Rock magnetic detection of the pyrite-­to-pyrrhotite reduction: applications to hydrocarbon maturity, mineral resources, and biogeochemistry (Invited)

Although pyrite is capable of reducing to pyrrhotite at room temperature in a flowing hydrogen atmosphere, in geological systems the transformation reaction is considered to be relevant only at mid-­‐hydrothermal temperatures (i.e., >300 Celsius). However palaeomagnetists recognise pyrrhotite-­‐borne magnetisations in carbonaceous and reduced sedimentary rocks throughout the timescale, and at all maturity grades, including oil-­‐window. It is likely that the intersection of these observations lies in trace transformation of pyrite to pyrrhotite as a solid-­‐ state diffusion-­‐limited reaction, or in the formation of pyrrhotite inclusions in other sulfide minerals at low (diagenetic, anchizone) temperatures. Because pyrrhotite has a ferrimagnetic response whereas pyrite is weakly paramagnetic, rock magnetic techniques are capable of detecting pyrrhotite, and potentially characterizing its full crystal morphometrics in a sample, at ppm (inexpensive magnetic susceptibility meters), ppb (superconducting rock magnetometers), or even ppt (scanning SQuID microscopes) levels. At such trace levels, pyrrhotite domains may not be recognizable using bulk or even microprobe-­‐scale compositional analysis of pyrites. We present a rock magnetometry detection test for the ~140-­‐200 degrees Celsius-­‐range initial reaction of pyrite + chloritoid to pyrrhotite in an iconic laboratory sulfide sample from Spain's La Rioja district. We then survey some aspects of applications of pyrite-­‐pyrrhotite rock magnetic discrimination for thermogenic hydrocarbon gas generation, for coal discrimination, for stratiform base metal sulfide ore paragenesis, and for trace metal biogeochemistry techniques and questions.