Barrett, T.J., Tennant, S.C. and MacLean, W.H., 1999.
Unpublished Report for Anglesey Mining plc, Bangor, Wales. Volume 1 (text) + Volume 2 (appendices).
At the Parys Mountain deposit, owned by Anglesey Mining plc, several different styles of base metal mineralization, including polymetallic massive sulfide lenses and Cu-bearing vein systems, are hosted within Ordovician to lower Silurian shales/mudstones and mainly rhyolitic volcanic rocks. The rhyolites extend about 3 km along strike, and almost 1 km across strike. In plan, they form a 'hairpin' - the northern and southern rhyolite limbs, which are ù50-100 m thick, 'merge' into a 300 m-thick mass in the western part of the property. A basaltic unit is also present in the latter area. The rhyolite limbs are flanked to the south, west and north by Ordovician shales (Abstract Fig. 1). However, lower Silurian (Llandovery) shales, known as the Central Shales, occur between the rhyolite limbs. A commonly but not universally accepted model for the overall structure invokes an E-W-trending syncline with the northern limb overturned to the south. Lower Paleozoic rocks on Anglesey lie unconformably on a regional metamorphic basement of late Precambrian age.
In the late 1700s to early 1800s, Parys Mountain was one of the world's leading Cu producers. The Cu was recovered mainly from 'lodes' within the Central Shales via shafts and pits, and from the contact between Northern Rhyolites and Northern Shales via underground workings. Information on the mineralization in the old open-pit workings is scanty, but in addition to the Cu-rich 'lodes' which were mined, Zn-Pb-rich 'bluestone' masses and veins were also present, although the Zn-Pb mineralization generally was discarded as waste at that time. Much of the Cu along the northern contact was in the form of cp-py-qtz veins hosted by silicified shales and an unusual quartz-rich rock. In the 1960s and 1970s, drilling by Canadian and British companies discovered polymetallic massive sulfides in the western part of the property, in an area known as the Engine Zone, at the contact between the Southern Shales and overlying rhyolites, i.e. on the normal-facing limb of the inferred synclinal structure. Massive sulfides have also been located along parts of the southern and northern contacts in the central part of the property. The eastern part of the property remains largely unexplored, as do some of the deeper central parts.
The Engine Zone comprises high-grade Zn-Pb-Cu-sulfide beds and masses within a series of altered and veined shales, thin felsic volcaniclastic beds, and heterolithic mudflows. In 1990, the Robertson Group estimated that the Engine Zone contained probable reserves of 1.41 Mt grading 1.99% Cu, 3.42% Pb, 6.65% Zn, 99 g/t Ag and 0.79 g/t Au; and possible reserves of 2.83 Mt at 3.20% Cu, 1.93% Pb, 4.54% Zn, 22 g/t Ag and 0.14 g/t Au. Probable reserves in the nearby White-Rock Zone were 0.84 Mt grading 0.49% Cu, 3.43% Pb, 6.72% Zn, 78 g/t Ag and 0.66 g/t Au. These areas formed part of an overall estimated geological reserve for the property of 6.45 Mt at 2.34% Cu, 2.60% Pb, 5.35% Zn, 39 g/t Ag and 0.23 g/t Au.
Recently, as part of Anglesey Mining's exploration and research programme at Parys Mountain, the volcanic rocks have been dated for the first time, by R. Parrish of the British Geological Survey. The results indicate that the northern and southern rhyolite limbs are both of Llandovery age (R. Parrish, pers. comm., 1998) (Abstract Fig. 1). An assessment of existing paleontological data by M. Howe of Leicester University strongly supports overturning, to the south, of the Northern Shales and much of the Central Shales (and thus also the Northern Rhyolites) (M. Howe, pers. comm., 1998). The mineralization of the Engine Zone, although lying on Ordovican shales, is probably also of Llandovery age (i.e. the age of the altered rhyolite immediately above the sulfides).
A large-scale lithogeochemical, relogging and petrographic program was carried out in 1995 and 1997-98 by Ore Systems Consulting. Overall, some 700 samples from 60 drillholes, and 80 outcrop samples, were analysed by XRF techniques. About 80 samples were also studied petrographically. In the basis of immobile element ratios such as Al2O3-TiO2, TiO2-Zr, and Nb-Zr, five distinct rhyolite types can be identified, termed rhyolites A, B, C, D1 and D2, as well as two mafic, and three main mudstone types. Although two rhyolite groups may lie on the same trend in a given plot, they can be separated out on another. Thus, rhyolites A and B lie on almost the same trend in an Al2O3-TiO2 plot, but on much different trends in a Nb-Zr plot, while Rhyolite C is identified by its much higher Zr/Al2O3 ratios relative to the other rhyolite types. In the case of rhyolites D1 and D2, there appears to be a trend from one end-member to the other, which suggests they are chemically related (e.g. through fractionation). The mafic rocks, which are volumetrically minor, fall into two chemical groups (both include basalt to basaltic andesite). One type corresponds to a syn-mineralization sill-like mafic sheet which occurs near the base of the Southern Rhyolites, and the other to 'late' mafic sills which intrude the Northern Shales.
All rhyolite types, except B, have elevated Nb (34-56 ppm) but moderate Zr contents (210-370 ppm, which suggests a high-K subalkaline affinity (Leat et al. 1986). They have fairly low Zr/Y ratios (2-5) and relatively flat REE patterns, which suggests a 'tholeiitic' affinity. All types except B have similar Zr/Nb ratios, probably due to derivation from a common source, and show little evidence for a subduction influence. Rhyolite B has a transitional affinity. Basalts near the base of the rhyolite sequence are enriched in Ti, Zr, Nb and light REE relative to normal MORB, and have a transitional, within-plate affinity; such features are commonly found in basalts emplaced in continental rift settings. Primary geochemical features of the rhyolites and basalts, as well as the fact that the Ordovician shales regionally lie unconformably on a late Precambrian metamorphic basement, suggest that the Parys Mountain deposit formed during a phase of volcanism (Llandovery) which accompanied intra-plate rifting of submerged continental crust.
Rhyolite A is the volumetrically dominant type in the western part of the property, with rhyolites D1 and D2 dominant in the eastern portion. Rhyolite C outcrops only in the southwestern corner of the property, where it lies above the Southern Shale, but below the main mass of rhyolite A. Rhyolite C thins downdip to the north, and has the overall form of a tapering wedge (maximum thickness of about 80 m). It can be traced downdip about 400 m, by which point it has thinned to a 10-20 m or less, and it can be traced east-west for about 800 m. A thin sheet of basalt, usually 10-20 m thick, generally occurs between rhyolites C and A, although in places it crosses rhyolite C. Where rhyolite C is absent, the basalt is usually absent. The Engine Zone massive sulfides are intimately associated with rhyolite B, which occurs as thin beds of volcaniclastic material or as thin massive lavas. Rhyolite B commonly is either so chloritized that it resembles shale, or so silicified it appears to be 'quartz-rock'. In the nearby Chapel Zone, massive sulfides occur along the same shale contact, but are overlain by rhyolite C and basalt. In this zone, the sulfides mainly occur marginal to, or beneath rhyolite C. In the Engine and Chapel Zones, rhyolites B and C (and the basalt) are overlain by thick sequences of rhyolite A.
Rhyolite B makes up the surface rhyolite outcrops west of the Penymynydd fault, and is the main volcanic rock at depth in the White-Rock Panel (west of this fault), which hosts massive and vein sulfides. As noted earlier, Rhyolite B also occurs in the deep Engine Zone as massive lavas up to 15 metres thick which lie above the shales and sulfides. These relations suggest that the White-Rock Panel is partly correlative with deep Engine Zone stratigraphy, although the former has probably been inverted and faulted. Finally, rhyolite B also occurs in a separate area between the Northern Rhyolites and the Northern Shales, where it is commonly strongly altered, and hosts an important massive sulfide occurrence. The presence of rhyolite B and sulfides on the northern flank supports the idea that deep Engine Zone stratigraphy is structurally repeated in this area. Rhyolite A extends to the east as a component of both the northern and southern rhyolite limbs. Conversely, rhyolites D1 and D2 become more abundant in this direction. A thick interval (ù200 m) of flow-banded rhyolite A in the western part of the property, and one of rhyolite D1 (ù80 m thick) in the eastern part suggest that these two areas mark the locations of eruptive centres. Although commonly flow-banded and flow-brecciated, rhyolites A, C and D1 also can have a fragmental or pyroclastic appearance. Rhyolite D2 appears to be a pyroclastic interval.
Shales at Parys Mountain have been divided into 3 main chemical types: N, X and C. In addition, there are smaller groups of shale which fall compositionally between the main groups. Most of the Northern Shales are of N-type, while most of the Central Shales are C-type. X-type shales and thin C-type shales locally occur below the first rhyolites (near the massive sulfide horizon).
Mass changes have been calculated for about 600 samples by relating each sample back to its appropriate precursor lithology and using single-precursor mass change methods (MacLean and Kranidiotis, 1987; Barrett and MacLean, 1991) The results have been plotted downhole and contoured on several sections across the property. In the Engine Zone, strong alteration commonly occurs in the upper part of the footwall mudstones, the rhyolite B volcaniclastic beds, and the lowest part of the rhyolite A sequence. On sections across the western and central parts of the property, alteration increases with depth and in the downdip direction, as shown by substantial mass additions of Fe and Mg (mainly as chlorite) to the rhyolites and shales. The most intensely chloritized rocks have also lost K. There is a general correlation between areas of Fe+Mg gain and the known locations of sulfide lenses. Locally, Fe+Mg has been added as ankeritic carbonate, which occurs as clots and veins within shales and rhyolites. Silica shows a wide range of mass changes, with large gains in some of the shales and the rhyolites (e.g. in the White-Rock Panel), but large losses in strongly chloritized or sericitized zones. Areas of Fe+Mg gains and Si gain have likely experienced a phase of chloritization at higher temperatures followed by a phase of silica precipitation at lower temperatures. Mass changes also have been calculated for 65 outcrop samples from across the property. A zone of increased alteration occurs along the northern flank, which may reflect the presence of a deeper hydrothermal system (e.g. the one which formed the massive sulfides and altered rhyolite B and shales in holes H-30 and A-15).
At Parys Mountain, the first volcanism after a long period of Ordovician shale sedimentation produced rhyolite B. Although volumetrically minor, it is important as a marker horizon, as the first polymetallic sulfides were deposited at this time, as high-grade Zn-Pb-Cu sulfide beds and masses (within shales and volcaniclastic rhyolite B beds). There is evidence that the composition of the associated shales was changing at this time, which may reflect the establishment of local grabens. At the west end of the property, rhyolite C was emplaced, probably partly within these shales, and partly above them. Its emplacement may have disrupted some of the sulfide-shale intervals to produce local mud-rich debris flows, some with sulfide clasts. Areas which were marginal to rhyolite C are more likely to host undisturbed sulfides. Where the sulfide beds are not mixed with other material, they are very high in base metals (30-40% Zn+Pb+Cu), with the remaining material consisting of pyrite, quartz and carbonate. An interesting feature of some of the base metal-rich ores in the Engine Zone is their high Ag and Au contents, which respectively are in the 200-1000 g/t and 1-5 g/t ranges (these ores contain (10% iron). Given that some of the high-grade sulfide beds are clastic, it would be important to locate their source area.
The western part of the property apparently was a site of active uprising of magmas, probably along fractures, and of related hydrothermal activity which produced strong alteration of shales and of the lower parts of rhyolite C and A (with local sulfide veining). At more or less the same time as rhyolites C and A were accumulating in the western part of the property, rhyolites D1 and D2 were erupting in the eastern part. The latter area has not been systematically explored to date, although it may represent a second locus of volcanic activity, and thus of seafloor faulting and sulfide mineralization. Mid-Llandovery rhyolite volcanism at Parys Mountain was followed by the deposition of mid-Llandovery Central Shales, which also hosted polymetallic mineralization (in the opencast pits). This suggests that the hydrothermal systems which formed the pre-rhyolite massive sulfides locally became re-established after volcanism and deposited more metals. The Parys Mountain massive sulfide lenses are closely similar to many Kuroko-type deposits in terms of: 1) an association with felsic volcanic and locally basaltic rocks; 2) the occurrence of several laterally separate sulfide lenses along one main time horizon; 3) the generally Zn-Pb-rich nature of the sulfides, which are also Cu-rich in places; 4) the presence of clastic (transported) sulfides; 5) the general absence of pyrrhotite. Parys Mountain differs in terms of the nature of its immediate footwall (shale versus felsic volcanic rocks), the general lack of barite, and the absence of footwall sulfate alteration. The lack of sulfates at Parys Mountain may simply indicate that circulating fluids and local bottom waters were more reducing. The Kuroko deposits probably formed in a volcanic back-arc, whereas the overall tectonic setting of Parys Mountain seems more akin to that of rhyolite-dominated VMS settings in rifted continental crust, e.g. the Iberian Pyrite Belt. Although the deposits in this belt commonly occur as large, single, pyrite-rich lenses, there are also numerous smaller Zn-Pb-rich orebodies. The Iberian deposits generally occur above rhyolite, but locally within shales. At Parys Mountain, precious metal enrichment occurs in parts of the Engine Zone; a well known example of highly Ag-Au-rich clastic sulfides occurs at Eskay Creek, British Columbia, where the ores occur in shales (above rhyolites chemically similar to those at Parys Mountain, and below basalts).
Significant portions of the southern rhyolite-shale contact in the central and eastern part of the Parys Mountain property, from 5000E to about 6500E at Penysarn (AMC grid, in metres) and a depths below about 400 m below mine datum, have not been drilled, although this is the same contact as that hosting massive sulfides in the Engine and Chapel Zones to the west. Several shallower areas of the southern contact are also untested. The downdip mineralization west of the Penymynydd Fault (White-Rock Panel) also has not been drilled off. In addition, the northern contact has the potential to host massive sulfides, as shown by the intersections in H-30 and A-15. The deep northern contact is almost untested east of 5000E, although it may be cut out by the Corwys Fault east of 5800E. Areas of Cu-rich quartz veins in the Northern Shales (i.e. the Northern Copper Zone) are interpreted as stockwork veins which may be potentially related to undiscovered massive sulfides situated along deeper parts of the northern rhyolite-shale contact. Although data are limited, the contents of gold in veined and silicified shales along the northern flanks is commonly anomalous, and should be further investigated. An unexplored and undrilled tract of shales extends east from the Penysarn rhyolite for two km, to the Rhosmynach rhyolite near the coastline. In the past, polymetallic mineralization was locally worked at Rhosmynach, although the area is still undrilled. The Rhosmynach rhyolite is probably correlatable with rhyolites in the eastern part of Parys Mountain, based on chemistry. It is conceivable that rhyolite-shale contacts are present in the subsurface between Penysarn and Rhosmynach. Such contacts could also be present under the shales which extend for at least 1 km to the west of the Engine Zone at Parys Mountain. Drilling is recommended in several areas, firstly to systematically explore the untested known and projected rhyolite-shale contacts on the Parys Mountain property, and secondly to search for further contacts in the areas to the west and east of this.
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