ALBERT

Description: The Mesozoic deposits in the Central Andes, especially represented by the dominant iron oxide copper-gold (IOCG) mineralization in southern Peru and northern Chile coastal IOCG belt, has emerged as one of the major exploration targets in the Central Andes in the last two decades. These deposits mainly formed during the Middle-Late Jurassic and Early Cretaceous. The major Cu-rich IOCG deposits are located in the Early Cretaceous mineralization belt, which is also the main Phanerozoic example of this type of ore deposit. The Central Andean IOCG deposits lie in a linear array of interconnected Mesozoic continental margin rift basins that record a major phase of extension accompanying subduction along the western margin of Gondwana. The Mesozoic tectonic evolution of the Central Andes since the initial phase of IOCG mineralization can be subdivided into the following: the Tethyan period (165–155 Ma); the South Atlantic period (145–135 Ma) and the Pacific period (120–100 Ma). Central Andean IOCG mineralization was initiated in the Middle Jurassic (165–155 Ma), associated with the high-angle subduction of the Phoenix plate and coeval with the early stage of Gondwana breakup. Following a relatively weak tectonomagmatic stage (135–120 Ma), the peak in Meso-zoic IOCG deposits occurred during the inversion of extensional basins (120–100 Ma), corresponding to the final separation of African and South American tectonic plates.

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 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: Mount Polley is a Late Triassic (~205 Ma) alkalic porphyry Cu-Au-Ag deposit (226.3 thousand tonnes (t) Cu, 21.5 t Au, and 65.1 t Ag), hosted by silica-undersaturated to silica-saturated monzonitic intrusions of the Mount Polley Complex, located in British Columbia, Canada. The Northeast ore zone at Mount Polley is hosted by magmatic-hydrothermal breccia. Copper and precious metals occur in sulfide minerals primarily as coarse- to fine-grained breccia cement. Local wall rocks include equigranular to porphyritic diorite, monzodiorite, and monzonite. Alteration, breccia cement, and veins of the Northeast ore zone formed in five paragenetic stages: prebreccia (stage 1), brecciation and main-stage mineralization (stage 2), late-stage mineralization (stage 3), unmineralized postbreccia dikes and veins (stage 4), and epithermal-style veins (stage 5). Intense pervasive K and Fe metasomatism ± calcite and calc-silicate alteration occurred prior to brecciation caused by the intrusion of megacrystic K-feldspar-phyric monzonite. Stage 2 fluids were silica undersaturated, high temperature (〉350°C), CO 2 enriched, and of near-neutral to alkaline pH. Potassic, sodic, and calc-potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4. Evidence for more acidic and lower-temperature conditions is preserved in stage 5 veins. The 34 S sulfide isotope compositions of stages 2 and 3 chalcopyrite, pyrite, and bornite range from –7.1 to +1.4. Sulfur isotope compositions of anhydrite and gypsum are mostly between 6.2 and 9.8. These values, together with the presence of hematite, are consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid. Limited sulfur isotope geothermometry indicates that Cu sulfides precipitated at temperatures from ~480° to ~250°C. Hydrothermal calcite occurs in all paragenetic stages at Mount Polley. Calcite 13 C values range from –0.2 to –10.5, and 18 O values from 4.0 to 20.9. The enriched C-O isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid. Interaction of the fluid and/or magma with limestone is considered a likely process to explain the C and O isotope signature. Lead isotope data suggest mixing of mantle and crustal sources during mineralization. Main-stage chalcopyrite and pyrite as well as late-stage galena have 206/204 Pb values of 18.77 to 18.92, 207/204 Pb of 15.56 to 15.59, and 208/204 Pb of 38.22 to 38.32. Strontium isotope data (0.70331–0.70371) provide evidence of a strongly depleted mantle source of Sr with minor crustal input. Epsilon Nd values for main-stage apatite range between 5.9 and 6.5, also indicating a depleted mantle source. Stage 5 carbonate 206/204 Pb values of 18.96 to 19.04, 207/204 Pb of 15.57 to 15.59, and 208/204 Pb of 38.26 to 38.36 suggest superposition of an epithermal system onto the Northeast ore zone, potentially as late as ~100 m.y. after breccia formation. The data presented are consistent with the hypothesis that the silica-undersaturated alkalic Mount Polley Complex formed due to carbonate assimilation prior to mineralization. This process can explain the 13 C- 18 O isotope data, calcite precipitation concurrent with Cu-Au mineralization, and silica undersaturation of the magma. The CO 2 released during assimilation of carbonate also could have promoted magmatic-hydrothermal brecciation. Silica-undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate-bearing substrate.

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 46-million-ounce Ladolam gold deposit, the largest alkalic epithermal gold deposit in the world in terms of contained gold, is composed of four ore zones: Minifie, Lienetz, Kapit, and Coastal. This detailed lithofacies study of the Minifie ore zone recognized three evolutionary stages: (1) volcanosedimentary strata (part of an alkalic composite volcanic island), (2) a porphyry-style breccia dike, and (3) epithermal-style breccias. The Plio-Pleistocene volcanosedimentary stratigraphy reflects the transition from subaerial deposition of pyroclastic surge deposits close to vent to a subaqueous, quiet depositional environment into which a cryptodome was emplaced. The stratigraphy is predominantly composed of polymictic, matrix-supported breccia and sandstone interbedded with lavas and shallow intrusions. The Minifie strata have a shallow southward dip as a result of Quaternary uplift. Overprinting the volcanosedimentary stratigraphy are three hydrothermally cemented breccia facies—one deposited in the porphyry environment and two deposited in the epithermal environment. Porphyry-style alteration assemblages and a 3- to 5-m-wide biotite-K-feldspar-calcite-anhydrite–cemented breccia dike are focused around the central "Minifie shear" fault. The porphyry alteration assemblages are overprinted by shallow-level and deeper-level epithermal-style features. The deeper-level (below the present mining surface; 〉140 m below sea level) epithermal vein stockwork is composed of quartz-calcite-adularia-anhydrite–cemented breccias and shallowly northward to near-horizontal dipping veins that have gold grades of 1 to 〉60 g/t Au. Shallow-level epithermal facies (adularia-quartz-pyrite–cemented breccias, veins, and associated alteration) host the bulk mineable gold ore, and typically yield 〉4 g/t Au over 12-m blast hole samples.

Description: Endeavour 41 is a deep-level, structurally controlled epithermal gold deposit hosted by Early Ordovician subaqueous volcanosedimentary rocks. Calc-alkalic to shoshonitic, mafic to intermediate sills, dikes, and stocks intruded the volcanosedimentary units in the Middle Ordovician. The most felsic intrusions, together with pyroxene-bearing dikes, are temporally related to gold mineralization. Postmineralization intrusions are exclusively of mafic character. Endeavour 41 evolved from early, high-temperature porphyry-style veins and alteration to lower-temperature epithermal-style gold mineralization. Early magnetite and garnet-bearing veins (stage 1 and 2, respectively) associated with actinolite, magnetite, and biotite-bearing alteration assemblages have been cut by gold-bearing veins and associated alteration assemblages. There were two main epithermal-style gold mineralizing events: (1) quartz-pyrite ± calcite ± adularia ± chlorite veins (stage 3) and (2) carbonate-base metal sulfide veins (calcite, ankerite, quartz, pyrite, sphalerite, galena, chalcopyrite, Ag tellurides, arsenopyrite, apatite, hematite, illite-muscovite, and chlorite [stage 4]). Gold occurs principally as a refractory phase in pyrite. It also occurs as grains of Au-Ag tellurides and as inclusions of free gold in pyrite, sphalerite, and chalcopyrite. Hydrothermal alteration associated with gold-mineralized veins produced early epidote and K-feldspar-epidote–bearing alteration halos and later-stage illite-muscovite-K-feldspar and calcite-rich alteration halos. The highest gold grades are associated with muscovite and illite alteration. Stable isotope analyses and fluid inclusion data provide evidence of a magmatic-hydrothermal component to the mineralizing fluids. Fluid inclusion data suggest that gold precipitated from boiling saline waters (~9.0 wt % NaCl) at temperatures of about 310°C. Stage 3 veins are estimated to have formed approximately 1 km below the paleosurface at hydrostatic pressure (~90 bars). Stage 4 illite formed at temperatures below ~280°C. Stage 3 calcite has 13 C calcite and 18 O calcite values that range from –5.2 to –4.6 and from 11.6 to 12.1, respectively. Calculated fluids for these mineral values at 300°C ( 13 C fluid = –3; 18 O fluid = 6) are consistent with a magmatic-hydrothermal source of carbon and oxygen during stage 3. A component of meteoric waters is inferred for stage 4, because 13 C carbonate and 18 O carbonate values range from –6.9 to –0.5 and from 10.9 to 30.1, respectively, corresponding to 13 C fluid and 18 O fluid values of –5 and –2 at 200° to 250°C. The 34 S sulfide values for early vein stages range between –4.9 and –0.5. Stage 3 has 34 S sulfide values ranging from –5.2 to +0.8 with the most 34 S enriched values deposited away from the mineralized center. Stage 4 sulfides have isotopic compositions from –7.5 to +2.5. The negative isotopic values are consistent with oxidized (sulfate-predominant) magmatic-hydrothermal fluids. Sulfur isotopic zonation patterns show that the most negative 34 S values correlate with gold-enriched domains and also with areas that contain high-temperature, porphyry-style alteration facies. The negative sulfur isotope values define zones of upflow for the mineralizing magmatic-hydrothermal fluids. The paragenetic history of Endeavour 41 records a transition from deep-level to shallow-level magmatic-hydrothermal activity. This transition implies erosion and unroofing of the system synchronous with mineralization. High-temperature assemblages (e.g., actinolite-magnetite, biotite, and K-feldspar-epidote) indicate that epithermal mineralization occurred proximal to a magmatic-hydrothermal center and that there is potential for the discovery of porphyry copper-gold mineralization below the current level of diamond drilling.

Description: Halloysite, a phyllosilicate clay mineral chemically similar to but structurally different from kaolinite, occurs in a variety of mineral deposit types. It is, however, difficult to identify and dehydrates quite readily. When identified in a porphyry or epithermal environment, halloysite is often interpreted to be a low-temperature polymorph of kaolinite and an indication of supergene processes. The occurrence of halloysite in the Cerro la Mina Au (Cu-Mo) prospect, Chiapas, Mexico, was investigated to determine if halloysite in porphyry-epithermal deposits might also be the product of hypogene processes. The prospect consists of Quaternary volcaniclastic breccias intruded by monzodiorites and trachyandesites crosscut by a volcanic-hydrothermal breccia pipe. Gold-copper-molybdenum mineralization at Cerro la Mina is structurally and lithologically controlled by matrix-rich breccia pipes hosting all of the significant alteration and mineralization drilled to date. Average grades within the prospect are 0.4 ppm Au, 0.16% Cu, and 131 ppm Mo. A total of 100 samples were obtained from 22 diamond drill holes over a range of stratigraphic levels. X-ray diffraction (XRD) and SWIR techniques were employed to identify the different clay minerals found within these samples, including halloysite, kaolinite, dickite, and illite. Identification of halloysite and kaolinite were facilitated by the use of formamide intercalation to produce distinctive peaks for the two minerals in X-ray diffractograms. Halloysite and kaolinite were found to occur from the current erosional surface to depths of over 800 m. Halloysite occurs as both the hydrated 10Å and dehydrated 7Å forms within the deposit. The morphology of the halloysite present is that of well-developed tubes and spheroids. Results of the study show that halloysite associated with gypsum and jarosite is of supergene origin, whereas halloysite occurring with quartz, alunite, dickite, kaolinite, and pyrite is of hypogene origin.