METEORIC FLUID INFILTRATION INTO THE CENTRAL ALPINE FAULT (NEW ZEALAND) REVEALED BY ISOTOPIC INVESTIGATION OF CLAY GOUGE

Preliminary results of an isotopic and mineralogical study of fault gouge samples collected from 8 locations along the central Alpine Fault, in the vicinity of the DFDP-2 drillsite, reveal disparate sources for fluids that have acted as the mineralization agent of two distinct populations of the mineral illite. This study utilizes recent advances in stable isotopic methodology applied to clay-gouge bearing brittle fault zones from around the world. These advances have proved the viability of a new isotopic method that uses authigenic illite to constrain the source and timing of paleofluid activity in the earth’s upper crust—illite uniquely lends itself to both hydrogen isotopic analysis and 40Ar/39Ar geochronology. The method utilizes grain-size separation techniques and illite polytypism to generate size fractions with varying authigenic and detrital ratios, which in turn are used to extrapolate to true end-member values. Together, these powerful methods can provide information, both about the timing of neoformation as well as the composition of the mineralizing fluid that produced a given population of clay minerals.

The suite of samples of the current study includes 4 samples from the principal slip zone, and samples from subsidiary faults in both the hanging wall and footwall of the deformation zone. X-ray characterization and polytype analysis indicates that some of the minor faults may be shallow in nature, without the displacement and exhumation needed to promote illite authigenesis. In contrast, samples from the principle slip zone preserve neoformed illite, and corroborate our hypothesis of meteoric fluid infiltration at depths of several kilometers into the fault. End-member isotopic values for sample SC, for example, are -88‰ for authigenic material and -68‰ for detrital material. Assuming a mineralization temperature window of 100-150°C, authigenic illite precipitated from water of a meteoric nature, with values in the range of -61 to -76‰. With a minimum temperature of 300°C, detrital illite precipitated from a water of a metamorphic nature, with values on the order of -50‰These results have implications for fault mechanics, crustal-scale fluid circulation models, and economic deposit generation and exploration.