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The Caijiaying zinc-gold deposit, located in Zhangbei County, Hebei Province, P, R. China, approximately 180 km NNW of Beijing (#Location: 41° 25' 49"N, 115° 16' 9"E).
The Caijiaying deposit is located on the northern margin of the North China Craton, within the Neimeng Axis which is mainly characterised by high grade Paleoproterozoic metamorphic rocks, overlain by Yanshanian volcanic rocks and small related intrusions. This section of the North China Craton was stable from the Paleoproterozoic until the 208 to 90 Ma Mesozoic Yanshanian orogeny when intensive magmatism occurred.
The Paleoproterozoic is represented by the 2330 to 1800 Ma Hongqiyingzi Group granulite and gneiss with marble lenses. The metamorphic rocks, which have been strongly folded about near north-south trending axes, are unconformably overlain by the Late Jurassic Baiqi and Zhangjiakou Formation volcanic rocks which occur as north dipping, tilted layers that strike east-west. Volcanic rocks from the Zhangjiakou Formation some 180 km to the east of Caijiaying, have been dated at 135 to 130 Ma.
The intrusive rocks in the area include several stocks and numerous shallow dykes, mostly quartz porphyry or granite porphyry with minor dioritic and lamprophyre dykes. The relatively large, 10 sq. km, 141 Ma Xuejiagou Stock, which is predominantly composed of granite porphyry, is exposed some 3 km to the north-east of the Caijiaying deposit. Dykes close to Caijiaying appear to have been emplaced in multiple stages, spanning a period from before the main mineralisation to late phases which clearly cut across the ore. Pre-ore quartz porphyry dykes have been dated at 119 Ma. Some of the late dykes have flow-banding on their margins.
An ENE-trending, regional fault zone cuts across the southern tip of the Caijiaying deposit. To the north of the structure, the Paleoproterozoic metamorphic hosts have no volcanic cover, although they contain numerous dykes, while to the south, the Mesozoic volcanic rocks dominate at the surface, implying oblique uplift of the northern side of the fault.
The ores are predominantly hosted by the Paleoproterozoic granulite and gneiss with marble lenses. The main Zn mineralisation lies within north-south trending altered corridors separated by zones of barren rock. The zinc ore bodies are predominantly elongated north-south, with widths of 10 to 50 m and persisting over lengths of 20 to 500 m long.
The main ore bodies mostly occur as disseminated sulphides with local banded, folded or breccia textures. In the brecciated ores, the ore minerals and gangue cement angular fragments of the altered granulite/gneiss wall rock. Some of the ore bodies locally have margins rich in disseminated pyrite but poor in sphalerite.
The major ore mineral is sphalerite, with lesser pyrite, arsenopyrite, hematite and galena. The sphalerite is typically coarse-grained, from > 3 mm, up to a few cm in diameter, and ranges in colour from light brown to black indicating Fe-poor to Fe-rich, with the Fe-rich sphalerite dominating. These Fe-poor and Fe-rich sphalerite phases coexist and do not show any spatial patterns. The amount of galena varies from being subordinate in sections of the orebodies to being equal to sphalerite in other parts. Pyrite is locally arsenical. The sulphide minerals typically coexist with a gangue of coarse (>3 mm), euhedral to subhedral quartz and minor carbonate.
Away from the main orebodies, some 1 to 10 cm thick veins of quartz and/or siderite + sphalerite and minor galena are found.
Small irregular, rounded patches of wall rock are dispersed throughout the ore body with varying degrees of alteration, dominated by chlorite + fine grained quartz + siderite in some locations and by sericite + siderite at other places. Some of these altered patches form the cores of cockade quartz textures, while some sphalerite fills the interstices between euhedral quartz. Within the ore bodies there also are local patches of hedenbergitic pyroxene, ilvaite, epidote, actinolite, and fluorite.
The boundaries between Zn ore bodies/veins and wall rocks is generally sharp, with only very minor quantities of ore minerals occurring outside the ore bodies. The adjacent wall rocks display chlorite and/or sericite alteration, and locally fuchsite, while some of the bands within impure marble lenses are replaced by chlorite and epidote. Epidote veins are also found in some pre-mineralisation diorite porphyries and barren siderite, quartz, or chlorite veins cut across both the ores and wall rocks.
Gold in the form of native gold or electrum is mostly hosted in sphalerite, galena, and pyrite, with minor occurrences in actinolite, calcite and quartz as independent inclusion, along micro-cracks in pyrite, or at the contact between sphalerite and gangue minerals. Gold-rich zinc ores are visually indistinguishable form gold poor varieties.
Within, and in volcanics adjacent to, the main fault zone, epithermal-style chalcedony occurs as veins, breccia cement, or as clasts cemented by later chalcedony. The host volcanic rocks surrounding the veins are strongly altered by adularia.
There is no strong evidence for a metamorphic or metamorphosed origin for the mineralisation. Chalcedony veins and breccias with adularia alteration outcropping along the major fault zone indicate an epithermal association, although the abundance of Zn and the presence of hedenbergitic pyroxene in the orebodies are not consistent with epithermal mineralisation.
Pre-mining resource estimates comprise:
Indicated resource - 16.9 Mt @ 7.84% Zn, 0.75 g/t Au, (4% Zn eq. cut-off)
Inferred resource - 6.68 Mt @ 8.69% Zn, 0.49 g/t Au, (4% Zn eq. cut-off)
Total resource - 23.6 Mt @ 8.08% Zn, 0.59 g/t Au, (4% Zn eq. cut-off)
Indicated resource - 6.95 Mt @ 11.58% Zn, 0.64 g/t Au, (7% Zn eq. cut-off)
Inferred resource - 3.60 Mt @ 11.73% Zn, 0.5 g/t Au, (7% Zn eq. cut-off)
Total resource - 10.56 Mt @ 11.63% Zn, 0.68 g/t Au, (7% Zn eq. cut-off)
Total gold resource - 2.61 Mt @ 6.78 g /t Au.
Summarised from: Zhaoshan Chang, White N C, Crowe R W A and Woodhouse W, 2006, AESC2006, Melbourne, Australia.
Remaining JORC compliant resources (1% Zn cut-off) at June, 2013 (Griffin Mining website, 2014) for Zone II and Zone III combined, were:
Measured resource - 14.0 Mt @ 5.0% Zn, 0.3% Pb, 26.6 g/t Ag, 0.8 g/t Au,
Indicated resource - 12.1 Mt @ 4.0% Zn, 0.3% Pb, 23.3 g/t Ag, 0.5 g/t Au,
Inferred resource - 23.2 Mt @ 3.6% Zn, 0.6% Pb, 22.5 g/t Ag, 0.3 g/t Au,
Total resource - 49.4 Mt @ 4.1% Zn, 0.4% Pb, 23.9 g/t Ag, 0.5 g/t Au.
Total resource in Zone II - 29.8 Mt @ 4.7% Zn, 0.2% Pb, 23.4 g/t Ag, 0.7 g/t Au,
Total resource in Zone III - 19.6 Mt @ 3.3% Zn, 0.7% Pb, 24.6 g/t Ag, 0.3 g/t Au.
The most recent source geological information used to prepare this summary was dated: 2006.
This description is a summary from published sources, the chief of which are listed below.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.
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