Bendigo Gold Field Research Articles

Structural Constraints and Numerical Simulation of Strain Localization in the Bendigo Goldfield, Victoria, Australia

Strain localization and fluid flow patterns adjacent to the massive quartz-hosted reefs of the Bendigo gold-field have been investigated using new structural observations, three-dimensional geometry of fold-fault relationships, and gold distribution. Quartz reef and vein formation in the Bendigo goldfield is associated with an initial E-W compression event that produced a penetrative upright cleavage and N-S–trending fold hinges. In the immediate hanging wall of quartz reefs with high gold grades, the S1 cleavage is locally deformed by an E-W–trending S2 cleavage that is attributed to a low-strain N-S–oriented compressive event. Further evidence of this low-strain event is provided by gently S plunging quartz-carbonate slickenline lineations on S1 cleavage surfaces and the reef margin surfaces and visible gold within the outer portions of the reef. The reefs are commonly situated on or immediately to the south of a left-flex (sinistral) change in strike of the axial surface of the folds, and the flexure occurs close to an anticlinal culmination defined by the double-plunging N-S–striking folds. The folds become deformed with meter to decimeter wavelengths, which generally affect the stiff units, in a disharmonic style. Accompanying this is a reactivation of reef margins by oblique slip and the formation of dilational sites and tensional vein arrays that provide the conduits for fluid flow and gold deposition. Numerical models that best simulate the observed distribution of strain, based upon the location of quartz reefs, involve NW-SE–oriented contraction with a significant sinistral strike-slip component. This contraction event has an overall N-S–sense orientation and is therefore consistent with deformation features in the immediate hanging wall of quartz reefs. The models also demonstrate a geometrical dependence on the localization of strain and, thus, the potential localization of fluid flow. Volume and shear strain increase with the flex angle, and variations in the magnitude of strain are also associated with variations in the strike angle of the flexure. This analysis highlights that the modification of preexisting fold structures and quartz veins by a low-strain event may enhance the localization of dilation sites within individual quartz reefs where there are subtle changes in the orientation of the reef margins. Such numerical modeling can also improve ranking of exploration targets based on fold geometry and provide a better understanding of fluid flow in hydrothermal mineral systems.

Timing of gold mineralisation in the western Lachlan Orogen, SE Australia: A critical overview

The western sub-province of the Paleozoic Lachlan Orogen in Victoria is dominated by thick turbidite sequences overlying Cambrian basement volcanics. The region was subjected to multiple Cambrian to Late Devonian regional deformation events, followed by extensive post-tectonic granitic magmatism. The western Lachlan Orogen is considered a typical ‘orogenic’ gold province and hosts a large number of goldfields, including the world-class Bendigo–Ballarat goldfields. A variety of geochronological methods (e.g. U–Pb zircon; Re–Os sulfide; 40Ar/39Ar whole-rock, mica) have been used to constrain the timing of gold mineralisation, and the relationship to metamorphism/deformation/magmatism. Regional granitic magmatism is relatively well constrained from U–Pb zircon dating, with the timing of deformation/metamorphism and gold mineralisation reliant largely on 40Ar/39Ar dating results. Owing to inconsistencies in the available 40Ar/39Ar data and recent revisions to 40Ar/39Ar monitor ages and decay constants, we recalculate and re-evaluate all existing 40Ar/39Ar age results. These revisions confirm that the western Lachlan Orogen is characterised by multiple deformation/metamorphism events, with the Stawell structural zone deformed during the ca 500 Ma Delamarian and ca 445 Ma Benambran orogenies, the Bendigo Zone deformed during the Benambran orogeny (with minor Tabberabberan overprinting), and the Melbourne Zone affected by the ca 380–370 Ma Tabberabberan orogeny. Post-tectonic granitic magmatism occurred in two main time intervals, the Early Devonian (ca 400 Ma) and the Late Devonian (ca 380–370 Ma), with the former limited to the Stawell and northwest Bendigo Zones, and the latter distributed throughout the Bendigo and Melbourne Zones and southeast Stawell Zone. Gold mineralisation occurred in two main episodes at ca 445 Ma and ca 380–370 Ma, with another possible (minor) event at ca 410–400 Ma. The ca 445 Ma event is prevalent across the Stawell and Bendigo Zones, with Late Devonian gold mineralisation restricted to the Melbourne and eastern Bendigo Zones. The timing of the two main events is supported by geological constraints, the reproducibility of 40Ar/39Ar results and, in the case of the Bendigo goldfield, coincidence with Re–Os data. Suggestions of a single Devonian age gold mineralisation event are not supported by the available data. The two main gold mineralisation episodes (ca 445 Ma; ca 380–370 Ma) coincide with the waning stages of the Benambran and Tabberabberan orogenies, respectively. Crustal thickening and consequent metamorphic devolatilisation during the Benambran orogeny may have been the main cause of fluid flow related to gold mineralisation at ca 445 Ma. In contrast, crustal anatexis is considered responsible for metamorphic fluid generation and Early Devonian gold mineralisation.

DEPLETION AND ENRICHMENT ZONES IN THE BENDIGO GOLD FIELD: A POSSIBLE SOURCE OF GOLD AND IMPLICATIONS FOR EXPLORATION

A regional lithogeochemical survey in the Bendigo region of central Victoria, Australia, covering an area of 3,750 km2 and including the giant Bendigo gold field and other large regional gold fields such as Maldon, Castlemaine, and Fosterville, was completed in 2004 by Ionex Pty Ltd. A total of 142 rock samples were collected and analyzed for gold and associated elements. Extensive areas having an anomalously low gold content (termed “depletion zones”) were detected in the vicinity of known gold fields and constitute a large proportion of the surveyed area. The results show that the gold content in the depletion zones is 70 percent below regional background levels. The Bendigo gold field itself is located within a positive gold anomaly (enrichment zone) with an average value of 14 ppb in all rock types and an extent of approx. 100 km2. To the north is a depletion zone on the order of 700 to 800 km2, with an average gold content of less than 0.5 ppb in all rock types. In total, the gold enrichment and depletion zones occupy an area of more than 1,000 km2 against a regional background of 0.7 to 4.7 ppb Au (avg 1.29 ppb). If this depletion zone extends to a depth of 2 km, the volume of depleted rock would be at least 1,400 km3 and the extent of apparent depletion would be 3,000 to 4,000 tonnes (t) of gold. The quantum of apparent gold depletion at Bendigo is on the same order of magnitude as the gold enrichment in the Bendigo gold field and associated dispersion haloes (3,000 t Au). Substantial depletion zones are also present adjacent to the other important gold deposits within the survey area. The presence of extensive gold depletion zones adjacent to the major gold deposits of central Victoria raises the possibility that these rocks were a source of gold in the deposits. These findings provide possible new criteria for the appraisal of prospective regions.

Numerical models of gold‐deposit formation in the Bendigo‐Ballarat Zone, Victoria

We present coupled mechanical, fluid‐flow and chemical numerical models that place constraints on some of the processes that formed gold deposits in the Bendigo‐Ballarat Zone of Victoria, Australia, and specifically the Bendigo goldfield. Although many goldfields in this region are located close to major intrazone faults, these major faults are rarely mineralised, and such goldfields are commonly located in the hangingwall of the faults, 5–10 km from the fault plane. Individual gold deposits are hosted within quartz veins associated with reverse faults and chevron folds. Regional‐scale models are aimed at determining if the intrazone faults were more or less permeable than the surrounding host rocks, if these faults acted as conduits to supply fluids to goldfields, if the presence of a blind fault below the Bendigo goldfield was necessary to deliver fluids to the area, and if the upper units of the Castlemaine Supergroup (Darriwilian and Yapeenian) acted as a low permeability cap to facilitate gold and quartz precipitation in the lower units (Castlemainian to Lancefieldian).The models indicate that the combination of permeable intrazone faults, and relatively low permeability units at the top of the Castlemaine Supergroup, allows the greatest fluid flux to occur in the area of the Bendigo goldfield. The presence of a blind fault beneath the Bendigo goldfield is not critical to the supply of fluids to this area. Small‐scale models of a chevron fold with a permeable fault simulate fluid pumping and suggest that fluids transported along the fault mixed with those in the host rocks. Fluids are expelled from, and drawn back into, the fault in a cyclical manner and are controlled by deformation‐induced changes in volume. Intermittent high fluid pressures in the host rocks relative to the fault cause the rocks to yield in tension, simulating hydrofracturing and the formation of quartz veins associated with gold‐bearing reverse faults. Models coupling deformation, fluid flow and chemical reactions demonstrate that gold is precipitated within the fault region when CH4 and H2S are sourced at depth and CO2 is transported along the fault. In the models presented, gold precipitation is strongly controlled by H2S, although the effects of fluids interacting with graphite have not been modelled. Rates of gold mineralisation of 0.09–0.023 g/t over 1 million years suggest that the permeability of the fault was at least two orders of magnitude higher than the permeability modelled here.

Stable Isotope (O, H, S, C, and N) Systematics of Quartz Vein Systems in the Turbidite-Hosted Central and North Deborah Gold Deposits of the Bendigo Gold Field, Central Victoria, Australia: Constraints on the Origin of Ore-Forming Fluids

Stable Isotope (O, H, S, C, and N) Systematics of Quartz Vein Systems in the Turbidite-Hosted Central and North Deborah Gold Deposits of the Bendigo Gold Field, Central Victoria, Australia: Constraints on the Origin of Ore-Forming Fluids

Stable Isotope (O, H, S, C, and N) Systematics of Quartz Vein Systems in the Turbidite-Hosted Central and North Deborah Gold Deposits of the Bendigo Gold Field, Central Victoria, Australia: Constraints on the Origin of Ore-Forming Fluids

The Central and North Deborah gold deposits are located in one of 15 mining centers of the Bendigo gold field, central Victoria, Australia, a major world-class gold province developed in a turbidite terrane. Gold mineralization occurs primarily within domes and reverse-fault systems in multiply deformed Lower Ordovician metaturbidites. Deformation and thrusting occurred between 450 and 420 Ma coevally with regional metamorphism at zeolite to greenschist facies. In the Central and North Deborah deposits six stages of quartz veining, each with characteristic geometry and mineral paragenesis, developed sequentially: stages 1, 2, and 4 are gold-bearing, whereas stages 3, 5, and 6 are barren. Gold-related quartz veins were deposited at temperatures of 350° ± 25°C based on oxygen isotope geothermometry of silicate phases (quartz-muscovite) and sulfur isotope compositions of coexisting sulfide mineral pairs (sphalerite-galena). This result is consistent with temperatures inferred from fluid inclusions. The δ 18O values of gold-bearing vein quartz and coexisting hydrothermal muscovite are uniform within and between the six vein stages ranging from 15.9 to 17.2 per mil and 11.6 to 12.8 per mil, respectively. The δ D values of hydrothermal muscovites are also uniform at –68 to –55 per mil. Ore fluids have calculated δ 18O values of 8 to 11 per mil and δ D values of –37 to –17 per mil. Pyrite of synsedimentary-diagenetic origin in slate and sandstone displays sulfur isotope compositions of –23.5 to –8.0 per mil and –0.1 to +11.9 per mil, respectively. Sulfides from gold-bearing quartz veins have a range of –7.4 to +8.1 per mil (median ca. +2.5‰). The relatively uniform vein pyrites indicate a large, uniform sulfur source, most likely from desulfidation reactions of sulfide minerals; interaction of the ore-forming fluid with depleted sulfides at the site of deposition may have shifted the sulfur isotope signatures of source sulfides to more negative values. Carbon isotope compositions of carbonates range from –5.1 to –9.0 per mil (median ca. –6.5‰). Nitrogen concentrations and nitrogen isotope compositions of hydrothermal muscovite from quartz veins are 652 to 895 ppm and 2.84 to 4.49 per mil, respectively. Collectively, the data define a band in N content vegh rsus δ 15N coordinate space from higher N and lower δ 15N in the Phanerozoic metasedimenary-hosted Central and North Deborah gold deposits to lower N and higher δ 15N in predominantly meta-igneous-hosted Neoarchean gold provinces. Collectively, the H, O, S, C, and N isotope compositions of ore-related hydrothermal minerals indicate that the dilute, aqueous carbonic and N-bearing ore-forming fluids were generated from metamorphic dehydration reactions at deeper crustal levels and were variably influenced by exchange with wall rocks during transport to the site of deposition. This genetic model for the Central and North Deborah gold deposits is consistent with other turbidite-hosted lode gold deposits and counterparts in volcanic-plutonic terranes. In contrast, these data are not consistent with either mantle-derived fluids or magmatic fluids, nor do they support a deeply circulated meteoric surface water model for vein formation.

A Fluid Inclusion Study of Au-Bearing Quartz Vein Systems inthe Central and North Deborah Deposits of the Bendigo Gold Field, CentralVictoria, Australia

The Central and North Deborah gold deposits, with a production of about 7 tons Au, are located in one of 15 domes in the Bendigo gold field, central Victoria, Australia, an important mesothermal lode gold province. The gold-bearing quartz vein systems are hosted predominantly by reverse faults in Lower Ordovician turbidites, which underwent lower greenschist facies metamorphism. In the Central and North Deborah mines, there are six stages of quartz veins that developed in the following sequence: (1) laminated veins, with a metal association of Au-As-Sb-Pb-Zn-Ni; (2) spurs (Au); (3) massive barren veins (main dilation stage); (4) brecciated veins, with Au-As; (5) quartz-ankerite veinlets; and (6) late, pure quartz or calcite veinlets. Of the three mineralized vein stages (1, 2, and 4), only the first one predated or was coeval with the main deformation event, whereas stages 2 and 4 postdated the main deformation. The vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing. Fluid inclusions in vein quartz contain C-O-H fluids of variable compositions. Three main types of fluid inclusions are recognized at room temperature: type I, two-phase, primary (CH4)V + (H2O)L fluid inclusions; type II, two- or three-phase, primary-pseudosecondary (CO2)V or L + (CH4)V or L + (H2O)L fluid inclusions, including IIa, CH4-CO2-H2O, and IIb, CO2-H2O fluid inclusions; and type III, (H2O)V + (H2O)L fluid inclusions. Type III fluid inclusions are further divided into three groups: IIIa, primary-pseudosecondary, vapor-rich; IIIb, primary-pseudosecondary, liquid-rich; and IIIc, secondary, liquid-rich. Data from fluid inclusion distribution, microthermometry, and Raman spectroscopy indicate that fluids associated with Au mineralized quartz veins (stages 1, 2, and 4) have moderate salinity ranging from 0 to 12 wt percent NaCl equiv (modeled salinities around 7–8 wt % NaCl equiv). These veins formatted at temperatures from 325° to 375°C, and pressures of 200 to 300 MPa, with emplacement depths of about 8 to 12 km. Fluids associated with barren quartz veins (stages 3, 5, and 6) have a low salinity of about 0 to 3 wt percent NaCl equiv (modeled salinities about 2–3 wt % NaCl equiv) and lower temperatures. There is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce, but differing fO2 may also have influenced gold deposition (Cox et al., 1995). Hydrothermal ore-forming fluids responsible for gold mineralization are in the CH2-CO2-H2O-NaCl system, whereas those of massive barren veins (stage 3) and veinlets (stages 5 and 6) are in the CO2-H2O-NaCl and H2O-NaCl systems, respectively. The carbonic phases of type I and II fluid inclusions range from pure CH4, through mixed CH4 and CO2, to pure CO2. Gold ore-forming fluids were weakly acidic, with f O2 estimated to have been 10–25 to 10–37 bars, and generally above the CO2/CH4 and below the SO2/H2S buffer boundaries throughout gold mineralization. The relatively reduced mineralizing fluids are similar to those of other turbidite-hosted lode gold deposits and counterparts in volcanic-plutonic terranes.

A Fluid Inclusion Study of Au-Bearing Quartz Vein Systems in the Central and North Deborah Deposits of the Bendigo Gold Field, Central Victoria, Australia

A Fluid Inclusion Study of Au-Bearing Quartz Vein Systems in the Central and North Deborah Deposits of the Bendigo Gold Field, Central Victoria, Australia

Wallrock alteration in the Bendigo gold ore field, Victoria, Australia: Uses in exploration

The Bendigo goldfield, which has produced most of Victoria's gold (22 million ounces=684.3 ton), consists of numerous deposits located mainly along narrow, parallel-trending anticlinal domes separated by 100 to 400 m. The individual domes are parts of a regional anticlinorium in Lower Ordovician sedimentary rocks. Highest gold ore values are generally located in the eastern limbs and particularly in the apical areas of the smaller individual anticlinal domes and in the central part of the more regional-anticlinorium. Alteration present in metasandstone units in the Nell Gwynne anticline consists of phengitic sericite (to 15 vol%), chlorite (to 7%), carbonates (siderite, sideroplesite, ankerite and calcite, to 10%) and sulphides (arsenopyrite, pyrite, chalcopyrite and sphalerite, to 5%). The alteration generally follows the trend of the structure extending upward and above mineralisation in anticlinal areas and below largely unmineralised synclinal areas. Phengitic sericite extends furthest, to at least 150 m above mineralised saddle reef positions, chlorite to 130 m, sulphides to 80 m and carbonate to 50 m. Chlorite geothermometry suggests temperatures of 260 to 290°C for the alteration. The sequence of carbonate precipitation generally is from siderite (early), sideroplesite, ankerite and calcite. The more Fe-rich carbonates tend to occur near the apical parts of the anticlinal domes. Arsenopyrite occurs within 10 m of the veins, while hydrothermal pyrite, chalcopyrite and sphalerite all extend to 50 m. Framboidal pyrite (possibly greigite) present in unaltered metasediments becomes progressively recrystallised and replaced by hydrothermal pyrite, chalcopyrite and sphalerite as mineralisation is approached. Hydrothermal pyrite is generally euhedral and anomalous in having high As contents when compared to framboidal pyrite. Maximum Au grades (205 ppm) occur in the saddle reef position and below a thick shale unit. Lower-grade mineralisation (0.5–6.0 ppm) occurs above this in the overlying stratigraphy. As and Pb anomalies cross cut the alteration zoning and occur as plume-like dispersion haloes above the mineralisation. The Au occurs in and near the As `plume'. Cu concentrations reach their maximum values adjacent to high Au values. Measured CO 2 values confirm the observed carbonate distributions. H 2O + and Zn values in the section show relatively random distributions. Two separate stages of mineralisation are indicated, the earlier localised within classic saddle reef structures and containing the highest Au, Cu and some As and a later, cross-cutting stage of mineralisation associated with high As. The alteration is mainly associated with the former.

Origin of the Bendigo saddle reefs with comments on the formation of ribbon quartz

Several textural varieties of quartz, together with ore textures and structural relations, indicate that (1) open-space filling, (2) replacement, and (3) growth by accretion were active processes in the formation of saddle reefs and related quartz bodies of the Bendigo goldfield, Australia. The relative importance of each process varied with the type of quartz body. . . . Book structure, ribbon structure, and 9crinkly banding9 probably resulted primarily from the replacement of previously sheared or foliated zones in wall rock; the individual leaves and ribbons are usually residues from the replacement process.

The structural environment of the Bendigo gold field [Victoria

The Bendigo, Vic., goldfield has produced over 16,000,000 ounces of gold, chiefly between 1851 and 1915. It is one of a number of gold-mining districts in southeastern Australia in a region underlain by a thick series of Ordovician slates and sandstones that is closely folded and intruded by Devonian (?) granitic rocks. At Bendigo gold-bearing quartz reefs occur close to the axes of several anticlines, especially on domes.The folds, faults, and quartz reefs are discussed and the conclusion reached that all three were developed during a long period of compressive stresses that affected Bendigo and a large surrounding region. The slates and sandstones yielded at first by folding but later by reverse faulting as well, especially in the axial portions of the folds. The connecting system of faults along each anticline provided an independent channelway and locus of deposition for ore solutions. Evidence is presented that compression and faulting continued during deposition of the reefs. It is believed that the reefs grew mainly by accretion. Tensional rifts roughly parallel to the axial planes of the anticlines and later than the quartz deposits are occupied by lamprophyre dikes that have been considered Tertiary but may belong to the Devonian (?) period of folding, intrusion, and mineralization.

Report on the Bendigo Gold-Field. By E. J. Dunn, F.G.S. Folio, pp. 20, with numerous Plans and Sections. (Melbourne, 1893.)

Report on the Bendigo Gold-Field. By E. J. Dunn, F.G.S. Folio, pp. 20, with numerous Plans and Sections. (Melbourne, 1893.)

II.—The Bendigo Gold-Field Registry: comprising Introduction for the Year, Comparison of Yields, Digest of Dividends, Table of Depths, Progress and Present Position of the Chief Reefs, and Lists of Companies. Terms, etc. By J. Neill Macartney. With Plans, by H. B. Nicholas, Also notes on the Bendigo Gold-field (Illustrated), by W. Nicholas; and a description of Fryer's Creek Claims, with

II.—The Bendigo Gold-Field Registry: comprising Introduction for the Year, Comparison of Yields, Digest of Dividends, Table of Depths, Progress and Present Position of the Chief Reefs, and Lists of Companies. Terms, etc. By J. Neill Macartney. With Plans, by H. B. Nicholas, Also notes on the Bendigo Gold-field (Illustrated), by W. Nicholas; and a description of Fryer's Creek Claims, with