Abstract The majority of world metal and fuel resources are contained in a very small proportion of the total number of world deposits that represent the upper end of the size spectrum of deposits. To survive in the increasingly competitive resource industry, mining companies must focus on finding or acquiring deposits of this type. The purpose of this workshop is to attempt to identify geological features that are characteristic or diagnostic of these very large deposits and their regional and local geological setting. One possibility is that very large deposits are just abnormally large “normal” deposits, being generated by essentially the same ore-forming process. Alternatively, the ore-forming process which generates very large deposits may be significantly different. The concern here is not with the well-known size differences between deposits of different geological type that contain the same metal (i.e., the difference between porphyry copper deposits and volcanogenic massive sulfide deposits as copper resources).

Abstract Natural diamond occurrences are reviewed to show that kimberlites and lamproites, though our principal source rocks for economic diamonds, are not primary deposits in the sense that diamond originated within them. The igneous hosts simply provided the transport medium for diamonds that formed within harzburgitic and eclogitic source rocks in the subcontinental lithosphere. A distinction is made between diamond source rocks and “primary” diamond deposits defined as kimberlites, lamproites, or any other igneous rock type originating deep enough to have sampled the diamondiferous source rocks. “Primary” deposits are then analysed in terms of sources, transport media, and depositional sites. Although sizes and grades among “primary” deposits vary greatly, “giant” deposits that are governed by different geological processes than smaller deposits cannot be identified. However, the enormous dilution in diamond grade between source rocks and “primary” deposits suggests that concentrations of eclogitic source rocks in the upper mantle may represent truly “giant” deposits that would dwarf even the largest and richest “primary” deposits.

Abstract Although of smaller tonnage, the higher grade of the Ni-Cu ores of the Noril’sk region contain almost as much nickel metal as those of the Sudbury district. They also contain at least twice as much metallic Cu and 15 times as much total PGE. Five principal magma types characterize the 3.8 km vertical thickness of flood basalt of the Noril’sk region. From base to top these are (1) a high TiO 2 type with alkalic affiliations (Ivakinsky and Syverminsky suites), (2) a TiO 2 -rich picrite-bearing magma (Gudchichinsky suite), (3) a picrite-bearing TiO 2 -poor magma (Tuklonsky suite), (4) a crustally contaminated TiO 2 -poor magma (Lower Nadezhdinsky suite), and (5) a picrite-free magma (Mokulaevsky, Kharayelakhsky, Kumginsky, and Samoyedsky) similar but not identical in its chemistry to the Tuklonsky and separated from the Lower Nadezhdinsky by transitional suites (Upper Nadezhdinsky and Morongovsky). The Ni-Cu ores are associated with olivine-rich intrusions that are geochemically similar to the Mokulaevsky magma. They are associated with about 1/15 to 1/10 of their mass in PGE-rich sulfides; this requires that these sulfides equilibrated with 15 to 200 times more magma than is represented by the intrusions themselves. Following the model of Naldrett et al. (1992), it is proposed that crustal contamination of original Tuklonsky-type magma occurred at the top of a vertically extensive, fault-controlled magma chamber and caused segregation of immiscible sulfides with low R factor. These depleted this magma in chalcophile elements and settled deeper in the chamber. As they settled, they reacted with less contaminated magma, scavenging additional chalcophile elements. They finally came to rest near the base of the chamber to form a zone of sulfide- (and thus chalcophile element-) enriched magma. Eruption, and in some instances intrusion, of progressively stratigraphically lower, less contaminated and thus less depleted layers in the chamber gave rise to the observed sequence of basalts and intrusions. The answer to the strong enrichment in Ni, Cu and PGE associated with the ore-bearing intrusions is that these metals came from the magma that is now represented by the basalt magma with which the sulfides have come into contact and which show chalcophile-element depletion. Lessons to be gained from the Noril’sk region with regard to exploration for similar deposits elsewhere are the importance of (1) a hot region in the mantle giving rise to vast amounts of basalt, (2) continental scale rifting, (3) major faults and the intersection of these with the flexed margins of upwarped and downwarped areas coinciding with the base of the volcanic succession, (4) study of the basalt stratigraphy with a view to establishing the presence of chalcophile element depletion, (5) study of the intrusions, with special attention to their age relationships with respect to the different basalt suites, so that one can concentrate on intrusions that are rich in MgO and that just post-date eruptions of chalcophile-depleted basalt, (6) concentrating on those intrusions closely related to major faults, particularly where upwarping has exposed the “keel” of the volcanic basin. While it is not firmly established how important sulfate assimilation has been to the origin of the Ni-Cu ores at Noril’sk, the presence of sulfate evaporites, or other sulfur-rich horizons in strata beneath flood basalts in any area can only be a positive sign. The simplest way to recognize chalcophile element depletion is to plot Ni/MgO vs Mg number [= atomic (MgO)/(MgO+0.85*FeO total )]. A strong indication that basalts have undergone crustal contamination is the correlation of high La/Sm with high SiO 2 . The Lake Superior region shows many of the key metallogenetic characteristics of the Noril’sk region, including continental rifting, a triple junction, huge thicknesses of basalt, and Cu-Ni sulfide-bearing intrusions (Duluth Complex, Crystal Lake Gabbro). Some of the Keweenawan basalts of the region show Ni-depletion correlated with chalcophile element depletion. The region is an example of the potential application of the Noril’sk model to exploration.

Abstract Giant gold deposits are defined as those containing > 200 metric tons Au, and bonanza deposits as those containing > 30 metric tons Au averaging at least 30 g/t (1 oz/t). Sixteen giant and/or bonanza gold deposits around the Pacific rim are compared and contrasted in an attempt to determine the factor(s) controlling their development in the epithermal environment. Metallogenic (regional) controlling factors seem to be excluded by the wide variety of tectonic, crustal, and petrochemical settings in which the deposits occur. The deposits are present in neutral, extensional, and, less commonly, compressive arcs. Extension may be concentrated within or behind principal arcs either during active subduction or immediately after cessation of subduction. Mineralized crust ranges from cratonic to primitive island arc. The petrochemisty of genetically related igneous rocks may be calc-alkaline or alkaline, with some extension-related suites being bimodal in character. Deposits are not confined to the proximity of major crustal faults or lineaments. The data base does suggest, however, that unusual arc settings characterized by tectonic complexity and relatively uncommon igneous rock compositions, especially the alkaline suite, are more prospective for giant and bonanza gold deposits than “normal”, andesite-dominated arcs. Deposit- or district-scale factors also cannot be isolated as unique controls of giant or bonanza status. All epithermal deposit types, both high and low sulfidation, may develop exceptional tonnages and/or grades. No single mineralization style or local structural or lithologic setting acts as a universal control. Most deposits are parts of volcanic centers, but these may be dome complexes, maar-diatreme systems, calderas, or stratovolcanoes. Nor do the number or sequence of mineralizing events, the depth of gold deposition, or the inferred mechanism of gold precipitation influence profoundly the formation of all giant and bonanza gold deposits. In the absence of any specific causative factor(s) of a regional or local character, it is difficult to avoid the empirical truism that supply of an unusually large amount of gold to the deposit site is the basic requirement. Large volumes of auriferous fluids may explain giant deposits, but exceptionally gold-rich fluids need to be invoked for generation of bonanzas. However, it is proposed that these fluid requirements must be combined with fortuitous geologic parameters or circumstances at the site of gold deposition if giant and/or bonanza rather than ordinary gold deposits are to develop in the epithermal environment.

Abstract Mesothermal gold deposits occur in all ages of volcanic-dominated and sediment-dominated supracrustal belts in collisional orogenic belts intruded by granitoids. The major difference between well- and poorly-endowed belts is that the well-endowed belts contain anomalously large deposits. The same pattern occurs within belt, with mining camps standing in stark contrast to the poorly-endowed regions between camps. To investigate controls on giant deposits, we review here the geological characteristics and setting of forty seven giant (> 60 tonnes Au) mesothermal gold deposits. 80% of production and deposits are in volcanic-dominated belts, and 20% in sediment-dominated belts. There is little evidence that deposit-scale ore controls explain the occurrence of giant deposits. For example, the structural complexity of deposits appears to be correlated with lithological complexity of their host-rock environment, and bears no relationship to deposit size. At a regional scale, however, giant deposits are associated with particular favorable rock assemblages and possibly earlier-formed gold mineralization. Most giant deposits in volcanic dominated belts occur in belts with high-Mg volcanic (komatiitic) or intrusive rocks; 20% are associated with felsic volcanic rocks and of these, 75% have a pre-metamorphic, high-Al hydrothermal alteration mineral assemblage. One giant and many smaller deposits are associated with porphyry-type Cu-Au mineralization. Giant deposits in Precambrian sediment-dominated belts are all hosted by iron formation, but those in Phanerozoic belts are not. To explain the common characteristics of mesothermal gold deposits, in particular the fact that the mineralogy of deposits is partly related to the host-rock environment, and the provinciality of the isotopic signature of deposits, it has been suggested that mesothermal gold deposits are formed by CO 2 -rich fluids that originated by devolatilization of subcreted volcanic and sedimentary rocks, and were modified by interaction with the crustal column between the sites of fluid generation and ore deposition. To explain the particular features of giant deposits, we suggest that the fault-valve model for mesothermal deposits be extended to involve addition of gold to fluids when fluids were stored in mining-camp-scale reservoirs during periods of supralithostatic pressure in the regime of cyclic fluid pressure variations involved in the fault-valve process. This model implies that the gold content and/or availability of gold to be leached in the rock assemblage is critical to distinguishing areas prospective for giant deposits. The general geotectonic environment of mineralization is generally thought to be a prograding arc-trench complex.

Abstract Porphyry Cu (-Mo,Au) deposits constitute an ideal subject in the search for differences between “giant” and modest mineralization: these major magmatic-hydrothermal systems have been extensively, if somewhat erratically, analysed and documented, and embrace both a great range of size, from small (< 100,000 t Cu) to gigantic (> 45,000,000 t), and a corresponding diversity of paragenesis and fabric. Moreover, the lithotectonic and metallogenic contexts of several major porphyry provinces have been clarified. “Anatomical” variations among deposits are herein calibrated graphically against the intensively-studied El Salvador center, emphasizing those aspects of the temporal evolution of the mineralization most likely to reflect significant variations in magma-aqueous fluid equilibria, and hence fundamental ore-genetic parameters. It is regretfully concluded that, at least on the basis of the criteria selected, there are no systematic qualitative differences between outsize and smaller examples of the porphyry clan. Thus, significant departures from the El Salvador template are represented in all tonnage categories, and there is no correlation, between mineralization “complexity” and size. Whereas the two largest-known examples, Chuquicamata and El Teniente, display unusual features, they differ greatly from each other, and their specific attributes are not unique, and would probably not aid in the identification of other outsize deposits. The intensity of ore formation in porphyry deposits overall bears no simple or universal relationship to the P-T conditions prevailing in the broad environment in which retrograde boiling of granitoid magmas occurs. The underlying controls on deposit size must therefore reside mainly in larger-scale lithotectonic conditions; i.e., metallogenic parameters override the ore-genetic. However, it remains difficult to define the former. Whereas the ensialic central Andean orogen of Chile and southeastern Peru has been remarkably fecund as a porphyry copper nursery since the Paleogene, each of the world-class porphyry-dominated metallogenic episodes represented therein, viz. , early Eocene, late Eocene-early Oligocene, and Late Miocene-Pliocene, occupies a significantly different geotectonic niche with respect both to plate convergence mode and tectonic/magmatic conditions at the continental margin. It is tentatively suggested that the giant , super-giant and behemothian porphyry deposits of this region may ultimately owe their origin to an unusually protracted (+200 m.y.) and fundamentally consistent magmatic history in the course of which only limited migration (≤ 150 km) of the main subduction-related arc occurred. Thus, parental melts generated in a poorly-defined but persistent and geochemically homogeneous (but increasingly Cu-rich?) zone in the lowermost crust or lithospheric upper mantle could periodically rise rapidly to the epizonal environment through a cordon sanitaire of essentially cogenetic precursor rocks. This minimized assimilation and fractional crystallization processes and hence precluded loss of Cu and Mo to the progressively-thickening crust. It is not clear to what extent such arguments may be germane to other less important porphyry provinces, but provenance and process considerations would be reconciled if the magmas associated with large-scale, central Andean, porphyry mineralization were significantly enriched in Cu, Mo, Cl and, possibly, S as a result of their unusual setting.

Abstract Giant porphyry molybdenum deposits are best exemplified by the Climax and Henderson deposits in Colorado. The high grades of these deposits are probably inherited from magmatic molybdenum concentrations of about 4 to 5 ppm, which are high for metaluminous rhyolitic magmas that average about 2 ppm molybdenum. High magmatic molybdenum concentrations in metaluminous rocks appear to be related to high magmatic oxygen fugacities (2 or 3 log units above QFM oxygen buffer) and are correlated with high niobium concentrations. High oxygen fugacities are likely inherited from calc-alkaline or lamprophyric predecessors. High niobium and molybdenum are related to extreme fractionation of rhyolitic magmas. Much higher concentrations of molybdenum (> 1,000 ppm) in the ore fluid (and the cupola magma) are probably achieved by crystallization in the deeper portions of a magma chamber accompanied by convection of the evolved liquid to the cupola and volatile fluxing. Exploration criteria for a giant, high-grade deposit include: 1) a tectonic setting that indicates a changeover from compressional to extensional tectonics, 2) thick continental crust at the time of deposit formation may encourage extreme differentiation and crustal contamination, 3) an isotopically zoned magma chamber indicative of a long-lived heat source, 4) a large, sub-volcanic, central-vent ash flow/dome system that erupted less than 100 km 3 of rhyolite, and 5) high niobium concentrations (> 75 ppm) in a subalkaline, magnetite-bearing rhyolite.

Abstract Volcanic-associated or “VMS” deposits represent a subdivision of the more general “massive sulfide” class of deposit. The size distribution of VMS deposits defines a natural geometric progression with approximately 80% of Archean, Proterozoic and Phanerozoic deposits falling within the 0.1 to 10 Mt size range. ”Giant” massive sulfide deposits containing >50 Mt have a frequency of occurrence of < 4%. All VMS deposits share many essential characteristics, regardless of size, however the giant deposits tend to be further characterized by: 1) a more complex metallic mineralogy; 2) Zn-(pyrite)-rich ore; 3) fragmental footwall rocks that are predominately felsic in composition; 4) an occurrence within volcanic subsidence structures; 5) a sedimentary hangingwall sequence; 6) indistinct footwall alteration zones characterized by sericite, quartz, carbonates and locally aluminous assemblages; and 7) poorly-developed stringer zones. Factors which favor the formation of giant VMS deposits include: 1) a location within large- to medium-scale synvolcanic subsidence structures such as calderas or grabens that are characterized by high heat flow, effective cross-stratal permeability and sustained hydrothermal fluid discharge; 2) efficient metal containment by sub-seafloor precipitation of sulfides and sulphidization of permeable fragmental rocks; and 3) a veneer of impermeable sediment atop the sulfide deposit that could have acted as a thermal and hydrologic cap that insulated the hydrothermal system from incursion by cooler seawater and facilitated sub-seafloor sulfide precipitation and subsequent zone-refining whereby early formed Fe ± Zn-sulfides are replaced by higher temperature, base metal-rich sulfide assemblages.

Abstract The majority of world metal and fuel resources are contained in a very small proportion of the total number of world deposits that represent the upper end of the size spectrum of deposits. To survive in the increasingly competitive resource industry, mining companies must focus on finding or acquiring deposits of this type. The purpose of this workshop is to attempt to identify geological features that are characteristic or diagnostic of these very large deposits and their regional and local geological setting. One possibility is that very large deposits are just abnormally large “normal” deposits, being generated by essentially the same ore-forming process. Alternatively, the ore-forming process which generates very large deposits may be significantly different. The concern here is not with the well-known size differences between deposits of different geological type that contain the same metal (i.e., the difference between porphyry copper deposits and volcanogenic massive sulfide deposits as copper resources). A number of different deposit types are being considered because there is very little useful quantitative data on the specific problem that we are addressing for any one deposit type. We hope that our lack of data on individual deposit types can be overcome by putting together parts of the puzzle from different deposit types. The paucity of data on the relationship between size and geological characteristics is unfortunate, but not surprising. Rarely is research concerned with widely accepted assumptions, no matter how important the implications of those assumptions. Few assumptions are more deeply ingrained in geology than the idea that ore-forming processes are independent of scale. In this introduction, four general issues will be considered that are pertinent to the topic that is the focus of this volume: 1. ) what is a mineral deposit? 2.) how is size to be defmed so that it approximates as closely as possible a purely geological variable (as opposed to a mixed geologic-economic variable)? 3.) how are polymetallic deposits, and deposits of different geological type to be compared? 4.) what are the geological factors, and the nature of their interaction, that control the size of ore deposits? A mineral deposit is defined as a single mineralized body, or a group of spatially associated mineralized bodies. In most cases, ore zones forming part of a mineral deposit are close enough to be developed and mined from one set of underground workings. In the case of deposits mined by open pit, however,

Abstract The field trip departs from the Bon Air Motel in Kirkland Lake, cumulative mileage from the Bon Air Motel is signified by the round brackets ( ). Distances between field trip stops are also provided. Take Highway 66 west from the Bon Air Motel in Kirkland Lake to Highway 11 (15. 5 km). Continue west on Highway 66 to Matachewan. Cross the bridge over the West Montreal River, in Matachewan, where Highway 566 begins (61. 5 km). Continue west along Highway 566 for 3. 2 kilometres. Then turn right and park just beyond the gate to the Matachewan Consolidated Mine (64. 8 km).

Abstract The Matachewan camp is located in the southwestern Abitibi Greenstone Belt of the Superior Province, 55 kilometres west of Kirkland Lake, northeastern Ontario. Gold production from the camp has come entirely from two contiguous mines, the Young-Davidson mine (YDM) and the Matachewan Consolidated mine (MCM). Between 1933 and 1957, a total of 9. 6 million tonnes of ore with an average grade of 3. 1 g/t Au and 0. 93 g/t Ag was produced from these mines. In 1979-80 Pamour Porcupine Mines Limited removed an additional 18,000 tonnes of ore at >3.4 g/t from open pit operations on the Matachewan Consolidated property (Sinclair, 1982). Since the surface exposures of ore grade mineralization have long since been removed from the mines of the Matachewan area, one cannot examine the detailed controls of the mineralization. Thus the aim of this surface field trip is to examine the regional setting of the Matachewan gold mines, the geological history of the area, and the general style and mineralogy of the gold mineralization and associated alteration. These features may then be compared with those of other gold deposits associated with the major tectonic zones of the southern Abitibi belt. In addition, evidence of post-Archean reactivation of major structures, including the Larder Lake-Cadillac Break (LLCB), and its relevance to gold exploration will be examined.

Abstract Perhaps a dominant theme that has been expressed in the last few years concerning the petrogenesis and metallogeny of granitoids in western North America concerns the relative proportions and composition of various types of crustal material that have been incorporated in batches of magma (Keith et al., 1985a; Barton, 1987; Farmer and DePaolo, 1983; Ague and Brimhall, 1985, 1986, 1987; Christiansen et al., 1986; Stein and Hannah, 1985; Newberry and Swanson, 1986). Many workers suggest that these processes of crustal melting or contamination by crustal components exert one of the strongest controls on the character or grade of mineralization that ultimately may be produced by that magma. However, broad distinctions between classes of mineralization (i.e. porphyry Cu versus W skarn) may largely be a function of depth of crystallization and correlative water content of the magma (Einaudi et al., 1981; Burt et al., 1982; Barton et al., 1988; Newberry and Swanson, 1986). The controversy over how metal ratios in a deposit are related to granitoid composition is particularly applicable to W, Mo, and Sn deposits because of their close spatial association to granitoids and strong evidence that the dominant portion of the metals and the hydrothermal fluid are derived from the magma. This being the case, the question that must be asked is which magmatic characteristics are set (or modified) during assimilation of crustal material, that have some control on the metal ratios or grade of mineralization, versus those which are "inert" in terms of effecting mineralization. Perhaps the

Abstract Gold mineralization at the Victory mine, Kambalda, is associated with discrete metasomatic alteration zones around quartz breccia zones, shear zones, and quartz vein arrays. The mineralogy, textures, and whole-rock chemistry of the wall-rock alteration zones are described for several different host rocks. Mineral assemblages at zone boundaries, calcite-dolomite geothermometry, and amphibole geobarometry have been used to estimate the temperature, pressure, and fluid composition associated with metasomatism. Fluid inclusion data have been used to estimate independently these conditions. Wall-rock alteration zones extend up to 3 m from veins and breccias at the Victory mine. Textures indicate that the zoned wall-rock alteration and associated gold mineralization postdated regional metamorphism and outlasted retrograde carbonation. Chemical variations across zoned alteration profiles indicate that alteration occurred at approximately constant volume. Outer alteration zones are characterized by the addition or loss of H 2 O, CO 2 , Na, and K whereas Al, Mg, Ca, Fe +2 , and Fe +a were mobile in the inner alteration zones. Chemical changes and mineralogy of the alteration envelopes depended critically on the initial composition of the host rock which affected the resultant mineral assemblages. Assuming that local equilibrium conditions existed at alteration zone boundaries, mineral compositions from microprobe data have been used to model equilibria in the system SiO 2 -Al 2 O 3 -MgO-CaO-K 2 O-H 2 O-CO 2 . The mineral equilibria together with calcite-dolomite geothermometry provide an estimate of 390° ± 40°C for metasomatism which is similar to a minimum temperature estimate of 370° + 30°C from fluid inclusion data. Mineral equilibria and fluid inclusion data suggest that pressure during metasomatism was approximately 1.7 to 2 kbars. Fluid inclusion data indicate that metasomatism was associated with a homogenous H 2 O-CO 2 -NaCl fluid containing 19 to 36 wt percent CO 2 CX CO2 = 0.1-0.2) and 8 to 9 equiv wt percent NaCl. The data presented in this study indicate that metasomatism occurred at considerably lower temperatures and pressures than those estimated for peak metamorphic conditions at Victory. Thus after peak metamorphism, substantial uplift occurred before the hydrothermal emplacement of gold.