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Volcanic processes in ore genesis
I. G. GASS Two major themes pervaded this joint meeting of the Volcanic Studies Group and the Institute of Mining and Metallurgy. On the one hand were the 'aca- demics' who, using trace element and isotope geochemistry and even naked thermodynamics, erected hypotheses and models for the tectonic setting, origin and circulation of ore forming fluids. At the other extreme were those who pro- vided the critical evidence as to whether these data applied to a particular ore deposit. Fortunately, speakers such as J. P. Hunt, T. Sato and G. Constantinou with experience in both areas more than adequately bridged the all-too-common gulf between the two. Stable isotope studies strongly suggest that the origin of the ore-carrying solutions was either sea-water, in the case of the massive sulphides formed at or near constructive margins, or meteoric waters for ore bodies such as porphyry coppers, emplaced above subduction zones. The role of magmatic waters is minimal in the former and minor in the latter; magmatic processes seem to provide the thermal energy and very little else. Both thermodynamic models of geothermal systems (J. W. Elder), or modification of stable isotope ratios (E. T. C. Spooner & T. H. E. Heaton) indicate that the circulation of the solu- tions that cause mineralization follow immediately after the magmatic thermal event and that they are short-lived in terms of thousands rather than millions of years and vigorous. There are therefore signs of a semi-quantitative break- through in the understanding of geothermal processes. This has been brought about by the use of a wide spectrum of analytical techniques on, for instance, the ophiolite massive sulphides of Cyprus and the porphyry copper of E1 Salvador. Their use elsewhere will enhance understanding of the processes involved in ore genesis. Happily, the days when it was considered almost indecent for 'pure' academic scientists to concern themselves with ores are over. A period of fruitful collabo- ration between hard-headed practicality and high quality academic research is hopefully here to stay. Summary of joint meeting of The Institution of Mining & Metallurgy and The Volcanic Studies Group, held in Burlington House 21-22 January 1976.
Global tectonlcs-llulds-ore deposits W. S. for forced flow, a structure to focus flow and an Fyfe appropriate site for deposition. Energy sources are normally supplied by gravity or igneous intrus- The requirements for the formation of an ore ions. Fluid sources are normally from the hydro- deposit are a suitable solvent, an energy source sphere, and halide-bearing fluids seem appropriate
Jl geol. Soc. Lond. vol. x32, x976, pp. 563-575 Printed in Northern Ireland.
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564 Conference report for most situations. Examples of such flow Joma, Gjersvik, Roros, Norway; Boliden, Sweden; regimes can be drawn from the ocean ridge envi- Parys Mountain, Wales; Buchans, Newfoundland) ronment, the subduction environment, high-level Geologically and geochemically, these deposits plutons, faults-thrust-shear zone environments, fall into two categories. Category ~ deposits are burial metamorphism and lateritic weathering. found within a sequence of submarine calc- Geochemical cycles require modification on alkali volcanics of island arc geochemical charac- account of the new importance attached to the teristic.s. These range from Cu-Zn-bearing types linkage continental weathering, ocean chemistry, associated with basalts and andesites to Pb-Zn- spilitization, subduction of spilite and sediments. Cu-Ag-bearing (Kuroko) types associated with New data that suggest more eclogite in the litho- acid rocks. 'Stable' trace-element geochemistry sphere make sediment subduction more plausible. shows that, compared with an average OFT, the Metal transport is no longer a chemical problem. basic lavas have lower concentrations of Ti, Zr, Y, New prospecting methods are focussed on evidence Nb, Ta, I-If, heavy rare-earth elements and Cr, of mass relations and depositional environments. but sometimes more La, U, Th and (in fresh rocks) Extensive rather than intensive parameters should alkali elements. Lavas associated with some be used. Particular emphasis will be placed on Archaean deposits also had these characteristics. gangue mineral volumes, stable isotopes and fluid Category 2 deposits are less common. They inclusions. are found in interbedded basic volcanic-sedimen- tary sequences. Compared with OFT these lavas contained higher concentrations of alkali elements, Ident/ficatlon of ore-depos|tlon environment U, Th, La, Nb, Ta and (usually) Ti, Zr, I-If; from trace-element geochemistry J. A. they contained similar concentrations of Y, Pearce & G. H. Gale heavy rare-earth elements, Cr and Sc. These have Geochemical studies of igneous rocks genetically a within-plate chemical character-indicating a associated with ore deposits can provide infor- continental margin or 'failed rift' environment. mation on the tectonic environment of ore On the basis of studies of unmineralized as formation. This approach is applied to three well as ore-bearing lavas, most massive sulphide types of deposit. deposits are probably formed in island arc or Cyprus-type deposits (example used: Troodos Massif, marginal basin environments and may all be Cyprus; Lansail prospect, Oman; I.~kken, Nor- dependent in some way on the subduction process way; York Harbour, Newfoundland). These for their formation; no deposit has been found massive Cu-Zn-bearing sulphide deposits are within lavas that have 'normal' ocean-floor basalt found in sequences of predominantly basic characteristics. pillow lavas at the boundary between two distinct Tin deposits (examples used from Cornwall, lava units. Evidence from Cyprus suggests that Malaysia, Bolivia, Nigeria). The environment of the lower of these units was erupted at a ridge eruption of intermediate and acid igneous rocks axis, the upper away from the ridge axis. can be deduced by use of diagrams based on the 'Stable' trace-element geochemistry shows that element Nb--such as SiO~ versus Nb. The acid- the lavas tend to have lower concentrations intermediate volcanics and granitic rocks from tin of Ti, Zr, Nb, Y, Ta, Hf, rare-earth elements provinces have typically high Nb concentrations and Cr compared with an average ocean-floor and classify either as within-plate or evolved tholeiite (OFT). Also, the concentrations of volcanic arc settings. In contrast, igneous rocks these elements (except Cr) decrease up the lava associated with porphyry copper deposits usually sequence. Empirical studies show all gradations have lower Nb concentrations (less than I o ppm between apparent ocean-floor tholeiite and in intermediate rocks). Petrogenetic calculations primitive island arc tholeiite characteristics; indicate that the tin may have a source in a part the island arc character increases up the sequence. of the mantle that is enriched in incompatible trace The deposits were not formed in a 'normal' elements, and is partially melted at 'hot-spots' mid-ocean ridge environment; if an equivalent or in rifting situations behind subduction zones. of this environment exists today, it is in a marginal The tin is further concentrated during crystal basin; water and perhaps copper from a sub- fractionation processes as the magma rises through duction zone may have been involved in the gene- a great thickness of sialic crust; or, alternatively, sis of the lavas; the ridge axis may have been tin-bearing fluids from the magma enrich the slow-spreading and higMy faulted. base of the sialic crust, which then undergoes Other massive sulphides (examples used include partial melting. 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Conference report 565
Identification of the origin of ore-form;ng rocks with relatively low water/rock ratios (less solutions by use of stable isotopes than 0. 5 atom ~o oxygen) subsequently collapses S. M. F. Sheppard in on a waning magmatic-hydrothermal system at about 35o-2oo°C. These fluids generally have mod- The origin of most constituents (magmatic, erate to low salinities (less than x5 wt ~o equiv- leaching of country rocks, fluid source reser- alent NaC1). Differences among these deposits voir) of hydrothermal solutions is indeterminate are probably in part related to variations in because (I) during reactions of the fluid with the relative importance of the meteorlc-hydro- the wallrocks, element concentrations are con- thermal versus the magmatic-hydrothermal events. trolled by solubility and ion-exchange equilibria The sulphur comes from the intrusion and the and (2) most elements, except notably H, C, O, S, country rocks. Sr and Pb, do not have stable isotope ratios that Deposits where meteoric or sea water is the vary measurably and that can be used as 'finger- dominant constituent of the hydrothermal fluids prints' of their origin. The stable isotope ratios come from epizonal intrusive and sub-oceanic of hydrogen (D/H), oxygen (leOf180) and environments where the volcanic country rocks sulphur (z4S/S2S) vary in minerals and waters. are fractured or well jointed and highly permeable. Most unaltered igneous rocks, and waters of Integrated water/rock ratios are typically high different origin--magmatie, metamorphic, meteo- with minimum values of o. 5 or higher (atom ~o ric, connate and ocean water--have charac- oxygen)--the magmatic water contribution is teristic D/H and tsO]tSO ratios. At equilibrium often 'drowned out'. Salinities are low to very low the isotopic composition of a hydrothermal min- (less than to wt ~o equivalent NaC1), and tem- eral is controlled by the physico-chemical con- peratures are usually in the range 35o-t5o°C. ditions of the solutions (T, fo2, pH), the isotopic The intrusion supplies the energy to drive the composition of the fluids and exchanging rocks, large-scale convective circulation system. The and the mass of the element in the fluid relative sulphur comes from the intrusion, the country to the rock (water/rock ratio). A combined rocks and/or the sea water. oxygen and hydrogen isotope approach is necessary, Argillic alteration, occurring to depths of in general, to determine the origin of water in several hundred metres, generated during super- magmatie and post-magmatic processes. Know- gene weathering in many of these deposits is ledge of the origins of water and sulphur is isotopically distinguishable from hydrothermal fundamental to any theory of ore genesis. Waters clays. of several origins are involved with ore deposits associated with volcanic and epizonal intrusive C/rculatlon of water in the crust, deductions environments. Waters of a single origin dominate from studies of modern hydrothernml main-stage mineralization in some deposits: systems J. w. Elder magmatic--Casapalca, Peru (Ag-Pb-Zn-Cu); rr~- The dynamical nature of the mechanisms of te0r/c--Butte, Montana (Cu-Zn-Mn), epithermal operation of hydrothermal systems and their deposits, e.g. San Juan Mts. District, Colorado key parameters were described. A simple model (Ag-Au-Pb-Zn), Comstock--Goldfield--Tono- suitable for a gross analysis of the role of water pah, Nevada (Ag-Au), Pachuca, Mexico (Ag-Au) ; circulation on the solution, transportation and sea water--Troodos, Cyprus (Fe-Cu), Kuroko, deposition of ores was considered in detail. Japan (Fe-Cu-Pb-Zn-Ba), Echo Bay, NWT, Canada (U-Ni-Ag-Cu). Solutions of more than Nature and distribution of porphyry copper one origin are important in certain deposits: deposits J. P. Hunt magmatic and m~teoric--porphyry copper and/or Efforts to explain the origin of porphyry coppers molybdenum deposits, North and South America; by global tectonic and igneous processes must and present in many. account for the nature of these deposits and their In the porphyry Cu-Mo deposits the initial distribution in space and time. Data from mine major ore transportation and alteration proces- studies were reviewed, including texture, com- ses (K-feldspar-biotite alteration) are magmatie- position, size and shape of mineralized in- hydrothermal events occurring at 75o-5oo°C. trusives; amount and proportions of copper, These fluids are typically highly saline Na-K- sulphide sulphur, sulphate sulphur and associated Ca-C1 rich brines (more than 15 wt ~o equivalent minor metals; the evolution of mineralization NaC1). The convecting meteoric-hydrothermal and alteration with respect to magrnatic proces- system that develops in the surrounding country ses and the effects of later groundwater; the Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
566 Conference report timing and duration of mineralization with res- and emplaced in outer arcs on margins of con- pect to associated vulcanism and geothermal tinents. These wedges may contain mineral deposits activity; the size and position of co-magmatic previously formed in the ocean, including Cyprus- calc-alkaline batholiths and depth of emplace- type massive sulphides, podiform chromite, ment. Knowledge of the absolute age and world- platinum, magnesite and asbestos. wide distribution of porphyry coppers is also The collision type of destructive plate margin summarized with respect to position of convergent is formed during the final stage of subduction of plate boundaries, the locations of giant deposits ocean floor between two continents, between two and gross amounts of copper deposited. The island arcs or between a continent and island arc. distribution of porphyry copper deposits with Slices of tectonically emplaced crust and upper respect to seismicity, regional petrologic provinces mantle, forming ophiolites, are obducted or and major structure features is obscure. Some thrust on to the continental crust or inactive fundamental genetic problems that are not yet island arc on the subducting plate to form elon- resolved include the sources of both magmas gate belts at the junction of the two plates. These and metals and possible relations, if any, to collision belts contain mineral deposits interpreted 'normal' igneous rock-forming processes and as having formed at oceanic ridges and in island subduction zones. arcs, and other related deposits may include tin-tungsten, iron-titanium ores associated with Mineralization at destructive plate margins anorthosites in the lower continental crust, M. S. Garson & A. H. E. Mitchell native silver-nickel-cobalt arsenides, gemstones Three main types of destructive plate margin and strata-bound uranium-vanadium-copper min- are recognized--island arc type, Andean type and eralization in molasse derived from associated collision type, each displaying distinctive mine- mountain belts. ralization. The first, characterized by an ensimatic island Igneous geology and evolution of hydro- arc magmatic belt developed within oceanic thermal systems in some sub-volcanlc crust, has associated deposits which include tin deposits ofBolivlaJ. H. Grant, C. Halls, & diorite-type porphyry, copper and gold, Kuroko- G. Avila type zinc-copper-lead sulphides, mercury, native sulphur-pyrite and Besshi-type cupriferous sul- The tin deposits of the southern part of the phides. Cordillera Oriental of Bolivia consist of complex The second, characterized by an ensialic or vein systems and pervasive disseminated mine- Andean-type magrnatic belt developed within ralization within zones of strong hydrothermal continental crust, has associated porphyry copper alteration in, or at the margins of, eruptive centres and molybdenum and, locally, tin-tungsten de- of late Tertiary age. At some centres erosion has posits and Chile-type strata-bound copper sul- removed the volcanic superstructure completely phides. Where complete, mineral zonation away and only intrusive rocks are preserved (Llallagua, from the plate margin into continental crust San Pablo de Morococala). At others, both comprises iron, copper-molybdenum, silver-lead- the intrusives and coeval volcanics are preserved zinc, tin-tungsten and antimony, the source of the (Chorolque, Oruro, Potosi), whereas in some cases metals possibly being related to progressively only the partially eroded volcanic superstructure deeper levels in the underlying Benioff zone. A is exposed and no unequivocally intrusive rocks modification of this type of plate margin results are known (Chocaya, Tatasi). from the formation of a marginal basin by outward In general, the geological relationships at all migration of incipient Andean-type mountain these deposits suggest that the mineralization is belts bordered on the ocean side by a subduction the product of hydrothermal systems generated zone. Granite plutons with associated tin, tungsten in the inner, deeper regions of terrestrial strato- and fluorite are emplaced in this setting. volcanoes. Centres of porphyry copper mineralization may The igneous rocks most closely associated be controlled locally by transform faulting at with mineralization are strongly altered quartz both types of plate margin, and Kuroko-type porphyries. Their original composition was deposits in some island arcs may also be situated probably rhyodacite. Where stocks are preserved, on transform faults. they are complex bodies, often conical in vertical Tectonic wedges of oceanic crust and upper section, narrowing with depth. They show evidence mantle are scraped up with overlying sediments of several phases of intrusion, igneous brecciation Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
Conference report 567 and explosive hydrothermal brecciation, which zation related to a retreating hydrothermal pre-date the formation of the major vein systems. focus can lead to a clearer understanding of the Silicate alteration assemblages include quartz- xenothermal or 'telescoped' type of mineral and tourmaline, tourmaline-sericite and serieite-clay, temperature zonation in sub-volcanic systems. which often show rough concentric zonation in Temperature data obtained from fluid inclusion that order outward from the centre. An outermost studies show, however, that mineral paragenesis aureole of propylitic alteration is sometimes pre- is not an unambiguous criterion of temperature sent where there are remnants of the volcanic zonation and that the two have to be considered superstructure. Alteration is pervasive and not con- independently before a combined synthesis trolled by the major veins. Low-grade cassiterite becomes meaningful. mineralization is dispersed throughout the inner Geochemical model for Kuroko mineral/- zones. zation T. Sato Fluid inclusion studies at Chorolque show that Kuroko deposits are stratabound massive the hydrothermal system was initiated by the sulphide deposits associated with Miocene felsic separation of a highly saline brine, or melt, of volcanic rocks of the green tuff region of Japan. complex chemistry, which formed at temperatures They are characterized by lenticular, massive above 5oo°C. The pervasive quartz-tourmaline Pb-Zn-Cu orebodies with abundant barite, alteration, initial fracturing of the igneous rocks resting directly on highly silicified stockwork of the vent and the hornfelsing of the adjacent zones. General vertical mineral zoning is distinct, sedimentary rocks were accomplished while but the zoning is concentric near the mineralizing temperatures fell to below 45o°C and intermittent centres. A model for physical and chemical boiling took place. Widespread disseminated processes involved in the submarine deposition cassiterite was deposited during this phase. The of Kuroko orebodies is proposed, based on the earliest stages of growth of the major quartz- assumptions that heavy metals dissolved in the cassiterite veins took place at temperatures of ore fluid were complexed with chlorine and that around 4oo°C, from a fluid similar in composition the fluid was equilibrated with pyrite, chal- to that of the earliest fluid, though substantially copyrite, sericite and kaolinite. By use of available diluted (about 4 ° wt % NaC1 equivalent). Vein thermodynamic data the ore fluid is calculated growth continued while temperatures fell to below to have been cooled from about 25o°C at the ooo°C and salinity decreased. Most cassiterite bottom of the stockwork zone to about i5o°C deposition seems to have taken place in the at the top of the massive orebody, which shows temperature range 3oo-25o°C and may have been a good agreement with fluid inclusion data. accompanied by a major decrease in the salinity Gradients of pH and oxygen fugacity and initial of the fluids. heavy metal concentrations of the ore fluid are The data obtained from the fluid inclusion also calculated. The zoning pattern observed in studies, taken in conjunction with the gross Kuroko deposits is explained quantitatively by geometry of the mineralization, suggest that differential precipitation of ore metals from a hydrothermal processes at the volcanic centres polymetallic fluid that was discharged into were initially controlled by the balance of con- and mixed with sea water. Application of the fining lithostatic pressure and the pressure of model to other stratiform massive sulphide the hydrous fluid residuum in the differentiated deposits was attempted, and the results were dis- parts of the rhyodacite magma, leading to per- cussed in terms of the difference of tectonic and vasive hydraulic fracturing, brecciation and geologic environments. alteration, of which the early generation of cassiterite mineralization was an integral part. Stable isotope stud/es on Bougalnville and The upper levels of the mineralized volcanic in Matupi Harbour, New Britain, Papua, structure appear to have stabilized while magmatic New Guinea J. H. Ford, D. C. Green, J. R. activity and hydrothermal generation at that level Hulston, I. H. Crick & S. M. F. Sheppard declined and the focus of generation retreated to Two case histories were presented in which greater depths from which mineralization was con- stable isotope ratios (D/H and xso/teO) place trolled by the interplay of tectonic stresses in important constraints on the origins of ore- the volcanic infrastructure and surrounding forming fluids in the Panguan porphyry copper basement and the build-up of hydrothermal deposit (upper Pllocene) and on the nature of the fluid pressure in linear vein-fault systems. volcanic exhalative environment at Matupi Har- This concept of a bimodal style of minerali- bour, near Rabaul. Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
568 Conference report
At Panguna the overlap in 6D values between close to the meteoric water line and demonstrate local meteoric water and inferred primary mag- that the thermal springs away from the shoreline matic waters poses problems in the interpretation are of meteoric origin and that sea water only of secondary biotite and sericite isotope data. enters the springs at the shoreline. If allowance is The range of hydrogen and oxygen isotope ratios made for the vapour-liquid separation at the of the secondary biotites associated with the temperature of collection (Ioo°C), the fumarole mineralized areas is essentially identical to those results from Tavurvur may be interpreted as from other magmatic hydrothermal deposits, steam separating from a water phase at N xoo°C. including porphyry copper deposits. The sericites The calculated water-phase isotopic compositions from alteration haloes adjacent to mineralized suggest that evaporation and possibly a small quartz veins have ~ xsO values of 5.4 to 8.6 ~ and amount of mixing with sea water are involved. 6D values of --6t to -77. A combination of These conclusions are in conflict with those thermodynamic and fluid inclusion techniques drawn from anion ratio and trace metal contents indicates that ~-~5oo°C is a realistic average inferred by previous authors to be consistent with temperature for much of the mineralization- an hypothesis of modified sea-water origin. We quartz veining-biotitization process. The isotopic contend that the chemistry is a reflection of the composition of the fluids causing this minerali- later history of these acid, mineralized, geo- zation and alteration was dD ~ --20 to -45~ thermal waters and does not necessarily give a and 61sO "" 6 to 7 %. If a meteoric water com- correct picture of their ultimate origin. The ponent existed at this stage of alteration, meteoric markedly enhanced Fe, Mn and Zn values of the water/rock ratios were extremely small (less than Matupi springs are purely a function of the 0.05 at ~). leaching potential of geothermal fluids at ele- Assuming that sericitization took place at vated temperatures and of the chemistry of temperatures down to 3oo°C, as suggested by the porous rocks through which they pass. the fluid inclusion data, then fluids responsible for this process had a composition of 6D = --3 ° to 5o% and d} tso of '~3-5~. These fluids prob- Modern subm~_rine hydrothermal mineral- ably would be largely of meteoric-hydrothermal izatlon: examples from Santorinl and origin, produced by tso shifting of groundwaters the Red Sea D. S. Cronan, P. A. Smith & during the formation of the propylitic zone and R. D. Bignell with a water/rock ratio of about o. x at ~. Quartz d 1sO values from veins associated with the Submarine hydrothermal mineralization is different alteration assemblages increase away known to occur on parts of the World Mid- from intrusive centres, consistent with an out- Ocean Ridge System, and in association with wards temperature decline, a decrease in ~ xso island arc volcanism. content of associated water and a progressive Two areas of submarine hydrothermal activity increase in the meteoric water component have been examined---one in the Atlantis II associated with the later and (?) lower-tempera- Deep of the Red Sea and the other off the volcano ture sericitization. There is no isotopic evidence of Santorini in the Cyclades Volcanic Arc. In to suggest that sea waters or connate waters each case hydrothermal solutions are mixing formed a recognizable component of the hydro- with sea water, resulting in the precipitation of a thermal system at any stage. Calculations with variety of phases as the physico-chemical pro- possible 6 1sO depletions of o.5-I.o % in whole perties of the mixture change. rocks from the Panguna region indicate that a Early-formed precipitates from the Atlantis II meteoric leaching model for the genesis of the Deep brines include sulphides and silicates. Panguna copper depoeit is theoretically possible, On continued mixing, iron is oxidized to the despite the small water/rock ratios involved. Fe wt- state and precipitates as ferric oxide and The area around Matupi Harbour includes hydroxide. Finally, Mn ~+ is oxidized to Mn 4+ the dormant volcanoes Rabalanakala and Tavur- and can scavenge considerable quantities of vur, set in loosely consolidated, Quaternary ash trace metals out of solution on precipitation. deposits. Tavurvur last erupted in x94t and x942 Manganese precipitated in the waters over the and has since reverted to a quiet solfataric con- Deep is probably redissolved on falling back in- dition. D/H and xsO/aeO ratios were determined to the brine. By contrast, manganese and some on fumarole condensates, hot springs and meteoric minor dements that escape from the Deep can waters. The data are grouped into distinct areas precipitate up to xokm from it. Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
Conference report 569
Sulphides occur locally as a minor com- Although the definition is purely descrip- ponent of the newly precipitated sediments off tive and no genetic implications are intended or Santorini, which are principally composed of implied, the presence of radiolarian sediments iron oxides. Iron precipitates close to the hydro- and the pillowed form of the basic volcanics thermal outlets and decreases in concentration strongly suggest that ophiolites were formed in a away from the volcanic source. By contrast, submarine environment. Furthermore, this en- manganese is low in the sediments dchest in iron, vironment must be extensional to explain the but increases in concentration more than twenty- presence of sheeted dyke complexes, and one fold away from the iron-rich zone. The distri- where large volumes of basic magmas can occur bution of certain minor elements in the sediments close to the surface. Few would question the ocea- follows that of Mn (e.g. Zn) and others Fe nic setting for ophiolite genesis, and many, (e.g. Mo), suggesting that, here too, scavenging pointing to the need for an extensional environ- may be a control on their distribution. ment, see only the oceanic lithosphere as pro- The selective dispersion of hydrothermally duced at constructive plate margins as fitting introduced metals on mixing with sea water these requirements. in these two environments results in a geochemical Field relationships and chemical and mineral zonation in sediments away from the metal composition of the tectonized peridotite suggest source in each case. The principal fractionation is that it is material from which a liquid basaltic between Fe and Mn, but some minor elements fraction has been fused and removed to form the also show distinctive concentration gradients. higher levels of the ophiolite sequence. It seems These observations may be of great potential that the magma body collects at shallow depth value in geochemical exploration for submarine (x-2 km) below the sea/rock interface. In an hydrothermal orebodies, as the extent of the extensional environment magma is tapped off, dispersion of the most mobile elements can be injected as dykes and extruded as pillow lavas. many times the extent of the deposit itself. At the same time, the magma remaining within Ancient volcanic orebodies sometimes show the chamber fractionates on cooling to give a similar selective dispersion haloes to those des- layered sequence ranging upwards from dunites cribed heremindicating that in the modem with chromite, through peridotites and olivine environment we may be observing a not un- gabbros to gabbros and minor soda-rich granites. common volcanic ore-forming process in opera- The model erected to explain the formation tion. of ophiolites and that deduced from geophysical studies for processes at constructive margins are so similar that the observational ophiolite data Orlg/n and emplacement of opldoHtes have been used to elaborate the skeletal geo- I. G. Gass physical model for present-day constructive mar- gin processes. So long as a four-layer sequence The most widely accepted definition of an of lavas, dykes, cumulate plutonics and tecton- ophiolite was that erected in x972 by the Geo- ized peridotite is produced, however, the loca- logical Society of America's Penrose conference tion of the constructive margin within the ocean on ophiolites. This describes an ophiolite as an basin is irrelevant, as is the size of the ocean assemblage of basic and ultrabasic rocks, a basin. Ophiolites could equally well be produced complete assemblage being: at major constructive margins, such as the mid- Basic volcanics Atlantic Ridge or East Pacific Rise, in minor (commonly pillowed) 'oceans', such as the Red Sea or Gulf of Aden, merging into at a behind-arc spreading axis, such as that in Sheeted basic dyke complex the Scotia Sea, or in marginal seas similar to (Trondhjemites those of the Western Pacific. By the same reason- (Gabbros ing, ophiolites could be produced near to or Layered cumulates (Olivine gabbros far from continents and island areas from which (*Peridotites they can be separated by a destructive plate (*Dunites (with chromite) boundary. Indeed, most ophiolites seem to have originated in small behind-arc ocean basins. Harzburgite (sometimes lherzolite) As it spreads away from the constructive with tectonized fabric margin, the newly created lithosphere undergoes (*commonly serpentinized) certain changes, seemingly brought about by the Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
57 ° Conference report circulation of vast quantities of sea water through Pre-tectonic segregations, veins and vesicle the still hot oceanic crust. This process imposes fillings of epidote, albite, chlorite, carbonate a horizontally disposed low-pressure/high-tem- and quartz related to primary volcanic flow perature zeolite-greenschist-amphibolite facies me- structures in the lava pile provide evidence of tamorphism on the basic rocks from which the pervasive in-situ sea-floor metamorphism, and circulating brines remove transition metals this interpretation is verified by the abundance (e.g. Cu, Fe, Mn). On returning to the rock/water of nearly monomineralic epidote clasts in the interface, these solutions react with the sea water derived conglomerates. to precipitate metal-rich sediments. The relationship of the eruptive and sedi- The precise mechanism(s) whereby part of an mentary suites is interpreted in terms of the oceanic plate is emplaced on to a continental evolution of an ensimatic island arc of lower margin or island arc is (are) not known. What Ordovician age, which underwent uplift and should normally go down goes up. The special erosion prior to emplacement on the Fennoscan- term obduction, the converse of subduction, has dian basement during the climactic stages of been coined to describe the overall process. The collision tectonism of the Caledonian orogeny in least deformed ophiolite sequences appear to be Silurian times. vertically uplifted pieces of ocean floor that The entire igneous and sedimentary assemblage have not undergone much horizontal deformation. has been affected by the tectonic stages of allo- Most other ophiolites are much more deformed. chthonous emplacement, but the gross differences Their gross structure is commonly that of a series in competence between the component lithologies of complexly deformed tectonic sheets stacked has resulted in a particularly heterogeneous style one on top of the other. Most of these processes of deformation in which folding, componental are poorly understood. Nevertheless, they prob- sliding, fracturing and penetrative metamorphic ably involve hydration reactions in the mantle, refabrication have been governed largely by the gravity sliding and/or tectonic pushing, together geometry of the most competent lithologies, with other processes as yet unknown. notably gabbro, diorite and granodiorite (trondh- jemite) intrusives and, within the extrusive Geological setting of the Skorovas orebody sequence, compact dacitic flows and their spili- withln the allochthonous meta-volcanic tized aphanitic equivalents (keratophyres). The strat/graphy of the Gjersvlk Nappe, Cen- heterogeneous pattern of deformation is resolved tral Norway C. Halls, A. Reinsbakken, in terms of two main stages of folding complicated I. Ferriday, A. Haughen & A. Rankin by componental sliding movements. Mineralization occurs at two levels in the erup- The Skorovas orebody is one of the chief tive sequence. The layered gabbros and lensoid stratiform base-metal deposits within the allo- metagabbros of the plutonic infrastructure con- chthonous greenstone belt of the Central Nor- tain small cumulus bodies of nickel, copper- and wegian Caledonides. It is contained in the platinum-bearing pyrrhotite-pyrite-magnetite ore volcanic level of a complex eruptive association of magmatic derivation. Mineralization of this of lower Ordovician age defined by Foslie and type is at present only known in sub-economic Oftedahl as the Gjersvik Nappe. The rocks of quantities. this nappe are contained as a depressed segment The Skorovas orebody, in common with other of the larger Krli Nappe and defined to the north widely dispersed volcanic exhalites in the Gjersvik and south, respectively, by the Brrgefjell and Nappe, occurs within the volcanic sequence at a Grong-Olden basement culminations. The prin- level marked by episodes of explosive dacitic cipal components of this nappe are a plutonic infra- volcanism and associated fumarolic activity. structure of composite gabbroic intrusions within The Skorovas orebody consists of approximately which has been emplaced a series of dioritic to I o oooooo tons of massive and disseminated granodioritic (trondhjemitic) bodies, which form predominantly pyritic ore with an approximate the roots of a consanguineous submarine poly- average grade of z.3% Zn and I.o~ Cu, together genie volcanic sequence. The eruptive rocks are with trace amounts of Pb, As and Ag. The complex overlain unconformably by a sequence ofpolymict lensoid geometry of the orebody is resolved in conglomerates and calcareous flysch sediments, terms of the disjunction of a single stratiform the composition of which suggests immediate unit by tight isoclinal folding and componental derivation by erosion from the underlying movements, probably involving both translation igneous complex. and rotation. Enrichment ofsphaerite, chalcopyrite Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
Conference report 571 and, locally, galena within the magnetite-pyrite produced by regional hydrothermal metamorph- ores at the stratigraphic top and margins of the ism. Mineralogical changes in the stockwork lavas ore lenses is interpreted as a primary feature. include the complete pseudomorphism of the The banded magnetite-pyrite ores are commonly primary minerals plagioclase and pyroxene by associated with magnetitic cherts or jaspers and quartz, the sulphurization of magnetite and the are thus transitional in aspect to the thin, iron- alteration of the glassy mesostasis to a mixture of and silica-rich, base-metal-depleted, exhalative chalcedony, chlorite, illite and less common sedimentary horizons that occur extensively within kaolinite. The chemical changes in the stockwork the extrusive sequence of the Gjersvik Nappe. lavas involved the dramatic decrease of Ca, Na These are interpreted as the products of settling and, to a lesser extent, A1 and K, and considerable of colloidal iron and silica hydrosols following increase of S and Fe. Magnesium decreases explosive dispersal into an oxidizing submarine considerably in the intensely mineralized lavas, environment. They are valuable time-stratigraphic whereas it increases in the weakly mineralized markers and indicators of way-up in complicated lavas. Whereas the stockwork lavas are invariably structures and are a potentially valuable tool in intensely silicified, there was no appreciable exploration for massive sulphide bodies formed in removal or introduction of silica in these lavas. limited reducing environments. Silicification of these lavas was probably produced by the selective leaching of the bases from the Massive sulphide deposits of the Troodos primary minerals and the glass, leaving behind Massif. G. Constantinou residual silica in the form of quartz and chalcedony. In the stockwork lavas Cu, Zn and Co increased, The Cyprus massive sulphide deposits are whereas Ni and Mn decreased compared to the restricted to and constitute and integral part of unmineralized lavas. Field, chemical and minera- the pillow lava succession of the Troodos Ophio- logical relationships suggest that the stockwork lite Complex. Extensive geophysical, geological zones represent the roots of the massive ores and and geochemical data collected in the last 2o years the channels through which the ore-bearing fluids suggest that this complex represents an uplifted have ascended to form the massive ores and the sea fragment of oceanic lithosphere and underlying water--pillow lava interface. The persistent upper mantle, which evolved in the Mesozoic at presence of illite and absence of montmorillonite divergent plate boundaries. The massive sulphide as a hydrothermal alternation product in the deposits consist of the basaltophilic elements S, stockwork zone, combined with experimental data Fe, Cu and, less commonly, Zn. They have been and studies of the active geothermal areas, formed at the original sea water--lower Pillow suggest that the temperature of the ore-forming Lavas or Basal Group interface and are char- fluids was 25o-26o ° and its pH 6.6-6.8. Recent acterized by distinct vertical zoning comprising, detailed stable isotope studies suggest that these in downward succession, a zone of massive ore, a fluids were mostly heated marine waters, which zone of sulphide with silica and the stockwork zone. leached the ore metals from the underlying Sheeted The latter consists of veins and disseminations of Dyke Complex. pyrite in brecciated and/or pillow lavas which have Rare-earth element evidence for the genesis undergone extensive silicification, chloritization of the met~_!l|ferous sediments of Troodos, and argillization. The veins represent fillings of Cyprus. A. H. F. Robertson & A. J. Fleet fractures in the lavas by sulphides, whereas the disseminated pyrite is believed to have been The origin of metalliferous sediments associated essentially produced by sulphurization of the with active ocean ridges continues to be debated. titanomagnetite of the basaltic lavas and it often The various types of metalliferous sediment that contains inclusions of euhedral leucoxene. Part of occur on and within the lavas of the Troodos the disseminated pyrite formed from the iron Massif, Cyprus, which is accepted by many present in the glassy mesostasis and the ferro- workers as a slice of Mesozoic ocean crust formed magnesian minerals of the lavas. at a constructive plate margin, enables the ficld Detailed optical, chemical and X-ray diffraction relationships of such sediments to bc studied at studies indicate that the sulphide ore-forming first hand. Umbers, which are chcmically and solution produces extensive mineralogical and mincralogically comparable with thc mctallifcrous chemical changes in the stockwork lavas. These sediments of prescnt-day active ocean ridges, are distinctly different from the mineralogical overlie the uppermost lavas of the massif. Beneath and chemical changes in the unmineralized lavas thc umbers there are cxtcnsivc zones of veined, Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
572 Conference report
brecciated and chemically altered lavas, which are mineralization and metamorphism of the ophio- interpreted as channelways for ascending hydro- litic rocks of the Troodos massif that involves thermal solutions, within which metalliferous hydrothermal convection of sea water. During sediments occur. Manganese-poor metalliferous water/rock interaction in the recharge part of the sediments known as ochres occur intimately associ- cycle of convective heat and mass transfer, the ated with the massive sulphide ores of Troodos. Pillow Lavas, Basal Group and Dyke Complex The rare-earth element (REE) contents and, were hydrothermally metamorphosed in zeolite hence, relative fractionation of samples of these facies to amphibolite facies conditions. Base metals, metalliferous sediments and of some associated in particular Fe, Mn, Cu, Zn, Pb, Ni, Co, Ag and lavas and sulphides have been determined by Au, were leached, and, at the localized positions instrumental neutron activation analysis in an of discharge of hot fluid, sulphide ore deposits attempt to identify the sources of these materials and/or ferromanganoan oxide sediments (umbers) and/or the different igneous and sedimentary were precipitated. geochemical processes that give rise to these Evidence for sea-water interaction during downflow materials. All the metalliferous sediments are through Pillow Lavas, Basal Group and Dyke Gomplex. enriched in the REE relative to chondrites and in This sequence of rocks was strongly hydrotherm- the light relative to the heavy REE. In the case of ally metamorphosed and hydrated (mean increase umbers this is to a greater degree than normal in H20 + content = approx. 2"2 wt.%). It there- pelagic days; and the bentonitic clays, which fore interacted with an aqueous fluid. overlie the umbers and are thought to be contin- That this fluid was sea water is shown by the ental in origin, are enriched. fact that the altered rocks are strontium isotopically All these sediments exhibit a marked negative contaminated relative to fresh rocks. The age- Ce anomaly, the size of which decreases with the corrected 87Sr/S6Sr ratios (nine analyses) of fresh increasing lithogenous content of the sediments. gabbros and plagioclase concentrates from fresh Such a marked negative Ce anomaly is a gabbros define a narrow range from o-7o344 -+- 5 characteristic of sea water and a feature of the to o.7o386-4-4 (mean ----o-7o362 ~ x3; E & A East Pacific Rise metalliferous sediments, but standard = o'7o8oo). In contrast, STSr/S6Sr ratios contrasts with the positive anomaly of manganese for altered rocks vary from o.7o338 -4-Io to nodules. The REE contents of the lavas analysed values as high as o.7o69 (mean of 17 determina- are difficult to interpret, but are enriched in La tions =o'7o49-+-9). These data show that relative to the other REE, probably because of during hydrothermal metamorphism the rocks alteration. were also strontium isotopically contaminated. The REE contents of the Troodos metalliferous The only reasonable reservoir of isotopically sediments appear to have been incorporated into heavy strontium was sea water (STSr/S6Sr = o'7o76 the sediments from sea water. This supports the in Campanian-Maestrichtian time). Hence, sea idea that the major components of the sediments, water was the source of the hydrothermal fluid. iron and, in the case of the umbers, some man- This interpretation is confirmed by the fact that ganese oxides and hydroxides, were derived by the the isotopic composition of the strontium in an pervasive leaching by sea water of the igneous rock interstitial zeolite sample taken from about 8oo m sequence. The REE contents of the interstitial below the top of the pillow lavas is statistically sediments imply deep penetration by sea water far indistinguishable from Campanian sea water into the pillow lava succession in agreement with (zeolite STSr/SeSr = 0.70760 ~ 3). the results of oxygen and hydrogen isotope The whole rock oxygen isotope ratios (6xsO) analyses. All the differing types of metalliferous are also strongly modified relative to the ~tsO sediment may be interpreted as related to differing value of fresh deep-sea basaltic rocks (OtsO---- redox conditions and events in the evolution of the ~, +6%). Enrichments in the upper part of the Troodos ocean ridge. sequence (highest ~xso = + 12"4o%) change to depletions deeper down (lowest olSO = + 3"31%). Isotopic evidence for the origin of xn/nerali- Mineral and whole rock combined D/H and zatlon associated with the ophlo]itlc rocks xso/xeO data, together with data on mineral of the Troodos ~/asslf, Cyprus. E. T. C. assemblages, enable the H and O isotope com- Spooner & T. H. E. Heaton positions of the Troodos metamorphic waters to The information presently available--in parti- be calculated. During greenschist facies meta- cular, that derived from the use of isotopes as morphism of the Sheeted Intrusive Complex and geochemical tracers--indicates a model for the upper parts of the gabbros, at temperatures Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
Conference report 573 between 200 and 45o°C, the OD value of the which is there underlain by a stockwork feeder pipe. waters was close to 0%, demonstrating a sea- The Planes pipe consists of sulphur-rich- water origin with little or no detectable 'primary copper-poor sulphide ore formed by the almost magmatic water'. CMorite and actinolite formed complete replacement of a stockwork conduit from sea-water fluids that had undergone an O which traverses lower Carboniferous felsic pyro- isotope shift (from O up to ,~ +5%) through ciastic rocks. Overlying the pipe, and extending exchange with rocks, implying low effective water/ far beyond its margin, is the genetically related rock ratios (less than x, wt ratio). In contrast, Planes-San Antonio cuprifcrous pyrititc, a 'scdi- vein epidotes formed from relatively non O rnentary-volcanogcnic' deposit precipitated as a isotope exchanged sea waters, implying high chemical sediment derived from sea-floor hot effective water/rock ratios (greater than t, wt springs associated with the lower Carboniferous ratio). If hornblendes in the gabbros formed at volcanic island arc of southwest Ibcria. Most of temperatures higher than 5oo°C, a 'primary the pyritite bodies of Rio Tinto wcrc deposited magmatic water' component in the fluid is directly above or close to their underlying feeder suggested. stockworks. The Planes--San Antonio pyrititc Evidencefor formation of ore de~Oositsduring discharge displays well preserved stratification, slump of geochemically modified sea water. From D/H and structures and intcrdigitatcd tuff bands, and was xso/xeO data on cMorites and quartzes from the mainly precipitated comparatively distant from Troodos mine stockworks, the H isotope data the nourishing Planes stockwork. again demonstrate that the discharging hydro- Across-layer base-metal zoning in the Planes- thermal fluids were of sea-water origin, and lack San Antonio sheet is usually characterized by an of change in the O isotope composition of these increase in copper, and to a lesser extent in zinc fluids implies high effective water/rock ratios and lead, towards the base of the deposit. Much of (greater than x, wt ratio). Stockwork hydro- this enrichment may have been due to late thermal alteration temperatures on the order of hypogcnc processes involving the ascent of 25o-35o°(3 are suggested (i.e. higher than the juvenile-meteoric solutions which leached base surrounding zeolite facies). The data suggest that metals from the underlying volcanic pile and the stockwork fluids were isotopically similar to deposited them as chalcopyrite, sphalcritc and the deeper vein epidote fluids. 'Primary magmatic galena by partial replacement of the lower part water', if present in the stockwork fluids, was so of the preexisting pyrite-rich stratiform sulphide diluted that it is isotopicaUy undetectable. sheet. The ~S values of pyrite from massive ore A notable feature of the pyrititc is the abun- and stockworks define a range from +3"o% to dance of colloforrn and framboidal pyrite. The +7.0% (mean of t 9 analyses = +4"8%). Re- mean isotopic composition of the sulphide sulphur interpretation of these data according to the of the Plancs stockwork (6~aS~ +xo%) is theory of sulphur isotope fractionation formulated distinctly heavier than that of the stratiform by Ohmoto suggests that the combined sulphur Planes-San Antonio pyritite (6alS ~_ +2"7%). in the ore deposits is a mixture of approximately The cupriferous pyritic mass of Planes--San one-third reduced sea-water sulphate (OwaS= Antonio exhibits some of the features character- ,~+i6% in upper Cretaceous time;) and istic of pyritic deposits formed during the initial two-thirds remobilized basaltic sulphur (6~S of stages of arc development ('Cyprus' or 'ophiolite' sulphides in deep-sea basalts = ~ + i %). type ores). It was actually formed, however, at a Conclusion. A sea-water hydrothermal convection late stage of arc evolution, during a period of system model for mineralization and meta- relative quiescence within the waning episodes of morphism of the ophiolitic rocks of the Troodos submarine felsic volcanism, and it displays Massif is reasonable. enviromental traits analogous to those of the Japanese 'Kuroko' ores. The Planes-San Antonio pyritic deposit Rio Tinto, Spain; its nature, environment Volcanogenle ,,,h, erallzatton at Avoca, Co. and genesis. D. Williams, R. L. Stanton Wicldow, Ireland and its regional ira. & F. Rambaud plications. J. W. Platt The San Antonio stratiform cuprifcrous deposit The northeast-southwest-strikingvolcanic rocks of Rio Tinto, discovered in the early 196os, is now that make up the lower Palaeozoic volcanic being exploited as an eastward extension of the province of southeast Ireland arc, in respect of bedded pyrititc sheet in the old Planes mine, their cconornic geology, the poorest known Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
574 Conference report outcropping section of the mobile belt of the within the Avoca belt. Two and possibly three Appalachian-Caledonian orogen. separate volcanogenic ore systems are recognized. The belt in Scandinavia and east North The orebodies are compositionally zoned. America has been relatively well studied in the economic sense. Southeast Ireland has been Zone (a) A siliceous cMoritized stockwork of relatively unstudied owing to the history of disseminated chalcopyrite (o.8o~ Cu) marginality and presumed epigenesis of the (stringer ore) passing laterally into a Avoea orebodies (Avoca is the only operating sericitized alteration envelope with base-metal mine in the Irish Caledonides) and footwail rhyolite (probably domical and more geological emphasis in Ireland being accord- containing magnetitic, potash-rich ed the Carboniferous ore deposits. Studies of areas). published information on the geology and plate tectonic relationships of Avoca-related Scandi- Zone (b) A massive sulphide facies overlying the navian and North American ore deposits (e.g. stringer ore and itself overlain by Bathurst Cap, New Brunswick, Canada; Buchans, carbonaceous shales and magnesium- Newfoundland, Canada; Skorovass, Norway; rich tufts. The massive sulphide orebody Stekkenjokk, Sweden) have led to the diagnosis is zoned into an underlying Pb-Zn-Cu- of Avoca orebodies as 'volcanogenic'. pyrite zone. Contacts between these The association of Avoca orebodies with a zones and their wallrocks are sharp, but felsic, calc-alkaiine volcanic pile, their ore they tongue laterally into ferruginous, mineralogy, age and tectonic style classify Avoca manganiferous, carbonate-rich shales in type (b) of Hutchinson's three-category and tufts, perhaps representing a back classification ofvolcanogenic sulphide ore deposits: arc type of basinal sedimentary facies (a) Zn-Cu-Py (b) Pb-Zn-Cu-pyrite (c) Cu-pyrite. and containing units of the diagnostic The deformational history and plate tectonic New Brunswick style 'bird's eye' crystal model association of the Avoca volcanogenic tuff. The carbonates are mainly per- ores have been qualified. The oceanic plate vasive oolitic dolomite. of the lower Palaeozoic Proto-Atlantic Ocean was destroyed along subduction (or Benioff) zones at An Avoca volcanogenic ore model in its its margins. A subduction system striking north- undeformed state is presented and is shown to east-southwest has been recognized, parallelling have some resemblance to respective ore models the Caledonian thrust front and defining the proposed for the Rosebery orebody, Tasmania, trace of the Caledonian mobile belt--this was and the classic Kuroko deposits of Japan. It is located northwest of a zone now extending from suggested that the oolitic dolomite-rich facies of the English Lake District to Co. Waterford in Avoca may be the lower Palaeozoic equivalent of Ireland. The island arc vulcanism that has made the Kuroko 'Sekkoko' or Gypsum zone. One up the calc-alkaline volcanic province of southeast deformed Avoca volcanogenic ore unit (West Ireland developed along this subduction zone. Avoca) is compared with the undeformed Avoea Subsequent tectonics (including regional meta- ore model. Strike attenuation, isoclinal folding morphism to lower greenschist facies) have left and graphitization of carbonaceous horizons have the fossil island ares of southeast Ireland exposed caused dissociation of the overlying Pb-Zn-Cu- in two northeast-southwest-striking synclinal cores, pyrite zone of the massive sulphide orebody from one stretching from Tramore in Co. Waterford to its counterpart Cu-pyrite zone. Strike-slip has the Irish Sea at Arklow in Co. Wicklow and moved the Pb-Zn-Cu-pyrite zone to the south- another in Co. Wicklow, known as the 'Avoca west to leave it largely enclosed in the oolitic belt' and hosting the Avoca mines in a complex of dolomitized shale/tuff sequence. quartzo-feldspathic tufts, lavas and volcanic Historically, Pb-Zn mineralization at Avoca breccias, intercalated carbonaceous and dolo- has been avoided by producers. Modem mining mitized clastic sediments and related dioritic (post I945) has been conducted almost entirely intrusions. at West Avoca in the Cu-pyrite zone of the massive An understanding of the volcanogenic ore sulphide orebody and its underlying stringer zone. geology of the Avoca mines is the key to the The mine has been described as a marginal copper assessment of the ore potential of the Lower mine, and the potential prize of the Pb-Zn-Cu- Palaeozoic volcanic province of southeast Ireland. pyrite zone awaits evaluation to amend this The Avoca mining area covers 2 miles of strike situation. Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021
Conference report 575
The development and application of the Avoca These deposits consist of angular (indurated) and volcanogenic ore model has led so far to the wispy (plastic) sulphide fragments intimately partial discovery of the dissociated Pb-Zn-Cu- mixed with a large variety of angular to rounded pyrite orebody at West Avoca, with consequent lithic fragments. Abundant evidence exists to implications for the volcanic province of south- suggest that these ores were transported as east Ireland. The 'volcanogenic diagnosis' has also rapidly moving sub-aqueous debris flows triggered led to the discovery of extensive tonnages of by surface volcanic explosions and by earth- stockwork ore in the East Avoca sector of the quakes. They were preferentially deposited in Avoca Mines. palaeotopographic depressions along the path of The tendency of voleanogenic ore deposits of debris flow and maintain economic grade for Avoca type to occur in clusters leads to further distances in excess of 2 km from their source. Other speculation on the ore potential of southeast deposits appear to have moved as slowly advancing Ireland and its generally unrecognized position 'mud glaciers'. In most cases the source of these of importance in the study of Caledonian volcano- transported ores can be identified as in-situ genie sulphide ores. massive sulphide deposits, but at least one sub- economic prospect has no known source orebody. Occurrence, origin and sign;f~cance of Parameters defining the direction of transport mechanically transported sulphide ores were discussed. On the basis of the Buchans at Buchans, Newfoundland. J. G. Thurlow example and numerous other occurrences of transported ores it is suggested that transported The Buchans orebodies are high-grade, poly- ores in deformed terrains may be more common metallic massive sulphide deposits associated with than has previously been suspected. In terms of Silurian (?) felsic volcanism of the northern exploration, economic sulphide accumulations Appalachians. Since 1928, 17 ooo ooo tons of ore may occur as transported orebodies well beyond have been mined, of which c 5o% consists of the limits of stockwork mineralization and mechanically transported sulphide accumulations. significant alteration.
The paper presented at this meeting will be published as a joint publication of The Geological Society of London and the Instruction of Mining and Metallurgy; further information can be obtained from M. J. Jones, Instruction of Mining and Metallurgy, 44 Portland Place, London WIN 4BR. IAN GORDON GASS, Department of Earth Sciences, The Open University, Milton Keynes, MK76AA.