Low Sulfidation Epithermal System Research Articles

Cenozoic Island Arc Magmatism in Java Island (Sunda Arc, Indonesia): Clues on Relationships between Geodynamics of Volcanic Centers and Ore Mineralization

Abstract. Java island, regarded as a classic example of island arcs, is built through multi events of Cenozoic arc magmatism produced by the subduction of Indian-Australian oceanic crusts along the southern margin of Eurasian plate. Regional crustal compositions, subducted slabs, and tectonics determined the spatial-geochemical evolution of arc magmatism and regional metallogeny. Tertiary geodynamics of island arc was dominated by backarc-ward migrations of volcanic centers. Only after the Miocene-Pliocene roll-back effects of retreating slab, slab detachment, and backarc magmatism took place in central Java. The source of arc magmas is mainly partial melting of mantle wedge, triggered by fluids released from dehydrated slabs. Increasing potassium contents of arc magmas towards the backarc-side and younger magmas is typical for all magmas, while alkali and incompatible trace elements ratios are characteristics for different settings of volcanic centers. The oceanic nature of crust and the likely presence of hot slab subducted beneath the eastern Java determine the occurrences of adakitic magmas. Backarc magmatism has a deeper mantle source with or without contributions from subduction-related materials. The domination of magnetite-series magmatism determines the sulfide mineralization for the whole island. District geology, geodynamics, and magma compositions in turn control particular styles and scales of precious metals concentrations. Deep-seated crustal faults have focused the locations of overlapping volcanic centers and metalliferous fluids into few major gold districts. Porphyry deposits are mostly concentrated within Lower Tertiary (early stage) volcanic centers in eastern Java which are not covered by younger volcanic centers, and whose sulfides are derived from partial melting of basaltic parental materials. On the other hand, high-grade low-sulfidation epithermal gold deposits formed in later stages of arc development and are spatially located within younger volcanic centers (Upper Miocene-Pliocene) that overlap the older ones. Gold in low-sulfidation epithermal system is likely to be derived from crustal materials. The overall interacting factors resulting in the petrochemical systematics that are applicable for exploration: 1) early-stage volcanic centers with high Sr/Y and Na2O/K2O ratios are more prospective for porphyry mineralization, while 2) later-stage volcanic centers with high K2O, total alkali, and K2O/Na2O ratios are more prospective for low-sulfidation epithermal mineralization.

The Jinxi–Yelmand high-sulfidation epithermal gold deposit, Western Tianshan, Xinjiang Province, P.R. China

The Jinxi–Yelmand gold deposit is located in the late Paleozoic Tulasi intra-arc volcano-sedimentary basin in the Xinjiang Uygur Autonomous Province of northwest China. Although all known deposits in the Tulasi Basin were previously described as low-sulfidation epithermal systems, the recently discovered Jinxi–Yelmand deposit is interpreted here to be a modified high-sulfidation epithermal gold deposit based on its hydrothermal alteration assemblages, fluid inclusions and isotopic data. Hydrothermal alteration at Jinxi–Yelmand covers an area of about 4 km 2 and is well zoned. The inner zone (Zone 1) is characterized by intense silicification, with a remnant quartz+dickite assemblage. Vuggy quartz textures are locally preserved in this zone. The intermediate zone (Zone 2) has advanced argillic alteration with a dickite+kaolinite+quartz+smectite assemblage. The outer zone (Zone 3) is characterized by an argillic alteration assemblage of montmorillonite+kaolinite. Gold is hosted mainly by quartz and pyrite in alteration zones 1 and 2. Fluid inclusion study indicates that the hydrothermal alteration and mineralization formed at temperatures between 198 and 275 °C and relatively low salinities (<5% NaCl equiv.). Oxygen and hydrogen isotopic data from fluid inclusions and alteration mineral assemblages, as well as geological relationships, suggest that the ore-stage hydrothermal fluids were of magmatic–hydrothermal origin, with early stage input of SO 2-rich magmatic vapors. This study reports the first recognized occurrence of a modified high-sulfidation style hydrothermal system in the western Chinese Xinjiang province, enhancing the potential for the exploration of epithermal gold in this region.

Geochronology of the Midas Low-Sulfidation Epithermal Gold-Silver Deposit, Elko County, Nevada

The Midas deposit is the largest known high-grade Au-Ag vein deposit located along the northern Nevada rift. It belongs to a suite of middle Miocene low-sulfidation epithermal systems associated with magmatism and faulting along the rift. Interpretation of new 40Ar/39Ar dates for volcanic rocks and hydrothermal minerals related to gold mineralization and additional isotopic dates throughout the Midas region constrain the timing of volcanic, tectonic, and hydrothermal activity. The Midas hydrothermal system developed following a change from mafic-dominated bimodal volcanism and basin formation to felsic volcanism, folding, and faulting at about 15.6 Ma. From 15.6 to 15.2 Ma, sediments and tuffs were deposited on a relatively impermeable rhyolite flow. During this period, faulting and tilting of the volcanic edifice created pathways for hydrothermal fluids that flowed to the surface forming sinters and hydrothermal breccias. Approximately 200 kyr after the change in volcano-tectonic regime, oblique-slip faulting took place along zones of preexisting weakness, creating dilational zones and additional channelways for mineralizing fluids. At 15.4 Ma, high-grade veins formed in fault zones throughout the region, depositing at least 4 million ounces (Moz) of gold and 40 Moz of silver. The timing of high-grade Au-Ag mineralization is identical to the age of rhyolite intrusions whose source magma chamber likely provided the heat necessary to drive the hydrothermal system. The age of an unaltered tuff that unconformably overlies opalized sediments establishes that tilting of the units and the hydrothermal system had ceased by 15.2 Ma. The temporal and spatial coincidence of rhyolite volcanism, faulting, and high-grade mineralization may reflect the importance of contributions from deeper fluid reservoirs containing magmatic components or highly exchanged meteoric waters.

Early Tertiary epithermal gold mineralization, Bahcecik prospect, northeastern Turkey

The Bahcecik prospect is located in the eastern Pontide tectonic province of the eastern Black Sea region of Turkey. It is one of the first low sulfidation epithermal systems to be described from this area of Turkey. Gold mineralization occurred within Late Cretaceous to early Tertiary volcanic rocks and is localized along east-trending normal faults and lithologic contacts. An early quartz–sericite alteration event was focused along the major fault in the area and resulted in precipitation of arsenian (?) pyrite with anomalous gold. A later, more spatially extensive, advanced argillic alteration event overprinted the quartz–sericite event. Native gold was precipitated during this event as disseminations and fracture fillings in quartz. Gold mineralization is restricted to the high-temperature portion of the alteration zone characterized by quartz veins with diaspore–pyrophyllite selvages. The prospect area was subjected to Tertiary to Holocene supergene alteration.

Miocene Epithermal Au-Ag Vein Mineralization, Dixie Claims, Midas District, North-Central Nevada; Characteristics and Controls

The Dixie Zone Au-Ag deposit is a low sulfidation (adularia-sericite) epithermal system located 1.4 km from the producing Ken Snyder mine (7.0 M oz Au equivalent reserve/resource), Midas Mining District, north-central Nevada. This WNW striking, steep south dipping, fault system is hosted by mid-Miocene (17 Ma to 14 Ma) felsic volcanic rocks that were erupted from a vent(s) to the northeast. The system was mineralized by approximately 250 ± 20°C (n = 86), near-neutral pH, ascending fluids of predominantly meteoric origin [volatile corrected salinity = 0.52 ± 0.35 eq.wt% NaCl (n = 86)]. Bulk analytical fluid inclusion gas chromatography identified a mineralizing fluid volatile composition of 99.68 ± 0.18 mole % H 2 O, 0.16 ± 0.09 mole % CO 2 , 0.11 ± 0.05 mole percent N 2 , 0.05 ± 0.04 mole % CH 4 , and 0.01 ± 0.01 mole % C n H n+i (n = 7). Sulfur species were not analyzed due to absorption within the chromatography system; however, a H 2 S concentration of 16 ppm to 160 ppm molar is estimated. The volatile content contributes approximately 50% of the microthermometrically measured freezing point depression. Cl − concentrations up to approximately 9000 ppm suggest an additional, probably magmatic, fluid component. Ascending fluids were channelled along approximately NW striking faults related to the same primary extension (17 Ma to 14 Ma) responsible for Nevada basin and range physiography. Boiling induced by CO 2 effervescence led to precipitation of Au and Ag over a 150 m vertical interval. At the time, this interval was about 460 m below the water table (approximately 38 bars), but because of erosion, it is now within 90 m of the surface. A Ag:Au ratio of 25:1 has been calculated for the Dixie Zone.

Pongkor (west Java, Indonesia): a Pliocene supergene-enriched epithermal Au-Ag-(Mn) deposit

The Pongkor gold-silver epithermal deposit with reserves of at least 98 tonnes of gold and 1026 tonnes of silver, average grades 16.4 g/t Au and 171.2 g/t Ag is one of the most recent and largest gold and silver discoveries in Indonesia, proven within a short period (1988–1991). 40Ar/39Ar dating on adularia samples give an age of 2.05 ± 0.05 Ma. The deposit is of the low-sulfidation epithermal type and consists of four main mineralized quartz veins located close to the internal rim of a volcano-tectonic depression (caldera). This resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks. This volcanic unit unconformably overlies Miocene subaqueous volcanic andesitic rocks with interbedded epiclastic rocks. The mineralized bodies are thick (average 4.2 m), steeply dipping, quartz-carbonate-adularia veins with a very low sulfide content (<0.5 wt.%). Their genesis is related to an extensional episode within a tectonic corridor showing NW-SE and NNE-SSW conjugate strike-slip faults, the major vein being located on the inner rim of the caldera. The vein fill reveals four successive stages of deposition marked by a specific facies: (1) carbonate-quartz breccia with dominant quartz and calcite and minor kutnahorite, rhodochrosite, and rhodonite (CQ facies), (2) a network of banded quartz and former carbonate transformed into manganese oxides through supergene alteration (MOQ facies), (3) banded opaline milky quartz (BOQ facies), and (4) grey, locally banded, sulfide-rich quartz breccia cutting all the other types (GSQ facies). Adularia was deposited at the same time as the quartz. The mineralogy and internal structures of the veins (crustiform banding, vugs, collapse breccia) clearly indicate a dilational context, which is common in low-sulfidation epithermal systems. Gold and silver grades, as well as sulfide mineral abundances, increase steadily through stages 1 to 4, locally reaching 1 kg/t in the GSQ facies. The sulfides are dominated by pyrite, accompanied by common acanthite-aguilarite, polybasite-pearceite and electrum in which the gold content ranges from 48 to 74 wt.%. Sphalerite, galena, chalcopyrite and hessite are fairly rare, although present within the CQ facies. The fluid inclusions of the four facies show homogenization temperatures ranging from 150 to 382 °C, indicating boiling of a hydrothermal fluid with an initial temperature of around 205 °C; no marked difference is seen in the GSQ facies, which has the highest gold content. Salinities are low, generally below 1 wt.% eq. NaCl. Lead isotope compositions of the associated volcanic rocks and the mineralization are very similar, 206Pb/204Pb between 18.706 and 18.814␣and between 18.744 and 18.801 respectively, demonstrating a genetic link between the Pliocene volcanism and the auriferous hydrothermal activity. The isotopic signature suggests that the source of the mineralization and associated volcanic rocks is an underlying ancient continental crust that melted and remobilized during the Pliocene volcanic and hydrothermal events. These conclusions seem applicable to the entire Bayah Dome. The existence of both a tectonic corridor and a caldera favoured channelling of the hydrothermal fluids and the deposition of primary ore in the veins. Late intense weathering of the ore deposit, to depths of 250 m below the surface, has given rise to manganese oxide layers, limonite zones, and silver micronuggets within the veins, as well as to gold enrichment.

"Extreme boiling" model for variable salinity of the Hokko low-sulfidation epithermal Au prospect, southwestern Hokkaido, Japan

The Hokko prospect is located in the Minamikayabe area southwestern Hokkaido, Japan, where gold-bearing quartz veins of Pliocene age are exposed at the surface. The alteration mineral assemblage is typical of low-sulfidation epithermal systems, with the quartz veins associated with adularia alteration overprinted on Late Miocene propylitic alteration. Fluid inclusion studies of the vein quartz reveal mean homogenization temperatures of approximately 220 °C, and the co-existence of low-salinity (<2 wt.% NaCl equivalent) and moderate salinity (2 to 12 wt.% NaCl equivalent) fluid inclusions within the same veins. The moderate salinity fluid inclusions (2–12 wt.% NaCl equivalent) typically have relatively low homogenization temperatures between 150° to 200 °C. The results obtained from stable isotope analysis of δ18O in quartz vein material showed a gradual decrease in δ18O signatures with increasing depth. The majority of the samples have calculated fluid source signatures (δ18OH2O) between −8.0 and −10.0‰, but there is a significant change in the composition above 185 m drill depth. The shallower samples in particular show a wide range of oxygen isotope signatures that are associated with the moderate salinity fluid inclusions. We interpret that low-salinity inclusions within the Hokko system represent the composition of the liquid phase of the fluid, before boiling, and that the moderate-salinity inclusions are representative of the residual liquid phase, after extensive non-adiabatic boiling and vapor loss in an open system. This mechanism resulted in the entrapment of fluids with variable salinities at the same time, and in close proximity to each other. This is also reflected in the δ18OH2O values which become more variable and heavier where the moderate-salinity inclusions occur. Deposition of ore minerals within the Hokko vein system also occurred at this time as a result of boiling and gas loss.