167 / 2015-09-02 11:21:43

Fluid Inclusion and Isotope Studies on the Chalapu Gold deposit, South Tibet in China

7732,7733

Introduction
The Chalapu gold deposit (2.54g/t~4.72 g/t @11.66t gold resource) located in Longzi country, Southern Tibet in China. The deposit is ~2km east from yelaxiangbo Granitic complex contact at the northern part of Southern Tibet Antimony-Gold Belt (STAGB). The STAGB hosts more than 50 Au, Sb, Sb-Au and Pb-Zn-Ag deposits excluding placer deposits varying from giant (e.g. Zhaxikang deposit, Pb-Zn-Ag-Au resource >1.26Mt) to small-size (<1t gold) scattered throughout this area. Deposits in this belt have been variably classed as Carlin, epithermal, orogenic style and hydrothermal overprinted SEDEX style in nature. These deposits are focused along N-S striking normal faults or E- striking thrust faults, with or without a genetic link to felsic magmatism (Zheng et al., 2004; Nie et al., 2005; Yang et al., 2009;Hou et al,2006; Sun et al.2010; Zhai et al,2013).

Chalapu deposit is one of representative gold deposits in STAGB. The principal objectives of this abstract are placed on the discussion the nature and source of fluid and sulfur. These results contribute to the broader understanding of fundamental processes involved in the generation of gold - antimony systems in the STAGB.

Deposit Geology
Au mineralization in Chalapu gold deposit occurs primarily in upper Triassic Nieru Formation dominated by sandy slate, carbonaceous slate, interbedded with some muddy micrite and calcareous siltstone. Metamorphism is dominantly of low-pressure green schist grade. The Nieru formation is locally intruded by diabase dikes in the mine area. Most of these dikes strike in NWW and E-W, extending up to 1500 m and ranging from 5m to 10m in width. Field relations show that the sedimentary rocks of the mine sequence and some diabase dikes are locally brecciated, locally mineralized and affected by carbonate, quartz, chlorite and sericite alteration along faults.

Gold mineralization occurs primarily in organic-rich black shale sequence of the Nieru Formation and along diabase dikes. Sulphide-quartz veins crosscuts cleavage and are largely confined to fault-controlled veins or dilation fractures striking NWW and NE. The veins are typically several millimetres to more than 5 m thick and extend for 50m to >2000m along strike and to >200m in depth. The gold - bearing veins are filled with quartz, sulfides and carbonates. Wall rock alteration surrounding the veins is generally intensive and includes development of hydrothermal quartz, carbonate, chlorite, illite and muscovite, but without orpiment and realgar. The mineralization occurs as open-space filling as shown by comb, breccia and seal textures. The ores are brecciated and consists of angular to subrounded centimetre-size wall-rock fragments. Disseminated subeconomic mineralization occurs within quartz veins and brecciated organic-rich black shale.

Four paragenetic sequences, each characterized by a particular mineral assemblage and/or texture, are identified in Chalapu gold deposit. (1) Pre-ore stage: consists of metamorphosed disseminated graphite and pyrite with fine grain, strawberry and colloidal texture. (2) arsenopyrite-pyrite stage: consists of disseminated arsenopyrite, pyrites, mica, quartz, sericite and illite. (3) quartz- arsenopyrite-pyrite vein stage: quartz, arsenopyrite, pyrite and fine grain mica deposition in brecciated host rock. (4) quartz-natural gold stage: only observed in a few cases, is represented by large, subhedral quartz crystals associated with native gold. Stage 2 and 3 are the main gold mineralization stages. The most common sulfides are pyrite and arsenopyrite, where sphalerite, galena, chalcopyrite, marcasite, stibnite are less abundant. Quarts are the major non-sulfide minerals. Pyrite and arsenopyrite are the main gold-bearing sufides, and gold are existed as “invisible” or submicrometre-size inclusions.

Fluid inclusion
All fluid inclusions examined were hosted by quartz from stage 3 and 4. Two main types of fluid inclusions have been recognized during this study. A range of H2O-CO2(±N2,CH4) fluid inclusions was identified, with three main subvarieties. Type 1A are three-phase aqueous-carbonic inclusions [LH2O+ LCO2+ VCO2]. Type 1B are two-phase carbonic inclusions [LCO2 +VCO2] that contain no visible aqueous phase. Type 1C inclusions are two - phase aqueous - carbonic inclusions with no separated carbonic phase [LH2O+LCO2]. Type 2 are two phase [liquid - vapour (L-V)], liquid - rich (V< 20%) fluid inclusions [LH2O+ VH2O] that homogenized by vapour disappearance. Primary and pseudosecondary fluid inclusions are predominantly of type 2, type 1A, with a few 1C, and rare type 1B inclusions. In hydrothermal veins, type 1A, Type 1B, 1C and type 2 inclusions commonly occur in the same cluster or plane. The fluid inclusions are characterized by moderate salinity, ranging from 2.7wt%Nacl - 9.3 wt%Nacl, and by a low to moderate homogenization temperature of 164℃-308℃, with calculated fluid density of 0.77 g/cm3 - 0.99g/cm3 and estimated depth of 1.1 km -3.8km.

Isotope
Four δD values of water extract form fluid inclusions are heterogeneous, ranging from -124‰ ~ -101‰. Nine Quartz δ18O values are heterogeneous, ranging from 11.6‰ ~ 19.9‰,with average value of 17.4‰. The calculated fluid isotope composition plotted on δ18O vs.δD diagram showing that the fluid composition associated with the arsenopyrite-pyrite deposition are quite similar with mix of metamorphic water and organic water.
The sulphur isotope of pyrite and arsenopyrite display uniform δ34S value, ranging from 2.87 to 3.72(n=3), 4.24 to 5.54(n=4), respectively. A lead isotope composition of pyrite with206Pb to 204Pb, 207Pb to 204Pb, 208Pb to 204Pb ratios are 18.577, 15.562, 38.567. On the 207Pb to 204Pb vs 206Pb to 204Pb, 208Pb to 204Pb vs 206Pb to 204Pb diagrams, pyrite plots close to the orogene curves.

Conclusions
Dome-centered granite intrusions, development of extension normal fault system in a post-collisional crustal extension setting are significant metallogenic setting for generation gold ore deposit. Fluid inclusion study indicate that the Chalapu gold deposit are formed at depths of up to 3.8km from carbon dioxide-rich, low to moderate temperature, moderate salinity, reducing fluids. High CO2 content of fluid inclusions, shallow depth of formation, intimate cross-cutting relationship between gold-bearing faults and diabase dike, lower δ34S values of ore-stage pyrite, distinctive low-temperature fluid, illustrate that metamorphic fluid, reactive pyrite and carbonaceous slate, calcareous siltstones and faults was the critical elements in triggering gold formation in the district.