Sphalerite geobarometry in metamorphic terranes: an appraisal with implications for metamorphic pressure in the Otago Schist

Abstract

Sphalerite geobarometry has long been known to give poor results when applied to regional metamorphic terranes. Application of the sphalerite geobarometer to three lowto medium-grade sulphide deposits-the Moke Creek and Waitahuna deposits, Otago, and the Goose Cove deposit, Newfoundland-yields pressures up to 9 kbar, which appear to be too high when compared with other geological data. Textural and mineralogical relationships suggest that the Goose Cove and, possibly, the Moke Creek deposits lacked the required equilibrium assemblage (pyrite + hexagonal pyrrhotite + sphalerite) during peak metamorphic conditions, rendering the geobarometer inapplicable. In addition, all three deposits show evidence of re-equilibration at T < 300"C, which has resulted in decreased FeS contents and high apparent pressures. Analyses of sphalerites from very low-grade metachert from South Georgia Island, which contains the assemblage sphalerite + pyrite + monoclinic pyrrhotite + chalcopyrite, confirm that low-temperature equilibration of this assemblage results in approximately 10-11 mol. % FeS in sphalerite. Comparison of these results with published descriptions of other deposits suggests that lack of the appropriate assemblage and retrograde re-equilibration of sphalerite probably account for most anomalously high-pressure estimates. Erratic compositions of sphalerites containing chalcopyrite inclusions may result from replacement of hightemperature intermediate solid-solution by chalcopyrite during cooling. Strain may enhance retrograde re-equilibration of sphalerite by grain-size reduction or dislocationassisted diffusion and/or nucleation. Re-evaluation of the data from Moke Creek suggests that the sulphides experienced pervasive greenschist facies re-equilibration at pressures of about 4.5 kbar, with late stage mobilization at about 2.8 kbar, and thus sphalerite compositions are not likely to reflect blueschist facies conditions. Pressure estimates based on sphalerite geobarometry should take into account at what stage in the history of a metamorphic terrane the sphalerite composition equilibrated.

Diagenetic pyrite as a source for metals in orogenic gold deposits, Otago Schist, New Zealand

“…Metamorphic mobilisation of metals from pyrite and rock Diagenetic pyrite recrystallised first to metamorphic pyrite and then to pyrrhotite with prograde metamorphism in the Otago Schist (Jamieson & Craw 1986;Pitcairn et al 2006Pitcairn et al , 2010. These iron sulfide transformations were controlled by host rock compositions, particularly redox conditions, at a range of metamorphic grades (Jamieson & Craw 1986;Pitcairn et al 2010).…”

“…These iron sulfide transformations were controlled by host rock compositions, particularly redox conditions, at a range of metamorphic grades (Jamieson & Craw 1986;Pitcairn et al 2010). The transformation of pyrite to pyrrhotite occurred mainly under greenschist facies conditions, and pyrite is rare as a metamorphic mineral in the amphibolite facies.…”

Metamorphic discontinuities in orogenic belts: example of the garnet-biotite-albite zone in the Otago Schist, New Zealand

“…4) indicates minimum pressures of 8±10 kb at approximately 400 C (Massonne and Schreyer 1987). This estimate is a little higher than pressures given by Yardley (1982) and Jamieson and Craw (1987; labelled 3 and 5 in Fig. 6a).…”

“…Only a few estimates exist for the Otago Schist, based on scattered metachert, greenschist and sulphide assemblages (e.g. Devereux 1968;Yardley 1982;Jamieson and Craw 1987; labelled 3±5 in Fig. 6a).…”

Lithology, geochemistry, and structure of Moke Creek sulphide deposit host rocks, Otago Schist, New Zealand

“…Numerous earlier workers (Wright 1966;Wood 1967;Henley 1975;Lowery 1979;Jamieson & Craw 1987) have described various aspects of the Moke Creek sulphide deposit itself, focusing on sulphide mineralogy. However, apart from generalised regional studies (Park 1909;Hutton 1940;Mortimer 1993), there is no published information on the host rocks of the Moke Creek sulphide deposit.…”