Abstract Among the Cu-Ni-PGE occurrences hosted by the footwall units of the Sudbury Igneous Complex, low-sulfide systems possess the lowest sulfide content while being significantly enriched in Pd and Pt. Although the contribution of hydrothermal processes to the formation of this type of mineralization has already been recognized, no detailed studies have previously focused on the mineralogy and zonation of mineralization-related silicate assemblages and their distinction from regional processes postdating ore formation. Here, the results of detailed alteration mapping carried out on two recently discovered low-sulfide occurrences in the footwall of the Wisner area in the North Range of the Sudbury Igneous Complex are described to address these questions. In both areas, disseminated sulfides, S-shaped sulfide veins, and extensional silicate veins trending northwest-southeast to north-northwest−south-southeast formed from hydrothermal activity driven by the heat of the Sudbury Igneous Complex. Hydrothermal vein assemblages in mafic to intermediate host rocks are dominated by actinolite whereas epidote and quartz predominate in granitic host rocks. Compositional similarity (high Ni, low K, and relatively low Mg contents) of actinolite rims of vein-filling amphiboles and actinolite in ore-bearing assemblages, coupled with anomalous PGE contents of amphibole veins and their spatial proximity to mineralized zones, suggest a direct genetic relationship between silicate veining and low-sulfide mineralization. Shear-type epidote veining postdates the Sudbury Igneous Complex-related hydrothermal alteration and slightly redistributes metals from the low-sulfide footwall ores on a local scale. Both Sudbury Igneous Complex-related and regional hydrothermal assemblages of the Wisner area show characteristics identical to similar occurrences elsewhere in the Sudbury structure.

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.

Abstract The Owl Creek gold mine, now closed, lies in the Archean Abitibi greenstone belt of the Superior structural province of the Canadian shield, 18 km northeast of the city of Timmins, Ontario. The major rock types observed at the Owl Creek mine are tholeiitic basalt, carbonaceous shale, arkosic graywacke, and mudstone. All rocks have been subjected to middle greenschist facies regional metamorphism. Most gold mined occurs in the form of inclusions in vein and wall-rock pyrite in steeply dipping, highly strained tholeiitic basalt and carbonaceous shale; however, considerable free gold was also mined from quartz veins located mainly within brittle, altered basalt. Most of the buff to gray basalt is high magnesium tholeiite that is massive to locally pillowed and intensely sheared. Basalt is intensely carbonatized (ankerite-dolomite; up to 16.7% CO 2 ) and the gray color is the result of postcarbonatization addition of chlorite and carbon. Sericite and pyrite alteration of basalt followed this addition. Most auriferous pyrite is between 0.1 and 1.0 mm and resides in basalt, where it forms 5 percent of the rock; such pyrite averages 58 ppm gold (n = 21). Carbonaceous shale contains as much as 14.8 percent amorphous carbon, and carbon content is highest adjacent sheared tholeiitic basalt. Carbonaceous shale contains fine-grained disseminated euhedral, nodular, and skeletal pyrite, in order of decreasing gold content. Local accumulations of larger pyrite nodules (up to 3 cm) and small lenses of massive pyrite (up to 2 m thick) occur close to shale-basalt contacts. Gold content of shale pyrite averages 3.63 ppm (n = 19). Nodules of diagenetic and hydrothermal origins are recognized. Although the former type contains low levels of gold (x = 1.72 ppm, n = 13), sporadic zone enrichment is not uncommon (up to 80 ppm). Hydrothermal nodules are significantly enriched in gold (x = 31 ppm, n = 4). Mudstone has a much lower amorphous carbon content (<2%) than shale; pyrite occurs as scattered euhedra (<1 mm) and as disseminations in bands up to 5 cm thick which may make up as much as 15 percent of the rock. However, both types are relatively devoid of gold (x = 1.47 ppm, n = 12). Two major exposures of arkosic graywackes and a 5-m-thick interflow unit within basalt make up the bulk of enveloping sedimentary rocks at Owl Creek. Individual beds range from centimeters to tens of meters in thickness. These are interbedded with mudstone laminae which are ripped up, indicating overturned strata which young to the south. Pyrite is present as well-preserved cubes up to 10 mm in size. Graywacke pyrite contains no appreciable quantities of gold. Widespread, diffuse, but sustained hydrothermal activity in the district led to accumulation of strongly arsenical and nickeliferous pyrite in sediments that bear low but significant levels of gold; however, later epigenetic activity induced pervasive carbonate, sericite, and chlorite alteration in tholeiites and introduced auriferous pyrite via myriad thin quartz veinlets. Development of a heterogeneous deformation zone was responsible for juxtaposing of altered, brec-ciated basalt against carbonaceous sediments and promoted increased permeability in these rocks. This deformation zone was the principal conduit for gold-rich fluids, and its development controlled auriferous wall-rock pyrite and gold-bearing quartz vein precipitation at Owl Creek. Quartz veins in the highly strained rocks exposed in the Owl Creek open pit are (1) subvertical; (2) south dipping, en echelon ten-sional; and (3) more shallowly dipping to subhorizontal. All three types of quartz veins contain ore-grade concentrations of gold. Subhorizontal quartz veins may themselves be brecciated, resulting in subvertical quartzose-breccia veins consisting of angular quartz fragments and a matrix of either chlorite and auriferous pyrite or goethite and chalcedony. The presence of nonfragmented auriferous pyrite confirms that there was a gold-precipitating event which postdated emplacement of gold-bearing quartz veins. Gold in altered basalt is most closely associated with pyrite, pyrrhotite, sphalerite, chalcopyrite, and ar-senopyrite in order of decreasing abundance; however, pyrite represents no less than 95 percent of the total sulfides present. With the exception of rare euhedral arsenopyrite, all inclusions in gold-bearing pyrite are anhedral. Gold grains occur as isolated blebs or as fracture fill in pyrite but may be found adjacent to chalcopyrite-pyrrhotite inclusions or at grain boundaries between arsenopyrite and pyrite. Gold was also observed as inclusions in arsenopyrite and other sulfarsenides. Altaite and rare gold tellurides were observed only with gold inclusions in hydrothermal pyrite nodules. Distinctive assemblages of accessory minerals in pyrite separates from basalt and shale suggest that there is no clear genetic relationship between gold in euhedral pyrite in carbonatized basalt, gold in crystalline pyrite nodules in carbonaceous shale, or gold in quartz veins. Rather, a succession of discrete hydrothermal events is envisaged. Sulfur isotope ratios of all types of pyrite were measured and permit the following observations. Ore-grade auriferous pyrite has a distinctive narrow range of S 34 S signatures (1.8-3.5%) which is independent of rock type. However, a narrow positive range is not exclusively indicative of auriferous pyrite (e.g., certain types of nodules). Weakly positive correlations between gold and S 34 S values for pyrite from basalt and graywacke (in contrast to negative correlations between gold and shale pyrite) support a direct hydrothermal source for epigenetic gold in the Owl Creek ore zone.