The mineral factory: how to build a giant quartz reef
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The mineral factory: how to build a giant quartz reef
Lisa Tannock* Marco Herwegh Alfons Berger
UNSW Minerals and Energy Institute of Geological Sciences Institute of Geological Sciences
Resources Engineering University of Bern, Switzerland University of Bern, Switzerland
Sydney, NSW 2052, Australia marco.herwegh@geo.unibe.ch alfons.berger@geo.unibe.ch
l.tannock@student.unsw.edu.au
Klaus Regenauer-Lieb
UNSW Minerals and Energy
Resources Engineering
Sydney, NSW 2052, Australia
klaus@unsw.edu.au
Using this information, a set of conditional criteria and a ‘quartz
reef window’ is established, which gives important information
SUMMARY for targeting the lode and accessory minerals.
Giant quartz reefs can develop as part of a larger scale
GEOLOGICAL SETTING
‘mineral factory’ within a specific space and time, due to
a series of interlinking processes and mechanisms.
Ascertaining the criteria for giant quartz reef formation is Within a Caledonian folded Proterozoic-Silurian basement, the
based on data obtained from multidisciplinary, multi-scale Heyuan fault formed as a result of widespread extension within
analysis. The results show that a ‘quartz reef window’ is the South China block during the late Mesozoic, along with a
met when the following conditions are optimal (i) series of NNE and NEE-striking faults (Ruoxin et al., 1995).
accommodation space; (ii) permeability; (iii) significant Significant magmatism accompanied this period of extension
fluid supply; (iv) considerable Time Integrated Fluid (Wang and Shu, 2012), producing large granite plutons,
Fluxes; (v) temperature conditions; (vi) SiO2 including the Xinfengjiang pluton (an extension of the larger
oversaturation; and (vii) cap rock/seal is present. These Fogang batholith to the west), through which the Heyuan fault
elements are key to understanding the evolution of giant transects (Chen and Talwani 1998; Qiu and Fenton 2015).
quartz reef formations, as well as identifying and targeting Overall, the strike of the fault is NE trending for ~700 km (Lee
these mineral lodes. et al., 1997) with a variable dip to the southeast of ~25-55°.
Key words: quartz reef window, hydrothermal, As a result of the fault movement during the Mesozoic to
microstructures, mineral systems. Cenozoic, numerous hot springs developed along the length of
the fault (Wang et al., 2014), which continue to discharge hot
hydrothermal fluid (24-76°C) till the present-day. This
INTRODUCTION continued hydrothermal fluid flow has resulted in the
precipitation of a giant quartz reef formation within the core of
This study focusses on a giant quartz reef in the Guangdong the Heyuan fault, which has since been uplifted, providing large
province of South China. Here an opportunity is presented in exposures along its length in which the evolution processes can
the form of significant exhumed exposures (detailing the paleo- be investigated.
fluid flow) as well as continued present-day fluid flow to the
surface from the hot springs. The reef is over 75 m wide, and at Towards the Cenozoic a change from extensional to
least 40 km long, as ascertained from the limited exposures and compressional stress regime (Tang et al., 2014), resulted in the
reconnaissance study to date. development of a series NW-orientated strike-slip faults, cross-
cutting the Heyuan fault. The largest of these faults is the
While many giant quartz reefs exist around the world, some of Shijiao-Xingang-Baitian fault; with the intersection of the
which have been investigated for their roles in mineral systems faults exhibiting increased seismicity, particularly since the
(Gandhi et al., 2000), or from a geodynamics point of view 1960’s as a result of reservoir-induced seismicity (Cheng et al.,
(Kerrich and Feng, 1992), there is little knowledge on the 2012; Qiu and Fenton, 2015).
geological conditions, formation mechanisms, and the space-
time evolution in which these giant quartz reefs form. METHOD AND RESULTS
The aim of this study is to establish the set of formation Fieldwork and Sampling
conditions which are prime for building a giant quartz reef,
based on multi-scale analysis carried out at the Heyuan fault A geological field campaign was carried out on a section of the
(South China) in comparison to other settings. Here we present Heyuan fault to ascertain the fault structure, facies and
just a snapshot of the vast data collected in this wider reconstruct the paleo-neotectonics. We collected 50 samples
multidisciplinary study which further includes geochemistry, across the fault zone, including drill core samples through the
hydrology and geomechanics. From the wide array of data core reef to a depth of ~60 m. Two of the key study sites include
analysed, here we focus on the macroscale, e.g. fracture (i) Leiyao quarry (24° 00' 56"N 115° 04' 45" E) which mines
mapping, through to microstructural analysis. the quartz reef; and (ii) a road cut at state route 205 near
Liucheng Town (23° 59' 12" N 115° 02' 05" E). The latter of
these sites transects through the structural facies of the fault
zone, providing a cross-sectional view through the host granite
AEGC 2019: From Data to Discovery – Perth, Australia 1
How to build a giant quartz reef Tannock et al.
footwall, granitic mylonite and granitic and hydrothermal plagioclase, 30 vol% K-feldspar, 5 vol% biotite), feldspar
cataclastic zones. Quartz vein frequency increases towards the alteration and seritisation, plus minor hydrothermal
quartz reef; which is observed above this location at the Leiyao minerals and extensive syn-post kinematic veining
quarry. A second, finer ultracataclastic/phyllonite zone with generations of chlorite, epidote-rich and quartz. Multiple
well-developed foliation is observed at the top of the quarry, viscous granular flow events. >90 vol% grain size
overlying the quartz reef. reduction. Increasing quartz veining towards the quartz
reef.
Macroscale Fracture Analysis 2) Hydrothermal cataclasite: exhibiting 80-85 vol% quartz,
and adularia. Fine grained, with quartz veins and
Photogrammetry was performed on multiple rock outcrops in fragments exhibiting large (~5 cm) idiomorphic quartz
the field using 3GSM’s ShapeMetriX 3D software and analysis crystals.
to characterise the faults and fractures at meso-macro scale. 3) Quartz reef: composed of >97% quartz, with minor white
Analysis provides 3D characterisation of the fractures in field mica, Cr-oxide, Haematite and locally Au-Ag metals and
outcrops (https://3gsm.at). pyrite. The internal structure shows a complex history of
fractures, healing, and neo-crystallisation cycles.
Table 1. Dynamic recrystallisation thermometry in the Multiple generations of various formations mechanisms
Heyuan fault rocks. Quartz microstructures observed are represented by different grain sizes, ranging from
across the fault zone, and their corresponding deformation large idiomorphic crystals (>1-10 mm), through
temperature ranges. nucleated growth on cataclasized grains (~40-300 µm), to
ultra-fine shear fracture abraded material (How to build a giant quartz reef Tannock et al.
Fluxes (TIFF) are required in order to transport the volumes of evident at the Heyuan fault and within the quartz reef on both
fluid necessary to precipitate the huge quartz volume. This fluid the macroscale in multiple fracture generations (Figure 1) and
can originate from several sources, as outlined below. cataclasites, and on the microscale.
Significant Fluid Supply Cap Rock/Seal
Fluid sources may come from meteoric recharge (e.g. An overlying seal or ‘cap rock’ must exist in order to trap the
Lemarchand et al., 2012), metamorphic dehydration (e.g. fluids and build the reef (Figure 2). If all other criteria except
O’Hara, 1988; Hippertt and Massucatto, 1998; Lemarchand et the seal are met, the result will be simple breakout veining.
al., 2012) and deeper crustal or mantle reactions (e.g. Kerrich
and Feng, 1992; Schaarschmidt et al., 2018). In the case of the CONCLUSIONS
Heyuan fault, there is evidence for contribution from all three
at some point in time: The Heyuan fault initiated as a normal fault in the late
i. Present-day hot spring isotopes signify higher meteoric Mesozoic, with accompanying widespread meso-microscale
input (Qiu et al., 2018) in line with the change in stress fracturing creating the necessary space and permeability to
regime and new fluid flow pathway. channel and accommodate growth of a giant quartz reef. The
ii. Deep mantle-source minerals are present and abundant hanging wall phyllonite likely contributed significant SiO2, as
(i.e. Cr) within early vein generations in the quartz reef as a consequence of feldspar dissolution and white mica
well as mineral species within the mylonite (e.g. precipitation; further resulting in a low permeability barrier or
fuchsite). ‘cap rock’, trapping the hydrothermal fluids and building the
iii. Phyllonite from the hanging wall of the quartz reef quartz reef through multiple cycles of precipitation, fracturing
appears to have undergone complete feldspar dissolution and healing events. Subsequent change to the compressional
with replacement by white mica, generating aqueous stress regime in the Cenozoic has exhumed the quartz reef and
SiO2. (e.g. Hippertt and Massucatto, 1998; Regenauer- provided a telescoped view of the mylonite beneath.
Lieb et al., 2015).
The ‘quartz reef window’ is achieved when the following
The resulting silica-rich fluid is then able to migrate to a conditions are met: (i) considerable Time-Integrated Fluid
location where conditions are suitable for precipitation. Fluxes; (ii) significant fluid supply; (iii) optimum temperature
conditions; (iv) SiO2 oversaturation to enable quartz
Optimum Temperature Conditions precipitation; (v) sufficient accommodation space to building
the quartz reef; (vi) permeability to channel the fluid flow; and
The temperature window for formation of a quartz reef appears (vii) a cap rock/seal must be present to ensure no pervasive
to be in the order of approximately 200-350°C. While the upper fluid flux.
bounds of this is more constrained (via quartz dynamic
recrystallization deformation textures and confirmed by These giant quartz reef bodies can essentially develop within a
chlorite and mica geothermometry), the lower bounds are still large-scale ‘mineral factory’, with a range of economically
to be confirmed pending fluid inclusion analysis. However, a important minerals (e.g. Au) which are hosted in the quartz
review of formations and studies at other locations (e.g. Kerrich lode. Identifying the ‘quartz reef window’ and understanding
and Feng, 1992; Lemarchand et al., 2012; Schaarschmidt et al., the parameters, can enable a deeper insight to the formation and
2018) is consistent with this guide. evolution of quartz reefs and enhance mineral targeting.
Quartz Precipitation Conditions ACKNOWLEDGEMENTS
An oversaturation of SiO2 within the fluid as a result of either This work has been carried out under the support of an
(i) pore fluid pressure drop, or (ii) chemical change (e.g. fluid Australian Government Research Training Program
mixing). A sudden drop in pressure may occur immediately Scholarship. We would like to acknowledge Jie Liu and Kerry
following a seismic fracturing event; creating low pressure pore Zhang of Sun Yat-sen University in Guangdong for facilitating
space within the local stress field, thus resulting in precipitation. the fieldwork and help with this research.
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Figure 2. Schematic illustration of the processes evolving with depth through the Heyuan fault. Brittle cataclastic deformation
progresses to ductile deformation at the frictional-to-viscous transition. A phyllonite “cap rock” develops overlying the
cataclasite, acting as seal to the silica-saturated fluids. The quartz reef continues to build under the active phyllonite shear
plane post-mylonitisation. Quartz deformation recystallisation textures indicative of temperature and strain rate (BLG:
Bulging grain recrystallisation, SGR: Subgrain rotation recystallisation, GBM: Grain boundary migration recrystallisation).
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