Bulgera Gold, Western Australia


  • Gibb River Diamonds Limited has acquired 100% of the Bulgera Gold Project, situated in the multi-million ounce producing Plutonic Well greenstone belt of Western Australia.
  • Bulgera was last mined in 2004 by Barrick Gold of Australia.
  • Shallow gold drilling targets have been identified within this underexplored greenstone belt.
  • Bulgera is just 10km from the Marymia gold mine and only 48km by existing haul road to the currently active Plutonic Gold treatment plant where previous Bulgera production was milled. The mill at Plutonic is currently under-utilised.
  • Tenements have been recently granted, there are no third party agreements in place and no private royalties. Tenements were acquired by pegging at minimal cost.

Figure 1: Bulgera Gold Project Location Plan and Drill Targets over Satellite Imagery
Bulgera Gold Project Location Plan and Drill Targets

1.0 Introduction

Gibb River Diamonds Limited (ASX: 'GIB' or 'Company') is pleased to announce the acquisition of the Bulgera Gold Project, situated in the Plutonic Well Greenstone Belt in Western Australia. The Project lies 10km east of the Marymia gold mine and 48km via existing haul road to the operating Plutonic Gold Mine that has produced over 5 million ounces of gold since 1990.

The project was pegged by the Company at minimal cost and has no private royalty obligations. The Bulgera Project consists of two tenements (E52/3276 and E52/3316) with a combined area of 37.3 km2, both tenements were recently granted and there are no heritage or other agreements in place over the tenements.
Previous mining on the project includes the former producing pits of Bulgera, Venus, Mercuri and Price. The total production from these four pits (over two phases of mining) was reported as 440,799 tonnes @ 1.65 g/t Au for 23,398 ounces.

Figure 2: Bulgera Gold Project Satellite Imagery Showing the Bulgera, Venus, Mercuri and Price Pits.

For scale, the Bulgera Pit is approximately 240 metres long.

Regional Gold Targets 'A' and 'B': Greenfields Drilling for New Discoveries
There are multiple 'Regional Gold Targets' on GIB ground that have the potential to  generate significant new discoveries. An 8,800m drilling program (RAB or aircore) has been designed to explore these targets. (The Regional Gold Targets discussed here are separate to the 'JORC Exploration Target').
The Regional Gold Targets have been identified from a combination of geology, structure, mineralised trends, geochemistry and geophysics. After a review of the various geological maps available for the project, including regolith and outcrop geology maps, it  was  concluded that soil sampling in the area, although useful in places, should not be used to sterilise any target areas, due to the potential for transported cover.
The Regional Gold Targets have been divided into two main areas 'Regional Gold Target 1' to the northeast and 'Regional Gold Target 2' to the southwest (Figure 3).
To test these targets, a total of 220 holes are planned to a maximum of 40m downhole  depth, with a hole spacing of 40m. A total of 8,800m drilling is planned. Partners are being actively sought to fund this drilling.

Regional Gold Target A
Regional Gold Target A lies in the north-eastern area of Bulgera (Figure 3) where the greenstone belt is partially truncated by a granitic intrusive. Targets include the ultramafic unit that hosts the Bulgera deposit, numerous banded iron formations and quartz veins.

Figure 3: Bulgera Regional Target Areas and Exploration Target Areas
Bulgera Regional Target Areas and Exploration Target Areas

Undrilled gold targets lie on the Mercuri mineralised trend along strike from the Venus and Mercuri pit mineralisation. A major fault structure intersects the ultramafic and has not been tested, despite gold mineralisation being present just to the north in mafic rocks (Section A).
At the far western edge of Regional Gold Target A, the Bulgera mineralised trend ultramafic unit (that hosts the Bulgera pit ) has been thrown further north by a regional fault and has never been drilled, despite there being gold mineralisation in the mafic unit immediately to  the south (Section A).
Five drill lines have been planned to test gold targets along the Mercuri and Bulgera mineralised trends. Regional Gold Target A is a very encouraging area and warrants further drilling, the proposed drilling is outlined on Figure 3.

Regional Gold Target B
Regional Gold Target B lies in the south-western area of the Bulgera Project (Figure 3) and consists of a large undrilled area approximately 1400 by 800 metres where soil sampling is likely to have been ineffectual due to transported cover. The area sits immediately along strike from the three Mercuri deposits and covers the same prospective stratigraphy.
The proposed drilling targets gold mineralisation in flexures in the ultramafic units as they approach a regional fault. The drilling also tests on-strike extensions of the mineralised shear south west of the Mercuri and Price pits.
The south-western end of Regional Gold Target B is an area of structural complexity with the greenstone thinning as it approaches a larger regional fault that is trending northwest to southeast. This fault has offset the Bulgera area from the rest of the Plutonic Greenstone Belt. Therefore if reconstructed, a gold target lies immediately along strike between the Marymia and Bulgera deposits. The trend has been partially tested at the Wanga Prospect, but an additional line is warranted between Wanga and the tenement boundary (Figure 3).

3.0 JORC Exploration Target on Areas of Known Mineralsiation

The Company has calculated a JORC Exploration Target at Bulgera on areas of known mineralisation from existing prospects that have been previously drilled and have intersected significant mineralisation. The JORC Exploration Target is:
1.9 to 2.1 Mt @ 1.3 to 1.6 g/t Au for 80,000 to 120,000 contained ounces of Au*

*The potential quantity and grade of this exploration target is conceptual in nature, there is currently insufficient exploration completed to support a target of this size and it is uncertain whether continued exploration will result in the estimation of a JORC resource.
The JORC Exploration Target assumes the expansion of mineralisation and in some cases improvement of grade at depth or along strike at each of the nine separate prospects/deposits (Figure 3).
The JORC Exploration Target was calculated by assessing individual cross sections on 25m centres through each prospect and measuring a down dip length. Figures 3 through 8 give  an indication of how down dip length and thickness were determined and illustrate some of the proposed holes designed to test the JORC Exploration Target.

The section spacing was assigned as the strike length (section window) and the thickness was estimated using the existing drilling data and the extrapolation of mineralisation. These three dimensions allow for the estimation of a volume for each section.

Density data was available but is variable depending on your depth or position in the weathering profile. The density was therefore estimated taking into account the depth of the mineralised material. With this information a tonnage can be calculated for each cross  section which can be summed to provide the overall tonnage.
Grade was estimated in two ways, firstly a conservative visual estimation when viewing the drill section, this value was combined with the tonnage per section to calculate the contained ounces on each section. This method resulted in a global grade estimate of 1.3 g/t Au which is the lower grade limit in the Exploration Target. The second method was based on the approximate head grades from the pits during production. The Bulgera trend was assigned 1.5 g/t Au and the Mercuri trend was assigned 1.7 g/t Au. Then a weighted average of 1.64 g/t Au was calculated which was rounded down to 1.6 g/t Au and used as the upper grade limit in the Exploration Target. The exception is Rainbow South which was assigned 1,000 ounces by assuming 65m strike, 40m down dip, 4m thick, SG of 2 and grade of 1.5 g/t Au.
Some historical block models were queried against the pit shells to work out the remaining unmined mineralisation with no cut off. These models are not compliant with JORC 2012 but served as an excellent starting point for the Exploration Target.
A drill program has been designed to test this JORC Exploration Target, the  program consists of 123 drill holes for 8,545m and is shown in Figure 3. With one drilling rig, this amount of drilling could be reasonably estimated to take around forty days.

Figure 4: Cross Section – Mercuri Deposit
Cross Section – Mercuri Deposit

Figure 5: Cross Section - Bulgera Deposit
Cross Section - Bulgera Deposit

Figure 6: Cross Section – Venus Deposit
Cross Section – Venus Deposit

Figure 7: Cross Section – Venus South Prospect
Cross Section – Venus South Prospect
Refer to Appendix B for assays of intersections which are not shown on section (due to space restrictions).

Figure 8: Cross Section – Price South Prospect
Cross Section – Price South Prospect

Figure 9: Cross Section – Rainbow Ridge North Prospect
Cross Section – Rainbow Ridge North Prospect

4.0 Bulgera Project: Discovery, Exploration and Mining

The Bulgera Gold Project was discovered by Resolute Resources Limited ('Resolute')  in 1988 when rock chip samples across an outcropping ferruginous chert returned gold values of 3 to 7g/t Au. Follow-up stream sediment, soil and rock chip sampling outlined high order gold anomalies in the district including, Area 203, East Bulgera and Bulgera Creek Shear prospects. RC drilling in October-November 1988 was not successful but RAB and RC  drilling during 1988 to 1999 over the Bulgera Creek Shear anomaly outlined a 260m long zone of gold mineralisation. Resolute then consolidated the area by purchasing the nearby Marymia discovery.
RAB drilling conducted in 1993 defined strong gold mineralisation at Mercuri. Several programs of RAB, RC and diamond drilling were undertaken between 1993 and 1996 to outline a maiden mineral resource. Resolute Resources mined the Bulgera area as three open pits, the Notice of Intent to Mine (NOI) was submitted in 1996. The Bulgera  pit produced a total of 48,972 tonnes @ 1.64 g/t Au for 2,596 oz of gold. Mercuri consisting of the Mercuri and Price (Mercuri South) pits produced 85,366 tonnes @ 2.17 g/t Au for 5,966 oz of gold. The ore was trucked to the Marymia Treatment Plant, some 10km to the west. Mining was completed prior to January 1998.
In late 1998 Homestake Gold of Australia Limited ('Homestake'), the then owner of  the nearby Plutonic Gold Mine acquired all of Resolute's Marymia property and assets consolidating the entire Plutonic Well Greenstone Belt. In 2001 Homestake became Barrick Gold of Australia ('Barrick') as result of the North American merger of their parent  companies.

In 2003 Barrick conducted a review of its open pit development assets including the Bulgera area. Development drilling in 2003 consisted of 5 RC drilling programs totalling 105 holes for 4,900m including sterilisation drilling. A RAB program was drilled to the north of the existing Mercuri pit, consisting of 42 holes for 1,869m.
The drilling validated the down dip gold mineralisation in the existing pits and further proved up mineralisation at Venus, the deposit 100m to the west of Mercuri and east of Bulgera. The existing Price Pit, 200m to the south of the Mercuri pit, has potential for further gold mineralisation.

Table 1: Bulgera Gold Project: Historic Production Summary
Historic Production
Company Pit Year Tonnes Grade Ounces Au
Resolute Bulgera 1996-97 48,972 1.64 2,596
Resolute Mercuri 1996-97 74,748 2.21 5,311
Resolute Mercuri South
1996-97 10,618 1.92 655
  Initial Mining Sub Total 134,338 1.98 8,562
Barrick Bulgera Cutback* 2003-04 106,685* 1.4* 4,802**
Barrick Mercuri Cutback 2004 199,776 1.56 10,034
Barrick Venus 2004
  Cutback Phase Sub Total 306,461 1.50 14,836
  Total Mined   440,799 1.65 23,398

* This grade was not found therefore it was estimated at 1.4 g/t and the tonnage reverse calculated from the ounces** and grade. 1.4 g/t was chosen as because an in-house Barrick production model was queried and the grade was 1.42 g/t, it was reasonable that  the cutback be a slightly lower grade than the original mine, as Mercuri was.
** A Barrick divestment document from June 2009 states the total ounces produced at Bulgera/Mercuri was 23,398 oz. The ounces mined at Bulgera in phase 2 were determined by subtracting the known production at the remaining individual pits from the total production.

Table 2: Bulgera Project Historic Drilling Summary
Type of Drilling Number of Holes Number of Metres
Diamond Core 6 292
RAB/Aircore 1,097 45,184
RC 585 29,412
TOTAL 1,688 74,888


Regional Geology
The Bulgera Gold Project is situated in the northeast corner of the Plutonic Well Greenstone Belt, which forms part of the Marymia Inlier (Figure 10). The Plutonic Well Greenstone Belt is a northeast trending belt approximately 50km long and 10km wide, consisting of mafic and ultramafic volcanic rocks, fine to coarse clastic sediments, and felsic to intermediate volcanic rocks. These units generally dip towards the northwest at shallow to locally steep dips. Multiple suites of felsic to intermediate porphyries intrude the greenstone sequence.
The greenstone belt is enclosed within an envelope of granites and gneisses, is complexly deformed and has experienced multiphase metamorphism and hydrothermal activity. The structural evolution of the area includes the development of early thrusts and shear zones associated with several generations of folds in the Archaean, followed by several stages of brittle-ductile deformation, correlated with the Paleoproterozoic Capricorn Orogeny.
The gold deposits at Marymia are Late Archaean, epigenetic lode-gold deposits, which are synchronous with, or postdate by a short time, regional peak low to mid-amphibolite facies metamorphism. Gold was deposited in structures during a progressive compressional event.

Figure 10: Geology, Plutonic Well Greenstone Belt
Geology, Plutonic Well Greenstone Belt

Figure from Barrick data

Bulgera Gold Project Geology

The Bulgera deposit (Figure 3) consists of a shallow dipping sequence of amphibolites with narrow intercalated layers of ultramafic schist and metasediment. The ultramafic layers increase in frequency towards the south end of the deposit. The amphibolites are of tholeiitic and high magnesian parentage.
The Mercuri deposit also consists of a shallow dipping sequence but lithologies consist of interlayered felsic volcanics, mafic volcanics, mafic sediments and minor felsic sediments underlain by an ultramafic unit. The mafic volcanics are altered and also high magnesian.
Locally, the sequence strikes southeast to northwest and dips at 40 degrees towards the east. Outcrop is common in the area although a thin cover (<5m) of colluvium is present over the mineralised part of the deposit with some laterite appearing at the southern end. The sequence is intruded by minor semi-conformable quartz porphyries.
The Mercuri/Bulgera areas have been interpreted as a faulted extension of the K1 area (Marymia) across a system of curved thrusts, the thrust faults are shown simplified in Figure 11 where Marymia and Bulgera are offset.

Bulgera Gold Project Mineralisation
The Bulgera Gold Project mineralisation mainly occurrs in two distict sub-parallel trends, the Bulgera Trend and the Mercuri Trend (Figure 3).
Bulgera Trend Mineralisation
The Bulgera Trend is a broad mineralised shear structure which extends over a strike length of 550m (23,750 – 24,300N). It lies on the western side of the Bulgera Gold Project and represents the main mineralised area in the Bulgera pit.
The mineralised shear structure is about 45m thick. Within this structure, the interpreted  lodes dip at 40 degrees to the west, subparallel to the layering. The higher grade gold lodes (>1.0g/t Au) have an average thickness of 5m but can be up to 17m thick. This is a multiple lode orebody with up to 8 lodes occurring on a section. The length of the lodes down-dip can be up to 140m.
All gold observed in the field and petrographic studies occurs as disseminated particles of free gold in quartz.  Host rock alteration consists of silica-biotite metasomatism.

Mercuri Trend
The Mercuri Trend lies on the eastern side of the Bulgera Gold Project. Mineralisation has been developed in 3 areas of the Mercuri trend:
  • Main Zone (Mercuri & Price Pits)
  • Western Zone (Venus Pit)
  • North-eastern Zone
The mineralisation style is similar in all 3 zones. The rocks which host the mineralisation are predominantly felsic volcanics and quartz porphyry intrusives. These rocks are intensely foliated and contain multiple shear zones. Gold mineralisation is related to quartz veining within these shear zones. The veins are typically 1-50cm wide and contain 1-5% pyrite.
The interpreted lodes dip at 35-45 degrees to the west.. This is a multiple lode orebody with up to seven lodes occurring on a section within a zone. The higher grade  lodes (>1.0g/t) have an average thickness of 5-6m but can be up to 18m thick. Host rock alteration consists of silica-biotite metasomatism.

Weathering Profile

The weathering profile is shallow over the Mercuri / Bulgera deposits. The average depth to the base of complete oxidation (BOCO) is between 15 and 25m and to the top of fresh rock (TOFR) is 45 to 50m. The BOCO approaches the surface on the periphery of the mineralisation. Outcropping rock is common over the general area. However, there is  virtually no outcrop in the mineralised areas, where transported cover and laterite are up to 5m thick.  Therefore the effectiveness of any soil sampling is questionable.

Adjacent Properties
Plutonic Gold Mine
The operating Plutonic gold mine lies 48km by haul road to the south west of the Company’s Bulgera Gold Project. The Plutonic processing circuit is a conventional CIL plant with a hard rock processing capacity of approximately 2.0mtpa. Reported processing at the current rate of 0.85mtpa indicates spare capacity in this processing circuit. The second phase of mining at Bulgera conducted by Barrick in 2003 was processed at the Plutonic facility which would indicate a compatibility with the ore type (mined at that time) from Bulgera.
Marymia Gold Project
Vango Mining Limited is an ASX listed junior mining company who control the Plutonic Dome Gold Project (formerly the Marymia Gold Mine) consisting of the old Resolute Marymia licences. This group is seeking to develop this mine.

Conclusion and Recommendations
The Bulgera Gold Project is hosted within a highly prospective, under-explored greenstone belt previously mined by Barrick. The proximity of the under-utilised Plutonic gold mill, (2 Mtpa) only 48km away by existing haul road, gives this project considerable commercial potential. Two separate avenues of exploration drilling are planned to be undertaken at Bulgera:
  1. Drilling Regional Gold Targets A and B, provides an excellent opportunity for a transformative, shallow, high-grade greenfields discovery. Proposed program of 220 holes for 8,800 metres of aircore or RAB drilling.
  2. Drilling of the JORC Exploration Target provides an excellent near mine opportunity for drill testing known areas of mineralisation. Proposed program of 123 drill holes for 8,545m of RC or aircore drilling.
The Company is currently seeking a partner to provide the funding for this exploration drilling at Bulgera.
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