Passive Structural Control on Skarn Mineralization Localization: A Case Study from the Variscan Rosas Shear Zone (SW Sardinia, Italy)

Fabrizio Cocco,Matteo Luca Deidda,Antonio Funedda,Stefano Naitza,Dario Fancello,Antonio Attardi

doi:10.3390/min12020272

Abstract

The case study presented here deals with the Pb-Zn-Cu skarn ores hosted in the Rosas Shear Zone (RSZ), a highly strained domain located in the external zone of the Sardinian Variscan chain. The RSZ is characterized by several tectonic slices of Cambrian limestones within a strongly folded and foliated Cambrian-Ordovician siliciclastic succession, intruded by late Variscan granites and mafic dykes. Based on geological mapping, structural and microscope analyses, our results show that the skarn ores in the RSZ are an example of passive structurally controlled mineralization. The RSZ was structured close to the brittle–ductile transition and, once exhumed to shallower crustal levels, acted as plumbing system favoring a large-scale granite-related fluid circulation. The paragenesis and the mineralization style of the skarn vary slightly according to the peculiarity of the local structural setting: a tectonic slice adjacent to the mafic dyke; an intensely sheared zone or a discrete thrust surface.

Highlights

A proper identification, quantification and exploitation of ore deposits cannot be separated from a full comprehension of the structural control of the mineralization processes
This view assumes a particular declination in the case of districts characterized by skarn metasomatic deposits, in which (1) magma inflow and fluid circulations are favored by opening of pathways in large scale shear zones; (2) shear zone structural set-up through tectonic slicing and cataclastic/mylonitic zones development results in a definite control on the spatial distribution and extension of reactive lithologies, susceptible to metasomatic phenomena and ore mineralization [15]
The structuring of the Rosas Shear Zone (RSZ) during the collisional phases of the Variscan Orogeny originated at the brittle–ductile transition crustal level and acted as the plumbing system exploited, at shallower crustal levels, during the extensional phases related to the collapse of the Variscan chain, by the mafic dyke intrusion and by extensive magmatic fluid circulation related to the granite emplacement

Summary

Introduction

A proper identification, quantification and exploitation of ore deposits cannot be separated from a full comprehension of the structural control of the mineralization processes. In the framework of complex structures such as polyphasic shear zones, the two control modalities can repeatedly occur in several moments, but they are frequently related to dominantly extensional regimes [7] This view assumes a particular declination in the case of districts characterized by skarn metasomatic deposits, in which (1) magma inflow and fluid circulations are favored by opening of pathways in large scale shear zones; (2) shear zone structural set-up through tectonic slicing and cataclastic/mylonitic zones development results in a definite control on the spatial distribution and extension of reactive lithologies (e.g., carbonate rocks), susceptible to metasomatic phenomena and ore mineralization [15]

Methods

Findings

Discussion

Conclusion