 1 normal diorite 21 93.58 4 Ab z, tiny 0.3547 9 10 6.297 9 22 2081.1 9 3.6 6.9 0.001 267 1.78 10.2 1051 0.3072 9 6 5.291 9 14 2027.5 9 2.0 16.9 2.2 2 z, cracked 22 1.2 309 0.003 6.2 2 z, cracked 357.5 0.3189 9 8 5.541 9 20 2043.1 9 3.8 14.5 0.006 136 1.06 23 1.28 3.8 1224 0.3413 9 32 5.989 9 58 2060.6 9 4.4 9.4 24 7 z, cracked 0.01 163 OM 95 - 3 gabbro dyke 25 485.1 1 Baddeleyite 0.3166 9 14 4.766 9 23 1786.0 9 4.5 0.8 0.001 103 0.05 0.5 2099 0.3187 9 10 4.808 9 15 1790.0 9 2.5 0.4 0.7 1.29 26 1 Ab zr 0.0015 418 340.9 27 0.3403 9 14 1 Unab zr 5.240 9 22 1826.7 9 4.6 -3.9 0.004 763 1.37 21.5 a Notes: Errors are two standard deviations. Pb blank–1 pg zircon, 2 pg titanite and apatite; U blank–0.1 pg. Ab, abraded, z, zircon. Pb com , Common Pb, including blank; calculated using blank isotopic composition. ThU calculated from radiogenic 208 Pb 206 Pb ratio and 207 Pb 206 Pb age assuming concordance. Disc. = per cent discordance for the given 207 Pb 206 Pb age. Decay constants are from Jaffey et al. 1971. Fig. 6. U-Pb concordia diagram showing OM95-11 zircon analyses. The two sigma error ellipses are highlighted by circles. The line represents a regression of all data except analysis 16. Only analysis 12 was from an abraded fraction. Fig. 8. U-Pb concordia diagram showing zircon analyses from OM95-1 Diorite. The two sigma error ellipses are highlighted by circles. The upper line 2096 9 11 Ma was regressed with- out analysis 21 the only analysis from an abraded fraction. The lower line 2110 9 6 Ma is a regression through all four data. Fig. 7. U-Pb concordia diagram showing zircon analyses from OM95-28 vein wall rock. The two sigma error ellipses are highlighted by circles. The line represents a regression of all data. Analyses were from abraded fractions except for 20. Fig. 9. U-Pb concordia plot showing analyses of zircon and baddeleyite from a late mafic dike. The line is a regression through the three data. clude pyrite and chalcopyrite, and titanite is found as small subhedral zoned crystals scattered throughout the matrix. Table 3 Pb isotopic analyses for secondary minerals from hydrothermally altered rocks at Omai a Fraction Number Weight mg U ppm PbCom pg Corr. Coef. 206 Pb 204 Pb 207 Pb 204 Pb 208 Pb 204 Pb OM 95 - 2 Hornblende Diorite Colourless 5 0.050 9.5 36 0.9989 306.0 9 16.4 295.4 9 14.8 51.2 9 2.0 titanite Colourless 0.290 6.3 266 6 0.9491 157.74 9 1.05 104.01 9 0.76 32.81 9 0.15 titanite 7 Colourless 0.226 11.0 243 0.9744 236.82 9 1.78 43.30 9 0.25 179.5 9 1.3 titanite 0.553 0.16 3180 0.9781 28 15.682 Feldspar 15.356 35.55 9 0.12 9 0.036 9 0.024 0.29 31 7415 0.8606 29 17.595 Feldspar 15.491 40.30 9 0.16 9 0.040 9 0.034 OM 95 - 26 Quartz Diorite 0.175 30 1.6 3224 0.9838 17.010 15.518 Apatite 36.44 9 0.11 9 0.036 9 0.027 OM 95 - 28 Quartz Diorite 0.048 4.8 31 766 0.9338 21.430 54.30 9 0.19 16.042 Rutile, dark 9 0.038 9 0.039 0.402 6.6 5042 0.9693 Rutile, dark 25.981 16.635 32 82.70 9 0.26 9 0.039 9 0.043 16.694 0.683 24.0 30426 Rutile, yel- 0.9971 26.465 33 64.45 9 0.23 9 0.041 low 9 0.038 a Footnotes to Table 2 apply. Two fractions of plagioclase feldspar were analysed from this sample, to be used for com- mon lead corrections. Fig. 4 insert shows the Pb-Pb plot of the two points, with the Stacey and Kramers 1975 curve for reference. Analysis 28 Table 3 contains the most primitive Pb and plots just above the curve, whereas analysis 29 plots below it but shows much more evolved 206 Pb 204 Pb. The more radiogenic Pb data from other minerals plot colinearly with datum 28, but do not regress successfully with the more evolved feldspar datum 29. The feldspar fractions were picked to be as fresh as possible but slight turbid- ity was evident in many of the grains, indicating the presence of some alteration. The high U con- tent of analysis 29 Table 3 suggests that it was from an impure feldspar fraction that may not have preserved a primary Pb isotopic composition. Analysis of one fraction of brown titanite gave a data point only 0.5 discordant with a 207 Pb 206 Pb age of 2093 9 2 Ma when corrected for the initial composition of the feldspar analysis 4, Fig. 5. The brown titanite is relatively high in U and its Pb is fairly radiogenic. Three fractions of colourless titanite fragments were also analysed from this sample. These contained much lower U concentrations and had less radiogenic Pb. Using the feldspar datum to correct for common Pb, the data points are 2 – 3 discordant and show vari- able 207 Pb 206 Pb ages of 2048 9 4, 2008 9 5 and 1999 9 6 Ma Table 2. The two younger data regress within error of the feldspar datum on a Pb-Pb diagram analyses 5 and 6, Fig. 4, and also regress within error of data from hydrothermal rutile and apatite from other samples see below. The 2048 Ma titanite datum lies distinctly off this line and is probably the result of inadvertently mixing the younger hydrothermal or metamorphic titanite with older igneous titanite, which some- times forms cores of crystals. Because of the relative U concentrations, any accidental mixing of phases would bias the result disproportionately toward the age of the older component. 5 . 3 . OM 95 - 26 Altered hornblendite OM95-26 is a hornblendite with strong hy- drothermal alteration chosen from the southeast hornblende-rich margin of the pluton. It contains bladed amphibole crystals that are pseudo- morphed by fine-grained carbonate, sericite, chlo- rite and some minor plagioclase and opaque minerals including pyrite with microscopic gold inclusions, magnetite and chalcopyrite. The ma- trix consists mostly of quartz and plagioclase, usually in a granophyric intergrowth. The quartz is filled with H 2 OCO 2 fluid inclusions and the plagioclase is strongly altered to fine-grained sericite and calcite. Apatite crystals were found in the matrix, as were fine rutile needles and pleochroic haloes indicating possible zircons. Apatite was the only potentially datable min- eral that could be recovered from this sample. A fraction of well-formed, clear apatites : 250 mm long was analysed. The resulting datum analysis 30, Fig. 4 insert is non-radiogenic and plots close to the feldspar data from OM95-2. 5 . 4 . OM 95 - 4 Marginal hornblendite OM95-4 is a hornblendite taken from the west side of the pit, where field relations show that it crystallized in situ. Large, zoned and twinned hornblende blades that are partly altered to chlor- ite and actinolite make up 75 of the sample. The matrix is quartz and albite, occasionally with granophyric texture, and the sample shows mod- erate hydrothermal alteration to carbonate, epi- dote and sericite. Opaque minerals include pyrite, magnetite, chalcopyrite and bornite. Zircon was not seen in thin section. Accessory minerals in- clude large, zoned euhedral titanites, associated with magmatic hornblende that grew in situ in- ward from the intrusive contact, and large euhe- dral apatite. Both appear to be of the same generation as the hornblende. Fragments of titanite up to 150 mm in size were recovered. Dark red-brown fragments were from the cores of crystals, colourless fragments were from the edges, and mottled, medium-brown frag- ments were presumably from areas between the centres and the edges. One multi-grain fraction of each colour was analysed Table 2. Uranium contents increase with depth of colour. One data point is within error of concordia while the others are only slightly discordant, but the 207 Pb 206 Pb ages are essentially the same and average to 2094 9 1 Ma Fig. 5: 57 probability of fit. One fraction of apatite crystals was also analysed from this sample. Grains chosen for analysis were generally colourless, well-formed prisms : 300 mm long, with slightly pitted sur- faces. Correcting for initial common Pb using the feldspar datum from OM95-2 gives a 207 Pb 206 Pb age of 2089 9 13 Ma. The data point on a concor- dia diagram is several percent discordant Fig. 5. 5 . 5 . OM 95 - 11 Southeast quartz diorite Sample OM95-11 is a quartz diorite with strong hydrothermal alteration from the hanging wall of a major quartz vein system in the south end of the pit. The matrix consists of quartz and plagioclase with chlorite, sericite, carbonate and rutile as alteration minerals. Opaque minerals include pyrite and minor magnetite, with the former often containing blebs of gold 30 mm in size. Zircons were found in thin section only as very small grains 30 mm long in chlorite, and often only by their pleochroic haloes so it is unclear if the grains seen in thin section are the same generation as those recovered by mineral separation. Zircons recovered from this sample are smaller than those from the metavolcanic unit, colourless, and often cracked with cloudy interiors. Five fractions were dated, but only one was abraded due to the poor quality of the grains. The abra- sion did not improve concordance. One highly discordant datum that is not colinear with the rest was not used in the regression Fig. 6. The lead loss line gives an age of 2094 9 6 Ma, with a 50 probability of fit and a lower intercept close to zero. 5 . 6 . OM 95 - 28 Central quartz diorite OM95-28 is vein wall-rock collected in the mid- dle of the pit, in an area of high grade mineraliza- tion. It consists of plagioclase, carbonate and quartz, with minor apatite, chlorite and pyrite. The vein material directly adjacent to the sample contains visible gold. Zircons about 50 mm in size were found in thin section. OM95-28 also con- tained rutile crystals large enough to be recovered. In thin section rutile is usually found as clusters of striated, twinned crystals, dark brown to reddish brown in colour, inside or bordering the matrix quartz and plagioclase. Four fractions of colourless, slightly cracked zircons, three of which were abraded, were analysed. All data are highly discordant, with the unabraded fraction giving the most discordant datum, which is not quite collinear with the other three. The three most concordant data were used to plot a Pb-loss line. The upper intercept with concordia is 2092 9 11 Ma, with a 36 probabil- ity of fit and a near-zero lower intercept age Fig. 7. Rutile was recovered from this sample as small 0.2 – 0.25 mm bundles consisting of brown elon- gate crystals, some of which show elbow twinning. Rutile has previously been used to provide mini- mum age constraints on mineralizing or signifi- cant hydrothermal events e.g. Davis et al., 1994b. Three fractions were separated for analy- sis. The freshest looking consisted of ten clusters of euhedral dark brown crystals with shiny stri- ated faces. A second fraction consisted of a large number of similar clusters of brown crystals that were less shiny and euhedral than the first frac- tion. The third fraction consisted of anhedral yellow-brown rutile. Pb from the rutile is only moderately radiogenic. On a U-Pb concordia dia- gram, the data points are very imprecise because of the small proportion of radiogenic Pb. On a Pb-Pb diagram the three analyses 31 – 33, Table 3 define an age of 2020 9 56 Ma when regressed together with the most primitive feldspar datum Fig. 4, insert. 5 . 7 . OM 95 - 1 North diorite OM95-1, is an altered diorite from the north end of the pit. It consists mostly of sericitized plagioclase, quartz, chlorite and epidote. Opaque minerals include magnetite and pyrite, with no observable gold. The zircons found in thin section are similar to those in OM95-11 i.e. they are extremely small and were seen in thin section only when they formed a pleochroic halo in chlorite. Zircons recovered from this sample are mostly small, cloudy and cracked. Discordant data from the three unabraded fractions define a Pb-loss line with an upper intercept of 2096 + − 11 Ma Fig. 8: 13 probability of fit. The abraded fraction gives the most concordant datum but is not colin- ear with the three other data points. When re- gressed with the two most discordant points, this datum defined an age of 2110 9 6 Ma Figs. 8 and 73 probability of fit. The lower intercepts for the regressions are between 500 and 600 Ma. Abrasion of cracked and altered zircons can produce scattered or even reversely discordant data, possibly due to within-grain Pb and U mo- bility in altered domains followed by U-Pb frac- tionation when part of the grain is removed by abrasion Davis et al., 1982. Therefore, the inter- cept age determined from the three unabraded fractions is considered to be the most reliable in spite of its lower probability of fit. 5 . 8 . OM 95 - 3 Gabbro dyke Sample OM95-3 is a drill core sample of the gabbroic dyke that cuts through the Omai pluton at depth. The sample was obtained from beneath the main pit. In thin section no dateable minerals were seen, although a few pleochroic haloes were observed, indicating the possible presence of zircon. Mineral separation yielded only a few zircon and baddeleyite grains. The zircon mostly consists of cracked and altered elongate grains, whereas the baddeleyite grains are very tiny. Two zircons were analysed from this sample, a crack-free grain that was abraded prior to analysis, and a second elongate, prismatic, cracked crystal that was not abraded. One flat, fresh-looking grain of badde- leyite was also analysed. The data from the un- cracked abraded zircon and the baddeleyite Table 2 are nearly concordant, whereas the da- tum from a cracked unabraded zircon analysis 27 is reversely discordant Fig. 9. The regression of the three data points defines an age of 1794 9 4 Ma, with a 94 probability of fit. Reversely discordant data have only rarely been observed from zircon. Lack of equilibration be- tween spike and sample can produce spurious reversely discordant data, but this is unlikely to be the case with analysis 27 because its 207 Pb 206 Pb age is older than those of the near concordant data and the three data define a statistically con- sistent regression with a fairly old lower concordia intercept age. The reversely discordant grain showed a relatively high U concentration, and lower concordia intercepts near 1000 Ma are typi- cal for high U zircons from mafic rocks, probably because they begin to accumulate radiation dam- age-induced Pb loss earlier than low U zircons. It appears that this may be a real case where U loss from zircon exceeded Pb loss. Unfortunately, the few remaining zircon grains from this sample are quite altered and, since they probably underwent pronounced secondary Pb loss, they are likely to give complex discordia that would not clarify the cause of the reverse discordance.

6. Discussion