Metamorphic belts are complex regions where accretion or collision has added to, or thickened, continental crust. Gold-rich deposits can be formed at all stages of orogen evolution, so that evolving metamorphic belts contain diverse gold deposit types that may be juxtaposed or overprint each other. This partly explains the high level of controversy on the origin of some deposit types, particularly those formed or overprinted/remobilized during the major compressional orogeny that shaped the final geometry of the hosting metamorphic belts. These include gold-dominated orogenic and intrusion-related deposits, but also particularly controversial gold deposits with atypical metal associations. Orogenic lode gold deposits of Middle Archean to Tertiary age are arguably the predominant gold deposit type in metamorphic belts, and include several giant (>250 t Au) and numerous world-class (>100 t Au) examples. Their defining characteristics and spatial and temporal distributions are now relatively well documented, such that other gold deposit types can be compared and contrasted against them. They form as an integral part of the evolution of subduction-related accretionary or collisional terranes in which the host-rock sequences were formed in arcs, back arcs, or accretionary prisms. Current unknowns for orogenic gold deposits include the following: (1) the precise tectonic setting and age of mineralization in many provinces, particularly in Paleozoic and older metamorphic belts; (2) the source of ore fluids and metals; (3) the precise architecture of the hydrothermal systems, particularly the relationship between first- and lower-order structures; and (4) the specific depositional mechanisms for gold, particularly for high-grade deposits. Gold-dominant intrusion-related deposits are a less coherent group of deposits, which are mainly Phanerozoic in age, and include a few world-class, but no unequivocal giant, examples. They have many similarities to orogenic deposits in terms of metal associations, wall-rock alteration assemblages, ore fluids, and, to a lesser extent, structural controls, and hence, some deposits, particularly those with close spatial relationships to granitoid intrusions, have been placed in both orogenic and intrusion-related categories by different authors. Those that are clearly intrusion-related deposits appear to be best distinguished by their near-craton setting, in locations more distal from subduction zones than most orogenic gold deposits and in provinces that also commonly contain Sn and/or W deposits; relatively low gold grades (<1–2 g/t Au); and district-scale zoning to Ag-Pb-Zn deposits in distal zones. Outstanding problems for intrusion-related deposits include the following: (1) lack of a clear definition of this apparently diverse group of deposits, (2) lack of a definitive link for ore fluids and metals between mineralization and magmatism, (3) the diverse nature of both petrogenetic association and redox state of the granitoids invoked as the source of mineralization, and (4) mechanisms for exsolution of the CO 2 -rich ore fluids from the relatively shallow level granitoids implicated as ore-fluid sources. Gold deposits with atypical metal associations are a particularly diverse and controversial group, are most abundant in Late Archean terranes, and include several world-class to giant examples. Most are probably modified Cu-Mo-Au porphyry, volcanic rock-hosted Zn-Pb-Ag-Au massive sulfide, or Zn-Pb-Ag-Au or Ba-Au-Mo-Hg submarine epithermal systems, overprinted or remobilized during the events in which orogenic gold deposits formed, but there is lack of consensus on genesis. Outstanding problems for these deposits include the following: (1) lack of a clear grouping of distinctive deposits, (2) lack of published, well integrated studies of their characteristics, (3) generally a poorly defined timing of mineralization events, and (4) lack of assessment of metal mass balances in each stage of the complex mineralization and overprinting events. Both orogenic gold deposits and gold deposits with atypical metal associations contain a few giant and numerous world-class examples, whereas the intrusion-related group contains very few world-class examples, and no giants, unless Muruntau is included in this group. Preliminary analysis suggests that the parameters of individual world-class to giant gold deposits of any type show considerable variation, and that it is impossible to define critical factors that control their size and grade at the deposit scale. However, there appears more promise at the terrane to province scale where there are greater indications of common factors such as anomalous subduction-related tectonic settings, reactivated crustal-scale deformation zones that focus porphyry-lamprophyre dike swarms in linear volcanosedimentary belts, complex regional-scale geometry of mixed lithostratigraphic packages, and evidence for multiple mineralization or remobilization events. There are a number of outstanding problems for all types of gold deposits in metamorphic belts. These include the following: (1) definitive classifications, (2) unequivocal recognition of fluid and metal sources, (3) understanding of fluid migration and focusing at all scales, (4) resolution of the precise role of granitoid magmatism, (5) precise gold-depositional mechanisms, particularly those producing high gold grades, and (6) understanding of the release of CO 2 -rich fluids from subducting slabs and subcreted oceanic crust and granitoid magmas at different crustal levels. Research needs to be better coordinated and more integrated, such that detailed fluid-inclusion, trace-element, and isotopic studies of both gold deposits and potential source rocks, using cutting-edge technology, are embedded in a firm geological framework at terrane to deposit scales. Ultimately, four-dimensional models need to be developed, involving high-quality, three-dimensional geological data combined with integrated chemical and fluid-flow modeling, to understand the total history of the hydrothermal systems involved. Such research, particularly that which can predict superior targets visible in data sets available to exploration companies before discovery, has obvious spin-offs for global- to deposit-scale targeting of deposits with superior size and grade in the covered terranes that will be the exploration focus of the twenty-first century.

Shear-zone-related gold–quartz veins in granitoid intrusions are commonly intimately associated with mafic dikes, which may have a profound influence on the localization, orientation, and kinematics of auriferous shear zones. The Bourlamaque pluton of the Val-d'Or district contains several economic auriferous shear zones, most of which follow and overprint diorite dikes. Mineralization in all deposits consists of quartz–tourmaline–pyrite veins in reverse- oblique orientation with a significant range of strike, dip, and slip direction. The geometry and kinematics of shear zone and vein array within the pluton is more complex than the simple conjugate pattern predicted for a deforming homogeneous intrusion. The stress tensor determined from the auriferous shear zones within the pluton indicates the same northerly-directed compression recorded by similar shear zones outside the pluton. This indicates that the complex shear zone and vein pattern within the pluton reflects the influence of diorite dikes, which acted as weak layers that were activated during subsequent deformation, showing the importance of layer anisotropy in auriferous shear zone development.The plunges of orebodies bear simple geometric relationships to the slip direction along a host shear zone: these are generally perpendicular to, or in some cases parallel to, the slip direction. Knowledge of the slip directions along activated dikes would therefore allow prediction of the possible plunge(s) of orebodies at early stages of exploration programs. Slip direction along an activated layer is controlled by the orientation of the layer with respect to the stress field and by the relative magnitudes of the three principal stresses. Using techniques developed for analysis of fault slip data, both parameters can be determined, provided there is a sufficient database, and slip direction can be predicted for activated layers of any orientations.

Certain trends in alteration are commonly observed near Archean mesothermal gold deposits. This study was designed to monitor the changes in composition of fluid inclusions using solid probe mass spectrometry by: (1) establishing and verifying systematic changes in the composition of the fluid during fluid-rock interaction, (2) characterizing the composition and behavior of fluids in the vein and the wall rock in order to establish a link between them, (3) elucidating new insights on the role of fluid immiscibility in gold deposition, and (4) scrutinizing the results for potential use in exploration.Three representative sites of gold mineralization were chosen: a large deposit with appreciable amounts of free gold (Sigma mine in Val d'Or), a second deposit with a smaller amount of free gold (Norbeau mine in Chibougamau), and a third area of gold mineralization with altered wall rock and little veining (Tadd prospect in Chibougamau). Fluid inclusions were carefully selected to sample different alteration zones and the gold mineralization at each site. In most of the cases, varying degrees of unmixing of CO2-H2O were noted.Mass spectra results show that during alteration all generations of inclusions at all sites contained CO2 and H2O. For the alteration-hosted deposit a decrease in X(CO2) could be mapped outward from the mineralized zone. The most striking result was that fluid inclusions associated with the main mineralizing episode at Sigma showed a nearly complete segregation into CO2 and H2O end members. This can be interpreted as the result of extreme fluid pressure fluctuations. This characteristic was absent from the Norbeau veins where free gold exists in very limited amounts. A test on a high-grade ore shoot from the Doyon mine resulted in a pattern similar to Sigma. We suggest extreme pressure fluctuations were limited in vein-hosted deposits like Norbeau and gold was precipitated essentially from fluid-rock mineral interaction. Unmixing was the most likely cause of free gold deposition. Mineralizing systems may have undergone major fracturing events at time of free gold deposition.This study suggests high CO2/H2O ratios are related to gold mineralization in altered rock-hosted deposits but that the ratio may be an unreliable guide in vein-hosted deposits because of the variable unmixing pattern of CO2-H2O fluids. Decrepitometry may prove a useful and rapid method for identifying CO2-H2O unmixing and bonanza gold potential.

The Casa-Berardi gold deposits are located 180 km north of Rouyn-Noranda within Archean rocks of the Abitibi Subprovince. The Casa-Berardi deformation zone (CBDZ), which contains these deposits, is an east–west trending structure that is recognized along a minimum distance of 80 km. Ductile deformation has affected the lithologies in the central portion of the CBDZ. The CBDZ juxtaposes distinct sedimentary and volcanic lithostratigraphic packages which locally display opposite facing directions. Within the CBDZ the Casa-Berardi fault has been recognized; it is a brittle structure with a reverse motion. This fault represents a distinct element associated with the progression of deformation in the CBDZ and appears late in the geodynamical evolution of this deformation zone. Two other deformation zones have been identified in this region: (i) the east–west-trending Boivin-Paradis deformation zone, which has limited lateral extent and is located at the periphery of a granitoid intrusion, and (ii) the Laberge deformation zone, which has a northwest to east–west trend and crosscuts the regional structural grain. Most of the economic gold mineralization in the Casa-Berardi deposits is found in the second set of four recognized vein and fracture sets. The mineralized veins are found within zones of intense deformation which are preferentially developed close to contacts between sedimentary and volcanic units. The geometry of the different sets of quartz veins, their relative chronology, and their respective hydrothermal alterations can be explained by the progression of deformation within the CBDZ and bracket the gold mineralization as syn- to late-tectonic. The CBDZ is distinguished from the other two deformation zones by its greater lateral extent, the juxtaposition of distinct lithological domains, the complexity of its internal fabrics, and the relative length of the deformation event. These features indicate that in contrast with the other two deformation zones, the CBDZ was the site of a major mineralizing event. The recognition of contrasting characteristics between the different deformation zones has repercussions on the understanding of the different criteria that control gold deposition and, ultimately, on gold exploration.