Mass Transfer During Volatile Exsolution in Magmatic Systems: Insights from the Analyses of Silicate Melt and Magmatic Fluid Inclusions
We analyzed silicate melt and magmatic fluid inclusions in numerous magmatic systems to quantify the distribution of elements between silicate melts and magmatic volatiles. LA-ICPMS analyses of co-existing silicate melt and fluid inclusions, entrapped in miarolitic quartz crystals, allowed direct quantitative determination of fluid/melt partition coefficients. Investigations of various granitic systems (peralkaline to peraluminous in composition, log fO2=NNO-1.7 to NNO+4.5) exsolving fluids with various chlorinities (1 to 13 mol/kg) allowed us to assess the effect of these variables on the fluid melt partition coefficients. Partition coefficients for K, Pb, Zn, Ag and Fe show a nearly linear increase with the chlorinity of the fluid. This suggests that these metals are primarily dissolved as Cl-complexes, and neither oxygen fugacity nor the composition of the melt affects significantly their fluid/melt partitioning. Partition coefficients of B, Mo, As, Sb and Bi are highest into low salinity (1-2 mol/kg Cl) fluids indicating dissolution as non-chloride (eg., hydroxy) complexes. Fluid/melt partition coefficients of copper are highly variable, but highest between vapor like fluids and silicate melt, indicating important role of ligands other than Cl. In-situ quantitative determination of fluid/melt partition coefficients in mafic systems with the use of natural samples is limited due to the lack of well preserved fluid inclusions. However, very high excess concentrations of Cu and Ag have been analyzed in plagioclase-hosted silicate melt inclusions in basaltic andesites compared to those in co-genetic pyroxene and olivine (up to 4800 ppm Cu vs. 40-200 ppm Cu in plagioclase vs. pyroxene and olivine hosted melt inclusions). The excess Cu is due to heterogeneous entrapment of a high temperature (~1000 oC) vapor phase rich in Cu, Ag and S. This suggests that high-temperature magmatic vapors may play an essential role in the transport of Cu, Ag and S from mafic to felsic magmas in complex magmatic reservoirs.
Age, Geochemical, and Fluid Characteristics of the MAX Porphyry Mo Deposit, Southeastern British Columbia
MAX is a porphyry-style Mo deposit located 5 km southwest of Trout Lake village in southeastern British Columbia. Molybdenum mineralization is hosted within several steeply-plunging granodiorite dykes and a well developed quartz-vein stockwork. Major and trace element data indicate that MAX is typical of the low-F porphyry Mo deposit classification. Crosscutting relationships between the four intrusive phases and mineralized quartz veins reveal a temporally complex history of magmatic and hydrothermal activity. Mineralized quartz veins have been subdivided into paragenetic assemblages based on vein style and crosscutting relationships. Sheeted quartz ± feldspar ± molybdenite veins crosscut irregular molybdenite stringers within granodiorite, and are in turn crosscut by variably oriented quartz ± molybdenite ± feldspar veins within the stockwork. These Mo veins are cut by post-magmatic quartz-carbonate veins containing sphalerite, argentiferous galena, and rare molybdenite. Three molybdenite samples were collected from early and late Mo-stage veins for Re-Os dating to constrain the first and last Mo mineralizing events within this paragenetic sequence. All three dates overlap within analytical error, and yield a weighted average age of 80.3 ± 0.4 Ma. Samples from each of the paragenetic vein sets were collected for fluid inclusion study. Saline aqueous and more dilute aqueous-carbonic inclusions are the two most common inclusion types at MAX. Both types are present in all veins; however, a progression towards increasingly carbonic fluids is evident from the Mo- to Ag- Pb-Zn-bearing veins. Fluid inclusions from Mo-bearing veins have homogenization temperatures (Th) ranging from 160°C to 350°C (mode at 310°C), and salinities ranging from 1 to 12 wt.% NaCl equiv. with two modes at 4 and 9 wt.% NaCl equiv. Fluids with these temperatures and salinity ranges are typical of the low-F porphyry Mo deposit type. Peripheral Ag-Pb-Zn veins are dominated by aqueous-carbonic fluid inclusions, but with similar Th (160°C to 330°C) and salinities (1 to 12 wt.% NaCl equiv.) to the Mo veins. These similarities between the Mo- and Ag-Pb-Zn-bearing vein fluids suggest a genetic link between porphyry molybdenum mineralization and peripheral base-metal-silver showings at the MAX.
Integrated Studies of the Genetic Linkages Between Jurassic Porphyry Cu-Au (Mo) and Epithermal Au-Ag deposits in the Toodoggone District of North-Central British Columbia, Canada
Possible genetic linkages between proximal porphyry Cu-Au (Mo) and epithermal Au-Ag deposits in the Toodoggone district of north-central British Columbia were investigated during a 3-year NSERC-CRD project (2004-2007) by integrating district-scale geological mapping and geochronological studies with detailed deposit models of the key porphyry and epithermal systems. Episodic plutonism from ca. 218 to 190 Ma coincided with the formation of the largest porphyry Cu-Au (Mo) systems from ca. 202 to 197 Ma, with only minor porphyry mineralization occurring from ca. 197 to 194 Ma. The Fin porphyry Cu-Au-Mo deposit differs from the other porphyry systems by having host-rocks with distinctly lower REE and immobile element abundances and an age that is 16 m.y. older than any other porphyry occurrence in the district. Porphyry systems are spatially restricted to exposed Asitka and Takla Group basement rocks, and more rarely, the lowest member of the Hazelton Group (i.e., the ca. 201 Ma Duncan Member). These country rocks are most commonly exposed in the southern half of the district, where high rates of erosion and uplift have resulted in their preferential exposure. In comparison, low- and high-sulfidation epithermal systems are more numerous in the northern half of the district, where younger, overlying Hazelton Group rocks are mainly exposed. Any cogenetic porphyry systems in northern areas are likely to be buried beneath Hazelton Group rocks. High-sulfidation epithermal systems formed at ca. 201 to 189 Ma, whereas low-sulfidation systems developed at ca. 196 to 186 Ma, demonstrating a temporal coincidence with porphyry systems elsewhere in the district. Amongst all studied epithermal systems, the Baker low-sulfidation epithermal deposit displays the strongest demonstrable genetic link with magmatic fluids. Fluid inclusion studies indicate that its ore fluids were hot (>468 C), saline, and deposited metals at depths >2 km. Sulfur, C, O, and Pb isotope data confirm the involvement of a magmatic fluid. In contrast, the Shasta, Lawyers, and Griz-Sickle low-sulfidation epithermal systems do not display a clear association with magmatic fluids. Instead, their fluid inclusion data indicate the involvement of low- temperature (175 to 335 C), low-salinity (1 to 11 equiv. wt. % NaCl) fluids that deposited metals at depths of <850 m. Their corresponding isotope (S, C, O, H, Pb) data suggest the interaction between meteoric and/or metamorphic ore fluids with basement country rocks. The three largest porphyry deposits studied (Kemess South, Kemess North, and Pine) have halite-bearing fluid inclusions (brines) that homogenize by halite disappearance. Such behavior precludes the possibility that these brines and vapors are conjugate end- members of a supercritical fluid undergoing immiscibility at the site of fluid entrapment. These brines may be direct magmatic exsolutions, but it is more likely that they are the result of fluid immiscibility at depth, with subsequent mechanical remixing and entrapment of ascending vapor and brine phases higher up in the hydrothermal system. This non-equilibrium fluid entrapment process is common in porphyry systems, and it complicates interpretation of in situ microanalytical studies of fluid inclusions to explain vapor phase metal transport in natural porphyry-epithermal systems such as occur in the Toodoggone.
Gold-rich sulfide melt inclusions in xenocrysts from a mid-crustal magma chamber, Mt. Milligan porphyry deposit, British Columbia, Canada
Very coarse-grained amphibole xenocrysts (potassian magnesiohastingsite) hosted in an early monzonite stock at the Mt Milligan Cu-Au porphyry deposit, British Columbia, Canada contain coeval sulfide and silicate melt inclusions of primary origin. The sulfide melt inclusions have a bulk composition comparable to Cu-rich ISS. Late growth zones in the amphibole are devoid of sulfide inclusions and contain only low salinity, chalcopyrite-bearing fluid inclusions(average 7.4 wt% NaCleq.). Thermobarometry constrains the minimum conditions of sulfide entrapment (amphibole crystallization) to ∼8 kbar and ∼700°C. LA-ICPMS analyses of 22 sulfide melt inclusions show that it was highly enriched in Au (50± 20 ppm, 1σ), Ag (140± 70 ppm, 1σ) and Ni (5000 ± 3000 ppm, 1σ). Ratios of Cu/Au (7500± 2500, 1σ) and Au/Ag (0.45± 0.24, 1σ) are identical to metal ratios in porphyry- stage veins, demonstrating that these metals were not fractionated from one another during suspected volatile exsolution, fluid-melt partitioning, and subsequent transport and precipitation of ore metals. The extremely Au- rich composition of the sulfide melt may reflect fractional crystallization of the sulfide liquid prior to entrapment in the amphibole. Both the xenocrysts and rare, high Mg, alkali basalt xenoliths hosted in the intrusions are depleted in Cr, Co, Ni and Cu, reflecting the sequestering of the base metals into a sulfide liquid in a mid- crustal magma chamber where amphibole and Cr-spinel were cumulus phases. The results of this study show that a Cu-Au-rich sulfide melt coexisted with a amphibole-saturated alkalic basaltic liquid in mid-crustal magma chamber prior to the emplacement of the main intrusions and associated porphyry stage mineralization at Mt. Milligan. This sulfide melt appears to have destabilized with the appearance (exsolution) of a single-phase low salinity aqueous fluid. Identification and analysis of ore metals in sulfide melt inclusions in relatively common xenocryst phases may serve as a useful exploration tool for predicting the metal ratio of undiscovered Cu-Au porphyry deposits in the Canadian Cordillera.
Hydrothermal Concentration of Zr, Y + HREE in the Lake Zone of the Thor Lake Rare Metal Deposit, Northwest Territories
The Thor Lake rare-metal (Zr, Y, REE, Nb, Ta, Be, Ga) deposit in Northwest Territories contains Canada's largest resource of Zr, Y and HREE and is one of the largest resources of these elements on the planet. Much of the mineralization was clearly concentrated by hydrothermal processes, providing compelling evidence for Zr mobility. Geologically, the deposit is situated at the southern edge of the Slave Province of the Canadian Shield, within the 2150 Ma alkaline to peralkaline Blachford Lake complex consisting of an early suite of gabbro, quartz syenite and granite, which was intruded by the Grace Lake granite and, in turn, by the Thor Lake syenite . A layered alkaline intrusion dominated by nepheline syenite occurs below the Thor Lake syenite and is thought to represent the youngest phase of the complex. This intrusion comprises numerous sub- horizontal layers of sodalite syenite, alkali syenite and lujavrite. Evidence of cumulate and adcumulus textures, cyclic magmatic differentiation and rhythmic layering of mafic and felsic units on scales ranging from tens of centimetres to several metres indicate a complex history of pulsed injection of magma and magmatic differentiation. The upper part of the layered intrusion and the overlying Thor Lake syenite contain the bulk of the rare metal mineralization, with Zr hosted primarily by zircon, Nb primarily by ferrocolumbite and fergusonite, and Y + HREE by fergusonite and zircon. The LREE are hosted by monazite, allanite, bastnaesite and synchisite/parisite. The precursor rocks to the ore were pervasively altered by Fe- and K-rich hydrothermal fluids, which replaced much of the primary mineralogy by magnetite, biotite and K-feldspar and redistributed/concentrated the rare metals including Zr (as secondary zircon). This alteration was overprinted locally by intense sodic alteration. In other rare-metal-rich alkaline complexes such as Strange Lake, Ilimaussaq and Lovozero, the Zr, Nb, Y + REE mineralization has been attributed to magmatic processes alone or a combination of both magmatic and hydrothermal processes. The Thor Lake rare metal deposit may be an unusual example of a deposit of this type where hydrothermal processes were dominant.  Davidson, A. (1978) Geol. Survey of Canada Paper 72-1A, pp. 119-127.
Vent Complexes above Dolerite Sills in Phanerozoic LIPs: Implications for Proterozoic LIPs and IOCG Deposits
New insights into the origin of IOCG (iron oxide copper gold) deposits [e.g., 1, 2, 3] follow from recent studies of Phanerozoic Large Igneous Provinces (LIPs). Detailed seismic studies of the 62-55 Ma North Atlantic Igneous Province and complementary studies in the 183 Ma Karoo and 250 Ma Siberian LIPs reveal thousands of hydrothermal vent complexes (HVCs). Up to 5-10 km across at the paleosurface, these vents connect to underlying dolerite sills at paleodepths of up to 8 km [4, 5, 6, 7]. They originate from explosive release of gases generated when thick sills (>50 m) are emplaced into volatile-rich but low-permeability sedimentary strata. HVCs are phreatomagmatic in origin. Their architecture, economic potential for IOCG-type deposits, and effects on climate strongly depend on the type of host rocks (black shales at Karoo and evaporites at Siberian LIPs) and its fluid (brines) saturation at the time of emplacement. About 250 HVCs associated with the Siberian LIP are mineralized having magnetite in the matrix. Some are being mined for Fe (Korshunovskoe and Rudnogorskoe), but their economic potential for copper and gold mineralization is understudied. These observations from the Phanerozoic LIP record suggest that HVCs should also be an essential component of sill provinces associated with Proterozoic LIPs, with a potential for causing major climatic shifts and IOCG-type deposits, particularly if the host sediments include substantial evaporites. Two examples are discussed here. The 725 Ma Franklin LIP covers 1.1 Mkm2 in northern Canada ; in the Minto Inlier of Victoria Island, this event comprises volcanics, sills, and breccia pipes [9, 10]. The breccia pipes appear identical to HVCs and, furthermore, the presence of evaporites in the host sediments of the Shaler Supergroup suggests (based on the Siberian trap example) the potential for IOCG-type mineralization. Could 1.59 Ga sills, as exemplified by the exposed Western Channel Diabase sills on the eastern side of Great Bear Lake , be the cause of both the Wernecke Breccias of the Yukon (with their hematite, Cu, Co, U and Au mineralization) and the Olympic Dam giant IOCG deposit of the Gawler craton of Australia, which was probably an adjacent block at this time [12, 11]? A dramatic expansion of new targets for IOCGs potentially could be achieved via a systematic survey of sill provinces associated with Proterozoic and Paleozoic LIPs from around the world, with a special focus on those in which the host sediments are evaporate-rich, with the goal of identifying mineralized HVCs.  Hitzman, 2000, In: Porter (ed.) v.1; PGC Publishing;  Williams et al., 2005, Econ. Geol;  Corriveau, 2007, GAC Min. Dep. Div. Spec. Pub 5;  Jamtveit et al., 2004, In: Geol. Soc. London, Spec. Publ. 234;  Planke et al., 2005, In: Dore & Vining (eds) Geol. Soc. London;  Svensen et al., 2006, J. Geol. Soc. London;  Svensen et al., 2008, EPSL;  Buchan & Ernst, 2004 GSC. Map 2022A;  Jefferson et al., 1994, GSC. OF 2789;  Rainbird, 1998, GSC OF File 3671;  Hamilton & Buchan 2007, GSA Ann. Mtg ;  Thorkelson et al., 2001, Prec. Res.
The Role of Fractional Crystallization and Magma Mixing/Mingling in the Genesis of Karacaali Magmatic Complex (Central Anatolia, Turkey) Fe, Mo-Cu Mineralizations
This work brings into focus different metal associations (Fe and Mo-Cu) characteristic for the Karacaali Magmatic Complex (KMC), in Central Anatolia, Turkey. The Mo-Cu mineralization is widespread hosted in rhyolitic-rhyodacitic/granidoid rocks or is related to N-S striking vertical quartz-calcite veins. The Fe mineralization, on the other hand, is hosted in gabbroic/basaltic rocks. Field relations and geochronologic studies on single zircons (U-Pb) point towards a coeval temporal relation between plutonites and volcanites. The relatively overlapping ages between monzonite (73.1 Ma) and rhyolitic rocks (67 Ma) reflect a long lasting gradual crystallization within a zoned magma chamber. This is confirmed by progressive transitional contacts from plutonites into volcanites. Based on detailed field, textural and petrographic studies, granitic and monzonitic rocks have been subdivided into four different facies. These are: porphyritic quartz monzonites, quartz-monzonites, fine-grained granites and porphyritic leucogranites. Furthermore, highly diverse textures and structures, which are typical for hybrid rocks, reveal important magma mixing/migling and fractional crystalization processes. From additional geochemical studies, granitic rocks show high Rb/Sr (1.52), nearly flat REE patterns and strong Eu negative anomalies. However; monzonitic and hybrid rocks have relatively low Rb/Sr ratios (0.37 and 0.32) and depleted HREE patterns. Thus, in this complex, granitic rocks are considered as evolved products from the felsic magma. Strong positive Mo-correlation within the granitoids can be explained by a high degree of magmatic fractionation (Ishihara and Tani, 2004). Therefore, last evolved granitic melts are enriched in Mo- rich volatiles giving rise to molybdenite-quartz-calcite veins. Field, macro-micro and chemical studies evidence a co-magmatic origin for the gabbroic/basaltic-hosted Fe-mineralization. Despite the very close relation between compositional character of granites and ore element associations (Blevin and Chappell, 1992, 1995), the correlation between granite composition and ore elements may be highly complex, mainly due to the very different physical-chemical characteristics of different metals (Fe, Cu and Mo). In the case of the KMC the association of magma mixing/mingling and fractional crystallization processes played a decisive joint role enhancing metal enrichment processes. The intrusion of a Cu- and S-rich basic magma into a semi-evolved acidic magma chamber seems to have caused the sudden segregation of iron- rich melts and re-mobilization of Mo to shallow depths in the magma chamber, into the felsic magma layer.