The geology and genesis of the Gortdrum Cu-Ag-Hg orebody.

Abstract

Superficially similar in structural setting to the Tynagh and Silvermines orebodies, there is here a close spatial relationship between mineralization and high-level basic magmatism. In the near-surface part of the deposit, chalcopyrite and Hg-rich tennantite occur with calcite and ferroan dolomite in veinlets within shattered bioclastic limestones of Courceyan 2 and 3 age. At greater depth bornite and chalcocite are the most abundant ore minerals, occurring within shaly dolomitized Courceyan 1 and 2 limestones. Pyrite is a common accessory, galena and sphalerite are rare, and accessory minerals include cinnabar, cobaltite, arsenopyrite, native amalgam, stromeyerite, wittichenite and gortdrumite. Fluid inclusion data show that the main chalcopyrite-tennantite mineralization occurred at 140-180oC from mildly saline fluid, in an area of high geothermal gradients related to, but later than, basic magmatism. The presence of hydrocarbons in some fluid inclusions indicates that these may have catalysed abiogenic reduction of sea-water-derived sulphate, thus causing preferential deposition of metallic sulphides within the organic-rich Courceyan marine beds. -R.E.