Ore-forming Model And Primary Halo Prospecting Pattern Of Phabon Gold Deposit In Laos

Laos Phabon Gold Deposit is located in the NE-strike Luang Prabang arc fault belt. The belt is bound with Phongsali-Simao micro-continental block by Luang Prabang arc suture line in the NW and bound with Truong Son fold belt by Prairie Hill arc fault in the SE. The NE-strike Luang Prabang arc fault belt is an important metallogenic belt. The studied area has favorable conditions for mineralization.The ore-forming model and prospecting pattern of this paper is based on the geological settings studies of Phabon Gold Deposit in Laos, as well as fluid inclusions, zircon geochronology and primary halo axial zoning study. The main achievements obtained are as follows:1. According to the study of zircon geochronology on diorite, the U-Pb isochron age is 99.1±2.9Ma, and MSWD=1.4, it belongs to the early time of Late Cretaceous.2. The petrographic observations of fluid inclusions show that, inclusions are mostly native, the host mineral is calcite. Inclusions are mainly liquid-rich phases. Gas-liquid ratios are 2%-25%, and the lengths of inclusions are 3～24μm. Freezing temperatures of inclusions are -6.7～0.5 ℃ and Homogeneous temperatures are 101.3-313.7 ℃. Salinities are 0.88%-9.77%. The densities of inclusions are 0.84-0.98g/cm3. The ore-forming fluid belongs to a low-salinity, low-density unsaturated solution. Mineralization pressures are 8.46-29.11MPa and mineralization depths are 0.85～2.91km. So Phabon Gold Deposit is formed in a middle-shallow condition. Inclusion laser Raman spectroscopy displays the vapor is mainly composed of CO2, the fluid inclusions formed in an oxidized environment.3. According to the geological characteristics, combining with characteristics of fluid inclusions, geochronology and stable isotope studies, Phabon Gold Deposit is an epithermal gold deposit.4. Factor analysis of 16 elements in drillings of No.l exploration line in Phabon Gold Deposit shows five factor combinations:Fl:V-Co-Bi-B-Sn; F2:Cu-Zn-Pb-As; F3:Sb-Mo; F4:Ba-W-Au; F5:Ag. These factors have shown a combination of elements with different temperatures, indicates a multi-stage superpositioned mineralization.5. After processing to the data of 16 elements from drillings, element primary halo sections in No.l exploration line section are finished. Primary halo axial zoning study of drilling cross-section is based on the basic principles of Grigorian’s index zonation. The exact axial zonation sequence of elements in drillings of No.1 exploration line in Phabon Gold Deposit (from shallow to deep) is:W-Ba-Au-Ag-Sn-Mo-F-Sb-Bi-V-Co-Cu-As-Zn-Pb-B. Coexistence of front halo and tail halo elements indicates blind orebody exists in the deep.6. (Ba×As×Sb×F)D/(W×Mo×Bi×Co)D is used to evaluate and predict deep blind orebody based on the study of zonation index and zonation sequence. The index varies from 8.47 at the upper part of the orebody (870m elevation) to 53.61 at the central part (800m elevation), to 13.69 at the the lower part (730m elevation) and to 28.81 at the tail of orebody (600m elevation). The index changes from low to high, and to low and at last reaches a high level. It becomes bigger at the tail all out of a sudden, and suggests that in the deeper position under the present orebody exists another gold deposit.