Atoll Archaeology in the Pacific

As islands formed by biogenic agents (unconsolidated carbonate sediments deposited by waves on reef platforms), atolls and table reefs, or low coral islands without lagoons, can be regarded as especially constraining habitats for human settlement. The challenges faced by people, both past and present, include low soil fertility, absence of perennial surface freshwater, and extreme vulnerability to flooding by stormsurge and sea-level rise due to low elevation of the highly fragmented landmass, only a few meters above mean sea level. There are about 300 atolls and low coral islands in the Pacific and thousands of individual islets (motu). Several archipelagoes are dominated by these limestone islands, such as the Tuamotus, the Central and Eastern Carolines, the Marshall Islands, Kiribati, and Tuvalu. Because of their small size, limited and at time fluctuating resources, and relative isolation, atolls are often perceived as marginal environments. While some human communities did not endure (Di Piazza and Pearthree 2001), a remarkable number were sustainable for centuries, taking advantage of opportunities many atolls provided and indicating a long history of resilience to environmental variability (Fitzpatrick et al. 2016). Despite these achievements, atolls have received relatively little coverage by Pacific archaeologist

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)