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Volcanic Processes in Ore Genesis

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Volcanic Processes in Ore Genesis Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021 Conference report 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. 7 Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021 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. Downloaded from http://jgs.lyellcollection.org/ by guest on September 25, 2021 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).
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