Abstract
This study investigates the contact zone between the Ordovician Greenland Group and the Devonian Reefton Group in the Crushington area of the Reefton Goldfield, West Coast, New Zealand. Previous studies have suggested that this contact is structurally controlled, but detailed field, petrographic, and geochemical information from the contact zone remains limited. In addition, recent exploration data from the Stony Creek area have identified anomalous Au and As values close to the mapped and inferred Greenland Group-Reefton Group contact zone, raising questions about the relationship between lithology, deformation, fault reactivation, and possible mineralisation processes.
The study combines field mapping, structural observations, petrography, pXRF analysis, and SEM investigation to examine rocks in and around the contact zone between the Greenland Group and the Reefton Group. Field observations from Murray Creek, Lankey Creek, and Stony Creek show abrupt juxtaposition of contrasting lithologies, local breccia-like textures, veining, sulphide occurrence, and alteration close to the mapped or inferred boundary. Petrographic observations further show that the two lithological groups record deformation differently. The Greenland Group is mainly characterised by fine-grained metasedimentary rocks with well-developed foliation and cleavage, whereas the Reefton Group is more commonly affected by fracturing, quartz deformation, and vein-related modification. Samples collected close to the contact also preserve mixed and disrupted textures, with contrasting lithological components occurring together within structurally disturbed material.
Geochemical and SEM results support the interpretation that the contact zone is not a simple lithological boundary. pXRF data show that mixed lithology samples are compositionally heterogeneous and overlap with both Greenland Group and Reefton Group values, consistent with local mixing and structural disruption. Mineralised samples show stronger As and S enrichment than unmineralised samples, although enrichment is unevenly distributed. SEM observations show that pyrite occurs in several textural forms, including isolated grains, clustered aggregates, and grains with core-rim variation or repeated As-rich rim development. These relationships suggest that sulphide development near the contact zone was not produced by one simple event.
Overall, the Greenland Group–Reefton Group contact in the Crushington area is best interpreted as a fault-controlled and structurally reworked contact zone. Although a continuous fault gouge zone was not observed, the repeated association of abrupt lithological juxtaposition, breccia-like textures, lithological mixing, veining, sulphides, and local geochemical enrichment supports a fault-related interpretation. The contact zone likely acted as a structurally favourable pathway for localised fluid flow and sulphide development during later reactivation. However, the available evidence does not show that the contact itself forms a continuous lode structure or the direct source of gold mineralisation. Instead, the Au–As anomaly in the Stony Creek area may be more closely related to mineralised Greenland Group rocks close to the fault zone, with the contact acting as part of a broader structurally complex system.