Periodico di Mineralogia

Soil geochemistry survey for gold exploration at Kısacık area (Çanakkale, Ayvacık, Türkiye)

Alaaddin Vural — 2024-05-20

The Kısacık gold deposit (Çanakkale, Türkiye) is a recently discovered area identified through soil geochemistry studies. Soil geochemistry surveys in the region commonly employed high detection limits for gold (40 ppb) and silver (5 ppm). Despite this disadvantage, a significant gold mineralization was discovered at the Kısacık site. This study has two main goals: establishing reliable threshold values for analyzed elements with high detection limits, and determining if the multi-element halo technique can reveal subtle anomalies that may be missed using single-element mapping. For this purpose, a total of 305 surface-soil samples collected from the area were investigated for the concentration and spatial distribution of gold and its seven pathfinder elements (Cu, Pb, Zn, As, Mo, Sb, and Ag). Frequency-based conventional methods and fractal/ multifractal-based concentration-number (C-N) method were used to calculate the threshold values, and robust threshold values were obtained with the C-N method. To prepare the distribution of element concentrations on a contour map, the data were used to calculate the variograms (ordinary Kriging prediction) of elements in soil in the area for spatial prediction. When the single element anomaly maps of all elements were assessed, no sufficient surface enrichment/anomaly halos were seen in the area. To enhance the data, the multi-element halo technique was used to create an anomaly map. From the anomaly maps formed via the multi-element halo technique, it can clearly be seen that they are an improvement on the single-element anomaly maps. Consequently, the multi-element halo technique seems to be more efficient in the assessment of vulnerable (weak) data and insufficient geochemical anomalies for geochemical explorations.

Fractal analysis and formation of biotite clots within double-layered mafic magmatic enclaves in the Bamuni Pluton, Mikir Massif, Northeast India

Gaurav Hazarika — 2024-06-10

Biotite clot is a rare magma mixing texture and its occurrence may provide valuable information to understand magma mixing processes. Back-veining, a common yet underappreciated phenomenon is emphasized here, which was liable for the formation of biotite clots within double-layered mafic magmatic enclaves (MME) in the Bamuni Pluton, Mikir Massif, Northeast India. From field observations, the existence of two distinct portions in this pluton, viz. lower portion comprising coarse-grained granite and upper portion consisting of fine-grained variety, depicts that the pluton represents a fossilized vertically zoned granitic magma chamber. Moreover, many MME of varying sizes are distributed throughout the pluton. The presence of quartz ocelli, sieve-textured plagioclase, acicular apatite, and biotite clots within MME at the upper portion of the pluton suggests mixing between mafic and felsic magmas. Biotite clots are distributed within outer layer of double-layered MME at the upper part of this pluton. Composition of biotite found in clot-bearing MME, composition of biotite found in granitic rocks surrounding the MME, and composition of biotite found in granitic rocks away from the MME, all plot in the field of siderophyllite. Compositions of plagioclase from the three zones plot in the fields of oligoclase and albite. Moreover, feldspars from the granitic rocks surrounding the MME plot in fields of anorthoclase and sanidine. Furthermore, fractal dimension (Dbox) and viscosity ratio (log VR) calculations reveal that the outer layers of the double-layered MME with lower Dbox and log VR values correspond to higher degree of hybridization between the MME and felsic host in comparison to the inner layers of the MME. Viscosity model also reveals that the viscosity of outer layer plot in between the inner layer of the MME and host granitic rocks. From our results, we infer that biotite clots within double-layered MME were formed due to the back-veining of felsic melt during the interaction between hotter MME and granitic host.

Potential Alkali-Silica reactivity of Italian natural aggregates for concrete: assessment according to UNI 11530 and UNI 11604 standards

Gabriele Vola — 2024-06-17

The study focused on natural aggregates used in concrete, sourced from six different production sites in Italy, and assessed their potential alkali-silica reactivity through two national standard methods. The first method, UNI 11530, involved a detailed petrographic examination, utilizing modal analysis for identifying and quantifying potentially alkali-reactive lithotypes. The second test method, UNI 11604, involved an accelerated expansion test at 38 °C and high relative humidity on concrete including an established combination of size fractions of aggregates. This method allows the assessment of aggregate reactivity based on concrete expansion after 1 year of testing and an expansion limit value of 0.04%. A correlation analysis was done on the results from both test methods aimed to establish a relationship between the type and content of potentially reactive lithotypes of aggregates and concrete expansion. Chert and Chalcedony were identified as the primary contributors to concrete expansion. The sum of Chert and Chalcedony contents in the combined aggregates was found to vary between 1.8 vol% and 10.1 vol%. A critical content of approximately 5.5 vol% for the sum of Chert and Chalcedony was identified for deleterious expansion development in concrete subjected to UNI 11604 test method. This limit proves valuable in predicting the expansive behavior of aggregates where petrographic analysis has identified the presence of lithotypes potentially reactive to alkalis.