Paragenetic and geological setting of the Starra iron oxide copper–gold deposits, Mount Isa Inlier, Queensland, Australia: constraints on IOCG deposit models

Figures & data

Figure 1. (a) Geographical location of the Mount Isa Inlier in northern Australia, highlighting the three main orogenic belts. (b) Geological map showing the main lithological units of the Eastern Fold Belt (EFB). The Starra deposits are in the southern part of the EFB (after Gibson et al., 2016).

Figure 2. (a) Geological map showing the five Starra deposits and locations of sampled drill holes outside the main ore deposits (after Murphy et al., 2017). (b) Positive anomaly of the total magnetic intensity (nT) along the Starra shear highlighting the occurrence of magnetic minerals. The barren Eastern Hematite ironstones show no significant magnetic anomaly (after Crosswell, 2014).

Figure 3. Cross-section through Starra 222 based on drill holes STD1119, STQ 1042 and to the south STQ 1091. Drill hole STQ1119 and STQ1042 both intersect the main ore body. The ore body crops out at the surface and reaches down to ∼470 m in drill hole STQ1042. At ∼650 m depth, magnetite ironstones, with minor chalcopyrite mineralisation, are intersected. Sills of intermediate to mafic composition occur from 700 m onwards and become progressively more mafic and finer-grained with depth.

Figure 4. Main lithological units from the Starra Line. (a) Unaltered metasedimentary rocks of the Staveley Formation, consisting of layered quartz and biotite (MH_STA_086, STQ1042, 617.2 m). (b) Coarse-grained intrusion I with albite, pyrite, and anhydrite overprinting the original texture (MH_STA_110, STQ1042, 794.8 m). (c) Coarse-grained intrusion I crosscut by calcite–albite–pyrite–magnetite–epidote vein (MH_STA_107, STQ1042, 782.9 m). (d) Fine-grained mafic intrusion II lacking visible alteration (MH_STA_122, STQ1042, 906.3 m). (e) Magnetite–albite–quartz–chlorite schist from the Starra footwall (MH_STA_274, STQ1099W1, 216.85 m). (f) Mineralised ironstone from Starra 222 with magnetite–quartz–hematite–chalcopyrite (MH_STA_316, STQ115, 733.9 m). (g) Brecciated meta-arenite from the Starra 276 hanging wall with calcic matrix (MH_STA_194, STQ1070, 253.5 m). Mineral abbreviations: Ab, albite; Act, actinolite; Cal, calcite; Ccp, chalcopyrite; Ep, epidote; Mag, magnetite; Py, pyrite.

Figure 5. Alteration assemblages affecting the main lithologies at the Starra deposits. The thickness of the bars indicates the relative abundance of each phase. Mineral abbreviations: Ccp, chalcopyrite; K-Fsp, K-feldspar; Wit, Wittichenite. (1)Age data for Na–Ca and Main Cu–Au stage after Duncan et al. (2011).

Figure 6. Representative drill core samples showing alteration and paragenetic relationships at the Starra Line. (a) Magnetite and biotite formed during K–Fe alterations overprinting albite–quartz assemblage associated with Na–Ca alteration. Chloritisation of biotite coincides with pyrite and chalcopyrite mineralisation. Calcite vein carrying chalcopyrite crosscuts Na–Ca and K–Fe alteration assemblages (MH_STA_091, STQ1042, 635.9 m). (b) Mineralised ironstone from Starra 222. Specular hematite is intergrown with chalcopyrite, while chloritisation of the wall rock replaces previous ironstone texture (MH_STA_041, STQ1119, 280.2 m). (c) Hematite-dominated ironstone from Starra 244 ore zone with disseminated calcite and bornite (MH_STA_175, STQ97-746, 337.5 m). (d) Barite–carbonate vein from the Starra 222 hanging wall carrying chalcocite/bornite, minor chalcopyrite and native bismuth. The wall rock consists of muscovite and K-feldspar (MH_STA_028, STD1119, 232.2 m). (e) Rounded hematite–quartz ironstone crosscut by barite veins containing minor amounts of native gold from the hanging wall to the Starra 222 main ore body (MH_STA_026, STD1119, 215.2 m). Mineral abbreviations: Ab, albite; Ank, ankerite; Bn, bornite; Brt, barite; Bt, biotite; Cal, calcite: Ccp, chalcopyrite; Cct, chalcocite; Chl, chlorite; Hem, hematite; Mag, magnetite; Py, pyrite; Qz, quartz.

Table 1. Samples shown in this study, and their geological context.

Sample ID	Map grid Northing^a	Map grid Easting^a	Elevation of drill collar (m)	Locality no.	Sample depth (m)	Type
MH_STA_026	7 599 216	445 075.70		222	215	Calcite veins
MH_STA_028	7 599 216	445 075.70		222	232	Ironstone
MH_STA_041	7 599 216	445 075.70		222	280	Ironstone
MH_STA_051	7 599 216	445 075.70		222	323	Ironstone
MH_STA_066a	7 599 283	445 160.10	364	222	399	Ironstone
MH_STA_086	7 599 283	445 160.10	364	222	608	Unaltered
MH_STA_091b	7 599 283	445 160.10	364	222	636	K–Fe alteration
MH_STA_094	7 599 283	445 160.10	364	222	660	Ironstone
MH_STA_107a	7 599 283	445 160.10	364	222	783	Intrusion I
MH_STA_110	7 599 283	445 160.10	364	222	795	Intrusion I
MH_STA_122	7 599 283	445 160.10	364	222	906	Intrusion II
MH_STA_175	7 601 390	445 758.40		244	337	Ironstone
MH_STA_194	7 604 400	446 079.90	376	276	254	Breccia
MH_STA_199	7 604 400	446 079.90	376	276	271	Ironstone
MH_STA_215	7 604 400	446 079.90	376	276	389	Ironstone
MH_STA_274	7 605 799	446 021.00	378	North	217	K–Fe alteration
MH_STA_316	7 599 225	445 268.50	353	222	734	Ironstone

^a The GPS Datum is GDA94.

Focus on samples from the Starra 222 ore body and proximal drill holes from the South and Eastern Hematite, compared with previous studies that targeted mostly Starra 251 and 276.

Figure 7. Reflected light photograph (a, c, e) and backscattered electron images (b, d, f) of intrusive units from the Starra 222 footwall. (a, b) Coarse-grained intrusion I showing rutile and ilmenite inclusions in titanite and albite overprinting actinolite (MH_STA_107a). (c) Anhydrite and pyrite reflect hydrothermal overprint of coarse-grained intrusion I (MH_STA_110). (d) Inclusions of Au–Te–Bi in pyrite from intrusion II resembles gold mineralisation in the main ore zone (MH_STA_110). (e, f) Fine-grained intrusion II consists of actinolite, oligoclase and magnetite. Ilmenite inclusion occur in titanite (MH_STA_122c). (g) Automated mineral identification and characterisation system (AMICS) map of intrusion I showing a similar region as in (c). (h) AMICS map of intrusion II highlighting mineral phases shown in (e). Mineral abbreviations: Ab, albite; Act, actinolite; Anh, anhydrite; Ap, apatite; Bt, biotite; Cal, calcite; Ccp, chalcopyrite; Chl, chlorite; Ep, epidote; Ilm, ilmenite; Krs, kaersutite; Mag, magnetite; Oli, oligoclase; Py, pyrite; Qz, quartz; Rt, rutile; Ttn, titanite.

Figure 8. Reflected light photograph (a) and backscattered electron (BSE) images (b–d) of magnetite-dominated ironstones below the Starra 222 main mineralisation. (a, b) Hematite partly replaced by magnetite in the ironstone leading to the formation of scheelite inclusions (MH_STA_091b, STQ1042, 635.9 m). (c, d) Mushketovite texture in the magnetite-dominated ironstones with scheelite inclusions (MH_STA_094, STQ1042, 660.25 m). Mineral abbreviations: Ccp, chalcopyrite; Hem, hematite; Mag, magnetite; Muk, mushketovite; Sch, scheelite.

Figure 9. Reflected light photographs (a–c) and BSE Image (d) of mineralised ironstones from Starra 276 and 222. (a) Chalcopyrite and Hem-II form, while magnetite remains intact (MH_STA_215, STQ1091, 388.9 m). (b) Relict magnetite in porous, rounded Hem-I and specular Hem-II associated with chalcopyrite precipitation (MH_STA_066a, STQ1042, 364.2 m). (c, d) Chalcopyrite overprinting pyrite and granular, porous Hem-I. Precipitation of free gold coincides with chalcopyrite ± bornite formation. Both fill pyrite cracks (MH_STA_051, STD1119, 322.9 m). Mineral abbreviations: Ccp, chalcopyrite; Chl, chlorite; Hem, hematite; Mag, magnetite; Py, pyrite; Qz, quartz.

Figure 10. Reflected light photographs (a, c, e) and BSE images (b, d, f) showing Au–Cu mineralisation at Starra 222. (a, b) Bornite replacing chalcopyrite is strongly associated with Au and Au-tellurides (MH_STA_041). (c, d) Calcite veins from the hanging wall to the Starra 222 ore zone carrying chalcocite, bornite, native bismuth and wittichenite (Cu₃BiS₃, MH_STA_028). (e, f) Free gold from porous hematite ironstone. Sulfides are absent in that assemblage (MH_STA_026). Mineral abbreviations: Bn, bornite; Brt, barite; Cal, calcite; Cav, calaverite; Ccp, chalcopyrite; Cct, chalcocite; Hem, hematite; Mag, magnetite; Ms, muscovite; Py, pyrite; Qz, quartz; Syl, sylvanite; Wit, wittichenite.

Figure 11. (a) Eigenvalue diagram showing the contributions to the first two principal components (PC) for selected elements in the whole-rock geochemistry dataset (see online data repository for complete dataset). (b) Four clusters defined by k-nearest neighbour classification separating low grade sample into clusters two and three, whereas mineralised samples are grouped into cluster one and four. (c) Principal component 2 distinguishing low Cu concentration from high Cu concentration. (d) Au vs Te diagram highlighting the close relationship between those elements.

Figure 12. (a) Reflected light photograph of hematite-dominated ironstone hosting chalcopyrite mineralisation from Starra 222. (b) Results from the principal-component analysis (PCA) highlights chemical differences between Hem-I and Hem-II. The black/white rectangle in (a) indicates the area imaged by LA-ICPMS shown in (c–k) (MH_STA_066, STQ1042, 399.4 m).

Figure 13. (a) Bornite replacing specular hematite. Both phases are overprinted by chalcopyrite. The black/white rectangle in (a) indicates the area imaged by LA-ICPMS shown in (b–g) (MH_STA_041, STD1119, 280.20 m). (h) Bornite with relict chalcopyrite carried on barite–carbonate vein above the Starra 222 ore zone. The black/white rectangle indicates the area imaged by LA-ICPMS shown in (i–n) (Sample MH_STA_028, STQ1119, 232.25 m).

Figure 14. Radiogenic lead isotopes ²⁰⁶Pb vs ²⁰⁷Pb from the hematite and bornite/chalcopyrite LA-ICPMS images shown in Figure 12 and 13. Hem-I and Hem-II have a radiogenic lead composition expected for 1600 Ma (Stacey & Kramers, 1975). Bornite and chalcopyrite from stage III and IV have a ²⁰⁶Pb/²⁰⁷Pb composition lower than the common lead trend at 1600 Ma, indicating remobilisation of ²⁰⁶Pb. Three ²⁰⁶Pb/²⁰⁷Pb trends are defined by stage III and IV sulfides.

Figure 15. Plots of ²⁰⁶Pb vs ²⁰⁷Pb from calcite. (a) Calcites from paragenetic stages II and IV have a mostly common Pb composition at 1600 Ma (Stacey & Kramers, 1975), whereas stage I, III calcites follow a radiogenic trend. (b) Calcite from the barren Eastern Hematite follow the common Pb trend, whereas calcites from the mineralised Starra line follow a radiogenic Pb and the common Pb trend.

Figure 16. Diagrams of two-stage paragenetic model for the Starra Cu–Au deposits. (a) During the first stage Ironstone lenses form along the Starra shear from a sulfur poor fluid near magnetite–hematite stability. (b) Gold–Cu mineralisation during the second stage formed via mixing of a mildly reduced fluid with an oxidised fluid. Previously formed ironstones provided pathways for fluid ascent. At deeper levels, early Hem-I is transformed to magnetite, increasing the fO₂ of the initially reduced fluid leading to Au mineralisation in the upper part.

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Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials. The geochemical data that support the findings of this study are openly available at xxx. Data for LA-ICPMS images are available from the corresponding author M.H. on request.