1. Introduction
Studies of hydrocarbons, including hydrocarbon bearing fluid inclusions, trapped in igneous and metamorphic rocks conclude that they can be either abiogenic or biogenic in origin. Reference [
1] reported on several occurrences of hydrocarbons considered to be abiogenic in origin, for example, (a) hydrocarbon bearing inclusions in the Ilimaussaq igneous complex of southern Greenland [
2]; (b) gas seeps in the Zambales Ophiolite, Philippines [
3]; (c) hydrocarbon inclusions in pegmatite quartz in granite at Strange Lake, Canada [
4]; (d) methane-bearing fluid inclusions in igneous rocks of the Kola igneous province [
5] and (e) hydrocarbon bearing fractures in crystalline rocks of the Canadian shield [
6].
In contrast, the widespread occurrences of biogenic hydrocarbons in fractured granitic and other crystalline basement rocks are well documented in the literature [
1,
7,
8,
9]. Globally, productive oilfields hosted by fractured crystalline basement rocks (including granites) occur, for example, in Europe, North and South America, China, Eastern Siberia and Africa [
7,
9]. The petroleum system components of basement reservoirs comprising granites or metamorphic rocks are, according to [
9], no different to those of conventional clastic reservoirs. The basement trap is typically a four-way dip closed structure, e.g., a fault block or the trap can be formed as the flank of a tilted fault block. Source rocks occur proximal to the trap as onlapping or capping successions. The reservoir’s poro-perm system comprises a network of faults and fractures. Basement reservoirs have geological histories that reflect fracture formation that resulted from tectonic, hydrothermal and epithermal processes. The Lancaster Lewisian basement reservoir, west of Shetlands, provides an excellent example of oil bearing fracture networks and their spatial relationship major fault zones [
9].
Granite plutons are thermal foci and are often highly fractured and therefore are hosts to the migration of water and biogenic hydrocarbon fluids from outside of the plutons [
7]. For example, hydrocarbons occur in several British granites and according to [
7], they can be attributed to biogenic sedimentary source rocks and are related to hydrothermal mineralised veins. Thomson et al. [
10] also attributed the source of hydrocarbon fluids, associated with granite related epithermal polymetallic mineral veins, to be from overlying biogenic sediments. Here, we report the discovery and composition of aqueous and hydrocarbon bearing fluid inclusions in quartz-amethyst from the Green Ridge Breccia in the Snoqualmie Granite, Washington State, USA (
Figure 1).
Amethyst is the purple variety of quartz. The role of trace amounts of iron and radiation in the formation of its colour are generally established [
11,
12]. It is found in a variety of geological settings, e.g., epithermal veins, miarolitic cavities in granites and geodes in basaltic lavas. Gem quality amethyst, e.g., from Brazil and Uruguay come from geodes in basaltic rocks of the Parana Continental Flood Basalt Province [
12]. Published crystallisation temperatures of amethyst cover a wide range, e.g., (1) 370 to 420 °C from fluid inclusion studies of Brazilian geodes [
13]; (2) <50 °C fluid inclusion data from Brazilian amethyst [
14]; (3) stable oxygen isotope data from amethyst and agate hosted in the Prana–Etendka volcanics [
14] and in Devonian and Tertiary volcanics of Scotland [
15,
16] indicate temperatures <150 °C; (4) Granite hosted amethyst veins from Thunder Bay, Canada, are considered to have formed between 40 and 90 °C [
17]; (5) amethysts from epithermal polymetallic veins have formation temperatures ranging between 150 and 250 °C based on fluid inclusion studies [
18,
19,
20] and (6) higher fluid-inclusion homogenization temperatures (280–400 °C) were reported from amethyst in granite miaroles of the Eonyang deposit, South Korea [
21]. Finally, such high temperatures are also used in laboratory-based hydrothermal amethyst synthesis [
18].
We use a combination of fluid inclusion petrography, UV Microscopy, Microthermometry, Laser Raman Microspectroscopy and Gas Chromatography-Mass Spectrometry to investigate the source of the hydrocarbons trapped after crystallisation of the quartz–amethyst euhedra in contrast to the aqueous fluids trapped during crystallisation. The latter are used to constrain the temperature of crystallisation of the amethyst.
2. Geological Background
The Green Ridge breccia occurs in the Snoqualmie Batholith, the largest of all the granite plutons emplaced in the North Cascades occupying an area of ~700 km
2. It occurs along Snoqualmie Pass [
23] and forms a number of topographic highs including Granite Mountain [
24]—
Figure 1. The predominant lithology is a medium grained granodiorite, however, rock compositions range from gabbro to alaskite [
25]. These were emplaced at ~17–24 Ma [
26] into the Denny, Guye, Mt. Catherine Tuff, Naches and Denny Mountain formations [
23]. Later, however, Grant [
27] ranked the Guye Formation as the Guye Sedimentary Member of the Naches Formation. Interbedded leaf-bearing carbonaceous argillites characterise this member. The leaf fossils indicate an Eocene age for the unit and according to [
26] the redefined Naches Formation is considered to be middle Eocene to early Oligocene in age. The contact between the batholith and the country rock is sharp. The country rocks display extensive folding and faulting that are pre-, syn- and post-granite emplacement [
24,
25]. The batholith produced an aureole <800 m wide with pyroxene-hornfels rocks developed at the contact; Erikson [
25] concluded that the batholith was emplaced at a maximum depth of ~5 km. Sulphide mineralization occurs throughout the Snoqualmie batholith and zones of shearing, brecciation, and veining are characterized by copper porphyry-type deposits and have been described by [
28,
29,
30,
31,
32]. Subsequently, during the mid to late Miocene (~10–8 Ma), episodes of faulting, uplift and erosion occurred. Several kilometers of roof rocks were eroded exposing the Snoqualmie batholith and its country rocks [
24,
33].
The Green Ridge Breccia is but one of a suite of breccias that occur in the Green Ridge Lake area (
Figure 1; [
32]). These breccias formed during the end stages of batholith emplacement and are associated with granite related sulphide mineralization noted above. McBirney [
32] records the spatial relationship, in the Green Ridge area, between brecciation and a NW trending shear zone cutting the granite. These authors note that the quartz lined vugs in the breccias maybe indicative of hydrothermal activity. The Green Ridge Breccia is composed of angular blocks of granodiorite that range up to one metre in longest dimension. MacDonald et al. [
34] describe the field setting of the Green Ridge Breccia noting that the breccia blocks have not been rotated (unlike other breccias in the region of Green Ridge Lake) and concluded that it is similar to a collapse breccia or shatter pipe. Vugs containing exceptional quartz-amethyst crystals cement the granite blocks [
35]. The quartz–amethyst euhedra host a range of fluid inclusion types including newly discovered centimetric scale amber coloured oil inclusions (
Figure 2).
3. Analytical Techniques
Five samples of quartz-amethyst euhedra were supplied by Cascade Scepters, Maple Valley, WA. The following describes the analytical techniques used to study the range of fluid inclusions encountered in the euhedra.
(1)
Fluid inclusion petrography of five doubly polished wafers (~150 μm thick) of the quartz amethyst crystals was performed using a Nikon Eclipse E200 transmitted light microscope. This revealed the presence of three principal types, i.e., Type 1 (aqueous) and two (Type 2 and 3) hydrocarbon bearing fluid inclusions (
Figure 2 and
Figure 3).
(2) Ultra Violet (UV) Light Microscopy using a Nikon Eclipse E200 microscope with an epi-fluorescence attachment was used to record the fluorescence emission colours displayed by the hydrocarbon bearing fluid inclusions. The UV excitation was approximately 365 nm and was provided by a high-pressure mercury lamp. A 420 nm emission filter was also used.
(3)
Microthermometric analysis was carried out using a Linkam THMSG 600 heating and freezing stage, mounted on an Olympus BX51 transmitted light microscope. Calibration of the stage was performed following the method outlined by MacDonald et al. [
34] using synthetic fluid inclusion standards (pure CO
2 and H
2O). Precision is ±0.5 °C at 300 °C and ±0.2 °C at −56.6 °C.
(4) Laser Raman Microspectroscopy (LRM) of selected aqueous fluid inclusions was performed using a Horiba LabRam II which is coupled to an Olympus BX51 transmitted light microscope. Gas and liquid phases were analysed at room temperature using a 532 nm (green) laser focused through either a 50× or 100× microscope objectives. The spatial resolution of the laser is approximately 2 μm.
(5)
Confocal Laser Raman Microscopy (CLRM) using a Witec Alpha 500 confocal laser Raman microscope in upright configuration was used to analyse and image selected hydrocarbon bearing fluid inclusions. A Toptica 785 nm laser was used with a 100 micron fibre (providing the confocal pinhole) from objective to detection system, a 600 grooves/mm diffraction grating and an Andor Idus CCD camera (Andor Technology Ltd., Belfast, Ireland). The spatial resolution of the laser was approximately 1 μm. The system was calibrated to the standard silicon peak of 520 cm
−1. Spectral resolution for all measurements was 1.5 cm
−1. Spectra were first treated for cosmic ray removal and then classified into clusters of matching spectra using k-means clustering (Witec Project 4 software version 4.1) [
36]. The individual cluster sets were further processed using background subtraction and Savitzky–Golay smoothing.
(6)
Gas Chromatograhy-Mass Spectrometry (GC-MS) was used at the School of Geosciences, Aberdeen University, UK, where the amber coloured oil was separated from the Type 3 centimetric scale fluid inclusions and analysed (
Figure 3). The sample was prepared by rinsing twice with distilled water, and again with dichloromethane (DCM). The quartz was crushed and extracted using a soxhlet apparatus for 48 h. Solid bitumen and tar samples were ultrasonicated with DCM and methanol (MeOH). All glassware was thoroughly cleaned with a 93:7 mixture of DCM/MeOH. The crushed sample was weighed, recorded and transferred into pre-extracted thimbles. The extracts were then dried using a rotary evaporator, separated into aliphatic, aromatic and polar fractions via a silica column chromatography using hexane, hexane/DCM in the ratio 3:1 and DCM/MeOH respectively. Prior to the GC-MS analysis, an internal standard (5β-Cholane, Agilent Technologies, Santa Clara, CA, USA) was added to the saturate fraction before injection into the mass spectrometer. This was done using an Agilent 6890 N gas chromatograph fitted with a J&W DB-5 phase 50 m MSD (Mass Selective Detector) and a quadruple mass spectrometer operating in SIM (Selected-Ion Monitoring) mode (dwell time 0.1 s per ion and ionisation energy 70 eV). Samples were injected manually using a split/splitless injector operating in splitless mode (purge 40 mL min
−1 for 2 min). The temperature programme for the GC oven was 80–295 °C, holding at 80 °C for 2 min, rising to 10 °C min
−1 for 8 min and then 3 °C min
−1, and finally holding the maximum temperature for 10 min
−1.
Quantitative biomarker data were obtained for isoprenoids, hopanes, steranes and diasteranes by measuring responses of these compounds on
m/
z 85, 191, 217, 218, and 259 mass chromatograms and comparing them to the response of the internal standard. Diasteranes are formed by a rearrangement of steranes during diagenesis and thermal maturation. Biodegradation causes the breakdown of steranes at a faster rate than diasteranes, including in the shallow subsurface in aerobic conditions, thus the ratio can be used as a measure of shallow biodegradation [
37]. Evidence for biodegradation was also sought in 25-norhopanes, which form near the oil–water contact [
38] and have been recorded to be abundant in organic matter from veins in basement rocks elsewhere [
39]. Thermal maturity was estimated from the 20S/20S + 20R ratio for C
29 steranes, based on the increasing proportion of the S isomer with maturation [
40]. Thermal maturity was also determined from the relative proportions of hopane peaks Ts (C
27 18α (H)-22, 30-trisnorneohopane) and Tm (C
27 17α(H)-22,30-trisnorhopane) peaks, and the C
30 βα/αβ (moretane/hopane) ratio.