Structural Controls of Ore Mineralization in a Polydeformed Basement: Field Examples from the Variscan Baccu Locci Shear Zone (SE Sardinia, Italy)

Antonio Funedda,Andrea Dini,Cristina Buttau,Fabrizio Cocco,Stefano Naitza

doi:10.3390/min8100456

Abstract

The Baccu Locci mine area is located in a sector of the Variscan Nappe zone of Sardinia (the Baccu Locci shear zone) that hosts several type of ore deposits mined until the first half of the last century. The orebodies consist of lenses of Zn–Cu sulphides, once interpreted as stratabound, and Qtz–As–Pb sulphide ± gold veins; the implication of structural controls in their origin were previously misinterpreted or not considered. Detailed field mapping, structural analyses, and ore mineralogy allowed for unraveling how different ore parageneses are superimposed each other and to recognize different relationships with the Variscan structures. The sulphide lenses are parallel to the mylonitic foliation, hosted in the hinges of minor order upright antiforms that acted as traps for hydrothermal fluids. The Qtz–As–Pb sulphide veins crosscut the sulphide lenses and are hosted in large dilatational jogs developed in the hanging wall of dextral-reverse faults, whose geometry is influenced by the attitude of reverse limbs of late Variscan folds. The ores in the Baccu Locci shear zone are best interpreted as Variscan orogenic gold-type; veins display mutual crosscutting relationships with mafic dikes dated in the same district at 302 ± 0.2 Ma, a reliable age for the mineralizing events in the area.

Highlights

Structural controls of ore deposits hosted in poly-deformed low-grade metamorphic basements are often not easy to recognize, even more so when there are multiple generations of mineralization, mostly because the relationships between structures and ore bodies are often not clear
In Baccu Locci, the ores are exposed at different structural levels along the hinge zone of the Flumendosa Antiform, hosted by the strongly mylonitized siliciclastic metasediments and felsic metavolcanics of the Gerrei tectonic unit [3]
Zn–Cu–Pb sulphide lenses occur in the Su Spilloncargiu mine sector (Figure 5), hosted in phyllitic quartz–mylonite rocks (Figure 6a–e), whose protolith was made mainly by siliciclastic sediments, with a high occurrence of recrystallized quartz and muscovite, metamorphosed in lower green-schist facies during the Variscan Orogeny

Summary

Introduction

Structural controls of ore deposits hosted in poly-deformed low-grade metamorphic basements are often not easy to recognize, even more so when there are multiple generations of mineralization, mostly because the relationships between structures and ore bodies are often not clear. The difficulties in unravelling the tectonic structures prevent the understanding of the ore bodies’ geometry, leading to mistakes in mineral exploration, evaluation of ore deposits, mine planning, and even mineral exploitation. Passive structural controls imply that there are no direct correlations between tectonic and mineralizing events; the physical parameters that characterized the deformational. In the case of active controls, these features were critical for mineralization and shaped both the deformational context and the genesis of the ores

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Conclusion