Field trip: Geology and Metallogenesis of Mining Distric of Vetas-California
Luis and Hernando are professors of the Universidad Industrial de Santander and researchers of mineral deposits. Christoph A. Heinrich is a Professor of Economic Geology at the Institute of Geochemistry and Petrology at the ETH Zürich. Seventeen geology students from the Universidad Industrial de Santander and five professionals of different mining companies (such as AngloGold Ashanti and Aux) participated in this field trip which was sponsored by SGA, SGA-Colombia Student Chapter and SEG-UIS Student Chapter. The Vetas-California Mining District (VCMD), located in the central part of the Santander Massif (Colombian Eastern Cordillera) at 40 km NE from Bucaramanga in the Santander Department, Colombia. The area hosts important porphyry and epithermal style Au and base metal mineralization (Mantilla et al., 2013).
The Field trip took place at three key points for the recognition of hydrothermal and supergene alterations. First place was the road that leads from Vetas to California, the second was California-La Baja road and finally, the third place was in a tunnel 50 m near the second place. During fieldwork, three main lithological varieties were recognized. Precambrian rocks of Bucaramanga Neis, Late Triassic rocks such as “Alaskite I” and andesitic porphyries of the Miocene.
On California-Vetas road, locally are recognized a Mo + Qz veinlets or type-B Veins, which are hosted in Alaskitic rocks. Around these veinlets, it isn't recognized alteration minerals coetaneous with the formation of these hydrothermal structures, due to the subsequent hydrothermal alteration of pyrite- quartz-sericite type.
Sericitic alteration is the most predominant in the California-Vetas road and it usually looks slightly yellowish-white colored by the pervasive replacement of from feldspars. Several quartz-pyrite- sericite halos veinlets or type-D Veins are observed which are oxidized in the majority of cases. These veinlets exhibit straight geometry, with thicknesses between 0.5 and 3 cm. These veinlets represent low temperatures and crosscut alaskites and porphyritic rocks of Late Miocene.
On California-La Baja road there is a place called La Virgen. At La Virgen were observed advance argillic and argillic alteration assemblages, comprising zones of quartz-alunite alteration surrounded by kaolinite-illite alteration overprinting sericite-quartz alteration. Advance argillic and argillic alteration assemblages has been observed to be affecting the gneissic rocks. Alunite–quartz alteration is the main ore zone comprising vuggy quartz and massive alunite alteration. Kaolinite-illite alteration shows a pervasive alteration in which feldspars are completely replaced by those minerals. These hydrothermal alterations are due to the increase of acid fluids in shallow environment at epithermal deposits. At 50 meters from La Virgen there is a tunnel and some outcrops on California-Vetas road, which present a secondary copper enrichment as Cu2CO3. Malaquite (Green) – Azurite (blue) are formed by product of leaching of copper oxides and supergene alteration.
During the field trip, it was observed that the mining district has a high potential in precious metals such as gold, silver and copper. Through, cutting relationships, veinlets and hydrothermal alteration, we can infer that a typical deposit from an empleaced porphyry systems to a high sulfidation epithermal environment.