Multistage fluid history of a copper province with carbonatites, lamprophyres, and associated rocks

Costanzo A., Moore K.R.

Department of Earth and Ocean Sciences, School of Natural Sciences, National University of Ireland Galway, Galway, Ireland

A sequence of Lower Palaeozoic sedimentary rocks in the southwest of Ireland is cut by copper-rich vein mineralization (at Allihies) and also by hypabyssal intrusions of carbonated trachyte and lamprophyre (see Brady et al., this meeting).  The presence of copper-bearing minerals in the intrusive rocks, and a spatial association between the intrusions and copper-free quartz veining suggests that there may be a genetic relationship between igneous activity and mineralization.  This hypothesis is tested using fluid inclusions studies on mineral samples from three Allihies Mines, and quartz veins and a calcite related to a carbonatite intrusion at Black Ball Head.

The mines at Allihies operated between 1812 and 1884, with total ore production of 297.000 (Williams, 1993).  Those sampled for this investigation were the Mountain Mine, Caminches Mine and Coom Mine. The mineralogy consists primarily of copper, iron and molybdenum sulphides with their attendant secondary minerals in areas of oxidation.  Chalcopyrite is the main ore mineral and is often accompanied by pyrite, bornite, tetrahedrite, hematite (var. specularite), molybdenite, and chalcocite.  The molybdenite is often disseminated in the quartz giving it a grey colour.  The secondary mineral assemblage comprises copper, iron and molybdenum carbonates, oxides and some sulphides such as malachite, covelline, goethite and limonite.  Quartz is the main gangue mineral (Fig. 1).

A zoned diatreme at Black Ball Head has an outer zone rich in xenoliths of local sedimentary and igneous crustal rocks and an inner zone of tuffisite hosting mantle xenoliths and xenocrysts.  Coarse-grained calcite bodies, fibrous calcite veins and small strings of medium-grained calcite have been observed in a discontinuous elliptical pattern, dominantly in the inner zone of the diatreme, but also crossing the contact between the inner zone and the outer zone.  The variety of forms of calcite and their distribution indicate migration of extremely low viscosity carbonate magma or, more likely, fluid percolating through the diatremic tuffisite.  There are no other cases of carbonate veining in the region and it is interpreted as a carbohydrothermal carbonatite on compositional and isotopic grounds (see Brady et al., this meeting).  Regional quartz veining has a spatial association with the margins of igneous intrusions and quartz has been sampled from the contact of the outer zone of the diatreme with the sedimentary country rock.  Many fluid inclusions have been observed in both calcite from the largest of the carbonatite outcrops and the elliptically-arranged calcite strings and in the quartz veining (Fig. 2) from the margin of the diatreme.

Fluid inclusion studies were carried out on primary inclusions as these represent the mineralizing fluids during mineral deposition.  Fluid investigations are here used to:

·         classify and compare the different fluid inclusion types hosted in quartz and calcite;

·         identify the composition of fluid species;

·         resolve temperature and pressure of entrapment;

·         investigate the relationship between fluids involved in Allihies mineralization and both regional and magmatic fluids in the vicinity of the Black Ball Head intrusions.

In samples from Black Ball Head, primary fluid inclusions (FIs) are much more abundant in quartz than calcite and occur as large two-phase fluid inclusions (up to 40 μm in length) distributed in trails or isolated within the crystals parallel to crystal growth planes.  Primary calcite-hosted monophase liquid inclusions are also very common and range between 3 and 10 μm in length.  Fluid inclusions in quartz crystals are generally distributed along planes in clusters made of two-phase liquid-vapour inclusions and/or monophase liquid inclusions (Fig. 2).  Numerous solid-rich fluid inclusions are also observed.  They generally have a negative crystal shape and contain between one and two solid phases.  (Secondary fluid inclusions were also observed and occur in both host minerals as trails of smaller FIs (3 to 25 μm in length) with linear and curvilinear distribution related to later phases of fracturing.)

In Cu-Fe-metal bearing quartz veins from the Allihies area, a large number of fluid inclusions are hosted within quartz crystals, particularly those in proximity to sulphide minerals.  Large (up to 50 μm in length) two-phase fluid inclusions are the most abundant and are most commonly distributed along linear planes within the crystals or along crystal rims.  Solid-rich fluid inclusions generally contain only one solid phase and are also considered to be primary.  (Small secondary FIs (2 to 10 μm in length) are also very abundant and occur as either two-phase or monophase FIs that are aligned along annealed fractures.)

Three fluid inclusion types have been identified and classified according to their phase ratios at room temperature:

·         TYPE 1: two-phase liquid-vapour inclusions (L+V) with 80 to 95 % liquid by volume are the dominant type.

·         TYPE 2: single-phase liquid-only inclusions (L).

·         TYPE 3: solid-rich fluid inclusions (L+V+S) generally contain one or two small solid phases in addition to liquid or vapour.  The most common solid present is a low birefringent mineral, anhedral or prismatic, and it is observed in the largest FIs with size up to 40 μm across.  However, most of the inclusions containing solids are too small to show any distinctive optical characteristics.  Type 3 inclusions normally show consistent liquid to solid phase ratios indicating that the solids are true daughter minerals crystallised from the enclosed fluid after trapping.

Type 1 and Type 2 inclusions occur at both the Allihies and Black Ball Head localities and are hosted by both quartz and calcite, whereas Type 3 inclusions are restricted to the host mineral quartz.  Microthermometric results used to derive estimates of the PVTX state of the fluids at the time of trapping will be presented to facilitate further comparison between the fluids associated with magmatism and mineralization.

This presentation was financially supported by INTAS project 05-1000008-7938.

References:

Williams, R.A. The Berehaven Copper Mines, Allihies, Co. Cork S.W. Ireland. British Mining No.42.  Kenmare: A.B. O'Connor, 1993. 228pp.