The B-Zone 4611 Silver-Rich Pod—An Unusual Ag-Ge-Sb-As-Ni Assemblage Within the Irish-Type Zn-Pb Silvermines Deposit, County Tipperary, Ireland

Abstract

The Silvermines Pb-Zn-Ag-Ba orebodies comprise vein, replacement, cross-cutting and stratiform mineralization mostly hosted in Lower Carboniferous limestones in the vicinity of a major ENE and E-W trending normal fault array and represent a classic example of Irish-Type Zn-Pb mineralization. Historically the deposits have been exploited at various times, but the major limestone-hosted Zn-Pb-Ba mineralization was not discovered until the 1960s. Structurally controlled crosscutting vein and breccia mineralization represent pathways of hydrothermal fluids escaping from the Silvermines fault at depth that exhaled and replaced shallowly buried Waulsortian limestones creating the larger stratiform orebodies such as the Upper G and B-Zones. The B-Zone, comprising a pre-mining resource of 4.64 Mt of 4.53% Zn, 3.58% Pb, 30 g/t Ag has a locally highly variable host mineralogy dominated by pyrite, barite, siderite, within dolomitic and limestone breccias with local silica-haematite alteration. A small, highly unusual pod of very high-grade Ag-rich mineralization in the B-Zone, the 4611 Pod, discovered in 1978, has not been previously documented. Unpublished records, field notes, and mineralogical and chemical data from consultant reports have been assimilated to document this interesting and unusual occurrence. The pod, representing an irregular lens of mineralization ca 2 m thick and representing 500 t, occurs within the B-Zone orebody and comprises high grade Zn and Pb sulfides with significant patches of proustite-pyrargyrite (ruby silvers) and a host of associated Pb, Ag, Sb, As, Cu, Ge sulfide minerals, including significant argyrodite. Although evidence of any distinct feeder below the pod is lacking, the nature of the pod, its unusual mineralogy and its paragenesis suggests that it represents a small, possibly late source of exotic hydrothermal fluid where it entered the B-Zone stratiform mineralizing system.

1. Introduction

Irish-type (“IT”) carbonate-hosted Zn deposits in Central Ireland constitute a world class orefield recognized after the discovery of Tynagh in the early 1960s [1] and succeeding discoveries at Silvermines, Gortdrum, Navan, Galmoy and Lisheen (Figure 1). These deposits were all developed into successful mines and over ten, mostly smaller deposits, remain undeveloped giving the orefield an endowment of >300 Mt of mineralized rock [2]. The Central Irish Orefield is an important example of a relatively young metallogenetic tract, with the highest concentration of Zn mineralization discovered per sq km globally [3], that has been found using shallow-soil geochemistry in a populated western European country—that had been considered devoid of major mineral deposits. Additionally, it defines a distinct sub-class of carbonate sediment hosted Zn-Pb deposits that exhibit features of both SEDEX and MVT mineralization leading to considerable debate as to their genesis (e.g., [2,4,5]). While several deposits (Navan, Tynagh, Silvermines, Lisheen) have been well documented, most research work has been rather localized, resulting from sampling campaigns focused on locally accessible mine areas and drill core. The authors, having worked extensively on the Silvermines, Navan and other deposits in the Orefield stress that IT deposits are highly variable and that important aspects of the deposits remain under-researched. This paper addresses an unusual and fascinating silver-rich area in the Silvermines deposit that has not been subject to academic research but where old company and consultant reports, and sample collections have been examined to document this occurrence.

2. Regional Geological Setting and Metallogenesis

The Central Irish Orefield occupies an area of ca. 23,500 km² where Zn and Pb deposits, with subsidiary Ag, Cu and Ba occur in stratabound, tabular, locally stratiform lenses hosted within Lower Carboniferous Courceyan to Chadian limestones and dolomites (Figure 1 and Figure 2). These strata lie conformably on basal Carboniferous and Devonian red bed clastic sandstones and conglomerates that unconformably overlie a complex Lower Paleozoic basement comprising greywackes, siltstones, shales and volcanics and end Silurian Caledonian granites (Figure 2). The basement rocks are only weakly metamorphosed but are structurally complex and record the closure of the Iapetus Ocean in which Avalonian and Laurentian terranes became juxtaposed along a wide NE to ENE trending deformation zone running under the general locations of the Navan and Silvermines deposits [6,7,8,9,10,11,12] and may well have influenced the location of the larger deposits [2]. The mineralization generally occurs in the first non-argillaceous carbonate units above the base of the Carboniferous (generally <300 m), these being the pale micrites of the Waulsortian mudbank limestone, [13] in many of the deposits and within the Pale Beds of the Navan Group (pale micrites and grainstones) in the case of Navan. Generally, the mineralization is associated with significant brecciation, dolomitization, pyritization and locally, early silica-hematite-magnetite alteration. Virtually all deposits are located adjacent to major E-W to NE-SW trending, mostly north dipping normal faults with throws commonly in hundreds of metres. These usually composite structures display complex structural histories and variable local orientations generally showing some evidence of movement contemporaneous with deposition and thickening of the Waulsortian limestones and overlying rocks with formation of shelf-basin morphology in the Chadian to Arundian. Most fault zones exhibit superimposed wrench and reverse movement phases, that displace the mineralization and are considered to be of late Carboniferous Variscan age.

Mineralization comprises often chaotic multiple generations of usually fine-grained pale brown-yellow sphalerite interlayered with coarser galena and variable amounts of pyrite-marcasite, carbonates and barite although there is very significant inter-deposit variation that can be ascribed to geographical and host-rock variations in addition to the intensity and cyclicity of mineralization at the varying locations. Numerous textural assemblages have been recognized including massive, semi-massive, disseminated, replacement, vein, and open-space fill with breccia infill and re-brecciation of sulfides. S isotope and fluid inclusion investigations have been both widespread and detailed and demonstrate mixing of bacteriogenically reduced seawater sulphate with metal-carrying hydrothermal fluids carrying a smaller amount of isotopically heavy sulfur [5,14,15].

Two models have commonly been proposed for Irish-type Zn-Pb deposits. The first is syngenetic seafloor deposition: evidence includes the stratiform geometry of some deposits, the occurrence of bedded and clastic sulphides exhibiting sedimentary textures, and, where determined, similar ages for mineralization and host rocks. A second model involves diagenetic to epigenetic replacement; such evidence includes replacement and open-space filling textures, lack of laminated sulphides, alteration and mineralization above sulphide lenses, and a suggested lack of seafloor oxidation [5,14,16].

However, the consensus, supported by overwhelming evidence, is that mineralization occurred at depths ranging from the contemporary sediment-water interface to depths of no more than 300 m or so and at times close to the deposition of host sediments in the late Courceyan and into the Chadian-Arundian [2].

The results from Pb isotope analyses provide evidence of metal derivation by leaching of metals from the underlying Lower Palaeozoic basement by convection of sea-water sourced fluids during extensional activity that promoted normal faulting [5,14]. Extensive fluid inclusion, oxygen and carbon isotope studies have been used to investigate the nature of the ore forming fluids and depositional mechanisms [14]. The data have been interpreted to indicate mixing of metal - carrying hydrothermal fluid, resulting from deep brine convection, with more locally sourced sulfur-carrying fluids The metal carrying fluids are hypothesized to have formed from dense brines, perhaps from partially evaporated Lower Carboniferous seawater, that was circulated to depth, presumably during extension and resultant fracturing; these fluids were then heated enabling them to more effectively leach metals. These sulfur carrying fluids are considered to have been low temperature, high salinity brines, also hypothesized to have been initially sourced from shallow-evaporitic seawater and involved in the production of reduced sulphur in the near subsurface through biological (bacteriogenic) activity. Most of the academic research on IT deposits has rightly been focused on the Zn-Pb mineralization. However, more minor exotic phases of mineralization (Cu, Ag, Ni, Co, Sb and As) are known from several deposits and are important from both economic and ore genetic perspectives but have not been extensively documented. This contribution is focused on the description of one such occurrence of Ag-Ge mineralization at the Silvermines deposits.

3. Introduction to Silver Mineralization in the Silvermines Deposits

The Silvermines Zn-Pb-Ag-Ba deposits in County Tipperary, Ireland are typical of Irish-type Zn-Pb deposits [2,17,18,19].

Although the area was mined sporadically over a millennium or so, the principal deposits were discovered in the early 1960s by Mogul of Ireland Ltd., Dublin, Ireland and mined by underground methods, extracting ca. 10.8 Mt @ 7.36% Zn, 2.70% Pb between 1968 and 1982. A further 5.5 Mt @ 85% BaSO₄ was extracted from an open pit by Magcobar Ireland Ltd, Dublin, Ireland over a similar period (Figure 3A).

Although silver was not particularly rich during the historic mining from the 10th to 19th centuries, the silver content of the oxidized lead ores gave the area its name. A figure of 80 oz. of silver per long ton (~2500 g/t) of smelted lead is quoted by Rutty (1772) and Wynne and Kane (1861) [20,21], but this almost certainly referred mainly to the oxidized ores, although Apjohn (1860) [22] confirmed the same quantity of silver in galena being being mined contemporaneously. Mining historically targeted galena with its subsidiary Ag content and minor Cu in a variety of near-surface, mostly vein deposits hosted by Devonian sandstones and basal Lower Carboniferous (Courceyan) sandstones, limestones and dolomites [23]. Supergene Pb and Zn oxides occur in several areas and zinc oxide mineralization was historically mined in the 1950s with further tonnages discovered by more recent exploration in the 1980s in broadly similar areas [23,24].

More extensive mining since the 1960s exploited several large zones of Zn-Pb sulfide and barite mineralization lying on the northern, downthrown flank of the Silvermines Fault at depths of up to several hundred meters. These zones can be broadly classified into cross-cutting and stratiform types. The cross-cutting areas of veining, brecciation and mineralization are clearly structurally controlled near offshoots of the Silvermines Fault but are ‘stratabound’ in that mineralization is largely confined to basal Courceyan siliciclastics and dolomitized limestones (Lower G, K, C, P, Gortnadyne and Shallee Zones; Figure 3A). The stratiform zone, (subdivided by economic criteria and the presence of ore mineralization—Upper G, B, Magcobar and Cooleen Zones; Figure 3A) occur slightly higher in the stratigraphy but are all hosted at or near the base of a sequence of brecciated and dolomitized Waulsortian limestones of basal Chadian age. Several detailed studies of the stratabound and stratiform zones note the spatial location of the former, stratigraphically below the latter, and outline numerous genetic characteristics to conclude that hydrothermal fluids rose up feeder structures producing the stratabound mineralization and then formed the stratiform mineralization by partial exhalation of fluids onto the Lower Carboniferous sea-floor and by early diagenetic replacement of brecciated Waulsortian limestones [5,17,18,19,25,26].

We describe previously unpublished data on a small area of Ag-rich mineralization that was found in the B-Zone during 1978 and may represent preservation of a relatively high temperature proximal feeder type mineralization within a developing stratiform body.

Values for silver in galena from Silvermines mostly lie in the range 300 to 700 g/t and there is no great difference between galena from the upper stratiform and lower stratabound orebodies. Galena from the Shallee mine contained between 950 to 1350 g/t of silver (production data for 1949 to 1958), and from Gortnadyne between 500 to 620 g/t [21,23]. A remarkable feature of the galena at the Shallee mine was the multitude of inclusions of bournonite, boulangerite, and tetrahedrite [23]. In direct contrast, galena at Gortnadyne rarely contained any exsolved minerals. Graham [27] reported that the values for silver in galena from the Upper G-Zone all lay within the range 300 to 700 g/t Ag but were slightly higher for the B-Zone at around 800 g/t.

The Mogul of Ireland flotation plant operating between 1968 and 1982 typically recovered only around 28–30% of the contained silver in mill feed producing a lead concentrate containing around 240 g/t Ag. The silver recoveries were low due to oxide lead in the feed and due to complex silver-bearer mineralogy [28,29]. Some silver also reported to the zinc concentrate and this averaged 20% of the feed for Ag in concentrate grade of around 50 g/t. However, most silver reported to slimes (<5 micron) and as a result, silver recovery was always rather low and as a result the tailings from the Mogul plant typically assayed around 20 g/t Ag and 0.80% Pb, 0.81% Zn, 21% Fe [28,29].

During routine mining in July 1978, at a nominal rate of 4000 tpd, unusually high silver grades were recorded in the lead concentrate which rose from a historical norm of between 217 g/t to 310 g/t to around 715 g/t [28,29]. Shortly afterwards, an unusual podiform mass of white to pale buff crystalline barite within complex sulfides was noted by mine geologists in the 4611 Room (the “4611 Pod”) of the B-Zone. This pod contained a very unusual assemblage of silver minerals returning assays over 1.5 m intervals of up to 9.55% silver—not always with commensurately high Zn, Pb values. The dimensions of this small but very high-grade silver-bearing lens suggest a total tonnage of about 500 t, potentially containing up to 20 t of silver with significant germanium as described later in this paper.

4. Previous Work

The nature of this rich silver occurrence is unique within the Silvermines orebodies and the diverse and exotic argentian mineralogy is significantly different from that recorded elsewhere in the district by previous authors [17,23,27]. Very little published material exists on the 4611 Pod apart from a mention by Taylor [19], who noted that unlike the lead-rich areas of the B-Zone, where higher silver values were associated with minute inclusions of argentian sulfosalts in galena, the 4611 Pod comprised prominent patches of discrete silver and germanium minerals (see also [30]). Zakrzewski [31] described members of the freibergite-argento-tennantite series and associated minerals from the location whilst unpublished company reports by Gasparrini [32], Hall et al. [33], Carson [28,29] and Pattrick [34] identified the mineralogical associations and provided individual mineral analyses. Considerable unpublished data have also been gleaned from notes in the routine daily underground mine geologists’ notebooks of the authors made between 27 February 1978 and 19 July 1979. Further data was gathered from a series of 11 polished sections made from material collected by the authors.

5. The B-Zone

The geology of the B-Zone orebody comprises a tabular body of sphalerite-galena +/− barite mineralization up to 25 m in thickness developed at the base of the Waulsortian equivalent breccia sequence extending over an area of approximately 800 m by 750 m and comprising 4.64 Mt @ 4.53% Zn, 3.38% Pb, 30 g/t Ag [17,18,19]. The B-Zone occurs mostly downdip of the B-Zone Fault, a WNW trending normal fault considered to be part of the hanging-wall of the Silvermines Fault complex (Figure 3A,B). The B-Zone Fault exhibits a complex slump morphology and was probably a syn-depositionally active structure [35]. No cross-cutting stratabound mineralization is known immediately below the B-Zone, though drilling has been sparse. Abundant sedimentary features such as syn-sedimentary slump breccias, graded-bedding, interbedding of sulfide and shale layers, and geopetal structures confirm the sedimentary to early diagenetic origin of the stratiform sulfide deposits [2,17,18,19,25,26,27,35]. Sulfide textures in the stratiform orebodies are fine-grained and intricate and suggestive of rapid precipitation. A wide range of colloform textural fabrics is also evident.

Zn-Pb-Ag mineralization occurs within complex lenses and sub-lenses interdigitated and layered within variably pyritized dolomite breccias (and rarely, un-dolomitized Waulsortian limestone breccias) and very distinct areas of massive mineralization comprising barite, pyrite and siderite zones where the baritic material is locally associated with haematite and minor silica.

The dolomite breccias are variably pyritized and extend for tens of meters above the ore lenses representing complex alteration and brecciation of Waulsortian limestones during and after early lithification [25].

The distinct lateral distribution and zonation of gangue mineralogy is considered to represent syndiagenetic mineralization during and after sedimentation of the Waulsortian limestones which were brecciated by syn-depositional faulting and where topographic lows formed between the mudbank knolls of the Waulsortian [17]. Some authors have envisaged a later, epigenetic model for ore genesis, and this has been extensively discussed over the years [2,5,25,36]. However, the geometrical relationship of the stratiform B-Zone to cross-cutting mineralization in underlying Courceyan limestones is undisputed and implies that mineralization in the B Zone is the result of rising hydrothermal fluids sourced from the Silvermines Fault complex contemporaneous with active sedimentation.

6. The 4611 Pod

The 4611-mining block is located within the north-central part of the stratiform B-Zone orebody within an area of relatively thin (3–5 m thick), but high grade (>15% Zn+Pb) ore hosted within a complex package of massive sulfide, pyritic dolomite breccias and thin lenses of siderite and barite up to a total of 10 m in thickness (Figure 3, Figure 4 and Figure 5). The footwall to mineralization is formed by knoll-shaped mudbanks up to 15 m thick of pale gray undolomitized sparsely crinoidal Waulsortian mudbank micrites with poorly developed stromatactis fabrics [17]. In the thicker, richer sections of the B-Zone, mineralization occurs at the base of the Waulsortian equivalent breccias and the 4611 area is on the flank, rather than the center, of a former palaeo-topographic low between the mudbanks. This contact can be quite irregular with a distinct relief and locally shows evidence of erosion towards a form of Neptunian dyke development (see Figure 4 and Figure 5) or possible feeder structure. The hanging-wall to the mineralization is gradational and often irregular and patchy within the pyritic dolomite breccias with the quantity of pyrite and sparse base metal sulfides diminishing upwards (Figure 4 and Figure 5).

The massive sulfides comprise fine-grained massive, often brassy, pyrite typically comprising up to 90% of the rock mass with patches of rich Zn-Pb sulfides attaining grades of up to 40% metal with Zn to Pb ratios around 1:1 and average silver contents of less than 30 g/t (Figure 6A,B). Within these typical massive sulfides, a lens of buff to white barite occurs, conformable with the overall dip, with a feather edge to the north. This barite dominantly comprises white coarsely crystalline intergrown laths with minor pyrite and clasts of dolomitized and pyritic Waulsortian micrites (Figure 6D,E). Typically, this zone comprises around 60% barite, 30% sulfides and 10% dolomitized limestone and is up to 1.5 m thick. The hanging-wall contact of the barite is relatively sharp and passes upwards via a gradational contact over a few centimeters into fine grained massive sulfides with occasional bladed barite crystals and base-metal sulfides, overlapping onto and over the mudbank surface.

In the 4611 Room, the upper part of the white barite lens contains a very localized pod of highly unusual mineralogy extending for approximately 9.5 m by 8 m with the long axis trending at around 150° and up to 2 m in thickness. Within this pod, masses of pyritic dolomitized mudbank micrite breccia up to 100 cm by 25 cm occur along with sporadic patches of black silver sulfosalts up to 15 cm by 10 cm with some blebs of ruby silvers up to 1 cm in diameter along with galena and very fine-grained pale buff sphalerite (Figure 6C,E,F). Grab samples of this material returned up to 20% Zn+Pb and spectacular silver grades up to 95,000 g/t (9.5% Ag) over vertical intervals of up to 2 m. Whilst not analyzed during mining, the content of germanium within the pod was visually estimated to have exceeded 0.1% Ge based upon the content of argyrodite (Ag₈GeS₆) noted in daily face sketches and contemporary notes. Below the silver enriched zone, which is up to 2 m in thickness, the silver sulfosalt content of the crystalline barite diminishes rapidly with only sparse small blebs of proustite, seemingly commensurate with a fining of crystal size of the barite (Figure 6G).

Irregular cavities up to 50 cm by 25 cm are developed within the coarse white barite and the overlying massive sulfides. These cavities are largely infilled with semi-lithified granular pale brown barite with a sandy appearance with patches and blebs of black silver sulfosalts and ruby silvers (Figure 6C). Prominent N-S vertical jointing associated with this cavity development and recrystallization of the barite within these cavities and on joint walls is also often associated with distinctive ruby silver minerals. This is interpreted as localized post-main mineralization groundwater-induced alteration.

The contact between the footwall Waulsortian mudbank micrite and massive sulfides or barite is gradational with diminishing mineralization and brecciation downwards. Dislocation of the breccia clasts is commonplace as demonstrated by rotated stromatactis fabrics. Within the footwall Waulsortian mudbank micrites are a zone of strongly developed narrow (<1 cm) veins of crustiform carbonate and pyrite trending around 160° (±5°) and dipping steeply NE. Parallel to these veinlets a steeply dipping irregular elongate “pipe” up to around 1.5m by 2.0m in diameter is present, also orientated with its long axis at around 160° (±5°). It is infilled with brecciated fine grained granular pyrite with disseminated zinc dominant base-metal sulfides assaying up to 15% combined metal with silver values around 12 g/t (Figure 4 and Figure 7). Occasional angular clasts of crinoidal limestone are present and some carbonate occurs as intraclast cavity infill. The contacts of the pipe are sharp and there is no evidence of faulting controlling its location. It was not possible to follow it vertically for more than a few meters due to lack of exposure in mine workings and drilling. It is notable that the veinlets in the footwall trend in the same direction as the minor post-mineral faulting seen in the western part of the B Zone.

7. Mineralogy

Detailed descriptions are given in Appendix A of this paper.

In addition to Zakrzewski [31] several unpublished reports and company memoranda on the mineralogy of the 4611 Pod exist. Reports by Hall et al. [33], Gasparrini [32] and Patrick [34] utilized reflected light microscopy, electron microprobe analysis and X-Ray diffraction analysis. The pod comprised approximately 50% of coarsely crystalline white translucent barite with around 30% as lenses of very fine-grained pyrite with sphalerite and 20% galena-sphalerite intergrowths and patches within dolomitized breccias. Initial examination of the pod in 1978 included analysis of several large (~10 kg) samples of representative material from within the pod and its immediate environs.

A very rich ore sample (33308) analyzed by AAS in the mine laboratory assayed 57.05% Pb, 12.09% Zn, 4.30% Fe and 0.29% Ag (2950 g/t) was collected from the main ore horizon immediately adjacent to the apparent bounds of the silver-pod. It comprised mainly a coarse intergrowth of galena and sphalerite with veinlets and irregular crystals of pyrite [37]. Radiating spheroidal dendritic intergrowths of galena in sphalerite (50–100 μm diameter) also occurred and the galena contained abundant bladed inclusions of geocronite-jordanite, similar to that described by Graham [27] from the Upper G-Zone.

Sample #33403 taken from the silver pod comprises gray, fine grained massive sulfides assaying 24.85% Pb, 27.40% Zn, 6.00% Fe, 0.86% Ag (8553 g/t) 0.28% Sb, 23.8% S, 1.01% Ni, 2.6% Ca, 0.11% Mg and 0.09% Mn. Pyrite was poorly crystallized (melnikovite) in patches and contained abundant inclusions of galena and sphalerite. Argyrodite was associated with sphalerite and galena; close to the argyrodite, pyrite was replaced by marcasite and gersdorffite. Intergrowths of pyrite and barite were also present and inclusions of K, Ba, Al, Si -phase (hyalophane) occurred within the pyrite [37]. Sample #33309 comprised white crystalline barite with pyrite, argentite-acanthite and visible blebs of ruby silvers and assayed 6.79% Pb, 8.59% Zn, 8.20% Fe, 5.8% Ag, 0.29% Sb, 1.70% As, 0.45% Cu, 1.70% As, 0.76% Ni, 0.26% Mn. This sample contains abundant sulfides in mosaic carbonate with laths of barite. Coarse-grained pyrite included patches of melnikovite rimmed by marcasite. The iron-sulfides are veined by galena and sphalerite and contain abundant inclusions of proustite. Fine “graphic” intergrowths of pyrite and proustite were present and visible to the naked eye. Sphalerite and galena are associated in a variety of intergrowths—including colloform texture, fine radiating dendritic spheroids (~50 μm diameter), emulsion texture (galena grains~1 mm) and fine disseminations. Sphalerite also occurred as large colloform areas (1000 μm diameter) of radiating acicular sphalerite crystals [37]

The principal silver minerals identified by subsequent investigations included members of the ruby silvers (Figure 8). Despite their similarity, at least five distinct species of these minerals have been recognized. They include proustite-pyrargyrite, xanthoconite and two other minerals tentatively identified, as close in composition and optical properties to smithite and arsenian miargyrite. Argentite-acanthite, argyrodite, geocronite-jordanite, diaphorite, arsenian boulangerite, argento-tetrahedrite-tennantite (freibergite) and gersdorffite have also been identified [31,32,33,34].

Minerals of the argentite-acanthite type are very abundant, locally forming in excess of 10% of the ore as large blebs of almost colloform aggregates intergrown with argyrodite up to 10 cm in diameter and as finer grains that occur enclosed in the major sulfides. Proustite and pyrargyrite with occasional intergrowths of xanthoconite, smithite and arsenian miargyrite usually form <1 vol % of the mineralization.

Argyrodite is relatively abundant occurring as discrete blebs up to several centimeters in diameter myrmekitically intergrown with argentite-acanthite or as monomineralic blebs that exhibit very little internal structure and have narrow diffuse replacive margins with earlier sulfosalts and sulfides.

Gersdorffite mostly occurs as dense fine-grained intergrowths with geocronite and galena as idiomorphic crystals up to 20 μm associated with myrmekitic intergrowths of proustite-pyrargyrite and galena, and, locally, as overgrowths on pyrite.

In general, the mineralization assemblage is dominated by antimonial silver minerals although arsenical variants are present. Copper and bismuth-bearing sulfosalts are sparse. Other minerals observed are sphalerite, galena and pyrite-marcasite. In some samples massive fine-grained sphalerite with up to 10 mm thick crusts of fractured marcasite is overgrown by up to 4 mm sized yellowish-white crystalline barite with some of the fractures in the marcasite being infilled by ruby silvers. Galena occurs in aggregates up to several mm in diameter within the sphalerite.

Zakrzewski [31] recorded that the minerals recognized macroscopically in paragenetic sequence comprise galena, white dolomite, ‘honeyblende’ sphalerite, and tetrahedrite as crystals up to 10 mm in size—which were interpreted as being recrystallized. Textural evidence indicates that marcasite and pyrite precipitated first as colloform crusts or as granular masses. Iron sulfides are generally of a colloform fabric, occurring as complete spheroids or fragments of radially crystalline masses. Minor amounts of pyritohedral crystals and crystal aggregates are also evident within larger masses of argyrodite. Concentric schalenblende-type sphalerite, typically dark and pale brown, replaces the iron sulfides and forms botryoidal masses.

Both minerals are invaded by coarse-grained first-generation galena that replaces sphalerite. Geocronite and gersdorffite crystallized together with galena whilst tetrahedrite and ruby silvers were among the later-forming phases. Following a phase of minor and localized fracturing a second generation of galena, in the form of myrmekitic exsolutions within proustite-pyrargyrite and tetrahedrite also formed at this stage. Subsequent recrystallization produced the coarse crystals of galena, sphalerite and tetrahedrite interstitial to the host rock dolomite breccia clasts. Tetrahedrite, geocronite and gersdorffite occur as anhedral grains up to 50 μm in diameter, intergrown with geocronite, proustite-pyrargyrite and galena. They appear to replace geocronite and may be replaced by proustite-pyrargyrite [31].

The minerals of the ruby-silver type are all rather similar in optical properties and qualitative compositions and, apart from in areas where grains of two different species were close and comparison of the optical properties was possible, it was difficult to differentiate one from the other.

8. Paragenesis

Examination of several new polished sections under reflected light has confirmed the general paragenesis established by Zakrzewski [31]. Early marcasite-pyrite is followed by pale sphalerite and galena (with geocronite and gersdorffite) before a second phase of galena with ruby silvers and diaphorite-tetrahedrite. Argyrodite is late, with a final phase of argentite-acanthite (Figure 9). The silver minerals show successive enrichment in Ag indicative of a sequential increase in Ag activity and possible temperature reduction of the hydrothermal fluids (Figure 10A). In parallel with the increase in silver in the mineral species there is an equivalent reduction in the Sb content and increase in the As contents (Figure 10B). Many of the Sb-As species show a general paragenetic trend from antimonial to arsenian over time (Figure 9 and Figure 10B). This compositional variation is likely to be in response to spatial and temporal changes in the fluid chemistry.

9. Discussion

The mineralogically complex silver-rich 4611 Pod is apparently unique within the stratiform ore bodies at Silvermines although similar pods could have been mined without detection or remain undiscovered. This chemically and texturally complex assemblage of a large variety of fine-grained, intimately mixed minerals rich in silver, germanium, arsenic, antimony, bismuth and nickel, and the associated relatively high Pb-Zn ratio, suggest that the silver pod developed where a small feeder structure channeled high temperature fluids into the developing B-Zone stratiform orebody—perhaps defining a small exhalative center.

High silver grades are restricted to a small area and diminish rapidly both laterally and vertically to levels typical of the overall orebody. The concentration of silver minerals within the white crystalline barite and immediately enveloping massive sulfides suggests a specific and unusual control on the silver deposition and a highly localized up-flow of silver-antimony-germanium rich hydrothermal fluid. Hall et al. [33] postulated that the high silver grades suggested proximity to a feeder conduit as did Taylor [19] who also suggested that concentrations of sulfosalts of copper, arsenic, and antimony provide evidence of feeder zones. Taylor and Andrew [17], Taylor [19], Andrew [18], Lee and Wilkinson [25] and Kyne et al. [26] all used Zn to Pb ratios to show that the positions of the large exhalative centers are probably controlled by WNW- trending structures. However, the location of the 4611 Pod is enigmatic in that it is not located close to a known structure nor is it contained within a distinct area of high lead to zinc ratios, although localized patches of high Pb mineralization are a characteristic of the pod (Figure 11A,B).

In the 4611 Pod the assemblage of Ag-Ge-Sb rich minerals appears to have been superimposed upon the earlier Zn-Pb-Ba mineralization in a localized fashion. It is therefore conceivable that any feeder at 4611 was not only localized but developed later in the mineralizing process. This may have comprised a small extensional center following early movement on the B-Zone Fault that allowed the development of a brine pool within a quiescent depositional environment - where early barite was replaced by argentian and base-metal sulfides as suggested by Rajabi et al. [38] with regard to Mehdiabad and elsewhere in the Malayer-Esfahan metallogenetic belt.

Both the silver and germanium contents of the 4611 Pod are the highest recorded in the Irish Orefield. Elsewhere in the Irish Midlands carbonate-hosted IT deposits contain generally moderate silver values. The Ge grade of Zn ores from the northernmost Navan Group hosted deposits is low (a few tens ppm Ge in sphalerite samples); but this increases in the southern Silvermines–Lisheen group of deposits, which attain grades of above 100 ppm Ge in sphalerite samples [39]. Wilkinson et al. [16] reported Ge grades from drill core samples within the Lisheen deposit that had grades of 400 to 900 ppm in sphalerite: 200 to 1300 ppm in galena, and 200 to 1000 ppm in tennantite. Recently Group Eleven Resources have reported significant Ge along with enhanced Ag values from drilling at the Ballywire prospect (located 40 km SSW of Silvermines) largely ranging between 23 g/t Ge and 79 g/t Ge. (Group Eleven Resources Corp—PG West—press releases).

Does the extreme concentration of both Ag and Ge in the 4611 Pod indicate a localized specific source of these metals? Ge is uncommon, but it is not an extremely rare element in bulk continental crust averaging 1.5 ppm in oceanic crust and 1.6 ppm in continental crust [40]. Little is known about the behavior of Ge in S-bearing hydrothermal fluids, and it seems that there are two types of sulfide ore deposits that can be distinguished: i.e., (a) those where Ge is concentrated in sphalerite (up to 3000 ppm Ge) and (b) those where Ge forms its own sulfide minerals or substitutes for metal atoms (mainly As and Sn) in sulfosalts. The behavior of Ge appears to be dependent on the sulfur activity; Ge enters ZnS in low to moderate sulfur activity environments and forms its own phases under higher fugacity of sulfur [41]. fO₂ and fS₂ conditions may have played a key role as well in the remobilization of Ge, controlling which Ge-mineral crystallizes. High fO₂(and low fS₂) had an essential impact on remobilization of Ge in sphalerite to form Ge oxide, which is thermodynamically stable under highly oxidizing conditions [42,43].

The mineralogical assemblage in the 4611 Pod is unusual and there are few analogs in the literature. Most recorded instances of the of argyrodite occur in (1) low-temperature polymetallic deposits with silver sulfosalts (Freiberg, Germany); (2) in high-temperature Sn-Ag deposits (Bolivia); or (3) in epithermal vein Au-Ag base-metal deposits [44,45,46]. Studies on the physico-chemical conditions for the deposition of similar silver—germanium mineral assemblages at Himmelfurst and Bräunsdorf, Freiberg, Saxony, Germany [47,48] have shown that depositional temperatures of between 180 to 220 °C existed. Chil Sup So et al. [44] concluded that Ge deposition in the Weolyu Ag-Au vein was mainly a result of cooling of hydrothermal fluids (down to 175~210 °C), due to increasing involvement of cooler meteoric waters in the epithermal system. Zhurakova et al. [45] established that in the complex naumannite, acanthite, argyrodite, galena and sphalerite ores of the Rogovik Au-Ag deposit the Ag sulfo-selenides of the acanthite series were formed later than naumannite, but within the same range of logfO₂ values below 110–177 °C from solutions with high S concentration. The Daliangzi Pb-Zn deposit in China is one of the Ge-rich Pb-Zn deposits in the Sichuan-Yunnan-Guizhou Pb-Zn polymetallic metallogenic triangle area and the trace element signature of the sphalerite indicates that the temperature of the deposit was low-moderate: the formation temperature of the Ge-bearing sphalerite in stage II was 86–213 °C (134 °C on average) [49]. All of this thermometric data falls within the range of depositional temperatures well established for the Silvermines mineralization [50].

10. Conclusions

The fine-grained intergrowths, colloform and emulsion textures are interpreted as being indicative of slight recrystallization of fine sulfide mud or gel, whereas the coarse-grained phases have either formed by more extensive recrystallization or by direct crystallization from hydrothermal solutions.

The large number of minerals of complex chemistry and their textural association is compatible with a chaotic and telescoped mode of deposition and seems to be good evidence of a near- surface interpretation of the formation of the silver- rich pod.

The Ag-Ge mineralization seems to be temporally related to a phase of minor WNW-ESE trending fracturing that post-dates the major movement on the Silvermines fault complex and the principal early exhalative and near seafloor pyrite-barite-siderite hosted Zn-Pb mineralization (Figure 5 and Figure 11).

We conclude that this probably reflects an unusual phase of hydrothermal fluid uprising from depth into the exhalative and near sea-floor mineralizing diagenetic environment that controlled widespread Zn-Pb-Ba-Fe mineralization in stratiform ore zones at Silvermines.

The 4611 Pod occurrence is presently unique in the Central Irish Orefield. However, why this extremely rich, closely constrained, small body contains such high silver and germanium grades remains a mystery.