Mineralization, Alteration Assemblages, Geochemistry and Stable Isotopes of the Intermediate-Sulfidation Epithermal Kylo Deposit, Drake Goldfield, North-Eastern NSW, Australia: Evidence for a Significant Magmatic Fluid Component ^†

Abstract

The intermediate-sulfidation epithermal Kylo deposit is part of the Drake Goldfield of north-eastern NSW. The mineralization is gold-dominant with minor silver and significant levels of zinc, copper and lead. Kylo has a resource of 2.298 Mt @ 1.23 g/T Au and 1.35 g/T Ag. Mineralization mainly occurs in the form of vein stockworks. Petrographic and SEM analysis shows that there are at least three mineralization events, with Au mineralization strongly associated with at least a deposit-scale alteration event. Quantitative XRD analysis shows a positive correlation between Au-mineralization and argillic-phyllic alteration. Electrum was found as an inclusion in massive sphalerite in the main mineralization stage. Correlation analysis for the assay data indicates that Au has a strong relation with Ag and Pb. Petrographic and geochemical analysis has identified three lithologies: rhyolite, rhyodacite/dacite and andesite, with Au mineralization more associated with the rhyodacites, while at deeper levels some of the andesites also show a relatively strong correlation with Au. Strontium shows a significant strong depletion, due to the intense and pervasive alteration at Kylo. The andesitic volcanics show moderate LREE enrichment with small negative Eu anomalies, and relative depletion in Nb, Ta and Ti, indicating an island arc tectonic setting. The carbon and oxygen isotopes of late-stage vein carbonates suggest that the late-stage fluid was mostly derived from a magmatic source, but with a minor contribution from low-temperature fluids intimately associated with alteration processes. The sulfur isotopes indicate that the sulfide mineralization had a magmatic sulfur source.

1. Introduction

The Drake Goldfield, also known as Mount Carrington, is located in north-eastern New South Wales (NSW) Australia, ~5 km north-east of Drake Village, 44 km east of Tenterfield, and ~800 km north of Sydney (Figure 1). It contains a number of low sulfidation epithermal precious metal deposits (Figure 2), including Kylo, Strauss, Red Rock, Lady Hampden, Silver King, White Rock and White Rock North [1]. These deposits occur exclusively within the Drake Volcanics, which comprise a 60 × 20 km NW-SE trending sequence of Late Permian shallow volcanics and related epiclastics within the southern part of the New England Orogen (NEO). The first significant precious metal deposit discovered in the Drake Volcanics was the White Rock silver deposit in 1886 and its geology was first described by Andrews [2]. Subsequently, many deposits of differing styles were discovered and mined. Although being known and mined for over 100 years, there has been no detailed study on the deposits associated with the Drake Volcanics as a whole, with the only study of any detail being that of Perkins [3] on the deposits of the Red Rock Field which is one of the smallest of the fields within the larger Drake Goldfield. Recent work by White Rock Minerals Ltd. suggests that most of the economic precious metal deposits within the Drake Volcanics are centered upon a geophysical anomaly called “the Drake Quite Zone” (DQZ), interpreted to be a collapsed volcanic caldera structure. Recent estimates suggest that the Drake Goldfield contains an Indicated and Inferred Resource of 23.3 Moz of Ag and 352 Koz of Au [4]. Kylo has a resource of 2.298 Mt @ 1.23 g/T Au and 1.35 g/T Ag. Recent Honors’ projects include studies on the relationships between mineralization, grade and alteration assemblages/intensity for the White Rock deposit; on the Lady Hampden deposit; on the relationship between primary volcanic facies and mineralization and correlation between the White Rock and White Rock North deposits [5,6,7]. Additionally, as part of her PhD, Lay [8] worked on the primary Ag mineralogy for a number of the Ag-rich deposits within the Drake Goldfield.

This paper focusses on one of the largest Au-dominated deposits—Kylo. It not only identifies the links between the volcanic units and Au mineralization, but also provides a comprehensive analysis on the genesis and the evolution of the deposit. New insights into the Kylo deposit and knowledge of controls on the Au mineralization can be used strategically for proposed future mining and exploration programs of not only the Drake Goldfield, but also of other epithermal systems in the New England Orogen, and Au-rich epithermal systems in general.

2. Materials and Methods

Samples for this study were collected from selected relatively mineralized and highly mineralized drill cores based on the assay data (provided by White Rock Minerals Ltd.), in order to study gold mineralization in the high-grade and low-grade zones of the deposit. Samples from KYDD003, KYDD007, KYDD008 and KYDD015 were selected for sectioning to examine their mineralogy and associations, textures and chemistry. Pulp samples from the selected drill cores were also sampled for the alteration and geochemical study.

Mineral species were verified using a combination of a Leica microscopy, X-ray diffraction (XRD) and a scanning electron microscopy (SEM). As for the geochemistry, data were collected using laboratory X-ray fluorescence (XRF), inductively coupled plasma mass spectrometer (ICP-MS) and Portable X-ray fluorescence (pXRF). XRD, SEM, XRF and carbon & oxygen isotope were carried-out at the Mark Wainright Analytical Centre, UNSW, Sydney, NSW, the sulfur isotopes were analyzed at the University of Tasmania and the ICP-MS were analyzed at the School of Earth Sciences, University of Melbourne.

XRD was carried-out at UNSW using the PANalytical Empyrean II XRD, while the Olympus Terra 6400 pXRD was used to identify the carbonate species prior to stable isotope analysis. Data were subsequently converted using ConvX and analysed in HighScore (Plus)^TM and Siroquant^TM (v.4) commercial interpretation software [11]. Machine settings and operational procedures followed those of Burkett et al. [12]. XRF analyses were conducted using either the PW2400 WDXRF Spectrometer or the Axios Advanced WDXRF Spectrometer. For full details on the analytical procedure, see Norrish and Chappell [13]. The trace elements for the same two samples were analysed using the Agilent 7700x ICP-MS following procedures modified from Eggins et al. [14] and Kamber et al. [15]. pXRF analysis was performed using the Innov-X Delta P6000 pXRF. SEM was undertaken using a Hitachi 3400-X(I) SEM. Backscatter and secondary electron detectors were fitted which allowed for compositional and topographic imaging. The SEM was operated at an accelerating voltage of 15–20 kV, with a current setting between 30–50. Carbonate vein minerals from Kylo were collected as ground powders through the use of a tungsten-tipped micro-drill. They were then placed in small sample holders before being ground to <50 μm. After each sample was collected, the micro-drill head was thoroughly cleaned with 85% acetone solution. The procedure followed that of Markowska et al. [16]. For sulfur isotope analysis, the following procedure was undertaken: between 0.4 and 1.0 mg of finely powdered sample was weighed in tin capsules and analyzed for sulfur stable isotopes using a flash combustion isotope ratio mass spectrometry (varioPYRO) cube coupled to Isoprime100 mass spectrometer. The SO₂ produced during combustion was collected in a trap, then fed into the mass spectrometer and measured against a reference gas. Stable isotope abundances are reported in delta (δ) values as the deviations from conventional standards in parts per mil (‰). The δ³⁴S values are reported relative to the CDT (Canyon Diablo Troilite). International reference standards with known isotopic compositions were measured for instrument calibration and quality assurance after every six sample. The analytical performance of the instrumentation, drift correction and linearity performance were calculated from the repetitive analysis of these standards. Precision was 0.2‰ for isotopic measurements and 0.25‰ for elemental percentages.

3. Results and Discussion

3.1. Petrography, Alteration and Mineralisation

The host rock lithology at Kylo is dominated by crystal-lithic vitric tuffs varying in composition from andesitic to dacitic, with mineralization occurring primarily in veins (Figure 3a–c). The dacitic tuffs have a porphyritic texture with a fine-grained groundmass, while the andesitic tuffs have a hyalopilitic textured groundmass with illitized feldspars. Phenocrysts consist of abundant altered feldspar quartz, clinopyroxene and hornblende. Most of the feldspar phenocrysts have been replaced by illite and muscovite (Figure 3e), while clinopyroxene and hornblende have been replaced by chlorite and illite (Figure 3d). The lithic clast composition varies from rhyolitic to andesitic. Illitization and pyritization are pervasive throughout the host tuffs at Kylo (Figure 3d–f). Carbonate is the last phase to have formed. Hydrothermal alteration, as revealed by XRD, primarily includes silicification, chloritization, illitization and a small amount of sericitization, pyritization and carbonization (Figure 3), of which illitization and chloritization are related to the mineralization. Argillic and phyllic alteration assemblages are abundant at the top of the drill cores, while propylitic alteration is more common towards the base. Based on petrographic and laboratory XRD analysis, argillic alteration has a positive correlation with the mineralization in Kylo.

Based on microscopic textural observations, mineralization at Kylo is of a polymetallic type, with at least three hydrothermal mineralization stages. The first stage can be observed as the assemblage of early framboidal pyrite (Py1), then, later, inclusion-rich, subhedral-euhedral disseminated pyrite (Py2) with coarse-grained quartz (Q1) in the host rock. Galena (Gn1), sphalerite (Sp1) and chalcopyrite (Ccp1) all occur as minor inclusions in pyrite. The second stage is defined by the intergrowth of sphalerite (Sp2) and galena (Gn2), with the sphalerite containing exsolution of chalcopyrite blebs (Ccp2). The third stage is shown by the assemblage: chalcopyrite + pyrite + electrum + tennantite. Electrum occurs as inclusions/small blebs associated with Sp2 and Py2 (Figure 3h,i), tennantite (devoid of silver) is mostly associated with chalcopyrite. The last stage is a post-mineralization stage and comprises the assemblage: covellite + chalcocite + calcite, with late-stage covellite and chalcocite rimming/replacing stage III chalcopyrite (Figure 3i).

3.2. Assay Data Statistical Analysis

Due to the assay data not being normally distributed, Spearman’s correlation coefficients (ρ) were used for the statistical analysis, and these results are shown in Figure 4. The following analysis comprises 603 assay data from Kylo. Au has a high correlation with Ag (ρ = 0.71) and Pb (ρ = 0.71), while Ag shows a moderate correlation with Cu (ρ = 0.58). Ag also shows a strong correlation with Pb (ρ = 0.83) and Cu (ρ = 0.75), and a moderate correlation with Zn (ρ = 0.62). Pb also has a moderate correlation with Cu (ρ = 0.60) and Zn (ρ = 0.68). These correlations suggest the possibility of Au occurring as Au-Ag alloys and closely associated with Pb and Cu-bearing sulphides.

3.3. Geochemistry

3.3.1. pXRF

Due to the pervasive and intense alteration throughout Kylo, only the immobile elements (e.g., Ti, Zr, Nb and Y) were used to reveal the host rock type. The pXRF results show a classification of rhyolite, rhyodacite/dacite and andesite.

3.3.2. Whole Rock Geochemistry

Some of the whole rock geochemistry for KYDD008 and KYDD015 is plotted in Figure 5. Strontium shows a significant strong depletion, due to the intense and pervasive alteration at Kylo. The andesitic volcanics show moderate LREE enrichment with small negative Eu anomalies, and relative depletion in Nb, Ta and Ti, indicating an island arc tectonic setting [17].

3.4. Stable Isotopes

Using pXRD, two species of carbonates were identified at Kylo: magnesite and calcite. The results of stable isotopes from Kylo are listed in

Table 1. Carbon, oxygen and sulfur isotope compositions from Kylo.

Sample ID	Depth	Mineralogy	δ13CPDB (‰)	δ 13Cco2 (‰)	δ 18Osmow (‰)	δ18OH2O (‰)	δ34SCDT (‰)
KYDD003 188 A	188	Mgs, Cal	−7.51	−5.69	8.75	3.18
KYDD003 188 B	188	Cal, Mgs	−7.35	−5.53	15.88	10.31
KYDD003 194	194	Mgs, Cal	−7.07	−5.25	13.42	7.85
KYDD007 156.75	156.75	Mgs, Cal	−6.79	−4.97	10.78	5.21
KYDD007 158.7	158.7	Mgs	−7.83	-	12.07	-
Open cut	-	Sp-Py					−2.78
KYDD001 53.1–53.3	53.2	Py-Ccp					−4.58
KYDD001 58.4–58.6	58.4	Ccp-Py					−3.31
KYDD001 95.2–95.5	95.3	Ccp-Py					−6.96
KYDD008 10.5	10.5	Sp-Ccy					−2.9

Note: δ¹⁸O_SMOW = 1.03086 × δ¹⁸O_PDB+ 30.86 [19]. δ¹⁸O_water calculations use fractionation equations 1000lnα_{calcite – water} = 2.78 × 106/T²–2.89 [20]. δ¹³C_CO2 calculations use the fractionation equation: 1000lnα_{CO2 – calcite} = −2.4612 + (7.6663 × 10³/T) − (2.9880 × 10⁶/T²) [21]. T (T = 300 °C) is an estimate temperature value for the Kylo deposit that was used to calculate values, based on the exclusion of chalcopyrite in sphalerite indicating a temperature range of 200–400 °C and intermediate sulfidation deposits temperature range of 200–350 °C. Abbreviations: Cal-calcite; Mgs-magnesite; Ccp-chalcopyrite; Py-pyrite; Sp-sphalerite.

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The range of C and O compositions for the carbonates are δ¹³C_PDB from −7.83 to −6.79‰, and δ¹⁸O_smow from 8.75 to 15.88‰. The δ¹³C_PDB value is close to the mantle range, showing a deep source character, while the plot in the δ ¹⁸O–δ¹³C diagram shows the points drifting towards higher δ¹⁸O values, most likely due to a significant influence of low-temperature alteration. Carbon dioxide equilibrated with calcite from Kylo (late stage) has δ¹³C values of −5.69 to −4.97‰ (average −5.36‰), which agree well with δ¹³C of CO₂ in geothermal systems from New Zealand (−9.1 to −3.2‰; [22]), and are also consistent with carbon sourced from global volcanic CO₂ (−10 to 2‰; [23]). Due to the relatively narrow range of carbon isotopic composition of atmospheric CO₂ (−8 to −6‰; [23]), we suggest a dominantly magmatic origin for the carbon at Kylo. The calculated δ¹⁸O values of ore-forming fluids vary from 3.18‰ to 10.31‰, with the average value at 6.63‰ in the late stage, which are in agreement with many previous studies which indicate that the ore-forming fluids were gradually mixed with meteoric water during mineralization (e.g., [24]). The meteoric water is not only low in its δ¹⁸O value, but low in its δD value as well (~−120%). If the ore-forming fluids were mixed with meteoric water, a shift towards lower δD values should be observed. Thus, further works on the hydrogen isotope compositions are required for a more complete understanding, and these would have to be done on the associated silicate minerals. In this study, sulfides from Kylo have δ³⁴S values between −6.96 to −2.78‰, with the majority close to 0‰, similar to previous studies on epithermal deposits worldwide (−6~5‰; [23,25]). These sulfides have a relatively narrow range of δ³⁴S compositions, suggesting that they formed under stable physical and chemical conditions and were derived from a relatively homogeneous source [25]. The δ³⁴S values for Kylo suggest that the sulfur has a magmatic origin or has been leached from the host volcanic sequence (~1‰, [26]).

4. Conclusions

Kylo is host to an intermediate sulfidation epithermal deposit with a strong association between Au-mineralization and argillic alteration events. At least three mineralization events, and multiple alteration events, are evident at Kylo, and there is a clear relationship between the mineralization and argillic alteration. Rhyolitic, dacitic and andesitic vitric-lithic crystal tuffs were widely developed in Kylo, with late-stage hydrothermal fluids being largely magmatic in origin, but influenced by low-temperature fluids associated with the alteration processes. In contrast, the sulfur in the sulfides had a magmatic source.