| Abstract | Forty-two different Devonian granitoid intrusions in New Brunswick were studied for this project. They formed by crustal growth processes during the Acadian orogeny, post-Acadian uplift, and Neoacadian
orogeny and most are associated with granophile element deposits, such as Sn, W, Mo, Cu, Bi, In, Sb, Au, and possibly Ta and Li, as well as base-metals and U mineralization. These intrusions were emplaced pre-, syn-, late-, and posttectonically
between 423 and 360 Ma with affinities ranging from primitive to highly evolved A-, S-, and I-types granitoids.
The aim of this study is to find a way to differentiate barren and mineralized granitic systems using biotite compositional systematics
since it is highly sensitive to physico-chemical changes of its host, and continuously reequilibrates with host and derivative fluids. Therefore, core to rim studies of this mineral and analysis of its compositional zoning may reveal the origin and
evolution history of the hosting granitoid and the different types of associated mineralization. Mineralized and barren granitoids are characterized by different chemical variations in biotite. For instance, biotite from a mineralized granitoid is
characterized by lower Mg and Ti, and higher Al content relative to biotite from a barren granitoid. A combination of electron microprobe (EPMA) and laser ablation ICP-MS (LA-ICP-MS) was used to identify major, minor, trace elements, and halogen
contents of biotite; these results were used to calculate fluoride and chloride activity of aqueous fluids associated with these intrusions, based on F and Cl contents in the mineral.*** Microprobe studies indicated homogeneous intragranular major
element composition; crystallization temperature was calculated using Ti-In-biotite geothermometer, which gave a range of 670 to 750 ± 25°C . However, hydroxyl exchange biotite-apatite thermometer confirmed sub-solidus processes disturbed these
systems resulting in a lower temperature around 300. Biotite grains from the highly fractionated bodies of the Pleasant Ridge, Mount Pleasant, and Kedron granites show the highest fluorine contents, ranging from 4.5 wt.% to 6.5 wt.%.
Trace element
changes within the biotite lattice were measured using LA-ICP-MS. Interestingly, almost all of studied grains show Cs, Ba, and Rb zoning relative to K. These patterns were followed by Co, Cu, K, Li, Be, Sn, W, Ti, Sc, Ni, B, and V. Large ionic charge
and radius make it difficult for elements to join or leave any crystalline structure; therefore, any Cs or Ba zoning could be of an igneous origin. As a result, any other elemental pattern following those could be a result of magmatic evolution,
which has been recorded by the biotite crystal growth. Based on the results of this project, the concept of using biotite composition to help identify fertile Acadian magma systems has been established; however, more work needs to be done to define
characteristics of different magmatic processes. |