Chapter 4: Internal and External Deformation and Modification of Volcanogenic Massive Sulfide Deposits

Abstract Ancient volcanogenic massive sulfide (VMS) deposits formed in rifted arc, back-arc, and other extensional geodynamic environments and were deformed during later convergent collisional and/or accretionary events. Primary features of deposits influenced the development of tectonic structures. Except for pyrite, common sulfides in VMS deposits are much weaker than their volcanic host rocks. During deformation, strain is taken by the weak sericitic and chloritic alteration envelope surrounding the deposits and by the sulfide bodies themselves, which act as shear zones, undergo hinge thickening and limb attenuation during regional folding, and are deformed into elongate bodies parallel to regional fold hinges and stretching lineations. A tectonic foliation forms as a sulfide banding in the interior of VMS lenses due to shearing and flattening of primary textural and compositional heterogeneities and as a banded silicate-sulfide tectonic foliation along the margins of the VMS lenses due to transposition and shearing of primary silicate (exhalites)-sulfide layers. Other characteristic structures, such as cusps, piercement cusps, piercement veins, and durchbewegung structures (sulfide breccias), formed as a result of the strong competency contrast between the massive sulfide deposits and their host volcanic rocks. Some features of VMS deposits may have both primary and tectonic components, requiring careful mapping of volcanic lithofacies and primary and tectonic structures to assess the nature of these features. One example is the vertical stacking of VMS lenses. The stacking may be primary, due to the rapid burial of lenses by volcanic or sedimentary deposits as the upward flow of hydrothermal fluids continued and precipitated new lenses above the earlier formed lenses. Or it may be tectonic, due to thrusting or isoclinal folding and transposition of the VMS lenses. Metal zoning (Cu/Cu + Zn), produced by zone refining at the seafloor or subseafloor, is refractory to deformation and metamorphism and can be used to delineate hydrothermal fluid upflow zones and, together with stratigraphic mapping, determine if the stacking is primary, tectonic, or both. Similarly, the elongation of VMS lenses may have a primary component due to the deposition and coalescence of sulfide lenses along linear synvolcanic faults or fissures, as well as a tectonic component due to mechanical remobilization of sulfides parallel to linear structural features in the host volcanic rocks. Structural mapping of VMS deposits is hampered by low-temperature recrystallization of sulfides, which masks the effects of deformation, by discontinuous and abrupt lithofacies changes in the volcanic host rocks, and by the weak development of tectonic fabrics and strong strain partitioning in volcanic rocks. To mitigate these issues, mapping of volcanic lithofacies should be done concurrently with structural mapping to delineate repeated stratigraphic panels across reactivated faults and to identify, in the absence of well-developed fabrics, regional folds characterized by abrupt changes in strata orientation from limbs to hinge. Where well-layered sedimentary rocks are intercalated with volcanic rocks, structures should be mapped in the sedimentary rocks and then correlated with those in volcanic rocks to alleviate difficulties in mapping structures in volcanic rocks and defining the sequence of deformation events that affected the volcanic rocks and their VMS deposits.

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