The nine wires used in a USB 3.0 cable are shown below.
1 VBUS Power
2 D− USB 2.0 differential pair
4 GND Ground for power return
5 StdA_SSRX− / StdB_SSTX− SuperSpeed transmitter differential pair
6 StdA_SSRX+ / StdB_SSTX+
7 GND_DRAIN Ground for signal return
8 StdA_SSTX− / StdB_SSRX− SuperSpeed receiver differential pair
9 StdA_SSTX+ / StdB_SSRX+
Leads 1 through 4 are used for USB 2.0 connections. Leads 5 through 9 were added for USB 3.0.
VBUS and GND are power wires, and can be used to power a device, supplying either 100 mA or 500 mA at 5V.
Note that GND_DRAIN is not the same as GND. The two differential pairs are each wrapped with a ground shield, and GND_DRAIN is connected to this shield. GND_DRAIN is not internally connected to GND inside the cable. I assume that GND_DRAIN is connected externally to GND only at one end of the cable to avoid ground loops.
The differential pairs are outputs from one end, and inputs to the other, therefore the dual labels.
According to this article under the heading Bus Speed, the transmitter first tries to detect the termination of the differential pair on the receiver side. If none is found, then the host drops back to USB 2.0 and uses the D-/D+ pair for communication. From this I assume you might be able to get by without the D-/D+ leads 2 and 3.
If you are acting as a host, you must supply 5v on the VBUS line. If you are a device, you can ignore this line if you are self-powered.
So as a minimum, you might be able to get by with the six leads 4-9.