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Two pyrrhotite and two arsenopyrite crystals, taken from all three Randalls deposits, were imaged to determine (1) their gold concentrations; (2) the presence or absence of any trace element zonation; and (3) any relationships between gold and other trace elements.Below are the results.

CB37-179.5 (‘blebby’ pyrrhotite)

LA-ICP-MS mapping of one of the large pyrrhotite ‘blebs’ in Cock-Eyed Bob sample CB37-179.5 revealed internal zonation with respect to Zn, Sb, Ag, Tl, Pb, Bi, and U, in that the rim is slightly more enriched in these elements than the core (Fig. 3.10).However, Ni, Co, and Se are uniformly distributed throughout the crystal.No Au is present within the pyrrhotite structure, and no free gold appears to be associated with this pyrrhotite, either.Cobalt is roughly an order of magnitude more abundant than Ni, which is atypical of pyrrhotite elsewhere at Randalls (see below).

CB37-164.1-C (laminated pyrrhotite with free gold/electrum)

Figure 3.11 contains an image of gold-bearing banded pyrrhotite-quartz-chlorite iron formation from the same drill hole and deposit as the previous image (Fig. 3.10).The laser map was run across parts of two separate pyrrhotite bands, with a chlorite-quartz band dividing them; only the upper pyrrhotite band contained the gold.While the specific trace elements present in pyrrhotite are nearly identical between the two pyrrhotite bands (apart from obvious differences in Au, Ag, and Te), the concentration

0 2 4 6 8 10 12 14 16 18 2580 2630 2680 2730 2780 2830 R ela tiv e p ro ba bil ity N um be r Age (Ma) 1. 2. 3. 4.

Used in age calculation

Not used in age calculation

0.34 0.38 0.42 0.46 0.50 0.54 0.58 5 7 9 11 13 15 17 20 6Pb / 23 8U 207_Pb/235_U 2000 2200 2400 2600 2800

data-point error ellipses are 68.3% conf.

Mean = 2646.3±4.9 [0.18%] 95% conf. Wtd by data-pt errs only, 0 of 22 rej. MSWD = 1.14, probability = 0.29

Figure 3.9   A. U-Pb concordia plot of detrital zircons from Santa-Craze.Red symbols indicate analyses used in the age calculation; blue symbols were other zircons/analyses not used for this purpose.The four photos in the bottom right hand portion of the diagram are CL images of selected analyzed zircons.B. Histogram of zircons in SC46-163 with little to no Pb loss.Zircons with 207/206 ages younger than ~2630 Ma had high U (>500 ppm), significant Pb loss, or both.This resulted in spuriously young ages for these crystals; consequently, these were not plotted.

Figure 8.

Figure 3.10     LA-ICP-MS image from CB37-179.5 (Cock-Eyed Bob), showing trace element contents and distribution in ‘blebby’ pyrrhotite.Cobalt, Ni, and Se are uniformly distributed throughout the crystal, but other elements display preferential concentration in different zones.Zinc, Ag, Sb, Te, Tl, Pb, Bi, and U are elevated in the rim of the pyrrhotite, whereas the core is relatively depleted in these elements.

300 µm Figure 9. po po po chl sid

Figure 3.11   LA-ICP-MS image of pyrrhotite containing grains of gold/electrum (CB37-164.1-C; Cock-Eyed Bob).Pyrrhotite laminae are intergrown with chlorite-quartz laminae, along with minor siderite.There are multiple trace element associations in this sample: (1) all pyrrhotite in this image contains elevated Co, Ni, Se, Pb, and Bi; (2) Silver, Sb, and Tl, while present in both the upper and lower pyrrhotite bands, are clearly more enriched in the upper band, apparently due to the presence of free gold/ electrum; (3) Tellurium and Bi are strongly enriched around the native Au/electrum inclusions; and (4) Lead and Bi concentrations between the two pyrrhotite bands vary by a factor of at least two, with the upper band having more Pb and Bi, presumably because of the presence of gold.Cobalt ‘spikes’ are also present, but SEM reconnaissance revealed no Co-bearing phases.The cause of these spikes remains unclear.

of these elements display a bimodal distribution: the upper, gold-bearing pyrrhotite band has higher overall Co, Ni, Sb, Pb, and Bi than the lower, Au-barren band.Selenium, as in the previous image, is uniformly distributed in both pyrrhotite bands, regardless of Au content.Spikes in Co concentrations (red-white dots) are noted in the upper band, but the cause of these anomalies is unclear, since no Co- bearing phase was identified by reflected light or scanning electron microscopy.

SC44-117 (Santa-Craze arsenopyrite)

Arsenopyrite from Santa-Craze has a Co-Ni-Sb-Te-Se±Au trace element signature. Antimony, Te, and Se are enriched in the cores but depleted in the rims, while Co and Ni are depleted in the cores but enriched in the rims (Fig. 3.12).Small amounts of Au appear to be dissolved in the structure of the crystal (0.1–0.5 ppm); a gold grain may be present on the edge of the arsenopyrite (green-blue dot on image, corresponding to ~10 ppm Au).Lead and Bi are excluded from the crystal structure of arsenopyrite, concentrating on the rims or along fractures, although the light blue colors on the Bi image indicate slight enrichment in the crystal.The zonation of Co and Ni in this crystal mimics that of As and S (compare Fig. 3.12 with Fig. 3.7), whereas Sb-Te-Se zonation is slightly different.Some pyrrhotite is present in this map, which contain Co, Ni, Pb, Bi, and a little Se.

MX41-130.3B (Maxwells arsenopyrite)

Trace-element systematics in arsenopyrite at Maxwells are mostly the same as those in arsenopyrite from Santa-Craze, with some exceptions (Figs 3.12 and 3.13).The key difference between the Maxwells grain and the one from Santa-Craze is the Mo-Te zonation.Individual bands of high Mo alternate with bands of relatively high Te in this arsenopyrite.Antimony seems to follow Mo in the upper portions of the crystal (near the edges), but unlike Mo, Sb is enriched throughout the rest of the grain.Co and Ni are present in the main portion of the arsenopyrite grain, albeit at low levels (~10 ppm), but higher values occur in the barely-developed rim on the lower right-hand side of the grain.This rim is also an area of relatively low Sb (~10 ppm vs. ~100 ppm in the interior).Like arsenopyrite from Santa-Craze, Pb and Bi are mostly excluded from the crystal structure, forming concentrated zones around the edges.Pyrrhotite and chalcopyrite inclusions are also present in this grain (the high Pb areas inside arsenopyrite and high Ni zones outside arsenopyrite).Dissolved Au in the arsenopyrite structure is close to the detection limit (0.1–0.5 ppm), but free gold appears to be associated with at least one of the pyrrhotite inclusions (areas of ~10 ppm Au in the middle of the arsenopyrite).