Gold - sulphide mineralization in ultramafic-mafic-granite complex of Jashpur,Bastar craton,central India: Evidences from geophysical studies

Time Domain Induced Polarization (TDIP) investigation in conjunction with high resolution electrical resistivity tomography surveys carried out at seven sites along a 5.2 km line in Jashpur and Raigarh districts of Chhattisgarh State yielded promising results leading to the demarcation of mineralized zones. These sites are confined to ultramafic-mafic-felsic intrusive complexes representing gabbro-pyroxenite-granite sequences characterized by gold - sulphide mineralization. Two dimensional (2D) geophysical datasets yielded interesting results and a good correlation is found between 2D inverted resistivity tomography and time domain induced polarization (IP) models. Both high resistivity-high chargeability as well as low resistivity-high chargeability conditions have been inferred from these datasets. This observation is consistent with conductive metallic mineralization. Interpretation of the Pharsabahar-Pandripani site, reveals a chargeability magnitude of 9-14 mV/V, which represents a strong metallic conductor associated with gold-sulphide ore body. The results at Samarkachar site are equally significant, showing a good correlation with the presence of a low resistivity and high chargeability zone. Hence, the present study suggests a new scope for application of resistivity and IP tomography surveys for mineral prospecting in favourable geological settings.     

印度地质调查所记录第75卷至第77卷 法克斯等

本刊原名Records of the Geological Survey of India,每年发刊一卷,分四期出版。1940年为第75卷,应事实之需要,改由不定期之小册子(Professional Papers)合成,第一册予     

Gold Mineralization in Kottathara Prospect, Attappadi Valley, Kerala, India: a Preliminary Appraisal

The Kottathara gold prospect of Attappadi Valley in Kerala is located within the Southern Indian Granulite Terrain comprising charnockite and gneisses with enclaves of high-grade supracrustals. The gold mineralization associated with the basic members of the Attappadi supracrustals and the quartz veins traversing them are confined within the Bhavani Shear Zone. Primarily the gold-quartz lode is emplaced in rheologically preferred zones along the contact of the basic members with the enclosing gneisses subsequent to a period of retrogression and shearing. Ore-mineralogical studies reveal that gold got remobilized and this remobilization is identified with the regional Bhavani Shear. SEM studies indicate that gold occurs in free state and also within sulphides especially pyrite. Variation in grain morphology is clearly discernible in gold occurring within oxidised and in non-oxidised zones.Sequencing of deformational events with associated emplacements of known ages suggests the age of gold mineralization of Attappadi area as between 2 Ga. and 2.5 Ga. The secondary mobilization has to be <2.0 Ga or younger possibly of younger Pan-African age related with the Moyar-Bhavani Shear System. The inherent gold content of the komatiitic metapyroxenites together with the auriferous quartz lodes assigns a lithological control on gold mineralisation. Subsequent folding and remobilization due to the regional shear constrained the geometry of the lode zones implying structural control.     

Gold in the beach placer sands of Chavakkad-Ponnani,Kerala Coast,India

Although sporadic mining of placer gold from river beds is not uncommon in India, there is no documented literature on the occurrence of gold in the beach placer sand deposits of the country. While characterizing the heavy minerals of the Chavakkad-Ponnani (CP) beach placer sands along the North Kerala coast, the association of gold with the pyriboles in these sands has been observed. A native gold grain of about 25 μm was seen to occur as an inclusion within an amphibole of hornblendic composition. The pyriboles of the CP deposit are angular to sub-angular indicating a nearby provenance that may be in the upstream reaches of the Ponnani River in the districts of Malappuram and Palakkad or further north in the auriferous tracts of the Wynad-Nilambur or Attapadi regions. It is argued that the occurrence of native gold in CP deposit is not a freak occurrence and that it warrants thorough investigation of all the pyribole-ilmenite-rich placers to examine the possible presence of gold and its abundance in the beach placers of northern Kerala coast.     

Geochemistry, Petrogenesis and Tectonic Setting of Metavolcanics and their Implications for Gold Mineralisation in Gadag Gold Field, Southern India

Gold bearing metavolcanics of Gadag Gold Field (GGF) are represented by mafic (metabasalt, metabasaltic andesite), intermediate (metaandesite) and felsic (metadacite, metarhyolite) rocks. Mafic metavolcanic rocks are low-K Fe-rich tholeiites and were derived by partial melting of the upper mantle sources with high Fe/Mg ratios and low M values. Intermediate and felsic metavolcanics were formed by remelting of these tholeiites mainly in crustal regimes. Although a complete sequence of metavolcanic rocks from mafic to intermediate to felsic fractions occurs, these products were not the result of differentiation from a single magma, crustal contamination was involved in the formation of intermediate and felsic rocks. A clear gap in the chemical composition as well as index of differentiation among the mafic, intermediate and felsic fractions indicate that these metavolcanics constitute a typical bimodal character. It is suggested that these metavolcanics were emplaced in an active continental margin or a continental island arc setting. The petrogenetic processes of formation of Fe-rich tholeiites that evolved in an active continental margin or a continental island arc setting could have provided a favourable geochemical environment for gold mineralisation under the conditions of deformation and metamorphism.     

Morphology and Chemistry of Placer Gold from Attappadi Valley, Southern India

We report the morphological, textural and chemical characteristics of gold grains in stream gravels from the Siruvani River in Attappadi Valley, southern India. The placer gold deposits contain both primary grains with jagged grain contours and secondary grains with smooth grain margins. The primary and secondary gold grains are also distinguished by marked contrast in microtextures with the latter displaying a range of corrosion textures including striations, etch pits and chemical corrosion cavities that coalesce to form honey-comb patterns. Some of these cavities are filled with fine clay derived from lateritic weathering front. While the primary grains are characterized by high silver content (up to 35.77 wt.%) with marginal overgrowths of high purity gold, the secondary grains show exceedingly high fineness (1000 Au/Au+Ag) levels (up to 984) with no marked compositional variation indicating selective extraction of Ag and/or reprecipitation of Au. From morphological and chemical characteristics, we propose that the high purity gold grains were not derived directly from primary sources, but underwent chemical refinement in the weathering front before they were transferred to the fluvial systems. Our findings have important implications for gold exploration in the Attappadi Valley.     

Highly pure placer gold formation in the Nilambur Valley,Wynad Gold Field,southern India

Placer gold grains in the Nilambur Valley of Wynad Gold Field in southern India are characterized by very high purity levels (985–1000). Their Ag-depleted core compositions, enhanced grain size and microscale growth patterns correlate with gold grains associated with laterite profiles in the weathering fronts. From the morphological and chemical evolution of gold grains associated with primary, supergene and secondary deposits in this region, we identified a two-stage process for the evolution of the highly pure placers, which shows that gold in the primary veins was mobilized, chemically purified, and reconcentrated in the laterite profiles, effecting enhanced purity and grain growth before transfer to the fluvial system. Further refinement was achieved during fluvial transport, generating natural concentrations of pure gold in the placers.     

Efficiency of regression techniques for ore valuation at Kolar Gold Fields, India

The Kolar Gold Fields are some of the best known gold deposits in India. An example of ore valuation utilizing 49 ore blocks of the Oriental lode of the West Reefs, explored and developed in the Nundydroog mines, is given. In this reef system, there are large ore reserves of sulfidebearing quartz reefs, and the gold distribution is erratic both along strike and downdip. Ore valuation at present is based on the arithmetic mean of samples taken at peripheral positions of the blocks. Samples taken from internal portions of the blocks give a totally different picture of the value. To correct this discrepancy, normal regression and lognormal regression of internal block and total block values, over peripheral block values have been used to evaluate the deposits. The valuation efficiency criterion shows the logarithmic variance for distribution of ratios of unregressed and regressed block values with the corresponding arithmetic mean of internal stope values as observed inside the blocks. The studies have shown that the logarithmic variance is minimum if the logarithmic regression is used, thereby indicating maximum efficiency. Further, the undervaluation and overvaluation of low- and high-grade blocks is less for the logarithmic example. With help of the logarithmic regression equation an effective pay limit of 177.8 in.-dwt has been found for selective mining, for peripheral block values corresponding to the official pay limit of 240 in.-dwt.NGRI contribution number 71-281.     

The Ore-forming Fluid of the Gold Deposits of Muru Gold Belt in Eastern Shandong, China - a Case Study of Denggezhuang Gold Deposit

Abstract. Denggezhuang gold deposit is an epithermal gold‐quartz vein deposit in northern Muru gold belt, eastern Shandong, China. The deposit occurs in the NNE‐striking faults within the Mesozoic granite. The deposit consists of four major veins with a general NNE‐strike. Based on crosscutting relationships and mineral parageneses, the veins appear to have been formed during the same mineralization epochs, and are further divided into three stages: (1) massive barren quartz veins; (2) quartz‐sulfides veins; (3) late, pure quartz or calcite veinlets. Most gold mineralization is associated with the second stage. The early stage is characterized by quartz, and small amounts of ore minerals (pyrite), the second stage is characterized by large amounts of ore minerals. Fluid inclusions in vein quartz contain C‐H‐O fluids of variable compositions. Three main types of fluid inclusions are recognized at room temperature: type I, two‐phase, aqueous vapor and an aqueous liquid phase (L+V); type II, aqueous‐carbonic inclusions, a CC₂‐liquid with/without vapor and aqueous liquid (LCO₂+VCC₂+Laq.); type III, mono‐phase aqueous liquid (Laq.). Data from fluid inclusion distribution, microthermometry, and gas analysis indicate that fluids associated with Au mineralized quartz veins (stage 2) have moderate salinity ranging from 1.91 to 16.43 wt% NaCl equivalent (modeled salinity around 8–10 wt% NaCl equiv.). These veins formatted at temperatures from 80d? to 280d?C. Fluids associated with barren quartz veins (stage 3) have a low salinity of about 1.91 to 2.57 wt% NaCl equivalent and lower temperature. There is evidence of fluid immiscibility and boiling in ore‐forming stages. Stable isotope analyses of quartz indicate that the veins were deposited by waters with δ^O and δD values ranging from those of magmatic water to typical meteoric water. The gold metallogenesis of Muru gold belt has no relationship with the granite, and formed during the late stage of the crust thinning of North China.