LabVIEW

Paringa interflow shale nodule GMSP-003. This nodule is composed of at least four concentric pyrite-matrix or pyrite-only zones (Fig. 5.12). The matrix to the nodule is heterogeneous; minerals in these zones include quartz, white mica, chlorite, and Fe-carbonate. The innermost pyrite is characterized by fine ‘spindle’-shaped pyrite crystals intergrown with mica and lesser carbonate. Enveloping this irregular, sausage-shaped area is a zone of pyrite-poor, carbonate-rich material that is itself overgrown by a thick (~2 mm) rim of almost pure pyrite (small amounts of chalcopyrite and sphalerite also occur in the outermost rim, along with rare silicates and carbonates). The pyrite in the outermost rim is bladed (cf. Fig. 5.6H).

One-fourth of this nodule, encompassing all visible growth zones, was mapped via LA-ICP-MS; results are shown in Figure 5.12. The most striking feature of GMSP-003 is the well-defined and concentric nature of the growth zones, to which the trace elements correspond: Cobalt, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Au, Tl, Pb, and Bi all co-vary with each other in the different pyrite types, with the exception of the pyrites in the relatively pyrite-poor zone inside the thick pyrite rim; these are apparently more enriched in Se and less enriched in all other trace elements than the other pyrite types (Fig. 5.13). Key trace elements include Au (3-4 ppm average, with small electrum inclusions within the inner pyrite core and outer pyrite rim), Te (30-40 ppm; ~10 times higher concentration than gold), Ag (30-40 ppm, same as Te), Sb (>1,000 ppm in the core of the nodule), Tl (maximum >50 ppm in the rim), Pb (>1,000 ppm in the core), and Bi (maximum 20-30 ppm). Abundant chalcopyrite and sphalerite inclusions are present, as evidenced by the high Cu and Zn zones (Fig. 5.12); in general, where these are plentiful, pyrite is less common.

Paringa interflow shale nodule GMSP-001. Like GMSP-003, GMSP-001 has an exterior rim of radiating pyrite, but this rim is much thinner than that of GMSP-003, and the interior growth zones of GMSP-001 are not as well-defined as in GMSP-003 (Fig. 5.13). Pyrite in the innermost zones has the ‘spindle’ shape seen in GMSP-003, but in the outer zones euhedral to subhedral pyrite is prevalent. The matrix to the innermost nodule is largely iron carbonate, with additional fine-grained silicates, especially in one small zone near the center of the nodule. This domain appears to post-date the thin rim of pyrite in the middle. On either end of the nodule, pressure shadows have developed, which are filled by sulfides (pyrite, chalcopyrite, and sphalerite) on one end.

Approximately half of this Paringa interflow shale nodule was imaged, and like GMSP-003, it has significant enrichments in Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Au, Hg, Tl, Pb, and Bi, all of which are covariant (Fig. 5.13). Also like GMSP-003, there are two zones of high Au-Te-Ag-As-Sb-Hg-Tl-Pb- Bi, separated by a ‘moat’ of pyrite and matrix that contain lower amounts of these elements but higher Se (100 ppm in middle vs. 50 ppm in core and rim). Molybdenum shows the same bimodal concentration differences as in GMSP-003 (i.e., high values in the rim, low values in the core), along with Tl and Hg, although this behavior is masked somewhat in the latter element due to the plethora of sphalerite inclusions, particularly in the rim zone (Fig. 5.13). Chalcopyrite is also abundant within the nodule, where it is intergrown with pyrite.

Of particular interest in this nodule is the thin but somewhat continuous zone of very high Ni (>100,000 ppm) and Sb (>200,000 ppm) concentrations. SEM investigations reveal the presence of ullmannite (NiSbS) intergrown with pyrite and chalcopyrite in these areas (Fig. 5.14A-D). Ullmannite is a sulfosalt that forms a series with Willyamite ([Co, Ni]SbS) and is also related to the minerals gersdorffite (NiAsS), paracostibite (CoSbS), and cobaltite (CoAsS). Selenium and As are also enriched in ullmannite from GMSP-001 relative to the nodule pyrite. Ullmannite has been reported from the Golden Mile previously by Golding (1978), specifically in Green Leader ore.

Pyrite in the GMSP-001 pressure shadow is enriched in Co (which is also zoned), Ni, and Se, but depleted in all other trace elements. Chalcopyrite in the pressure shadow contains low-level Ag (~10

Fig. 5.13     Trace element image of GMSP-001. As this nodule is also from the Paringa interflow shale (like GMSP-003), the trace element contents and patterns are much the same, but there are a few differences. The biggest change between this nodule and GMSP-003 (Fig. 5.12) is the presence of ullmannite (NiSbS) in a crudely ovoid ring within the nodule. This nodule also contains more sphalerite inclusions, and has well-developed pressure shadows containing pyrite, chalcopyrite, and sphalerite. The pyrite in the pressure shadow has very different trace element characteristics from the nodular pyrite. Field of view is approximately 1.5 cm.

Fig. 5.14     Ullmannite (NiSbS) in GMSP-001. This sulfosalt is intergrown with pyrite, chalcopyrite, and the carbonate-silicate nodule matrix. In certain cases (see ‘D’), the ullmanite appears to be replacing the pyrite euhedra/subhedra.

ppm), Sn (~50 ppm), and Au (~0.1 ppm).

Paringa interflow shale nodule GMSP-008. GMSP-008 (Fig. 5.15A-C) is not as complex as GMSP-003 or GMSP-001 (with respect to pyrite), but it nonetheless has internal and external diversity that the other two lack. On the rim of the nodule, a clear distinction between nodule pyrite and later coarse-grained pyrite is evident by the slight change in color and shape of the grains (Fig. 5.15B-C). Intergrown with this later pyrite are other sulfides (e.g., chalcopyrite, sphalerite, and rare arsenopyrite) and electrum (Fig. 5.15C). Approximately one-fourth of this nodule, plus a large portion of the pressure shadow and surrounding shale matrix, was imaged. This nodule is very much like GMSP-003 and GMSP-001 in trace element geochemistry, but the interior zone of trace element-rich pyrite in -003 and -001 is not present here. However, the outer rim is well developed, along with several grains of free gold associated with a later generation of Ni-rich pyrite in the pressure shadow; these gold grains are not present in GMSP-001, which, like GMSP-008, has pressure shadows containing a later generation of pyrite. Other sulfide phases in this nodule include chalcopyrite and sphalerite. Chalcopyrite is enriched in Se (50 ppm), and Sn (~100 ppm) relative to the nodular pyrite, whereas the pressure shadow pyrite is still more enriched in Se (~100 ppm). Mercury is present in the sphalerite, as well as in the nodule pyrite, but at lower levels. The shale matrix contains a high proportion of the pyrite-bound trace elements due to the presence of very small (<10 µm) pyrite crystals.

Black Flag nodule BFB-004A. As is shown here and below in the next image, there is a clear difference in trace element concentrations between nodules in the Paringa Basalt interflow shale and those in the Black Flag Group and Kapai Slate. A portion of the rim and ‘core’ of a Black Flag nodule was imaged (Fig. 5.16), and as with the previous nodules, this Black Flag nodule (particularly the rim zone) contains high Co (1,000-2,000 ppm), Ni (~1,000 ppm), Cu (~1,000 ppm), Zn (~50 ppm), As (500-1,000 ppm), Se (50-100 ppm), Mo (1-5 ppm), Ag (~5 ppm), Sn (2-3 ppm), Sb (~100 ppm), Te (20-30 ppm), Au (1-2

A

B

Fig. 5.15     LA-ICP-MS imaging, GMSP-008. A. The investigated pyrite nodule. Field of view is 3 cm. B. Photomicrograph showing the association of electrum with the later pyrite in the pressure shadows. Where the nodule rim has been fractured, electrum can be present (as in ‘C’). Like the previous Paringa interflow shale nodules (Figs. 5.12 and 5.13), the outermost rim of this nodule is enriched in a large suite of trace elements (including Au, at around 5 ppm). However, the innermost zone of trace element-rich pyrite is not present here. The amount of chalcopyrite in the nodule is greater than GMSP-003 (Fig. 5.12) and GMSP-001 (Fig. 5.13); there is also more chalcopyrite in the pressure shadow. Electrum inclusions pepper the pressure-shadow pyrite; lastly, the fine-grained ‘matrix’ pyrite has a geochemical suite similar to that of the nodule rim.

A

Fig. 5.16     LA-ICP-MS image of BFB-004A, a Black Flag pyrite nodule from the Super Pit. Trace elements (including Au) in BFB-004A display coherent patterns, as in all previous nodules (Figs. 5.12-5.15), and there is a difference in geochemistry between the rim and the core pyrites. Selenium, Ni, Co, and Mo are enriched in the (presumably) later pyrite. Field of view is 2 cm.

Fig. 5.17     LA-ICP-MS trace element imaging of GMKS-002, from the Kapai Slate. Disregarding the cross-cutting chalcopyrite- sphalerite vein, all trace elements present in this nodule display coherent patterns. The highest value of Au occurs in the core of the nodule (≤1 ppm), and As, Ag, Sb, Te, Tl, Pb, and Bi are likewise enriched with Au in the core.

ppm), Hg (~7,000 counts per second), Tl (0.5-1 ppm), Pb (100-200 ppm), and Bi (10-20 ppm). However, a key difference between this nodule and the previous ones is the far lower Au (≤1 ppm) content, although this element is co-variant with the others in the structure of BFB-004A, as with the previous nodules.

A second generation of pyrite is also present in this nodule; like the later pyrite generations in the Paringa interflow shale pyrite nodule images, the type here is enriched in Se (~1,000 ppm), Co (~5,000 ppm), and Ni (~5,000 ppm), but also has appreciable Bi (~50 ppm) and is relatively depleted in all other trace elements.

Kapai Slate nodule GMKS-002. One-fourth of this nodule was LA-ICP-MS imaged, covering all zones within the nodule plus the rim of fine-grained pyrite enveloping the nodule and some of the coarser pyrites in the matrix (Fig. 5.17). The nodule is characterized by a core enriched in Au-As-Ag- Te-Sb-Tl-Pb-Bi (Au ≤1 ppm), surrounded by a region relatively depleted in these same elements (Au ≤0.5 ppm). On the left side of the image, the nodule is cut by a chalcopyrite-sphalerite vein; chalcopyrite is also disseminated throughout the nodule as intergrowths with the nodular pyrite. The latter form of chalcopyrite commonly has minor intergrown cassiterite (see Fig. 5.4C). Immediately surrounding the nodule is a zone of tightly clustered, fine-grained (<1 µm) pyrite euhedra, and the trace element character of these pyrites is similar to the nodule core pyrite. The rim of the nodule is depleted in all trace elements relative to the core, except for Co (~500 ppm), Ni (~500 ppm), Se (~50 ppm), and perhaps Tl (~30 ppm).

5.5.4  LA-ICP-MS geochemical investigation - spot/line data on proximal and