Microstructures, Mixed Layering, and Polymorphism of Chlorite and Retrograde Berthierine in the Kidd Creek Massive Sulfide Deposit, Ontario

Abstract

Abstract--Transmission electron microscopy (TEM) was utilized to determine the origins of berthierine and chlorite in the core of the footwall alteration zone of the Kidd Creek massive sulfide deposit, Ontario. TEM images show lamellar intergrowths of packets of berthierine, mixed-layer chlorite/berthierine, FeMg chlorite, and relatively Fe-rich chlorite that contain dislocations, stacking faults, kink bands, and gliding along (001). Interstratification of packets of berthierine and chlorite with one to several tens of layers commonly is associated with terminations of a layer of chlorite by two layers of berthierine. Layers in adjacent domains ofberthierine and chlorite are continuous across interfaces that transect their common {001 } planes. High-strain zones that cut across cleavage planes, consisting of distorted layers and lensshaped pores, are associated with stacking faults and gliding along cleavage planes in chlorite crystals. Similar features separate interstratified chlorite/berthierine of different structures and textures, implying development of such composite grains after deformation of chlorite. Electron diffraction patterns show that the chlorite is an ordered one-or two-layer polytype or a one-layer polytype with semi-random stacking, and that the berthierine is a one-layer polytype with semi-random stacking epitaxially intergrown with chlorite.Coexisting chlorite and berthierine have nearly identical ranges of compositions, containing Si ~ 5, Al 6, and Fe ~ 6.5-8.5 pfu, and minor, variable Mg and Mn contents, in formulae normalized on the basis of 20 total cations. This implies polymorphism among Fe,Al-rich members of the serpentine and chlorite groups. In one of the samples, berthierine and mixed-layer chlorite/berthierine coexist with chlorite having two compositional ranges, including Fe-rich chlorite with a relatively wide range of Fe-Mg contents, and a dominant Fe-Mg chlorite. In another sample, compositionally homogeneous Fe-rich chlorite is associated with berthierine and mixed-layer chlorite/berthierine; Fe-Mg chlorite was not detected.The "microstrnctural relations and the presence of coexisting polymorphs, complex mixed layering, heterogeneous potytypism, and wide ranges of mineral compositions are consistent with replacement of chlorite by berthierine under non-equilibrium retrograde conditions, in contrast to the generally assumed prograde origin for other berthierine occurrences.

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

“…A Philips CM 12 scanning transmission electron microscope equipped with a low-angle Kevex Quantum detector for X-ray energy-dispersive spectroscopy analyses was used for TEM imaging, electron diffraction, and AEM analyses. Instrumental settings and characterization procedures were described in Jiang et al (1992), Slack et al (1992), and Jiang and Peacor (1993).…”

Comparison of Evolution of Trioctahedral Chlorite/Berthierine/Smectite in Coeval Metabasites and Metapelites from Diagenetic to Epizonal Grades

“…They are instead inferred to be due to retrograde overprinting, as tlaey occur only at crystal rims, and never as layers within crystals. Such retrograde overprinting, due to interaction with fluids, has been proposed to explain the berthierine associated with a massive sulphide deposit in Ontario by Jiang et aL (1992). Such retrograde alteration of illite and muscovite, with textures similar to those of the samples of this study is not unusual; it cannot generally be detected by XRD, but has been recognized with increasing fre-quency with the application of TEM (e.g.…”

“…If so, Fe-rich berthierine may have a true stability field relative to chlorite. Some berthierine has been inferred to have the same composition as associated chlorite Jiang et al, 1992;Xu and Veblen, 1996;Abad-Ortega and Nieto, 1995;L6pez-Munguira and Nieto, 2000;Coombs et al, 2000). Such pairs are therefore true polymorphs.…”

“…Berthierine has been inferred to be a precursor of chlorite in diagenetic rocks. Although mixtures of berthierine with chlorite at very fine scales may easily escape detection during routine mineralogical analyses, intercalation of berthierine in chlorite has been described by several investigators (Lee and Peacor, 1983;Ahn and Peacor, 1985;Amouric et al, 1988;Hillier and Velde, 1992;Jiang et al, 1992;Hillier, 1994) and such materials have been inferred to be precursors of pure chlorite in diagenetic rocks.…”

Mineralogy and Origin of Spots in Spotted Slate From the Malaguide Complex, Betic Cordilleras, Spain: An Xrd, Empa and Tem-Aem Study

“…Furthermore, the fine-grained as-of the retrograde alteration of Fe-bearing phases such as cordierite (Abad-Ortega & Nieto 1995) and chloritoid (Banfield et al 1989), and the possibility that berthierine may form during a hydrothermal or metamorphic process has been pointed out by Slack et al (1992) and Jiang et al (1992). On the other hand, the persistence of berthierine, partially transformed into chlorite, in metapelites metamorphosed to the prehnite-pumpellyite facies (Coombs et al 2000) and in rocks of the granulite facies (Xu & Veblen 1996) lends support to the possibility of a stability field for berthierine, that overlaps that of chlorite with a somewhat similar composition.…”