Spatial Distribution of the Hydrothermal Black Matrix Breccia and its Impact on the Irish-type Zn-Pb Mineralisation

<p>The Mississippian Waulsortian Formation of the Irish Midlands hosts a number of significant Zn-Pb mines including Lisheen, Galmoy and Silvermines. Consisting predominantly of sphalerite, galena and pyrite, the deposits are intimately associated with hydrothermal dolomite and dolomitic breccias, commonly referred to as “Black Matrix Breccia” (BMB). The hydrothermal dolomite and associated breccias form a predominantly tabular body that largely envelops the zone of sulphide mineralisation. A wide variety of mineralisation styles and textures are recognized, however the majority of the mineralisation resulted through replacement of this hydrothermal dolomite. Recent geochemical evidence indicates that the hydrothermal dolomite contains distinct geochemical signatures that may be useful in sulphide exploration within the Irish Midlands. To date, little work has been conducted on the spatial distribution and variability of this significant hydrothermal dolomite and the role it plays in ore genesis within the Irish Midlands. Through detailed petrographic characterisation, this study documents the distribution of the hydrothermal dolomite. This distribution helps constrain the origin of the massive Zn-Pb deposits and forms an important tool for future mineral exploration in the Irish Orefield.</p>

Hydrothermal dolomite breccia: when pre-existing rock heterogeneities control fluid-mediated replacement patterns and mimic tectonic features.

<p>Breccia structure is a ubiquitous feature that is characterized by angular fragment in a matrix composed of smaller grain size, often associated to brittle tectonics or to specific sedimentary environment such as karst collapse. Many different studies across the world describe breccia related to dolomite geobodies, themselves associated to ore deposits occurring during major extensional events (Hungary, Spain, France, Canada, Poland, Canada). The mineralogical textures of these structures, i.e. angular fragments of dark dolomicrite bound by elongated blocky, white, dolomite crystals in veins, are interpreted as univocal markers of fluid overpressure and hydrofracturing, hydrothermal dolomite breccia (HDB) being a precious tool to help to reconstruct pressure history.</p><p>This contribution presents a case study that challenge this hydrofracturing origin of HDB, questioning the role of fluid-mediated replacement in the observed crystallographic textures. The Mano Formation located in the Mail Arrouy, an anticline related to Mesozoic hyper-extension of the crust located in the Chaînons Béarnais (Northwest Pyrenees, France), presents classical HDB, i.e. characterised by black dolomite fragment surrounded by a white dolomite-matrix supposedly related to hydrofracturing. Yet, in some places, it is possible to observe this angular black fragment in contact with a brown dolomite matrix. As attested by the presence of dolomitized fossils, the brown dolomite and black fragments constitute an initial sedimentary breccia structure, that is described regionally.</p><p>Textural and chemical analyses of the HDB and of the initial sedimentary breccia have been carried out by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and electron probe microanalyzer (EPMA) across different dolomite-dolomite interfaces. Quantitative data has been obtained by image processing, showing that oxide particles that are randomly distributed in the brown matrix appears pushed at the tips of the white crystals of dolomites, suggesting a cleaning process during growth. Also, the initial breccia comprises small-size around 1830 µm² (surface area) clasts that are absent from the HDB. Moreover, the contact between black, white and brown dolomite show a roughness similar to what is observed in fluid-mediated dissolution/replacement processes. Finally, EBSD results show that white dolomite crystal grew under local stress generated by a competition between grain growth, typical of slow, fluid-limited, grain growth.</p><p>This array of results leads us to propose that the HDB results from texturally controlled, fluid initiated hydrothermal recrystallization of initial sedimentary dolomicrite. This model is further tested by 2D numerical simulations of phase separation process using the modelling environment “ELLE” that reproduce the patterns observed in natural samples. Hence, a critical reappraisal of the origin and process behind HDB must be conducted, as we show that, in the case of the Mano Formation in the Mail Arrouy, no fluid overpressure were required to create HDB.</p>

Hydrothermal Dolomite Paleokarst Reservoir Development in Wolonghe Gasfield, Sichuan Basin, Revealed by Seismic Characterization

Hydrothermal dolomite paleokarst reservoir is a type of porous carbonate reservoir, which has a secondary porosity and can store a large amount of oil and gas underground. The reservoir is formed by magnesium-rich hydrothermal fluids during the karstification and later stages of the transformation. Due to the strong heterogeneity and thin thickness of hydrothermal dolomite paleokarst reservoirs, it is a real challenge to characterize the spatial distribution of the reservoirs. In this paper, we studied the hydrothermal dolomite paleokarst reservoir in the Wolonghe gasfield of the eastern Sichuan Basin. First, based on detailed observations of core samples, the characteristics and storage space types of the dolomite reservoir were described. Secondly, the petrophysical parameters of the paleokarst reservoirs were analyzed, and then the indicator factor for the dolomite reservoirs was established. Thirdly, using the time–depth conversion method, the geological characteristics near boreholes were connected with a three-dimensional (3D) seismic dataset. Several petrophysical parameters were predicted by prestack synchronous inversion technology, including the P-wave velocity, S-wave velocity, P-wave impedance, and the hydrothermal dolomite paleokarst reservoir indicator factor. Finally, the hydrothermal dolomite paleokarst reservoirs were quantitatively predicted, and their distribution model was built. The 3D geophysical characterization approach improves our understanding of hydrothermal dolomite paleokarst reservoirs, and can also be applied to other similar heterogeneous reservoirs.