Abstract Some sediment-hosted base metal deposits, specifically, the clastic-dominated Zn-Pb deposits, carbonatehosted Mississippi Valley-type (MVT) deposits, sedimentary rock-hosted stratiform copper deposits, and carbonate-hosted polymetallic (“Kipushi-type”) deposits, are or have been important sources of critical elements including Co, Ga, Ge, PGEs, and Re. Cobalt is noted in only a few clastic-dominated and MVT deposits, whereas sedimentary rock-hosted stratiform copper deposits are major producers. Gallium occurs in sphalerite from clastic-dominated and MVT deposits. Little is reported of germanium in clastic-dominated deposits; it is more commonly noted in MVT deposits (up to 4,900 ppm within sphalerite) and has been produced from carbonate-hosted polymetallic deposits (Kipushi, Tsumeb). Indium is known to be elevated in sphalerite and zinc concentrates from some MVT and clastic-dominated deposits, produced from Rammelsberg and reported from Sullivan, Red Dog, Tri-State, Viburnum Trend, Lisheen, San Vincente, and Shalipayco. Platinum and palladium have been produced from sedimentary rock-hosted stratiform copper deposits in the Polish Kupferschiefer. Sedimentary rock-hosted stratiform copper deposits in the Chu-Sarysu basin are known to have produced rhenium. Although trace element concentrations in these types of sediment-hosted ores are poorly characterized in general, available data suggest that there may be economically important concentrations of critical elements yet to be recognized.

Abstract Despite recent commercial interest in possible mining of sea-floor massive sulfide (SMS) deposits, there has been a great reluctance to attempt any estimate of their global abundance, owing to the limited exploration of the oceans and the general lack of knowledge of the deposits. The need for such an assessment is now more urgent, as a number of leading companies and international consortia have begun to invest in intensive exploration campaigns for SMS, and governments and other organizations have begun to establish the legal framework for sea-floor exploration and exploitation of mineral resources in territorial and international waters. A growing database of global SMS occurrences is beginning to provide clues to the likely distribution, size, and grade of the deposits. More than 300 sites of sea-floor hydrothermal activity and associated mineralization are now known on the ocean floor; about 200 of these are sites of confirmed high-temperature venting (black smokers) and associated polymetallic sulfide deposits. They occur primarily at mid-ocean ridges (65%) but also in back-arc basins (22%) and on submarine volcanic arcs (12%). More than 3,800 samples have been collected from 95 of the best studied deposits, and preliminary estimates of the sizes of the deposits have been made at 62 sites. The total amount of massive sulfide contained in the known deposits is estimated to be ˜50 million tons (Mt); the top 10 percent of deposits (≥2 Mt) contain about 35 Mt of massive sulfide or about 70 percent of the total. The largest deposits, excluding the Atlantis II Deep in the Red Sea, are on the order of 10 Mt in size. However, the median deposit size is only about 70,000 t. The average concentrations of metals based on analyses of surface samples are 3.6 wt percent Cu, 7.9 wt percent Zn, 0.4 wt percent Pb, 1.7 g/t Au, and 115 g/t Ag, although comparisons with drill core indicate that grades can be significantly lower below the sea floor in many, but not all, deposits. A number of independent datasets, including global heat flow, circulation models for high-temperature vent fluids, geochemical budgets of the oceans, and the incidence of hydrothermal plumes, all arrive at similar estimates of ˜1,000 active vent sites along the mid-ocean ridges. As many as 500 additional vent sites may be located on submarine volcanic arcs and in back-arc basins, for a total of ˜1,500 sites. However, an analysis of the spatial distribution of known deposits, both on the mid-ocean ridges and in subduction-related environments, suggests that this is likely a maximum and that the total number of significant SMS occurrences along the neovolcanic zones of the world’s oceans is closer to ˜900. If the size distribution of the known deposits is representative of what remains to be discovered, then the total tonnage of SMS, excluding the Red Sea, is expected to be on the order of 600 Mt (˜1,000 deposits with a minimum size of 100 t and a maximum size of 10 Mt). The total contained metal would be about 30 Mt, based on a grade of 5 wt percent combined Zn + Cu + Pb. This estimate is similar to the total discovered metal in Cenozoic VMS deposits on land. However, it does not include long extinct deposits that may be located far off-axis. If present-day rates of massive sulfide formation on the mid-ocean ridges and back-arc spreading centers are extrapolated to older crust, then significant tonnages of massive sulfide may be expected beneath off-axis sediments. In contrast to land-based exploration, where larger deposits are commonly discovered early in the exploration history of a VMS district, exploration of the modern sea floor has discovered a high proportion of small, widely spaced SMS deposits. Large, inactive deposits are more difficult to identify by current exploration methods but may exist in isolated areas that have yet to be fully explored, such as in heavily sedimented back-arc rifts. This raises the possibility of a dramatically different resource future for SMS if one or more large deposits or “districts” are discovered that contain a high proportion of the total metal.

Abstract The Tethyan belt extends throughout the circum-Mediterranean and eastward through Turkey, Iran, and Pakistan to China and Southeast Asia. The belt is characterized by rift zones with bimodal volcanic rocks and thick clastic sedimentary rock fill, passive-margin basins with platform carbonates, and calc-alkaline island-arc volcanic rock sequences. It is known primarily for its copper and gold endowment, but it includes several large and globally significant zinc-lead provinces. These include the Basque-Cantabrian basin (Réocin) in northern Spain, the Atlas zinc-lead district in Morocco, Algeria, and Tunisia, the zinc-lead-silver deposits in the Balkans—including the Trepča district extending south to Macedonia and Greece—the zinc districts in central Anatolia, Turkey, and Iran—such as Angouran and Mehdiabad—and deposits such as Padaeng, in Thailand, and Jinding, in southwestern China. Late Carboniferous to Triassic rifting in northern Gondwana and opening of the Neotethys Ocean marks the commencement of the most significant zinc-lead metallogenic cycle and initiated the break-up of Pangea. Rifting migrated eastward and broad Permian to Cenozoic carbonate shelf and passive-margin basin sequences were deposited. The thick carbonate sequences provided ideal trap settings for MVT deposits, with zinc- and lead-rich mineralizing fluid flow initiated by Cretaceous to Cenozoic inversion, collision, and uplift on the Tethyan margins. High-temperature carbonate-replacement and vein-type mineralization is also associated with magmatically induced hydrothermal activity during the Cenozoic compressional events. Unusual hybrid deposits involved both basinal and magmatic fluid inputs. Subsequent uplift and oxidation has resulted in the development of economically significant nonsulfide zinc deposits throughout the belt.

Abstract The late Paleoproterozoic upper McArthur Group (River Supersequence) is a dominantly shallow marine carbonate platform sequence. Deeper water shaley rocks of the Barney Creek Formation of this Supersequence host the supergiant HYC Zn-Pb-Ag deposit. Three higher order sequences, the Emmerugga Depositional Sequence, the Barney Creek Depositional Sequence, and the Lynott Depositional Sequence, make up the River Supersequence in the southern McArthur basin. Within the Barney Creek Depositional Sequence, there are 26 lithofacies that can be grouped into seven facies associations, each representing specific and coeval sedimentary environments. This complex facies mosaic formed in response to a sinistral transpression event during regional north-south extension. Basin architecture was controlled by major meridional strike-slip structures, such as the Emu, Tawallah, and Hot-Springs faults. Although these structures have a later, postsedimentation history, they also controlled the distribution of the different facies of the Barney Creek Depositional Sequence. North- to northwest-trending segments of these structures were transtensional, whereas structures oriented east of north were transpressional. Transtension resulted in the development of significant local accommodation and sub-basin development and allowed substantial thicknesses of deeper water, fine-grained sediments, which are potential hosts to Zn-Pb-Ag SEDEX deposits, to accumulate. Locally, in platform and slope facies of the Barney Creek Depositional Sequence, the orientation and vergence of kinematic indicators, such as neptunian dikes and intrafolial folds, may indicate the direction to deeper shaley facies.

Abstract Rabbit-ear anomalies in soil geochemical surveys over buried sulfide deposits are commonly reported and may be ubiquitous. A rotary air blast drilling transect over a deeply buried part of the Thalanga stratiform Zn-Pb-Cu deposit, Queensland, Australia, shows discrete, high-contrast chimney-like geochemical anomalies in ore-forming elements. The 6-m-wide massive sulfide zone, with 15 percent combined Zn, Pb, and Cu, is overlain by 50 m of flat-lying, semilithified, transported sediments of the Campaspe Formation. Total digestion analysis of Zn and Pb shows anomaly to background ratios of 100/1 and 15/1, respectively, starting in the upper weathered bed rock and extending for at least 10 m upward into the cover materials. The strongest part of the Zn and Pb responses are approximately 15 and 10 times the width of the deposit, respectively, and concentrations are highest in the outer part of the chimney and diminish toward the center. The responses in both elements show a bias toward the hanging wall in terms of position and magnitude. Although the magnitude of the anomaly diminishes with thickness of sedimentary rocks, its contrast remains high to the surface, where previously published soil geochemical responses show distinct rabbit-ear anomalies in all four of the surface transects. The surface rabbit-ear responses appear to be a direct expression of the subsurface halo anomalies. The halo and rabbit-ear responses are proposed to result from electromigration of ore-forming cations due to redox-induced spontaneous polarization at the edges of a reduced chimney over the ore. Reduced chimneys occur where buried reduced features are actively oxidizing. A strong redox gradient at the edges of the reduced chimney induces the electrical polarization of conductive and semiconductive mineral grains in overburden with their negative poles pointing in the oxidizing redox direction, which is outward from the chimney. The electrical fields of all these microscopic dipoles are additive in series and result in a macroscopic electrical field throughout the redox gradient that is positive inward and negative outward. The magnitude of the horizontal redox gradient is strongest at surface and diminishes rapidly with depth and, therefore, dipole development is strongest at surface. Current, in the form of a cation flux, migrates inward and downward. At depth, where dipole development is weakest, the current migrates outward and upward completing the electrical circuit and inducing upward movement of ore-forming cations. Electrical dipole-forming bacteria within the redox gradient may contribute to current in a similar fashion.

Abstract The Toqui district is located in southern Chile, 1,350 km south of Santiago. The total geological resource for the district is 20 million tonnes (Mt) grading 8.2 percent Zn and 1.5 g/t Au, with zones of significantly higher Au grades. All orebodies in the district are being developed by the Toqui mine, an underground room and pillar operation that has an average annual production of 500,000 t per year. The Toqui district contains a series of skarn and replacement orebodies within a 24 km 2 area. Oldest rocks include Jurassic andesite and Cretaceous volcanic sandstone and tuff of the Toqui Formation, with a basal 5-to 30-m-thick limestone unit, rich in oyster fossils and forming the main ore host. Above these units is 800 m of black shale of the Katterfeld Formation, overlain by andesite of the Cretaceous lower Divisadero Group, which is then overlain unconformably by rhyolite ignimbrite of the upper Divisadero Group. Intrusive rocks include rhyolite, dacite, and andesite sills emplaced into all the Cretaceous rock units. Multiple periods of magmatic and hydrothermal activity have been documented from 120 to 105 Ma. At district scale, Fe, As, Au, Bi, and Co are highest in the southeast, associated with garnet, pyroxene, and amphibole alteration, whereas Pb and Ag are highest in the northwest, associated with chlorite and sericite. Zinc grades are fairly uniform across the district, but sphalerite is zoned from high Fe in the southeast to low Fe in the northwest. Economically significant gold mineralization was superimposed on earlier base metal-rich skarn in the southeastern part of the district. Late hydrothermal fluids entered the skarn system along pre-existing northwest-trending structures. Gold occurs as electrum associated with native bismuth, cobaltite, and a variety of sulfosalts. Gold-rich ore generally contains abundant arsenopyrite, but arsenopyrite-rich ores are not necessarily gold rich. Gold and cobaltite deposition was accompanied by extensive retrograde amphibole formation, with clay minerals more abundant at the periphery of the gold zones. Deep drilling has encountered two areas of subeconomic pyrite-chalcopyrite-molybdenite stockworks. One is beneath the skarn orebodies in the southeastern part of the district and the other is beneath mineralization in the northwestern part. The emerging picture is one of a large porphyry-skarn district with multiple pulses of intrusion and alteration, resulting in multiple orebodies and mineralization styles.

Abstract The Little Whiteman prospect is located in the western part of the historic Fortymile mining district of the Yukon-Tanana Uplands of east-central, Alaska. The prospect consists of steeply dipping Zn-Pb-Ag-(Cu) massive and semimassive sulfide chimneys and mantos that replace marbles of the greenschist-grade Nasina assemblage of the Yukon-Tanana terrane. A prominent northeast-trending, sinistral strike-slip fault and accessory structures occur within a complex structural zone referred to as the Kechumstuk fault. Normal dip-slip displacement on the southeast-dipping Kechumstuk fault juxtaposed unreactive metavolcaniclastic footwall rocks adjacent to reactive hanging-wall carbonate rocks. Transtension along the Kechumstuk fault has resulted in left-lateral dilation at the northern part of the Little Whiteman prospect. Hydrothermal fluids were channelized along the Kechumstuk fault and vertically restricted by an overlying quartz diorite sill. Hydrothermal alteration ranges from dolomitization of the marble near fault contacts to a distal siliceous zone often containing abundant manganese-oxide stockwork veinlets. Acidic and partly oxidized hydrothermal fluids caused strong local alteration of porphyry dikes, resulting in a muscovite, quartz, pyrite, and kaolinite mineral assemblage. Sulfide bodies extend for >700 m along strike and to depths >300 m. Replacement-style sulfide deposition is localized near and along contacts of steeply dipping structures and felsic porphyry dikes. The sulfide-rich replacement bodies display a sulfide mineral paragenesis of early iron- and subordinate arsenic-bearing sulfide minerals, followed by zoned sphalerite with iron-rich margins containing abundant chalcopyrite inclusions. Continued sulfide mineral precipitation formed a galena and sulfosalt mineral assemblage that became increasingly silver rich through time. Most silver resides in tetrahedrite, which forms inclusions in galena or in late-stage carbonate-sulfide veinlets. Mineralization at the Little Whiteman prospect is interpreted to be the result of hydrothermal fluids driven by Late Cretaceous volcanism. The spatial relationship between the sulfide bodies and felsic porphyry dikes suggest they are related and, perhaps time equivalent to the adjacent Middle Fork caldera that has an age of 69 Ma.

Abstract Exploration for world-class Ni-Cu-(PGE) deposits in mafic and/or ultramafic igneous rocks has focused on extensional environments where high degrees of mantle melting have occurred in association with mantle plumes. Where continental rifting has been involved, the interaction between large volumes of mafic magma and crustal rocks in either intrusive or extrusive settings may have resulted in contamination that triggered sulfide saturation or melting of sulfides within country rocks. Staging chambers and conduits in the subvolcanic environment and embayments associated with channels in the volcanic environment are localities where immiscible sulfide liquid may accumulate. The large-tonnage, high-grade deposits in conduit and magma chamber environments, such as those at Noril’sk, Siberia, remain high priorities for greenfields exploration, and it is now clear that intrusions with even small footprints may be important exploration targets. Examples of small footprint deposits include the large-tonnage ore systems at Voisey’s Bay in the Nain plutonic suite, Labrador, and the low-tonnage, high-grade mineralization at the Eagle deposit in the Keweenawan of northern Michigan. The high-grade mineralization in small deposits is particularly attractive as incremental feed if smelters are located nearby and transportation routes are available. Low-tonnage, high-grade deposits can also be mined using underground methods, and having lesser environmental impact and remediation is typically more straightforward. Although convergent margin environments have not been universally viewed as viable target areas for magmatic sulfide-rich Ni-Cu-(PGE) deposits, suprasubduction zone environments have high degrees of mantle melting, and they provide locations for crust-magma interaction and conduit geometries where sulfides may collect. Deposits such as Kalatongke in China, Aquablanca in Spain, and the Turnagain and Duke Island Ural-Alaskan intrusions illustrate that convergent margins should not be dismissed as targets for magmatic Ni-Cu-(PGE) ores. New advances in hydrometallurgical techniques, particularly pressure leach methods, are making the extraction of Cu, Ni, and PGEs from large-tonnage but low-grade deposits economically promising. The large disseminated sulfide-rich Ni-Cu-(PGE) resources of the Duluth Complex are an example where advances in process technology may permit future development of low-grade occurrences that have traditionally been considered to be of marginal economic value.

Abstract Nickel laterites are thick weathering profiles derived by leaching of ultramafic rocks by meteoric water. Olivine or derived serpentine provides the nickel. Profiles with economically significant deposits derive their Ni from 40-m (15−100 m, 10 th −90 th percentile range) thicknesses of protolith grading 0.16 to 0.3 percent Ni and 5.5 to 10.5 percent Fe. The profiles may be preserved in situ or transported to form a sedimentary unit that may be buried, lithified, and metamorphosed. From bottom upward, in situ nickel laterites may be comprised of silicate saprolite, a nontronite clay zone, high Co and Mn limonite or ferruginous saprolite, low Co and Mn limonite, and allocthonous cover. Any of these units may be absent due to erosion or nondeposition and, importantly, one or all may be siliceous, usually due to quartz precipitation in the saprolite zone. Nickel is leached downward from the limonite zone, added to the saprolite and nontronite zones, and left residually enriched in limonite. Strong supergene enrichment requires downward leaching into saprolite and fractured rock above a deep water table. Zones of strong passive jointing and pre- or synweathering fracture zones all may lead to an order of magnitude increase in the rate of advance of the weathering front. The rate of advance of the weathering front in tropical rain forest covered highlands is about 50m/m.y., regardless of whether the bed rock is ultramafic, dioritic, or felsic. Weathering fronts advance at progressively slower rates in terranes with less relief. Nickel laterite deposits accumulate on terraces or plateau landforms in karstlike basins or under semiarid peneplains. The topographic controls of in situ nickel laterite deposits can be understood in terms of structural controls and three long-term climatic and topographic scenarios. The scenarios include: (1) permanently wet rain- forest setting in tectonically active terrane with moderate relief, (2) a formerly wet peneplain that has evolved toward aridity, and (3) a formerly arid peneplain setting that has evolved into a permanently wet environment.

Abstract The Ntaka Hill nickel sulfide deposits are hosted in the peridotitic to pyroxenitic Ntaka ultramafic intrusion located in the Nachingwea area and are the first significant occurrence of nickel sulfides in the Tanzania portion of the Late Proterozoic Mozambique belt. High-grade nickel sulfide mineralization was first discovered at Ntaka Hill in 2006. Six near-surface sulfide deposits have since been delineated containing a measured and indicated mineral resource of 1.8 million tonnes (Mt) @ 1.82 percent Ni and 0.31 percent Cu. The recent discovery history can be traced back to the presence of a historic surface copper oxide malachite showing. Subsequent soil sampling defined a large coincident Ni-Cu anomaly that provided the impetus for airborne and ground geophysical surveys and ultimately diamond drilling leading to the initial discovery. Further ground electromagnetic surveys were successful in defining additional moderate to high conductance anomalies resulting, upon drill testing, in the discovery of five additional nickel sulfide zones. The Ntaka intrusion is postulated to have formed from a relatively primitive, high MgO magma, dominated by the crystallization and accumulation of olivine and pyroxene. The intrusion is characterized by high MgO contents, low CaO, Al 2 O 3 , Cu, PGE, and incompatible element contents, and relatively flat chondrite-normalized REE profiles lacking Eu anomalies. The geologic setting is similar to that of the Early Proterozoic Thompson Nickel belt in Canada. There supracrustal rocks formed on a continental margin platform and were intruded by ultramafic sills that interacted with the sulfidic metasedimentary rocks to produce the resulting nickel deposits. The Ntaka Hill sulfide zones occur in separate south-plunging lenses but may represent remnants of a dismembered original basal sulfide zone. The zones consist of magmatic, remobilized, and graphite-bearing mineralization with variable nickel grades of as much as 17 percent. Mineralization consists of disseminated, net-textured, and massive magmatic sulfides, as well as remobilized semimassive and massive sulfide veins and stringers composed of pyrrhotite, pentlandite, pyrite, chalcopyrite, and violarite. Pentlandite is the main nickel-bearing sulfide mineral occurring as coarse grains and eyes that are as large as 5 cm in diameter. Pyrrhotite-rich, nickel-poor, graphite-bearing, disseminated to massive sulfide mineralization occurs at several locations within the Ntaka intrusion and is thought to have formed by assimilation of graphitic metasedimentary rocks. The Ntaka intrusion possesses a number of elements critical to the formation of nickel sulfide deposits and good potential exists to discover additional nickel sulfide deposits, both in the Ntaka Hill area and regionally. Exploration challenges in this underexplored belt include a complex deformation history, an abundance of graphitic metasedimentary rocks, and a paucity of outcrop.

Abstract The Ni-Cu-(PGE) deposits of the Thompson nickel belt in the Circum-Superior boundary zone of northern Manitoba define the second largest Ni-Cu-(PGE) mining camp in Canada and one of the premiere Ni-Cu-(PGE) camps of the world. Despite a complex deformation and metamorphic history, the deposits in the Thompson nickel belt exhibit many fundamental characteristics similar to those of other major magmatic Ni-Cu-(PGE) districts: they are hosted by or associated with ultramafic intrusions that appear to represent dynamic feeders, the ores occur at or near the bases of the intrusions, and there is evidence for incorporation of significant amounts of sulfur from the Ospwagan Group metasedimentary country rocks. However, they differ from most other deposits of this type in being metamorphosed to much higher grades, in being much more complexly deformed, and in being mobilized to much greater degrees into the country rocks. The ultramafic intrusions are generally lensoid in shape, reflecting the effects of superimposed deformation on the enclosing metasedimentary rocks, range in composition from komatiitic dunite to komatiitic pyroxenite, are variably serpentinized, and are interpreted to represent a series of sills and low-angle dikes that intruded and interacted with the Ospwagan Group metasedimentary rocks. High Fo contents in relict igneous olivine (as much as Fo 92 ) indicate a low Mg komatiitic parental magma with 22 to 24 percent MgO. Mineralization occurs as type II disseminated sulfides within the ultramafic rocks (e.g., William Lake), as type V tectonically modified massive sulfides within or adjacent to the ultramafic bodies (e.g., Pipe and Birchtree), and as type IV magmatically and metamorphically mobilized sulfides within metasedimentary rocks of the Ospwagan Formation (e.g., Thompson). Intrusions occur at all almost all levels within the Ospwagan Group, but mineralized intrusions are localized exclusively within the lower and middle parts of the Pipe Formation, which contains abundant sulfide-facies iron formation. Density-driven magma emplacement models indicate that the Ospwagan metasedimentary rocks were likely partially lithified prior to magma emplacement and the absence of significant thermal aureoles suggests that they were being metamorphosed. Stratigraphic correlations between ultramafic intrusions, S-rich rocks of the Pipe Formation, and Ni-Cu-(PGE) sulfide mineralization, together with nonmantle δ34S values and S/Se ratios in the ores and nonmantle Th/Yb and Th/Nb ratios in the host rocks, collectively suggest that the mineralization formed by incorporation of S-rich sedimentary rocks by high-temperature komatiitic magmas. Postore deformation and metamorphism have significantly modified the primary characteristics of many of the Thompson nickel belt ore deposits, mobilizing Cu, Au, and Pt. The best exploration tools appear to be aeromagnetic surveys to identify serpentinized ultramafic bodies, which are the heat and metal sources; stratigraphic studies to recognize appropriate levels of the Pipe Member of the Ospwagan Group, which is the S source; lithogeochemical studies to identify the most magnesian and most contaminated host units; which provide the evidence of magma-sediment interaction; and recognition of areas of anomalous Cu, Au, and Pt dispersion halos.

Abstract The Eagle’s Nest Ni-Cu-PGE deposit was discovered in the McFaulds Lake area of the James Bay lowlands of northern Ontario, Canada, in 2007 by Noront Resources Ltd. It is a magmatic sulfide deposit hosted by mafic and ultramafic rocks interpreted to be a feeder conduit beneath an extensive complex of sills and related volcanic rocks, which range in composition from dunite through ferrogabbro to rhyolite. The complex, called the Ring of Fire, has been dated at 2734.5 ± 1.0 Ma and it was emplaced into 2773.37 ± 0.9 Ma felsic plutonic rocks. The felsic rocks form a sill complex structurally beneath metasedimentary and metavolcanic rocks considered to have formed along a passive margin at ca. 2800 Ma within the Oxford-Stull domain of the North Caribou superterrane in the Archean Superior province. In its original configuration, the Eagle’s Nest deposit formed in a shallowly plunging or subhorizontal keel structure at the base of a dike-like chonolith, but subsequent deformation has turned it into a vertically plunging rod of sulfide mineralization along the northwestern margin of a north-south–striking dike. The most magnesian chilled margin is a picrite with MgO content near 18 wt percent, placing it at the boundary between komatiite sensu stricto and komatiitic basalt. Modeling suggests that the parental magma contained at least 22 percent MgO and was derived from previously melt depleted mantle. Sulfide saturation was attained following extensive contamination of the magma, resulting in the accumulation of a slurry of olivine crystals with variable amounts of interstitial sulfide melt and postcumulus orthopyroxene at the base of the conduit, locally producing significant pools of massive sulfide at or near the lower contact. The sulfide segregation occurred at moderate degrees of sulfide supersaturation from a magma rich in chalcophile elements, leading to high base and precious metal tenors in the resulting deposit. Minor fractionation of the sulfide magma is evidenced by the dispersion of massive and net-textured sulfide compositions along a tie-line between Ni-rich monosulfide and Cu-rich intermediate sulfide solid solutions, as well as by minor quantities of vein-hosted massive sulfide with extremely enriched base and precious metal tenors throughout the deposit. The former are interpreted as sulfide cumulates, whereas the latter are possible remnants of highly evolved sulfide liquids. Extensive metamorphic remobilization of Pt is considered to be responsible for wholesale depletion of Pt in much of the massive sulfide and for the local generation of sulfide veins carrying >1,100 ppm Pt.

Abstract Geophysical surveys have played a defining role in the discovery and subsequent delineation of many nickel, copper, and platinum group element (Ni-Cu-PGE) deposits. The high conductivity of pyrrhotite and the robustness of the electromagnetic methods that have been developed to directly detect this mineral are responsible for the exploration success. The introduction of concentric time domain electromagnetic (EM) systems towed by helicopters (known as HTEM systems) has led to direct drilling programs, providing more timely feedback on the nature of conductive sources, and, therefore, an increased ability to test more targets in a given field season. The EM techniques have also evolved to better penetrate conductive overburden allowing for more confidence in areas with no outcrop. In this paper, we summarize a number of geophysical surveys from two Ni-Cu-PGE occurrences in the McFaulds Lake of northern Ontario. The discovery was made during a period of time in which HTEM systems were not fully accepted for direct-drill programs. As a result, exploration began using traditional methods including ground geophysics and later migrated toward modern airborne methods. The future of Ni-Cu-PGE exploration using geophysics will continue to be evolutionary. There will be a gradual decrease in the reliance on ground geophysics because surface methods have not kept pace with airborne methods and offer little to no additional information on the nature, position, and orientation of the target conductor. Infield interpretation with additional flight lines designed to better define discrete targets will be slowly implemented as more geophysicists become familiar with real-time profile interpretation. Multiple flights over conductive sources at different flight heights, will reduce the uncertainty between small targets near surface and deeper sources that are only partially resolved. Closer spacing of the flight lines will provide improved strike direction estimates and will help resolve the nature of the conductor (e.g., a continuous source versus a series of discrete lenses). Geophysical technology will ultimately lead the geologist in an interesting direction, one where geophysical surveys will be followed by drilling and then geological mapping methods, in an effort to develop a working exploration model for the discovery of buried mineral deposits in areas with little to no surface exposure and thus geologic information.