Pegmatites

Pegmatites REE Granitic pegmatites What is a Pegmatite? “A very coarse grained igneous or metamorphic rock” Evans (1993) Two genetic types Metamorphic vs igneous Only Granitic pegmatites form significant ore bodies since they form major concentrations of REE and incompatible elements that otherwise occur only at trace levels.

Granitic Pegmatites: occurance      

Occur singularly, fields, belts or provinces Closely associated with late orogenic granitic plutons. Abundant above subduction zones and in orogenic belts Localised in faults systems, lithological or pre-existing contacts Hosted at all crustal levels, shown by their association with all metamorphic grades of rock Generated in a single tectono-magmatic events Associated with all major tectonic events from >3000 Ma to present day

Histogram, age distribution of pegmatites

Classification of granitic pegmatites

General characteristics Structure and shape - controlled by host rock and degree of fractionation of parent fluid. - Range from dykes, tabular, lenticular, ellipsoidal, to turnip or mushroom shaped also fracture filled Pegmatites can be unzoned or zoned - Zonation is both internal and/or regional Hydrothermal alteration - Core zone of some pegmatites show extensive hydrous and carbonate alteration Internal structure: - Unzoned - Mineralogically simple, uniform in compositon and texture usually metamorphic in origin - Rarely of economic value - Zoned - Nine characteristic mineral zones - Order of zones consistent though not all always present - Increased geochemical complexity will increase the complexity zoning - Four major zones, the core, intermediate, wall and border. - Layered zoned and homogeneous pegmatites are end-members

Pegmatite Zonation Border to Core

1. 2. 3. 4. 5. 6. 7. 8. 9.

Plagioclase Quartz Muscovite Plagioclase Quartz Quartz Perthite Plagioclase Perthite Quartz Quartz Perthite Plagioclase Spodumene Amblygonite Quartz Spodumene Quartz Micrcline/Perthite Quartz Lepidolite Li Mica Plagioclase Quartz Microcline

Internal structure of zoned pegmatites

Regional Zoning - Distinct chemical association to parent pluton - Regional zoning also occurs, increasingly fractiuonated groups occur further from parental granite - Distal pegmatites are most intensly fractionated and enriched in incompatible volatiles and REE

Regional zoning in Osis Lake Granite Complex and Rush Lake Pegmatite Group, Manitoba (Canada)

Mineralogy - Fundmental assemblage is QUARTZ , PERTHITE, ALBITE and accessory MUSCOVITE/BIOTITE. - Complex mineralogy generated due to increased concentrations of REE and Incompatible elements like Be, Li, Sn, (