Volcanogenic Massive Sulphide Deposits in the Southern Tulks Volcanic Belt, Central Newfoundland: Where do They fit Within the Tectonostratigraphic Architecture?
To help clarify their tectonostratigraphic affinity within the Victoria Lake supergroup, and to better understand mineralizing environments, U-Pb geochronology, trace-element lithogeochemistry and Sm/Nd isotopic geochemistry were applied to the host rocks surrounding the Tulks East, Tulks Hill and Boomerang VMS deposits, in central Newfoundland. A subvolcanic porphyry from the Tulks Hill deposit, dated previously at 498 +6/-4 Ma, provides a minimum age for the nearby Tulks Hill and Tulks East deposits. New U-Pb zircon ages were obtained from both the felsic tuff that hosts mineralization at the Boomerang deposit and from a felsic dyke interpreted to be broadly synvolcanic. The combined TIMS and SHRIMP data for these two samples indicate an identical U-Pb age of 491 +/- 3 Ma. This date is younger than the 498 +6/-4 age from Tulks Hill, although the errors do overlap at their older and younger limits, respectively. Inheritance patterns in the Boomerang samples suggest the presence of older crustal material having Cambrian (514 - 510 Ma) ages akin to those reported from the Tally Pond group, an older sequence within the Victoria Lake supergroup. The new results suggest that VMS mineralization in the Tulks area and at Boomerang may represent temporally discrete events, despite some apparent similarities. The age determined for the Boomerang deposit is closer to (but not identical with) a U- Pb date of 487 +/- 3 Ma, obtained some 30 km to the southwest of the Boomerang deposit, from a unit termed the Pats Pond group. The comparison of lithogeochemical patterns from the three deposit areas, with published data from the Pats Pond group, is complicated by the effects of hydrothermal alteration near the VMS mineralizing environments. Nevertheless, examination of immobile trace-element signatures suggests that these sequences cannot be distinguished on the basis of their geochemistry. The volcanic and pyroclastic rocks are all broadly arc-related, and show a mixture of calc-alkaline and tholeiitic signatures that perhaps record the construction and later rifting of individual arc sequences. Nd isotope signatures from felsic rocks in the Boomerang area and the Pats Pond group are generally higher (epsilon Nd of +4 to +5.5) than those from the Tulks Hill and Tulks East areas (epsilon Nd of around +3), although there is significant overlap in the signatures on a regional scale. Collectively, the new results, when taken with the results to date, suggest that the tract of rocks known as the Tulks Volcanic Belt includes rocks of more than one age, but of generally similar geochemistry and tectonic setting.
Petrogenesis of the 1.9 Ga Mafic Hanging Wall Sequence to the Flin Flon, Callinan, and Triple 7 Massive Sulphide Deposits, Flin Flon, Manitoba, Canada
Paleoproterozoic volcanic rocks of the Hidden and Louis formations comprise two shield-like volcanic edifices within the hanging wall to the volcanogenic massive sulphide (VMS) deposits at Flin Flon, Manitoba. These edifices have variable geochemical and Nd, Hf, and Pb-isotopic characteristics that provide insights into the magmatic and tectonic evolution of ancient arc volcanoes. Basaltic to andesitic (crypto) flows and minor rhyodacitic domes/flows of the Hidden and Louis formations yield epsilon Nd1.9Ga from +1.7 to +5.9, epsilon Hf1.9Ga from +7.0 to +11.8, and 204/206Pb values from 16.2 to 23.9. All units within the Hidden and Louis formations have distinct negative Nb anomalies (i.e. Nb/Th (mantle normalized) = 0.15-0.58). Light rare earth element (LREE)-depleted rocks occur at the base of the Hidden formation, LREE-enriched rocks occur at the top of the formation and there is a return to more LREE-depleted rocks in the overlying Louis formation. Thus, the Hidden and Louis formations have geochemical attributes similar to modern island-arc tholeiites with varying amounts of light-REE (LREE)-enrichment combined with positive initial epsilon Nd and epsilon Hf values. These geochemical and isotopic variations within the Hidden and Louis formations also record a change from depleted mantle sources to more enriched mantle sources and a return to a more depleted mantle source during emplacement of the units. Variations in geochemical characteristics also indicate broadly increasing amounts of crustal contamination from the base of the Hidden formation to the top of the Louis formation consistent with growth of the arc. The geological and geochemical characteristics of the 1920 unit, which occurs at the base of the Hidden formation, suggest it formed in a localized rifted arc environment though the melting of hydrated crust followed by low-pressure fractionation in a high-level magma chamber with associated, and variable amounts of contamination. This environment would have provided the structures and heat source required to drive high temperature hydrothermal cells needed for the formation of the Flin Flon VMS deposits. Therefore, the presence of icelandite or similarly evolved rocks in a bimodal volcanic sequence combined with evidence of a rifting is a useful tool in exploring for VMS deposits.
Low Sulphide Mineralisation of the Sudbury Igneous Complex, Canada: Examples of Fractionated Magmatic or Hydrothermal Systems?
Low Sulphide mineralisation occurs within the footwall of the Sudbury Igneous Complex. Samples of mineralised material have been collected from five properties and analysed using petrographic, microthermometric, point and bulk geochemical techniques. Two distinct mineralisation types have been identified for the North range (NR), a magmatic chalcopyrite (ccp) + millerite (mi) + pentlandite (pn) ± pyrrhotite (po) assemblage displaying epidote + chamosite + actinolite rich margins and haloes; and a hydrothermal ccp + mi + pyrite (py) + quartz + carbonate assemblage. South range (SR) mineralisation displays the same pattern with a po + pn + ccp ± cubanite magmatic assemblage and a hydrothermal ccp + po + pn + quartz + carbonate assemblage. Both types display a preference for exploiting lithological contacts, structures, fabrics, granophyric veins and Sudbury breccia matrix - clast contacts; and occur as cm and mm scale veins, blebs and disseminations. Analysis of mineralised material has revealed elevated precious and semi-metal concentrations dissolved within sulphides and as discrete crystals, intergrowths or aggregates of Pt, Pd, Au and Ag bismuthinides, tellurides, arsenides, antimonides, selenides and combinations thereof. Sizes range from 1-75μm, occurring interstitially or fully enclosed in sulphides and silicates. Magmatic material displays elevated concentrations of Pd in pn, Ag in ccp and Se in all sulphides when compared to contact and ''traditional" footwall mineralisation, suggesting precipitation from a strongly fractionated sulphide liquid down to temperatures of 379 to <282oC, inferred from the presence of interstitial millerite and polydymite. Hydrothermal mineralisation displays the same elevations suggesting a link to the magmatic material, with primary fluid inclusions in associated quartz giving Th (NR) of between 350-407oC and salinities of 30-35% NaCl and 18-24% NaCl-CaCl2 and tentative Th (SR) of 350-388oC and salinities of 15-28% NaCl-CaCl2 for mineralised material . The data above will be combined with thermodynamic modelling, bulk and mineral halogen analysis, outcrop mapping, acid etching, geothermometry and geobarometry to produce a genesis and exploration model for this challenging new deposit type.
Evaluating the Consequences of Edifice Instability-Related Processes in Hydrothermal Ore Genesis at Composite Volcanoes
Composite volcanoes intrinsically evolve toward instability, which is resolved through sudden (e.g. flank/edifice failure) or gradual (e.g. volcano-basement interaction) processes. They commonly host hydrothermal systems and related ore deposits within their edifices and shallow basement. The nature and extent of the influence instability-related processes exert on these hydrothermal systems and ore genesis are as yet poorly understood. Short-term effects are basically related to sudden depressurization of the system. The key factors determining the response of the hydrothermal system are its depth and maturity, and amount of depressurization. Deep excavation will lead to evisceration of the edifice-hosted hydrothermal system, dispersion of its volatiles in the atmosphere and incorporation of solid-phase components in the resulting debris avalanche deposit (DAD). When mature, such a system may provide DAD-hosted ore deposits. The fate of the deeper, basement-hosted hydrothermal system depends on its maturity. The evolution of an immature system will be aborted as a consequence of premature depressurization-driven boiling, and no ore-grade mineralization forms. Mature systems, however, will benefit from pressure drop and induced boiling by massive deposition of pressure-sensitive ore minerals and formation of high-grade ore. Long-range effects of edifice-failure are related to increase of the meteoric input into the hydrothermal system due to the formation of a large depression and reorganization of the surface hydrologic regime. Shift from high-T vapor-dominated regime to low-T dilute hydrothermal regime is its expected outcome. The influence of gradual release of edifice instability by volcano spreading and related phenomena on the hydrothermal system has not been studied so far. Deformation induced in both edifice and basement would result in change of fluid pathways according to the shift of local stress regimes between compressional and tensional, in turn depending on a number of factors (e.g. the presence, depth and thickness of a plastically deformable layer in the basement). That would determine the actual location of ore mineral deposition. Thorough understanding of instability-related processes greatly helps in mineral exploration activities at extinct and old composite volcanoes.
Genetic Model of an Aborted Porphyry-copper System
The Neogene Sturzii shallow intrusion from the East Carpathians (Bargau Mts., Romania), hosted by Paleogene-Miocene sediments of the Transcarpatian Flysch, developed as an immature porphyry copper structure. It consists of small volumes of dacites, andesites and related contact breccias, having a surface exposure of a few km2. Hydrothermal alteration occurred in the inner part of the intrusive body. The related mineralization consists of pyrite and chalcopyrite either as small veins or disseminated within the rock. A genetic model of the intrusive structure has been developed based on an integrated petrographic, geochemical, isotopic, fluid inclusion and geophysical study. The rapidly ascending calc-alkaline magmas that generated the intrusion are mantle-derived and contaminated with lower-crustal material. Pressure estimations for amphibole reveal significant differences between values corresponding to the crystal cores and rims, suggesting that decompression occurred during its crystallization. The occurrence of exploded fluid inclusions, as well as of primary igneous garnet, also indicate decompression regime during magma uplift and/or storage. All fluid inclusions identified in dacites are aqueous; C-N-S species were not detected. The general evolution of the fluids is toward decreasing salinity with decreasing temperature. Early high-T, high salinity fluids, most likely of magmatic origin, were subjected to a boiling event, related to a change of fluid pressure from litho- to hydrostatic, and followed by dilution with meteoric fluids as indicated by low salinities. These characteristics of the fluids suggest the tendency of the intrusion to evolve towards a porphyry copper system. We estimate that the evolution stopped due to decompression that allowed cold and dilute external fluids to enter the system and because of the small size of the intrusion that cooled down rapidly and could not induce extensive and long-lasting fluid circulation. Since there is no evidence for volcanic activity at the surface and presence of volcanic edifice whose collapse could trigger such decompression, a distensional tectonic event is to be invoked. Such a situation can be used as a comparative case for depressurization-induced effects on the hydrothermal system at unstable volcanic edifices.
Late Miocene Debris-Avalanche Deposit At The Gutai Shield Volcano, NW Romania. Re- Evaluation Of Geological Mapping And Mineral Deposits
The recent identification of debris avalanche deposits (DADs) originating from the southern edge of the Ignis peak (1306m, highest of the Gutai Mts.) has important implications for understanding its genesis in the geological context of the broader area, rich in hydrothermal intrusive-related base metal and gold-silver deposits closely connected to the Dragos Voda - Bogdan Voda strike-slip fault system. Pyroxene andesite lavas are exposed below the Ignis peak followed by hornblende and pyroxene andesites the only ones found in the DAD. The flank failure event has left an E-W-oriented horseshoe shaped scar with an estimated volume of material removed of at least 0.35 km3 and an estimated area covered by DADs of 4,345 km2 as a minimum. The deposit is a mega breccia with a variable amount of coarse matrix with jigsaw-fractured blocks, large boulders, and several southward-elongated hummocks up to 1.8 km distance from the scar. Between 720-850 m altitude the DADs contain megablocks of 5-12 m thick and up to 100 m long of layered fine-grained poorly consolidated pyroclastic materials of interlayered ash and lapillistone of fallout origin, and clay beds rich in vegetation remnants(known as the 'Chiuzbaia flora' of similar age as the surrounding lava flows, i.e. ca. 10-7 Ma) and diatoms. These megablocks found in various positions, suggest a lithological discontinuity likely representing the detachment surface of the gravity-driven instability phenomenon and the deep excavation of the volcano flank by the sector collapse event. The clayey material of these blocks acted probably as an efficient barrier to water infiltration and helped destabilization of the overlying rock mass. Since no explosive products have been observed to follow the DAD, it is possible that the sliding was triggered by pressure release of hydrothermal system along an E-W fault parallel to the Dragos Voda-Bogdan Voda fault system, with related high-grade ore deposits. This suggests the possible presence of unidentified hydrothermal ore deposits at depth in connection with the volcano instability-related tectonic features.
Hydrothermal Zoning of Rift Zones Inferred From Magnetic Susceptibility Variations: Implications for the Collapse of Hawaiian Shield Volcanoes, and for Ore-genesis Processes.
Hawaiian shield volcanoes have experienced large scale landslides throughout their history. These collapses are due in part to the failure of the surrounding sea floor to support the weight of the spreading volcano as it grows. Nevertheless, these collapses also might be promoted by the weakening of the volcanic edifice due to the injection of dykes within rift zones, and by the alteration of the rock due to hydrothermal activity along these zones. In turn, hydrothermal alteration modifies the rock bulk magnetic susceptibility, and such relationship provides a good opportunity to estimate the zoning of alteration by completing measurements of magnetic susceptibility. In this work we show preliminary evidence suggesting that a hydrothermal zoning can be inferred to have existed in the Hawaiian Shield volcanoes, probably reflecting the variation of the optimum temperature for alteration as a function of distance from the magma center. The mechanical destabilization of the volcanic edifice due to dyke injection and that related to alteration of the rocks seems to have been inversely related, therefore resulting in an average destabilization of approximately equal magnitude along the whole extension of the rift zone. Such uniform destabilization seemingly favors the collapse of large sectors of the volcanic shield once a critical mass is achieved. In the context of ore-genesis, zoning is known to be related to paleogeography and temperature variations among other factors. Actually, different patterns of orebody zoning are known to take place depending on the conditions prevalent in each region, and it is of interest to determine the details of zoning of the deposit to understand its genesis. Despite the fact that Hawaiian volcanoes are not the most economically important places to study ore-genesis processes, the better understanding of the processes of hydrothermal alteration gained in these settings should contribute to gain a better knowledge of the distribution of epigenetic hydrothermal deposits elsewhere, and in particular the relation of these deposits with collapses of other volcanic edifices around the world.