ALBERT

Description: SHRIMP-RG U-Pb zircon geochronologic data together with published U-Pb ages from the succession of quartz diorite porphyry intrusions in the western porphyry Cu-Au centers at Reko Diq, western Balochistan, Pakistan, indicate the emplacement of four spatially distinct porphyry Cu centers over ~1 m.y. in the Miocene between ~12.9 and 11.9 Ma. Intrusive activity in the western porphyry cluster began with emplacement of hornblende-rich quartz diorite porphyry associated with the northern and uneconomic H79 porphyry Cu center with U-Pb ages of 12.87 ± 0.27 and 12.72 ± 0.3 Ma. To the south of the H79 porphyry Cu center, successively younger porphyry intrusive centers formed along a south-southwest trend. The economic H15 center has U-Pb zircon ages for four quartz diorite porphyry intrusions between 12.58 ± 0.29 and 12.18 ± 0.19 Ma with three of the porphyry intrusions that span the intrusive sequence in the H15 center having essentially identical ages between 12.58 ± 0.29 and 12.49 ± 0.20 Ma. The adjacent economic H14 center to the south has U-Pb ages on zircons for five quartz diorite porphyry intrusions between 12.22 ± 0.15 and 12.00 ± 0.16 Ma with four of the porphyry intrusions spanning the intrusive sequence having ages between 12.14 ± 0.14 and 12.00 ± 0.16 Ma. The southern end of the trend is the subeconomic H13 complex where mineralized quartz diorite porphyry has a U-Pb age on zircon of 11.93 ± 0.14 Ma. Two new Re-Os ages on molybdenite reported herein agree with the magmatic ages for the H15 (12.43 ± 0.05 Ma) and H14 complexes (11.95 ± 0.05 Ma), whereas previously published Re-Os ages on molybdenite are younger (H15: 11.73 ± 0.04 Ma; H14: 11.82 ± 0.04 Ma) than the youngest dated intrusion in those centers. Published K-Ar ages on hydrothermal biotite from the H79 center (13.0 ± 0.2 Ma), H14 (12.0 ± 0.2 Ma), and H13 (12.0 ± 0.2 Ma) indicate the individual system cooled rapidly. Overall the combined U-Pb geochronology dataset for the four porphyry intrusive centers suggests short-lived magmatic systems characterized each center. However, the available Re-Os ages on molybdenite suggest the potential for complexity in associated hydrothermal systems at the H15 and H14 porphyry Cu-Au centers.

Description: Porphyry Cu (±Mo ±Au) and epithermal Au-Ag deposits are major sources of mined metals and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings. The igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings, but while many have produced acidic magmatic fluids, very few, past or present, have produced sufficient metal, chlorine, and sulfur enrichments necessary to engender an ore deposit. The reasons for this remain uncertain. We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element (REE) abundances in zircon, a nearly ubiquitous and robust trace mineral in crustal magmas. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly Eu N /Eu N * ≥0.4). We interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to SO 2 degassing from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S 6+ ) to SO 2 (S 4+ ) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO 2 -rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration.

Description: The Lower Main zone of the Lorraine alkalic porphyry Cu-Au district, BC, Canada, is hosted in an intrusive complex that comprises pre- and late mineral biotite pyroxenites, monzonites, and syenites. Some of the biotite pyroxenites have interstitial sulfides, which appear similar to net-textured sulfides in magmatic ore deposits; however, the pyroxenites have undergone several stages of hydrothermal alteration. Main-stage mineralization produced sulfide zonation patterns consisting of a bornite-chalcocite or bornite-chalcopyrite core grading outward to domains of chalcopyrite, chalcopyrite 〉 pyrite, and a peripheral domain of pyrite with minor chalcopyrite. Syenite in the inner bornite-chalcopyrite zone typically contains abundant turbid K-feldspar (〉70%), whereas syenite marginal to the bornite-chalcopyrite core contains less K-feldspar (50–70%), indicating an increase in K metasomatism of syenites toward the core of the deposit. Main-stage mineralization predominantly occurs as fine-grained disseminated sulfides in syenite, biotite pyroxenite, and fine-grained K-feldspar biotite rock. Textural analyses have shown that (1) primary magmatic diopside in contact with sulfides has corroded and actinolite-altered margins, suggesting alteration of primary minerals, (2) scalloped relicts of biotite and diopside occur as inclusions in sulfides, implying that sulfides have replaced primary minerals, and (3) the deposit-scale sulfide zonation patterns overprinted numerous rock types, including the biotite pyroxenites that contain interstitial sulfides. These results provide evidence for replacement-style mineralization at Lorraine, where primary magmatic biotite and diopside were totally or partially replaced by sulfide minerals during main-stage alteration. U-Pb zircon dates from pre- and late mineral syenite dikes show that the timing of main-stage mineralization occurred between 178.8 and 176.0 Ma. Lorraine is hosted within the Quesnel island arc terrane, and mineralization and magmatism at Lorraine postdate accretion of the Quesnel terrane to ancestral North America by approximately 7 to 10 m.y. The Lorraine deposit represents the youngest known alkalic Cu-Au porphyry deposit within the Quesnel terrane, and appears to coincide with the last gasp of alkalic magmatism within the Quesnel terrane during the Mesozoic.

Description: The Late Triassic Southwest zone alkalic porphyry Cu-Au deposit in the Galore Creek district, northwestern British Columbia, is hosted in hydrothermally cemented breccias that overprinted older matrix-rich breccia and megacrystic orthoclase-phyric syenite and monzonite dikes. Synmineralization cemented breccias and biotite-phyric monzodiorite dikes are localized at the contact of the matrix-rich breccia and host syenite and monzonite dikes, as well as in crosscutting faults. Regional shortening tilted the Southwest zone deposit so that the cemented breccias now dip 50° to 60° southwest. Four paragenetic stages formed the deposit. A premineralization stage (1) consists of the emplacement of coherent syenite and monzonite host rocks that are cut by a matrix-rich breccia. The synmineralization stage (2) produced potassic-altered rocks and phlogopite-magnetite-K-feldspar–cemented breccia and veins, and most of the Cu-Fe sulfides. A halo of stage 2 diopside-magnetite-grandite-phlogopite calcic-potassic veins and altered rocks formed around the potassic domains and Cu ore zones. Alteration during stage 2 probably formed at temperatures of about 500° to 425°C from oxidized saline magmatic-hydrothermal fluids. Contemporaneous high (a Ca 2+ )/(a H+ ) 2 conditions peripheral to the potassic domains are required to form the marginal calcic-potassic and calcic assemblages. The potassic and calcic-potassic alteration domains were overprinted by propylitic and calcic alteration assemblages due to the retrograde influx of an admixture of cooler magmatic and nonmagmatic fluids. Sphene from a stage 2 calcic-potassic vein yielded a 202.8 ± 2.1 Ma age (U-Pb SHRIMP-RG) that is interpreted to reflect the approximate time of hydrothermal alteration. Late-mineralization stage (3) alteration overprinted the cemented breccias and comprises sericite-anhydrite (sericitic), grandite (calcic), and chlorite-epidote-carbonate-pyrite (propylitic) assemblages. Localized late-mineralization and Cu-poor stage (4) K-feldspar-Fe carbonate-specularite (carbonate-potassic) and quartz-pyrite veins (quartz veins) cut stage 2 and 3 assemblages. Chalcopyrite and bornite dominate the core of the deposit, and grade outward to chalcopyrite 〉 pyrite and to pyrite 〉 chalcopyrite, flanked by a pyrite-only halo. The core of the system is Cu-Au rich, grading outward to an Au-enriched, Cu-poor halo coincident with pyrite-bearing propylitic alteration. Gold-copper ratios increase upward within stage 2 potassic zones and outward from the core. Variations in pH, redox, and temperature along and across fluid flow paths best explain the change to higher Au/Cu values upward and away from the Cu-rich potassic domains.

Description: The moderately tilted and faulted Early Jurassic Mt. Milligan Au-Cu deposit provides a cross-section view of the hydrothermal alteration, sulfide mineralogy, and geochemical zonation of a silica-saturated alkalic porphyry system over a vertical distance approaching 700 m. Magnetite-bearing potassic alteration and associated Au-Cu form a core to the system in the central monzonitic stock and adjacent basaltic trachyandesite host rock. Lateral to the high-temperature core are sodic-calcic and inner and outer propylitic alteration assemblages. Chalcopyrite dominates the high-temperature potassic core whereas pyrite is the predominant sulfide within and outboard from the sodic-calcic assemblage. A funnel-shaped remnant of carbonate-rich phyllic alteration of the host supracrustal rocks in the fault-bounded 66 zone represents the upper auriferous alteration in the alkalic porphyry Au-Cu system. Alteration mineral assemblage, S isotope (ranging from 34 S –5 relative to Canon Diablo Troilite in the core to 34 S +0.5 in the periphery), and limited fluid inclusion data suggest mineralization at the Au-Cu alkalic porphyry system was derived from an oxidized, CO 2 -bearing magmatic fluid that rose upward along the margins and through the Magnetite Breccia stock. Laterally from the Au-Cu mineralized potassic core, magmatic fluid evolved through water-rock interaction, mixing with an external fluid as shown by a shift in calculated 87 Sr/ 86 Sr 0 for alteration minerals to values higher than magmatic values, declining temperature, or some combination of all. Variations in trace element concentrations of epidote (V, Mn, Sb, Zr, As, and Bi) and pyrite (Mn, As, Zr, Pb, and Bi) across the deposit show a local high degree of variability, but general increases or decreases in overall trends in their median values are inferred to reflect the hydrothermal evolution of the system. Pistachite ratios of epidote show an outward decrease in ferric iron as recorded in the pistachite ratio changing from P S36 to P S25 , suggesting less oxidizing conditions on the system periphery. Additionally, light rare-earth elements in epidote fractionate toward the core of the deposit, and the height of positive Eu anomalies also appears to have a similar spatial trend. In pyrite, there is a general increase in trace element concentration toward the epidote-pyrite–rich outer propylitic assemblage forming the system periphery. Alteration mineralogy and trace element signatures indicate the southeastern portion of the deposit, the 66 zone, is a down-dropped segment from higher in the paleohydrothermal system. S isotope signatures of sulfides within and surrounding the stratiform Upper Trachyte unit in the 66 zone indicate structural channeling of oxidizing fluids that likely were the distal and cooler expression of more oxidizing magmatic-derived hydrothermal fluids responsible for potassic alteration and Au-Cu in the subjacent Magnetite Breccia zone, the main orebody.

Description: Polyphase veins at the low-grade, structurally controlled epithermal gold deposit Endeavour 42 in the Junee-Narromine volcanic belt of the Ordovician to Silurian Macquarie Arc, central New South Wales (Australia), record multiple tectonically driven fluid-flow events during the mid-Paleozoic. Ordovician host rocks consist of a volcano-sedimentary succession intruded by trachyandesite and andesitic lavas and intrusions, as well as a volumetrically significant Ordovician diorite sill. The entire succession was tilted and faulted prior to the emplacement of multiple mafic to intermediate dike phases, commonly localized along faults. Gold-bearing veins cut all lithologic units at the Endeavour 42 deposit, with the exception of rare late-stage intrusions. Economic gold is restricted to faults, fault-hosted breccias, and veins with two distinct populations: (1) an older, steeply dipping conjugate vein set, cut by (2) two younger, moderately dipping vein sets. Both steep and inclined, moderately dipping vein sets contain a similar mineral assemblage, implying formation during a single epithermal system. Veins are generally 1 to 10 mm thick, commonly straight walled, but locally form overlapping veins with hard linkages. The steep veins reactivated two sets of older faults, fault-parallel fractures, and dike contacts, and accommodated subhorizontal extension. High gold grade, hydrothermally cemented breccias are associated with early gold-bearing veins and localized along deposit-scale faults. Crosscutting the early steep gold-bearing vein sets are inclined gold-bearing veins with moderate dips to the southwest and west. These opened as a new vein set and as veins localized along and at a slight angle to westward-dipping bedding planes, respectively. The geometry of the inclined, moderately dipping veins either represents a sudden change in the far-field stress state to near-vertical minimum compressive stress, thereby suggesting a change from an extensional to a contractional environment, or it may reflect local changes in the differential stress state with subhorizontal northeast-southwest tension. Late veins and thrust faults in the deposit indicate a postmineral contractional environment. The Endeavour 42 deposit underwent tectonic burial until it was exhumed in the early Carboniferous, as indicated by 40 Ar/ 39 Ar plateau ages of 362.7 ± 2.1, 356.3 ± 1.8, and 344.5 ± 2.1 Ma, toward the end of the Kanimblan Cycle.

Description: The Late Triassic Galore Creek porphyry district is the largest accumulation of Cu-Au prospects in British Columbia, Canada, in terms of contained metal. The principal economic resource is the Central zone deposit. It comprises three high-grade centers: the Au-rich North and South gold lenses and the Cu-dominated Central replacement zone. Alteration footprints, ore shells, and porphyry intrusions in these centers dip 45° to 60° to the west in the North gold lens and southwest in the South gold lens and Central replacement zone, indicative of significant postmineral tilting. This inclination created an oblique vertical expression of the Central zone at surface and at depth, providing a +700-m depth profile through the deposit. Galore Creek is considered to be the end-member of the silica-undersaturated class of alkalic porphyry Cu-Au deposits. It displays a unique hydrothermal alteration footprint, generally devoid of quartz veining. Despite the lack of crosscutting veins, a paragenetic sequence of several discrete potassic and calcic alteration events has been established. During main-stage alteration, two mineralization events occurred that formed the economic resource in the Central zone. Highly oxidizing fluids derived from porphyritic syenite to monzonite intrusions produced initial potassic alteration and sulfide mineralization in the North gold lens and South gold lens. Cu-Au ore shells are characterized by a gold-rich core dominated by bornite associated with hematite-dusted orthoclase, specular hematite, and anhydrite. The second period of hydrothermal activity caused brecciation and calcic alteration in the Central replacement zone. The calcic fluids formed hydrothermal cement grading from Ti-rich andraditic to grossularitic garnet in the core of the breccia to a diopside and magnetite-dominated assemblage at the margins. Calcic alteration also formed in the surrounding host rock. This alteration event was followed by potassic alteration and mineralization, characterized by biotite, anhydrite, and chalcopyrite in the Cu-rich ore shell of the Central replacement zone. Most of the Au is hosted in bornite that is spatially associated with oxidized (reddened) rock units dominated by an alteration assemblage of orthoclase ± hematite ± specular hematite ± garnet ± anhydrite. High-grade Cu in the Central zone is related to abundant chalcopyrite that has commonly replaced an assemblage of biotite ± magnetite ± diopside. This distinct spatial separation strongly suggests that the local redox environment within the wall rocks governed sulfide and gold distributions. The switch from early Au-rich mineralization to late-stage Cu-dominated mineralization appears to have been controlled by varying redox conditions within the magma. The change from more oxidized alteration assemblages (Fe 3+ 〉 Fe 2+ ) to more reduced assemblages (Fe 3+ ≤ Fe 2+ ) is indicative of a change in oxidation potential that may have propelled sulfide saturation in the magma and, thus, the subsequent depletion of most of the Au.

Description: The volcanosedimentary rocks that host the Au-rich porphyry Cu deposits of the Cadia Valley preserve the products of episodic volcanism that erupted into a large sedimentary basin. Volcanogenic sedimentation, including the Forest Reefs Volcanics, overwhelmed the fine-grained sedimentary component that characterized much of the Weemalla Formation. The Forest Reefs Volcanics evolved as a relatively low relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes, and subvolcanic intrusions (dikes and sills). Stacked lava sequences, including hyaloclastites, massive lavas, and their reworked equivalents, are up to 1 km thick, forming significant intrabasinal topography. Explosive volcanism occurred during the late stages of Forest Reefs Volcanics deposition. These air-fall deposits, combined with coexisting shallow-water faunal assemblages, imply that volcanism became locally emergent. Continuity of sedimentation between underlying deep marine basin deposits of the Weemalla Formation and Forest Reefs Volcanics, coupled with the predominance of sheet-like, laterally continuous debris flow and other coarse-grained sedimentary deposits, implies that volcanism and related sedimentation persisted in an active sedimentary basin marginal to an oceanic island arc. Deposition of the Forest Reefs Volcanics spanned the Late Ordovician to Early Silurian. Monzonite fragments (identical to the ore-related intrusions) are abundant in sedimentary breccias found at the top of the preserved volcanic stratigraphy. This finding, combined with available absolute ages of crosscutting intrusions and associated hydrothermal alteration and mineralization, suggests that some volcanosedimentary units were deposited synchronously with or immediately after the last known porphyry-related hydrothermal event in the Cadia Valley. The Au-rich porphyry deposits were therefore emplaced into an evolving sedimentary basin with episodic intrabasinal magmatism. Permeable horizons and volcanic lithofacies can preferentially host alteration and mineralization that can extend over several kilometers in lateral extent. This finding suggests that hydrological models of fluid flow in porphyry systems need to take basin architecture into account.

Description: The Paleocene to Eocene southern Peru porphyry belt contains three significant porphyry Cu-Mo deposits at Cuajone, Quellaveco, and Toquepala. Ten new zircon U-Pb Sensitive High Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG) ages for Cuajone and Toquepala, together with published ages for Quellaveco, establish a magmatic history characterized by episodic events. Punctuated magmatism at Cuajone is distributed over approximately 13 m.y., at Toquepala over 8 m.y., and at Quellaveco over 6 m.y. The ages of the porphyry intrusions hosting or associated with the introduction of Cu and Mo at the three deposits show remarkable similarity, with emplacement beginning and ending at approximately 56.5 to 53.0 Ma at Cuajone, 57.0 to 54.0 Ma at Toquepala, and at 58.4 to 54.3 Ma at Quellaveco. Field relations coupled with the U-Pb ages for synmineral intrusions suggest very similar timing of the cupriferous hydrothermal systems, with the youngest pyritiferous and Cu-poor hydrothermal systems being associated with porphyry intrusions as much as 2 m.y. younger than significant Cu introduction. Overall, the porphyry Cu-Mo intrusive complexes represent the youngest magmatic complexes formed during the Late Cretaceous and early Tertiary arc, having formed prior to eastward migration of the magmatic locus.

Description: Rapidly changing ore deposit types in the Late Cretaceous and early Tertiary in the Southeastern Anatolian orogenic belt in east-central Turkey, from Baskil on the south to Divrigi on the north, broadly reflect shifting tectonic environments in an evolving collisional orogen, a part of the extensive Tethyan orogen. Late Cretaceous (~83–79 Ma) subduction-related arc magmatism in the Baskil arc was followed by extension that exhumed the metamorphic basement, unroofed the obducted ophiolites, and filled basins with Maastrichtian to Paleocene subaerial and submarine sedimentary rocks. Calc-alkaline (Divrigi pluton, ~76–73 Ma) and alkaline igneous rocks (Hasançelebi and Keban districts, ~76–74 Ma) accompanied post-Baskil arc extension. In the early to middle Eocene, shallow and locally voluminous intrusions of intermediate composition (Çöpler, Kabatas, Bizmisen-Calti, Dedeyazi-Polat, Karamadazi, and Horoz), localized mainly along major strike-slip faults, formed during closure of the Neotethyan Ocean. These intrusions are dated between ~50 and 44 Ma, peaking at 48 Ma. Geochemical data from a suite of igneous rock samples across the transect reflect temporally based changes from arc-type to late orogenic environments, as demonstrated by Late Cretaceous rocks in the Baskil area on the south and the more deeply eroded Late Cretaceous plutons in the Divrigi area on the north (Divrigi). These magmatic products were followed in the Eocene by calc-alkaline magmatism related to either incipient slab-rupture or crustal-scale faults that cut from northeast (Çöpler, Kabatas, Bizmisen-Calti, and Polat-Cavuslu) to southwest (Karamadazi and Horoz) obliquely across the transect. The multielement patterns for the Late Cretaceous magmatic rocks resemble the patterns for magmas generated in enriched source regions compared to primitive mantle and mid-oceanic-ridge basalt. Locally, at Divrigi-Hasançelebi and Keban, the geochemical characteristics are suggestive of the influx of low field strength elements into the mantle source regions from a subducted slab, and an arc to postcollisional to late-orogenic setting for their generation. Eocene magmatic rocks display similar geochemical characteristics regardless of the geographic setting and associated hydrothermal system. Their characteristics favor a subduction zone enrichment that was inherited either from an older event, or from an older slab, or from a mantle wedge contaminated by a contemporaneous subduction component. 40 Ar/ 39 Ar and U/Pb geochronology on biotite, zircon, K-feldspar, hornblende, and sericite from a suite of magmatic rocks and alteration zones defines three magmatic-related metallogenic events in the Southeastern Anatolian orogenic belt between 78 and 44 Ma. The oldest metallogenic event between ~78 to 74 Ma is related to arc-type calc-alkaline magmatic rocks (83–79 Ma) intruded into the Bitlis-Poturge and Malatya-Keban metamorphics and overlying Komurhan-Ispendere ophiolites of Tauride platform. Uneconomic porphyry-type Cu-Au systems formed at Topalkem-Baskil (Elazig) and Ispendere-Sisman (Malatya). A second metallogenic event, between ~74 and 69 Ma, is contemporaneous with postcollisional exhumation and extension-related magmatism. During this event, calc-alkaline followed by alkaline magmatism (76–73 Ma) was accompanied by formation of iron oxide copper-gold (IOCG) systems at ~74 to 73 Ma at Divrigi (Sivas). Alkaline magmatism at ~76 to 74 Ma is associated with IOCG systems at Hasancelebi (Malatya) and porphyry Cu-Mo and Pb-Zn veins at Keban (Elazig). The youngest metallogenic event between ~50 and 40 Ma is represented by porphyry Cu-Au deposits associated with calc-alkaline intrusive complexes at Çöpler and Kabatas and Fe skarns at Karamadazi, Horoz, Dedeyazi, and Bizmisen-Calti.