Alteration

Geology

 - Property Geology

 - Alteration

The main characteristics for each of these alteration types are discussed below. Zonation of the alteration is roughly concentric, with a strong to intensely altered central core area, where mainly intrusive subvolcanic rocks are located, grading outward to less altered rocks in the surrounding areas. Rocks in the outlying areas are dominated by intermediate volcaniclastic rocks (IVc) and red bed sedimentary rocks (RB) which often exhibit clay and/or chloritic or weak propylitic alteration. Diorite intrusive rocks found to the east and inside the core area are almost pristine, usually showing only selective chloritic alteration after mafic minerals.

Near the core of the Rio Grande system, the rocks exhibit variable degrees of potassic, K-feldspar alteration. The innermost zone of the main igneous centre contains broad zones of moderate to intense calcic-iron-sodic (CaFeNa) alteration that commonly consists of variable amounts of diopside, magnetite, scapolite and actinolite. Limited areas of potassic biotite altered rocks are confined to the central area of the Rio Grande system.

The following alteration descriptions were derived from a variety of sources, including: field observations, core logging, rock staining, and thin sections. In addition to the six (6) major alteration types, a number of common “accessory” alteration minerals were also observed, including: sericite, silica, calcite, anhydrite, apatite, and titanite.

Sericite appears to present in many altered rock types based on thin section studies. In rare cases individual, secondary white mica crystals can be seen with naked eye. Anhydrite occurs as soft, translucent veinlets in many different rock units. In drill core, some of them are accompanied by sulphides, however good cross-cutting examples between pyrite-chalcopyrite veinlets and later anhydrite have been documented. White to translucent calcite occurs as fracture infillings in all rock units. Thin section results show that it is commonly associated with presence of chlorite. Apatite and titanite are present in the majority of thin section samples studied, with important secondary hydrothermal growth of apatite crystals in some cases. It is notable the presence of these calcic minerals and calcic clinopyroxene (diopside?) in several samples. Also one thin section sample contained fluorite.

The relative chronology of alteration events established at Rio Grande, from oldest to youngest, is as follows:

1. Potassic – K-feldspar (PK) - An early, pervasive, moderate to strong K-feldspar alteration event affected rocks within the central part of Rio Grande system. The distribution of this alteration type is widespread, with almost every sample sent for thin section showing this type of alteration.
   
   
2. Silicification (SIL) - Silica or silicification appears to be related to quartz veinlets, or as selective replacement of the groundmass. In some cases, quartz veinlets are seen together with magnetite or sulphides, and can develop sheeted veinlets and stockworks. The exact timing of this alteration is not known.
   
   
3. Calcic-Iron-Sodic (CaFeNa) – The CaFeNa alteration event occurs in a variety of intensities and habits (see below). Thin section studies describe cross-cutting relationships between clinopyroxene (diopside?) veinlets and the early potassic K-feldspar alteration. Cross-sections interpreted with surface and drill-hole information suggest that the CaFeNa alteration forms a relatively shallow “blanket like” or “cap like” feature. Some evidence, such as K-feldspar alteration envelopes on diopside-magnetite veinlets, suggests that there may be continuum or contemporaneous relation between the K-feldspar and CaFeNa events.
   
   
4. Potassic – Biotite (PB) – Finely disseminated biotite and biotite ±magnetite ±K-feldspar veinlets are clearly seen cross-cutting the previous alteration types.
   
   
5. Chloritic / Propylitic (CHL/PRO) – Chloritic and/or propylitic alteration is developed mainly in the rocks surrounding the main central Rio Grande alteration system.
   
   
6. Iron Oxide / Clay (FeOx / CLAY) – Fe-oxide/clay alteration, largely controlled by surface supergene processes, is the youngest event and overprints all the other alteration types. The downward penetration of this alteration is important and drill holes show that secondary iron oxides can be found at greater than 300 metres depth below surface.