Spectral properties of weathered and fresh rock surfaces in the Xiemisitai metallogenic belt, NW Xinjiang, China

2.1 Geological Setting

The West Junggar region of Xinjiang, located between Altay Shan and Tianshan, extends westward to the Junggar-Balkhash system adjacent to Kazakhstan, and eastward to the Junggar Basin in China (Figure 1a) [29]. The West Junggar tectonic unit consists of several terranes such as island arcs and accretionary complexes, including, from north to south, the Zharma-Saur arc, the Tarbagatay accretionary complex, the Chingiz arc, and the West Junggar accretionary complex (Figure 1b) [30, 31].

Figure 1

(a) Simplified schematic geological map of China and adjacent areas (modified by 29); (b) Generalized geological map of the northwestern Junggar region (modified by 29): (c) Geological map of Xiemisitai area showing the location of main rock samples.

Xiemisitai Mountain, which belongs to the Chingiz arc, is one of several E–W-striking mountains in the northern part of West Junggar, and is bounded by two major faults, the northern Hong Gullah fault and the southern Xiemisitai fault [29]. In this region, Silurian island-arc-related felsic volcanic lavas and tuff, as well as granitic intrusions are widespread, and several newly discovered mineral deposits in the region are related to this magmatic event (Figure 1c). The Xiemisitai copper deposit is a volcanic-related porphyry copper deposit controlled by a caldera fracture system, which is superimposed on E–W-striking regional faults [28, 32].

2.2 Sample Analyses

Two surfaces of all lithological units were analyzed: a fresh surface obtained by sectioning the sample, and a weathered surface sampled in the field. Fresh surface analyses were conducted on rock fragments with little or no visible signs of surface alteration. The same samples were also used for thin section analyses and subsequent petrographic description. Weathered surfaces were analyzed as exposed in the field or on weathered parts of rock hand samples. Mineral assemblages of selected samples were confirmed by microscopic study, portable near-infrared mineral analyzer (BJKF-II) and X-ray fluorescence (XRF). For fresh sample surfaces, mineral composition was determined using plane-polarized and cross-polarized transmitted light and reflected light petrography with a Carl Zeiss Axio Scope A1 microscope. Maximum and minimum magnification were 50x × 10x and 2.5x × 10x, respectively. Mineral composition of the weathered surfaces was determined by BJKF-II portable near-infrared mineral analyzer (Nanjing Instrument Co., Ltd., China). BJKF-II is a portable instrument specially designed for field mineral exploration. The instrument adopts diffuse reflectance spectroscopy, scanning by grating spectrophotometer, integrating sphere detection, and microcomputer for spectral data processing to guide field mineral exploration [33].

Spectral data were acquired with a spectroradiometer (FieldSpec 4 Hi-Res, ASD, USA). The band extent varied from 350 nm to 2500 nm, and the spectral resolution was 3 nm and 8 nm at band ranges of 350–1000 nm and 1000–2500 nm, respectively. Samples were collected from the West Junggar and were measured at the laboratory in Urumqi. Spectral measurements were made using a fiber optic (FOV 25°) along the direction normal to the surface at a distance of 1 cm, which results in an approximately circular footprint of 0.45 cm on the sample surface. A100-W Reflecta halogen lamp mounted at an angle of 45° was used as an illumination source. The reflectance spectrum was obtained by calculating the ratio of the radiance of the sample to the radiance of a 99% reflective white reference panel (Spectralon, Labsphere, North Sutton, NH, USA) under the same illumination and viewing conditions. The spectrum recorded for each surface location is based on an average of 10 scans [5, 19].

Qualitative mineralogical analysis of rock surfaces was carried out using a hand-held XRF device Thermo Scientific NITON XL3t950, XRF, USA). Simultaneous analyses of up to 25 elements in the analytical range between sulphur (atomic number 16) and uranium (atomic number 92), as well as of light elements (Mg, Al, Si, P, S and Cl) were carried out. All analyses were performed in bulk mode (“mining mode”). Measement time was 120 seconds, using four different energy settings: light (Mg, Al, Si, P, S, Cl), low (Ca, K, Sc, Ti), main (K-lines from V to Ag and L-lines from Au to Pb), and high (Ag to Au). Analytical conditions included a maximum excitation source beam current of 80μA, acceleration voltage of 45 kV, and a beam diameter of 10 mm. The XRF is factory “calibrated” with a Fundamental Parameter algorithm-based program. As this software is propriety software of the Niton company, it is not possible to discern how it compensates for variable sample thicknesses, irregular-shaped surfaces or other matrix and geometric effects. In addition to the analyzed elements, the XRF software also estimates the non-measurable part of each spectrum, named “balance” (“Bal”). The balance is composed of the oxide portion of the compounds, organic matter, and – if present – carbonate and water.

2.3 Description of Sampled Materials

We paid close attention to the distribution of the samples (Table 1 and Figure 2), making sure that most of the lithological units, their weathering rinds, and rock coatings would be included. Some of the major lithological units were not considered due to difficulties in obtaining representative fresh and weathered rock samples. These limitations are due to the friable nature of sediments such as marls and conglomerates belonging to the Quaternary. The macroscopic description of mineral composition and the type of metamorphic and/or hydrothermal alteration (if present) of the lithological units selected for this study is given below.

Figure 2

Hand specimens and microscopic photographs, (a)(b): rhyolite, (c)(d): andesite, (e)(f): granite, (g)(h): Tuffaceous sandstone, (i)(j): basalt, and (k)(l): diorite; (Qtz – quartz, Pl – lagioclase, Hbl – hornblende, Tuff – tuffaceous). (b): plane–polarized light, 10–fold magnification; (f): plane–polarized light, 10–fold magnification; (l): plane–polarized light, 10–fold magnification; (d): cross–polarized light, 10–fold magnification; (h): cross–polarized light, 10–fold magnification and (j): cross – polarized light, 10–fold magnification.

Eighty rock samples were collected from bedrock outcrops. Six representative samples from the dominant lithological units in the study area include rhyolite, andesite, granite, tuffaceous sandstone, basalt and diorite. A summary of the sampled lithological units with their respective petrographic description is given in Table 1.

Rhyolite is a felsic volcanic rock, which is brick–red in the Xiemisitai area. It contains 70–80% quartz phenocrysts, 5-10% plagioclase, 10-25% glass, and is characterized by a porphyritic texture (Table 1 and Figure 1a, 1b). Feldspar and volcanic glass show signs of sericitization.

Andesite is an intermediate extrusive rock, often showing porphyritic texture. It is composed of phenocrysts (~20%), many of which contain biotite (~50%), hornblende (~20%), plagioclase (~30%), and glass (~80%) (Table 1, Figure 1c and 1d). Magnetite occurs along the borders of the phenocrysts and is altered slightly in the remainder of the samples.

The weathered surfaces of granite are largely pink–brown in color, with visible minerals beneath the surface. Under the microscope, the major mineral composition of the granite samples is identified as quartz (~40%), plagioclase (~20%), potassium feldspar (~30%), and biotite (~10%) with a distinct foliated fabric (Table 1, Figure 1e and 1f). The feldspar minerals show signs of sericitization and alteration to clay minerals, while biotites are characterized by chloritization.

Tuffaceous sandstone samples are medium -grained–poorly sorted, and are dominated by sub angular grains of feldspar (~10%), quartz (~10%), and a matrix (~80%) of volcanic glass. The tuffaceous sandstone in the Xiemisitai area is black, and can be categorized as lithic arkose tuff with cements consisting of tuffaceous material (Table 1, Figure 1g and 1h). The tuffaceous sandstone is strongly weathered, and most of the surface is covered with a layer of loess.

Basalt in the area is extremely fine-grained (<100 µm) and exhibits a uniform gray color on fresh surfaces. The basalt samples are composed of plagioclase (20–30%) and matrix (70–80%) (Table 1, Figure 1i and 1j). Minor fractures are partially filled with quartz with some minor rust staining on the edges. Weathered surfaces are characterized by a mixture of shades of gray or brown with a mesh of fine veins of quartz and calcite. Visible alteration of the interior of samples is limited.

Diorite is an intrusive igneous rock, mainly composed of quartz phenocrysts (20%), plagioclase (60-65%), and melanocratic minerals (15-20%). The diorite samples are grey to dark-grey in color with a greenish cast, but black or bluish-grey varieties also occur in the Xiemisitai area (Table 1, Figure 1k and 1l). The melanocratic minerals are altered into magnetite and chlorite.

All rock samples were analyzed for 25 elements (i.e., Ag, Al, Ba, Ca, Cd, Cr, Cs, Cu, Fe, K, Mg, Mn, Ni, Rb, S, Sb, Si, Sn, Sr, Te, Ti, V, Zn, Zr, and Bal) by XRF analysis (Table 2). The XRF analysis of weathered and fresh rock surfaces show that Si is the dominant element (>100000 ppm) followed by Al > Ca > Fe > K > Mg (>10,000 ppm). Elements with concentrations > 100 ppm are Mn > Ba > Sr > Te > Zr > Cs > V > Cu > Ni (>100 ppm). In general, element concentrations of weathered surfaces are similar to those of fresh surfaces.

3.1 Spectral Features of Rhyolite

The weathered surfaces of all rhyolitic lithologies show an overall increase in reflectance and a steep profile of the reflectance curve (Figure 3a). The diagnostic absorption features of the weathered surfaces show similar spectral behavior as the fresh surfaces, differing only slightly owing to their lower reflectance values. The spectral curve for rhyolite tends to be flat until a broad increase in absorption is visible around 1000 nm (Figure 3a). This absorption is attributed to the effect of ferrous iron and electronic transitions in discrete ions in crystal field absorption (CFA) [37, 38]. Weak absorption is visible at 480 nm, 1900 nm, and 2200 nm. Absorption by Fe ions at 480 nm produce a broad and strong peak in the absorption spectrum. The difference between fresh and weathered surfaces is the result of a weak absorption of Fe ions at 870 nm on the fresh surface. A maximum in absorption at about 1900 nm is probably due to absorption by water. Weak absorption is observable at 2200 nm, which could be caused by sericitization of plagioclase, most likely related to the presence of hydroxyl groups such as Mg–OH.

Figure 3

Weathered and fresh surface spectral curves of rhyolite (a: spectral curve; b: spectral curve with continuum removed).

The differences between fresh and weathered surfaces are primarily related to spectra brightness, and are not affected by the intensity of the absorption features. In fact, the continuum-removed spectral curves have similar characteristics. However, there is an exception in the visible region, where the fresh surfaces show a more pronounced slope indicating differences in iron oxide content between the two surfaces (Figure 3b). According to XRF analysis, the Fe contents of the weathered and fresh surfaces are 13460.43±202.85 ppm and 10699.34±180.42 ppm, respectively. It is observed that the Fe content of the weathered surfaces is 1.26 times higher than that of the fresh surface.

3.2 Spectral Features of Andesite

The andesite spectral reflectance curves exhibit significant differences in spectra brightness (the whole reflectance value is much lower than other rocks) as well as in the intensity of the absorption features between fresh and weathered surfaces. It can be observed that the weathered surface reflectivity is greater than the fresh surface reflectivity from 350 nm to 1400 nm. In contrast, the reflectivity of the fresh surfaces is larger than that of the weathered surfaces (Figure 4a). The envelope diagram shows that the surface spectral absorption and change of relative reflectance values varies significantly between weathered and fresh surfaces between 350 nm and 2500 nm (Figure 4b).

Figure 4

Weathered and fresh surface spectral curves of andesite (a: spectral curve; b: the continuum removed spectral curve).

In the visible light bands, the significant absorption spectrum of the fresh surfaces exhibits an absorption edge at 530 nm, a weak absorption valley near 645 nm, and a well-defined band centered near 880 nm. The wavelength position of the chief ferric iron absorption band (near 880 nm) and the shape of the absorption edge and shoulder at 530 nm and 645 nm are most consistent with lepidocrocite and goethite [39–41].

In the short-wave infrared bands, there is a broad absorption shoulder from 1000 nm to 1950 nm with two strong absorption valleys around 1450 nm and 1940 nm. The absorption bands found near 2210 nm, 2350 nm, and 2450 nm are combinations of a ligand-OH bend and an OH stretch. Usually only two of these three bands appear. The bands appear near 2200 nm and 2300 nm when the aluminum is the bonding ligand (e.g., in dioctahedral clays). Meanwhile, Mg–OH molecules (e.g., in trioctahedral clays) give rise to absorption bands near 2300 nm and 2400 nm [42].

3.3 Spectral Features of Granite

Figure 5a shows no significant difference in spectral absorption features between weathered and fresh granite surfaces other than the slightly varying intensity of the absorption features. The spectrally characterized exterior surface was covered with a veneer of pink–brown oxidation products through which the underlying mineralogy was visible. The fresh surface appeared to be free of any alteration and the constituent minerals were readily apparent.

Figure 5

Weathered and fresh surface spectral curves of granite (a: spectral curve; b: spectral curve with continuum removed).

The granite sample differs from the others by a lower abundance of iron-bearing minerals. According to XRF analysis, the Fe content of the weathered and fresh surfaces is 34060.01±433.02 ppm and 37310.42±467.42 ppm, respectively. This is also revealed in the absence of detectable ferric iron absorption features in the spectrum of the weathered surface (Figure 5). The broad reflectance decline towards shorter wavelengths in the exterior spectrum is probably attributable to a series of charge transfers involving in a number of atomic species [42]. The inflection at 590 nm may be due to a very small amount of ferric iron, which might have derived from weathering of the biotite. While quartz is spectrally featureless, feldspar often shows a broad and weak absorption band near 1250 nm which is not evident in the weathered surfaces sample spectrum [43]. Asymmetric hydroxyl absorptions are present around at 1400 nm and 1900 nm in both spectra. Their shape is due to multiple hydrated mineral species at a range of 2205 nm, 2350 nm, and 2450 nm.

3.4 Spectral Features of Tuffaceous Sandstone

The overall shape of the reflectance curves indicates that spectral differences for this rock type were found only in the intensity of the absorption features, which are more pronounced in the weathered spectrum than in the fresh one. This could be attributed to a higher content of the iron oxides (as a result of the weathering process) and/or hydrates on the weathered surfaces (hematite or goethite) (Figure 6).

Figure 6

Weathered and fresh surface spectral curves of tuffaceous sandstone (a: spectral curve; b: spectral curve with continuum removed).

The absorption spectra of iron ions at 460 nm are in the wavelength range of visible light near the infrared region (Figure 6b). Water molecules and ferric iron together result in the formation of a new compound visible from 680 nm to 1300 nm, which is the possible reason for the slight right shift of the absorption valley. Spectral bandwidth is narrow and strong at 1400–1900 nm near a strong absorption. However, as a result of the effect of water, the two spectral bands do not have significance for mineral recognition. In the 2000–2500 nm region, the reflectance change is lower and the two spectra tend to be similar to each other. Nevertheless, the short-wave bands at 2200 nm and 2350 nm have different spectral absorption characteristics, caused by hydroxyl, CO32−, and composite spectra among which the carbonate ion absorption bands are relatively stronger [6, 44]. Meanwhile, the strong absorption at 2350 nm in CO32− is due to a higher content of sheet silicates on the fresh rock surfaces [16, 42]

3.5 Spectral Features of Basalt

The reflectance spectrum of weathered basalt surfaces is characterized by ferric iron absorption bands visible as an abrupt rise in reflectance near 530 nm, and two broad absorption bands near 730 nm and 920 nm. At longer wavelengths, an absorption feature is present near 1090 nm and there is an additional broad, intricate absorption feature between 1900 nm and 2500 nm. The reflectance shows maxima at 2250 nm and 2350 nm, which are followed by a reflectance decrease toward longer wavelengths with a shoulder at 2200 nm, and minima at 1910 nm, 1990 nm, and 2450 nm (Figure 7).

Figure 7

Weathered and fresh surface spectral curves of basalt (a: spectral curve; b: spectral curve with continuum removed).

The ferric iron absorption bands are approximately focused at 480 nm, 730 nm, 920 nm, and 1090 nm. However, their position cannot be precisely determined. The broadness of this feature is consistent with multiple, hydrated ferric iron phases such as goethite, feroxyhyte, and ferrihydrite [44, 45]. Strong ferric iron absorption bands are expected for weathering products of iron-rich materials such as basalt. The weathered surface spectrum is not as strongly affected by ferric iron absorption bands as the fresh spectrum. Relatively, a broad band is centered near 1090 nm, which shows the presence of ferrous rather than ferric iron in the fresh spectrum [37, 46].

At longer wavelengths, the fresh and weathered surface spectra exhibit similar spectral features. All of these bands can be due to various hydrated mineral phases. The bands are relatively more intense in the weathered surface spectrum. The 1900 nm band is broad and shows a complex shape, which is consistent with multiple hydrated species and/or water adsorbed onto the surface of samples from a number of sites. Structural OH is probably present in only one or two sites, which may contribute to the sharpness of the 1400 nm feature. There is also a contribution of adsorbed water to this band; however, the relative abundances of the two types of water (bound water and structural water) cannot be determined. The 2200–2500 nm regions show strong absorption with discrete absorption features at 2200 nm, 2250 nm, 2350 nm, and 2450 nm, which are characteristics for Al–OH and Mg–OH. Both Al and Mg are present in basalts, and consequently this type of spectral behavior is expected in the weathered surface spectra. Although precise spectral matching is not feasible, mineral phases such as gibbsite, chlorite, and serpentine provide the best spectral matches to these bands. Overlaps and interferences of absorption bands occur in multicomponent mineral mixtures [47], which makes it difficult to determine the relative contributions of each phase.

3.6 Spectral Features of Diorite

The diorite spectral reflectance curves show that the reflectivity of fresh surfaces is greater than that of weathered surface from 350 nm to 990 nm. In contrast, the reflectivity of fresh surfaces is larger than that of weathered surface from 990 nm to 2500 nm (Figure 8a).

Figure 8

Weathered and fresh surface spectral curves of diorite (a: spectral curve; b: spectral curve with continuum removed).

As observed from the continuum removal diagram (Figure 8b), there is an absorption edge near 390 nm, and the weathered surfaces have lower reflectance values than the fresh surfaces. A broad and symmetric band that appears in the 1000 nm region could be caused by Fe²⁺ –Fe³⁺crystal field transitions on the weathered and fresh surfaces [48]. Broad hydroxyl bands are present near 1400 nm and 1900 nm. These bands are considerably stronger and broader than those of biotite and cordierite, suggesting the possibility of an optically significant weathered zone. A local reflectance peak is present at 2200 nm, followed by a decrease in reflectance with absorption features superimposed at 2250 nm, 2300 nm, 2380 nm, and 2450 nm. Correlation of these bands with specific minerals is hampered by the observation that different samples of the same mineral can have absorption bands at different wavelength positions and of different relative strengths [36, 37, 49–51]. The absorption bands in the sample spectrum appear to be caused by weathering products that consist of both Al–OH and Mg–OH absorption bands. This is supported by the observation that the lattice -OH bands are not very distinct and a certain degree of disorder must be present in weathered materials.

As the differences between fresh and weathered surfaces are mainly related to spectra brightness and do not affect the intensity of the absorption features, the continuum removal curves for both weathered and fresh surfaces are similar. The fresh surfaces present a more pronounced slope, indicating the differences in iron oxide/hydroxide content between both surfaces.