Geology
- Property Geology -
Lithology
The Haquira area is dominated by Jurassic-Cretaceous sedimentary rocks which are intruded by Oligocene-Miocene aged intrusive rocks. The following descriptions have been condensed from an Antares Minerals Inc. internal consultants report by Pratt (2006).
SEDIMENTARY ROCKS
Chuquibambilla Formation
The Chuquibambilla Formation is Tithonian (Upper Jurassic-Berriasian to Lower Cretaceous) in age (Gutierrez, 1981). This formation is the lowest stratigraphic unit found on the Haquira property. Within the Haquira property, the Chuquibambilla Formation forms the core of a major anticline; with only the northern flank of the anticline (overturned towards the north) being observed during this survey, around Cerro Juñacota. The formation is affected by tight parasitic folds and faulting/thrusting. The INGEMMET (1994) map depicts the contact with the stratigraphically overlying Soraya Formation as a fault.
On the Haquira property, the lowest observed part of the formation (>250 m) (‘Areniscas olivas’) comprises interbedded sandstones, siltstones and mudstones. The sandstones are mostly medium- to thickly-bedded, fine-grained olive green/grey feldspathic sandstones (sub arkoses). There are also a few 1 m-thick quartz arenite beds, which form bedding features in otherwise poorly exposed ground. Limonite staining is widespread, particularly in the quartz arenites. The sandstone beds commonly contain swarms of quartz tension gash veins. The intervening rocks vary from light grey fissile mudstones to olive green siltstones. Both mudstones and siltstones are commonly weakly cleaved
The Areniscas olivas are overlain by dark grey fissile mudstones (Lutita negra) with very thin laminated siltstone beds. This mudstone sequence, about 50-100 m thick, is an important stratigraphic marker horizon and forms a distinctive gully with few natural exposures. The contact with the underlying olive-green sandstones appears to be a thrust and is everywhere associated with strong shearing, duplexed sandstones and water springs.
The dark grey mudstones are overlain by a distinctive sequence (150-200 m) of tough flaggy olive green siltstones and silty mudstones with scattered olive green fine-grained sandstone beds (Limolitas olivas). These beds frequently show signs of contact metamorphism, with very small andalusite (?) porphyroblasts or fine-grained secondary biotite. Ammonites up to 0.25 m in diameter were encountered near the top of the formation, in the Lutita negra, and much lower down stratigraphically, in olive green sandstones.
Soraya Formation
The Soraya Formation is Valenginian-Hauterivian (Lower Cretaceous) in age (Gutierrez, 1981). The Soraya Formation is the main stratigraphic unit found on the Haquira property and consists of multiple packets of massively bedded quartz arenites in the order of 10-50 m thick. The quartz arenites are white- to light grey and range from fine- to coarse grained, locally pebbly. They are generally well sorted, though the coarser varieties show poorer sorting. Planar lamination and large-scale dune cross-stratification is widespread. The arenite bodies are very continuous and tabular in form, akin to braided-, rather than meandering, river systems.
The base of the formation is placed at the first appearance of significant beds of quartz arenite. Exposure is not good in the critical area, but the quartz arenites seem to overlie the Limolitas olivas of the Chuquibambilla Formation. The quartz arenites are generally very indurated and cemented by diagenetic quartz overgrowths. The detrital feldspar content varies from a few percent, in the cleanest quartz arenite, to 20% in some of the stratigraphically lowest packets.
Sedimentary rocks near the top of the formation comprise very fine grained, limonite-stained quartz arenites and light green/grey to purplish siltstones. These show a much greater variety of bedforms, with small-scale dune, and ripple, cross stratification. They are typified by strong goethite staining. Unusual Liesgang-type patches and halos of brick red silicified (?) material are widespread and characteristic of the sequence.
There is an upward trend of declining feldspar content and increasing grain size in the Soraya Formation sandstones. This culminates in coarse-grained channelised pebbly quartz arenites on the western flank of Cerro Tocone. Structural complications make correlations uncertain, but these appear to be the highest strata in the Soraya Formation.
Mara Formation
The Mara Formation is Aptian-Neocomian (Lower Cretaceous) in age (Gutierrez, 1981). The Mara Formation lies conformably above the Soraya Formation and consists of about 350 m of reddish sandstones and mudstones with horizons of dark mudstone and white ‘quartzites’; yellow sandy limestones occur near the top. Within the Haquira property, the Mara occurs as thinly bedded, laminated purplish silty mudstones and siltstones grading upward into light green- to olive green siltstones with packets of fine grained sub-arkoses (i.e., 20% detrital feldspar) within the Tocone Syncline; and as footwall to the Tocone Thrust.
Ferrobamba Formation
The Ferrobamba Formation is Middle Cretaceous in age and consists of limestones that host the Cu-skarn deposits of the Tintaya and Las Bambas areas. These limestones do not appear to occur within the immediate Haquira property area, however they have been observed in the core of a major upright syncline about 2 km to the east of the project, close to village of Ccasa.
Pyroclastic Flow Lithic Tuff (Ignimbrites)
The Jurassic-Cretaceous sedimentary sequence is blanketed by very young (Pliocene?), columnar jointed dacitic ash flow tuffs and ignimbrites. Within the Haquira property this unit not been widely recognized in the drill holes and is typically found in topographic lows and infilling valleys. These volcanic rocks consist of pyroclastic flow rocks that include fine ash tuffs, and heterolithic breccias which contain abundant sub-rounded to sub-angular heterolithic clasts of both intrusive and sedimentary rocks described above. Clasts range in size from a few millimetres to several centimetres. The majority these rocks are massive, poorly sorted, and poorly bedded. These rocks are earthy to greyish-yellow coloured. Sometimes they can be easily misidentified with colluvium, especially in RC drill holes; because it is very common to find them clay altered due to supergene weathering.
INTRUSIVE IGNEOUS ROCKS
Two major (Haquira & Pararani) and two minor (Lahuani & Ccahuanhuire) intrusive phases have been identified within the Haquira property to-date. Most of the porphyries are dykes; they occupy N- to NW-striking, sub vertical conduits. There is a distinct concentration of dykes, and other phenomena such as breccias, quartz + sulphide stockworks, massive sulphide lodes, and faults, within crudely north-south corridors. There are numerous examples of minor sills that are contemporaneous with dykes. Several examples of dykes with sill-like apophyses were seen in drill platforms. Magma was clearly intruded along the paths of least resistance, but in a dominantly E-W extensional regime. Sills were intruded along thrusts and within significant fine grained sedimentary units within the Soraya quartz arenites.
Haquira Porphyry
The Haquira Porphyry and its chilled-margin equivalent are volumetrically the most important intrusive rocks identified to-date on the Haquira property. Petrographic work indicates this intrusive unit is compositionally a monzonite. The Haquira Porphyry is the oldest intrusive unit recognized to-date on the property and is typically equigranular to porphyritic, leucocratic, medium- to coarse-grained, with locally developed, narrow aplite dykes; which appear to be a late-stage of the main intrusive phase, based on irregular, intergrown boundaries between the two phases. The unit contains: 30-40% tabular-shaped, subhedral plagioclase phenocrysts, 5-8% biotite phenocrysts, occasional amphibole phenocrysts, and rare quartz phenocrysts. The Haquira Porphyry chilled-margin phase is variably porphyritic, with plagioclase and biotite phenocrysts set in an aphanitic groundmass.
The Haquira Porphyry contains abundant disseminated chalcopyrite, pyrite, and molybdenite in the groundmass and associated with classic “A” and “B” type quartz-sulphide veins and veinlets; both as stockworks and sheeted vein systems. The majority of the better hypogene copper grades are associated with this intrusive unit.
Pararani Porphyry
The Pararani Porphyry and its chilled-margin equivalent are volumetrically the second most important intrusive units on the Haquira property after the Haquira Porphyry. The Pararani Porphyry occurs as discrete dykes who strike NW and WNW. Petrographic work indicates this intrusive unit is compositionally a quartz monzonite. The Pararani Porphyry is typically porphyritic, light to medium grey coloured, with 40-45% subhedral, white plagioclase phenocrysts, 1-3% biotite (and locally amphibole) phenocrysts, 2-3% quartz phenocrysts, and traces of disseminated magnetite; all set in a fine-grained groundmass. A distinctive characteristic of the Pararani Porphyry is the presence of large (i.e., 1 to 3 cm sized), potassium-feldspar megacrysts and oval-shaped mafic cognate fragments ranging up to three centimetres in size. The Pararani Porphyry chilled-margin phase is porphyritic, finer-grained, and dominantly aphanitic groundmass material. Both phases typically contain abundant disseminated pyrite (and very rare chalcopyrite) in the groundmass.
This intrusive unit clearly crosscuts the other intrusive rocks recognized on the Haquira property, based on intrusive and chilled contacts. In addition, this intrusive phase seems to be a late-stage in the mineralizing events recognized on the Haquira property; however weak chalcopyrite veining has been described for this unit in some drill holes. This suggests that the Pararani Porphyry is a late intra-mineral intrusive phase. Although typically poorly mineralized in the hypogene portion of the deposit, these dykes can be extremely well mineralized with secondary copper mineralization in the supergene oxidized and enriched zones of the deposit.
Lahuani Porphyry
The Lahuani Porphyry and its chilled-margin equivalent are volumetrically limited; with only small, NW-striking dyke bodies having been identified to-date. Petrographic work indicates this intrusive unit is compositionally a quartz monzodiorite. The Lahuani Porphyry is porphyritic, leucocratic, medium-grained, with 10-15% subhedral plagioclase phenocrysts, 2-3% amphibole and biotite (locally up to 10%), and 5% quartz phenocrysts; all set in an abundant aphanitic groundmass. The Lahuani Porphyry chilled-margin phase is porphyritic, finer-grained, and dominantly aphanitic groundmass material.
The Lahuani Porphyry contains disseminated chalcopyrite, pyrite, and molybdenite in the groundmass and mineralized quartz veins; although less abundant then the Haquira Porphyry phase described above. The textural and mineralization similarities between the Lahuani and Haquira Porphyry units and the fact that no cross-cutting relationship between the two phases has been observed, suggests that the two phases may be related.
Ccahuanhuire Porphyry
The Ccahuanhuire Porphyry forms a large outcrop of deeply weathered rock in the Tocone Syncline. The intrusion is the coarsest and most mafic-rich igneous rock found to-date on the Haquira property. Compositionally, it is probably a quartz diorite. The texture is moderately porphyritic, with a fine-grained plutonic groundmass. There are abundant small euhedral amphibole (hornblende) phenocrysts, with scattered large poikilitic euhedral biotites. The Cchuanhuire Porphyry is shows more plutonic, higher temperature characteristics such as: 1) common aplitic dykes, locally pegmatitic with k-feldspar megacrysts up to > 20 mm long, 2) extensive biotite hornfelsed country rocks (Mara?) (with later actinolite + epidote stockworks), and 3) vein-dykes.
BRECCIAS
A significant body of breccia occurs in the vicinity south-central portion of the Haquira deposit. The breccia comprises mainly clast-supported quartz arenite blocks, but is locally more polymictic, with scattered quartz + muscovite-altered porphyry clasts, finer grained sedimentary rocks, and wispy green fine grained rock. Clasts vary in sorting, rounding, and diameter, but rounding is generally good. Sill-like bodies of breccia penetrate the country rock in many places, giving the impression of laccolith-like bodies. Overall, the geometry of the main body of breccia seems to be intermediate between a sill and a dyke. It appears to cut across the bedding at a low angle, but sends off minor sills into the country rock in several places. Thin N-S-striking sub vertical pebble dykes, rarely more than 0.5 m thick and narrow sills occur in many places.
The breccia matrix, commonly < 10% by volume, comprises sugary granular quartz. It seems to be derived by milling of the quartz arenites. The matrix is generally not friable. Instead, it is commonly strongly cemented and difficult to break with a hammer. Elsewhere the matrix includes voids with limonite (after sulphides) and drusy euhedral quartz and coarse grained muscovite with sparse quartz veins.
The breccias are widely invaded by a deeply-weathered, clay-altered, porphyry (probably Pararani?) with a weak sulphide (now limonite) stockwork and scattered quartz veins.
A syn-mineral origin is proposed for the breccias since they include clasts of porphyry, cut the Ccahuanhuire Porphyry, but are also intruded by Pararani Porphyry dykes and cut by scattered quartz veins.
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