Geol 135 Sedimentation
J Bret Bennington
Heavy Minerals and Provenance
Heavy minerals are detrital grains of minerals with high density (>2.9) that occur as accessory minerals in quartz sands. These grains are derived from the eroding source rocks and may be distinctive of particular types of rock.
Heavy mineral grains can be identified based on their physical properties. Usually, they are first separated from the majority quartz grains (this can be done using heavy liquid separation) and then they can be further separated based on their degree of attraction to a strong magnetic field (magnetic separation).
On Long Island, wave swept winter beaches along the south shore barrier islands develop concentrations of heavy minerals as quartz grains are winnowed, leaving the denser grains behind. These heavy mineral sands can be black or red in color, depending on the dominant heavy mineral (magnetite vs garnet).
Heavy mineral sands include ore minerals that may be concentrated sufficiently to form economic deposits - called placer deposits. Placer deposits can form in alluvial channels, on beaches, or on marine abrasion surfaces. Important placer deposits include zircon and illmenite sands in New Jersey and Georgia / north Florida, cassiterite deposits on the Sunda shelf of Indonesia, and extensive illmenite, zircon, and rutile deposits on Quaternary beaches of Queensland, Australia.
Provenance - refers to the source region of detrital sediments.
Stable minerals such as zircon and rutile can survive multiple cycles of weathering and deposition, becoming rounded. These are indicative of a sedimentary source area. Less stable minerals such as magnetite, pyroxenes, and amphiboles do not survive recycling and are thus diagnostic of the composition of proximate source rocks.
If a clast within a sedimentary rock can be attributed to a particular source area then the provenance of the sedimentary deposit can be established. This provides information on the paleosource area and can also lend insight into the timing of uplift and erosion (unroofing) of the source area. Generally, uppermost source rocks contribute detrital grains to older sedimentary layers. As the layers get younger upsection they preserve progressively deeper-derived source rock grains.
Examples of Provenence Studies
Dartmoor Granite, southwestern England.
The Dartmoor granite occurs in a swarm of plutons in the soutwest of England. These granites contain a distinctive suite of minerals that weather into detrital heavy mineral grains (zircon, octahedrite, anatase, monazite, brookite, and manganiferous garnets). The granite intrudes folded and metamorphosed marine strata of Devonian and Carboniferous ages of the Variscan fold belt that trends E-W across southern England. These folded rocks are overlain unconformably by nonmarine red sandstones of Permian and Triassic ages.
The earliest indication of unroofing and exposure of the plutons occurs in the Permian sands. These contain minerals characteristic of the contact metamorphic aureole around the granite, but not from the granite itself. These minerals are found in Triassic and Jurassic strata as well. In the Early Cretaceous sands occur the first abundant grains of Dartmoor heavy minerals, and all younger sands also contain these minerals. Thus, exposure and erosion of the plutons can be dated to the Early Cretaceous.
Eastern margin of North America, northeastern US
Along the eastern margin of the US, two belts of crystalline rocks occur with distinctive heavy mineral assemblages.
Piedmont - Newark Basin - Triassic / Jurassic igneous sills and lava flows
Blue Ridge - Proterozoic metamorphics
One hypothesis for the geomorphic development of the eastern continental margin posits that erosion created a westward peneplain in the Late Jurassic - Early Cretaceous and that Late Cretaceous to Early Cenozoic coastal plain strata completely covered the present day Appalachian fold belt.
Studies of mineral sands in these strata cast doubt on this hypothesis because the sands contain distinctive minerals from the Proterozoic rocks but show no contributions from the Mesozoic Newark rocks. This suggests that, while the Piedmont may have been covered by coastal plain strata, the Appalachian fold belt remained exposed farther to the west.