Presentation on theme: "ASTARTE PROJECT University of Aberdeen Heavy mineral analysis: applications and limitations S.E. Poynter."— Presentation transcript:
ASTARTE PROJECT University of Aberdeen Heavy mineral analysis: applications and limitations S.E. Poynter
Summary Heavy minerals are more provenance-specific than quartz and feldspar. They are a powerful tool for assessing the provenance and dispersion of modern and ancient river and coastal sediments. They have been used successfully to correlate barren sandstones and reconstruct palaeogeography. There are four potential important processes that can modify a heavy mineral assemblage: source area weathering, transport abrasion, hydraulic sorting and post-depositional diagenesis. The latter two processes are the most significant. The effects of hydraulic sorting can be minimised by analysing the very fine sand fraction of a sample, and by discounting platy minerals such as mica and chlorite. Progressive chemical dissolution of minerals occurs by near-surface acid weathering and during deep burial diagenesis. The two processes have different orders of mineral stability. Post-depositional diagenesis can be difficult to distinguish from provenance variation. Sands which have undergone significant post-depositional diagenesis have a high proportion of ultrastable minerals (zircon, tourmaline and rutile); whereas unstable minerals (olivine, pyroxene) will be present in sands which have undergone minimal mineral dissolution. Three methods can be used to recognise provenance changes in altered mineral assemblages: 1.Mapping the presence/absence of diagnostic stable mineral species such as chloritoid. 2.Determining changes in the ratio of two stable mineral species (e.g. rutile and zircon). 3.Varietal mineral chemistry. Changes in the abundance and chemistry of an individual mineral species can indicate fluctuations in provenance. Garnet and tourmaline are particular suited to a study of this type as they have provenance-specific endmembers and are chemically stable.
1.Introduction Heavy minerals have density of > 2.8gcm -3 and typically comprise less than 1% of a sandstone. They are more provenance specific than the light mineral fraction (Table 1, e.g. sillimanite found in contact metamorphics, garnet found in medium-high grade regionally metamorphosed source rocks). There are over 50 non-opaque heavy minerals, including ferromagnesian rock forming minerals, such as amphiboles and pyroxenes, as well as accessory minerals such as zircon, apatite and rutile. During the early part of the 20 th century, heavy minerals were commonly used as sediment provenance indicators (e.g. Milner, 1923). The technique fell into decline during the 1930s and 1940s, as it became clear that transport and depositional processes could significantly modify heavy mineral assemblages. The decline of heavy mineral studies was accelerated by the introduction of newer sedimentological and correlatory techniques such as biostratigraphy. Although work using heavy minerals continued during the 50s and 60s (e.g. Van Andel, 1950; Hubert, 1962), it was not until the 1970s that there was a more widespread revival, and the problems of transportational and post-depositional modification were partially resolved (e.g. Morton, 1985a). Heavy minerals are separated from the light mineral fraction by passing the sample through a separation column containing a heavy liquid. The light minerals float on top of the liquid and the heavy minerals settle to the bottom where they can be removed. Detail of this methodology is summarised in Appendix A. The optical identification of heavy mineral species is difficult, and experience is required to identify the less common species.
Table 1: Paragenesis of 14 heavy minerals species, showing their provenance-specific characteristics (data from Mange and Maurer, 1992).
Heavy minerals are widely used for sediment provenance dispersal and correlation studies in both modern and ancient sediment. Examples of the application of heavy mineral analysis are outlined below : 1.Used to examine sediment flux and transportational processes in modern river systems (Russell, 1937; Shukri, 1949; Van Andel, 1950; Morton and Smale, 1990; Morton and Johnsson, 1993). –In small catchments, like the Cascade River, New Zealand, fluvial heavy mineral assemblages faithfully record the provenance of their source rocks (Morton and Smale, 1990). –In larger systems, the localised input from tributaries is shown by downstream variations in heavy mineralogy (Van Andel, 1950). 2.Can also be applied to ancient river systems to determine their provenance, spatial distribution and palaeodrainage pattern (Allen and Mange-Rajetzky, 1982; Preston et al., 2002). 3.Used successfully to map modern and Tertiary sediment dispersal and provenance along coastlines and the continental shelf (Flores and Shidler, 1978; Mange-Rajetzky, 1983). 4.Heavy minerals have also been used to map the bedrock geology in areas with deep soil or glacial cover (Callahan, 1980). 5.Stratigraphic changes in heavy mineral suites can be used for palaeogeographic reconstructions or to determine the denudation history of the source area (Weissbrod and Nachmias, 1986; Morton et al., 1994). –e.g. Miocene basin-fill sandstones in the North Sumatra Basin, Indonesia, record a shift from granitic to pelitic source rocks, caused by the uplift of the Barisan Mountain Range in the early Middle Miocene (Morton et al., 1994). 6.Heavy minerals can be used to correlate barren sandstones, where biostratigraphic control is poor. This technique has been used successfully in the North Sea to correlate over large, reservoir scale areas and between boreholes (Morton, 1992; Lihou and Mange- Rajetzky, 1996). 2.Application of heavy minerals
Figure 1: Five major processes that can modify a heavy mineral assemblage during transport and deposition. The heavy mineral assemblage of a sediment is principally a function of the source area from which it was derived. For example, an assemblage rich in kyanite and sillimanite indicates high-grade metamorphic source rocks. However, there are four key processes, which have been recognised as potentially important modifiers of a primary heavy mineral suite: source area weathering, transport abrasion, hydraulic sorting and post- depositional dissolution (Fig. 1, Morton et al., 1994): 3.Controls on heavy mineral assemblages
i)Source area weathering The relative proportion of different heavy mineral species in weathered rock crust can differ significantly from the proportions of heavy mineral species in the unweathered parent rock (Rubey, 1933; Dryden and Dryden, 1946). Most modern river sediments have diverse heavy mineral assemblages, which do not appear to have been significantly altered by source rock weathering (e.g. Russell, 1937; Shukri, 1949; Van Andel, 1950). However, most of these studies are in weathering-limited systems, which may explain the diverse heavy mineral suites observed (Johnsson et al,1991 and Morton et al, 1994). –e.g. the Merida Andes and Caribbean Mountains of Venezuela and Columbia are considered to be in weathering-limited systems (Morton and Johnsson, 1993). The steep mountain slopes in this region have thin soils and exposed bedrock, which means that once eroded, detritus is quickly removed from the area. In transport-limited studies, material is eroded at a greater rate then it can be removed, and therefore source area weathering may be a more important process. ii)Transport abrasion During the 1920s and 30s abrasion was considered to be the most important control on a heavy mineral assemblage, by increasing the proportion of smaller grains with increasing transport distance and changing the ratios between harder and softer minerals (e.g. Rubey, 1933). However, studies of the Nile (Shukri, 1949) and Mississippi (Russell, 1937) do not show any change in the proportion of mechanical weak minerals with downstream distance. Therefore, abrasion during transport has only a very minor effect on the heavy mineral assemblage of a sediment.
iii)Hydraulic sorting Hydraulic sorting can significantly modify the heavy mineral assemblage of a sand (e.g. Rubey, 1933; Van Andel, 1950; Morton and Hallsworth, 1999). Minerals behave differently in a fluid flow depending on their density, size and shape. Density The size distribution of a heavy mineral species is a function of the mineral’s density relative to quartz (its hydraulic equivalence). e.g. a 0.63 mm grain of magnetite has the same settling velocity as a 1 mm grain of quartz. The average heavy mineral grain size is therefore always smaller than the total average grain size of the sand, and heavier grains are deposited in the finer fraction. Shape Minerals with a platy or bladed habits have a very different hydraulic equivalence to rounded minerals of the same density and size. Size The grain size availability of a mineral can determine how it is hydraulically sorted (Rubey, 1933; Russell, 1937). e.g. zircon and rutile form small crystals of less than 250 m (Poldevaart, 1955; Mange and Maurer, 1992); kyanite and sillimanite form larger grains of m (Van Andel, 1950). Reducing the effects of hydraulic sorting Separating the m very fine sand fraction of a sample minimises hydraulic sorting (Morton and Hallsworth, 1994) and eliminates apparent variations in mineral populations caused by different grain size. Because of their different hydraulic behaviour, platy minerals are not included in heavy mineral counts (Morton, 1985a).
iv)Post-depositional diagenesis With the exception of source area, diagenetic mineral dissolution has the single-most important influence on a heavy mineral assemblage. Post-depositional dissolution has modified all pre-Quaternary sediment to some degree. Extensive dissolution can reduce a diverse assemblage of more than twenty minerals to an assemblage composed only of ultrastable zircon, tourmaline and rutile. e.g a study of the Oligocene deposits of the ancestral Rio Grande showed that post-depositional dissolution had removed 90% of the original heavy mineral assemblage (Milliken and Mack, 1990). Progressive chemical dissolution of minerals occurs by near-surface acid weathering and during deep burial diagenesis. Both acid weathering and deep burial dissolution have distinct orders of heavy mineral stability (Table 3). Table 3: Chemical stability of heavy minerals under acid weathering and deep burial conditions (Morton, 1985). Deep Burial Diagenesis olivine, pyroxene andalusite, sillimanite amphibole epidote sphene kyanite staurolite garnet apatite, chloritoid, spinel rutile, tourmaline, zircon Acid Weathering olivine, pyroxene amphibole sphere apatite epidote, garnet chloritoid, spinel staurolite kyanite andalusite, sillimanite rutile, tourmaline, zircon Increasing mineral stability
Acid weathering Acid weathering occurs where meteoric groundwaters percolate through subsurface sediments, such as in a flood plain or delta top environment. e.g. In Neogene sands of Jylland, Denmark, the proportion of unstable mineral species (amphibole, pyroxene) decreases rapidly beneath rooted peat horizons (Friis, 1976). More stable grains show surface etch patterns associated with the early stages of mineral dissolution. This pattern of etching and mineral abundance can be explained by intense weathering below a vegetated surface. Deep burial diagenesis Heavy mineral diversity decreases with increasing depth of burial (e.g. Pettijohn, 1941; Milliken and Mack, 1990; Cavassa and Gandolfini, 1991). e.g. Late Palaeocene sands of the central North Sea, containing apatite, amphibole, epidote, garnet, kyanite, rutile, staurolite, sphene, tourmaline and zircon at shallow depths, progressively lose amphibole, then epidote, sphene, kyanite and staurolite as burial depth increases Morton (1984). Grain surface etching also occurs in the early stages of mineral dissolution. The order of stability of minerals under deep burial conditions is controlled by: pressure and temperature of pore fluids; connectivity and movement of pore fluids; pH of pore fluids; presence or absence of oil; length of the time the sediment resides in deep burial conditions; variations in the chemical composition of a mineral species.
4.Determining the provenance signature of altered mineral assemblages Differential mineral dissolution generates stratigraphic variations in heavy mineralogy that can mask changes caused by a shift in provenance. The ZTR ratio (% of ultrastable zircon, rutile and tourmaline in the total non-opaque heavy mineral fraction) gives a measure of the degree of dissolution that has occurred (Hubert, 1962). In greywackes and arkoses, the ZTR index is between 2-39%, and usually exceeds 90% in quartz arenites (Mange and Maurer, 1992). The presence or absence of diagnostic unstable mineral species can also be used to assess the extent of post-depositional dissolution. e.g. olivine and pyroxene are highly unstable, therefore indicate of that little mineral dissolution has occurred. Three techniques can be applied to distinguish provenance change from variations caused by mineral dissolution: 1.Mapping the presence or absence of stable minerals such as spinel or chloritoid. Variations in the abundance of these minerals cannot be explained by differential weathering (as they are chemically stable), and provides evidence of actual rather than apparent provenance change. 2.Variation in the ratio of two or more stable minerals with the same hydraulic behaviour (Table 4) corresponds to provenance change rather than mineral dissolution or hydraulic sorting (Morton and Hallsworth, 1994). 100 grains of the two minerals must be counted to generate a statistically significant ratio.
Index DeterminationIndexMineral Pair ATi GZi RZi CZi MZi 100x apatite/(total apatite + tourmaline) 100x garnet/(total garnet + zircon) 100x TiO 2 group/(total TiO 2 group + zircon) 100x chrome-spinel/(total chrome-spinel + zircon) 100x monazite/(total monazite + zircon) Apatite, tourmaline Garnet, zircon TiO 2 group, zircon Chrome-spinel, zircon Monazite, zircon Mineral index ratios are useful for determining provenance change and/or correlating sand bodies, which have stable or ultrastable mineral suites. They can also indicate the relative importance of different lithological provinces e.g. GZi index - garnet is primarily metamorphic; zircon primarily granitic. However, they are less suitable for unstable mineral assemblages, where stable mineral species are rare. The ATi ratio of a suite may not reflect changes in provenance if acid weathering has occurred, as apatite is unstable in these conditions (Table 2). The ATi ratio is therefore commonly used to determine the intensity of near-surface weathering rates. 3.Varietal mineral studies. Changes in the abundance and chemistry of an individual mineral species can indicate fluctuations in provenance. For study of this type, microprobe analyses of 50 individual mineral grains/sample are obtained. Detail of the methodology used is summarised in Appendix A. Garnet and tourmaline are particularly suited for this type of study as both are common, have variable provenance specific chemistries and are stable. Amphibole, epidote and pyroxene also have variable chemistry and can be used if post-depositional dissolution has been insignificant Table 4: Provenance sensitive mineral index ratios, from Morton and Hallsworth (1994)
Garnet geochemistry Garnets have provenance specific endmembers which make them a powerful tool in provenance and correlatory studies (Fig. 2, e.g., Morton, 1991; Preston et al 2002). The main disadvantage with using garnet is that although moderately stable, it does undergo dissolution during both acid weathering and deep burial (Hansley, 1987). There are four common provenance specific garnet end members: Almandine (Fe 3 Al 2 Si 3 O 12 ) - found in a wide range of rock types, including granites, pegmatites, and biotite and amphibole schists Pyrope (Mg 3 Al 2 Si 3 O 12 ) - common in ultramafic rocks, including peridotites, eclogites and kimberlites Spessartine (Mn 3 Al 2 Si 3 O 12 ) - less common than almandine and pyrope and is typically found in skarn deposits, metacherts, low- grade metasediments and gneisses Grossular (Ca 3 Al 2 Si 3 O 12 ) - characteristic of thermally and regionally metamorphosed impure limestones and metavolcanics Figure. 2: Compositional fields indicate provenance (data from Wright, 1938; Deer et al, 1997; Preston et al, 2002).
Tourmaline geochemistry Tourmaline is an ultrastable mineral and therefore ideal for determining correlating sandstones that have been significantly modified by post-depositional diagenesis(Morton, 1991). Tourmaline forms in a variety of rock types including granite, pegmatites, metasomatosed and metamorphic rocks (Krynine, 1946; Deer et al., 1997). It is a complex borosilicate mineral with eleven endmembers corresponding to variations in sodium, calcium, aluminium, lithium, magnesium, iron, manganese and chromium. Dravite, uvite, elbaite and schorl are the most common endmember species. Figure 3: Tourmaline geochemistry provenance plot, Henry and Guidotti, 1985.
5.Conclusions Heavy mineral analysis used in conjunction with light mineralogy can be used to refine understanding of provenance and intrabasinal processes. Heavy minerals are more provenance-specific than quartz or feldspar, and are therefore particularly useful in studies of arkosic or quartz-rich arenites. They may provide information on subtle provenance changes that are not reflected in the light mineralogy. Post-depositional dissolution and intrabasinal weathering can significantly modify the heavy mineral assemblage of a sandstone. Stratigraphic and spatial changes in mineral abundance within a unit may therefore be the result of differential weathering and groundwater flow, rather than a change in provenance. Hydraulic sorting may also significantly alter the heavy mineral assemblage of a sand. Varietal studies of individual mineral species and stable mineral ratios can be applied to altered sandstones to overcome these problems. Although it destroys provenance signature, the degree of heavy mineral dissolution can be useful, yielding information on weathering rates, alluvial storage, sediment recycling and maximum burial depths.
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Appendix A: Methodology Mineral separation Heavy minerals are separated using the method proposed by Morton (1985). To reduce any hydraulic sorting effects a standard very fine to fine ( m) sand fraction is separated from each sample by wet sieving. Clay coatings from around the mineral grains are removed by using an ultrasonic probe on the sample soaked in water. The cleaned sample is then air dried at 100 o C and its heavy minerals are gravity separated by passing the sand fraction through bromoform (tribromoethane); a heavy liquid. A fraction of the heavy mineral residue is mounted in Canada balsam and minerals identified using a standard polarising binocular microscope. To determine the relative abundance of heavy mineral species, between 200 and 600 grains (depending on the complexity of the assemblage) are counted per slide, using the line counting method proposed by Galehouse (1971). The slide is then scanned for rare minerals.
Mineral geochemistry Mineral geochemistry is determined using the whole grain method proposed by Morton (1985). Using a petrographic microscope, individual mineral grains are picked from the remaining loose heavy mineral fraction scattered on a glass slide. A fine brush dipped in distilled water is used to pick the grains. To avoid bias, grains are picked systematically by selecting all grains that pass beneath the crosshairs. Between 60 and 70 grains are mounted in rows on the probe slide and coated with between m of carbon to make them conductive. All geochemical analyses in the study of the Amur were acquired at the Department of Geology and Petroleum Geology, University of Aberdeen, using a Microscan MK5 electron microprobe, made by Cambridge Scientific Instruments Ltd. The system used for Energy Dispersive analysis was a Link Analytical AN10/25S and analyses were obtained and processed with the Link ZAF4/FLS program.