Reply to Discussion: The timing of gold mineralization across the eastern Yilgarn Craton using U–Pb geochronology of hydrothermal phosphate minerals

This is a reply to the discussion article doi: 10.​1007/​s00126-015-0609-

Silver-rich telluride mineralization at Mount Charlotte and Au-Ag zonation in the giant Golden Mile deposit, Kalgoorlie, Western Australia

The gold deposits at Kalgoorlie in the 2. 7-Ga Eastern Goldfields Province of the Yilgarn Craton, Western Australia, occur adjacent to the D2 Golden Mile Fault over a strike of 8 km within a district-scale zone marked by porphyry dykes and chloritic alteration. The late Golden Pike Fault separates the older (D2) shear zone system of the Golden Mile (1,500 t Au) in the southeast from the younger (D4) quartz vein stockworks at Mt Charlotte (126 t Au) in the northwest. Both deposits occur in the Golden Mile Dolerite sill and display inner sericite-ankerite alteration and early-stage gold-pyrite mineralization replacing the wall rocks. Late-stage tellurides account for 20 % of the total gold in the first, but for &1 % in the second deposit. In the Golden Mile, the main telluride assemblage is coloradoite + native gold (898-972 fine) + calaverite + petzite ± krennerite. Telluride-rich ore (>30 g/t Au) is characterized by Au/Ag = 2. 54 and As/Sb = 2. 6-30, the latter ratio caused by arsenical pyrite. Golden Mile-type D2 lodes occur northwest of the Golden Pike Fault, but the Hidden Secret orebody, the only telluride bonanza mined (10,815 t at 44 g/t Au), was unusually rich in silver (Au/Ag = 0. 12-0. 35) due to abundant hessite. We describe another array of silver-rich D2 shear zones which are part of the Golden Mile Fault exposed on the Mt Charlotte mine 22 level. They are filled with crack-seal and pinch-and-swell quartz-carbonate veins and are surrounded by early-stage pyrite + pyrrhotite disseminated in a sericite-ankerite zone more than 6 m wide. Gold grade (0. 5-0. 8 g/t) varies little across the zone, but Au/Ag (0. 37-2. 40) and As/Sb (1. 54-13. 9) increase away from the veins. Late-stage telluride mineralization (23 g/t Au) sampled in one vein has a much lower Au/Ag (0. 13) and As/Sb (0. 48) and comprises scheelite, pyrite, native gold (830-854 fine), hessite, and minor pyrrhotite, altaite, bournonite, and boulangerite. Assuming 250-300 °C, gold-hessite compositions indicate a fluid log fTe2 of -11. 5 to -10, values well below the stability of calaverite. The absence of calaverite and the dominance of hessite in the D2 lodes of the Mt Charlotte area point to a kilometer-scale mineral and Au/Ag zonation along the Golden Mile master fault, which is attributed to a lateral decrease in peak tellurium fugacity of the late-stage hydrothermal fluid. The As/Sb ratio may be similarly zoned to lower values at the periphery. The D4 gold-quartz veins constituting the Mt Charlotte orebodies represent a younger hydrothermal system, which did not contribute to metal zonation in the older one. © 2012 Springer-Verlag

The timing of gold mineralization across the eastern Yilgarn craton using U–Pb geochronology of hydrothermal phosphate minerals

The highly mineralized Eastern Goldfields of the eastern Yilgarn craton is an amalgamation of dominantly Neoarchaean granitoid-greenstone terranes and domains that record a history of early rifting, followed by westward directed collision with initial arc formation, collision and clastic basin formation, and final accretion to the western Yilgarn proto-craton between 2.66 and 2.60 billion years ago. The gold deposits that define this region as a world-class gold province are the product of orogenic processes that operated during accretion late in the tectonic history, after initial compressional deformation (D1–D2) and the majority of granitoid magmatism. Minor gold was also deposited throughout the entire tectonic history in magmatic-hydrothermal-related systems. However, such mineralization (mostly < 0.3 g/t gold) is nowhere economic unless it overprints, or is overprinted by, much higher-grade orogenic gold lodes.Robust SHRIMP U–Pb geochronology of gold-related hydrothermal xenotime and monazite supports structural studies that gold mineralization occurred during late transpressional events (D3–D4), shortly before cratonization. However, westward migration of collision and accretion produced a complementary diachroneity in the timing of gold mineralization of 5 to 20 m.y. between c. 2.65 Ma in the east (including Laverton District, Kurnalpi Terrane) to c. 2.63 Ma in the west (including Kalgoorlie Terrane) across the eastern part of the craton. The robust geochronology refutes previous suggestions that significant gold mineralization events extended from DE to D4 in the evolution of the orogen and that the Kalgoorlie gold deposits formed over a period of 45 m.y. The crustal continuum model is applicable within terranes where orogenic gold depositional events were penecontemporaneous, but must be modified to account for diachroneity of orogenic events and gold mineralization across the Eastern Goldfields

Neoarchean orogenic, magmatic and hydrothermal events in the Kalgoorlie-Kambalda area, Western Australia: constraints on gold mineralization in the Boulder Lefroy-Golden Mile fault system

The Boulder Lefroy-Golden Mile (BLF-GMF) fault system is the most intensely mineralized structure (>2150 t Au to 2015) in the Archean Yilgarn Craton, Western Australia. The fault system links the Kalgoorlie and Kambalda mining districts in the Eastern Goldfields Province, a continental-margin orogen subdivided into the western Kalgoorlie ensialic rift and the eastern Kurnalpi volcanic arc. After rifting, the 2.73–2.66 Ga greenstone-greywacke succession in the Kalgoorlie-Kambalda area underwent five phases of orogenic deformation, predominantly during ENE-WSW shortening: D1 upright folding at ca. 2680 Ma, D2 sinistral strike-slip faulting at 2678–2663 Ma, D3 folding of late conglomerate-turbidite successions at 2665–2655 Ma, D4 dextral strike-slip faulting at 2655–2640 Ma and D5 east-northeast-striking normal faulting. Regional prehnite-pumpellyite to greenschist facies burial metamorphism took place during D1 and D3 crustal thickening, and amphibolite facies aureoles formed around granite batholiths during and after D3 at 400 ± 100 MPa pressure. The D2 BLF offsets D1 folds by 12 km SW-side south and contains a porphyry dyke (2676 ± 7 Ma) boudinaged by transtensional oblique-slip along a line pitching 21° southeast. The BLF is linked by transverse D2 thrusts to other sinistral faults recording strike-slip until 2663 ± 7 Ma. Late D2 strike-slip movement alternated with early D3 shortening. D3 thrusts accommodated strain in fault blocks of rigid mafic-ultramafic volcanic rocks consolidated during D1, while the sedimentary rocks in D3 synclines were foliated at high strain.Biotite-sericite alteration and gold-pyrite mineralization in the BLF-GMF system took place at 11 ± 4 km burial depth in faults active during D2 and D3. The Golden Mile (1708 t Au) and other deposits are associated with stocks and dykes of high-Mg monzodiorite-tonalite porphyry, part of a late-orogenic (2665–2645 Ma) mantle-derived suite of adakitic affinity. Hornblende and apatite compositions indicate that these intrusions are characterized by high water contents (5–6 wt% H2O in melt), by high oxidation states (dNNO +1.0 to +2.4 log units) and by igneous anhydrite. Some stocks contain pervasive anhydrite-pyrite mineralization of low gold grade (0.4 g/t). Biotite-sericite-pyrite ore bodies such as those at Kanowna Belle (140 t Au) also replace faulted metamorphic rocks above batholith domes cored by plutons of the monzodiorite suite. The D4 strike-slip faults are barren at Kambalda but control gold quartz-vein ore at Kalgoorlie (2651 ± 9 Ma), and Au-Ag breccia ore at Black Flag (<2648 ± 6 Ma)