Linking Magmatism and Mineralization using In-Situ Nd- and Sr-isotope Systematics of Vein-Hosted Apatite at the Morila Au Mine, Mali
Abstract
At the Morila Au Mine, Mali, auriferous arsenopyrite, scheelite, and alloys of Au-Sb and Bi-Te occur in polymineralic veins with timing relationships that match those of nearby calc-alkaline plutons. Although an intrusion-related origin is implicated spatially and temporally, robust isotopic links between alteration and ore deposition and the adjacent 'smoking gun' pluton have yet to be established. Previous attempts to fingerprint the mineralizing fluids at Morila using S-isotopes for sulfides in host metasediments (R. Quick, unpublished data) yielded an equivocal range of values between +2 to +6 permil (relative to CDT) reflecting mixing and isotopic re-equilibration during the growth of Aspy at the expense of pyrrhotite within a tortuous micro-porosity network. As an alternative, radiogenic isotope systematics for mineralized veins that contain accessory minerals amenable to in-situ isotope measurements can be used to help minimize potential homogenization of isotope signatures. At Morila, LREE- and Sr-bearing apatite occurs in calc-alkaline intrusions and mineralized polymineralic veins. This allows us to compare the isotopic signature of mineralized veins to that of nearby plutonic rocks. Three distinct vein assemblages were targeted for in-situ work: 1) scheelite-bearing veins hosted by granodiorite stocks, 2) Au-Loellingite-Aspy-bearing veins in the upper footwall of the deposit, and 3) late-stage peraluminous granitic veins distal to the ore body. Apatite was analyzed by LA-MC-ICPMS in standard thin sections thereby providing complete control on the textural context of each grain. The results, calculated at t = 2095 Ma, yield a narrow range of epsilon-Nd values between -1 to +2 but a wide range of initial 87Sr/86Sr between 0.7015 to >0.7030. Whereas the Nd-isotope data overlaps with the full range of intrusive rocks at Morila and with the host metasediments, the bulk of the Sr-isotope data for the veins matches the isotopic signature of late-stage granites. Integrated with macro- and microscopic evidence for AFC processes prior to granite emplacement, this in-situ Nd- and Sr-isotope dataset strengthens the genetic links between magmatism and mineralization and highlights the importance of magma hybridization in intrusion-related Au deposits.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSM.V73B..07M
- Keywords:
-
- 1034 Hydrothermal systems (0450;
- 3017;
- 3616;
- 4832;
- 8135;
- 8424);
- 1040 Radiogenic isotope geochemistry;
- 1042 Mineral and crystal chemistry (3620)