Jamie J. Wilkinson

Fluid inclusions in hydrothermal ore deposits

Abstract The principal aim of this paper is to consider some of the special problems involved in the study of fluid inclusions in ore deposits and review the methodologies and tools developed to address these issues. The general properties of fluid inclusions in hydrothermal ore-forming systems are considered and the interpretation of these data in terms of fluid evolution processes is discussed. A summary of fluid inclusion data from a variety of hydrothermal deposit types is presented to illustrate some of the methodologies described and to emphasise the important role which fluid inclusion investigations can play, both with respect to understanding deposit genesis and in mineral exploration. The paper concludes with a look to the future and addresses the question of where fluid inclusion studies of hydrothermal ore deposits may be heading in the new millenium.

Pressure fluctuations, phase separation, and gold precipitation during seismic fracture propagation

Gold mineralization at Croagh Patrick, western Ireland, occurs in quartz veins associated with three synchronous oblique tensile shear systems. The veins evolved by progressive thickening of isolated, en echelon, lensoid tensile fractures that show a power-law relation between their length and thickness. A change in power-law exponent marks the linking of the en echelon arrays by large, oblique-tensile veins. Fluid-inclusion data show that gold was deposited due to unmixing of an H 2 O-CO 2 -NaCl fluid at 320–240 °C and 160–30 MPa primarily due to decreases in fluid pressure. Evidence for phase separation is only observed in the large linking veins and is abundant close to intersections of the vein arrays. These sites are where high gold grades occur. The distribution of gold is believed to reflect both spatial and temporal variations in fluid pressure fluctuation during seismic fracture propagation, controlled by the vein growth mechanism and zones of maximum dilation.

Anomalously Metal-Rich Fluids Form Hydrothermal Ore Deposits

Hydrothermal ore deposits form when metals, often as sulfides, precipitate in abundance from aqueous solutions in Earths crust. Much of our knowledge of the fluids involved comes from studies of fluid inclusions trapped in silicates or carbonates that are believed to represent aliquots of the same solutions that precipitated the ores. We used laser ablation inductively coupled plasma mass spectrometry to test this paradigm by analysis of fluid inclusions in sphalerite from two contrasting zinc-lead ore systems. Metal contents in these inclusions are up to two orders of magnitude greater than those in quartz-hosted inclusions and are much higher than previously thought, suggesting that ore formation is linked to influx of anomalously metal-rich fluids into systems dominated by barren fluids for much of their life.

Ore-forming processes in Irish-type carbonate-hosted Zn-Pb deposits: Evidence from mineralogy, chemistry, and isotopic composition of sulfides at the Lisheen mine

Lisheen is a strata-bound zinc-lead deposit formed during the Mississippian by replacement of hydrothermally dolomitized, grossly stratiform breccia bodies located near the base of the Carboniferous Waulsortian Limestone. It represents one of a number of carbonate-hosted massive sulfide ore deposits in the Irish ore field that, due to several unique features, have been classified as Irish type. Disseminated pyrite occurs in preore dolomite and around the margins of preore dolomite clasts within dolomite breccias. Early fine-grained sphalerite-pyrite mineralization occurs as infill of intergranular dolomite porosity. Locally, massive to semimassive iron sulfide is observed, mainly comprising pyrite with lesser marcasite. A complex polymetallic sulfide assemblage typifies the main ore stage, dominated by fine-grained disseminated, massive or colloform sphalerite and galena, with minor pyrite, chalcopyrite, arsenopyrite, tennantite, nickel- and cobalt-bearing minerals. Silver occurs in solid solution in tennantite, galena, and sphalerite. Dolomite and barite dominate the gangue, with lesser calcite. Main-stage mineralization involved the progressive replacement of preexisting iron sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Coarse crystalline sphalerite and euhedral galena crystals are generally restricted to fracture-fill mineralization or vugs within main ore-stage assemblages where they occur with euhedral dolomite and calcite. Barite intergrown with main ore-stage sulfides has δ 34S values of 14.3 to 18.1 per mil, consistent with the derivation of sulfate from coeval Carboniferous seawater. The δ 34S values for sulfides range from –44.1 to +11.8 per mil, with a mean value of –13.7 per mil, typical of the Irish ore deposits. The dominant low δ 34S signature is considered to be the result of bacterial reduction of coeval seawater sulfate. Extremely low δ 34S values, in the range of –38 to –44 per mil, are only observed in preore disseminated pyrite; such extreme fractionations are thought to be due to low bacterial sulfate reduction rates coupled with oxidative cycles in near-sea floor pore waters. Main ore-stage sulfides have δ 34S values in the range of –4 to –18 per mil, with a mode of –10 per mil, consistent with a typical bacterial fractionation from coeval seawater sulfate. Isotopic equilibrium between cogenetic sulfides is not observed. The bacteriogenic sulfur component was probably transported from bacterial colonies fringing the ore system by low-temperature brines. The δ 34S values of late ore-stage sulfides mainly range from –20.2 to +12.0 per mil, with the majority having relatively high values (mean = –3.0 ± 8.5‰, 1 σ ) interpreted as being due to the presence of a hydrothermal sulfur component, leached from the lower Paleozoic basement. For galena and sphalerite there is a general increase in δ 34S values with depth in the system, with time, and with proximity to east-west– and northwest-trending faults. These relationships suggest that input of hydrothermal sulfur from depth via fractures became increasingly important. Hydrothermal sulfur appears to be more important at Lisheen than the other major Irish deposits. Galena lead isotope analyses gave average 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.183, 15.594, and 38.080, respectively. These data do not correlate with ore-stage, galena texture or δ 34S. The results are comparable to previous data from Lisheen and from Silvermines, 35 km to the west, implying a common lead source in the lower Paleozoic basement. The textural, mineral, chemical, and isotopic evidence suggests that main-stage ore was precipitated as a consequence of rapid supersaturation, caused by fluid mixing within the permeable dolomite breccias. This process involved relatively high temperature (ca. 240°C), metal-bearing solutions derived from a basement-equilibrated fluid reservoir (carrying Zn, Pb, Fe, Cd) and shallow, saline (ca. 25 wt % NaCl equiv) formation waters rich in bacteriogenic H2S. Minor metals (Cu, As, Ni, Co) are thought to have been stripped from the footwall Old Red Sandstone during hydrothermal alteration around fault conduits. The availability of abundant seawater sulfate, operation of open-system bacterial sulfate reduction, and episodic availability of free oxygen imply that ore formation cannot have occurred at significant depth below the paleosea floor. Cessation of mineralization was due to a cut-off of the sulfur-rich brine supply, possibly by deposition of impermeable hanging-wall sediments. This process of ore formation is consistent with evidence from the other economic Irish-type deposits in the ore field.

Colloidal gold and silica in mesothermal vein systems

Some of the textural features of mesothermal gold-quartz veins may be best explained by the initial precipitation of amorphous silica gel (colloid), which subsequently crystallizes to quartz. This can occur in brittle-ductile shear zones where a significant fluid-pressure drop occurs during stick-slip failure. Such a process rapidly supersaturates the hydrothermal fluid with respect to amorphous silica, which precipitates instead of quartz, owing to favorable kinetics. Depressurization also commonly leads to fluid unmixing and destabilization of soluble gold complexes. However, the presence of colloidal silica can stabilize gold colloid, allowing further transport of particulate gold in suspension in the hydrothermal fluid. Silica gel would be highly unstable under mesothermal conditions and would undergo rapid syneresis and crystallization to form quartz; solid impurities would tend to be expelled toward grain boundaries. This model can account for the primary anhedral aggregate textures typical of mesothermal quartz veins, the concentration of gold along grain boundaries and the formation of discrete gold nuggets, and the rare occurrence of low-order silica polymorphs and relict spheroidal structures. The transport of gold in colloidal form may be one reason for the frequently consistent bulk grade distribution in gold-quartz vein systems over many hundreds of metres (in some cases kilometres) of depth. In addition, the formation of charged colloidal particles may help to explain the attraction of gold grains to specific mineral surfaces.

Intracratonic crustal seawater circulation and the genesis of subseafloor zinc-lead mineralization in the Irish orefield

We have determined the chemical composition of ∼350-m.y.-old solutions extracted from fluid inclusions, and strontium isotopic compositions of hydrothermal minerals from the Irish zinc-lead orefield. These data show that ore-forming fluids were derived from evaporated seawater and acquired metals by deep circulation within fractures in continental crust. Mineralization occurred in the near-seafloor environment when these solutions returned to the surface via thermohaline convection and mixed with brines rich in H 2 S produced by bacterial reduction of seawater sulfate. The results indicate that deep penetration of seawater or evaporated seawater into the continental crust can occur in rift zones or extending passive margins and that this process can generate large volumes of base metal ore-forming solutions. Our results are inconsistent with topographic flow models for mineralization in the district, and support deep convection models for ore formation. The widespread development of evaporitic brines on the Laurussian continental margin under late Paleozoic greenhouse conditions is likely to have been critical for generating numerous accumulations of base metals in sedimentary basins at this time.

Fracture-controlled fluid flow in the Lower Palaeozoic basement rocks of Ireland: implications for the genesis of Irish-type Zn-Pb deposits

Abstract A fluid inclusion study has been carried out to test whether circulation of fluids within fractured very low grade metasedimentary basement rocks was an important process in the genesis of the carbonate-hosted base metal deposits of Ireland. In the Silvermines district, Silurian greywackes and mudrocks are host to several different cross-cutting vein types which are locally abundant in east-east-northeast and northwest-north-northwest trending, high angle, brittle fault zones. Structural analysis indicates that the vein orientations are consistent with formation under a northeast-southwest dextral trans-tensional regime, controlled locally by reactivation of pre-existing faults. Three vein types have been identified: (1) early hematitic calcite-quartz ± pyrite; (2) quartz-calcite ± sphalerite, galena, chalcopyrite, pyrite, barite; (3) ankerite-ferroan dolomite-quartz ± sphalerite, pyrite. Vein types 2 and 3 are associated with weak to locally intense sericite-chlorite-carbonate alteration. Primary fluid inclusion data from vein Types 2 and 3 show homogenization temperature-salinity characteristics (123–238°C, 9.7–20.6 wt% NaCl equivalent) overlapping significantly with the deposit data. In addition, primary inclusions hosted by quartz and sphalerite commonly contain CO2 as indicated by the formation of gas clathrate and, rarely, solid CO2 on cooling. Bulk fluid inclusion analyses on quartz vein samples show that the fluid composition is comparable with experimental data on fluids equilibrated with greywackes at temperatures of 200–350°C. Alkali geothermometry gives temperatures (158–219°C) not significantly different to homogenization temperatures suggesting that the veins formed under low fluid pressures. New inclusion data from Navan brown sphalerite confirm the importance of a relatively high temperature, moderate salinity (187–220°C, 15–18 wt% NaCl equivalent) mineralizing fluid as observed at Tynagh, Silvermines, Lisheen and in the Lower Palaeozoic basement. In these inclusions, gas clathrate has been identified. This is the first reported occurrence of this phase in fluid inclusions from the Irish base metal deposits. These data provide convincing evidence for the regional flow of high temperature fluids, similar to the Lower Carboniferous ore-forming fluids, within the Lower Palaeozoic metasedimentary basement. Systematic regional variations in homogenization temperature-salinity characteristics imply variable infiltration of local, low temperature, surface-derived fluids, consistent with a density driven convection model. In combination with existing isotopic data and thermal constraints, the new data strongly suggest that deep circulation of fluids was a fundamental process in the genesis of the Irish deposits. Therefore, purely topographically driven fluid flow models solely utilizing the Old Red Sandstone as a regional aquifer are inappropriate for the Irish orefield.

Oxygen and hydrogen isotopic evolution of Variscan crustal fluids, south Cornwall, U.K.

Structural analysis of quartz vein systems and fluid inclusion criteria were used to distinguish five different fluid types which flowed through a segment of Palaeozoic crust in southwest England during the Variscan orogeny. Mineralogical constraints in combination with fluid inclusion thermobarometry enabled the temperature of vein formation to be estimated, and isotopic compositions of fluids were determined by analysis of vein material and direct measurement on fluid extracted from inclusions. Peak, low-grade (pumpellyite-actinolite facies) metamorphic fluids had a high 6D and S’*O signature (SD = - 18 to - 10%0, 8’0 = + 10.6 to + 11.9%0) which evolved to compositions in the range 6D = - 28 to - 13%~ 8’0 = + 7.9 to + 11.4%0 during later retrogression and uplift. Fluids in the contact aureole of the Comubian batholith had SD-values intermediate between typical magmatic compositions and the regional metamorphic fluids ( - 23 to - 43%0), and a similar range of S180-values to both magmatic and the regional metamorphic fluids (between + 5.6 and + 14.0%0). These compositions are comparable with those of fluids responsible for Sn-W mineralisation in the province. Post-erogenic fluid chemical and isotopic compositions were exotic and indicate significant infiltration of externally-derived fluids during late- to post-erogenic brittle faulting. Low-temperature, low-salinity fluids which circulated in ENE-WSW-trending brittle normal faults had low #*O-values ( - 0.3 to + 7.4%0) suggestive of a significant meteoric component. Low-temperature, high-salinity fluids, which flowed through N-S- to NNWSSE-trending strike-slip faults and fractures and were responsible for Pb-Zn mineralisation, had significantly D- and rsOdepleted compositions (SD = - 80 to - 49%0, St*0 = - 0.1 to + 4.7%0), typical of basinal brines. These data document the isotopic evolution of fluids in an external (Rhenohercynian) part of the Variscan orogen, through the complete cycle of foreland thrust-belt development and low-grade regional metamorphism, S-type granite emplacement and associated hydrothermal systems, post-erogenic collapse and low-temperature fluid flow in regional fractures. There is limited overlap in isotopic composition between the different fluid types, indicating that fluids flowing through the same host rocks at each stage of orogenesis may be distinguished on the basis of their oxygen and hydrogen isotopic compositions. These data provide a framework for future studies involving fluids of unknown origin in the Variscan and are a reference for comparison with the isotopic evolution of fluids in other erogenic belts.

The origin and evolution of base metal mineralising brines and hydrothermal fluids, South Cornwall, UK

Abstract A fluid inclusion geochemical study has been carried out on quartz from post-Variscan quartz ± carbonate ± base metal sulphide ± anhydrite ± fluorite veins hosted by Palaeozoic basement (Porthleven, Menheniot, Cornwall) and Permo-Triassic sediments (Western Approaches). Data indicate that the base metal mineralising fluids have a similar bulk chemical composition to the saline fluids found in the Permo-Triassic basinal sequence and support the hypothesis that these basins are the source of the mineralising fluids. Cl and Br systematics suggest that the brines were formed either by the evaporation of seawater or a seawater–meteoric water mixture past the point of halite precipitation. The major cation composition (Na, Ca, K, Mg) of the brines is not consistent solely with evaporation processes but may be explained by dolomitisation processes, albitisation processes, or both, which are recognised in the basinal sequences. The presence of seawater in the base metal mineralised veins suggests that the first marine incursions (Late Triassic) into the region must act as a lower age limit for the mineralisation. The halogen chemistry of a second, hotter (200°C), more dilute (0–5 wt.%) fluid identified in fault-hosted E-W trending veins in the Porthleven area, suggests that the chlorinity of these fluids has a magmatic origin. Circulation of these fluids in post-Variscan extensional structures was driven by the local high-heat-producing Cornubian batholith. The local high-heat-producing granites provided fracture permeability and a heat source that heated the base metal mineralising fluids as they entered the horst block and the dilute fluids circulating around the granites. Petrographic evidence suggests that both palaeohydrologic systems were active contemporaneously. However, each flow system was isolated in differently orientated structures, and there is little evidence for fluid mixing.

Silicothermal fluid: A novel medium for mass transport in the lithosphere

New experimental data from synthetic fluid-inclusion studies in the system K 2 O-CO 2 -SiO 2 -H 2 O (KCSH) show that a potassic, silica-rich (≈ 90 wt% SiO 2 ) fluid can coexist immiscibly with a supercritical, alkaline, aqueo-carbonic fluid and quartz from temperatures as low as 300 °C to more than 750 °C at relatively low geologic pressures ( 2 -rich fluids, if they form in the lithosphere, are likely to be important in the mobilization and transport of silica and large ion lithophile elements (e.g., K, Cs, Ba) and metals of economic significance (e.g., Au, Ag, U).