This was a five day field trip to Sichuan in southwest China, flying from Hong Kong to Kunming and then driving four hours in a convoy of 4WD vehicles on a newly built four lane toll highway to the city of Panzhihua.
The highway reminded me of those in Europe with tunnels going through hills and bridges spanning valleys, making a fairly level road in hilly countryside. The following three days were spent visiting three mines of magmatic and hydrothermal related ore deposits: the Panzhihua and Hongge Fe-Ti-V deposits hosted in mafic-ultramafic plutonic rocks and the Lala Fe-Cu-(REE-Mo-Au) IOCG (iron oxide copper gold) deposit in schist.
The tour was led by Prof Zhou Mei-Fu of the University of Hong Kong, Prof Ma Yuxiao of Chengdu University and Prof Liu Lixin of Panzhihua University. They were assisted by Dr Wei Terry Chen and Yan Wang Christina of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, and several other university staff and research students, Geological Survey staff and the mine staff. There were six registered PACRIM participants from NZ, Australia and South Africa, plus about 16 others from the Universities of Hong Kong, Panzhihua and Chendu, all fluent in English.
Panzhihua city grew up adjacent to the Panzhihua mine in the 1970s and now has a population of about 1.2 million. It has a spectacular setting at the confluence of the Jinsha and Yalong Rivers with the city built on both sides of the river valley – the locals must be very fit to walk around the city.
Our group stayed in the Panzhihua Xuefu Hotel owned by the university. It was very plush, but with very hard beds! Our arrival at the hotel was televised (along with one of the mine visits) and on the first night we were hosted to a banquet organised by the university, and on the second night another banquet hosted by the geological survey. We also had a special ceremony at Panzhihua University where we were conferred as Honorary Visiting Professors at the university – we have the certificates to prove it! To record the event, we assembled for a group photo at the university the following morning.
Day two of the field trip was our first mine visit to the Panzhihua mine immediately west of the city. The Panzhihua deposit (ca. 260 Ma) is one of the largest magmatic Fe-Ti-V oxide deposits in the world, containing 1,333 Mt with an average grade of 43% FeO, 11.7% TiO2 and 0.3% V2O5. The intrusion is a 19 km long, c. 2 km thick sill that strikes NE-SW and dips 50-60oNW. It is a gabbroic layered intrusion with iron-rich layers in a 60 m thick zone near the base (Figure 1). These layers contain titanomagnetite and titanium-bearing magnetite (both with ilmenite exsolution), and minor ilmenite, pyrrhotite and pentlandite. The footwall is marked by marble with local development of skarn minerals.
We started with an overlook stop followed by viewing a video of the development of the mine. The deposit was known from the 1930s, but was not explored until the 1960s when it was officially discovered. It came into production around 1974, and that is when the city was developed. We had several stops in the mine including a picnic lunch. There were excellent examples of compositional layering that generated lots of discussion and many samples were taken.
On leaving the mine we went to an overlook well above the mine to view the change in dip from the mine sequence to faulted juxtaposed younger rocks. Then we walked down the road to look at the upper section of gabbros cut by pale-coloured dikes dominated with plagioclase plus minor pyroxene (some with epidote veins). Then down into another part of the mine to look at another pit for an overlook at the last stop of the day.
Day three was a visit to the nearby Hongge Fe-Ti-V deposit that is hosted in a 16 km long, 3-6 km wide (but 2.7 km thick), lopolith-like intrusion (ca. 260 Ma), dipping at shallow angles to the E and NE, and intruding metasediments, metavolcanics and schists. It consists of a lower olivine clinopyroxenite zone, middle clinopyroxenite zone, and upper gabbro zone. Thus the Hongge intrusion differs from the Panzhihua intrusion in having the ultramafic components. The ores consist of titanomagnetite, ilmenite, and minor apatite and hornblende. They are best developed in the middle zone and lower part of the gabbro zone. Pre-mining, the Hongge intrusion contained 4,572 Mt of ore with 1,830 Mt Fe, 196 Mt Ti and 14.7 Mt V.
We viewed gabbros, compositional layering and abundant titanomagnetite. There were also some spectacular examples of breccia.
A break for a picnic lunch, then on to another part of the pit where we spent a long time looking through stockpiles and in various discussion groups. We left the pit and drove up a hill for a lookout from the roadside for mine vistas and a walk to weathered outcrop of some ultramafic layers. At the last stop, we walked down the hill, very steep in parts, to a former small pit to look at fresher olivine-bearing ultramafics with titanomagnetite (unusual because titanomagnetite is usually in the gabbros). On the way back to Panzhihua, we took a side trip to the Ertan Dam with dam abutments in syenite that intrudes flood basalt.
On day four we visited the Lala Fe-Cu (IOCG) deposit, a giant hydrothermal Fe-Cu deposit in the Kangdian IOCG province, containing 200 Mt of ore with an average grade of 13% Fe, 0.92% Cu, 0.018 % Mo, 0.022 % Co, 0.25 % REE2O3 and 0.16 ppm Au. The main magnetite stage of mineralisation at 1,100 Ma is strata-bound and occurs as NW-SE striking, south dipping, bands and lenses conformable to the host schist. Some of this was remobilised around 850 Ma to form veins cross cutting the conformable mineralisation. The veins are very prominent because of abundant chalcopyrite. This remobilised coarse chalcopyrite was the most obvious feature in looking at the rocks in the pit. The two styles of mineralisation collectively have a wide variety of minerals including quartz, K-feldspar, magnetite, pyrite, chalcopyrite, pyrrhotite, bornite, molybdenite, carbonates (siderite, calcite, ankerite), fluorite, apatite, biotite, muscovite, allanite, titanite and the REE-bearing minerals uraninite, monazite, zenotime, bastnaesite, churchite and parasite.
The Lala mine visit required a three hour windy drive from Panzhihua through hilly countryside and many villages. It was a great opportunity to see the rural environment. On arrival at the mine village, we had lunch hosted by the mine. Then we drove up a very short distance for a pit overview and lecture from Terry. We went down into the pit and looked and sampled some of the stockpiles and viewed some of the pit walls. Then we drove to an upper bench to look at a very weathered area with molybdenite.
Back at the village we visited the mine museum which had numerous photos, cross sections and plans on the walls. We finished the visit at the drill core storage area where several trays had been laid out on the ground for us to view. We had a very fast drive back to Panzhihua reducing our return travel time to two-and-a-half hours. It was a very bumpy ride and very exciting especially passing lots of slow trucks, even on blind corners. In the evening, we had a very pleasant meal at a chicken restaurant, specialising in a cook-your-own chicken meal in a wok over a fire within the brick table.
Following farewells, day five was a return three hour drive to Kunming and flight back to Hong Kong for some of us, whereas others went to different destinations. Many thanks to the tour guides who provided an enjoyable and informative field trip and cultural experience. The mine visits were excellent, unhurried with lots of discussion amongst the tour group and guides, and a real insight into the processes that form magmatic ore deposits. Lots of reading material was provided in the form of a guide book containing many published papers on the deposits.