Using analytic hierarchy process (AHP)-based index and geographic information system (GIS) in geochemical exploration of gold

In this study, a gold exploration index (GEI) that reduces financial expenditures and time losses during exploration studies has been developed using the Analytical Hierarchy Process (AHP) in a region where a high sulfidation epithermal Au deposit exists. The GEI can be used to predict the location of the target element by evaluating the maps obtained from related element distributions together with a GEI-based prediction map. The hierarchical structure of the index has been established based on geochemistry of the rock samples. The elements used in the design of the hierarchical structure are arsenic (As), silver (Ag), antimony (Sb), copper (Cu), manganese (Mn), lead (Pb) and zinc (Zn), which are determined by the correlation analysis and experts’ opinions. The efficiency scores of the alternatives were converted into prediction maps called GEI-based anomaly distribution maps. They were compared with the maps derived from both GIS-based overlay analysis of the rock samples and spatial gold distribution. The efficiency scores of the alternatives in these maps were categorized into three groups as "high,” "medium," and "weak" in terms of gold potential. Comparison of the results with those derived using Principal Component (PCA), Weighted Total (WS) and Weighted Product Models (WPM) methods showed that the produced index yields reliable information that can be used to determine where gold enrichment occurs, especially in high sulfidation epithermal environments.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5443218

New perspective Reza gold deposit (Gedabek ore district, Lesser Caucasus, Azerbaijan)

The article describes Reza gold deposit of Ugur exploration area located in Geda- bek Ore District of the Lesser Caucasus in NW of Azerbaijan. It is established that main mineralization in the Reza gold deposit consists of hematite-barite-quartz-kaoline veins- veinlets and breccia, pyrite stock-stockverk and quartz-sulfide veins. On the main orebody surface center secondary quartzites with vein-veinlets barite-hematite mineralization have occured over which remain accumulations of hydrous ferric oxides cementing breccias of quartz and secondary quartzites. “Reddish mass” is also observed in erosion parts, being an oxidation product of stock and stockverk limonite-hematite ores. Representing typical gossans, these accumulations by the data of trenches for thickness about 5-10 m contain gold 0.3-3.5 ppm and silver 1.0-45.0 ppm. There are three zones of gold mineralization within the Reza gold deposit: oxide mineralization; transition zone mineralization; sulfide mineralization. The oxide gold mineraliza- tion consists of clay-gravel weathering crust of kaolinite type. The gold-bearing mineralization has been oxidized to a depth of ap- proximately 50-100 meters. Typically, the gold mineralization is coarser and a minor increase in gold grade occurs within the oxides compared to the original rocks. The nugget effect increase in the gold grade of the oxides does not exceed approximately 10%. Deposit alteration signature has characteristics which suggest the current outcrop level may be near the top of a mineralized, gold-bearing high sulfidation epithermal (HSE) system. The gold mineralization at the deposit is interpreted as forming in shallow high sulfidation epith- ermal systems. The mineralization has been noted in well-confined hydrothermal breccia and associated with pyrite stock-stockwork. The majority of the deposit material and current estimates are formed within the barite-hematite-quartz-kaoline mineralization in the secondary quartzite rocks. The main brecciation and stockwork are hosted within secondary quartzite, sometime massive silicified andesite porphyritic rocks.

Comment to "Porphyry-related high-sulfidation mineralization early in Central American Arc Development: Cerro Quema deposit, Azuero Peninsula, Panama" by Perelló et al., (2020)

The Cerro Quema Au-Cu deposit is hosted by a dacite dome complex of the Río Quema Formation, a Late Campanian-Maastrichtian volcano-sedimentary sequence of the Panamanian magmatic arc. Its formational age is constrained at ~49 Ma by field evidences, crosscutting relationships and 40Ar/39Ar geochronology (Corral et al., 2016, Corral, 2021). The recent molybdenite Re-Os dates by Perelló et al. (2020) claim that ore is spatially and temporally related to the host volcanic domes at ~71 Ma. After a thorough review of the geologic, geochemical and geochronological data from the Cerro Quema area, it is concluded that the Re-Os dates of Perelló et al. (2020) are not representative of the Cerro Quema formational age. Their proposed formational age at ~71 Ma is significantly older than the age of the host rock (~67 Ma). Furthermore, they invoke a previously unrecognized regional-scale magmatic event solely based on their molybdenite Re-Os dates. Instead, the Cerro Quema genetic model discussed here, in which magmatic-hydrothermal fluids derived from porphyry copper-like intrusions associated with the Valle Rico batholith produced the Au-Cu mineralization at ~49 Ma, is consistent with the geology, geochemistry and geochronology of the Azuero Peninsula.

Reply to “Porphyry-related high-sulfidation mineralization early in Central American Arc development: Cerro Quema deposit, Azuero Peninsula, Panama” by Corral (2021)

Dear Editor, we thank Corral (2020) for his anticipated interest in our paper on the timing of the porphyry-related high-sulfidation epithermal mineralization at Cerro Quema in the Azuero peninsula of southwestern Panama. Our study, based on three Re-Os ages for molybdenite intimately associated with Cu-bearing sulfide minerals from the hypogene roots of the La Pava center (Figure 1), shows that the main event of high-sulfidation Cu mineralization took place during the earliest Maastrichtian at ~71 Ma. The reported ages, together with the geologic relationships described in our paper (Perelló et al., 2020), plus a series of regional geologic, structural, petrochemical, and geotectonic considerations, not only precisely date the porphyry-related nature of the Cerro Quema high-sulfidation mineralization, but are also significant in that they confirm the rapid evolution of the earliest stages of the Central American Arc – from subduction initiation at 75-73 Ma to arc stability and maturation at 71 Ma (e.g., Buchs et al., 2011a and references therein) – and place the mineralization in a regional geodynamic setting. Irrespective of the regional geologic arguments reiterated by Corral (2020) in support of his previous genetic interpretation (e.g., Corral et al., 2016) and to invalidate our conclusions, Corral´s real concern is the reliability of our molybdenite ages, which are much older than his preferred age of mineralization for Cerro Quema. We believe that many of the points raised by Corral (2020), including the regional and local geologic backgrounds of the deposit and the dated samples, were properly addressed in Perelló et al. (2020), and that it would be redundant to repeat them here. Additional petrochemical evidence in support can be found in Whattam and Stern (2015, 2020) and Whatam (2018).

Auriferous pyrite formed by episodic fluid inputs in the Akeshi and Kasuga high-sulfidation deposits, Southern Kyushu, Japan

Pyrite geochemistry has proven useful for tracking changes in the composition and physico-chemical conditions of hydrothermal fluids in ore-forming environments. Here, we investigated the microtextural features and chemical composition of pyrite, a main Au-bearing phase in the Akeshi and Kasuga deposits (Southern Kyushu, Japan), to better constrain the ore-forming processes in these high-sulfidation epithermal Au deposits. Despite the widespread distribution of Au-bearing pyrite in both deposits, no visible Au minerals coexist with pyrite. However, in situ laser ablation inductively coupled plasma mass spectrometry results show that Au concentrations in pyrite vary from below the detection limit to 41 ppm and are positively correlated with Cu (r = 0.4; up to 7400 ppm) and Bi concentrations (r = 0.44; up to 640 ppm). In both deposits, high Cu and Au concentrations occur in small (< 25 μm) anhedral grains of pyrite, which are interpreted to have rapidly crystallized from the ore-forming hydrothermal fluid. In addition, dissolution–reprecipitation textures and thin, concentric, Cu-rich overgrowths were identified in a number of larger (> 25 μm) pyrite grains and aggregates. These abrupt changes in the trace element compositions of pyrite grains likely record episodic metal-rich fluid inputs. We also propose that gold adsorption onto growing pyrite surfaces played a key role in the mineralization of these deposits.

Hydrothermal Alteration of High Sulfidation Epithermal Deposits in Secang Area, Tulungagung, East Java, Indonesia

This research aims to determine geological condition and alteration in an epithermal high sulfidation mineralization as an initial guide for further exploration stages. Detailed geological mapping with scale of 1:12,500 is conducted to identify geological aspects and distribution of alteration zones. Selected rocks samples were prepared for laboratory analysis which are petrography, XRD (X-Ray Diffraction), and FA-AAS (Fire Assay-Atomic Absorption Spectrometry) analyse. Geological condition of the study area consists of six rock units including andesite lava, andesite breccia, diorite intrusion, polymict breccia, limestone, and alluvial deposit. Geological structures found are left strike-slip fault with right strike-slip fault as accompany. Result of XRD analysis shows the presence of clay minerals group: smectite, kaolinite, illite, diaspore, alunite, and pyrophillite. The alteration zones of study area are propylitic, argillic, advance argillic, and silisification zones. The further exploration stage is recommended to focus at the southwest and northeast of study area.

Copper-Gold Fertility of Arc Volcanic Rocks: A Case Study from the Early Permian Lizzie Creek Volcanic Group, NE Queensland, Australia

The Early Permian Lizzie Creek Volcanic Group of the northern Bowen Basin, NE Queensland, Australia, has compositions that range from basalt through andesite to rhyolite with geochemical signatures (e.g., enrichment in Cs, Rb, Ba, U, Th, and Pb, depletion in Nb and Ta) that are typical of arc lavas. In the Mount Carlton district the Lizzie Creek Volcanic Group is host to high-sulfidation epithermal Cu-Au-Ag mineralization, whereas farther to the south near Collinsville (~50 km from Mount Carlton) these volcanic sequences are barren of magmatic-related mineralization. Here, we assess whether geochemical indicators of magma fertility (e.g., Sr/Y, La/Yb, V/Sc) can be applied to volcanic rocks through study of coeval volcanic sequences from these two locations. The two volcanic suites share similar petrographic and major element geochemical characteristics, and both have undergone appreciable hydrothermal alteration during, or after, emplacement. Nevertheless, the two suites have distinct differences in alteration-immobile trace element (V, Sc, Zr, Ti, REE, Y) concentrations. The unmineralized suite has relatively low V/Sc and La/Yb, particularly in the high SiO2 rocks, which is related to magma evolution dominated by fractionation of clinopyroxene, plagioclase, and magnetite. By contrast, the mineralized suite has relatively high V/Sc but includes high SiO2 rocks with depleted HREE and Y contents, and hence high La/Yb. These trends are interpreted to reflect magma evolution under high magmatic H2O conditions leading to enhanced amphibole crystallization and suppressed plagioclase and magnetite crystallization. These rocks have somewhat elevated Sr/Y compared to the unmineralized suite, but as Sr is likely affected by hydrothermal mobility, Sr/Y is not considered to be a reliable indicator of magmatic conditions. Our data show that geochemical proxies such as V/Sc and La/Yb that are used to assess Cu-Au fertility of porphyry intrusions can also be applied to cogenetic volcanic sequences, provided elemental trends with fractionation can be assessed for a volcanic suite. These geochemical tools may aid regional-scale exploration for Cu-Au mineralization in convergent margin terranes, especially in areas that have undergone limited exhumation or where epithermal and porphyry mineralization may be buried beneath cogenetic volcanic successions.

NEW 40Ar/39Ar DATING OF ALUNITE FROM THE CERRO QUEMA Au-Cu DEPOSIT, AZUERO PENINSULA, PANAMA

Cerro Quema is a high-sulfidation epithermal Au-Cu deposit located in the Azuero Peninsula, southwestern Panama. It is hosted by a dacite dome complex of the Río Quema Formation, a volcano-sedimentary sequence of the Panamanian Cretaceous-Paleogene magmatic arc. Cerro Quema has oxide resources of 24.60 Mt at 0.71 g/t Au and 0.04% Cu, and sulfide resources of 11.38 Mt at 0.41 g/t Au and 0.31% Cu. Alunite 40Ar/39Ar dating of a sample from Cerro Quema yielded a final age of 48.8 ± 2.2 Ma (weighted average of plateau age) and 49.2 ± 3.3 Ma (weighted average of total gas age). This age is interpreted to represent the formational age of the Cerro Quema deposit at ~49 Ma, linking it to the Valle Rico batholith intrusive event. Based on the new alunite 40Ar/39Ar data and a reexamination of published geochronological data, magmatic-hydrothermal deposits such as the Río Pito porphyry copper and the Cerro Quema high-sulfidation epithermal deposit formed during the early arc stage (68–40 Ma) in the Chagres-Bayano arc (eastern Panama) and the Soná-Azuero arc (western Panama), respectively. They formed in a similar geodynamic setting at ~49 Ma, when diorites and quartz-diorites intruded Cretaceous volcano-sedimentary sequences. Cerro Quema and Río Pito provide evidence for the exploration potential of Cretaceous-Paleogene arc segments. Exploration should focus on Cretaceous volcanic and volcano-sedimentary sequences intruded by Paleogene batholiths of intermediate to felsic composition.

Silicate-replacive high sulfidation massive sulfide orebodies in a porphyry Cu-Au system: Bor, Serbia

The Cu-Au deposit of Bor (Serbia) represents a continuum of mineralization styles, from porphyry-style ore occurring in quartz-magnetite-chalcopyrite veins and chalcopyrite disseminations to high-sulfidation epithermal Cu-Au ores in pyrite-chalcopyrite and anhydrite-sulfide veins. Decisive for the great economic importance of Bor is the presence of exceptionally rich high-sulfidation massive sulfide orebodies, composed of pyrite + covellite + chalcocite/digenite and minor anhydrite and enargite. They form irregular bodies measuring 0.5–10 million tons of ore grading up to 7% Cu, hosted by andesites and surrounded by intense argillic alteration. This study focuses on a small but rich underground orebody mined out recently, where limited drillcore is preserved for quantitative geochemical study. This paper documents the vein relationships within the deep porphyry-style orebody of Borska Reka, the transitional porphyry-epithermal veins, and the overlying and laterally surrounding epithermal massive sulfides of the Bor deposit. Geological observations indicate that the formation of massive sulfide orebodies concludes the ore formation. Mass balance calculations, recast into geologically realistic bulk fluid-rock reactions, confirm textural evidence that near-isovolumetric replacement of andesite host rocks is the dominant formation mechanism of massive sulfide orebodies at Bor, whereby all lithophile elements including Si are dissolved and only Ti stays relatively immobile. While net volume changes for individual mineralization styles within the massive sulfide orebody vary from − 16% volume loss to + 127% volume gain, overall volume change for the whole massive sulfide orebody was probably slightly negative. Brecciation is important only as means of creating channelways for reactive fluid that turns the andesite protolith into massive sulfide, whereas net breccia infill occurred only locally.