Volcanic sequence and alteration at the Parys Mountain Volcanic-hosted massive sulfide deposit, Wales, United Kingdom: Applications of immobile element lithogeochemistry

Lenses of massive Zn-Pb-Cu sulfides at the Parys Mountain deposit in Angles ey, northwestern Wales, occur at and near the contact between Ordovician sh ales and overlying rhyolites. The rhyolites, which have been dated as Lower Silurian (Parrish, 1999), have a thickness of a fem; hundred meters and ar e overlain by Lower Silurian shales. The rhyolites and shales strike northe asterly for 2 to 3 kin and dip to the north, Minor basalts are present but there re no intermediate rocks, Cu-bearing stockwork veins occur in the upp er portion of the Ordovician shales, and Cu-bearing mineralization was mine d from the Lower Silurian shales in the 18th century. Precambrian basement rocks are present to the south and north of Parys Mountain. Application of immobile element lithogeochemical techniques to >1,000 drill core and outcrop samples allows division of the rhyolites into five main c hemical types (A, B, C, D1, and D2), the shales into three main types, and the mafic rocks into two types. The distribution of these units has been ma pped across the property. Where individual volcanic chemical units have lat eral continuity, they are considered to represent singular geologic events and thus are time markers. Correlations based on volcanic facies alone are of limited use, because a chemical unit can pass laterally through differen t volcanic facies, whereas in strongly altered zones the original volcanic facies cannot be recognized at all. The chemostratigraphic units, on the ot her hand, can be traced through even the most altered zones. Consequently t hey provide an improved means of correlating units on local to property sca les. The first volcanic rocks to accumulate above the Ordovician shales were thi n flows and volcaniclastic beds of rhyolite B and a laterally adjacent lens of largely massive rhyolite C. The main massive sulfide lenses in the west ern part of the property are associated with these volcanic events. These u nits were covered by thick flows of rhyolite A in the western part of the p roperty. Rhyolites D and A are the main chemical types ii the eastern part. Rhyolite D has two subtypes: D1 is generally massive and flow banded, wher eas D2 is pyroclastic. It is suggested that rhyolite D1 was erupted first, followed by less fractionated rhyolite D2, which was the last volcanic even t prior to the accumulation of the Silurian shales. The main vents for rhyo lite A in the west and D in the east were separated by about 1 km. The alteration minerals at Parys Mountain are sericite, chlorite, quartz, p yrite, and Fe-bearing carbonates. Mass changes calculated for the rhyolites and the shales show a general property-scale increase in alteration in the downdip direction. FeO and MgO additions of up to 10 and 20 wt percent, re spectively, are present in footwall shales up to 50 in below the massive su lfide lenses (in areas where stockwork veins are present). Similar FeO and MgO gains occur in rhyolites up to 50 in above the ore lenses. In rhyolites with Fe + Mg gains, Si and K have generally been lost. In areas lateral to the massive sulfides, shales and rhyolite B have gained large amounts of s ilica. Most Parys Mountain rhyolites have moderately high Nb contents and slightly inclined rare earth element (REE) patterns, whereas the basalts are enrich ed in the high field strength elements (HFSE) including the light REE, The primary geochemical features of the volcanic rocks, together with regional geologic relationships in northwestern Wales, suggest that volcanism occurr ed during intraplate rifting of a small marine basin, which was underlain b y Precambrian continental crust. The small volumes of all rhyolite types (a nd basalt) at Parys Mountain suggest that they were derived from small, dis crete pods of magma, which were tapped by basement faults. Most of the mass ive sulfides accumulated immediately before, and during, the first outpouri ngs of rhyolitic lava.