Fluid chemistry of veining associated with an ancient microearthquake swarm, Benmore Dam, New Zealand

Steeply dipping strata in the vicinity of Benmore Dam, Otago, New Zealand, are complexly deformed metasedimentary rocks of the Torlesse Supergroup (Mi ddle Triassic age). Over an exposed area similar to 100 m wide x 25 m high, these strata are disrupted by a fault-fracture mesh comprising conjugate C oulomb shears interlinked by extensional and extensional-shear fractures, a ll formed in a common stress field and hosting quartz + prehnite +/- epidot e +/- calcite veining. The combined effect of these structures is shortenin g perpendicular to beddings and subvertical extension so that in their pres ent attitude, they correspond to a set of conjugate thrust faults with asso ciated extension fractures. On the evidence of incremental vein textures, t he development of this distributed fault-fracture mesh is interpreted as re sulting from a fluid-driven microearthquake swarm, which postdated regional low-grade metamorphism. Mechanical considerations suggest that the migrati ng hydrothermal fluids were significantly overpressured, possibly to approx imately lithostatic values, if the mesh structure developed in its present attitude. Fluid-inclusion microthermometric studies show that Benmore vein quartz con tains two-phase aqueous inclusions with salinities between 1.4 and 2.9 wt% NaCl equivalent and homogenization temperatures (T-h) between 189 and 217 d egreesC. The assemblage quartz + prehnite + epidote suggests trapping tempe ratures (T-t) of similar to 280 degreesC, requiring the addition of an simi lar to 70 degreesC correction to T-h values. Late calcite contains inclusio ns with noticeably lower salinity (0.0-0.9 wt% NaCl) and T-h values (129-17 5 degreesC). Studies on quartz + pumpellyite +/- calcite veins from nearby Lake Aviemore show similar fluid-inclusion salinity and T-h, values. Fluid-inclusion gas analyses show all the vein samples to be dominated by H 2O (99.3-99.9 mol%) with few other gases apparent, including CH4 (less than or equal to0.5%), N-2 (less than or equal to0.1%), CO2 (less than or equal to0.1%), and C-2-C-4 hydrocarbons. Cation and anion analyses, when combine d with the gas data, show that NaCl dominates the fluid-inclusion salinitie s. Oxygen isotope results, when combined with calculated T-t values, indica te that the water responsible for the deposition of Benmore and Aviemore qu artz had delta O-18 compositions of 9.4 parts per thousand and 4.8 parts pe r thousand, respectively. Calcite delta C-13 values between -25.3 parts per thousand and -38.0 parts per thousand are indicative of oxidation of CH4 t o CO2 as a result of hydrothermal fluids interacting with organic-rich sedi ments. Fluid-inclusion deltaD(H2O) values for Benmore range between -73 par ts per thousand and -89 parts per thousand compared to -109 parts per thous and for the one Aviemore sample. This research has demonstrated that (1) water of meteoric origin, probably from subantarctic latitudes, penetrated to greater than or equal to6 km dep th and underwent an oxygen isotope shift before depositing the Benmore-Avie more veins; (2) the migrating hydrothermal fluids were likely overpressured well above hydrostatic to near lithostatic values if the mesh structure wa s active in its present orientation; and, (3) fluid migration was coupled t o distributed brittle failure in the prevailing stress field, "self-generat ing" a permeable fault-fracture mesh.