Abstract The Noranda camp in the southern Abitibi greenstone belt comprises over 20 volcanogenic massive sulfide deposits hosted by volcanic rocks of the 2704–2695 Ma Blake River Group. Decades of research and exploration have provided a firm understanding of the characteristics of these deposits as well as the geological controls on deposit location. Observations made on the deposits of the Noranda camp significantly contributed to the syngenetic model of massive sulfide formation and shaped the current understanding of ancient and modern sea-floor hydrothermal systems. The Horne and Quemont deposits, which are the largest deposits in the Noranda camp, are hosted by 2702 Ma felsic volcanic successions dominated by volcaniclastic rocks. The massive sulfide ores of these deposits largely formed through processes of subseafloor infiltration and replacement of the highly permeable wall rocks. Laterally extensive hydrothermal alteration halos dominated by chlorite and sericite surround the replacement ores. The Horne deposit formed in an extensional setting in a graben bounded by synvolcanic faults. Rapid extension accompanying deposit formation resulted in the upwelling of mantle-derived mafic melts and the emplacement of a thick package of mafic rocks in the stratigraphic hanging wall of the deposit. Most of the massive sulfide deposits in the Noranda camp are hosted by a 2700–2698 Ma bimodal volcanic succession that formed in a large volcanic subsidence structure to the north. The ~2,000-m-thick lava flow-dominated volcanic package is floored by the large, multiphase, synvolcanic Flavrian pluton. The deposits in this part of the Noranda camp are small (<5 million tonnes) and primarily formed as sulfide mounds on the ancient sea floor. Synvolcanic structures provided cross-stratal permeability for the hydrothermal fluids and controlled the location of volcanic vents. Thin tuffaceous units mark the sea-floor positions hosting the massive sulfide mounds within the flow-dominated volcanic succession. The concordant massive sulfide lenses overlie discordant alteration pipes composed of chlorite- and sericite-altered rocks. Contact metamorphism associated with the emplacement of the ~2690 Ma Lac Dufault pluton converted the hydrothermal alteration pipes into cordierite-anthophyllite assemblages. Recent brownfields exploration successes have demonstrated that massive sulfide discoveries are still possible in one of Canada’s most mature mining camp through three-dimensional geological modeling performed at the camp scale. Geologic target generation through computer modeling has reversed the general trend of progressively deeper exploration with time in the Noranda camp. Deep exploration currently focuses on the reevaluation of a previously uneconomic low-grade ore zone at the Horne deposit.

Abstract The La Pitarrilla Ag-Zn-Pb deposit, Mexico, is hosted by Cretaceous, Eocene, and Oligocene strata that record a complex volcano-sedimentary, structural, and hydrothermal history. Deformed Cretaceous rocks form the basement to unconformably overlying Eocene and Oligocene volcanic strata. The Eocene volcaniclastic strata were derived from arc volcanism and from the erosion of subaerial arc volcanoes, with the clastic material transported by sedimentary gravity flows and deposited into a below storm wave base basin that developed within a back-arc extensional setting. Uplift of the arc during the Eocene was accompanied by extension and voluminous silicic pyroclastic volcanism, which is manifested by ignimbrite and pyroclastic surge deposits dated at 49.8 ± 1.0 Ma. Erosion during the Eocene and early Oligocene was accompanied or followed by northeast-and north-northwest–trending faulting, the emplacement of rhyolitic and andesitic sills and dikes, and a 31.59 ± 0.52 Ma rhyolitic dome. The La Pitarrilla Ag-Zn-Pb deposit is characterized by iron oxide- and sulfide-associated mineralization, whichch10 defines a vertically stacked mineralized system centered on rhyolitic dikes and sills that constitute the feeder system for an early Oligocene volcanic center manifest by a rhyolitic dome. The sulfide-associated mineralization is rooted in the basement Cretaceous sedimentary strata and is represented by an areally restricted but vertically extensive zone of disseminated and vein-hosted Ag-Zn-Pb (-Cu-As-Sb) sulfide mineralization and strata-bound replacement mineralization within conglomerates that occur at the Cretaceous-Eocene unconformity. The sulfide mineralization extends upward into the overlying Eocene and Oligocene volcaniclastic strata and rhyolitic sills, where it abruptly grades into a laterally more extensive, supergene zone of disseminated iron oxide-associated mineralization that replaced the sulfides. The main Ag-Zn-Pb mineralization event is interpreted to have occurred during or after emplacement of the early Oligocene rhyolitic dome.

Abstract Quartz and quartz-feldspar porphyritic rhyolite (QP rhyolite) overlies a succession of bedded volcani-clastic deposits and massive sulfide lenses at the Kidd Creek mine and represents the last significant phase of felsic volcanism at the deposit. Isopach maps clearly illustrate that the QP rhyolite consists of two separate ridges, a north ridge exposed in mine workings east of the mine shafts, and a south ridge located south of the shafts. The north and south ridges have a combined volume of approximately 0.1 km 3 to the 4700 level. Aspect ratios of less than 10, the predominance of massive and flow-banded rhyolite with an intact re-crystallized spherulitic groundmass, the lack of broken phenocrysts and bedding, and the internal lobelike flow morphology indicate that the ridges are subaqueous rhyolite flows or domes and not pyro-clastic deposits. The ridgelike morphology of the QP rhyolite is best explained by eruption from two parallel fissures that are now subparallel to the northeast-trending F 1 fold axial plane. Features that support this interpretation include: (1) the parallel and linear orientation of the ridges, (2) the occurrence of distinct domes along both ridges, (3) the presence of coarse breccia flanking the domes, and (4) inferred flow direction vectors that indicate flow away from the domes. Thus, the QP rhyolite is interpreted as a fissure-fed, vent-proximal flow-dome complex that erupted from two parallel fissures located approximately 800 m apart. Passive fissure eruptions and endogenous dome growth resulted in the construction of the two ridges that extend for at least 1.9 km. Two rhyolite domes constructed above the north fissure and the one elongate dome constructed above the south are interpreted to mark specific vent sites along the ridges that had high lava extrusion rates and/or sustained eruptions. The inferred fissure beneath the north ridge may have acted as a conduit for ascending hydrothermal fluids before and after QP rhyolite volcanism and is interpreted to have controlled the location and deposition of the South orebody. Hydrothermal activity continued after QP rhyolite volcanism, as indicated by patchy to pervasive alteration, primarily sericitization, silicification, and feldspar destruction, as well as sphalerite mineralization within the QP rhyolite and lesser alteration in basalts above the north ridge. The QP rhyolite was emplaced at the end of hydrothermal activity; structures which controlled its emplacement may have controlled the location of underlying massive sulfide deposits. The QP rhyolite may be geochemically distinguished from underlying rhyolites. This may prove to be important, since QP rhy-olite volcanism marks the end of hydrothermal activity at Kidd Creek and exploration in, and outside of, the mine area should be focused on strata below the QP rhyolite or time-stratigraphic equivalent units. The Southwest orebody, initially interpreted to occur within the QP rhyolite, is now interpreted to occur within underlying volcaniclastic rocks of the Middle member.

Abstract At the Kidd Creek mine, massive and stringer sulfide orebodies occur within a felsic volcanic center containing both preore and postore rhyolites. These rhyolites have undergone extensive hydrothermal metasomatism, but the rare earth and high field strength elements have remained relatively immobile, except in the footwall stringer zone. Comparisons based on rare earth elements and high field strength elements indicate that least altered Kidd Creek rhyolites have a close geochemical affinity to the felsic igneous products of anomalous midocean ridge spreading centers such as the axial rift zones of Iceland, the Galapagos spreading center, and the mid-Atlantic ridge at 45° N. Rhyolites erupted from the central volcanoes Askja and Krafla in Iceland’s eastern axial rift zone are particularly close chemical analogues of the Kidd Creek rhyolites. Other similarities between Iceland’s axial rift zones and Kidd Creek include fissure-controlled eruptions, incompatible element-enriched tholeiitic basalts and gabbros, lack of terrigenous sedimentation, absence of a continental chemical component in igneous rocks, and the presence of Mg-rich volcanic rocks (picrite in Iceland, spinifex-textured komatiites at Kidd Creek). The strong similarity to Iceland suggests that the Kidd Creek rhyolites were derived by partial melting of juvenile crust, probably hydrated basalt, in response to subsidence and a steep geothermal gradient, possibly related to an upwelling mantle plume. Geochemical and volcanological similarities make it reasonable to speculate that many of the processes occurring in the central volcanoes of Iceland’s axial rift zones also occurred at Kidd Creek. The most important of these, with respect to volcanogenic massive sulfide formation, are the coupling of anomalous heat flow and subsidence in an oceanic environment. The high-temperature hydrothermal convection system that formed the Kidd Creek deposit was driven by heat derived from the mantle, either directly or by intrusion of mantle-derived magmas. Ultramafic rocks in the footwall to the Kidd Creek formation are a manifestation of high mantle temperatures. Faults bounding subsidence structures, either rift-related grabens or calderas, provided cross stratal permeability essential for focused hydrothermal discharge. Fluid discharge into volcaniclastic debris flooring the subsidence structure promoted sulfide deposition beneath the sea floor, thereby maximizing metal retention; low accumulation rates of volcanic material favored a long-lived hydrothermal system. An additional process in the formation of the Kidd Creek ore-bodies, and some other volcanogenic massive sulfide deposits in bimodal successions, may have been a shift in the location of volcanism resulting from the introduction of siliceous magma into a mafic-ultra-mafic magmatic system.

Abstract The volcanogenic massive sulfide ores of the Kidd Creek mine occur within a succession of rhyolites and rhyolitic volcaniclastic rocks lying stratigraphically above a komatiite-bearing ultramafic succession and below basaltic rocks. Eleven rhyolite samples from the Kidd Creek mine area yield a regression age of 2733 ± 260 Ma, within error of previously determined U-Pb ages of zircons in Kidd Creek rhyolites which range from 2717 ± 2 to 2710.5 ± 1.1 Ma. With the exception of two intensely chloritized and seric-itized footwall samples, e Nd (2712 Ma) values range from 1.1 to 4.2 and show no systematic variation with height through the stratigraphic succession. These values bracket the depleted mantle curve and do not suggest contamination by older crust. The Kidd Creek e Nd values are similar to those obtained for igneous rocks of similar age elsewhere in the southern Superior province and are consistent with the eruption of the Kidd Creek rhyolites in an oceanic environment dominated by rocks of recent mantle derivation. Postmagmatic disturbance of the Sm-Nd isotope system is recorded in two intensely altered, light REE-depleted samples from the footwall rhyolitic succession, an Fe-rich chloritite and a sericitite, which return anomalously low € Nd (2712 Ma) values of -3.6 and –5.1.