Geology and genesis of the Permata-Batu Badinding-Hulubai and Kerikil Au-Ag low sulfidation epithermal deposits, Mt Muro, Kalimantan, Indonesia

The Permata-Batu Badinding-Hulubai (PBH) vein and Kerikil breccia-hosted deposits of Mt Muro, Kalimantan, Indonesia (10.4 Mt at 3.8 g/t Au and 101 g/t Ag) represent two styles of Au-Ag, low sulfidation epithermal deposit. These two systems provide important information on the processes and mechanisms of metal deposition under epithermal conditions. PBH and Kerikil volcanic host rocks range from andesitic to basaltic in composition and are correlated with Early Miocene Sin tang volcanism and Pliocene Metalung volcanism of Kalimantan. PBH and Kerikil exhibit similar structural trends and northnorthwest dilational settings that are the result of north-northwest directed compression and dextral movement on major northwest striking basement structures. The different characteristics of the two deposits are attributed to different structural, lithological and hydrological controls that effected the nature of ore deposition. The PBH deposit is hosted within extrusive and intrusive coherent volcanic rocks with minor volcaniclastic and sedimentary rocks. These units were deposited on the slopes of a stratovolcano and into valleys and pull-apart basins. Structure is dominated by northnorthwest, northwest and northeast striking fractures, faults and veins on both a regional and deposit scale. The main deposit at PBH is hosted by a 2.2 km long, mineralized, cymoid structure which strikes north-northwest to north-south and dips steeply. Six stages of vein infill are recognized at PBH: stage 1 jasper; stage 2 microcrystalJine quartz; stage 3 microcrystalline quartz + sulfide + sulfosalt; stage 4 base metal sulfide + sulfosalt + quartz; stage 5 amethyst and stage 6 carbonate. Early infill stages are typically fine-grained and microcrystalline with colloform, cockade and crustiform textures. Later infill stages are coarse-grained and crystalline with crustiform, colloform, cockade and dogstooth textures. Infill stage compositions and textures are linked to the dilation history of the vein and Riedel-style mechanics. Gangue mineralogy is dominated by polymorphs of silica (quartz, chalcedony and amethyst) with lesser adularia and clays. Carbonate is only present in the last vein stage. Ore mineralogy consists of pyrite, sphalerite, galena, Ag-Sb sulfosalts, Ag sulfides, Ag tellurides, native Ag and electrum. J alpaite, freibergite and acan thite are all important hosts of Ag. Electrum ranges from 219 to 761 flne and contains trace amounts of Hg and Cu. PBH exhibits vertical metal zonation, with Au and Ag deposited at bonanza grades at higher elevations with Cu, Pb and Zn deposited below. Alteration is developed principally in the hanging-waH to the deposit and is well zoned, with disruption to zonation occurs where hydrothermal fluids have exploited more permeable and/ or reactive beds. Alteration ranges from halloysite + kaolinite + silica assemblages at shallow depths to illite + sericite + pyrite + adularia+ quartz surrounding the deposit to phengite/ sericite + adularia + pyrite + quartz and chlorite + carbonate + albite + epidote + quartz, both distal to the deposit and at depth. Evidence for boiling within the hydrothermal system is recognized from the presence of bladed quartz after carbonate, adularia and two phase (liquid-vapor) fluid inclusions. Sulfur and carbon isotope data indicate a magmatic source for sulfur in pyrite and carbon in carbonate. 1)18 0 values of infill stage quartz show a trend towards lower values with successively later infill vein stages. 1)18 0 values of whole rock alteration facies have lower values closer to the vein and higher values associated with younger overprinting alteration assemblages. Based on these characteristics, PBH can be classified as a sericite/illite-adulariaquartz, Ag-Au low sulfidation epithermal vein deposit. The distribution and zonation of alteration, mineral textures, mineral composition and metals within the mineralized structures are a direct result of the mechanical and physico-chemical processes of depressurization (through structure dilation) and consequent boiling, mixing and cooling of the hydrothermal fluids. PBH is a single dilating conduit which effectively focused fluid flow and boiling is the dominant mechanism of metal deposition. Alternating periods of boiling produced the banded, colloform, crustiform and cockade vein textures observed at PBH. The physico-chemical processes of boiling-related mineral deposition resulted in discrete zoning of metals. Bicarbonate fluids, create.d above the boiling zone, were excluded from the system by temperature and buoyancy effects. After the system waned the bicarbonate fluids were able to migrate down into the system and deposit carbonate in the last inflll stage. The Kerikil deposit is hosted by coherent volcanic lavas and intrusions of a stratovolcano vent environment. Kerikil is divided into three main deposits that total over 900m in length and are confined by north-northwest and north-south striking structures. Eight vein and breccia stages are recognized within three main periods of mineralization at Kerikil. During period 1, in fill stages 1 to 4 are dominated by quartz gangue. In period 2, infill stages 5 to 7 are characterized by the presence of rhodochrosite as an important gangue mineral. In period 3, inf.Ul stages 8 and 9 are represented by base metal and pyrite veins, respectively, which crosscut all earlier infill stages. The main ore stages are stage 2 (microcrystalline quartz + sulfide + sulfosalt), stage 5 (rhodochrosite + sulfide + sulfosalt) and stage 8 (base metal sulfide + quartz). Ore mineralogy is dominated by pyrite and chalcopyrite with minor sphalerite, galena, Ag sulfosalts and electrum. Selenian jalpaite, acanthite, and native Ag are important hosts of Ag. Electrum is 480 to 764 fine and is typically observed as inclusions in pyrite and association with chalcopyrite. Metal zonation is poorly developed at Kerikil with Au, Ag, Cu, Pb and Zn precipitating at the same level within the system. A brecciated system and multiple fluid pathways, allow the downwards migration and mndng of oxidizing ground waters and bicarbonate waters with geothermal fluids, thus favoring both Au and base metalprecipitation together. A broad alteration zonation with depth is apparent at Kerikil. Alteration ranges from halloysite + kaolinite+ quartz at shallow depths to illite/ sericite + adularia + pyrite + quartz proximal to the deposit and chlorite + carbonate + albite + epidote + quartz distally and at depth. At Kerikil, there is overprinting of the illite/sericite + adularia + pyrite + quartz assemblages by the kaolinite + halloysite + quartz facies at shallow levels and deeper in the deposit. Evidence for boiling within the conduit comes from the presence of bladed carbonate, adularia and two phase fluid inclusions. Sulfur and oxygen isotope values indicate a magmatic source for sulfur in pyrite and carbon in carbonate. Carbon and oxygen isotope values suggest that rhodochrosite at Kerikil was precipitated from surficial bicarbonate waters. 18 0 values of inflll stage quartz are relatively constant indicating a fluid in equilibrium with andesite host rocks. 1)18 0 values of whole rock alteration facies, display a trend towards lower values with depth and higher values at surface, associated with late stage alteration. Kerikil is an illite, Au-Ag, quartz-carbonate, low sulfidation epithermal breccia and stockwork deposit Kerikil consists of breccias, veins, faults and stockwork. Hydrothermal fluids have been able to boil, cool and mix with bicarbonate waters through enhanced permeability facilitated by repeated sealing, brecciation and re-brecciation of the coherent volcanic host rocks. Sealing of multiple fluid conduits and subsequent rupturing gives rise to complex overprinting mineralogical and textural relationships, complex mineral paragenesis, metal and alteration zonation. Boiling is an important process when fluid pathways are open. However, sustained boiling precipitates microcrystalline quartz which seals fluid pathways, allowing the influx of earlier boiling derived bicarbonate fluids into the former up flow zone. Subsequent over-pressurization and seismic rupture leads to seal failure and the direct contact of bicarbonate waters above the seal with boiling hydrothermal fluids from below the seal. Precious metals and base metals then precipitate together due to the combined physico-chemical processes of boiling and mixing. Study of the volcanological, structural, mineralogical, metallogenic, alteration and isotopic characteristics of the PBH and Kerikil deposits has led to geological and geochemical vectors being established to aid in mineral exploration at Mt Muro