Summary:
The Tres Cruces property hosts gold deposits that have characteristics of a low- to intermediate- sulphidation type epithermal system. The Tres Cruces deposit, as well as fitting a low-sulphidation classification, is also considered to have intermediate-sulphidation characteristics, such as alteration consisting of vuggy silica and illite at depth in the system. Rocks resembling silica sinter have been observed in surface exposures that may post- date and overprint the underlying mineralization and alteration assemblages.
PROJECT GEOLOGY
The Tres Cruces area is underlain by Calipuy Group calc-alkaline volcanic rocks, predominantly andesitic, with minor dacitic and locally rhyolitic units. At depth, unconformably underlying these volcanics, is a Mesozoic sedimentary sequence which includes the Cretaceous Chimu formation, comprised of quartz sandstone, interbedded siltstone, shale, and minor coal. These Mesozoic rocks have been folded and faulted by early Paleogene compressional and late Paleogene to Neogene extensional tectonic events. Regional mapping by Lyons (1999) suggests that the erosional surface of the Chimu unit is highly irregular, similar to today's surface relief. A composite unit of andesitic volcaniclastic rocks, including lahars, breccias, tuffs, and flows, of the Calipuy Group was deposited on this surface and is the oldest volcanic unit at Tres Cruces. A sample from an andesite dike just north of the Property, believed by Lyons to be of equivalent age to the andesitic volcaniclastics, was dated at 31.1 Ma.
Underlying and intruding the andesitic clastic rocks are at least three andesite porphyry plugs (Tcap). They lie in the northeast corner of the Property, as well as below the mineralized zones in the core of the Property and in the southeast quadrant. Two small intrusions are relatively well exposed; however, their overall dimensions at depth are not well understood. The porphyry body below the central mineralized zones is not exposed. Where drill tested, porphyry contacts are steeply dipping (>70°), and usually dip outwards from the intrusive center. A sample of andesite porphyry intrusive from drill hole DTC-077 at a depth of 302 m, beneath the South Zone, was dated by Lyons (1999) using K-Ar radiometric dating at 25.1 Ma.
Following a period of significant erosion that leveled the surfaces of both the porphyry bodies and the volcaniclastic units, a later phase of Calipuy Group volcanism emplaced dacite porphyry (Tcdp) dikes and small domes, with associated ejecta, rhyolitic (Tcr) flows and volcaniclastic rocks. These felsic intrusive units appear to have been emplaced adjacent to the pre-existing contact zone (APV Zone) between andesite porphyry and andesitic volcaniclastic rocks.
A 50-m-wide by 80-m-long breccia pipe (Tchb) is present in the northeast part of the Property adjacent to a dacite porphyry body. Chimu quartz sandstone fragments locally constitute over 50% of some of the hydrothermal breccias, with rounded clasts up to 0.5 m in diameter. Drilling in the nearby Cuevas area intercepted beds of well-sorted sandstone and conglomerate with a significant quartz sandstone component. These beds lie on top of the andesites and were probably formed by sedimentary reworking of the diatreme ejecta (Battle Mountain Canada Ltd., Tres Cruces Staff, 1999b).
Hydrothermal fluids with associated mineralization followed the emplacement of the felsic volcanics, and hydrothermal centers with extensive breccia development were preferentially located in permeable channel-ways along the APV contacts. Heyl and Livingstone (1998) noted that rhyolites are often silicified by opaline and chalcedonic quartz and in the south part of the deposit this forms a silica cap over the mineralization.
During the waning stages of mineralization, a closed topographic depression formed near the center of the Property. Lacustrine andesitic to rhyolitic sediments, intercalated limestone, shale, chert and a second unit of lacustrine andesitic sediments, were deposited into this basin. The fresh-water limestone probably was derived from warm-spring bicarbonate solutions venting into the shallow lake. These units are not mineralized.
MINERALIZATION
Gold mineralization at Tres Cruces is mainly associated with pyrite, with occasional traces of other sulphide minerals. The auriferous pyrite is very fine grained, gray to blackish gray (commonly called "fine black pyrite”) and is present as disseminations and in very thin veinlets that locally form areas of stockworks (Macedo et al., 2012). Black pyrite also occurs as rims on earlier pyrite forming a colloform-banded texture. Clemson (Battle Mountain Canada Ltd., Tres Cruces Staff, 1999b) noted colloform-banding for this stage of pyrite, often with repetitive oscillatory bands of sooty and crystalline pyrite. The late-stage black pyrite is directly associated with the main-stage gold event. Early formed, non-gold-bearing pyrite is coarsely crystalline, anhedral to euhedral, poikilitic, and is evenly disseminated through the andesitic rocks. A second stage, also coarsely crystalline, is subhedral to euhedral and occurs primarily in veinlets and fractures. In addition, minor amounts of marcasite, arsenopyrite, galena, stibnite, realgar, orpiment, and enargite have been identified in some mineralized core. Some carbonate minerals and rhodochrosite are also found in areas of pyrite veining. Age of mineralization is bracketed between measured age dates of 25.1 Ma from andesite porphyry host rocks and 22.2 Ma from a post-mineral flow; however, there may have been multiple pulses of mineralizing fluids during this period.
Gold deposition appears to have been controlled by both structural and stratigraphic features. Subvertical controls of the mineralization are located along the intrusive contacts of andesite porphyry and dacite porphyry bodies. Steeply-dipping, commonly northeast-trending faults that cut the intrusive bodies may have provided deep-seated conduits for mineralizing hydrothermal solutions driven by heat from underlying magma. The rising high-pressure fluids caused the formation of breccia “pipes” containing fragments of sandstone that were carried up from older ynderlying rocks. Hydrothermal fluids carrying mineralization rose through the breccia and emanated laterally into adjacent flows, flow breccias and pyroclastic rocks. Rhyolitic pyroclastics in the upper part of the pile may have been less permeable than the underlying andesitic rocks, thereby causing the mineral-rich hydrothermal fluids to pool under the rhyolite and spread laterally for considerable distances. Decreasing temperature and/or pressure of the hydrothermal fluids initiated the deposition of gold and sulphide minerals within stockwork veins in the fractured host rocks. Mineralization has been found to extend only short distances into andesitic and dacitic intrusive rocks, which may be due to lower permeability or less favorable chemical characteristics.
The Tres Cruces deposit contains both oxide and sulphide mineralization. Gold-bearing sulphide mineralization is mainly hosted in the older andesitic pyroclastic rocks and to a limited extent in andesite porphyry. The rhyolites also host mineralization but only near or along the andesite-rhyolite contact. Most of the rhyolite hosted mineralization along this contact has been oxidized, possibly due to strong fracturing of the siliceous rhyolite, which allowed deep circulation of surface waters. The underlying andesitic rocks have developed only weak oxidization of the sulphide minerals, believed to be due to the abundance of clay minerals resulting in poor permeability (Battle Mountain Canada Ltd., Tres Cruces Staff, 1999b).