Au-Ag-Te–RICH MELT INCLUSIONS IN HYDROTHERMAL GOLD-QUARTZ VEINS, XIAOQINLING LODE GOLD DISTRICT, CENTRAL CHINA

We present petrographic and microthermometric evidence for precipitation of Au-Ag-Te–rich melt directly from hydrothermal fluids and subsequent entrapment as primary melt inclusions within pyrite from quartz veins of the Xiaoqinling lode gold district, southern margin of the North China craton. We propose the formation of Au-Ag-Te–rich melt through adsorption-reduction mechanisms on pyrite and subsequent growth of the melt nuclei via direct scavenging of metals from fluids. Because neither initial formation nor later growth of the melt require saturation of the ore fluid with respect to the constituent metals, this mechanism offers a new understanding of the enrichment of low-abundance ore components, such as gold. Our model may thus partly explain the discrepancy between the high gold solubilities reported from experimental studies and the much lower gold concentrations usually measured in natural fluids. This study also implies that Au-Ag-Te–rich melt has probably gone unrecognized in other lode gold deposits in which Au-Ag tellurides are present.

The Lode Gold Occurrence in the «Territory» of Oleg Kuvaev (North of the Chukotka Upland)

A sample of a gold-bearing quartz vein from the Upper-Ichuveem ore occurrence, discovered in the Triassic terrigenous complex in the north of the Chukotka Upland, the Oleg Kuvaev's “Territory”, had been comprehensively studied. The gross gold grade was estimated at 3–4 g / t. The vein includes quartz, native gold, sulfide-goldsilver solid solutions, alumoseladonite, aluminium-sulfate-phosphates Fe-Pb-Mg-Ca compound, apatite, pyrite, iron-titanium oxides, litharge, native phases composed of Fe (Ni), Ag-Pb-Bi, Fe-Al-PS-As, multicomponent ocher of hypergene origin. Native gold ranges from medium to fine. The vein contains particles of non-crystalline organic matter, which is close in composition of organic groups to polysaccharides. It is possible that the carbon particles found in the gold-quartz vein are of an abiogenic nature and can be compared with abiogenic condensed organelles in the products of modern volcanism. The data obtained make it possible to attribute the Upper- Ichuveem gold ore occurrence to a low-sulfide gold-quartz formation, but with additional signs of gold-silver and polymetallic formations, which can be regarded as a favorable prerequisite for prospecting and exploration in Oleg Kuvaev's «Territory» not only of gold-placer deposits, but also of lode gold deposits.

GENETIC CLASSIFICATION OF ARCHEAN LODE GOLD DEPOSITS

Main features of the Archean gold deposits spatial distribution are outlined. Geology and ore composition of two deposits within the Canadian shield distinguished by tectonic position and host rock lithology are described. Delineation of two separate genetic types of the Archean gold deposits, orogenic and granitoid-related, similar to Phanerozoic accretion-collision belts is validated on that basis.

Epithermal Mineralization in the Busang Southeast Zone, Indonesia: New Insight into the Au Prospect at the Center of the Bre-X Fraud

The Busang mineral prospect in Kalimantan, Indonesia, was reported to host a large Au resource until 1997 when it was revealed that drill core samples had been deliberately and systematically contaminated (“salted”) with extraneous Au to falsify resource estimates. One month before the fraud was uncovered, Dr. G. Milligan, then professor emeritus of geology, visited the site to collect a suite of core samples for academic study that was deemed representative of the host rocks, alteration, and mineralization of the Busang Southeast Zone. These samples were re-examined here by optical microscopy, electron microprobe (EMPA), whole-rock geochemistry, and fluid inclusion microthermometry to characterize the subsurface geology and hydrothermal mineralization, and to assess reasons why the system is of uneconomic character. The host rocks were variably altered calc-alkaline porphyritic subvolcanic diorites, typical of the lithological units along the mineralized trend in the Kalimantan Gold Belt. Early hydrothermal mineralization with quartz-sulfide (pyrite, chalcopyrite, Cu-sulfosalts) stockwork veinlets associated with pervasive phyllic and propylitic alteration was overprinted by crudely banded quartz-carbonate-sulfide/sulfosalt (pyrite, sphalerite, chalcopyrite, galena, tennantite-tetrahedrite, bournonite-seligmannite) veins. The stockwork veins were associated with up to 140 ppb bulk rock Au, some of which was hosted by Cu-sulfosalts. Microthermometry on quartz-hosted aqueous fluid inclusion assemblages (FIA; n = 13) and single inclusions (non-FIA; n = 20) in quartz-carbonate-sulfide/sulfosalt veins yielded an overall range in homogenization temperatures (Th) between 179 °C and 366 °C and bulk salinities between 1.1 wt.% to 8.6 wt.% NaCl equivalent, with much smaller data ranges for individual FIA (e.g., FIA 3; 239.1 °C to 240.5 °C and 0.5 wt.% to 1.4 wt.% NaCl equivalent). Primary FIA along growth zones in quartz were identified, providing constraints on fluid characteristics at the time of quartz growth. Carbonate-hosted FIA (n = 3) and single inclusions (non-FIA; n = 3) in the same veins yielded Th between 254 °C and 343 °C and bulk salinities of 1.1 wt.% to 11.6 wt.% NaCl equivalent. Likewise, data ranges for individual FIA were much smaller. Many of the geological characteristics of the Busang Southeast Zone were compatible with a telescoped, intermediate-sulfidation epithermal system, having formed from diluted magmatic fluids that precipitated weak base metal mineralization. However, the system was unproductive with respect to Au and Ag, at least within the studied area. Of note, vein textures and fluid inclusion characteristics indicative of boiling or efficient fluid mixing—processes both considered critical for the formation of economic lode gold deposits—were absent in the samples.