Castelo de Sonhos Gold Project
Project Location and Infrastructure
The Castelo de Sonhos project is situated in the southwestern Pará state. The property has easy road access all year round. The Cuiabá-Santarém road is a paved highway through the village of Castelo de Sonhos that goes south to Mato Grosso and north to Santarém. The Castelo de Sonhos property encompasses a 15km by 12km plateau with an average elevation of 650 meters (a.s.l.) and around 300-350 meters above the surrounding plains.
Castelo de Sonhos was one of the most famous and gold rich alluvial garimpos in all the Tapajós. The main gold rush took place there between 1985 and 1990 and it is estimated that 300,000 ounces of gold has been produced. By early 1995, Barrick Gold began to pay attention and soon recognised that the source of the alluvial gold was the Castelo de Sonhos plateau. So, in June 1995, after optioning the property, Barrick started an aggressive exploration program and discovered the conglomerate hosted gold mineralization.
Geology and Mineralization
The Castelo de Sonhos Formation is an isolated package of sandstones and conglomerates which form a roughly circular plateau rising 300 m above the surrounding plains. The plateau lies within the Amazon Craton, bounded on the east by volcanic, plutonic and sedimentary rocks of the Iriri-Xingu domain of the Central Amazon Province with ages dating back to ~3Ga (Santos et al., 2001; Bizzi et al., 2003), and on the west by the 2,030 – 1,860 Ma Tapajós-Parima Province, consisting of granodiorites, monzogranites and syenogranites that belong to the Creporizao and Parauari Suites. Both are intruded by the slightly younger monzo- and syeno-granite stocks of the Maloquinha Suite. The predominantly granitic Tapajós-Parima Province borders the Castelo de Sonhos Formation on the north and west.
The gold mineralization is intimately associated with the conglomerates. Strong gold mineralization is often seen with hematite alteration, and sometimes with silicification as well. But not all hematite-rich layers are gold-bearing. Although gold is predominantly found in conglomerate, it also occurs within an iron-rich hangingwall quartzite at Esperança South. Furthermore, gold is sometimes also found in iron-poor sediments of all types. In both conglomerates and quartzites, gold occurs as sub-millimeter grains within the matrix.
On January 29, 2016, TriStar Gold Inc. (TriStar or TSG) reported the following as the exploration target range for its Castelo de Sonhos (CDS) gold project in Brazil.
Tonnage: 50 – 84 Mt
Gold Grade: 1.3 – 1.6 g/t
Metal Content: 2.1 – 4.3 Moz
Note: This is conceptual in nature and is not an estimate of mineral resources. There is no guarantee that the further drilling required to estimate a mineral resource will be done. Nor is there any guarantee, if further drilling and exploration are done, that the project’s mineral resources will eventually reach this target range.
Exploration Target Range Methodology
The exploration target range presented in above is based on a quantitative risk assessment (QRA) that uses a probabilistic method usually referred to as “Monte Carlo”. The approach used in TriStar’s QRA follows the approach used by the petroleum industry for establishing the range of potential resources for oil and gas reservoirs. 100 plausible computer models are created, each one of which honors all of the available data and geologic information. Each of these models is different in detail, but since each of them honors all the data and geologic information, they are all equally likely as models of what might occur in the sub-surface. The collection of 100 scenarios defines a distribution of possibilities. Once these are ranked from the least promising to the most promising, the 10th percentile (P10) and the 90th percentile (P90) are retained as the two that represent pessimistic and optimistic scenarios.
For the Castelo de Sonhos QRA, the 100 models were developed using geostatistical simulation of the lithologies and grades in the original depositional environment. These models were then structurally deformed to conform with the present day CDS configuration: folded into a syncline that plunges to the southwest, with its axial plane tilted, and faulted to create the separation of Esperança East. In its current configuration, geostatistical simulation was used to model the location and intensity of hematitic alteration.
The geostatistical simulations require assumptions about the large-scale trends in the variables being simulated. The proportions of the lithologies were assumed to change in the way they do in well-studied fluvio-deltaic systems, with high-energy lithologies (the pebble-supported conglomerates) dominating near the shore and lower-energy lithologies (the sandstones) becoming more dominant as one moves away from the shore in the seaward direction.
Software from the petroleum industry was used to model the geometry of the sinuous channels at Castelo de Sonhos. These simulated channels then provided the local directions of maximum continuity used for geostatistical interpolation of gold grades.
The combination of the lithology, grade and alteration simulations allows the creation of block models that not only honor the available drill hole data, where it exists, but that also captures the geometry and style of mineralization that is consistent with the original depositional environment and with subsequent structural deformation of the deposit, and with the remobilization of gold during metamorphism related to nearby intrusions.
From this set of 100 plausible models of the sub-surface of the plateau, the P10 and P90 scenarios were identified by ranking the models per the metal content inside an ultimate pit shell. The values reported in above are the in-pit tonnages, grades and metal content for those two specific scenarios: the P10, which has a 90% chance of being more pessimistic than reality, and the P90, which has a 90% chance of being more optimistic than reality.