Shallow, texturally complex granites can be associated, spatially INTRODUCTION and temporally, with hydrothermal alteration and mineralization. Granite-related ore deposits include porphyry and skarn The formation of a granite-related ore deposit represents the confluence type ores, and other hydrothermal deposits that are of many chemical and physical geological factors, including: magma spatially and temporally associated with granitic (sensu composition (e.g. concentrations of ore metals, chlorine, and water); lato) rocks. In this paper, I emphasize characteristics magma oxidation state; the relative timing of crystallization, magma found in porphyry style (Cu‐Mo‐Au) deposits, and some ascent, magmatic volatile phase exsolution; and the depth of em- related systems; however, my treatment here is not meant placement. Early magmatic crystallization of ore-metal-rich minerals to cover the full range of granitic rocks and mineralization coupled with late exsolution of the magmatic volatile phase is that make up the granite-related ore systems. The forpromoted by deeper emplacement levels and lower magmatic water mation of granite-related ore deposits by orthomagmatic‐ concentrations, and leads to low eYciencies of removal of compatible hydrothermal processes is dependent upon many physical elements from melts into the magmatic volatile phase(s). Water and chemical magmatic factors, including (but not limited concentrations in the melt and pressures of crystallization can be to) the composition and evolution of the associated estimated from phase equilibria and textural relationships of horn- magmas, the rate of magma ascent, the depth and blende in some felsic systems. Physical factors aVect the dynamics geometry of magma emplacement, and the local temof fluid (magma, melt and volatile phase) movement, which in turn perature and stress fields at the site of magma emaVect the size of zones of alteration and mineralization; granite placement. Detailed studies of the igneous rocks textures can record the partial history of some dynamic processes. associated with ore deposits can therefore yield critical Miarolitic cavities are good evidence for magmatic volatile phase information regarding the source of metals and ligands exsolution; both miarolitic cavities and pegmatitic texture exhibit in the ore-generative hydrothermal solutions that are evidence for external nucleation of crystals. Nucleation-controlled operative in the formation of granite-related ores. In this phenomena, expressed as variations in granite textures, can result review, I will present a series of narratives, consistent, in from a multiplicity of causes including undercooling of the melt; my view, with prevailing field and experimental data on textural elements such as pegmatite, aplite, skeletal crystals, and the orthomagmatic‐hydrothermal theory for the genesis graphic textures can form as a result of undercooling. Interconnected of some granite-related deposits. volumes of the magmatic volatile phase allow for advection of Some high-level granites (i.e. those that are emplaced hydrothermal ore fluids through the magma. In shallow granitic at depths of ~8 km or less) represent the crystallized systems, miarolitic cavities can be interconnected, consistent with remains of magmas that were associated with subvolcanic, this hypothesis. ore-generative, hydrothermal systems. Therefore, studies of shallow granites, aimed at understanding magmatic evolution and volatile phase exsolution at shallow levels, are necessary if we are to understand ore genesis and the deep inputs to present-day geothermal systems as well as evaluate hazards associated with the volcanic

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A Review of Shallow, Ore-related Granites: Textures, Volatiles, and Ore Metals

The partitioning of trace elements between ilmenite, ulvospinel, armalcolite and silicate melts with implications for the early differentiation of the moon

Precise Re-Os molybdenite ages for the Endako porphyry
molybdenum deposit and the Nithi Mountain occurrence, British Columbia,
Canada, yield new information on the timing of molybdenite deposition. Three
episodes of mineralization are indicated: (1) ~154 Ma, (2) ~148 to 146 Ma,
and (3) ~145 Ma. Re-Os dates agree well with new U-Pb and 40 Ar/ 39 Ar
data from granitoids of the Nithi phase (155.2 ±
1.5 Ma), the Endako phase (148.4 ±
1.5 Ma), and a preore dike (147.4 ±
0.6 Ma), and the Casey phase (145.1 ±
0.2 Ma), plus hydrothermal biotite (144.5 ±
0.75 Ma; Villeneuve et al., 2001). The excellent correlation between the
Re-Os dates from molybdenite and the ages of granitoids from the area
indicates a direct genetic relationship between these plutons and
molybdenite mineralization. These Re-Os data also provide further insight into the
systematics and robustness of Re-Os in molybdenite. The Endako molybdenum
deposit is the product of two episodes of mineralization, both associated
with moderate-salinity (5–15
wt % NaCl equiv) and high-temperature (~440°C)
fluids; a third low-temperature (190°–300°C)
hydrothermal event is associated with Eocene faulting (Selby et al., 2000).
The clear distinction between two of the Re-Os molybdenite dates in the
deposit and the close agreement of these dates to the ages of granitoid
intrusions suggest that the Re-Os system in molybdenite is not reset by
high-temperature (~440°C),
moderate-salinity hydrothermal fluids. In contrast, magmatic biotite in a
~147 Ma preore dike has a 40 Ar/ 39 Ar age of ~145 Ma,
identical to the age of the hydrothermal biotite, indicating that ~440°C
fluids reset the Ar-Ar system in biotite. Hence, we interpret that the Re-Os
isotope system in molybdenite is resistant to resetting by high-temperature
fluids (~440°C),
in contrast to the 40 Ar/ 39 Ar system in biotite. The
Re-Os system in molybdenite is also unaffected by postore hydrothermal
fluids. These data show that molybdenite can provide critical information in
understanding sulfide mineral deposition in ore systems, which is not
recorded by other isotopic systems in alteration minerals.

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Re-Os Geochronology and Systematics in Molybdenite from the Endako Porphyry Molybdenum Deposit, British Columbia, Canada

The ore-element associations of granite-related ore deposits in the eastern Australian Palaeozoic fold belts can be related to the inferred relative oxidation state, halogen content and degree of fractional crystallisation within the associated granite suites Sn mineralisation is associated with both S- and I-type granites that are reduced and have undergone fractional crystallisation Cu and Au are associated with magnetite- and/or sphene-bearing, oxidised, intermediate I-type suites Mo is associated with similar granites that are more fractionated and oxidised W is associated with a variety of granite types and shows little dependence on inferred magma redox state The observed ore deposit-granite type distribution in eastern Australia, and the behaviour of ore elements during fractionation, is consistent with models of ore element sequestering by sulphides and Fe-Ti phases (eg pyrrhotite, ilmenite, sphene, magnetite) whose stability is nominally fO2-dependent Fractional crystallisation acts to amplify this process through the progressive removal of compatible elements and the concentration of incompatible elements into decreasing melt volumes The halogen content is also important S-type granites are poorer in Cl than I-types Cl decreases and F increases in both S- and I-type granites with fractional crystallisation Low Cl contents combined with low magma fO2 in themselves seem to provide an adequate explanation for the rarity of Mo, Cu, Pb and Zn type mineralisation with S-type granites Although such properties of granite suites seem adequately to predict the associated ore-element assemblage to be expected in associated mineral deposits, additional factors determine whether or not there is associated economic mineralisation

The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia

The late-collisional Erzgebirge granites (~325–318 Ma) were INTRODUCTION emplaced at shallow crustal levels in the Variscan metamorphic The Erzgebirge is a NE–SW trending antiformal structure basement shortly after large-scale extension caused by orogenic exposing Variscan crystalline basement rocks at the northcollapse. These granites comprise mildly peraluminous transitional ern margin of the Bohemian Massif. The region is famous I–S-types and strongly peraluminous S-type rocks, which can be among economic geologists for its extensive and highly subdivided into three major groups: low-F biotite granites; low-F variable metallic ore deposits, which have been mined two-mica granites; and high-F, high-P2O5 Li-mica granites. The since the Middle Ages (e.g. Stemprok & Seltmann, 1994; highest degree of differentiation is reached in the Li-mica granites, Tischendorf & Forster, 1994), and for the classic studies which exhibit strongly elevated concentrations of P, F, Li, Rb, Cs, in the fields of economic geology, mining and metallurgy, Ta, Sn, W and U; but very low Ti, Mg, Co, Ni, Sr, Ba, Y, Zr, chemistry and mineralogy (e.g. Werner, 1791; Breithaupt, Hf, Th and rare earth elements. Crystal–melt fractionation is the 1849; Agricola, 1974). dominant process controlling the bulk composition of all groups of As in many of the other Variscan provinces in central granites. However, metasomatic processes involving late-stage residual Europe, granitic rocks make up a large proportion of the melts and high-T orthomagmatic fluids became increasingly more exposed rocks in the Erzgebirge, and a great variety of important in highly evolved units and have modified the abundances compositional and textural types occur (Fig. 1). Knowof mobile elements (P, F, Li, Rb, Cs, Ba, Sr) in the Li-mica ledge of the processes involved in the generation and granites particularly. Isotopic and geochemical characteristics suggest evolution of these silicic magmas is essential for underthat the three granite groups cannot be derived from a common standing heat and mass transport during the Variscan precursor magma. Their discrete compositions are source related, orogeny, and also for the formation of metallic ore and are attributed to melting of quartzo-feldspathic and pelitic deposits. Several lines of evidence indicate that early, crustal lithologies in different proportions. Granites are common in high-temperature Sn–W deposits are associated with the the central European Variscides, but the Erzgebirge is unusual for emplacement of highly evolved, volatile-rich granitic the predominance of evolved Li-mica granites associated with melts (Tischendorf, 1986; Forster & Tischendorf, 1992; economically important Sn, W and U deposits. The abundance of Stemprok, 1993). The role of the granites in formation of Li-mica granites is attributed to a combination of favourable factors: later Permian (U, Pb–Zn) and younger ore mineralization (1) low degrees of anatectic melting of crustal protoliths; (2) wide (F–Ba) is controversial, but good evidence exists that the distribution of fertile lithologies rich in large-ion lithophile elements vein-type U deposits owe their uranium to leaching from and ore elements; (3) extended magmatic differentiation by crystal– the uraninite-rich granites (e.g. Tischendorf & Forster, melt fractionation and subsequent autometasomatism. 1994).

Late-Collisional Granites in the Variscan Erzgebirge, Germany

The partitioning of molybdenum and tungsten between ilmenite (ilm) and silicic melts has been determined at 800 degrees C, 1 kbar of pressure, and at two oxygen fugacities. At the oxygen fugacity of the graphite-methane(-water) buffer, D (super ilm/melt) Mo = 0.42, and D (super ilm/melt) W = 0.39. Near the nickel-nickel oxide buffer, D (super ilm/melt) Mo = 0.22 and D (super ilm/melt) W = 0.5. The variations in the partition coefficients with f (sub o 2 ) are significant at the 0.06 and 0.33 levels for molybdenum and tungsten, respectively. The observed variation with oxygen fugacity is tentatively extrapolated to more complex systems and implies that the oxygen fugacity which prevails during magmatic crystallization may play a role in determining the Mo/W ratio in porphyry, skarn, or other hydrothermal mineral deposits. Examination of the literature on molybdenum- and tungsten-bearing mineral deposits indicates that systems with high W/Mo ratios are rather reduced and exhibit high CH 4 /CO 2 ratios in fluid inclusions, low Fe (super +3) /Fe (super +2) ratios in iron-bearing minerals, and low oxygen fugacities in associated igneous rocks. On the other hand, deposits with high Mo/W ratios are associated with rather oxidized magmatic systems. We suggest that f (sub o 2 ) -dependent sequestration of molybdenum and tungsten in ferromagnesian phases during plutonic consolidation exerts a control on the relative efficiencies of removal of these metals from magmas into ore-forming fluids.

Stephen L. Bouton

Papers

The influence of oxygen fugacity on tungsten and molybdenum partitioning between silicate melts and ilmenite