Masbate, Philippines

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The Masbate gold deposits are located within the municipality of Aroroy, 25 km WNW of Masbate City and 360 km SE of Manila, near the northern tip of the island of Masbate, in the Philippines. The 3270 km2 Masbate Island lies in the central part of the Philippine archipelago, midway between the islands of Mindanao and Luzon (#Location: 12° 28'N, 123° 24'E).

The Philippine archipelago formed in response to ongoing convergence between the South China Craton and the Pacific Plate, which commenced during the Mesozoic. It is composed of accreted volcanic arcs, marginal basins, ophiolites and continental fragments that have evolved as a result of subduction, plate collisions and strike slip faulting.

There are currently three active elements in the Philippine archipelago, from west to east:
i). A gentle east-dipping trench system composed of the the Negros and Cotabato trenches in the south, and the Manila Trench in the north;
ii). The Philippines Fault, a sinistral strike-slip fault complex which lies midway between the western and eastern subduction zones, and roughly parallels them in strike. It is believed to have been active since the Late Pliocene (4 to 2 Ma), representing the reactivation of pre-existing structures, and is still seismically very active with 10 large earthquakes in the last 100 years.;
iii). The west-dipping subduction zone bordering the eastern margin of the Philippines, involving the Luzon and Philippine trenches.

The Philippine Fault passes just offshore from the Masbate deposit and is inferred to come onshore along the eastern coast of Masbate Island not far from Masbate City.

The oldest known rocks in the northwestern part of Masbate, are interpreted to be Cretaceous or older pillowed mafic lavas of the Mt Manapao Basalt, which outcrop near the east coast of the island, east of the Masbate gold deposits. These volcanic rocks are capped by deep-water mudstone of the Boracay Formation. The Late Eocene to Oligocene Mandaon Formation sandstones, 'red' calcarenites and volcanics of are unconformably and/or structurally juxtaposed over both preceding units.

A large intrusive complex of inferred Middle Oligocene age, the Aroroy Diorite, intrudes both the Mt Manapao and Mandaon Formations. The latter formation is unconformably overlain by the Sambulawan Formation, inferred to be of Late Oligocene to Early Miocene age, composed of shallow marine clastic sediments, andesitic volcanics and carbonate platform rocks. The Late Miocene to Early Pliocene Masbate Group in turn overlies the Sambulawan Formation and comprises various andesitic volcaniclastic rocks subdivided into the basal Lamon Andesite, Buyang Limestone, Lanang Conglomerate and Cawayan Clastics. The mainly intrusive Mt Nabongsoran Andesite cuts various clastic lithologies and is included within the Masbate Group. The sequence is capped by the Late Pliocene to Early Pleistocene Bugui Limestone.

Within the Masbate gold deposits, two NNW-striking fault structures are mapped, namely the Pinanaan Fault in the east, which hosts the Pajo Reef, and the Malubi Fault in the west that has the David Sun Veins developed within it. These faults are interpreted to represent a 'sinistral wrench couple' between which the Masbate Reef system fracturing was formed and mineralised, and are inferred to be shears sympathetic to the Philippine Fault.

The main rock types encompassing the Masbate gold deposits include andesite, basaltic andesite, epiclastic rocks derived almost exclusively from andesite volcanics, polymict clastic rocks derived from multiple source regions, and porphyritic intrusives of andesite composition.

The andesitic rocks of this sequence are massive, coherent feldspar-phyric andesite, often intermingled with andesite breccia. In some localities these andesitic rocks are pillowed, containing abundant amygdales of carbonate, with inert-pillow space filled by haloclastite breccia or mudstone. Pillow lavas are found in all of the pits. The characteristics of these rocks suggest they were erupted into a deep marine setting.

Conglomerate is present throughput the mine area, although it is more common in the southeast. Both polymictic and monomictic conglomerate are recognised. The former are characterised by clast types that are commonly rounded and include basalt, ultramafic, andesite, dacite, felsic porphyry and a variety of metasediment types. The matrix of the polymictic conglomerate is chloritic, indicating an intermediate to mafic bulk composition. They range from clast to matrix supported and rarely show any sorting, regularly containing boulders over 5 m diameter and a number of huge olistoliths of carbonaceous mudstone and sandstone, some over 250 m in length. These rocks are interpreted to be indicative of a submarine canyon setting with catastrophic sector collapse of seabed from the canyon walls into debris flows (Turner et al., 2011).

Turbiditic sandstone and siltstone occurs as thin (0.1 to 20 cm) units of feldspathic and variably carbonaceous sandstone, wacke and siltstone interbedded within the andesitic rocks and conglomerates. These interbeds are characterised by common cross-bedding in sandstone, scours, drop stones and shale rip-up clasts and occur between cycles of conglomerate and olistoliths in polymictic conglomerates.

The intrusive bodies recognised within the area surrounding the Masbate gold deposits have been correlated with the Mt Nabongsoran intrusives of the Masbate Group. These include a distinctive hornblende-augite porphyry which is present as both intrusive bodies and as clasts within polymictic conglomerate, and andesite dykes that intrude conglomerate units in several of the pits.

The dominant structural style influencing the Masbate gold deposits is cataclastic brittle fault deformation, particularly tectonic brecciation and fault gouge development, with fault zones up to 30 to 40 m in width recognised. Cataclastic zones are invariably bounded by high strain shears, the surfaces of which are commonly striated. However, the country rocks outside of the cataclastic zones are relatively undeformed, commonly with sharp boundaries between the high- and un-strained domains. Fault related structures predominantly strike WNW-ESE or NW-SE and typically dip steeply (i.e., >65°). Kinematic measurements indicate dextral, sinistral and normal movement on these structures, highlighting the complex and long-lived nature of faulting at Masbate. Sinistral dislocation is consistent with the sense of movement on the nearby Philippine Fault, and it is expected this represents the bulk of displacement within the Masbate gold deposit area. Sinistral offset of up to 700 m can be inferred along a major cataclastic fault zone linking the Montana and Holy Moses-Basalt pits, while ubiquitous dextral kinematics may only reflect a relaxation phase after a sinistral event. Normal movement at shallow crustal levels undergoing extension (e.g., orogenic collapse, gravity sliding, back-arc basin opening) is a common feature of epithermal settings (Turner et al., 2011).

Alteration associated with mineralisation has depleted all of the sodium and most of the calcium from the original andesitic lithologies, while mineralised rock is highly enriched in potassium. The principal alteration associated with mineralisation comprises microcrystalline adularia (not previously reported) and chlorite, which results in hard, green coloured rocks, commonly mis-identified as silicification. Sericite is also widespread in highly altered samples. There is a poor correlation between mineralisation and clay minerals, which comprise dominant mixed illite-smectite, although gypsum is very common in the Main Vein, Libra and Holy Moses-Basalt pits, while kaolinite is common at Colorado. The original clay-mica alteration mineralogy appears to have been zoned around mineralised reefs from illite to illite-chlorite to chlorite, although this has been strongly overprinted by the low temperature alteration assemblage. The overprint is pervasive, and particularly strong along mineralised structures.

The presence of thick, barren calcite veins and pervasive smectite-gypsum alteration at the Main Vein result from the collapse of a hydrothermal system, allowing the descent of steam heated water. H
2S from the boiled off vapour has condensed into groundwater, become oxidised to sulphate and them percolated back into the system to form gypsum. Colorado appears to have been located at a shallower level in this collapsing system. It is interpreted (Turner et al., 2011) that at the Colorado pit, the oxidation of H2S in the absence of significant groundwater has produced sulphuric acid which has reacted with the rock to form kaolinite.

The main Masbate gold deposits are distributed over an area of 4.5 km north-south and 1.5 km in a ENE-WSW direction. They comprise a series of generally NNW-SSE to NW-SE oriented, en echelon vein networks. These include the NNW-SSE oriented, 2 km x 350 m Colorado deposit in the north, including the Syndicate zone on its southern tip; the 2 km x 500 m, NW-SE trending Holy Moses-Basalt deposit, made up of eastern and western higher grade zones to the immediate south to SSW of Colorado; the 3 km x 800 m Main Vein deposit, parallel to and 200 m to the SW of the Holy Moses-Basalt deposit; and the 750 x 150 m Old Lady resource that is an extension of the Main Vein, ~400 from the SE tip of the latter. A series of other veins and anomlaies are found to the SSE of these systems.

The gold mineralisation of the Masbate deposits is hosted by quartz and quartz-calcite veining, with quartz veining developed in all of the lithologies described above. Individual veins may be traced for up to 3 km, with the known system extending over a strike of more than 10 km, from Balete in the south to Pajo in the north. The more important vein networks vary from 1 to 20 m in width. At the Main Vein, where it is intersected by cross-cutting structural orientations, broad zones of alteration or brecciation is often developed, resulting in mineralised zones up to 75 m wide. Mineralised vein networks extend to depths of at least 300 m below the topographic surface, with most yet to be closed at depth (2011).

A complex evolution of veining is evident, with multiple episodes of vein opening and infill that changes over time from quartz and chalcedonic quartz, to calcite dominant veins. Gold was introduced in several stages, including an early steep (>70° to the NE or SW), NW-SE striking, greyish, sulphide rich quartz, and in later sulphide poor chalcedony and calcite veining. Undeformed mineralised quartz veining is uncommon, with quartz reefs typically strongly overprinted by cataclastic deformation and alteration within fault zones, implying earlier structures hosting mineralisation in quartz veins have undergone renewed brittle deformation. Fluid inclusions from mineralised quartz veins are dominated by two-phase (liquid+vapour) liquid-rich inclusions that have homogenisation temperatures ranging between 219 and 286°C, averaging 250°C, with a low salinity (0 to 3.85 wt.% NaCl

Pyrite is the dominant sulphide associated with the quartz veining, comprising up 85 to 95% of all sulphides, with local marcasite. Other sulphide minerals only occur in trace amounts, and include chalcopyrite, galena and sphalerite. In oxidised rocks, goethite is the dominant ore mineral, with textural evidence suggesting it has formed from the replacement of pyrite.

Gold (electrum) is typically observed as <10 µm inclusions within pyrite or goethite, occuring at the margins of pyrite and other sulphide phases, or more rarely, as free particles up to 500 µm in size. A small amount of gold is also present within silicate minerals. On the basis of elevated Bi and Te contents it is inferred that gold telluride minerals may also be present. The Ag content of ore is variable, but is typically present in a ratio of 0.5 to 2 x the Au content. Dore produced by the Masbate processing plant typically contains 40 to 45% Ag.

Published NI43-101 compliant reserves and resources calculated at a 0.36 g/t Au cut-off, 20 May 2008 (Mining Associates 2008 in CGA Mining 2011) were:
    Proved + probable reserves - 92.2 Mt @ 1.0 g/t Au, containing 93 t (3 Moz) of gold;
    Indicated resources - 135.33 Mt @ 0.96 g/t Au, containing 141 t (4.55 Moz) of gold;
    Inferred resources - 127.15 Mt @ 0.79 g/t Au, containing 100 t (3.22 Moz) of gold.

Published NI43-101 compliant reserves and resources calculated at a 0.36 g/t Au cut-off, 31 December 2012 (B2Gold, August 2013) were:
    Proved + probable reserves - 103.923 Mt @ 0.97 g/t Au, containing 100 t (3.23 Moz) of gold;
    Measured + indicated resources - 91.453 Mt @ 0.78 g/t Au, containing 71.7 t (2.31 Moz) of gold;
    Inferred resources - 72.317 Mt @ 0.82 g/t Au, containing 93.3 t (3.00 Moz) of gold.

This summary is closely based on Turner, Vigar and Jones, 2011, submitted to CGA Mining Limited, available from the TSX SEDAR database.

The most recent source geological information used to prepare this summary was dated: 2011.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

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