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Interpreting magmatic processes from accessory phases: titanite—a small-scale recorder of large-scale processes
- Philip Piccoli, Philip Candela, Mark Rivers
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- Journal:
- Transactions of the Royal Society of Edinburgh: Earth Sciences / Volume 91 / Issue 1-2 / 2000
- Published online by Cambridge University Press:
- 03 November 2011, pp. 257-267
- Print publication:
- 2000
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In this study we examined variations in ore and other trace-metal concentrations in titanite, a ubiquitous product of magmatic (and subsequent sub-solidus) crystallisation in oxidised silicic magmas. Accessory titanite occurs in the Tuolumne Intrusive Suite (TIS), Sierra Nevada Batholith, as euhedral to anhedral, poikilitic, or interstitial grains. Zoned crystals of titanite were analysed by electron microprobe and synchrotron X-ray fluorescence for major and trace elements. Backscatter electron images reveal zoning, with bright areas correlating positively with total REE concentrations. REE concentrations generally decrease toward the edge of titanite crystals; however, some crystals are reversely zoned, and others exhibit oscillatory or patchy zoning; some grains contain discrete anhedral cores. Most elements in magmatic titanite decrease in concentration towards crystal rims, independent of host rock composition.
At least one major reduction event in the magma chamber(s) transiently stabilised ilmenite, now present only as inclusions in titanite, and resulted in a reduction in the REE concentration in titanite. We suggest the hypothesis that the reduction in the REE concentration in these zones is due to the diminished activity of the (REE)Fe3+ Ca−1Ti−1 exchange component; however, the scatter in the data, together with the operation of other exchange vectors for Fe and Al, did not allow us to test this hypothesis herein. Secondary (i.e. sub solidus, hydrothermal) titanite can be recognised on the basis of its chemistry, sometimes by its anhedral form, and by its position as an alteration rim around primary magmatic phases; however, secondary titanite growth on primary titanite crystals may be harder to discern. Secondary titanite rims on magnetite contain higher Cr, Zr and Mo, and lower REE, relative to magmatic titanite. U/Th ratios increase toward the rim of most titanite grains; however, Th decreases in concentration from core to rim. This is due, most likely, to complications resulting from the coupled substitutions necessary for replacement of Ca by tetravalent Th; factors of this sort are commonly overlooked in trace element analysis.
The analysed titanites are from rocks of the normally zoned TIS which ranges in 87Sr/86Sri, from 0·7059 (tonalite and quartz-diorite) to 0·7066 (granite). Many element ratios in the titanites exhibit little to no functional dependence on 87Sr/86Sri. However, log Mo/W increases with increasing 87Sr/86Sri, of the host unit from the equigranular quartz-diorite and tonalite, to the interior granodiorites, possibly reflecting the greater crustal contribution to the interior, more felsic units. Neither Mo nor W increase significantly from core to rim in titanite. If these trends are indicative of the general behaviour of these elements during in-situ fractionation, then these data suggest that Mo and W are not strongly incompatible, and indeed may behave compatibly, in some titaniteand magnetite-bearing granodioritic magmas.
Controls on ore metal ratios in granite-related ore systems: an experimental and computational approach
- Philip A. Candela
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- Journal:
- Transactions of the Royal Society of Edinburgh: Earth Sciences / Volume 83 / Issue 1-2 / 1992
- Published online by Cambridge University Press:
- 03 November 2011, pp. 317-326
- Print publication:
- 1992
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Size and composition (bulk metal ratios) of magmatic hydrothermal mineral deposits are affected by a number of chemical and physical processes including the nature of the source region and mode of emplacement. At shallow levels, rising plumes of vapour bubbles + melt, and the advection of water through interconnected vapour bubbles, allows access of the magmatic aqueous phase to the upper reaches of a magma chamber. These processes are operative at shallow levels where low water solubility and high molar volume for water make these processes more efficient.
Partitioning experiments suggest that oxygen fugacity-dependent crystal/melt partitioning of ore metals leads to different efficiencies of removal of Cu, W, and Mo from silicate melts into ore-forming aqueous fluids. For example, the Mo/W ratio in magmatic hydrothermal deposits should increase as the oxygen fugacity of the magma increases. Further, Cu should behave as a crystal-compatible element in H2O-undersaturated, sulfide-saturated felsic magmas with fO2 NNO + 1 due to the strong partitioning of Cu from the melt into pyrrhotite.
Cycling of oxidised, hydrated, sulfidised and Cl-enriched oceanic crust into mantle can give rise to magmas that contain S but are oxidised (≥NNO). The combination of high oxidation state, relatively hydrous but shallow conditions and a high Cl/H2O ratio leads to saturation with respect to H2O early during crystallisation, and loss of a large proportion of magmatic Cu to the aqueous phase. Ores formed from these oxidised magmas also possess high Mo/W ratios due to the effect of oxygen fugacity on the sequestering of Mo vs W.
In less oxidised magmas, Cu and Mo are partitioned into sulfides and Ti-bearing phases, respectively, resulting in lower efficiencies of removal of Cu and Mo from melts into aqueous fluids. Further, the partitioning of W into crystallising phases is reduced, producing a more efficient removal of W into ore-forming fluids. This ultimately leads to mineral deposits with higher W/(Mo + Cu) ratios relative to deposits associated with more oxidised systems. Silicic, high-F magmas with fO2 = NNO can be found in tensional environments (e.g. rocks associated with the Climax-type deposits of the Colorado Mineral Belt). High HF/H2O activity ratios in the source regions yield melts that evolve an aqueous phase late during crystallisation, leading to relatively low ratios of compatible/incompatible elements in the melt at H2O saturation.