Summary:
In the Punitaqui-Ovalle district mineral deposits are related to hydrothermal activity and are considered broadly as epigenetic Cu± Au-Ag-Hg deposits. There are a number of actively producing copper and/or gold mines as well as numerous and widespread prospects and small workings. These mineral occurrences and mines include manto style copper mineralization, generally hosted in calcareous pyritic sedimentary units, structurally controlled copper (gold) deposits, quartz sulphide gold veins, with or without, copper mineralization and structurally controlled massive magnetite deposits with some copper mineralization.
This mineralization is structurally controlled “feeder” structures within dilational zones and shears. Principle mineral types include pyrite, chalcopyrite and bornite. In many deposits, copper and gold mineralization occur with magnetite and hematite which (Sillitoe, 2003) links these deposits to the iron oxide copper-gold (IOCG) family of deposits.
Cinabrio
The Cinabrio copper deposit is hosted within sequence of early Cretaceous volcanic rocks with sedimentary interbeds. The volcano-sedimentary sequence has been designated the El Reloj formation (Thomas, 1967) and, more recently, the sequence has been included in the Arqueros formation (Emparan and Pineda, 2020).
Regional and district structures control the location of copper mineralization. There are three main systems, north-south, northwest-southeast, and east-west trending, which displace the stratigraphy of the Cinabrio mine into blocks. Five main blocks are recognized: Block IV (upper part of the mine) and Block III, II, I and Block 0 (deepest).
Copper mineralization at Cinabrio is largely hosted by the sedimentary sequence. Only small bodies of mineralization have been identified in the adjacent volcanics. The sedimentary sequence at Cinabrio extends for over 700 m along strike and drilling has shown that it extends for at least 1200 m downdip. Mineralization is concentrated in an elongate zone which is 100 m to 300 m wide along strike and extends down dip for at least 1,200 m and is open at depth. Near surface the zone of mineralization widens and extend along strike for over 700 m.
The mineralization consists of chalcopyrite, bornite and pyrite as fine disseminations and in veinlets and breccia infill. The sulphides in veinlets and breccia infill are commonly accompanied by calcite and lesser amounts of quartz.
Locally sphalerite occurs with the copper sulphides. The sphalerite is commonly distributed around the margins of the zones of copper mineralization.
The main mineralized host rocks are dark colored laminated and unlaminated shales. The shales host the bulk of the fine-grained disseminated mineralization. Locally, higher grade disseminated mineralization correlates with the presence of bornite as the dominant sulphide.
Mineralization in the tuffaceous rocks occurs as disseminated sulphides and veinlets and breccia infill with the sulphide minerals generally coarser grained.
A progressive zonation of sulphide mineralization is evident in some drill sections. From the center of mineralized zones to the margins this zonation is: 1. Bornite to bornite-chalcopyrite; 2. Chalcopyrite to chalcopyrite-pyrite ± sphalerite; and 3. Pyrite.
The mineralization at Cinabrio is related to the interaction of migrating copper-rich, sulphur poor fluids with pyrite-rich sedimentary rocks.
San Andres
San Andres is a zone of copper mineralization located 500 m southwest of the high-grade Cinabrio deposit. The host rocks and copper mineralization at San Andres is very similar to Cinabrio. The stratigraphic setting at San Andres is the same as the Cinabrio deposit.
The San Andres zone is interpreted to be a structural offset of the Cinabrio stratigraphy along an extensional fault known as the San Andres fault. The San Andres zone is the structurally offset, up dip part of the Cinabrio deposit. The San Andres fault strikes north-northwest and dips -30o to -40o to the west. The apparent offset along the fault is around 900 m.
The San Andres copper mineralization is hosted within an east dipping tabular sedimentary horizon within the volcanic sequence. This sedimentary horizon is variably mineralized and ranges in width from 5 m to 30 m. The horizon dips -40o to -50o east and is cut-off at depth by the moderately west dipping San Andres fault.
Like at Cinabrio to the east, the TSU sedimentary horizon consists of an interlayered volcano-sedimentary sequence composed of dark colored laminated and unlaminated shales, volcanoclastic sandstone, conglomerates and breccias and tuff breccias. There is a variable component of syngenetic pyrite.
The host horizon is also cut and offset by other faults with a wide range of orientations. The fundamental orientations identified to date include: Moderately west dipping splays of the San Andres fault, generally with downward and westward movement; Steep dipping northeast to northwest trending faults with both sinistral and dextral offsets.
The mineralization is predominantly chalcopyrite and bornite. It consists of veinlets and irregular disseminations in both the fine and coarse-grained clastic rocks and locally within the volcanic rocks above and below the host unit.
The intersection of the host sedimentary unit and the San Andres fault plunges toward the south. Because of this, the potential volume of ore within the host sedimentary horizon increases towards the south.
Dalmacia
Copper mineralization at Dalmacia over a 1.6 km strike with the bulk of the current resource delineated a Dalmacia North within an area of approximately 200 m x 400 m. The geometries of the bodies are tabular and irregular with an approximate strike of N330°E and dips of -30° to - 70° southwest. Controls of mineralization include lithology, lithologic contacts, and structures.
Mineralization occurs in ocoites, andesites, andesite porphyries and sandstone. The most common host rock is white ocoites with vesicular white ocoites often having the highest grades. Copper minerals include chalcopyrite and bornite and rare covellite, chalcocite, digenite and pyrite.
The sulphides occur as disseminations, veinlets, and infill. The textural relationships indicate the initial formation of magnetite, which was partially replaced by chalcopyrite and bornite. Late veinlets of chlorite ± sericite ± pyrite ± chalcopyrite suggest at least two copper mineralizing events.
Between surface and 50 m depth oxide copper minerals occur generally with partially oxidized sulphides. Below 50 m oxide copper minerals are rare.
The paragenetic relationship magnetite – bornite - chalcopyrite within the potassic alteration patches would indicate that the hydrothermal system originated at temperatures between 500°C and 350°C and intermediate sulphidation stages (Einaudi M., et al., 2003) as the first mineralization event. The second event would be related to the presence of veinlets of epidote + chlorite ± chalcopyrite ± pyrite ± sericite, associated with a phase of retrograde alteration related to metasomatism.
Cinabrio Norte
The Cinabrio Norte target is the northern extension of the main Cinabrio deposit. The Cinabrio Norte target is only 110 m north of the Cinabrio underground workings on the 220 m level.
Mineralization has a strike length of about 400 m and ranges between 10 m and 30 m thick. It is continuous to a depth of around 350 m. The Copper mineralization dips steeply to the east and is occasionally locally faulted with small offsets. The mineralization is predominantly chalcopyrite and minor bornite and pyrite.
The mineralization is variable and believed to be controlled by mineralizing fluids focused along structures within the footwall rocks. Syngenetic pyrite is a common constituent of the sedimentary unit.