Abstract The supergiant Chuquicamata porphyry Cu-Mo deposit in northern Chile is truncated on its west side by a N-S-trending regional fault (the West fault), juxtaposing its ore to a relatively barren granodiorite (Fortuna Igneous Complex). There has been much speculation about the fate of, and extensive exploration for, the “missing half” of the deposit. It has been proposed that the west side of the fault hides the ore at depth, or that it was uplifted and the ore eroded; however, regional geologic mapping suggests that the West fault had a postore left-lateral strike-slip displacement of ca. 35 km. Accordingly, exploration, so far unsuccessful, has been focused in an area 35 km south near the Loa River and the city of Calama. In 1989, the Mina Ministro Hales (MMH) deposit was unexpectedly discovered west of the fault, under thick gravels, only 7 km south of the main mine. A previous study at MMH had suggested that mineralization was as old as 39 Ma, hence its ores were correlated with deposits of that age near Calama. Our recent U-Pb and Re-Os dating indicates that MMH mineralization was formed between 35 and 31 Ma, thus concurrently with Chuqui. The geochemistry of host Triassic and Eocene porphyry intrusions, ore mineralogy, and common Pb isotope ratios of hypogene sulfides at MMH and Chuqui proper are indistinguishable. Fluid inclusion data for paragenetically early porphyry assemblages at MMH closely mimic T h -salinity data from earlier studies at Chuqui, showing little or no evidence of boiling but indicating widely fluctuating confining pressures, compatible with hydraulic fracturing and fault movement during and after mineralization at a minimum initial lithostatic constraint of 5- to 8-km depth. We propose that MMH is a sheared-off portion of Chuqui, wedged in a fault cymoid loop and spared the full 35-km displacement of the West fault.

Abstract The diamondiferous mantle root beneath the Lac de Gras area in the central Slave craton (northwestern Canada) is now one of the world’s best characterized lithospheric mantle sections with regard to geochemical and thermophysical information. Its most spectacular feature is its marked stratification. An ultradepleted, highly oxidized, shallow layer to ~150-km depth consists dominantly of granoblastic harzburgite with olivine Mg# (100Mg/(Mg + Fe)) of 92 to 94. Garnet in this layer has very low TiO 2 and Zr contents (avg 0.05 wt % and 9.5 ppm, respectively), and strongly sinusoidal REE patterns. The shallow stratum, which exhibits enhanced conductivity, is separated from a less conductive, less depleted, and less oxidized, dominantly lherzolitic layer by a seismically and geochemically imaged sharp discontinuity. The deep stratum features an olivine Mg# of 91 to 92, average garnet TiO 2 of 0.26 wt % and Zr of 33.4 ppm, and includes porphyroclastic varieties. It reaches the thermal and mechanical lithosphere-asthenosphere boundary at ~220 km. The ultradepletion of the shallow subcontinental lithospheric mantle (SCLM) may require an origin by polybaric melting at excess mantle potential temperature, accompanied by shallow plate interactions at ca 3.5 Ga, while the mild depletion of the deep SCLM could be explained by ca 3.3 Ga subcretion of upwelling mantle after a short melting interval, or may alternatively have formed by accretionary processes. The formation of both strata produced peridotitic diamond populations and was followed by amalgamation of the ancient (4.0−2.8 Ga) cratonic core with juvenile (2.7 Ga) domains in the eastern Slave craton, which may have led to the incorporation of coeval lithospheric mantle portions. Ancient (Proterozoic or Archean) interaction with fractionated high field strength element (HFSE)-poor fluids is inferred from garnet with strongly sinusoidal REE patterns and peridotite minerals with radiogenic Sr and Hf, but unradiogenic Nd, and was accompanied by diamond formation. A <350 Ma metasomatic event by an evolving and increasingly fractionated kimberlite liquid is indicated by a spectrum of garnet REE patterns from “normal” light rare earth element (LREE) depleted to increasingly sinusoidal, and by relatively constant 143 Nd/ 144 Nd at variable Sm/Nd. The unfractionated melt was either destructive to diamonds or at least not conducive to diamond growth, whereas the signature of fractionated melt is identified in diamondiferous peridotite xenoliths and may have produced some fibrous overgrowth on diamonds. Short-lived accretionary processes at the western craton margin are reflected in ca 1.85 Ga eclogite xenoliths that make up <5% of the lithosphere column beneath Lac de Gras and that have trace element systematics consistent with gabbroic-or boninite-like precursors. They are concentrated just below the intralithospheric discontinuity and their mode of emplacement into substantially older mantle lithosphere remains enigmatic. Some eclogitic diamonds were likely generated during the metasomatic episodes identified in peridotite samples. However, the accretion itself produced a disproportionately high (relative to the absolute eclogite/peridotite ratio) abundance of sulfide-included and perhaps also other diamonds and eventually helped to conserve the diamondiferous mantle root beneath the central Slave craton.