Gema R. Olivo

Transamazonian tectonism and Au-Pd mineralization at the Cauê Mine, Itabira District, Brazil: Pb isotopic evidence

Abstract Pb isotopic results are reported for mineral separates from iron formations that host Pd-Au mineralization at the Caue Mine, in the Itabira District of the Southern Sao Francisco Craton, as well as for an amphibolite that has been tectonically imbricated with the iron formations during the earliest D1 tectonic event in this area. Pb isotopic results were obtained for hornblende and ilmenite that were rotated within the regional S1 foliation of an amphibolite. These minerals yielded a metamorphic age of 2.66 ± 0.27 Ga and a time-integrated μ1 value of 8.35. This shows, for the first time, the involvement of late Archean to Paleoproterozoic basement rocks in the tectonic history of the Itabira District; this unit is tentatively correlated with the Archean Rio das Velhas Supergroup in the Quadrilatero Ferrifero. Pb isotopic results were also obtained for quartz, gold and specular hematite that were formed during the transformation of the iron formations into itabirite and jacutinga which occurred during the D1 event. These minerals, which are stretched within the S1 foliation plane, yielded an age 1.83 ± 0.10 Ga which corresponds to the Tranzamazonian Orogenesis in the southern part of the Sao Francisco Craton. Other Pd-bearing gold deposits hosted in Minas Supergroup iron-formations of the southern Sao Francisco Craton have many similarities with the Caue mine, suggesting that Pd-Au mineralization may have been a widespread process associated with the tectonic transport of the Minas sequence during the Transamazonian orogenesis.

The Santa Rita gold deposit in the Proterozoic Paranoá Group, Goiás, Brazil: An example of fluid mixing during ore deposition

Abstract The Santa Rita gold deposit (Central Goias, Brazil) is hosted by Middle to Upper Proterozoic carbonate-pelite sequences of the Paranoa Group that have been metamorphosed in the greenschist facies. The ore is contained in pyrite-bearing quartz-carbonate veins. The mineralization is structurally controlled by WNW-ESE high-angle faults and fractures resulting from the reactivation of older NE-SW lineaments. Pyrite is the sole sulphide and it shows growth zones enriched in Co, Ni and As (up to 4 wt%). Hydrothermal alteration zones are enriched in Co, Ni and As and are characterized by diffuse albitization, carbonatization, silicification and pyritization. A fluid inclusion study on quartz from pyrite-bearing quartz-carbonate veins led to the identification of two fluids: (1) a highly saline CO 2 N 2 -rich aqueous fluid with halite and ± sylvite daughter minerals, and (2) a CO 2 N 2 rich aqueous fluid with moderate salinity. The two fluid types occur in the same quartz domain and display great variation in the degree of filling and notable dispersion of the microthermometric data. On heating, all the inclusions decrepitate between 200° and 300°C. Raman spectrometry detected high concentrations of N 2 in the gas phase, with a CO 2 N 2 molar ratio between 1 and 19 and a small proportion of CH 4 (up to 2 mole %). The simultaneous entrapment of compositionally variable fluids in the system H 2 OCO 2 N 2 NaClKCl allow us to propose a mechanism of heterogeneous trapping. The entrapment may result from the mixing of a high-salinity fluid (H 2 ONaClKCl system) with a carbonic fluid (H 2 OCO 2 N 2 system) produced by the devolatilization process of carbonate and phyllitic host rocks. Considering the absence of spatially and temporally related igneous activity and the low P - T regional metamorphism in the Paranoa Group, the brines are inferred to result from leaching of evaporites occurring in the lower part of the Paranoa lithostratigraphic column. Gold was probably initially transported as an AuCl 2 -complex ( T >300°C, low pH, moderate ƒ O 2 - pyrite field stability). As temperature decreased below 290°C, the “switch-over” process would lead to the predominance of Au(HS) 2 − in the fluid. Pyrite precipitated in this temperature interval. The oscillatory zoning of the AsCoNi-bearing pyrites indicates episodic fluctuation of the fluid composition. Such changes in fluid composition are favoured by a mechanism of fluid mixing by intermittent supplies in the hydrothermal system. The proposed mechanism of heterogeneous trapping of two separate fluids in the system H 2 OCO 2 N 2 NaClKCl and the resulting changes in the physicochemical conditions caused by the fluid mixing appears as a conspicuous process for the Santa Rita hydrothermal fluid evolution. A model based on the existence of a Proterozoic geothermal system involving the regional thermal gradient is proposed.

Paragenesis and mineral chemistry of alabandite (MnS) from the Ag-rich Santo Toribio epithermal deposit, Northern Peru

Abstract The Miocene, epithermal, Ag-rich polymetallic Santo Toribio deposit is hosted by the volcanics of the Quiruvilva-Pierina subbelt, Northern Peru, which also comprises the world-class, high sulphidation Pierina deposit. The Ag-rich, alabandite-bearing veins of the Santo Toribio deposit formed during two major stages. The early stage is characterized by deposition of arsenopyrite, pyrite, quartz, Mnsphalerite, stannite, alabandite and minor miargyrite. Sphalerite associated with this stage is exceptionally enriched in Mn (up to 14.5 wt.%) and alabandite is optically and mineralogically zoned. Its brown zones have greater Fe+Sb and smaller Mn contents than the green zones and Fe+Sb replaces Mn in its structure. During this early stage, fS₂ must have been high to allow the stabilization of alabandite relative to rhodochrosite. In the second stage, the physicochemical conditions changed and the CO2/S ratio increased, causing dissolution of alabandite and the deposition of abundant rhodochrosite and a second generation of arsenopyrite, pyrite and quartz, sphalerite with chalcopyrite inclusions, mirargyrite, ramdohrite, and finally stibnite. This polymetallic ore probably formed due to an abrupt decrease in H+ and/or Cl- concentration caused by boiling or dilution of the high-salinity hydrothermal fluids and constitutes an example of ‘‘intermediate sulfidation-state’’ epithermal deposits.

Bayesian inference of spectral induced polarization parameters for laboratory complex resistivity measurements of rocks and soils

Abstract Spectral induced polarization (SIP) measurements are now widely used to infer mineralogical or hydrogeological properties from the low-frequency electrical properties of the subsurface in both mineral exploration and environmental sciences. We present an open-source program that performs fast multi-model inversion of laboratory complex resistivity measurements using Markov-chain Monte Carlo simulation. Using this stochastic method, SIP parameters and their uncertainties may be obtained from the Cole-Cole and Dias models, or from the Debye and Warburg decomposition approaches. The program is tested on synthetic and laboratory data to show that the posterior distribution of a multiple Cole-Cole model is multimodal in particular cases. The Warburg and Debye decomposition approaches yield unique solutions in all cases. It is shown that an adaptive Metropolis algorithm performs faster and is less dependent on the initial parameter values than the Metropolis-Hastings step method when inverting SIP data through the decomposition schemes. There are no advantages in using an adaptive step method for well-defined Cole-Cole inversion. Finally, the influence of measurement noise on the recovered relaxation time distribution is explored. We provide the geophysics community with a open-source platform that can serve as a base for further developments in stochastic SIP data inversion and that may be used to perform parameter analysis with various SIP models.

Publisher Correction to: Expanding the size of multi-parameter metasomatic footprints in gold exploration: utilization of mafic dykes in the Canadian Malartic district, Québec, Canada

The original version of this article contained a mistake.

SPECTRAL INDUCED POLARIZATION SIGNATURES OF ALTERED METASEDIMENTARY ROCKS FROM THE CANADIAN MALARTIC GOLD DEPOSIT BRAVO ZONE, QUÉBEC, CANADA

At the Canadian Malartic deposit, gold mainly occurs as fine inclusions in 1 μm to 1 mm pyrite grains hosted in altered metagreywacke. Previous time-domain induced polarization surveys conducted over this deposit have, however, failed to delineate known areas of pyrite alteration in the metasedimentary host rocks. To define the petrophysical footprint of the gold mineralization using an alternative approach, spectral induced polarization (SIP) measurements were conducted both on drill core samples in the laboratory and in situ, at a key outcrop zone. Three groups of SIP spectra are identified and interpreted using Mineral Liberation Analysis of polished thin sections and by performing Bayesian inference of SIP parameters using a Debye decomposition model. Values of Debye mean relaxation time (τ) and total chargeability (Σm) are used to characterize the respective SIP signatures of ilmenite-rich mudstones and monzodiorite (τ = 0.10 s and Σm = 17%), mineralized metagreywacke (τ = 0.05 s and Σm < 10%), and barren metagreywacke (τ < 0.03 s with variable Σm).