Paper Number: 1755
Geochemistry and Raman spectroscopy of micas associated with greisens in the Poimena granite, NE Tasmania.
van Moort, J.C.1, Allen, N.R. 2, Bennett, J. 3, Bottrill, R.4 and Mernagh, T. 4
1Earth Sciences, University of Tasmania, Private Bag 79, Hobart TAS 7001, Australia. jcvanmoort@bigpond.com
2 14 Station Lane, Exton. TAS 7303, Australia. <nallen@skymesh.com.au>
3ANSTO, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia. <john.bennett@ansto.com.au>
4MRT, PO Box 56, Rosny Park TAS 7018, Australia. <Ralph.Bottrill@stategrowth.tas.gov.au>
5Research School of Earth Sciences, ANU, Acton ACT 2601, Australia. <terry.mernagh@anu.edu.au>
__________________________________________________________________________________________The Poimena pluton of the mid-Palaeozoic Blue Tier batholith is a medium to coarse grained biotite granite/adamellite with occasional minor muscovite. The district produced 17 kilotons of cassiterite. The Sn content of the granite varies between 15 and 42 ppm Sn. This paper deals with tin-bearing greisens in this granite. At Gladstone in the Fly by Night Creek a 300 m wide quartz-muscovite-kaolinite greisen is in abrupt contact with the granite, of which fragments remain in the greisen. In the nearby Hardens Ravine the greisen is observed as quartz/muscovite veins in bleached granite, in the Anchor DDH BT52 core it shows as bleached zones and at Dorset Flats as altered granite with quartz sulphide veins and alluvial cassiterite and small particles of native Cu, Pb, Zn, Cr, Fe and alloys.
The bulk geochemistry of the granites, greisens and derived kaolinite has been analysed for fifty five elements by FUS-ICP, TD-ICP, INAA and FUS-ISE. In summary: the biotite granites are low Li and F, high in Na, Ca and Sr, and low in Sn and Ta ; the bulk rock chemistry does not gives away that they host, indirectly, so much tin mineralisation. The greisens are higher in Li and F, low in Na, high in Al, low in Ca and Sr, and high in Sn and Ta. CH4 occurs in quartz. The composition of the biotite and muscovite in granites and greisens were explored by INAA and followed up by duplicate LA-ICPMS 75 µm spot analyses, with quality control checks by the comprehensive FUS-ICP etc analyses mentioned above. Rare biotite remnants in the greisen are often attached to quartz and occasionally overgrown with muscovite. The data, in ppm, are summarised in the table below. The muscovite formed during destruction of feldspar during greisenisation is phengitic and contains three to two times more Li, Rb, Cs, Ta, Nb, Sn and Fe than the muscovite in the granites of the area without tin mineralisation. The biotite remnants in the greisens are unexpectedly enriched in Li, Rb, Cs and Ta.
| Li | ||||||
| Rb | ||||||
| Cs | ||||||
| Ta | ||||||
| Nb | ||||||
| Sn | ||||||
| Fe% | ||||||
| Ta/Nb | ||||||
| Biotites in granites without known tin mineralisation (n7) | ||||||
| 1274 | ||||||
| 2105 | ||||||
| 223 | ||||||
| 27 | ||||||
| 187 | ||||||
| 88 | ||||||
| 17.8 | ||||||
| 0.14 | ||||||
| Biotites in granites with known tin mineralisation (n11) | ||||||
| 2475 | ||||||
| 3591 | ||||||
| 271 | ||||||
| 28 | ||||||
| 260 | ||||||
| 358 | ||||||
| 18.6 | ||||||
| 0.11 | ||||||
| Muscovites in granites without known tin mineralisation (n16) | ||||||
| 445 | ||||||
| 1137 | ||||||
| 65 | ||||||
| 3 | ||||||
| 25 | ||||||
| 256 | ||||||
| 1.7 | ||||||
| 0.12 | ||||||
| Muscovites in greisens (n25) | ||||||
| 1360 | ||||||
| 3554 | ||||||
| 182 | ||||||
| 9 | ||||||
| 36 | ||||||
| 572 | ||||||
| 4.9 | ||||||
| 0.24 | ||||||
| Biotite remnants in greisen (n6) | ||||||
| 5170 | ||||||
| 7702 | ||||||
| 500 | ||||||
| 74 | ||||||
| 315 | ||||||
| 397 | ||||||
| 12.6 | ||||||
| 0.24 | ||||||
Pneumatolitic/hydrothermal alteration of the host-granite and hornfelsed slates hosting tin mineralisation appears to be responsible for the introduction of the elements above (TaF5 is water soluble). The greisens and phengites contain F <0.3% and CH4 in quartz making them somewhat comparable to some Brazilian greisens [2]. The elevated Ta content of the phengite and in particular of the altered biotites in the greisen suggests crystallisation/alteration from a metasomatic event involving late pegmatic fluids, derived from magmatic concentration Stepanov [3]. Changes in the Raman spectra of micas were found to correlate with the Fe content with the increasing phengitic nature of the muscovites. The most notable difference occurs in the OH-stretching where three bands at ~3575 cm-1, ~3621 cm-1 and ~3655 cm-1. Significant changes are also observed in the region from ~100 cm-1 to ~300 cm-1 which are attributed to the Fe substitution on octahedral sites. The effect of Ta substitution is also apparent in this region.
References:
[1] Allen et al. (2004) In : Dynamic Earth, Abstracts, 17th Aust Geol Convention
[2] Borges et al. (2009) Journal South American Earth Sciences 17:161-183
[3] Stepanov et al. (2014) Contrib Mineral Petrol 167:1009 (online)