Year

1996

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

A skarn association, representing one of the numerous mapped occurrences of generally small, uneconomic sulphide-bearing deposits temporally related to the Marulan Batholith, is developed along the southern contact of the I-type Glenrock Granodiorite with the Late Silurian, Lower Bungonia Limestone at Marulan South, New South Wales, Australia. Skarns occurring at Marulan South were formed over two different temperature-time stages. In the first high temperature stage (prograde phase) of skarn formation, calcite limestone was replaced by garnet, clinopyroxene, wollastonite, vesuvianite, and subordinate amounts of quartz, plagioclase, pyrrhotite, F-bearing titanite and epidote. The second stage or, low temperature retrograde phase, was characterised by the formation of disseminated hydrous silicates (including prehnite and fluoroapophyllite), calcite, quartz and also by crosscutting sulphide veinlets (predominantly pyrite and chalcopyrite) which overprinted the primary assemblages. This skarn displays the mineralogical and geochemical attributes of a 'reduced' skarn-forming environment characteristic of either tungsten or gold skarns. The skarn is believed to have evolved in a convergent margin tectonic setting.

Sangan iron ore deposit which is one of the largest economic magnetite skarn deposits in Iran, is located in the east-central part of the Khorassan province. The deposit is hosted by a sequence of Late Cretaceous calcareous sediments, intruded by I-type granitoids of the Sar Now Sar pluton, of probable Late Eocene to very Early Oligocene age. The tectonic setting of this deposit is not well established, however the deposit could well have evolved in a continental arc setting, based on the geochemical attributes of the plutons.

Skarns occurring in the Sangan area were formed during two major early higher temperature, prograde stages (Stages I and II) and two (Stage III, Stage IV) ,later lower temperature, retrograde events. The prograde stages of skarn formation were characterised by replacement of limestone (mostly calcic limestone) by contact skarn (Stage I) containing predominantly andraditic garnet, whereas Stage II was characterised by abundant development of andradite-hedenbergite assemblages, together with assemblages locally containing scapolite, phlogopite, quartz, ?K-feldspar, plagioclase, minor magnetite and rare biotite. Retrogression (Stage III) overprinted the primary phases and produced hydrosilicate minerals, chiefly aluminous amphibole (hastingsite), quartz, calcite and magnetite. Stage IVa was generally characterised by the development of, low-Al amphibole (ferroactinolite), chlorite, magnetite, pyrite, pyrrhotite and chalcopyrite. Stage IVb, was characterised by the growth of chlorite, sericite, quartz, pyrite, pyrrhotite and chalcopyrite. The bulk of the magnetite within the Sangan deposit formed during the later stages of skarn evolution.

A genetic model is proposed to account for the development of bimetasomatic skarn in the Marulan South deposit. Exoskarn geochemistry indicates addition of many components relative to an essentially pure limestone precursor, including Si, Al, Fe, Zr, Zn, S, Mn, REE (rare earth elements) and Cu, negligible transfer of K, Na and Rb and loss of C02. Strontium and Ca loss from the parent limestone is indicated by mass balance calculations at constant volume.

The data for exoskarn at Sangan indicate potential addition of the following components assuming a relatively pure calcic or dolomitic limestone precursor: Si, Ti, Al, Mn, Fe, K, Zn, Rb, Y and Zr. Carbon dioxide is lost during devolatization. REE data for the Sangan deposit were also indicative of metasomatic addition; the normal enrichment pattern was manifested by significant increase in LREE/HREE. Garnet skarns developed at the contact with granite bodies in the Sangan deposit were usually characterised by positive Eu anomalies, probably reflecting more oxidising conditions adjacent to the pluton.

Metasomatic transfer within both skarn deposits was probably accomplished by the formation of inorganic metal (or element) ligands, including neutral metal halide or carbonate complexes. Previously published experimental data, together with field and laboratory observations has indicated that high field strength elements (HFSE), for example Zr, Y and even REE formed neutral complexes in acidic solutions rich in F and that sulphate and carbonate complexing may provide other viable mechanisms for HFSE transportation in the absence of F-bearing fluids. Previously published experimental data also indicated that Cl complexation is probably not of prime importance in facilitating HFSE transport. In contrast, many metals, for example Fe, Mn, are probably transported as metal-chloride complexes, in acidic solutions.

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