Textural, mineralogical and geochemical variation in the zoned Timbarra Tablelands pluton, New South Wales*

Abstract

The Timbarra Tablelands pluton is an extensive (~550 km 2 ) complexly zoned intrusion forming one of many predominantly monzogranite I-type plutons, which constitute the Moonbi Supersuite in northern New South Wales, Australia. It comprises an outer rim of Rocky River monzogranite (Zones 1-3), an intermediate zone of Sandy Creek syenogranite (Zones 4A-4C), surrounding a core of Surface Hill syenogranite (Zones 5-7). The suite is calc-alkaline, high-K, and varies from mildly metaluminous to weakly peraluminous with increasing fractionation. Average Rb/Sr ratios range from 0.4 in the least evolved very coarse-grained monzogranite (Zone 3) to 46 in the most evolved very fine-grained biotite microgranite (Zone 6). Trace-element modelling indicates that the observed compositional variation could have been produced by crystal fractionation. New bulk rock major-and trace-element data for 71 samples are presented, and indicate that a compositional continuum exists that varies between 63 and 78 wt% SiO 2 . Importantly, there is no systematic chemical variation with spatial distribution of samples from the core of the pluton to its margin, requiring multiple separate pulses of an evolving magma to explain compositional discontinuities. The pluton is interpreted to have been emplaced at mesozonal levels (~180 Ϯ 60 MPa, 5-10 km depth) and crystallised at temperatures between 620 and 820 Њ C under moderately oxidising conditions (log f O 2 = -11.5 to -19). The association of goldmolybdenite mineralisation at Timbarra with moderately oxidised I-type magmas is consistent with fractionation-redox controls on ore-element behaviour in magmatic systems in other studies.

Gold and metal enrichment in natural granitic melts during fractional crystallization

“…The whole-rock geochemistry and trace element modeling, however, highlight a cogenetic fractionation series that displays higher Rb/ Sr values with increasing SiO 2 ( Fig. 2; Table DR1 [see footnote 1]), and thus indicates a common stratified source region (Mustard, 2004). The magma pulses are tapped from different zones of the stratified magma chamber and intruded each other.…”

“…ated coarse-grained hornblende-biotite monzogranite and zones 6 and 7 are the most fractionated rocks with textures (e.g., miarolitic cavities and unidirectional solidification textures) that are indicative of formation near the magmatic-hydrothermal transition where fluids coexisted with late-stage melts (e.g., Candela, 1997;Mustard, 2004). Based on petrographic contact relationships and geochemical analyses it is envisaged that zones 1-3, 4A-4C, 6A-6B, 5A-5B, and 7 represent individual pulses of magma (Mustard, 2004). The whole-rock geochemistry and trace element modeling, however, highlight a cogenetic fractionation series that displays higher Rb/ Sr values with increasing SiO 2 ( Fig.…”

“…The Au deposit at Timbarra in the New England Fold Belt, East Australia, is an example of an intrusion-related gold deposit with disseminated gold mineralization and minor late-stage vein mineralization in the roof zone of a composite pluton (Mustard, 2001(Mustard, , 2004. The Timbarra pluton consists of two major intrusions (Bungulla monzogranite and Stanthorpe syenogranite) that have several texturally and chemically different zones (for details see Mustard, 2001Mustard, , 2004; see Data Repository 1 ).…”

“…The Timbarra pluton consists of two major intrusions (Bungulla monzogranite and Stanthorpe syenogranite) that have several texturally and chemically different zones (for details see Mustard, 2001Mustard, , 2004; see Data Repository 1 ). Seven individual zones of the pluton can be readily distinguished based on texture, grain size, mineralogy, and field relationships.…”

“…Ulrich-Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. deposits (i.e., Fort Knox and Timbarra; Bakke, 1995;Mustard, 2001Mustard, , 2004Thompson et al, 1999; Fig. 1).…”