The Carbonate-Hosted Tullacondra Cu-Ag Deposit, Mallow, Ireland

The Tullacondra Cu-Ag deposit is located on the southern margin of the Lower Carboniferous Irish Midlands orefield and contains historical reserves of approximately 4.2 Mt at 0.7% Cu and 27.5 ppm Ag. The deposit is hosted within the hanging wall of a feeder fault, the EW-trending Tullacondra Fault, where sulfides and sulfosalts containing elevated Cu, Ag, As, and Sb deposited, whereas Zn and Pb are nearly absent. The deposition of Cu sulfides in Tullacondra took place along bedding and bedding-parallel dissolution seams, suggesting an epigenetic mineralization that formed: (a) the Transition Series-hosted mineralized zone containing elevated Cu associated with Ag, As, and Sb; (b) the Lower Limestone Shale-hosted mineralized zone, Cu-dominated and depleted in other metals, and (c) a near-vertical mineralized zone associated with fractures related to the Tullacondra Fault. Some similarities are shared with Irish-type Zn-Pb deposits, such as structural and stratigraphic controls, and elevated Cu, Ag, As, and Sb within feeder-fault proximal zones (such as in Lisheen and Silvermines). Whether Tullacondra mineralization was part of the Irish-type system or not, our deposit geometry evaluation, whole-rock geochemistry, paragenetic sequence, and texture relationships indicate that Cu-Ag deposition involved the reaction of metal-bearing fluids with carbonate rocks.

Magma Mingling in Kimberlites: Evidence from the Groundmass Cocrystallization of Two Spinel-Group Minerals

We present the results of a detailed petrographic study of fresh coherent samples of the Menominee kimberlite sampled at site 73, located in Menominee County, MI, USA. Our objective is to account for its unusual and complex paragenetic sequence. Several generations of olivine, ilmenite, and spinel-group minerals are described. Early olivine and ilmenite are xenocrystic and were replaced or overgrown by primary minerals. Zoned microcrysts of olivine have a xenocrystic core mantled by a first rim in which rutile, geikielite, and spinel s.s. (spinel sensu stricto) cocrystallized. The in situ U–Pb dating of a microcryst of primary rutile yielded 168.9 ± 4.4 Ma, which was interpreted as the age of emplacement. The groundmass consists of olivine, spinel s.s., a magnesian ulvöspinel–ulvöspinel–magnetite (MUM) spinel, calcite, and dolomite. An extremely low activity of Si is suggested by the crystallization of spinel s.s. instead of phlogopite in the groundmass. The presence of djerfisherite microcrysts indicates high activities of Cl and S during the late stages of melt crystallization. The occurrence of two distinct spinel-group minerals (spinel s.s. and qandilite-rich MUM) in the groundmass is interpreted as clear evidence of the mingling of a magnesiocarbonatitic melt with a dominant kimberlitic melt.

The relationship between mineral composition, crystal structure and paragenetic sequence: the case of secondary Te mineralization at the Bird Nest drift, Otto Mountain, California, USA

An unusually diverse array of 25 secondary Te oxysalt minerals has been documented from Otto Mountain, California, and 18 of these from the Bird Nest drift sublocality. A paragenetic sequence for these minerals is proposed, using observed overgrowth relationships plus spatial association data and data from other localities. Apart from Te and O, the components Pb, Cu and H are essential in the majority of the minerals. The atomic Cu/Te ratio decreases through the paragenetic sequence. This, and the occurrence of minerals with additional components such as Cl–, CO32–, SO42–and Fe3+at an intermediate stage, suggests nonmonotonic evolution of the parent fluids, reflecting differing access to or spatial distribution of various components. For the minerals with known crystal structures, two alternative 'structural units' were identified, one consisting only of the Te4+or Te6+oxyanion, while the other also included small, strongly-bound cations such as Cu2+. The degree of polymerization for the Te oxyanion correlated with the paragenetic sequence: the monomeric tellurate anions of early minerals were replaced progressively by dimers, chains and sheet structures, which may relate to a decreasing abundance of the 'network modifying' Cu2+cation, analogous to Bowen's discontinuous reaction series in igneous rock-forming silicates. No relationship was seen between paragenetic order and the larger type of structural unit, or structural complexity as defined by information content. This contrasts with results in the literature for evaporite sulfates and pegmatite phosphates. While structure–paragenesis relationships may be widespread, the exact nature of such relationships may be different for different chemical systems and different paragenetic environments.