Abstract How and when continents grew and plate tectonics started on Earth remain poorly constrained. Most researchers apply the modern plate tectonic paradigm to problems of ancient crustal formation, but these are unsatisfactory because diagnostic criteria and actualistic plate configurations are lacking. Here, we show that 3.5–3.2 Ga continental nuclei in the Pilbara Craton, Australia, and the eastern Kaapvaal Craton, southern Africa, formed as thick volcanic plateaux built on a substrate of older continental lithosphere and did not accrete through horizontal tectonic processes. These nuclei survived because of the contemporaneous development of buoyant, non-subductable mantle roots. This plateau-type of Archean continental crust is distinct from, but complementary to, Archean gneiss terranes formed over shallowly dipping zones of intraoceanic underplating (proto-subduction) on a vigorously convecting early Earth with smaller plates and primitive plate tectonics.

Abstract Although generally considered a poor cousin of Au-rich deposits such as orogenic or epithermal deposits, a significant number of volcanic-hosted massive sulfide (VHMS) deposits are significant repositories of Au. Several of these deposits had original Au resources exceeding 8 Moz and in some recently discovered deposits Au, not base metals, is the primary economic metal. Although most Au in volcanic-hosted massive sulfide districts is hosted by massive sulfide lenses, recent discoveries, both on land and on the ocean floor, indicate that significant Au occurs outside of these lenses. In most deposits, Au has a metallogenic association with either Cu or Zn. When associated with Cu, Au is concentrated toward the base of the massive sulfide lens. Gold-rich deposits of this metallogenic assemblage commonly are associated with (metamorphosed) advanced argillic assemblages and are inferred to have formed from acidic, high-temperature (>300°C), oxidized fluids. These deposits have been equated to high-sulfidation epithermal deposits and may be detected using recently developed spectral techniques such as PIMA (Portable Infrared Mineral Analyzer) and airborne hyperspectral scanners. When associated with Zn, Au is concentrated near the top of massive sulfide lenses, in some cases in baritic zones. Gold-rich deposits of this metallogenic assemblage tend to be formed from low-temperature (200° ± 50°C) and/or near-neutral fluids as indicated by fluid inclusion studies or by alteration assemblages (e.g., K feldspar or carbonate). A small number of deposits cannot be classified into the Au-Zn or Au-Cu association. In these deposits, Au is concentrated in pyritic zones that contain relatively low amounts of base metals. Moreover, a consistent relationship with Zn or Cu is not present. Although this group is small, it includes deposits such as Horne. Mineralogically Au can occur in electrum or native gold, Au tellurides, or auriferous pyrite or arsenopyrite. In deposits of the Au-Cu association, Au tends to occur as native gold or tellurides, whereas electrum and auriferous pyrite and/or arsenopyrite is more common in the Au-Zn association. Metamorphic recrystallization tends to liberate Au held in auriferous pyrite or arsenopyrite, potentially enhancing metallurgical recoveries.

Abstract Oxygen isotope mapping in rhyolitic rocks hosting the Kidd Creek volcanic-hosted massive sulfide deposit indicates that the ores are associated with a zone of relatively low δ 18 O values (9.7-12‰). Zones of higher δ 18 O values (13-15.8‰) occur 300 to 500 m stratigraphically above ore and are associated with massive rhyolite bodies as the footwall to the orebodies. The δ 18 O values (7.2-11.3‰) of mafic rocks are lower than those of rhyolitic rocks from the strongly silicified zone immediately underlying the massive sulfide bodies. Mafic rocks with the lowest δ 18 O values (<9‰) occur in the core of a diorite sill stratigraphically above the ore zone. Hydrogen isotope mapping indicates that a zone of low δD values (<–40‰) extends at least 500 m stratigraphically below the orebodies. Most chlorite associated with chalcopyrite stringers has lower δ 18 O (2.7-4.1‰) and higher δD (-47 to -41‰) values than chlorite from metamorphic veins (δ 18 O = 5.7-7.8‰; δD = -59 to –45‰). Quartz-chlorite pairs from metamorphic veins indicate temperatures of 370° to 400°C and a metamorphic fluid composition of δ 18 O ∼ 6.7 to 7.0 per mil and δD 17 ± 8 per mil. The isotopic composition of ore-forming fluids is inferred to have been δ 18 O ∼ 3.8 ± 0.5 per mil and δD ∼ –8 ± 5 per mil. The Kidd Creek ore-forming fluids are best interpreted as evolved seawater that exchanged with 18 O-enriched country rock; it may have contained up to 20 percent magmatic hydrothermal water.

Abstract Isotopic studies covering some 200 km 2 of the Kidd Creek Volcanic Complex, within about 10 km of the giant Kidd Creek deposit, include the analysis of 395 whole-rock and quartz phenocryst samples for oxygen isotopes and 87 whole-rock samples for hydrogen isotopes. All of the rocks of the Kidd Creek Vol canic Complex are enriched in 18 O relative to fresh or even mildly altered equivalents elsewhere, comprising a range for whole rocks of δ 18 Owhole rock = 6.3 to 15.7 per mil. Mapped distribution of δ 18 O whole rock values indicates several prominent zones of lower δ 18 O whole rock values located in the footwall of the Kidd Creek mine sequence and in footwall-equivalent sequences at the Chance deposit. Other zones located elsewhere suggest widespread hydrothermal activity throughout the complex. Broadly conformable zones of relative 18 O increase in mafic and rhyolitic rocks, primarily in hanging wall-equivalent sequences, mark waning hydrothermal activity and cooling temperatures. These broad zones are not spatially associated with either the Kidd Creek mine or the Chance deposit, but they are nevertheless related to the evolving hydrothermal activity in the Kidd Creek Volcanic Complex. Isotopic alteration of the crust was the result of long-lived hydrothermal activity (possibly on the order of 10 m.y.) that continued past the period of sulfide mineralization at Kidd Creek. The zones of 18 O enrichment are, in many cases, associated with uneconomic but anomalous occurrences of Zn that may represent the manifestation of a cooling hydrothermal system still able to mobilize minor amounts of metal. The minimum oxygen isotope composition of rhyolitic magma in the Kidd Creek Volcanic Complex inferred from analyses of phenocrysts (δ 18 O quartz ) was ca. 8.5 per mil due to melting or assimilation of 18 O-enriched, possibly low-temperature altered igneous crust. Quartz phenocrysts with δ 18 O quartz values as high as 15.4 per mil indicate subsolidus exchange with the rock matrix during regional greenschist facies metamorphism. Hydrogen isotope studies indicate narrow ranges in δD values for all rock types except several rhyolite flows. A rhyolite flow in the footwall ultramafics, about 1,000 m beneath the Kidd Creek mine, has δD value vs. wt percent HO characteristics that mirror rhyolites emplaced and degassed in very shallow to surficial environments. At least 1 km of subsidence is inferred to have occurred over a short period of time, prior to mineralizing hydrothermal activity at Kidd Creek. An extensional (i.e., rifting) tectonic environment would promote both subsidence of the crust and deep penetration of seawater-derived hydrothermal fluids.