Oxidized sulfur-rich arc magmas formed porphyry Cu deposits by 1.88 Ga

Most known porphyry Cu deposits formed in the Phanerozoic and are exclusively associated with moderately oxidized, sulfur-rich, hydrous arc-related magmas derived from partial melting of the asthenospheric mantle metasomatized by slab-derived fluids. Yet, whether similar metallogenic processes also operated in the Precambrian remains obscure. Here we address the issue by investigating the origin, fO2, and S contents of calc-alkaline plutonic rocks associated with the Haib porphyry Cu deposit in the Paleoproterozoic Richtersveld Magmatic Arc (southern Namibia), an interpreted mature island-arc setting. We show that the ca. 1886–1881 Ma ore-forming magmas, originated from a mantle-dominated source with minor crustal contributions, were relatively oxidized (1‒2 log units above the fayalite-magnetite-quartz redox buffer) and sulfur-rich. These results indicate that moderately oxidized, sulfur-rich arc magma associated with porphyry Cu mineralization already existed in the late Paleoproterozoic, probably as a result of recycling of sulfate-rich seawater or sediments from the subducted oceanic lithosphere at that time.

Evolution of the Magmatic-Hydrothermal System at the Santa Rita Porphyry Cu Deposit, New Mexico, USA: Importance of Intermediate-Density Fluids in Ore Formation

The Santa Rita porphyry Cu deposit in New Mexico, USA, is characterized by a stockwork of three vein types that differ in morphology, mineralogy, and associated alteration assemblages. Early quartz veins associated with potassic alteration are composed of recrystallized granular quartz grains that host ubiquitous hypersaline liquid and vapor-rich fluid inclusions. The early quartz likely formed at high (≳500°C) temperatures and lithostatic pressures. Hypogene Cu mineralization at Santa Rita is in sulfide veins that reopened or crosscut the early quartz veins. The sulfide veins are surrounded by alteration halos containing chlorite and K-feldspar. Rare quartz crystals are present in some of these chalcopyrite and pyrite veins. The cores of the quartz crystals contain hypersaline liquid and vapor-rich fluid inclusions, whereas the rims mostly contain hypersaline liquid inclusions. The quartz crystals are interpreted to have formed close to the ductile-brittle transition as a result of the pressure drop from lithostatic to hydrostatic conditions. Formation of the quartz crystals was postdated by the deposition of Cu sulfides. Grain boundaries between the quartz and the sulfide minerals are irregular in shape, with sulfide crosscutting growth zones in the quartz. The Cu sulfides are interpreted to have formed from intermediate-density fluids that form secondary fluid inclusion assemblages in all earlier-formed quartz types. Microthermometric investigations showed that these fluid inclusion assemblages homogenize at ~385° to 435°C by critical or near-critical behavior and have salinities of <10 wt % NaCl equiv. The precipitation of Cu sulfides occurred as a result of cooling of these fluids following their escape from the lithostatic into the hydrostatic realm. Retrograde quartz solubility caused the corrosion of earlier-formed quartz during Cu sulfide deposition. The youngest veins at Santa Rita are composed of quartz and pyrite. These veins are associated with intense sericite alteration that overprinted all earlier alteration assemblages. The late quartz hosts primary and secondary liquid-rich fluid inclusions, but no intermediate-density fluid inclusions were identified. This quartz vein type formed at temperatures <360°C and hydrostatic pressures. The paragenetic relationships show that hypogene Cu mineralization at Santa Rita postdated potassic alteration of the host rocks. The Cu mineralization was formed by cooling intermediate-density fluids with critical or near-critical densities as they escaped from lithostatic to hydrostatic conditions.

Three-dimensional weights of evidence modelling of a deep-seated porphyry Cu deposit

Given the challenges in data acquisition and spatial modelling at the detailed exploration stage, it is difficult to develop a prospectivity model, particularly for disseminated ore deposits. Recently, the weights of evidence (WofE) method has demonstrated a high efficiency for modelling such deposits. In this study, we propose a framework for creating a three-dimensional (3D) WofE-based prospectivity model of the Nochoun porphyry Cu deposit in SE Iran. The input data include qualitative geological and quantitative geochemical information obtained from boreholes and field observations. We combine ordinary and fuzzy weights of evidence for integrating qualitative and quantitative exploration criteria in a 3D space constrained by a metallogenic model of the study area for identifying a deep-seated ore body. Ordinary weights of evidence are determined for geological data, including lithology, alteration, rock type and structure. Moreover, we determine the fuzzy weight of evidence for each class of the continuous geochemical models created based on the factors analysis of Fe, Mo and Zn concentration values derived from boreholes. We integrate the input evidential models using WofE and create the posterior probability model. We also determine anomalous voxels in the probability model using a concentration–volume fractal model and validate them using a prediction–volume plot and test boreholes. The modelling results indicate the efficiency of the posterior probability model in identifying the anomalous voxels representing copper mineralized rock volumes. We provide open source software for the proposed framework which can be used for exploring deep-seated ore bodies in other regions.Supplementary data: Python scripts for implementing the proposed framework and supplementary files including more details on the evidential models are available at https://github.com/intelligent-exploration/3D_WofE.